924Board.org

[WEASEL149]

This thread is really aimed at all you cam gurus out there.

I understand 90% of the theory behind camshaft design but still find choosing one for a turbocharged car difficult.

It all came about from discovering damage to my cam and lifters. The price for a brand new stock cam from Porsche over here (with small club discount) is £396.
This got me thinking that maybe I could approach various experienced cam companies to help me design a new cam specifically for the 931, and have it ground on a brand new steel billet blank.

There are really 2 schools of thought (and both have their merits) that I have come across:-

1) Stick with the stock profile, or go with 10°-20° less duration, max valve lift 5%-15% less than NA equivalent. Basically less duration, less lift and little to no overlap to combat reversion in high back pressure systems and maintain low speed torque.

2) If the engine has been modified to improve inlet flow and also runs a low back pressure exhaust and turbine/housing then treat the engine as you would a tuned NA engine.
This would allow more duration, lift, and more overlap. Depending on how hot, you trade off low revs torque for higher power at higher revs (+3000rpm) without relying just on boost pressure alone ie. the power comes from the cam AND boost rather than just boost, as it would with stock cam.

I realise there's normally heated debate over this on other forums but that's mostly a good thing. I also realise there's the Integral profiles still available but these all go from stock and upwards in spec - none reduce duration etc. Also, the ~$700 with shipping/tax etc. is a little too steep for me.

Gegge has already discussed options with me via PM and suggested ENEM and AGAP cams in Sweden.
Enem offer a 931 grind similar to the Integral Stage 1, the V16T which offers 12.2mm lift and 268° duration on a 112° LSA. The guy I emailed described it as mild with a power band 2500rpm-6500rpm, but the Volvo guys say it well rev way past 7000rpm...

The guy at AGAP recommended the AGAP R37-251-14.0 from this list http://www.agap.se/res/Default/agap_volvo_b230_eng_61.pdf
or said he would go a step smaller and suggested designing me a cam that
would offer 244°@ 0.050" lift with roughly 13.5mm lift.

Out of the 3 options I think the last appeals to me the most but I'm still a little concerned it could be too much for a roadcar, and also I'd like to know how much valve to piston clearance there'd be at those higher lifts assuming everything (head etc.) is stock.

Anyway, as I said, this is a thread appealing to all the turbo cam gurus out there asking for your opinions and thoughts regarding what the best cam would be for a modified road-going 931.
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1979 UK 932

[Rasta Monsta]

I must say, the Integral I seems pretty dialed in to me.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ptheskil]

When you say "modified road going" what exactly are you after? As I'm sure you know, conventional fixed camshaft technology requires you to trade off different performance parameters against each other, so there is no one right answer to your question. My personal preference for a road car would be a flat spread of torque over the widest rpm range possible and I would accept a reduced max power at 6500rpm to achieve that. Others would be horrified at the thought of losing out on 20bhp. Turbos are good at this.

My personal view is that for a TC engine you want the breathing characteristics to favour low end torque (and incidentally run the highest CR you can get away with for the power you want to achieve). You can always use the turbo to force more air in at the top end and if this is a little less efficient that it would have been with a less low-end orientated cam then that's the price you pay. So keep the overlap low, relatively early IVC, and get the maximum valve lift you can. Modern cam profiles do a good job of minimising valve accelerations (I don't know how good the original 931 profile is in that respect) but decent springs will be a must with increased lift. Piston clearance will give you an upper limit of course.
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1981 931 series2 Euro spec

[Carrera RSR]

Thankfully I don't need to replace the cam in my rebuild, and I'm not sure i can afford it when adding in new springs etc. onto the very high price Im already paying! However, need and want is two different things. So interested how this pans out for you Wes. My demands of the cam would be classed as fast road/occaisional track use with lightened/balanced/ported engine.

Piper do 924 cams, pulleys etc. for the 924 championship. Do they not offer a 931 profile?
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252

[WEASEL149]

@Rasta - how does the Stage 1 cam compare to the stock cam? Did you notice much loss in low-end torque and when does the engine seem to come 'on cam'. Is there more power/torque at high revs?

@ptheskil - you have hit the nail on the head, there is no right answer. I have to buy a cam as I'm not a fan of regrinds so decided to consider all options. Thing is, the more I read, the more difficult it becomes as 10 different cam experts will give 10 different opinions on what you should run in a TC engine.
I do realise with a fixed single cam it's always either a compromise or you decide which way to go - you can't have your cake and eat it

The other reason I considered a cam change is that I'd like to equal or beat 924 GTS Clubsport power levels while maintaining reliability ie. more than 270hp.
All the 924 variants basically used the same cam apart from the GTS Clubsport and GTR. At least this is what I have read.

