How to Choose a Camshaft

[Floating VW]

I see a lot of people here on the samba asking for advice about choosing a camshaft for their engine, and I thought, wouldn't it be nice if we all pooled our knowledge into one spot so people could not only get good advice, but also learn something new in the process. So, I decided to start a thread with what little I know (or, at least, what little I think I know), and see where it goes.

NOTE: I'm posting this here, rather than in the Performance forum, because I figured the people that hang out mostly over there already know this stuff and more, so the people in this forum might benefit a little more by it. Also, I know that a few of the performance guys regularly check this forum too, so maybe they'll chime in.

As you can imagine, there's a lot involved in choosing a camshaft, so I'll try to break it up into a couple parts to keep it from getting out of hand. So here's part 1:

A BASIC UNDERSTANDING OF LIFT: The function of lift, in simplest terms, is to allow the engine to breathe. At low lift, gases are starting to enter or exit the cylinder, but flow is still somewhat restricted by the valve. As the valve continues to be lifted, eventually it reaches a point where it no longer presents a restriction to the flow of gases (research “curtain area = valve area” for a better explanation of this). Lifting the valve any higher beyond this point merely causes more wear and tear on the valve train and promotes valve float. So why would anyone want to increase the cam’s lift? By increasing lift, you increase the number of crank degrees that the valve is at or above the point where it is no longer a restriction. This helps the engine breathe, especially at lower RPMs, but the penalty you pay is shorter valve train life.

A BASIC UNDERSTANDING OF DURATION: The function of duration is to determine when and for how long the valve opens. At higher pressures and velocities, air acts like a big spring, so as the piston rapidly travels downward on the intake stroke, the air is being stretched as it enters the cylinder. This means that at the beginning of the stroke, the air closest to the piston immediately starts following the piston downward, but the air farthest from the piston has not started moving yet. And when the piston starts moving back up, even though the air at the bottom of the cylinder is now being compressed, the air at the top is still coming in. The faster the piston travels, the greater this effect will be and the more duration you will need to take of advantage of it. Increasing the duration increases the “overlap” between the intake and exhaust valve, which allows the evacuating exhaust gases to help pull the fresh intake charge into the cylinder, and also extends the amount of time available for air to enter and exit the cylinder. But there is a price to pay here, as well. At lower RPMs and velocities, the air is not as springy, so opening the valve too early and closing it too late will cause reversion, meaning that some of the air in the cylinder gets pushed back out. This can cause everything from poor emissions to an engine that is completely undriveable at low RPM, but at high RPM will absolutely scream. Opening the valve too late and closing it too early will have the opposite effect, making the engine unable to breathe at high RPM, but pull like a tractor at low RPM, and be more likely to pass emissions. There is no right or wrong in terms of duration; you merely have to decide what your goals are: high RPM screamer, low RPM stump puller, or something in between.

NOTE: All of the following values for duration, cam timing and crank degrees are in reference to the standard 0.050” lift point.

INCREASING BORE AND STROKE: When you increase the displacement of the engine, you increase the engine’s capacity to produce torque. Increasing the bore tends to increase the engine’s potential peak torque, while increasing the stroke tends to help the engine produce torque over a wider range of RPM. To take advantage of the higher torque potential, more valve lift and duration is needed. The greater torque potential also allows you to increase the duration for improved high RPM performance, with plenty of torque left over in the bottom end to keep it from feeling too soggy.

ROD LENGTH: Increasing the Rod/Stroke ratio slows the piston down slightly near TDC and BDC, requiring a slightly later intake valve opening and earlier closing. Slightly less duration and/or a slightly wider Lobe Separation Angle (LSA) is needed. Decreasing the R/S ratio will have the opposite effect, requiring a slightly earlier intake valve opening and later closing. In this case, slightly more duration and/or a slightly tighter LSA is needed.

MAX PISTON VELOCITY DELTA: Increasing the Rod/Stroke ratio also increases the piston velocity delta (i.e. the piston hits max velocity closer to 90° of crank rotation). I’m working on a theory I call the “Max Piston Velocity Relationship”. It concerns the Intake Valve Opening (IVO) angle, Intake Valve Closing (IVC) angle, intake Lobe Centerline Angle (LCA is just a fancy way of saying, “crank degrees where full valve lift occurs”), and their relationship to the max piston velocity angle. Basically, the theory is that if you subtract the angle of max piston velocity from the total number of degrees between the IVO and intake LCA, the result should be your ideal IVC. For example, the IVO and intake LCA of the stock cam are 0° BTDC and 104° ATDC (0° + 104° = 104°), minus the stock max velocity angle of 76.6°, which gives 27.4° (104° ‒ 76.6° = 27.4°). However, the IVC of the stock cam is 30°, which, according to the theory, means the cam timing is not ideal; it’s off by 2.6°. To be ideal, either the intake opening (OR the intake closing OR the max piston velocity) needs to occur 2.6° earlier, or the LCA needs to be 2.6° later, or some combination of these to split the difference. Now, say you install an Engle FK-41 cam with the 76mm crank and stock 137mm rod. IVO and intake LCA of the FK-41 are 14° and 108°, respectively. The 76mm crank with stock rod gives a max piston velocity angle of 75.5°. 14° + 108° = 122°. 122° ‒ 75.5° = 46.5°. The IVC of the FK-41 just happens to be 46°, which is a much closer match. Like I said, it’s just a theory and a work in progress, so it may be relevant, or it may mean nothing at all.

