924Board.org

[Euro924S2]

Hi guys. I've been recently pondering about cam timing. I have a vernier pulley fitted to my car but I've not experimented with it yet - nor tested out the effects on the rollers.

My supercharger gives great low down torque and I don't tend to rev it too hard but I'm interested to clarify the broad statements of "low down torque versus high end bhp" when I comes to cam phasing / timing.

I have a few questions which I'd appreciate anyone with prior experience or theoretical knowledge to contribute to a discussion!

Keeping a standard, stock 924 cam and 931 pistons, if I change the vernier from full retarded to full advanced (a change of 10degrees IIRC?) where is the change over point for low down torque versus high end power (ie more torque below 3k rpm and more bhp above 5k etc etc - I've just plucked those rev numbers out of thin air so some clarity there is what I need!)

Also, I read that the Franco variable cam pulley adjusted a total of 10degrees - is this the optimal amount between full retard and full advanced, or is there more to gain from say a range of 15 or 20 degrees? I know Franco's are highly regarded and I've read much about them but little to say whether the 10degrees is a particularly sensible number!

Final question for now, how much advance or retard is safe within the constraints of our interference engine (this is only ball park I know, because variables like deck height and HG thickness etc will affect this interference point - or is this irrelevant since the 'sweet range' for power is well within the 'safe' timing area?)

I'm keen to hear people experiences and better yet, see some dyno plots of different cam timing settings - the wholly Grail would be to see a dyno plot for a Franco pulley too!!

Thanks guys,
John.
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[fiat22turbo]

Adding a Franco to my car helped improve the bottom end after losing it from adding the 45mm ITB's and Integral cam.

So while there may be gains from going further than 10 degrees (requiring redesign of the solution), just the 10 degree change is sufficient for mostsituations since it has the ability to be configured at 0 base timing with 10 degrees gain or -3 base with 7 degrees gain at 3500rpm.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[Euro924S2]

Thanks, that's what I'd read elsewhere. I guess what I'm really wanting to know what amount of advance gives maximum low down torque and what amount of retard gives maximum top end bhp? I understand it's a trade off (presuming there is no VVT system employed) but I'm interesting to understand the number of degrees we are talking about, after which, no further gain is seen.

Any ideas anyone?
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[fiat22turbo]

Way too many variables to determine that. Each engine, cam and intake combination would need to be tested to determine that, multiple times and at different static timing settings along with different ignition timing and fueling to compensate.

For your car you'd have to test it back to back at different settings to find the best solution for your needs/situation. You may find that the distributor would need to be recurved to compensate (unless running programmable ignition) and the fueling would likely need to change as well.

If you have access to some of the really impressive engine modeling software available now, you may be able to get fairly close before trying it in real life, but from here to there is a jump that not many people can make due to a lack of funds and time.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[Cedric]

I work with engine simulation every day but with hd diesels(in gt power) . I actually started to build a fully working 931 engine, but i didnt finish it due to lack of time, i just didnt have the time to measure up the engine physically, and good flow data.. There is alot of work needed, but the possibilities are endless. Lots of assumptions have to be done though, normally i have a huuuge ammount of logged data to calibrate it to.

Cam timing would be a good exercise
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1980 924 Turbo
www.instagram.com/garagecedric/

[Euro924S2]

Thanks guys - I can see why there are lots of variables and I wasnt expecting a number to 2decimal places of a degree - but I hoped there would have been some parameters known for a stock 924 turbo engine - the distributor would only matter on a S1 though as the ignition timing is based on the flywheel trigger on the s2.

In the same way that we know there is no more power to be gained from an AFR of less than 11 on boost (in general terms atleast), I hoped a similar general number is known for the cam timing.
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[jason c]

[Carrera RSR]

Read Johns signature to see he's not running stock ignition or CIS

John, my advice is to advance and retard by 1/2 tooth increments and road test the results. I found you can feel the adjustments quite easily with the butt dyno on how the engine responds to this. Mine is not optimised as I haven't done it under controlled conditions but is currently acceptable without spending a lot of money on dyno time to fully optimise. Something that's on the to do list
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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

[jason c]