Camshaft advice wanted

[ber911]

I am in the process of rebuilding my 2.5motor with the following:

66mm counterweighted crank
10.5:1 JE 90mm pistons in Nickasil cylinders
Twin plug heads with 32mm ports

The car will be used for a mix of road driving, fun track work and tarmac rallys with a 901 gearbox with short ratios.

I now need to decide on which camshafts to use. I am tempted to use E cams which together with the 32mm ports will give me low down driveability but on the other hand dont want to lose out on the wonderful free reving nature of a short stroke motor. Would S cams be a beter bet? I would imagine that something wilder like a GE 80 grind would be wasted with small ports. If anybody who has built a similar motor could share their thoughts I would appreciate it.

Robert

[jluetjen]

My $0.02 is that with 32 mm ports you're looking at 160-180 HP max. With a 2.5 (the stroke doesn't really matter, the results will be pretty much the same with a 66 mm crank or a 70.4 mm crank) you should be able to get that sort of HP with an E rev range. I doubt that you will be getting the benefit of the extra lift and duration of a wilder camshaft, nor will the engine be able to pull the air required to generate HP at rev's higher then that. From the data that I've collected, here's a set of cam choices rangeing from E to S cams:



Some comments on the data; the numbers in red are estimates based on a regression analysis that I've done, and they assume adequate airflow capacity in the ports. With 32 mm ports and a 2.5 liter capacity, I would expect the peak torque and peak HP engine speeds to be slightly lower in your application for the cams towards the "S" side of the spectrum. Also, 32 mm ports hardly flow any more air at .5 inches of lift then they do at .45 inches, so any lift beyond that by the cam shaft will not be used by your configuration engine.

Given your engine's higher CR, you can most likely get away with some extra duration so you might want to look at something like a GE40 or Webcam 20/21. The radically wider lobe center of the 20/21 makes it ideal for a CIS car or also potentially an engine which is flow restricted like yours will be in the 6000 - 7000 RPM range. Porsche followed the same approach by increasing the lobe center angle in the later E's -- I suspect because they were becoming flow restricted also.

So I guess my recommendation would be a 20/21. I'm sure that others will have their own opinion.

[ber911]

Thanks for the info John. I would really like to get to at least 200 HP from the motor even if it means sacrificing some of the bottom end. What would you recommend I do in terms of cams and port sizes to achieve this?

Robert

[A Quiet Boom]

Robert, I'd be careful sacrificing lowend to get 200HP, even if you did get that high the car wouldn't be much fun on the street. I like the E cam for all around use and the S cam for more track than street. Free reving power is fun at first but it gets old after awhile and you wish you had a little more torque to move you around. On an all purpose motor you should be more concerned with build area under the HP and torque curves than just peak numbers. Remember it's important to cam an engine to work in the rpm range you'll use the most. I'd be very hestitant to lose power in the midrange where the engine spends most of it's time. This true for more than just P-cars, I used to have a radically cammed stroker V8 and while it was a blast above 3500 it was flat out obnoxious below than, barely idling at 1100 rpm and it would buck and shake at light throttle until about 3000, this motor made huge torque and power when "on cam" and quite a bit less off cam because the torque and HP curves were very steep rather than flat like you'd want in a street motor.

[jluetjen]

For 200 HP out of a 2.5, I'd suggest that you get your heads ported out to "S" spec (36 mm intake ports) and use something like a GE60 cam. While I haven't seen the exact profile for a GE60, I'm under the impression that while the lift and duration are similar to an S's, but that the profile is more agressive (faster valve accelerations). You may need stiffer springs to go with it and the wear may be more then a stock S, but it should provide a fatter torque curve while still providing the top end HP of an "S". With your higher CR and twin plugs, I suspect that the motor will run pretty well even when "off the cam".