LOW LIFT FLOW: Larger valves and multiple-angle valve grinds improve low-lift flow at the valve. If low-lift flow is greatly improved, a later valve opening and earlier closing is needed to prevent reversion. According to David Vizard, retarding the cam timing, even though this will cause the valve closing to happen later, is beneficial. If low-lift flow is not optimal, advancing the cam timing can help this. So, did you keep the old, stock heads, or did you splurge for something with ridiculously large valves and a trick 6-angle valve grind? Adjust your cam timing accordingly.

COMPRESSION RATIO: Starting with your static compression ratio, use the cam’s IVC angle to calculate what your dynamic compression ratio is. You want a dynamic ratio of not less than 7.5:1 and not greater than 8.25:1 for best performance and trouble-free motoring. This is the reason why you hear cam vendors say, “For cam X, set your static compression to between 9 and 9.5:1.” The later IVC angle of cam X will cause the static compression to drop a bit, resulting in a dynamic ratio in the 7.5 to 8.25:1 sweet-spot. The compression ratio also has an effect on the exhaust timing. Increasing the compression improves combustion efficiency and increases the flame-front propagation speed, thereby causing the combustion to terminate earlier. To accommodate the earlier ending of the combustion event, an advance of the Exhaust Valve Opening (EVO) angle is needed for an earlier “blowdown” event. The higher the compression, the earlier the EVO can be.

COMBUSTION CHAMBER VOLUME: In order to increase or decrease compression, a change in combustion chamber volume (including deck height volume and/or piston crown dish or dome) must be made. A smaller combustion chamber volume requires less time to empty and fill, therefore, less valve overlap at TDC is needed. Larger chambers will need more valve overlap. The stock 1600cc engine has a chamber volume of about 52cc and deck height volume (assuming a deck of 0.055” with no “step”) of just over 8cc, giving a total chamber volume of around 60cc. Most higher performance aftermarket heads have chambers over 60cc, and, for example, a 94mm cylinder with a 0.060” deck has a deck height volume of just under 11cc, giving total chamber volume of around 71cc, and possibly more. In this case, a few extra degrees of overlap to help pull those extra 11cc out of the chamber would be needed.

EXHAUST SYSTEM: A well-tuned exhaust system can help pull a substantial amount of fresh fuel charge into the combustion chamber during valve overlap at TDC. Too much overlap will allow an excessive amount of fuel to be pulled into the exhaust system in this situation. A poorly tuned or restrictive exhaust system might benefit from an earlier EVO to allow more time for the exhaust gases to be expelled from the cylinder, but again, too much valve overlap is a problem, as it may cause reversion back into the intake manifold due to excessive back pressure.

Part 2, coming up!
_________________
"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[Floating VW]

OK, part 2:

PUTTING IT ALL TOGETHER: Choose a familiar engine with known characteristics as a starting point. For example, the VW type 1, 1600cc engine. Some relevant data: Stock engine output is 50-60 BHP at 4000 RPM. Typical stock dual-port heads have 52-53cc combustion chambers and single-angle valve grinds. Stock deck height is 0.055”. Stock compression is 7.6:1 (static) and 7.26:1 (dynamic). Some stock cylinder heads come with a “step” in the chamber, resulting in a reduction of static compression to 7.3:1. The stock cam has a 107° Lobe Separation Angle (LSA), a duration of 210°/212°, virtually no overlap at TDC, and max valve lifts of 0.302”/0.287” at the cam. Stock cam timing is:

1. The first thing you need to decide is whether you want a stump puller, a hot street engine, a high RPM competition-only screamer, or something in between. For this example, let’s keep it simple and go with a well-rounded, solid street engine with a little bit of pepper in it. Let’s start by increasing the bore of the baseline engine from 85.5mm to 90.5mm, for a classic 1776cc (about a 12% increase in swept volume from stock). At this point you’ll probably only want a cam with a slight increase in lift, around 12°-16° of overlap at TDC, and a slightly later IVC, say, between 36°-40°. An increase in lift to 0.340”-0.360” at the cam (0.374”-0.396” at the valve with 1.1:1 rockers) and an increase in duration to 222°-230°, with the standard 107°-108° Lobe Separation Angle (LSA) should do the trick. If you want more of a high-RPM screamer, increase the IVC and overlap; if you want more of a stump puller, decrease them.

