Here's an interesting question...
Thread Starter
TECH Apprentice
Joined: Sep 2003
Posts: 380
Likes: 0
Here's an interesting question... Is there a known formula for calculating compression ratio from vacuum pressure? Just something I was thinking of when looking at a vacuum/boost gauge...
I'm sure there is a conversion equation somewhere out there, I just can't seem to find it searching the threads.
I'm sure there is a conversion equation somewhere out there, I just can't seem to find it searching the threads.
The two have very little to do with each other. The amount of vacuum in a setup is proximately determined by RPM, displacement, and the intake tract.
The only things that determine compression ratio are the displacement, piston, headgasket, and the combustion chamber.
The amount of vacuum in a setup has no affect on static compression ratio, as static compression is taken assuming 100% VE.
The only things that determine compression ratio are the displacement, piston, headgasket, and the combustion chamber.
The amount of vacuum in a setup has no affect on static compression ratio, as static compression is taken assuming 100% VE.
Teching In
Joined: Aug 2006
Posts: 26
Likes: 0
First thing is find out how much vac you are pulling @ WOT at whatever RPM you choose. Then know the size and how the carb/TB is rated 1.5"- 3.0" etc which means it will pull that vac at that flow. Most 4 barell carbs are rated at 1.5" hg not sure on TB's though.
Now the flow you are getting into the engine (assuming no filter losses) is the sqrt of the difference between the vac you have and the rated vac times the rated flow.
If you are getting .5hg and the TB is rated at 750 cfm at 1.5" hg then the engine is taking in ~432cfm. Now assuming it would take a full 750 CFM to obtain 100% VE in this case the VE would be lower than that and the compression ratio the engine "sees" would be lower as a result. This is how it was explained to me anyway and may not be 100% accurate but it makes sense.
Now the flow you are getting into the engine (assuming no filter losses) is the sqrt of the difference between the vac you have and the rated vac times the rated flow.
If you are getting .5hg and the TB is rated at 750 cfm at 1.5" hg then the engine is taking in ~432cfm. Now assuming it would take a full 750 CFM to obtain 100% VE in this case the VE would be lower than that and the compression ratio the engine "sees" would be lower as a result. This is how it was explained to me anyway and may not be 100% accurate but it makes sense.
