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Marv [UK]
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PostPosted: Fri Feb 13, 2009 3:46 pm    Post subject: Cam Theory Reply with quote

The majority of posts on here are queries about what head, what cam, what carb etc and for the most part, the search function does a good job of returning decent answers but the one thing i've not found, and most questers for knowledge can't seem to find, is a thread devoted to cam theory.

Now i'm no expert... more of a keen amateur.... and i'd say I have a fair to middling idea of just what goes on in an engine but the "dark art" if you will, of engines is always the cam.

Real experts will quite happily say things along the line of "you need a CR of x to 1 with that cam" which is something that is usually borne of experience.

What I think would be a good idea is to give a guide to cam theory that can be easy to follow and give the basics of what duration etc actually does to a motor..... I'm not saying the recommendations type of "an FK8 is perfect" type of theory but honest to goodness "what is overlap" type of theory.

so, i'll begin with what I hope is the correct type of information and then if the cam experts can either correct me, fill in the blanks or just add a little bit of their experience into the mix which will give that little bit of extra knowledge in one place for everyone to utilise..

Please, no recommendations of cams, just the theory so that its a reference tool Smile

I'm going to 'borrow' an image of a cam card for reference purposes from This site as it's quite good for the number of cams listed.

it's a card from an Engle W125 and here it is

Image may have been reduced in size. Click image to view fullscreen.


so the basics of a cam are

advertised duration
duration at 0.050"
lift at cam
lift at valve with x:1 ratio rockers
Lobe centres

then the cam is specified as

intake opens (degrees Before Top Dead Centre)
Intake closes (degrees After Bottom Dead Centre)
Exhaust opens (degrees BBDC)
Exhaust closes (degrees ATDC)

The cam in an engine runs at half the revolutions of the crank, but the cam is specified as duration of rotation of the crank... bear with me.... for example with the W125, the pertinent information on opening degrees for the intake is 26 degrees before the crank moves the piston up to TDC.

As the cam rotates at half the speed of the crank, for each roatation of the crank, the cam turns half. Bearing in mind that the 4 stroke cycle of induction, compression, combustion and exhaust takes 2 full revolutions of the crank, the cam rotates once.

The cam card illustration can mislead. the intake circle and exhaust circle only overlap once, not twice as it appears on the illustration.

The intake valve opens before TDC on the induction stroke, the cylinder sucks gas and reaches BDC and commences the compression stroke where the intake valve shuts. Compression of the intake charge increases towards TDC during which ignition occurs, usually somewhere between 40 to 30 degrees BTDC. This allows the spark to propagate throughout the mixture leading to the maximum amount of force being applied as the piston passes TDC and maximum compression. At this point both valves are still closed, however, as the piston approaches BDC and the combustion has fully, or at least mostly, occured, the exhaust valve begins to open so that as the piston passes BDC and commences the exhaust stroke, compression of the hot combustion gases does not occur and the waste products of combustion are free to pass through the exhaust.

As the piston continues to travel towards TDC allowing the combustion gases to escape, the Intake valve begins to open in anticipation of the induction stroke.

At this point, both valves are open which is the condition known as overlap and the whole cycle repeats

The key to a cam is volumetric efficiency, or the ability of your cylinder to extract the maximum amount of potential power from it's volume. this depends on the flow charactereistics of you heads and carburettors etc, but the basics are that when the engine is taking an intake charge into the cylinder, VE is less than 100% which creates the vacuum. As the intake stroke turns to the compression stroke, the intake valve remains open as the vacuum (or pressure differential between the atmospheric pressure and the pressure inside the cylinder) continues to bring in the intake charge. The trick is to pick a cam that will gain the greatest intake volume before the compression stoke begins to blow it back out of the intake. Similarly with the exhaust stroke. The pressure built up by squeezing the exhaust gasses out of the cylinder will continue after the cylinder has reached TDC. Overlap is where the exhaust gasses are on their way out and the intake valve is open.......

The duration of a stock cam is (and experts, please correct me if I'm wrong here)
Duration--- Dur @ 0.050"------Lift -------- Lobe Cent
250º---------214º-------------0,334"------- 108º

The duration of 250 degrees is where the start of the egg shape of the cam raises the lifter to move the valve to the end and is based on crankshaft degrees, i.e for 125 degrees of the cam , there is a lobe. For the remaining 235 degrees of the cam, the lifter is not raised.
For 250 degrees of one rotation of the CRANK, the valve is moving.

Still, this is not the impotant information relating to the performance of your engine. the duration at 0.050" is the more relevant as to all intents and purposes, until the valve is open 0.050" nothing is happening. It is effectively closed

The difference between full duration and duration at 0.050" will tell you how steep the cam ramp is or, more importantly, the shape of the cam lobe.

