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Thread: Cam tuning info and guide

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    Default Cam tuning info and guide

    On four stroke engines, it is important to realize that the cam rotates once for every two rotations of the crankshaft.

    Volumetric efficiency is based on cylinder fill. If a 2.0L engine is filled with 2.0L of an air/fuel mixture, we say its volumetric efficiency is 100%. If a 2.0L engine fills with 3.0L of an air/fuel mixture, we say its volumetric efficiency is 150%. A forced induction engine will have a larger than 100% volumetric efficiency since the intake charge and combustion chamber are being pressurized. A naturally aspirated engine can also have a slightly larger than 100% volumetric efficiency, but it will only happen for a short duration, and is usually only in the peak of the powerband.

    The art of designing camshaft profiles is meant to increase the volumetric efficiency in the RPM range that the customer requires. Camshafts don’t make magical horsepower from nowhere, they simply move the powerband around by changing the volumetric efficiency to attain the desired results.

    The four strokes of the engine are:
    Exhaust
    Intake
    Compression
    Combustion
    **The “start” is not important because it’s a CYCLE, meaning it repeats**

    Looking at a camshaft, the sequence would be as follows:
    The exhaust lobe pushes open the exhaust valve and the piston comes up to push the exhaust out, then starts to close. The intake starts to open, just as the exhaust is closing, piston goes down, and the intake valve closes. Then both valves stay closed for the compression and combustion strokes. This means that the first lobe to come through the rotation will be the exhaust lobe, immediately followed by the intake lobe.

    Overlap is the point where the exhaust valve is closing, and the intake valve is just opening.

    To increase overlap, you have to RETARD the EXHAUST, and/or ADVANCE the INTAKE.
    To reduce overlap, you have to ADVANCE the EXHAUST, and/or RETARD the INTAKE.

    Simple cam tuning rules for NATURALLY ASPIRATED engines:
    Advancing both cams => more low-RPM power, less high-RPM power
    Retarding both cams => more high-RPM power, less low-RPM power
    Less overlap => more low-RPM power, less high-RPM power
    More overlap => more high-RPM power, less low-RPM power

    In a naturally aspirated engine, the extra overlap is called "scavenging". Scavenging is using the out-flowing exhaust to help draw in the next intake charge (partially causing lumpy idle).

    Simple cam tuning rules for BOOSTED engines:
    Advance intake and exhaust => more low-RPM power, less high-RPM power
    Retard intake and exhaust => more high-RPM power, less low-RPM power
    Less overlap => lower EGTs, faster turbo spool, less fuel
    More overlap => higher EGTs, slower turbo spool, more fuel

    Boosted engines don’t like overlap. The incoming cold air and fuel cools down the outgoing exhaust charge, condensing the exhaust gasses. This is VERY counter-productive in a turbo application since the engine needs no help from scavenging to fill the cylinder. I've heard this being called "turbo chill".

    Cool, condensed gasses in the same space push less hard on the turbo, causing lag. HOT gasses are better at spooling the turbo, thus the advanced exhaust timing to open the valve sooner in the power stroke. This steals some of those hot, expanding exhaust gasses to help spin the turbo a little faster. When the piston is near the bottom of the bore, hardly any energy is going into rotating the crank anyway, so stealing expanding gasses won’t hurt anything. The retarded intake just helps cut down the overlap further.

    Retarding overall cam timing:
    Retarding overall cam timing is better for high-RPM power. This is because the valves are closing later. The intake valve is closing AFTER the piston has started to travel back up the bore for the start of compression stroke. This is terrible at low RPM because the intake air velocity is low, and air that was once in the cylinder is now being pushed back into the intake manifold and causing turbulence.

    At high-RPM, the rules change. Air has weight, and thanks to Sir Issac Newton, we know that once it is moving, it doesn’t want to stop moving. This means that the air can continue to flow into and fill the cylinder, EVEN AFTER the piston has begun to travel UP the cylinder bore. This can allow an engine to exceed 100% volumetric efficiency, if even by a small amount.

    Advancing overall cam timing:
    Advancing overall cam timing is better for low-RPM power. This is because the valves are closing a little sooner. The intake valve is closing AT or NEAR when the piston is at the bottom of the bore for the start of the compression stroke. This is great at low RPM because the intake air velocity is low and easily affected by changes in the direction of piston movement in the engine. Almost as soon as the piston gets to the bottom of the bore on the intake stroke, the valve gets slammed shut so no air can escape as the piston begins to travel back up the cylinder on the compression cycle.

    At high-RPM, this may become a restriction since the air has inertia and responds a little slower to pressure changes, potentially choking the air flow to the engine a little.

    Conclusion:
    This information is aimed at allowing tuners to understand what happens when cam timing is altered. When a larger duration camshaft is being installed, unless the lobe centerlines have been changed, the overlap will be increased. If installing larger camshafts in a turbo application, advancing the exhaust and retarding the intake will reduce the inherent increase in overlap caused by upgrading to a larger profile. Most cam grinders, especially regrinders, put the new profile in the same position as the old profile because it is easier, or the only way possible. This has to be changed when the cams are installed in an engine to attain the desired result.

