Don Terrill’s Speed-Talk

I have read,for every ten degrees of duration you add to a cam it moves the rpm (power) up 500,and for every ten degrees you subtract it moves the rpm down 500. I assume that is refering to both intake and exhaust. Also read for every four degrees you retard a cam it moves rpm up 200,and for every four degrees you advance a cam it moves rpm down 200.
Also what happens to the rpm if you go from say, ten degrees of standard split on duration to fifteen degrees of split ,and then to twenty by just adding exhaust duration? Would taking away intake duration affect the rpm the same as adding exhaust? Just trying to understand as much as I can about cams ,to me they are the most interesting part of an engine.

You are trying to make the explanation very simple, but it is much more complex than what you think. As an engine builder for over 50 years, this is a subject that is one of the most MISUNDERSTOOD aspects of making serious HP and torque. The biggest thing here is you must decide if you are talking about street or race, because they are not the same.


JOE SHERMAN RACING ENGINES

blackflag wrote: Just trying to understand as much as I can about cams ,to me they are the most interesting part of an engine.

Have you ever studied about exaust gasses/ intake dynamics? These are my favorites

Thanks for relpys, I am only interested in race engines, nothing wrong with street engines, just not my preference.

I just read a great book about big block chevy's in the SA series of books. It had a great section explaining to the beginning engine builder some of the concepts of camshaft design and how choosing the right one for a motor is achieved, it is much more complicated than just saying a given number will move your power band up or down a certain rpm. As said above it is one of if not the most complicated and mis understood parts of an engine, but could be something to look at if your serious about getting to know about how a vavletrain really works in relationship to the bottom end of the motor, and how max touque in the rpm range you want is accomplished. Camshafts and valvetrain geometry have always sparked an interest for me for years too. Ever since I found out as a kid that they are what really makes an engine have that lopey idle that was really cool in my uncles car

Let me give you a little info to think about.

The Intake seat duration controls the Peak HP RPM. How much a given amount of duration change will move the power peak depends on the engine size. The Intake Lift/Area is calculated to fill the cylinder at max RPM. The exhaust duration and lift is calulated to be long enough to evacuate the cylinder at max RPM, but also keep the exhaust velocity high enough to pull on the intake during overlap.

As far as where peaks occur, I think heads and manifolds change this way more than camshaft duration or area, I have seen 10-15 degrees of duration difference in camshafts on the same engine not change where the peaks occur. Just changed the concentration of torque and horsepower in different areas. Yet have seen very slight adjustments to CSA's, speeds and valve seat angles, change peaks by a couple hundred rpm.

Chad

CGT wrote:As far as where peaks occur, I think heads and manifolds change this way more than camshaft duration or area, I have seen 10-15 degrees of duration difference in camshafts on the same engine not change where the peaks occur.
Chad

That's only the case when the heads or manifold are a restriction.

If you have way too small of a port, or carb, you can add 40 degrees to the cam, and not change peak HP RPM.

When talking about camshaft changes effecting the engine, you have to assume that the engine isn't being restricted by other factors(port, carb, exhaust, etc). Restricted engine applications are a whole other beast.

CamKing wrote:When talking about camshaft changes effecting the engine, you have to assume that the engine isn't being restricted by other factors(port, carb, exhaust, etc). Restricted engine applications are a whole other beast.

Whats your definition of restricted? Can be a broad term. What I have seen this on wouldnt be considered restricted by me and others I dont think, but i do see your point on the restricted stuff. But I've seen engines peaks not go up by adding duration, and also very large CSA heads and manifolds peaks not brought down by reducing duration quite drastically, steers me towards the heads and intake tract in general dictating peaks for the most part. Although i dont doubt that duration in other applications has swung peaks much more than what i've witnessed personally.

Chad

CGT wrote: But I've seen engines peaks not go up by adding duration,

If you've seen that, then something else was restricting the engine.

PS, I'm talking about HP peak. Torque peak is effected a lot more by tuned lengths.

I understand both points,but camshaft changes and what it effect on power is what I am after. Induction and exhaust (header tuning) will and can change rpm (power) I have seen and used ,that first hand. Sounds like serious restriction ,if you change duration that much and no change in rpm.

blackflag wrote:but camshaft changes and what it effect on power is what I am after.

That's what I was trying to explain.
Adding Intake duration increases peak HP rpm, but the amount it changes is different on different sized engines, and different rpm ranges.

Example: A 12 degree increase on a 327ci when going from a 260-272 seat duration will move the power up 520 RPM, but increasing the same 12 degrees when going from a 320-332 seat duration will move the power 640 RPM.

If you move 12 degrees from 300 to 312 seat duration in a 327ci, the power will move up 600 RPM.
If you move the same 12 degrees from 300 to 312 in a 454ci, the power will only move up 490 RPM.

As for the exhaust duration. There is a duration required to evacuate the cylinder at max HP RPM. Going smaller then that will start to kill the top-end power, and if you go short enough, it'll move peak HP RPM down.
Going bigger will not increase max HP RPM, but can make more power beyond peak(not fall off so fast). Going too big can also hurt low end power.

Not to hijack,
Mike, how much does LSA change peak HP RPM, all other things being equal.

greywolf wrote:Not to hijack,
Mike, how much does LSA change peak HP RPM, all other things being equal.

Not a bunch. It's more like it moves where the engine is more efficient in the power band.

thanks.