If the difference between the two is small, then the ramp is steep and the cam is more rounded which allows the valve to be open to a greater extent for longer. If the difference is larger, the ramp is shallower and the cam more pointed.

A rounder cam allows more breathing for the same amount of lift but creates greater forces in your valve train. A steep ramp is less suited to large ratio rockers

The lobe separation is the number of degrees around the cam between the centres of the intake cam and the exhaust cam.

The lift of the lobe is the distance from the circular "back" of the lobe to the "tip" of the lobe perpendicular to the axis of rotation.

In General, the exhaust and intake lobes of a VW cam are the same but it's not a given. Cams are tuned to the application and can come with different chaped intake and exhaust lobes


Ok, so that is the basics of the cam...

But what happens when you alter the properties of the cam. Increased duartion etc.

This is where the experts on cam theory could contribute

From what I have gleaned, the following is a general guide under the "change one thing" principle

When comparing two cams
Assuming that the lift is the same and cam lobes are the same shape
Increasing the duration of a cam will increase the power and also move the power band up the rev range

Assuming that the duration remains the same and cam lobes are the same shape
Increasing the lift of the VALVE will increase power and torque generated by the motor but leave the rev range largely unchanged. It will move up the rev range slightly but not as much as increasing duration

Increasing the steepness of the ramp will have a similar effect (when isolated from other variables)



Please can the cam experts contribute their knowledge and experience to this thread to describe the effects of changing the specifications of a cam will have on a motor and why compression makes a difference with some cams and not others.

What difference does lobe separation make when changed
what kind of percentage difference does 10 degrees increase in rotation have
what defines the maximum capability of a cam, why do certain cams fall flat at 6000 revs
Why will some cams float valves at revs where others wouldn't

And variations on a theme, especially turbos!

Cam theory really interests me as the cam is the biggest single performance controller in the engine. Displacement is the producer of power, but without a compatible camshaft to control the valvetrain, the efficiency of the displacement can be sorely tested.
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ALB
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PostPosted: Fri Feb 13, 2009 5:54 pm    Post subject: Reply with quote

Marv- Great piece! I'm sure a lot of people will benefit from it.

One comment (and please someone correct me if I'm wrong); doesn't increasing the steepness of the ramp widen the power band or produce more torque in the low and midrange? Engle's VZ series and the FK40's series use steeper ramps and more lift to create more all around power.

Anyone that has a better understanding of camshaft design care to shed some light here? Al
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PostPosted: Sat Feb 14, 2009 4:19 am    Post subject: Reply with quote

ALB wrote:
Marv- Great piece! I'm sure a lot of people will benefit from it.

One comment (and please someone correct me if I'm wrong); doesn't increasing the steepness of the ramp widen the power band or produce more torque in the low and midrange? Engle's VZ series and the FK40's series use steeper ramps and more lift to create more all around power.

Anyone that has a better understanding of camshaft design care to shed some light here? Al


Al, I'd love to be in a position to confirm or deny whether increasing the steepness of the ramp widens the power band but this is one of those variables that a) I have no experience with and b) i've read contradictory things about.

Cam theory to a degree (no pun intended) is a fairly straightforward "it's egg shaped and it does this"

The reason I wanted to start a thread like this is to try to demystify the basics of the camshaft and what it does, but also to have the experts on the dark art put forward their experience and knowledge about what happens when you change a variable from stock

It's not an attempt to stop people asking the "whats best with my combo" questions, but more to share the understanding of what changing cam variables accomplishes.
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PostPosted: Wed Feb 18, 2009 2:13 pm    Post subject: Reply with quote

Whew! Reading through all that stuff gave me a headache. Couldn't you have written this simpler, Marv?

"DArk arts"...in camshaft design? I am not so sure if there is any dark arts in camshafts. It is a simple device.. a stick w/ lumps on it. It moves the valve head off its seat..ye amount of time. Big deal.

HEAD WORK... this is where all the dark arts lie. Very Happy

Or... making the lower-end survive abuse, make them bullet-proof. These are probably more of the topics we need. But what the hell.

Just do not choose the cam and build and engine around it. Design the engine and choose the appropriate cam for it. You do NOT need to lift the valve to 0.500 inch, if the ports are done flowing at say.. 0.300 inch.
Gotta watch that "hang time".

Either gather all the airflow using the valve head area (increasing valve size) or increasing lift at the valve head or both. Port characteristics, intake system, and how the lower end is configured...long rods vs. short rods all come into play when you choose the cam.

The head porter will tell you which cam you will need.

Just my 2 cents.
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PostPosted: Thu Feb 19, 2009 1:17 am    Post subject: Reply with quote

nsracing wrote:
Whew! Reading through all that stuff gave me a headache. Couldn't you have written this simpler, Marv?

