924Board.org

[lordam]

I am stuck and need advice on assembling an 931 engine for stock competition. 931s can compete in classic road events like Targa under FIA rules as a late classic (pre Dec 1981). Currently the rules allow freedom to modify the cam, and fix the boost at any level (variable boost controls I presume are not allowed). But they do not allow the fitting of an intercooler if one was not there originally.

So an intercooler cannot be fitted to the 931 and remain in the Stock class. With the handicap system being applied, there is no penalty for changing the cam, or changing the boost. Adding the intercooler would move the car into the Modified class, and impose a handicap of about 6 seconds per minute. I doubt that I am good enough to regain that sort of handicap.

Just for the record, these events can run over 5 days, with individual special stages on closed roads up to 40km long. Some of the stages in Targa Tasmania resemble a 10+ minute hill climb. Reliability is critical, both as there are long stages, and day after day of competition.

So I would like to know how to mix cam changes and boost changes to maximise the engine output without melting it. Or did Porsche get close with the design of the 931?

TIA

John

[Cedric]

I dont know if you have higher octane fuel like Shell-Vpower 99 octane over there, that could give you some extra margin. But If i would you I would be very careful with raised boost on a stock engine. Check everything in the fuel system and so son so everything works 100%. I would also do some wide band lambda testing to see if its running as it should before modifying.
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1980 924 Turbo
www.instagram.com/garagecedric/

[gegge]

For a street driven car a modest increase of boost is ok. The cast alu turbo pipe and manifold will absorb and soak some of the heat. But for a rally/race car durability is much more important, so stay away from any increase in boost. Max 0,7bar if you don´t have a IC or waterinjection.

But there is much to gain if you tune the engine as if it was a N/A. By porting the head, use a hotter cam and remove as much as possible of the exhaust/mufflers you will have 200hp.

(A cam with longer duration is actually decreasing your dynamic compression by 0,5-1,5 units, so increasing boost by a small amount is still safe.)
_________________
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

[Rasta Monsta]

If you are allowed to update the oil cooler and radiator, sink as much money as you can into those. A 924S-style cold intake might help as well.

And, some will disagree, but DEFINITELY change the cam. . .Porsche used the same cam as for the NA cars, and it is nowhere near optimal for a turbo.
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[Raceboy]

[gegge]

You need better VE and there are two ways getting the torque you want, increase the boost or improve the aspiration. If you want hp, add revs to the list.

The 931 engine i N/A state:

The S2 turbo: 177hp/1,65 bar= 107hp (without IC)
The Carrera GT 210hp/1,75bar= 120hp
The GTS: 245hp/2,0bar= 122,5hp (the Clubsport is another animal)

Asume the 931 engine got 120-125hp as N/A, go for a nice street set-up and then add the boost. The stock cam is a good mid-range alternative but is out of breath over 5000 rpms. I belive you need a 268 turbocam that will pull strongly to 6500, a ported head and as low restriction as possible in the intake and exhaust. Cold air to the turbo can be found without an intercooler; try an effective turbocharger, low boost, good ventilation under the hood, isolate the exhaust...

If possible use the #6 turbine from the S1 turbo, it is larger and better suited for a long duration exhaust cam. Overlap is not a major problem, the exhaust manifold is better than you think and the pressure balance ok.

Just as an example, take the 924 D engine:
A 2.0 liter block with the 931 head making close to 200hp@7500rpm (I know, 12:1 in comp) add the turbocharger and it will make 330hp with only 0,65bar. My point is - build a good N/A first and don´t compensate a bad design with crazyamount of boost.
_________________
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

[tuurbo]

I like Cedric's idea about the octane. Don't buy those little cans of it though, just mix race gas with your 92. 99 octane sounds good. I think you can take it to 10 psi without too much worry. Some guys have it at 12. I think you're in better shape, if you boost beyond 10, with a higher octane gas - imho. I've had mine at 12 and it's pretty cool.

You could also rig up a WOT switch and the alternator to reduce parasitic drag, just shut it off at WOT, and gather a couple of ponies that way.
_________________
1980 924 turbo, MSD, Meth. Inj, otherwise stock.