2. Now add a 76mm stroked crankshaft, giving us a tried and true 1956cc (about a 23% increase in swept volume from stock). With the increased stroke and flatter power band that comes with it, you can add even more duration without making the bottom end too soggy, maybe to around 232°-244°, which translates to an IVO of around 10°-16° and an IVC between 42°-48°. A greater increase in lift, perhaps to around 0.360”-0.390” at the cam, should help ensure plenty of power in the bottom end. And don’t forget, with longer strokes come higher mean piston speeds and higher flow velocities, which require larger valves and/or higher valve lifts, so if you don’t mind sacrificing a little valve train life for more power, a set of higher ratio rocker arms might be beneficial. Again, if you want more of a high-RPM screamer, increase the IVC and overlap; if you want more of a stump puller, decrease them.

3. The stock R/S ratio is around 1.986:1. If you’re like most people, you probably kept the stock-length rods with the 76mm crank, which dropped the R/S ratio to 1.803:1. To compensate, you’ll need to add an extra degree or two of intake duration or tighten the LSA. You could ask for a cam with a slightly tighter LSA ground into it, but it would probably be easier just to add a couple degrees of duration to the intake. Now your cam should have around 234°-246° of intake duration, with an IVO of 11°-17° and an IVC of 43°-49° ABDC.

4. Fortunately, you understand that any well-built machine is a synergy of components all working together in harmony, so you purchased a set of good flowing cylinder heads with proper sized valves and a decent three-angle valve grind to match your spicy, yet well-behaved 1956cc engine. Installing your cam with the timing a couple degrees retarded will help reduce any reversion caused by the improved low-lift flow.

5. You like a fairly tight 0.045” deck, and those new cylinder heads came with a chamber volume of 60cc, which you want cut down to 58cc for a static CR of 8.5:1. Up to this point, your engine is asking for a cam with an IVC of around 43°-49° ABDC, and with the 76mm stroke and 137mm rod, that puts you around a 7.6:1 dynamic CR, which is just where you want to be. Increasing the static CR closer to 9:1 (dynamic CR to around 8:1) would improve thermal efficiency even more, but at the cost of having to pay for premium fuel. Also, because the higher compression is increasing the flame front propagation speed, you can choose a cam with an earlier EVO than stock. The stock EVO is 36°, so an EVO between 40°-45° would be beneficial, or even 45°-50° with 9:1 static compression.

6. With the 90.5mm cylinder, 0.045” deck and cylinder head volume of 58cc, the total chamber volume comes to a little over 65cc (a 5cc increase in volume over stock). The total overlap at TDC the engine wants so far is 21°-27°, so increase the overlap a few degrees to, say, 26°-32° to help clear the larger chamber.

7. That tuned, properly sized 4 to1 header with the ceramic coating you got not only looks good, but also really helps pull the fresh intake charge into the cylinder. Subtract a few degrees of overlap to prevent those perfectly timed exhaust pulses from pulling an excessive amount of intake charge into the exhaust manifold. So, from 26°-32°, drop it down to around 24°-30°.

8. With the 76mm crank and stock 137mm rod, the max piston velocity on the intake stroke occurs at 75.5° BTDC (as opposed to the stock 76.6° BTDC). At this point, the engine is calling for a cam with an IVO between 12°-18° BTDC, and an IVC between 43°-49° ABDC on a 108° LSA. For a cam that fits the Max Piston Velocity Relationship, that works out to an IVO of 12° for an IVC of 44.5°, 14° for 46.5°, or 16° for 48.5°. The later the IVC angle is, the lower the dynamic compression, so let’s keep the middle value and go with an IVO of 14° and an IVC of 46°.

Stay tuned for part 3!
_________________
"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[Floating VW]

And, part 3:

THE RESULT: A good cam for this mild 1956cc engine would then have a 14° IVO and 46° IVC, for a total intake duration of 240°. For the exhaust, we want an EVO around 45° BBDC, and since we want an overlap between 24°-30°, the EVC works out to be between 10°-16° ATDC. Let’s avoid a split-duration cam and call it 46° EVO and 14° EVC, for a total exhaust duration also of 240° (although, having said all that, I don’t mind a slightly reduced exhaust duration, because I don’t feel you lose much performance and every extra degree the exhaust valve sits on the seat is that much better for cooling it). A good number for full lift at the cam would be upwards of 0.400” (again, slightly less on the exhaust side is a good trade between performance and longevity). If high-ratio rocker arms are used, lift at the cam can be reduced, as it will be compensated for by the higher-lift rocker arms, as well as be a little easier on the valve train with no reduction in performance. And don’t forget to try installing the cam a couple degrees retarded, especially if you’re worried about emissions.