    A forced-induction engine should idle smooth when properly tuned, and a naturally aspirated engine should be “lumpy” and have a lope if it is tuned aggressively towards the high-RPM range. If a forced induction engine is loping at idle, fuel is being wasted, turbo spool time is being increased, and power is being lost.

    I know this is a lot of info, but I wanted to write this in a way that it would be informative for someone who is a novice, or someone who is a full-on tuner, wanting some concrete info on how to tune cams.

    -Dave Atchison

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    Default Re: Cam tuning info and guide

    I have this posted on *another forum*, but I wanted to share this with this community as well. I hope it's useful enough to maybe become a sticky as this seems to be a fairly frequently asked question.

    If anyone needs any help or would like some pointers that aren't available in this post, please feel free to PM me. I was a cam grinder for 7 years, and designed several N/A and boosted profiles. I also worked at a shop dyno-tuning cars both a/f ratio, cam gears, and several other aspects.

    An important thing to note is that if you are tuning your cam gears after you've tuned your air-fuel ratio, you have to re-tune your fuel because the change in air-flow characteristics will change your air-fuel ratio.

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    Default Re: Cam tuning info and guide

    Thanks Dayve. Hopefully we can get this stickied either here or in the general mkiii section.
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    Default Re: Cam tuning info and guide

    Thanks Dave, and for those of you that don't know him, he has answered numerous questions for me reguarding my cam's on my 1jz. He's a little smarter than I am, o.k. he's a whole lot smarter than I am. Thanks again Dave, Jay

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    Default Re: Cam tuning info and guide

    Dave if I remember correctly, I had my exhaust advanced about 8degrees and my intake retarded about 4 degrees and this seemed to be the best for my application. I also noticed that changing the intake side didn't really affect that much. Is this normal or should I notice a substantial change when adjusting the intake side?

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    Default Re: Cam tuning info and guide

    Saw this on SF. Stickied!

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    Default Re: Cam tuning info and guide

    A couple notes:

    1) Volumetric efficiency = (Actual air ingested) / (theoretical air ingested) * 100%

    A 191ci motor spinning 6400 rpm @ 14.7psi boost should be ingesting 707.407cfm of air. If it is actually ingesting 656.436cfm of air, we say the engine's VE at that point is 92.79% volumetric efficiency.

    The *only* way to have greater than 100% VE is to have a tuned intake/exhaust system that allows proper air wave pulse harmonics IN ADDITION TO the actual pressurization of the intake.

    2) Overlap is the period that both exhaust and intake valves are open. (Just a little clarification)

    3) Boosted engines not liking overlap is a myth. Boosted engines with abnormally high exhaust pressures not liking overlap, is truth.

    4) The statement "A forced-induction engine should idle smooth when properly tuned, and a naturally aspirated engine should be “lumpy” and have a lope if it is tuned aggressively towards the high-RPM range. If a forced induction engine is loping at idle, fuel is being wasted, turbo spool time is being increased, and power is being lost." is wrong. See Duane Stephens for proof of concept.

    Just wanted to clarify a few incorrect statements. The generalizations of advancing/retarding the camshafts are crude, but correct enough for most.

    *edit* This - In a naturally aspirated engine, the extra overlap is called "scavenging". Scavenging is using the out-flowing exhaust to help draw in the next intake charge (partially causing lumpy idle). - is incorrect. Scavenging is using proper exhaust pulse harmonics to help draw out spent gases from the combustion chamber, at the moment overlap is present - this low pressure inside the combustion chamber is what draws the intake air/fuel mixture in.

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    Default Re: Cam tuning info and guide

    Quote Originally Posted by suprahero View Post
    Dave if I remember correctly, I had my exhaust advanced about 8degrees and my intake retarded about 4 degrees and this seemed to be the best for my application. I also noticed that changing the intake side didn't really affect that much. Is this normal or should I notice a substantial change when adjusting the intake side?
    That's pretty much right on. Bigger (performance) cams generally need a bit more advance on the exhaust and a little more retard on the intake in order to cut the overlap down more. The bigger the cam profiles, the larger the inherent overlap. If you tune an engine for a certain set of cams, then put in larger cams, you'll probably end up increasing whatever you did with the smaller cams (assuming they are ground on the same centerline).

    Believe it or not, the exhaust is where most of the power is made on performance cams. Of course, you do need a larger intake in order to flow more air, but the power gains are usually found in the exhaust. If you were to just upgrade one cam in a DOHC setup, the exhaust would make you more horsepower compared to an equal increase in size on just the intake camshaft.

    So, to clarify, most of the gains in performance camshafts are found in the exhaust, but at a certain point, the intake becomes the weak link and must be increased as well.

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    Default Re: Cam tuning info and guide

    Quote Originally Posted by Doward View Post
    A couple notes:

    1) Volumetric efficiency = (Actual air ingested) / (theoretical air ingested) * 100%
    If you want to be picky, that's correct, but the "theoretical air ingested" is usually the displacement of the engine (meaning 100% of the engine's volume). Since the theoretical displacement of the engine is 100%, if you were running boost, it would be much easier to be running higher than 100% volumetric efficiency. Even naturally aspirated engines can reach over 100% volumetric efficiency. That's all about what performance camshafts do. They don't create power, they re-tune your engine to be more volumetrically efficient at a certain point. It's not uncommon for an N/A engine to have a volumetric efficiency of even 106% at a certain point in the RPM.

