X2BOARD

I was wondering...and this prolly sounds like a stupid question, but could cam lift, duration, timing or overlap have ANY efffect on a motor's compression ratio ?

rtype11

yes, dynamic CR is affected, a big cam will also affect vaccuum and braking system, big block chevys with big cams need a vaccuum canister for brakes.

X2BOARD

I have not been able to find any info on this subject (on the net at least). Is there naywhere that you may know of that I can research this more (like theoldone.com or elsewhere) ? Just really curious.

bigTom

i dont exactly know the technical names... dynamic cr and whatnot... but cams will not effect the end compression ratio... compresion ratio is the the amount of compression the piston compresses the air inside the combustion chamber...

for example... 10:1
the 1 designates when the piston is essentially at its lowest point on the stroke... btdc... then as the piston moves up towards tdc... it makes the combustion chamber 10 times smaller... or if u divide it into 10 parts and compresses all the air into one part...

so at tdc... the same amount of air is in an area 10 times smaller then btdc... cams have nothing to do with this

rtype11

sorry never looked onliine for info like this, what do you want to know specifically?

X2BOARD

Just really want to know the principle/theory behind it. Curiosity...I like to know things.

rtype11

theory, well basically take your DOHC motor and look at one cylinder and intake and ex valves, look at the timing at which all the events take place like intake stroke as compared to when the exhaust is finally closed - the basic idea is how a motor scavenges air into its cylinders, it is possible to produce positive manifold pressure with race designed NA motor. static CR is simply BDC volume / TDC volume, dynamic CR is a measure when more air is forced into the cylinder at BDC by way of scavenging it in by playing with valve timing. look into F1 NA cars, they have been known to produce 3psi boost on an NA motor. a rule of thumb for turbos is that for every 4psi boost you can add one point to your base CR, this result is dynamic compression.