If you compare those numbers to some popular existing cam grinds, you’ll find that several come pretty close, such as the Engle FK-7, Engle FK-41, Web 118 and CB Performance 2232, among others. For a little more pepper, something closer to the Engle FK-42, Engle W-110, Web 163 or Scat C-35 would also be good choices (if you bump the compression a little to compensate for the later IVC angles of those cam grinds). Obviously, A LOT of information had to be left out, such as how to compensate for a restrictive intake or exhaust, how to choose the size of the carburetor(s), the diameter of the exhaust header, the type of ignition, the size of the valves, and a plethora of other things that affect the choice of camshaft. But this is a good starting point, and I’m sure someone who understands those things a lot better than I do could do a better job of explaining it, anyway.

Also, I couldn’t say from personal experience, so if anyone has an engine that closely resembles the hypothetical 1956cc engine in this example, I’d like to hear about what cam grinds you used and how well they performed. And, of course, if anyone disagrees with this information or has anything they’d like to contribute, I’d like to hear about that, too.
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"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[mark tucker]

wow

[Bob Brugge]

I am sure this makes sense to someone, but it is way out of my depth. Nicely written either way.
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Keep on Dubbin'!

[eashc]

So if youre lazy me and dont feel like putting in a new cam, but have heads with 46cc cambers, how much is too much compression for a stock cam?

[67rustavenger]

[[email protected]]

I guess it depends on who's dynamic compression calculator you use as to how far you can go. I've never come across an engine that didn't detonate on premium 93 octane pump gas on a DCR higher than 7.7 using this calculator.

https://www.uempistons.com/index.php?main_page=calculators&type=comp

To throw in another variable, Crower has grinds favoring the exhaust side with more lift/duration.

[Floating VW]

[Floating VW]

[Lingwendil]

Might want to ask a mod to move this to the performance/engine section, or at least add it to the performance sticky.

Good read though.
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73 super beetle thread http://www.thesamba.com/vw/forum/viewtopic.php?t=649622 Back on the Road!

Modify your Kadrons for SVDA http://www.thesamba.com/vw/forum/viewtopic.php?p=8115884#8115884

Cast iron VJU4BR8 SVDA reference thread- https://www.thesamba.com/vw/forum/viewtopic.php?t=...mp;start=0

Need replacement filters for original Kadron aircleaners? WIX #42087 is a perfect fit, as is Napa Gold #2087!

[Zundfolge1432]

[Floating VW]

Yeah, the real reason I wanted to post this here instead of the Performance forum was because I was hoping Mark Tucker wouldn´t see it and zing me for being a half-retarded windbag.

Oh well, better luck next time.

P.S. I´m just kidding. Mark is a good guy and I value his input (even if it is at my own expense). Then again, he does live in Florida, and you know how those people can be.
_________________
"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[Danwvw]

[Floating VW]

[modok]

Lift depends on valve size.
0.25 times valve size can be considered a minimum
0.3 is healthy
0.35 is radical, and
more than that is....just probably a bad idea.

it also depends a bit on the seat angles, and throat % relationship.
Sometimes I use a 37 degree seat if the lift is barely enough, like 1mm oversize valve SP heads and STOCK cam,
but lifting 0.35 with a 37 degree seat? NO!! That must be wrong.
but with a 52 degree seat.....maybe

There can be a sweet spot for lift, not enough....don't really have to explain, but too much and you aren't getting as fine a spray of fuel/air into the cylinder, since the inlet valve is also a bit like a sprinkler nozzle, in this sense. There will be a point of diminishing returns from that.

Overlifting the exhaust valve does not really HURT anything, but it doesn't help much, and wears the guide faster.

Quite a few folks have used a fk-41 in small/mild engines, report a LOT of low end.....and a lot of lash noise.

[Floating VW]

Thanks, Modok. That's good info right there.

The stock lift-to-size ratio comes to right around 0.25, maybe a fuzz over or a fuzz under, depending on who manufactured the components you're using, so there you go.

Keep it comin'.
_________________
"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[ALB]

[Floating VW]

Dremel with a tiny ball-shaped cutter, a pair of steady hands, and a whole lot of nothing else better to do.

I did the intake manifolds, too:

They flow pretty well. I haven't had them on the bench for an actual measurement, but I can say from experience that with no porting other than the dimples and a little blending, stock-sized valves, single-angle seats, and a bone-stock cam and rockers, the engine has no problem spinning to north of 5000 RPM. How far north? I couldn't say, exactly, because the valve train wasn't built to go much above that, so I've never had the courage to really find out. And besides, unless you're at the track, how often does someone need to go higher than 70 MPH in 3rd gear?
_________________
"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."