    Quote Originally Posted by Doward View Post
    A 191ci motor spinning 6400 rpm @ 14.7psi boost should be ingesting 707.407cfm of air. If it is actually ingesting 656.436cfm of air, we say the engine's VE at that point is 92.79% volumetric efficiency.
    This is correct ONLY if you mean that 14.7psi is atmospheric pressure (your boost gauge = 0). If you mean 14.7psi on a boost gauge, you are incorrect since that's psig (your boost gauge = 14.7)

    psia = the normal pressure around us compared to a vacuum (a psia gauge would read 14.7 at rest)
    psig = PSI of the gauge you are reading compared to atmosphere (a psig gauge would read 0 at rest)

    I think this is where the confusion lies.


    Quote Originally Posted by Doward View Post
    The *only* way to have greater than 100% VE is to have a tuned intake/exhaust system that allows proper air wave pulse harmonics IN ADDITION TO the actual pressurization of the intake.
    I completely disagree. Guys who have been telling me this are usually tuning a megasquirt where VE has almost become an abstract concept which changes whenever the injector size is changed. The VE in a megasquirt is more closely related to injector dutycycle.

    Quote Originally Posted by Doward View Post
    2) Overlap is the period that both exhaust and intake valves are open. (Just a little clarification)
    Agreed. Looking at the stroke sequence above, it's where the exhaust is just closing, and the intake is just opening.

    Quote Originally Posted by Doward View Post
    3) Boosted engines not liking overlap is a myth. Boosted engines with abnormally high exhaust pressures not liking overlap, is truth.
    There are all sorts of arguments to this. What I'm saying is what's true for 99.9% of all street driven cases that would be found on this or other forums like it. Engines that run abnormally high (for street use) RPMs utilize overlap, even in boosted applications. If you want to rev your engine to the moon, overlap and boost go quite well together. I haven't seen a street-driven car that was able to justify this.

    Almost every turbo engine I've ever seen has abnormally high exhaust pressures. It's what's know as the "turbo drive pressure", and is usually VERY close to your boost pressure (although a little higher). Turbos aren't magical devices that create pressure from nothing. What you are doing is using the expanded exhaust gasses (which are mathematically larger due to heat expansion) to blow the significantly cooler (and mathematically smaller) intake charge in. If your turbo was producing 20psi of boost on the intake, you'd be seeing something like 22 psi or more of exhaust drive pressure in the manifold. To me, this constitutes "abnormally high exhaust pressure".


    Quote Originally Posted by Doward View Post
    4) The statement "A forced-induction engine should idle smooth when properly tuned, and a naturally aspirated engine should be “lumpy” and have a lope if it is tuned aggressively towards the high-RPM range. If a forced induction engine is loping at idle, fuel is being wasted, turbo spool time is being increased, and power is being lost." is wrong. See Duane Stephens for proof of concept.
    I know Duane Stephens, his phone number is in my phone. I'm one of the electricians who does the electrical work at the CNC shop he works at. I haven't had this conversation with him yet, but I have spoken to him in depth about tuning, and we usually tend to agree. He's also in a different class than 99.999% of the people on here.

    I'm not making up these concepts, I've proven them on a dyno, time and time again on countless engines from domestic V8s to Hondas.

    I'd like to emphasize the fact that I'm talking to the 99% majority, not the 1% minority where the rules don't apply the same.


    Quote Originally Posted by Doward View Post
    *edit* This - In a naturally aspirated engine, the extra overlap is called "scavenging". Scavenging is using the out-flowing exhaust to help draw in the next intake charge (partially causing lumpy idle). - is incorrect. Scavenging is using proper exhaust pulse harmonics to help draw out spent gases from the combustion chamber, at the moment overlap is present - this low pressure inside the combustion chamber is what draws the intake air/fuel mixture in.
    Harmonics have very little to do with what I'm talking about. We're not designing intake and exhaust manifolds, we're talking about cam timing. Pressure equalization is what we're dealing with. Harmonics have to do with pressure waves at different reflective frequencies, based on the volume of the "container" in question... meaning the exhaust or intake. That's why flow testing is used, rather than harmonic sound waves to measure the value of a given intake or exhaust manifold... Not to say that harmonics don't play a part in the system though.

    Doward, I thank you for contributing to this thread, and I hope that we and others can continue this discussion in a meaningful and mature manner.

    -Dave

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    Default Re: Cam tuning info and guide

    I read your information from Supra Forums before. I am running 9mm cams and I do not have cam card so I could not degree the cam. But I found that running the Exhaust 4* advanced and the Intake *2 retarded to reduce overlap helped a lot. I had too much overlap at first and the car sounded aggressive but it didn't run well.

    Thanks for your information. I will be tuning my cams on a dyno some time in the near future.

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