"DArk arts"...in camshaft design? I am not so sure if there is any dark arts in camshafts. It is a simple device.. a stick w/ lumps on it. It moves the valve head off its seat..ye amount of time. Big deal.

HEAD WORK... this is where all the dark arts lie. Very Happy

Or... making the lower-end survive abuse, make them bullet-proof. These are probably more of the topics we need. But what the hell.

Just do not choose the cam and build and engine around it. Design the engine and choose the appropriate cam for it. You do NOT need to lift the valve to 0.500 inch, if the ports are done flowing at say.. 0.300 inch.
Gotta watch that "hang time".

Either gather all the airflow using the valve head area (increasing valve size) or increasing lift at the valve head or both. Port characteristics, intake system, and how the lower end is configured...long rods vs. short rods all come into play when you choose the cam.

The head porter will tell you which cam you will need.

Just my 2 cents.


You are of course entirely correct, but bear in mind that a lot of the posters on here don't understand what the camshaft does other than open and shut valves.

What my original post was intended to do was to properly describe the mechanism of the camshaft, then go on to discuss what changing each variable over and above stock has on a motor. It was intended to give people a bit more of a clue as to what sort of performance each variable dictates so that when the inevitable "what cam should i use" thread comes along, these guys are armed with a simple knowledge of what they are asking. "FK8" as an answer is perfectly acceptable, but no one ever says why.

What I initially hoped for was a discussion with contributions along the lines of "in general, add 10 degrees of duration and you'll move your peak power 250 revs up the range" sort of thing, but I have since come to realise with my reading that this is to simplified

I know heads are where the power lies but then there still has to be a combination that works, but this is not about power per se, more about what a cam will do to any given combination when you change any variable

I've researched a fair bit more on the dynamics of the humble bumpstick in the last few days and i'll be posting a couple of big posts about it fairly soon, for no other reason than I want to. Please feel free to disregard what I post or contribute.

It's always good to learn
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PostPosted: Thu Feb 19, 2009 8:19 am    Post subject: Reply with quote

Ok so here it is….

I basically decided that I’m going to have a bash at finishing off the thread myself and answer the questions I’ve asked. I’ll qualify everything I’ve written on the basis of the following.

This thread is written purely on a theoretical basis and none of the information I’m going to discuss is won from experience. It’s all gleaned from reading around the subject and from many freely available web articles. I’ll not stand by the accuracy of the information as I have not come by any of it myself through experimentation, just through a desire to know more about cams and why they are so important and when the characteristics of the cam change, what the effects are on the performance of an engine.

Any engine… I’m not going to go into displacement or heads which is where the power comes from, just what the cam does to the potential of any given combination when you change a thing or two.

Basically, what I’m saying is that this is not gospel and I may well have got large portions of it wrong.

Where possible (mostly to do with me forgetting to) I will reference where I came about the information but suffice to say, this is the way I see it. It may not be gospel and you may disagree. Feel free to disagree with me if it means I learn something Smile

We’ve discussed the information that a cam card contains ( namely lift, duration, lobe centres etc) and that the steepness of the ramp can be judged by the difference in the advertised duration and the duration at 0.050” but what does it all actually do

Well, stock lift allows for a small (very small) overlap of the intake and exhaust valves. We’ve said that the stock cam has advertised properties of

Duration--- Dur @ 0.050"------Lift -------- Lobe Cent
250º---------214º-------------0,334"------- 108º

But all of the above information is almost useless unless you know when the intake valve opens. Once you have this information the remainder of the cams properties can be worked out from first principles.

The stock camshaft opens the intake valve at -1 degree BTDC @ 0.050” of lift (from Webcamshaft cam card Here Before anyone objects, I know this is actually AFTER TDC technically but it’s a convention apparently

This allows your cam to be totally worked out as follows

-1 + 180 + 35 = 214
214 / 2 = 107
107 - -1 = 108 degrees for lobe centre of intake cam

Similarly for the exhaust valve

35+180+-1 = 214
214 / 2 = 107
107 - -1 = 108 degrees

This shows that the intake valve cam centre line is 108 degrees AFTER top dead centre
But it also shows that the exhaust valve cam centre line is 108 degrees BEFORE top dead centre which make sense

Just because you know the lobe centres are 108 degrees off the centre line of the lobes does not mean that they are 108 degrees either side of TDC. If you look at the W125 posted above you will note that if you do the maths, intake centre line is 105 degrees ATDC with exhaust being at 111 degrees BTDC. An overall offset of 216 degrees but NOT 108 degrees either side of TDC. Without knowing when your cam opens at least one of the valves, there is no way of knowing exactly where the lobes are. This is why you need a cam card with every cam in order to degree it properly. Note: the lobe separation of a cam is measured in CAMSHAFT degrees, not crankshaft. The above cam has lobe separation of 216 crankshaft degrees, and as rotation of the cam is half of the crankshaft, this equates to 108 degrees of total lobe separation on the camshaft. The lobe separation angle as quoted in the blurb does not mean that the intake lobe is that number of degrees after TDC.

