Dynamic Compression Ratio: Solving the Mystery with Excel

[kenikh]

I have been working for the last few weeks to solve the mystery of why some can run huge static CR numbers on their motors, yet get away with it without burning up their pistons. It always seemed to me that the answer wasn't in what static CR these motors were producing, but in what the actual compression of the motor is, given the fact that cam overlap can have a major effect on the net realized compression the motor sees.

Of course engine management is important in mitigating detonation at the bleeding edge, but my goal was to create a tool so that I could baseline a specific motor configuration against various cam choices and later, turbo boost levels. As long as you are comparing apples to apples with the only changing variables being cam choice and boost, you’ll get a nice picture. Changing the editable data in the spreadsheet will allow anyone to do the same analysis. Formulas are included for reference.

This work has resulted in easily being able to calculate the dynamic compression of any 911 motor, including motors with forced induction, to get the real net compression realized under operating conditions, by factoring in cam timing and boost. This allows known configurations can be compared against theoretical configurations for validation.

This project started as an effort for me to validate cam and piston choices for my motor build. It has resulted in a tool that I hope will significantly benefit the air-cooled Porsche community at large.

Caveats: there are numerous other variables at play like volumetirc efficiency that are taken for granted here. This spreadsheet is for generating generic guidelines only. I bear no responsibility for usage or the repercussions of usage. You are on your own if you use this to make critical decisions on your motor build.

You’ll need Microsoft Excel to open and use it. Also, for NA motors, make sure to set boost at 0 psi. Enjoy!



I'd like to thank John Dougherty and Aaron Burnham for their insights and being sounding boards for my ideas. I'd also like to give credit to the various gearheads on non-Porsche forums that posted spreadsheets for disparate pieces I ended up assembling into this final spreadsheet.

[kenikh]

This is also an interesting reference to use with the above spreadsheet:

[Zeke]

This is for turbo motors only? I thought on NA motors the dynamic would be greater.

[911st]

Way cool info! Good work!

Is there enough info here to calc the CFM the motor would pull? This would help w turbo and intercooer selection I would think.

From that could you then calc the intake port and exhaust manifold ideals?

Milt, I belive this should work for NA motors also. Howver with more agressive cams the dynamic CR will drop lower.

[911st]

I am wondering if a stock 930 w small ports running one bar of boost sees the full CR on Peak Boost pressure in the combustion chamber?

Two worthy bench marks on the CR x Boost chart is the 993 normally aspirated motor at a static CR of 11.3/1 and a 993TT's set up which is 8/1 CR w .8 bar boost.

Remember both these motors utilize knock sensors and probably overly rich AFR's near peak combustion pressure areas to operate safely.

The standard practices bench mark limit for a 930 seems to be SC cams, 7.5CR, and one bar boost or stock cams at 7/1 CR with 1 bar of boost. This is with fat AFR's I belive. On todays gas that might be up for questioning.

[Zeke]

Originally Posted by 911st

Way cool info! Good work!



Milt, I belive this should work for NA motors also. Howver with more agressive cams the dynamic CR will drop lower.

I now better understand what a 100% efficient engine is.

[fredmeister]

thanks for sharing man. if i could buy you a beer i would.
Now that is what this site is all about. great work.
Fred