As Gegge said, the stock cam is hard to beat up to 250hp. After this, it's either more boost, more cam+boost, or more cam,boost and revs.

I am actually leaning towards a stock-style mild road cam now after giving it much thought. You know what it's like now in the UK - high fuel price, roads packed to the hilt, so the car must make reasonable mpg when not at full chat and be able to run ok in heavy traffic.
The question remains though on whether a better profile could be designed for a road-going turbo. The stock cam is actually regarded as warm/hot for a TC car at 255 degrees.
It would be interesting to see what a very low overlap cam would do with a little less duration etc.

@Carrera RSR - I'd stick with your stock cam Steve if it's in good order. I've learned much but it's a right can of worms when you get into cam discussions, although interesting at least.
Inner springs are available from Porsche and aren't expensive - I'll get you the price if you want.

Regarding Piper, they did do a 931 profile a very long time ago but I don't know the specs and don't know if they still offer it.
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1979 UK 932

[ideola]

The only profiles I am aware of specifically for the 931 are from Integral. At last check, the Piper and WebCam specs were identical for NA and 931 applications. If Piper ever made a 931-specific grind, I've not been successful in finding data on it.

I have a Stage 1 Integral in my new 941 motor (this particular one happens to be one of the new hard-faced weld-and-regrinds, not an original Integral billet version). I have a lot of other minor tweaks to that engine (long rod, higher compression pistons, dual scraper, ported head and intake), so I can't be certain how much is attributable to the cam. But what I can say is that I have absolutely NO loss in low end response, and the car revs faster and freer than the 937 or any of my other turbos.

I've compared the 941 back-to-back with SPIKE (stock US-Spec S1), GEBLASN (stock US-Spec S2), and the 937. I don't have empirical evidence, but it sure feels quicker to rev from idle than any of the other cars; it makes the S1 feel downright sluggish from take off. The 937 overtakes it for now because of the intercooler and because I can't go full throttle due to a non-functioning wastegate on the 941, but even at 50% throttle and no intercooler, I can keep up with the 937.
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erstwhile owner of just about every 924 variant ever made

[ideola]

Two additional notes after re-reading your original post:

• The RPM limit on our engines isn't so much the cam as it is the lifter design. There are some interesting pix here somewhere of destroyed lifters where they cracked at the hole for the adjusting screw when the engine was accidentally over-revved in a downshift situation (something the rev-limiter doesn't prevent). Not sure you're planning to rev that high, but it's a point worth repeating since others who read this in the future may not pick up on the nuance. Unless you plan to run solid lifters, 6500 RPM is the practical rev limit you should be designing around regardless of the cam (in my opinion).
  
• You wrote: "I'd like to know how much valve to piston clearance there'd be at those higher lifts assuming everything (head etc.) is stock." The only way to get that information is to mock it up and check with clay. I've had this discussion multiple times with the guys over at Diamond, particularly as it relates to custom NA pistons where the valve reliefs are required due to the heron-style head. There is no way I am aware of to calculate or simulate what valve float and valve lift are going to do relative to the piston, you have to check it with clay on an actual assembly.

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erstwhile owner of just about every 924 variant ever made

[gegge]

Just two notes to Ideolas trext above:
- The design of the lifter is not bad, but the weight of the valvetrain is. That´s what causing the valvefloat smashing the lifter into pieces. Harder spring will solve that problem and create new ones.
- You can always calculate it with trigonometry if you know the cam profile which you do Then check 10 degrees ATDC, 20 and 30. IIRC is the chamber in the head 5mm deep and the valve is open about 2mm at TDC - give or take half a mm.
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[WEASEL149]

[WEASEL149]

Just to update this thread.

The cam spec I went for in the end (designed using same software as Integral I believe) was:-

237°@0.050" lift for the inlet, with max lift 12.2mm/0.480"

229°@0.050" lift for the exhaust, with max lift 11.9mm/0.469"

Ground on a 112° lobe separation angle.

This is being ground on a brand new camcore keeping the base circle close to 40mm.

Cam should have manners much like the Integral Stage 1 but has more lift 'under the curve'. In fact, the duration@0.050" is almost at Integral Stage 2 level.

Lift at TDC will be less than 2mm so no valve to piston conflict issues.
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1979 UK 932

[morghen]

how much did this cam cost you?
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Supercharger and EFI kits
https://www.the924.com

[WEASEL149]