With this stock cam, assuming an overall advertised duration of 250 degrees has an overlap of 34 degrees. Bear in mind that with both valves only open a fraction, if you work it out using the 214 degrees at 0.050” of lift, there is no overlap at all. This is because the general consensus is that not a lot happens at 0.050” lift


Now you can work out exactly what is going on with your cam, we can get back to the basics of what each change does over stock We’ve already said that:

Assuming that the lift is the same and cam lobes are the same shape
Increasing the duration of a cam will increase the power and also move the power band up the rev range

Assuming that the duration remains the same and cam lobes are the same shape
Increasing the lift of the VALVE will increase power and torque generated by the motor but leave the rev range largely unchanged. It will move up the rev range slightly but not as much as increasing duration

But how about a bit more of an explanation?


Last edited by Marv [UK] on Thu Feb 19, 2009 8:37 am; edited 1 time in total
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PostPosted: Thu Feb 19, 2009 8:20 am    Post subject: Reply with quote

Torque is power. There is no other measure of engine capability. All that ever happens on a dyno is the conversion of torque into the easier to understand horsepower. The commonly used conversion is thus:

(Torque x Engine speed) / 5,252 = Horsepower

The reason for this is that a dyno measures how fast you can turn a big roller and then converts that into torque, then horsepower. Horsepower is just another expression of torque and is 550 foot-pounds per second

Where does the cam come in? Well, cams control the valves hence they control compression. Stump pulling low end power is derived from having a huge amount of pressure in the cylinder when it is ignited which sends the piston down the cylinder at a fairly high rate.

Compression is controlled by having the inlet valve close very early on the compression stroke before the intake charge has a chance to pulse back out of the port, or a low duration if you will.

If you can imagine a cylinder head with no flow restrictions where the valves are either entirely open of completely shut with nothing in-between and a head that can flow whatever CFM you demand, you could happily achieve 100% volumetric efficiency, the perfect scenario would be for the valve to be open fully at TDC and then shut fully at BDC. This would give you a 100% compression ratio and you would be able to get 100% of the available power out of the combination of volume and explosive properties of the fuel. Similarly, when it came to exhaust, there would be no requirement for the valve to open before BDC nor be open after TDC on the exhaust stroke. Zero Overlap, Full compression, full power.

Unfortunately, these physical characteristics of the head cannot be achieved and any normal cylinder head cannot achieve anywhere near 100% volumetric efficiency as a valve is not an ON / OFF switch and until they invent solenoid activated inertia free valves, there will always be a need for a camshaft to actuate the valves. Similarly, there will always be a flow restriction on the heads that will create an efficiency loss whether it is a restrictive port volume, shape or just simply the valve itself.

This is why cams need overlap as a device for gaining performance, or at least, defining the performance characteristics. In an ideal world as above, each engine would have 100% of the performance of the engine per rev (please note per rev, it will be important later) at any time. In the real world, the engine is always going to be performing somewhere below it’s maximum and the cam choice will define where and when the optimal performance is achieved. (for those pedants among us, yes, this does not apply to turbo motors which have their own set of rules as they can achieve in excess of 100% VE)

This graph is entirely made up but it’s to prove the concept, not actual figures in any way (if you can't see it it's because I haven't edited it in yet)


Low duration camshafts, with little or no overlap allow the engine to achieve it’s peak torque at low revs as there is little to no compression lost as the intake valve closes early in the compression stroke. Converting the potential energy of the fuel air mix is relatively easy. When camshaft duration is increased, overlap increases significantly as does the closing point of the intake valve on the compression stroke. When there is significant increase in the duration, at low revs, intake charge is able to escape through the intake port during the compression stroke and compression is lost.

Please note that overlap is not a bad thing. Too much can be though!

Overlap allows the exhaust valve and intake valve to be open at the same time, and if you use an appropriate exhaust system, this can actually assist in bringing intake charge into the engine by increasing the vacuum created in the cylinder. The effect, when it works correctly is known as ‘scavenging’ and relies on an exhaust pulse from another cylinder creating a vacuum in the exhaust for this cylinder, pulling the exhaust gases out of the cylinder and helping to pull some intake in. The shape of the combustion chamber takes a role in scavenging too with ‘flow’ from the intake valve to the exhaust valve shaped by the cut. Getting it wrong can lead to intake charge backfiring in the exhaust, or worse, the intake, and also contamination of the intake charge.
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PostPosted: Thu Feb 19, 2009 8:22 am    Post subject: Reply with quote

Overlap does not affect the compression stroke in any way, however, the increased duration shifts the peak power output up the rev range by dint of the intake port closing later in the compression stroke. Due to the properties of air and the fact that intake ports are naturally restrictive, at higher revs with the pistons moving faster, compression is achieved before the intake valve is shut as the air simply can’t get out fast enough. Less compression is lost during the compression stroke the faster the pistons are travelling, or in simple terms, the higher the revs, the higher the actual compression.

