compression vs. compression ratio

is there a way to estimate CR, by checking compression and figuring cam specs????
or can i go to a major manufacturer and say "i have these specs, and this much cranking cylinder pressure"
and them be able to suggest a better cam?

because cylinder pressure is more a factor that compression ratio, right???

I have a buddy that has a 12.5-1 motor and he asked me to guess how much cylinder pressure he had and I think I shocked him when I guessed dead on. I think it would give you an Idea but when you get down to it i think you need some #s to figure out what you have. :twisted:

Cranking compression is an equation calculated by Compression ratio and Valve Overlap of the cam.

Have a look at this guide:
http://www.4secondsflat.com/Racer_Brown_Cam_Specs.html

This page is not yet in the public domain as it's still under construction, but it will give you a good idea of how cam duration and CR affect your tune-up, gearing and convertor stall.

I'll be finalizing it with Jim in a few days and we'll get it positioned into the public domain, you can bookmark it and we'll date it when complete.

There is no accurate way to do what you are asking. the only way to "know" is to take a head off and measure some stuff. Cranking cylinder pressure is effected by the static compression ratio of the piston/gasket/chamber combination, and the closing event of the intake lobe. It is not effected by overlap, the exh has nothing to do with it. What is your exact setup..heads, block, gasket, cam, work done, etc? and what do you have for a reading now..

what i've been told:
72 RR 400, 452 heads, .509 cam

what i know:
72 400 block, pistons have a pentastar and casting number on the inside of the skirt, so they MUST be stock, i assume, .509 cam IS true, 452 heads

what I don't know:
milling amount (if any), gasket, and current cranking compression (don't have a tester yet

so...looks like i'll have to tear it down eventually, but since it runs and there is no reason for me to go that far into it, i suppose i'll have to wait until i have to take a head off....

It's a safe bet that if it has not been rebuilt, you have lower compression than the books will show. Probabaly in the area of 7.5-8:1. Dont be surprised if the cylinder pressures are low, that cam doesnt agree well with low static compresion. All that really means tho is the power band will be higher, and the total amount will be slightly down. :wink:

yeah...so what i'm thinking....is if i have in _______area for a compression reading, as opposed to a _____area reading, then it's still low compression.....with the blanks being, for example, 90-130# and 180-200#, or something like that

the engine was "built" few miles ago....i was told 50.....i doubt it was that recent...probably more like 500
but the motor IS cleeean

and i don't think 452 heads came on a 72 400, did they??

my boss at work said i can check the intake mounting face on the heads to see if they've been milled, but i don't know what i'm looking for

72 400 would probably have 346 heads on it. They are all pretty much the same though.

yeah....the thought behind it, though, is that if different heads were put on when the engine was built, they MAY have been milled........

MOst of the 400s I've dismembered have 452s, but as Mike said, 346s are possible on an early one. I think you are confusing two different measurements here.

Static compression cant be approximated by "pumping" an engine. for two reasons, first, leakage would add volume to the reading, and second, because you'd have to add something and measure what you're adding to get the two volumes to compare. Some racing bodies use this test to determine displacement, but I havent ever done it or seen it done. Compression ratio is a ratio of volume before the piston moves up from TDC, and the volume of the space left after the piston stops at TDC. it is not dynamic at all, and assumes the cylinder is sealed from the moment the piston moves up, and there is no leakage past rings or valves. This is not the case, as cams close the intake after the piston begins up, and very few rings seal at 100%, more like 96% on a new but seated set.

Cranking cylinder pressure is what you are actually measuring by using a compression gage..taken into account by this test it ring seal, valve seal, and the time the cylinder is sealed, and the piston is moving up. That's why the exh valve timing is the only figure that matters. And this is a much more accurate way of seeing how a street engine will respond in the lower rpm band. Depending on the cam timing, the same engine may read 130psi, 150psi, or 175psi.

Dynamic compression ratio is another figure of importance. Think of it as the cranking cyl pressure, but taken at higher engine speeds. It's most useful in determining peak torque ranges, and octane requirements...Theres a link on another board to try to figure that out if you dare..it gave me a headache, but theres good info there.

Basically, I'd look for cranking pressure readings in the 115-130 range for a 509 cam in a fresh 8:1 engine. BTW, this test should be done with a good battery, no plugs in the engine, engine warmed up, and the carb held at wide open throttle.
:scratch: