Does anybody know the formula for figuring out the minimum recommended static compression for a certain cam. When I asked the cam maker, the answer was "I would say about 12 to 1" It sounded like he was taking a wild guess. I'd like to be a little more precise. Professor UDHAROLD? Wolfplace? anybody?

You can look at the DCR page linked in my sig. Using the DCR calculator available there you can calculate the Dynamic CR and get a good idea of what cam and CR work well together. Pretty much 7.5 DCR is the minimum for acceptable performance.

And I'm finding 8.6 is the max for 93 octane. Maybe even a bit over-max.

was told around 10:5 is close for 92 octane.
8:6 seems pretty low.

Originally posted by 70SS540:
Does anybody know the formula for figuring out the minimum recommended static compression for a certain cam. When I asked the cam maker, the answer was "I would say about 12 to 1" It sounded like he was taking a wild guess. I'd like to be a little more precise. Professor UDHAROLD? Wolfplace? anybody? ==
There is no set answer.
Depends on what you plan on using for fuel but if it is pump gas I would not build a 12.0 engine regardless of what the DCR is.
Dynamic Compression is a very good tool when used with a bit of common sense.
You can put a cam in that will give you an 8.0 or so number you should be able to live with on the street but,,,, you have to remember that as the RPM goes up the engine starts getting efficient & will begin seeing that 12.0 at some point.
If you don't have the fuel to support the compression when this happens your engine will not be happy ;)

Thanks for the replies everyone!

Originally posted by ovelle:
was told around 10:5 is close for 92 octane.
8:6 seems pretty low. The 8.6 is the DYNAMIC compression ratio figured using something like Pat Kelly's DCR calculator.
The 10.5 you mentioned is the meaasured STATIC compression ratio. It is affected at cranking and low engine speeds by the intake lobe closing point and rod length and stroke. This becomes the dynamic compression ratio.
10.5 static would be probably OK with aluminum heads and a moderately hot street cam on 92 octane as long as you watch the temp, fuel supply, and timing curve.

Randy

As Mike said, 12:1 SCR is pushing it. However, many engine are running successfully at 11:1 on pump gas. Although 10-10.5 make live much easier. To determine what the minimum SCR for a cam is, you'd need to look at the cam, particularly the intake closing point. If the cam is too big, the DCR, the CR of the engine after the intake valve closes, will be low, under 7.5 or so. Although the engine will run at lower DCR the power will be down, particularly at lower rpm (this is called overcamming). That's why it is important to match the cam with the CR. DCR helps do this. It gives you a number, the Dynamic Compression Ratio, that can help decide if the cam and CR match. For pump gas 8.5 is pretty much the upper limit, however for street use and less hassle 8.25-8.3 is a better upper limit. There are ways to run at and exceed 8.5 but for most it is not worth the extra expense and hassle. On the lower end, 7.5 works pretty well but as this number drops performance falls off rather quickly.

So, if the CR is too high for the cam, detonation is likely. If the CR is to low for the cam, power drops off. This all pretty much applies to street engines and low dollar racing.

You ask about 12:1. To run a engine on pump gas at this CR, you'd need a huge cam with very radical characteristic. Something over 310º. This would make a high rpm engine with weak lower rpm torque. Low gears, a rotating assembly able to withstand well over 7000 rpm, a convertor in the 4500-5000 (or more) rpm range, and heads and carburation to support this are needed. Very unfriendly for street use and somewhat expensive to setup for strip use (at least for low buck racers like me).

Does anybody know the formula for figuring out the minimum recommended static compression for a certain cam.I am no expert, but wouldn't you pick a cam based on CR (and other info), and not the other way around? Just seems backwards...

This is what Ed Staffel's book "How to build Max Performance CHevy Rat Motors" has to say about this.

On page 113.

Any cam that closes the valve so that 70 percent of the charge is trapped needs 12.4 to 1 static CR.

80 percent need's 11 to 1.

84 percent need's 10.5 to 1.

88 percent need's 9.5 to 1.

94 percent need's 9.0 to 1.

You have to calculate or draw where the piston is in crank degrees ABDC to find this out!!

AND this will also show you why stroke controls CR moreso then bore for a given rod length and a stroke change if you really want to think about what is happening. This is b/c if rod length and cam timing is held constant, the longer stroked motor will have the piston down in the hole farther when the intake valve is closed thus trapping more charge.

This is why a big cam seems smaller in a longer stroked motor!!

Excellent stuff to learn about, imho!!!

pdq67