This is the reason why many “wild” cams idle like a bag of nails as there simply is not enough compression for the fuel to ignite properly, whereas they tend to “come in” at higher revs where they achieve sufficient compression whether the valve is open or not.

A short duration cam will fall off earlier in the rev range than a longer duration cam as, at higher revs, the long duration will allow a greater length of time to suck in the intake charge. At high revs, a short duration cam will starve the cylinder of intake charge and the torque generated will decrease. The engine will choke on the limited window when the valve is open.

The total amount of torque per rev, or horsepower (torque per second) generated by a short duration cam will be greater than long duration at low revs and the opposite is true at high revs.

The measure of the cam in these circumstances is known as the dynamic compression.

The Static Compression Ratio (SCR) of any motor can be calculated using deck height, chamber cc’s, bore and stroke, but this will never be achieved. This is generally ascribed to the intake valve being open whilst under compression allowing charge pressure to escape. The Dynamic Compression Ratio (DCR) is calculated from when the intake valve closes (0.050” of lift) and uses the stroke of the piston remaining after the valve closes to calculate the compression ratio.

For example, on the case below I am assuming that a 2110 cc motor (90.5 x 82) running 10:1 static compression is using the W125 cam above.

Static compression ratio is 10:1

Using the DCR calculator HERE

DCR is then worked out as 8.47:1

Significantly less than static and definitely in the range of normal pump gas but possibly a little bit high. Dropping the SCR down to 9.5:1 then reduces DCR to just a fraction over 8 : 1 at 8.06. Much better (a matter of opinion of course)

The DCR of any engine will not factor in any scavenging effect the exhaust has on the intake charge, nor any contamination has on spark propagation. Nor will it account for insufficient flow on the head allowing the intake valve to close where there is still less than 1 atmosphere of pressure in the cylinder at the start of the real compression.

Even on the stock head with the stock cam, the intake valve is open for 35 degrees of rotation after BDC to allow for the vacuum in the cylinder to equalise and to allow the intake charge momentum to drag more in with it.

The skill in choosing a cam resides in knowing the flow of the head and being able to gauge the point where the intake valve closes just as the pressure in the cylinder reaches atmospheric pressure. This can also be expressed as 100% volumetric efficiency for the remainder of the stroke.

This explains a lot about why certain cams work with certain engine combinations.

A stock 1600 with a stock cam operates at 7.5 : 1 SCR. Factor in the cam characteristics and the DCR of a stock 1600 is 7.05

This demonstrates that a stock short duration cam loses less Dynamic compression than a long duration cam.

In short, this explains why a long duration cam moves the power band up the rev range and also why a large displacement engine with a long duration cam can handle a wild cam and a high static compression ratio.

The power per revolution is less, but the engine can breath so can produce more revs. 120 horsepower is 157.6 ft lbs of torque per second which equates to 2.364 ft lb per rev at 4000 revs but 175 horsepower is 114.8 ft lbs of torque which equates to 0.86 ft lbs per rev at 8000 revs. Less per rev but way more revs gives a greater total per second (or horsepower).


Last edited by Marv [UK] on Thu Feb 19, 2009 8:38 am; edited 1 time in total
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PostPosted: Thu Feb 19, 2009 8:22 am    Post subject: Reply with quote

So out of the list of:

Duration
Overlap
Lift
Ramp
Lobe centres
Compression

We have covered everything but Lobe centres, lift and ramp.

As discussed, lobe centres of the cam are the number of degrees apart that the centres of the lobe are apart. However, don’t allow yourself to fall into the trap of thinking that they are evenly spaced either side of TDC.

Changing the lobe centres on a camshaft from the stock separation of 108 degrees can have different effects depending on which way you go. I

If you decrease the separation, this will INCREASE the valve overlap but will decrease the amount of time after bottom dead centre that the intake valve is open on the compression stroke. This will have two effects, the most important of which will be an increase in your DCR. The other effects will be more exhaust escaping down the intake port or decreased scavenging or increased contamination coupled with the valve beginning to close right when you need it at it’s most open, namely at the pistons maximum velocity on the intake stroke where the maximum charge is taken in. This decreases the volumetric efficiency of the cylinder. You will find more normally aspirated cams are lower than 108 degrees of separation than above. The increase in DCR is the most important factor in reducing lobe centres but relies upon good head design to handle the overlap

If you increase the lobe separation, you decrease valve overlap which can have benefits with scavenging depending on your head and exhaust combinations but the downside is that it will decrease your DCR

For comparison, the same 2110 as I discussed earlier, with a 9.5 to 1 SCR on W125 cam with 108 lobe centres is compared below with the lobe centres changed both ways.

