Dynamic Compression Ratio: Solving the Mystery with Excel

[kenikh]

The link to the Tool

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 variable being cam choice and boost, you’ll get a nice picture. Editing the spreadsheet will allow the Excel experts among you do some other really interesting things, too.

This work has resulted in my 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.

You’ll need Microsoft Excel to open and use it. 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.

[304065]

OK, where do I find the sheet?

[jpnovak]

OOH pretty. Nice job Kenik. Now you need to graph the valve closing and then resulting cylinder pressure versus cam/crank angle to see what it is really doing.

Then expand it to VE and diff. EQ for changing VE with RPM and cam related peak torque and finally boost related peak torque. I have some of these numbers for static rpms but not yet fit for dynamic prediction. Numbers are fun.


What?! only 1bar boost? Where is your sense of adventure?

btw, That is a nice 3.3 spec motor.

[emcon5]

I found a dynamic compression ratio applet here, in Visual Basic:

http://www.empirenet.com/pkelley2/DynamicCR.html

Good discussion of what DCR is and why you should know about it as well.

Tom

[kenikh]

Originally Posted by john_cramer

OK, where do I find the sheet?

Pelican dropped my embed code; added a link to the top. Thanks for the heads up.

[kenikh]

Originally Posted by jpnovak

OOH pretty. Nice job Kenik. Now you need to graph the valve closing and then resulting cylinder pressure versus cam/crank angle to see what it is really doing.

Then expand it to VE and diff. EQ for changing VE with RPM and cam related peak torque and finally boost related peak torque. I have some of these numbers for static rpms but not yet fit for dynamic prediction. Numbers are fun.


What?! only 1bar boost? Where is your sense of adventure?

btw, That is a nice 3.3 spec motor.

I've been workin on these, too, as well as creating a graph to overlay compressor maps. Help is always appreciated!

[kenikh]

Originally Posted by emcon5

I found a dynamic compression ratio applet here, in Visual Basic:

http://www.empirenet.com/pkelley2/DynamicCR.html

Good discussion of what DCR is and why you should know about it as well.

Tom

The VB based tools are opaque - Excel allows everyone to see and manipulate the math and add to the discussion. I did use the VB tool to check my math.

[304065]

OK, I get 7.67:1 with John's Supercup 102s. What do I win? What do I do next?

What I really want to model is Bruce Anderson's scenario of the Short Stroke 2,5 with 911T cams- he says in the book that T cams work the best due to the higher dynamic compression. Should be easy to get the specs from the little white book.



PS isn't this an argument for throttle-by wire? I remember John's point about the old NASCAR motors that had 17:1 static compression- you could only open the throttle once the motor was above 5000 RPM where the volumetric efficiency had fallen off to the point where dynamic compression was below the detonation limits. So you had to only give it part throttle on the pit road lest the engine blow.

With throttle by wire (or whatever marketing name Bosch uses, -- blank-Tronic--) you could limit the VE of the motor down low automatically. Maybe this is how the modern engines have 13:1?

[kenikh]

Originally Posted by john_cramer

OK, I get 7.67:1 with John's Supercup 102s. What do I win? What do I do next?

What I really want to model is Bruce Anderson's scenario of the Short Stroke 2,5 with 911T cams- he says in the book that T cams work the best due to the higher dynamic compression. Should be easy to get the specs from the little white book.

All cam specs are on the 4th tab in the spreadsheet - of course there are no 'T' specs . These came from John's website. One of these days I'll add a lookup so you can auto populate the cams, rod length and head volumes from a dropdown in the spreadsheet.

[304065]

Our old friend VLOOKUP. The trouble with the factory specs is the lift is unspecified.

[kenikh]

Originally Posted by john_cramer

Our old friend VLOOKUP. The trouble with the factory specs is the lift is unspecified.

T cam specs from Elgin's site:

Carb'ed 911T

Seat Duration (I/E) 260 254
Lift (I/E) .263 .235
Lash: STD

[jluetjen]

This whole tool is very cool, and very professionally finished.

kenikh

One question -- where are you getting the "Centerlines Intake/Exhaust" numbers from?

[kenikh]

Originally Posted by jluetjen

One question -- where are you getting the "Centerlines Intake/Exhaust" numbers from?

John's site lists the centerlines. I was calculating them off of the setting at one time, but since some of Johns cams list separate centerlines for the I/E cams, I made the guess that these are accurate.

[jluetjen]

Anyone know the head volume for a 69 engine? Piston dome volume for 2.0S pistons?

[longhornchris04]

Would that be in Wayne's Engine rebuilding book? Its sitting on my coffee table at home, but I know the back has lots of tables - I've only really looked at the line for 930/04 (my engine) and never that closely at internal specs.

If so, I'd be happy to post later.

[kenikh]

2 liter head is 75cc, IIRC. Just back out the dome volume until you get the right CR.

[304065]

Originally Posted by jluetjen

Anyone know the head volume for a 69 engine? Piston dome volume for 2.0S pistons?

John 2,0 heads are 72cc. Dome volume for S pistons is a calculated 39.5cc.

[kenikh]

Originally Posted by john_cramer

John 2,0 heads are 72cc. Dome volume for S pistons is a calculated 39.5cc.

That's it!

[Eagledriver]

I probably just don't understand, so please be patient with me. Doesn't detonation occur because the cylinder pressure gets too high? Isn't dynamic compression ratio just a correction for for the valves being open for part of the compression stroke?

If the above is true then I don't thnk you can determine how high the peak pressure is with the "dynamic" compression ratio. Race cams for use at high RPM would appear to have a low dynamic compression ratio due to the valves being open for more time during the compression stroke. These cams however can allow high volumetric efficiency at high RPM due to all the things we haven't considered here. These would include intake and exhaust tuning primarily. If the engine is capable of a VE of 1.0 at 6000 RPM, then it is getting the effect of full compression ratio at that RPM, not the "dynamic" compression ratio calculated here.

What am I missing?

-Andy

[jluetjen]

Andy's right, without a full dynamic pressure model of the engine system you're still going to be missing a level of information.



That being said, this model definitely is a step-up from where most of us were before. So is it perfect? No. Is it better then most of us were? Absolutely!