[hr] Okay everyone, some clarification please. I am just an average rider trying to get a grasp on the basics of making my motor perform better than it does now and I am more than a little confused on one thing in particular. That is, compression numbers. I understand what it is in basic terms. Its the type of compression that confuses me and which one(s) is most important to planning a build and in what way.

I read somewhere lately that most people quote their uncorrected mechanical compression when speaking of their build (as in signatures on this site) which is simply the product of (head combustion chamber volume + deck height) divided by (bore and stroke) the piston sweep area. Because this would by default assume that the intake valve closes at BDC, it is a baseline number for reference, but does not take into account the variations of intake closing timing of different cams. It was stated that corrected mechanical compression was the number that was most important in planning a build and whether that build was prone to pinging and/or could be run on pump fuel or not. This was why cams with later intake closing angles needed higher compression in order to come alive. Their corrected mechanical compression ratio was to be kept at or just below 9:1 to be generally safe for all around riding.

An example:

A 95" BB with a straight mechanical CR of 9.5:1 running a Andrews TW60 cam. With an intake closing angle of 56 degrees ABDC, the corrected compression ratio would be a doggy 8.01:1, the same as the stock TC88 CR. To get this cam to where it is happy (as close to 9:1 corrected compression, without going over) you would need to increase the uncorrected mechanical compression to around 10.7:1. If this was actually built to have 10.5:1 ratio, it would yield a safe 8.8:1 corrected compression ratio and be able to run on mid-grade pump fuel (89) without any significant issues.

Talk to me motor builders! Is this true? What is the mathematical formula for finding the corrected compression ratio? Surely there is one as the software programs like Accelerator Pro and Desktop Dyno can compute these numbers by just knowing the above information. Short of installing a degree wheel, aligning the proper cam degree that you are researching at ABDC (when that cams intake valve closes) and then using the remaining bore and stroke piston sweep area to calculate mechanical compression...how do you find this number?[/align]

 

Okay, so simply...what is the mathematical formula for finding corrected compression ratio? The same way that a software engine simulator would?

 

Uh. . . I had it but then I lost it.

 

You locate where the piston is in the cylinder at the time the intake valve closes....take this measurment and substitute it for theSwept Volume in your formula.
Use of the degree wheel will be needed and knowing the intake valve closing number

 

Thanks Doc1, but I had that solution in my first post already. I was trying to find a substitute for the actual hands on. Some one elsewhere is telling me to just worry about the dynamic compression. I know there is a way to forecast the ammicable marriage of all parts (bore, piston, squish band, combustion chamber, etc.) that should keep you worry free from detonation and pre-ignition and get the most out of the build still, though I also realize that this only gets you an estimate of how it will run and is not a perfect solution. It also doesn't account for the efficiency increasing as the motor revs higher which throws things off a bit. Thanks for your input so far! Just trying to bounce some configurations around without having to buy some software to play...

 

Well if thats all you want I have it....this sitewon't let me download it so PM me your e-mail and I'll send it there, you'll like this I'm sure.

 

PM sent Doc1. Thanks.