108 centres = 8.06:1 DCR

106 lobe centres increases the opening of the inlet valve to 28 degrees BTDC but it closes earlier at 54 ABDC giving a DCR of 8.16 : 1

110 lobe centres does the opposite giving a DCR of 7.98 : 1

This doesn’t tend to have particularly big consequences so it’s probably best just to leave it as it is at this point.

Ramp is a very important concept in cam design. Ramp is expressed in lift per degree of rotation of the crankshaft and is the defining characteristic of the cam shape. If the ramp is steep and the duration long, the cam is going to be more oval than egg shaped especially if the lift is relatively low, and will leave the valve at higher lift for longer. This will allow greater breathing of the fuel and will increase power at a given max lift and compression. There are downsides insofar that a fast ramp will increase wear in the lifter bores due to the side loading and will increase wear on both the cam and the lifters due to the lateral forces. The steepness of the ramp can be guesstimated by subtracting the duration of the lift at 0.050” from the advertised duration and dividing all by 2.

A stock cam works out as (250 – 214)/2 = 18 degrees. When valve clearance is set at 0.006”, the ramp moves the lifter to close the lash and lift the valve to 0.050”, or 0.056” total in 18 degrees. 0.00311” per degree if you will

Looking at the W125 above, the ramp works out as being (301-262)/2 = 19.5 degrees but the valve lash is set as 0.004” meaning the ramp is only 0.00276” per degree. Milder than stock.

This is only a guide as to the ramp steepness. A stock cam only lifts to 0.334” whereas the W125 lifts to 0.430” so the stock cam will curve to flank a lot sooner than the W125.
If you look at the webcam 86a it works out as being somewhere inbetween the above two but has a huge amount of lift at 0.502”.

Once the cam is pushing the valve down, on a stock cam it then has half of 214 degrees to achieve 0.334” of lift, or 0.00312 per degree, on the W125 there is half 262 to achieve 0.430” of lift, or 0.00164” per degree. This is only a linear guide. A cam is Not linear and the lift will change over the stroke of the lifter some but as a rough guide, it is relevant.

The advertised duration is not an accurate measure of the start point of a ramp and can be quite a closely guarded secret, but it will give you a good clue as to whether a ramp is steep or not. A steep ramp with high ratio rockers will give big opening for a long time but will put a huge amount of stress on the valve train and lifter bores. A shallow ramp and low lift is much more beneficial to the bottom end but will affect performance somewhat.

Bearing in mind that ramp steepness is more involved with choosing your rockers than your cam for the valve lift you want, I’ll leave it there. Suffice to say, your duration is increased with a higher ratio rocker as is your valve lift which will ultimately move your power band further up the rev range but give you more power. Additionally, the higher lift for longer will benefit the lower reaches of your power band as it will allow greater breathing so your power band will be broader.

Cam lift is closely related to the steepness of a cam ramp as the steeper the ramp, the more lift is attainable for the same amount of duration. More lift equates to more breathing in of intake charge and consequently more power. Increasing the valve lift from stock by adding ratio rockers in effect will increase the duration of the cam and will have a marginal effect on moving the power band up the rev range, but apart from that, the big effect is simply more power at any given revs. The more accurate definition is that increasing the valve lift for any given cam duration will broaden the power band and increase the max power available. This is where high ratio rockers come in to multiply the cam characteristics without the huge stresses. There are cams with steep ramps and low lift specifically designed for high lift rockers that allow the valve to be at its highest lift for longest. These are where the high ratio rockers are most suited to give you the broad power availability.

So that’s pretty much it as I see it. As far as I know, everything about cams has been covered and at least considered. Not every effect may have been mentioned but then again, I’ve never pretended to know everything there is to know. I’ve used a lot of reference material to plug this little lot together and it’s only fair that my sources (probably not all, but the most relevant) are mentioned and that’s going to happen at the end.
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Marv [UK]
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PostPosted: Thu Feb 19, 2009 8:23 am    Post subject: Reply with quote

The summary and the entire point of the thread so far, is this…

An increase in duration over stock will push the power band up the rev range as it will increase valve overlap (allowing the engine to breathe more) but is likely to reduce compression at lower revs as the intake valves will be open later in the compression stroke. A large duration cam will allow the engine to be built with a higher static compression ratio as the dynamic compression ratio will be the yardstick to actual compression.

The affect of the longer duration will be to reduce the torque generated per revolution of the engine, but will lead to a higher overall amount of torque per second (horsepower) by allowing a higher number of revs to be attained.

