Pelican Head Peer Review - Cams

[jluetjen]

No, this isn't the opportunity to rate your fellow Pelican heads. I'd like the readers of this board to review the attached, try it out and give me some feedback. Did you find some any errors? Does it make sense? Questions?

Objective: If you've been reading this BBS for a while you'll know that I've been trying to understand the affects of porting, cam selection and other things on HP in 911 engines. One facit of that has been to get some sort of understanding of the impact of different cam selections. Such things as "What can I expect if I put my 69E camshaft on a 3.6 engine?" "How does a GE80 camshaft compare to a Webcam 104/102r?" This could then be combined with some other calculations such as BMEP and intake port diameter to spec out a motor and have a reasonable idea what sort of HP and Torque will result, as well as how fast you'll need to spin the engine to get the desired results. I've come up with a couple of calculations that I'd like you to review that I think accomplish this.

Methodology: Using the attached data, I ran some correlation analysis and regression analysis against the numbers to see what correlates with Peak HP Engine Speed and Peak Torque Engine Speed. This is the same methodology which is often used in medical studies as well as marketing analysis.

Results:

Predicted Peak HP Engine Speed (RPM) equals:
5176 + (24.6208*Overlap in degrees)+(39.8836*Intake Port in mm)+(-2280.72*Intake Lift in inches)

Predicted Peak Torque Engine Speed (RPM) equals:
-3970 + (32.1682* Int Duration in degrees)+(25.2757*Intake Port diameter in mm)

Comments:
First - I do not believe that these calcuations will be meaningful for any other engines except for Air cooled 911 engines because of differences in combustion chamber design, port design, valve train, etc. etc. etc. Conversly, the reason that I think this is appropriate for the entire family of air cooled 911 engines is because they are so similar.

Also keep in mind that the HP and torque peak can often be moved a few hundred RPM by advancing or retarding the cam. That was not taken into account in my analysis since I couldn't think of a way to quantify that adjustment. So the prediction will be within a couple of hundred RPM plus or minus as a result.

Once you have the peak HP engine speed and peak torque engine speeds, it becomes a question ensuring that your engine is built with the appropriate robustness to support the predicted rev's. You can also use the results as a starting point for carb sizing and induction selection. Finally, you can also use the results of these calcuations to come up with a predicted HP based on some BMEP assumptions. The predicted torque number can be calculated in a similar fashion.

Related Equations and other stuff:

Intake Gas Speed = (Stroke in mm * peak HP RPM)/3000*(Bore in mm/intake port diameter in mm)^2.

Estimated Peak HP = BMEP at max HP * (capacity in liters * peak HP RPM)/13000

Estimate Peak Torque = BMEP at max torque * capacity in liters * 62)/150.8

Some reference BMEP's:
* BMEP for Mild (T or CIS) cams: 144 psi at maximum torque and 127 psi at peak HP engine speed.
* BMEP for E type cams: 155 psi at maximum torque and 140 psi at maximum HP engine speed.
* BMEP for S type cams: 161 psi at maximum torque and 160 psi at maximum HP engine speed.
* BMEP for Race cams (Based on 906): 185 psi at maximum torque and 170 psi at maximum HP engine speed.

References:
Porsche 911 Performance Handbook by Bruce Anderson
Porsche Technical Specifications for 69,70 & 71 911T, E and S.
Porsche Shop Manual for 911's through 1971.
Four-Stroke Performance Tuning by A. Graham Bell
Auto Math Handbook by John Lawler

And finally this BBS!



PS: 5/25/02 - I tweaked the Torque equation slightly and removed the term having to do with intake gas flow. It was just adding noise.

[jluetjen]

Here's a copy of the data that I used. The first time I appended it didn't work too well.

[jluetjen]

Ok. Did I lose everyone? Is everyone just trying to work the math before answering? Was this just stupid?

Hello????

[jpnovak]

Bravo! I am considering livening up my 71/3.0 project and this helpsa bunch. Can you send me a copy of your spreadsheet with cam data? email to If you also have the calculations on there that would be great. Is this an Excel file?

Jamie