What compression ratio 140PSI

[1BIGJIM]

Any ball park figure on a compression ratio of 140PSI at 1,000 above sea level? I check my Corvette and it reads 150PSI and its supposed to be 9.5 1. Still trying to resolve my under carbed, over cammed issue:(

[DetroitJim]

Dude, if you are cranking at 140 that's about right for a mild aftermarket cam and 9.0 static or thereabouts. My 2 cents anyway.
DJ

[Hotcrusader76]

It's good for pump gas.

[1BIGJIM]

I am trying to determine If I have enough compression for the cam I installed.

[Tom Foolery]

I thougth the compression ration was determined by the change in cylinder volume when the piston is a TDC & BDC? Is this correct? or is it the change in cylinder pressure from atmospheric to pressre at TDC?

[058]

Camshaft profile has the most influence of cranking compresson. An engine with a big, lumpy cam and lots of compresson ratio can have 130 lbs cranking compresson or a 8 to 1 engine with a very small cam can have 170 lbs of cranking compresson. A compresson guage is not a good estimate of C/R.

[Tom Foolery]

So how is compression ratio calculated then?

[Hotcrusader76]

Tom,
Cranking compression, compression ratio (dynamic and static) are all different animals.
As 058 said....it all depends on cam timing events, atmospheric pressure, intake manifold type, chamber design, and temps (both engine and atmospheric).
Sounds confusing I know, but it's just physics.
~Ty

[Rubths]

1BIGJIM
Man you got to change that avitar I keep thinking your Roz :confused:

[Moneypitt]

figure out your cylinder volume. (cu in Divided by 8, converted to CCs ) plus the combustion chamber cc, then deduct the piston dome cc. (or add the dish cc) This number, BDC cylinder volume, converted to cc s is divided by the chamber volume, (minus dome cc volume). If you push 1000 cc of air/fuel into a 100 cc chamber, you get 10:1......Packing a cylinder looser/tighter with cam profiles and timing, tuned headers, rammed intake, will change the actual ratio a little, but these numbers will get you really close, math don't lie.................Moneypitt

[1BIGJIM]

Confused when I posted the question and more confused now! After reading the link dynamic ratio I can't stop thinking. Never delt into this this far:rolleyes: Still trying to figure out if my cam is to big..

[Moneypitt]

If you know the component specs, do the math, or post it here, the rest is really minor, ie: cam, intake etc. As far as cranking pressure, this method is primarily a tool to determine cylinder equality, to find a weak cylinder, bad valve, blown gasket etc. The results should be within 5-8 % accross the board.. Actual/ factual ratio can only be found in the numbers, using component specs......This is a small part of the blueprinting process, but you have to know what pieces you're dealing with. When dealing with CC s, very small units, the result depends on accuracy.....................Piston info is available,(dome volume) cylinder size is easy, chamber info based on casting #s is rough, thats why heads are CC'd, for more precision info.....Nobody said this level was going to be easy, but it does have its rewards. .....We're here to help you......Moneypitt