Cam Lobe Centers

[mark tucker]

hmm a month??poor cust service?? had my custom1 off hyd roller for my type 1 at my door in lessthan 2 weeks( about 10 days) and I live on the right coast.and that is counting 2 weekends& snail shipping.

[rosevillain]

Yeah, no returned calls, zero communication. I'll probably do it again, though.

Point was, custom cam grinders exist, and they can take all things in to consideration when designing a cam for your wants and needs.

[modok]

The tough thing about it is all the parts work together.
If the header and intake are doing something good at a X RPM, then opening up the overlap a bit beyond the usual should be good, at X rpm.
You can experament with advance/retard and try a few cams and based on how it reacts to the changes you may learn something useful. You may find that a custom cam is the answer or you may learn that some other part of the engine needs to be changed.

It's that simple, it's no majic bullet.
What "good" is and where "x" rpms are is something you'd have to experament to find out how much you have to lose or gain where. And then if you change the header or the cam or the heads and CR ect, then it'll act a little different.
It's been mentioned above that how much overlap an engine needs is probably related to the heads, like maybe you have big valves and a rampy cam and then you don't need as much to achieve the same effects, seems reasonable to me. Rod ratio probably factors in too somehow.
But one thing for sure the snap between the exhaust and intake stroke is a very important thing and what happens in that split second can make or break a combo.

So heck who knows. If you want to even TRY an altered cam the first question is: what special conditions might exist that we can take advantage of? What about the heads and intake and exhaust?

Can't complain about dells, I would use tall manifolds with them but short or tall they don't have any wierd side effects.
I'll take your word that your heads are well chosen.

Now, I see you have "a 1 3/4 sidewinder". So I'll ask you why you are running that PARTICULAR header, whose idea was that?

If a fellow was to combine a combo with:
big carbs
large diameter longtube four into one header, plenty of flow
a healthy amount of overlap
also maybe short rods

According to a theory I came up with a few years ago these are all ingredients for an exhaust reversion problem at cruise speeds.
I don't know if this is true for you, some people have problems and some don't, but you it appears you may have the conditions for such a condition. In which case reduced overlap would help it, but not actually fix the problem.

[mark tucker]

a bigger tube needs to be shorter than a small tube to scavange at the same rpm. to big of a tube will let the ex "stack up" instead of "marching inline and out the exit door. and when stack up accurs, scavange dosent work real good.and that = lost tq&hp.

[RockCrusher]

You are on the right track for sure guys.

Mark.....those big boat engines are inherently intake undersized. Intakes were probably in the 40-45% of cylinder diameter or less and that is prime for wider separation angles. Take a 90.5 cylinder and 44 valve and you should run less than 108LSA (like most of the off the shelf VW cams).
What if you trade a bigger intake for a smaller exhaust to get the intake flow back up to snuff? Then you need to open the exhaust sooner or earlier in the power stroke to get the same blow-down....it's all so complicated I just want to cry!

Modok.....if the exhaust is doing it's job correctly then the overlap is THE critical time as pressure depression is far lower than will ever be seen by the piston running down the bore so intake valve size and low lift flow become the dancing partners with overlap and exhaust closing is critical. So now we have IO, EC (overlap) and IC as the happening schiznit...but what about EO? Critical too combined with exhaust function as that tuning will ultimately determine how much depression the cylinder sees at IO. WHEW!

The old school choice based on duration and IC was for simplicity and the lack of real understanding of what was going on in all phases of cam operation.

RC
_________________
[email protected] Please use email for all general inquiries.

I will be happy to speak to anyone who has a serious inquiry (meaning real potential business for RC enterprises) or a parts order. Due to machining noise causing missed calls all calls will be returned promptly.

[modok]

oh yeah, the header is not for looks, they all do SOMETHING if they have tubes.

The trick is getting the header to pull vac at the right time and rpm.