A steep cam ramp will ultimately increase your duration by allowing the valve to be open for longer at higher lift. This too will increase your total horsepower and move your power band up the rev range, but it will give you a wider power band as the lower revs will receive more air and create more compression and thus, torque.

Cam ramp steepness is linked to the overall lift of the cam lobe. Increasing the cam lobe lift for the same duration of cam will broaden the power curve and increase the max power. It will marginally move the power curve up the rev range but the broadening will account for the low revs which should see increased torque.

From stock

Increase duration – increase available revs and overall horsepower.
Increase lift – broaden power band and increase overall power
Increase cam ramp steepness – increase duration
Increase or decrease lobe separation – alter valve timing with potentially beneficial results

I’ve not touched on increase of displacement and the effect that this can have on any given camshaft as it’s not that simple but as a rough guide, the following holds water

If you want low down torque, keep duration Low but DCR to around 8 to 1 or whatever suits your fuel
If you want to rev to twice the stock revs, increase duration but keep your DCR to around 8 to 1 or whatever suits the fuel you use
If you want to increase power for given revs, increase your valve lift but bear in mind what affect this will have on your DCR

One other thing I have learned in all of the reading I’ve done recently, is to keep your port velocity below 0.6 of the speed of sound!!! Shocked but that looks like a minefield to me!

I’ll apologise now for the length of this post and quite possibly for annoying some people with the sheer boredom of reading it but I think it’s something that was needed and to be honest, I’ve enjoyed writing it and learning more about cams. If you feel or know that I’ve done something wrong here or even contradicted myself, please let me know. As I’ve said, it’s a learning experience and is not advertised as being definitive and knowledge is power after all. Please keep it constructive though Wink

Also, please feel free to add to it Very Happy





















As for my references……and in no particular order ( and probably incomplete )

www.summitracing.com
http://www.phnet.fi/public/hefor1/eng/vwinfoca.htm
www.rbracing-rsr.com
www.webcamshaft.com
www.thirdgen.org
wikipedia
www.skunk2.com
www.empirenet.com
www.kennedysdynotune.com
How to Hot Rod Volkswagen Engines. By William Fisher
How to rebuild your Volkswagen air cooled engine by Tom Wilson
The Aircooled VW Engine Interchange Manual by Keith Seume
And many posts on The Samba and VZi by various people
And lots of conversations over the years
.
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PostPosted: Thu Feb 19, 2009 9:28 am    Post subject: Reply with quote

nsracing wrote:
The head porter will tell you which cam you will need.


Some of us do our own work and pick our own parts.
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PostPosted: Thu Feb 19, 2009 9:52 am    Post subject: Reply with quote

Marv [UK] wrote:
I’ll apologise now for the length of this post and quite possibly for annoying some people with the sheer boredom of reading it...


No need to apologize. You did a ton of work putting this information together. I found it to be super informative. I have always wondered about cam selection and it's effect. Now I have a better understanding. Thanks Marv.
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PostPosted: Thu Feb 19, 2009 1:06 pm    Post subject: Reply with quote

Your Post got me thinking, I have a lot of the same questions you have on the subject… and while researching cam Theory to help contribute to this post I came across an other post that was talking about Dyno simulators, now I want to get Dynomation 5 just to get more information on how I can maximize my combinations, Dynomation and it's built in engine build theoretical principals, is in a way, a quick method to test the effects of making cam and other changes all based on accepted automotive engineering theory :0)

It is available for $399.99, it would probably save more than its price in mistakes that could be made by making bad cam and component choices ;0)

Thank you for your great post and all your effort in trying to help us all learn why one came is a better choice than an other!
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PostPosted: Thu Feb 19, 2009 4:32 pm    Post subject: Reply with quote

Lionhart94010 wrote:
Your Post got me thinking, I have a lot of the same questions you have on the subject… and while researching cam Theory to help contribute to this post I came across an other post that was talking about Dyno simulators, now I want to get Dynomation 5 just to get more information on how I can maximize my combinations, Dynomation and it's built in engine build theoretical principals, is in a way, a quick method to test the effects of making cam and other changes all based on accepted automotive engineering theory :0)

It is available for $399.99, it would probably save more than its price in mistakes that could be made by making bad cam and component choices ;0)

Thank you for your great post and all your effort in trying to help us all learn why one came is a better choice than an other!


This seems a whole lot like spam. Confused
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PostPosted: Thu Feb 19, 2009 5:00 pm    Post subject: Reply with quote

Marv, that seems kinda lite, do ya think you could expand on that a little more? Just kidding! That's a LOT of info to think about, thanks!
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PostPosted: Tue May 10, 2011 2:20 am    Post subject: Reply with quote

This thread rocks!

