I was just using this website to calculate my compression ratio bassed off of my cranking 185-192psi readings I got off of a gauge. This sounded pretty high to me so I went to this website and used the calculator. It says that I have a 12.0:1 motor. thats not possible. The guy that built the motor for me a few years ago said it was about 9.5:1
any ideas about this wacky reading? does temperature effect the reading by that much? 60psi on a hot engine?
its a 1991vette block with stock pistons and 72cc heads
any ideas. Someone else should try that website and see if it works.

http://www.rbracing-rsr.com/comprAdvHD.htm

 

yeah i can believe 12:1 if you had a harley and not a SBC

 

Far too many things affect compression gauge readings besides just static compression ratio.

Remember, when you mechanically compress a gas, the temperature goes up; the rise in temperature causes the pressure to increase more than just the mechaincal compression would create; if you could compress the gas and then wait for it to cool back down with no leakage, you'd come closer to getting a reading that was the same as the static CR.

Also, the cam will affect it; since the intake closes after BDC, it will bleed off some of the pressure.

 

you cannot determine the compression ratio by using a compression gauge.

 

I think your gauge is off, most SBC owners on here will have a gauge reading of about 160

 

But he said the engine was built a few years ago. That basically means it isn't stock.

It sounds as though the engine's set up for low-mid range torque though. Either that or you do have A LOT of static compression. I doubt it's 12:1 though. You'd need a seriously nasty cam to pull that off with pump gas. Either that or absolutely no timing advance.

 

look at his cam and then look at his chamber size, I bet its gonna be right around 160, unless he has some monster doomed pistons in it. I looked at his sig before I said anything!

 

Or, the cam is installed advanced a few °. Or, the engine was a little bit warm when he took the readings.

There's just way too many variables involved.

 

I can read sigs also, it's just with dynamic compression that high doesn't mean that his guage is broke.

What all this means is he's relying on some calculation from a program that someone made that has no proof of being accurate.

That's all.

Don't get so mad. Ya'know, they shoot dogs for being mad. :sillylol:

 

Thats what I thought was so cool about that website. It has a calculator that takes into account bore,stroke,rod length, altitude,compression ratio.... It also takes into acount your cam specs. I have seen other people use it and it was pretty close.
The engine was warm when I took the reading and perhaps the gauge is off. cant forget the old science class PV=NrT (temp goes up pressure goes up)
Anyway If anyone knows there compression ratio and what they pull on a gauge try it out and see what you come up with.

I know my engine is not 12:1 but I thought if the website calculator worked then my hat is off to the guy who made it.

Thanks for the replies everyone!