The effect of the primary tube will pull a vac at some time soon after blowdown is done. A the top of the rpm range (maybe 5500 rpm for this guy) blowdown isn't over till near the end. So lets imagine the cylinder pressure is coming down as the piston is coming up, during the last 60 or 90 degrees of the stroke the cylinder goes down below atm pressure, intake opens and you got all kinds of PULL..... great right?

Example of what you DON't want:
rpm is 3500 rpm or so, blowdown is over at just a handful of degrees past BDC. After blowdown the header pulls it into vac. So the header is pulling a vac towards the BOTTOM of the stroke, it only pulls a vac for so long till flow stops and it may even start flowing back towards the engine. So it pulls a vac from just after BDC to 90 deg before TDC, then swings back to pressure as the piston is coming up.
now you have a pressure spike at TDC.........bad.......blows the intake back out for an instant when the valve opens, not good!

Now I've spent a good amount of effort trying to figure headers out, and my current conclusion is the best way to avoid this happening with a four into one is to size the pipes and port so VELOCITY is high enough to keep it from reversing before it reaches the point in the rpm range where this will happen. Where in the rpm range it will happen is related to many factors one of them being the primary length. Long length tunes lower SO
That is why a BIG LONG header can in some cases result in a reversion problem at cruise speeds. If you made it shorter then the point would be higher up in the rpm range, where there would be more velocity at the same tube size.

So yeah I agree with Mark Tucker, his theory is a lot more simple, but I think he's saying he's seen the same thing I have.

[RockCrusher]

You're stating trends based on empirical observation and that's great. This is one of the reasons the exhaust port CSA has to be carefully designed. You do get reversions with all header sizes but it is a slinky effect where the wave strength and timing are related to size and length (as you pointed out). The reversion wave doesn't just come back and die there, it travels back out again and that timing is what we need. Once a high negative is set up at overlap and as the valve continues to open and the exhaust closes we have set up a hi velocity flow well ahead of piston vacuum that has all ready set the stage for good cylinder filling. The simple theory does not make it the better theory although I have no argument about it being a good rule of thumb.
_________________
[email protected] Please use email for all general inquiries.

I will be happy to speak to anyone who has a serious inquiry (meaning real potential business for RC enterprises) or a parts order. Due to machining noise causing missed calls all calls will be returned promptly.

[modok]

I agree, factors such as the port, size of valve, how fast the valve lifts, and the collector/muffler design of the header are also factors that if they were right then maybe there is no problem noticed at all with a big long header.

If on the other hand all the factors line up in a bad way then there is going to be a spot in the rpm range where the engine is very unhappy, and it may be right at rpm where it spends a lot of time.

But it's all speculation till he puts a different header on it. Header is easier to change than the cam but cams ARE cheaper!

[RockCrusher]

Yee-ah.....so true.
_________________
[email protected] Please use email for all general inquiries.

I will be happy to speak to anyone who has a serious inquiry (meaning real potential business for RC enterprises) or a parts order. Due to machining noise causing missed calls all calls will be returned promptly.

[58 Plastic Tub]

I appreciate the discussion. However, the issue is not a dead spot, or any dissatisfaction with the power. The issues with what I've got right now are:

1) More blow-by and friction than I'd like- brought on by the short rods and cylinders that aren't of a known expansion rate.

2) Excessive dynamic compression, resulting in pre-ignition at anything more than 26 or so deg of advance (on 93 octane, under heavy load on a 95 deg day). I need to be able to run 91 octane with up to 32 deg advance if I need, and dynamic compression has to come down.

So- I'm going to replace the super-short rods with (moderately) longer ones and replace the piston/cylinders with genuine LN nickies and custom JEs. The pistons can be dished, so pretty much any static compression ratio I want is available to me. Choosing the cam last makes no sense to me because how much static compression is good is dictated by what cam is chosen, because the closing point of the intake is one of the perimeters that dictate the dynamic compression ratio. A later closing point means more static compression is possible- and running dynamic compression too low is just leaving power on the table.

Perhaps I've overstated my "low RPM" desires, but I really do want no noticeable flat spots on my way to 200 hp or so at 5500-6000 RPM. A big, flat, fat torque peak is ideal. I've got enough of everything (heads, carburation, exhaust) to be able to get it done.

The central issue is wrapping my mind around the big, general concepts of lobe angle and advancing (or retarding) the cam. General explanations are what I'm after- that generally, increasing lobe centers raises the torque peak (or vice-versa, or something like that).

For trackable, flat torque curves- is it better to have a more "mild" cam (one with shorter duration individual lobes) ground on wider centers, or a more aggressive cam (one with longer duration individual lobes) ground on more narrow centers? And secondly- what effect does advancing and retarding the cam have?

It's the concepts I care about right now.
_________________
Stan Galat
"A single point in isolation is a reference point. Two points is a line. Three points is a trend. Trends don't lie."

[Ken Taber]

I have dynoed these cams on 100 lobe centers in a 90X94 with 1 5?8 exhaust, 44 idf's , 44x37.5 oval port heads , fk-43 and fk-44 as compaired to a 108 lobe center the power band moves down 500 rpm. Changing the lobe center this much allows you the ability to short shift and you will not have to do shift as often.

[mark tucker]

are you talking intake centerline??just addvancing the cam??or lobe seperation??witch has to be ground into the cam.

I never thought of a 2.300-2.4 valve as being a small valve.but it is smaller than a 2.5.

[RockCrusher]

[modok]

The lobe center changes the overlap relative to the other cam characteristics.
You can't talk about overlap wothout talking about the header and intake.
A special cam can make the most of what you have, but it is still about what you have. Can't make the intake and exhaust do anything they don't by nature do, just allows or prevents them from doing whatever it is they do.

I assumed you have an understanding of what moving the IC point does, right? Changing the LC or advance lets you adjust the other stuff relative to the IC point.


At what rpms does it run hot and knock?
You think it is a problem with the chinese cylinders, and it may be, or not.

[58 Plastic Tub]

Modoc-

I'm aware that intake closing point changes the dynamic compression ratio, among other things, but I'm sure I'm not aware of everything it effects. I'd like to learn.

I understand that the lobe angle is going to effect overlap, and that scavenging dictates how much overlap an engine will tolerate. I know that different length header tubes scavenge differently at different engine speeds. Beyond that, I'm a pud.

On my engine, the manifolds are CBs "tall" intakes, although there isn't a lot of intake tuning available for these cars- a guy can have anything he wants (it seems), as long as it's "tall" or "short".

Likewise, the A1 was chosen because it was made by somebody who cares, and it fits under the apron of the car. The tubes are about 44" long (or so) from what I can tell, although I think there is some variance in the length of each individual tube.

The particulars of my engine seem to be engendering more conversation than I ever intended. I want to learn in general terms about what happens when lobe angles are opened up or closed down, and when cams are advanced or retarded. The specifics of my situation aren't my primary reason for asking in the first place. This is a hobby for me- and a hobby is made more enjoyable by having at least a cursory knowledge about what is going on.

I talked with Engle today. From what I understand from the conversation, reducing the lobe center narrows the powerband, lowers the RPM of the peak, but makes the engine "snappier". Playing around with a dynamic compression ratio calculator this evening showed that dynamic compression ratio also increases with a more narrow lobe angle.

An FK43 is advertised as making power from 2500 to 6500 RPM, closing the angle up to 106 deg means that the intake closes 2 deg sooner after BDC. Engle said that this would move the powerband down to 6000 RPM, but probably not a corresponding amount on the bottom. I've yet to understand what the purpose is of advancing or retarding the cam.

Everybody's level of understanding is different. Somehow the specifics of camshafts beyond, ".550 in of lift, 300 deg duration" were lost on me until recently.

That's OK, I want to learn.
_________________
Stan Galat
"A single point in isolation is a reference point. Two points is a line. Three points is a trend. Trends don't lie."

[modok]

ok, I see
The problem is advancing the cam or changing the lobe center changes like four things at once.
While this is just the nature of it, I think is is more practical to talk about it in terms of the four .050 timing points, and what changing each one could do, then you can decide what lobe center and advance will be needed to move the points where you want.