I stumbled upon it looking for what an increase in duration might have compared to lift, and ended up learning a whole lot more.

Thanks to the author for putting in the time to write this
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PostPosted: Tue May 10, 2011 6:42 am    Post subject: Reply with quote

Thanks for all your work Marv. All the info in one place will give everyone, especially budding engine builders a good idea of what a cam does and how to choose it to work with their engine combo.

THANKS, brad
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PostPosted: Tue May 10, 2011 7:42 am    Post subject: Reply with quote

and someone once told me I was a little long winded.....1 short thing to add in cam choice.everything matters.weight of car,usage of car,cc,cr,gear,alttitude, carbs, intakes,type of intakes ,size of intakes,heads,head flow graphs ploted out ,exhaust,fuel,loose nut behind the wheel,rpm,fuel,size of carbs,howmany carbs,stick or auto or lenco,just somany things. or just grab what you think you want.
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PostPosted: Tue May 10, 2011 5:18 pm    Post subject: Reply with quote

Marv [UK],

I bought Dynomation-5 Pro, and would be glad to enter data you have into it, and post the results to help clarify the effects of changes on Cam’s/CR/CC/ etc… let me know if that interest you, I can post/e-mail the input parameters… I entered what I could find on a stock T1 engine, it seems to be fairly close to published VW HP number, it came up with 59 HP @ 4,000 RPM & 78 ft/lb Torque(big torque #due to auto optimal timing feature), although not exact, as I do not have some of the stock data that may make it more accurate...

https://www.thesamba.com/vw/gallery/pix/805878.jpg

Image may have been reduced in size. Click image to view fullscreen.


FYI,
Some Examples of changing valve sizes (could be lift, duration, Lobe center, CR…) on different combinations of Bores & Strokes:

(Compression kept at 8:1, and Rod = 137.10mm, so ratio changed with Cranks; 69mm =1.987 76mm 1.804, 78= 1.758)


Int. 35.6 mm Ex. 32.10 mm Change HP/Torque Int. 40 mm Ex. 35 mm
--------------------------------------------------------------------------------------------------------------------------------------
1584cc @ 2,500 RPM 48 HP & 108 ft/lb 0/-7 47.9 HP & 100.6 ft/lb 94.7% Mec. Eff
(85.5x69) @ 4,000 RPM 59 HP & 78.2 ft/lb -10/8 49.2 HP & 86.7 ft/lb 86.3% Mec. Eff


1641cc @ 2,500 RPM 48 HP & 108 ft/lb 1.5/-2 49.6 HP & 104.2 ft/lb 94.7% Mec. Eff
(87x69) @ 4,000 RPM 59 HP & 78.2 ft/lb 7/8 66.2 HP & 86.9 ft/lb 85.5% Mec. Eff.


1776cc @ 2,500 RPM 48 HP & 108 ft/lb 5/4 53.6 HP & 112.6 ft/lb 94.7% Mec. Eff.
(90.5x69) @ 4,000 RPM 59 HP & 78.2 ft/lb 7/8 65.9 HP & 86.6 ft/lb 84.2% Mec. Eff.


1955cc @ 2,500 RPM 48 HP & 108 ft/lb 11/15 58.9 HP & 123.7 ft/lb 94.3% Mec. Eff.
(90.5x76) @ 4,000 RPM 59 HP & 78.2 ft/lb 5/6 64.7 HP & 84.9 ft/lb 81.7% Mec. Eff.


2007cc @ 2,500 RPM 48 HP & 108 ft/lb 12/18 60.3 HP & 126.6 ft/lb 94.2% Mec. Eff.
(90.5x78) @ 4,000 RPM 59 HP & 78.2 ft/lb 5/6 64 HP & 84. ft/lb 80.9% Mec. Efficiency

(Big torque # due to auto optimal timing feature I need info below to make it more accurate)

Ignition Timing (Spark Timing)
[ X] Estimate Ignition Timing (For Best Torque @ each RPM Point)

OR (enter info ??)
Basic Ignition Timing @ Crank: = (5.0 ??) Deg BTDC (?????????)
---------------------------------------------------------------
Timing Advance (Mechanical): = (10.0 ??) Deg Per 1000 RPM
Until: 3000 rpm
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PostPosted: Tue May 10, 2011 9:41 pm    Post subject: Reply with quote

hmm Ive got a program sorta like that, never tryied it on vw stuff.was pretty close on v8 stuff.it said my sbm made 622 hp, the flow bench said 618 hp and the time slip came out to 614 hp. I may try to find it again, haventseen it in a few years. motion pro I think.and they sponcered the IHRA world finals at rockingham in 2005 where we qualified #6 ,but only 80+ cars showed up for our 64 car field , top sportsman class. had a great week there.
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