Like my example 105LC 110/163 with 5 degree advance, i wanted a cam that opend the intake valve very early, so i used 105 LC and split duration and some advance to do it, while that sounds complicated all it does is open the intake sooner.
see what I mean? that's all it does.

[58 Plastic Tub]

How about a breakdown of those four points, and what they "do"?

As for your example- I understand what you were doing, and did when you mentioned it earlier. Why you want the valve to open earlier is the kind of thing I'm most interested in.
_________________
Stan Galat
"A single point in isolation is a reference point. Two points is a line. Three points is a trend. Trends don't lie."

[modok]

so lets say you change the lobe center 6 degrees tighter

The exhaust will be opening 3 degrees later
The exhaust stroke is like 230 degrees long, it starts as soon as the valve opens and hopefully ends when the piston is at TDC. the difference between having 227 and 230 degrees for the exaust to leave is a not a big deal, taht is why it is the least critical point. If all the stuff(valve, port, header) is designed in a close to ideal ratio, moving the EO point 5 degrees either way will have no effect on how it runs, if it DOES make a difference then you know something about the exhaust side is bigger or smaller than ideal.
Some engines that are known to have less than ideal exhaust flow will often use a cam with an early exhaust opening, like a lot of V8 and also our type4 engines. It is easy with smaller type1 engines to have too much exhaust flow, so we hardly ever need early EO type cam, maybe some would be helped with a cam with a late EO point.

[modok]

The exhaust will close 3 degrees later
EC is a more critical point. Now you have three degrees more time for the exhaust to keep flowing after the piston has come up, or three degrees more time for the exhaust to flow backwards into the cylinder and intake port(not good). when you close the exhaust valve is a lot more critical than when you open it. At what rpm it is good and bad depends once again on the valve/port/header combo. If an engine is near the top of the rpm range and running near the limit of it's exhaust flow, having the valve open a few extra degrees after the piston comes up can really help, but if the engine has more than enough flow for the rpms it's running you won't see any gain from this alone.

The intake opens three degrees sooner
One thing about the intake stroke is it's MAIN part is when the piston is going fastest, the highest piston speed is at like 80 degrees ATDC, while the highest valve lift is like 100-110(intake lobe centerline). So you might say it's the cam little late by nature. The sooner you start lifting the valve the higher it will be when the pistons is goin', and that can be a good thing for perfomance all around. So you do want to lift the valve as soon as you can without getting into trouble. The trouble once agin has to do a lot with the exhaust valve/port/header. if the engine is short on exhaust flow at the top of the rpm range you can be sure opening the valve too soon there will show no gain up there. In the midrange and where the header is working ok there probably won't be any disadvanage to it. On an engine with a plenum intake early IO (and overlap in general) can really screw up the idle and part throttle perfomance, but we run IR carbs so no problem with that unless the header is causing it.

Both these things together gives you more overlap total. If the header is working well at X rpms then more overlap can be a good thing at X rpms. If the header is up to no good at Y rpms, then it can hurt perfomance in some way at Y rpms.
if you have very little or no overlap(like the stock cam) the header is not very important, it can't hurt or help much. If you run lots of overlap the header can make or break the way it runs; like what kind of header you use determines the nature of the beast!
If the exhaust is limited at the top of the rpm range then you'd probably want to move the whole overlap period later(like retarding the cam). if the exhaust is a little on the big side compared to the rest of the engine than you'd want to move the whole overlap period sooner to help prevent any kind of reversion and get it "on the cam" sooner.

That.....is the overlap. open it up and you'll find out what is happening at the snap between the exhaust and intake stroke, big overlap allows the header and intake to play with each other for better or worse.

[58 Plastic Tub]

Thank you.

You said intake closing was the most important point. An explanation?
_________________
Stan Galat
"A single point in isolation is a reference point. Two points is a line. Three points is a trend. Trends don't lie."