[924guy]

hmm, if you cant add an ic, what about Water injection?

Id think about a two stage boost controller as well...
_________________
Eric
78 924
82 931 SE "smokey"
99' VehiCross
Y2K Honda Insight
http://www.cardomain.com/id/924Guy
Performance by Pasha

[Rasta Monsta]

[Raceboy]

What I'm saying is not based on what car manufacturer did on some models, but what are my personal experiences (and some others too).
No ofense, but you shouldn't make statements that are not based on actual experience, especially in this area.
Maybe we should use 928 K-Jet also like on GTS and not EFI because Porsche chose so? Sorry, that is not an argument.

And as for stock cam? Does this looks like running out of breath over 5000?



This is 924 NA with stock cam and maximum power is present in pretty damn long line.

With stock 924 NA cam and equal boost to Carrera GT, boosted NA engine is WAY faster than Carrera GT, I've proven that with V-Box test runs and is on par with 951 Turbo S.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[patand]

Before increasing boost I would add a knock indicator like e.g Knock Link: http://www.thor-racing.co.uk/Knock_Link-LNK_KNK_LNK.asp

I have a Knock Link fitted in my car and it really is a great tool.
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www.patandracing.se

[gegge]

Comply with FIA homologisation is tricky, you are not allowed to use modern technology and only equipment explicit written in the documentation. I read the homologization for the N/A 924 wich specify the exact weight of the flywheel, crank, rods, pistons as well as duration and lift of the cam(#3075). But the FIA for the GT does not (#672). What does the homlogisation for the 931 say? It would be very interesteing reading that...

If you are allowed to change the cam I would recommend that as well as blueprint, balance and lighten all internal engineparts. That is the only way to get more power under FIA rules. Porting the head is an alternative.

Intercooler is not an option in this case, therefore be careful with the boost. The heated compressed air from the turbo will result in detonation and kill your pistons.

I agree that the stock cam is good enough to 250hp@1bar with an effective intercooler. Raceboy has proven that and the GTS got similar result. Change to a hotter cam is not necessary as long as you can increase the boost.

Check the torque for the 924 N/A, absolutely dead beyond 5000 rpms
http://www.924board.org/viewtopic.php?t=26375
Boost is helping the engine to breath but it could be improved
http://www.924board.org/viewtopic.php?t=26375&postdays=0&postorder=asc&start=30
I know it is with the more restrictive N/A head, but I insist - the stock cam is an ok midrange cam.
_________________
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

[gegge]

Look at the form and curvature of the torquegraphs and resulting hp. The GT and GTS got the same duration as the 931 cam. But I belive that the GTS Clubsport had a different set-up in order to get that top end power. The turbo was the same and boost level alone can´t explain the caracter.

_________________
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

[ideola]

Re: the cam discussion, for what it's worth, every single expert I consulted with for my build felt that factory cam was more than adequate for a 931 due to the low overlap. I did end up going with a slightly "hotter cam" from Integral, and there are at least 2 or 3 add'l stages, but all they do is move the power band higher into the rev range.

Do you have to run stock turbo? If not, as gegge suggested, there are significant gains to be had by changing the cold side to one that breathers easier.

As Rasta suggested, there are significant gains to be had in the cooling system as well. the 924S/944 rad (or even better, the 951 rad) are nearly direct fit replacements with better cooling capacity. You could also consider reversing the coolant flow through the head, but that might require mods that are not allowed. At a minimum, you should also look at equalizing flow across the head by modifying the rear water flange. These measures should prove very worthwhile for reliability and suppressing detonation.
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erstwhile owner of just about every 924 variant ever made

[Raceboy]

Torque drop after 5000 rpm on non-boosted NA application is not only due to cam specs, but rather due to extremely poor port design that is hindering the flow. And BTW, torque is supposed to drop after max torque rpm (3500 on 924) because of the nature of the internal combustion engine and single non-adjustable cam. That is the very reason why max torque and max hp are not at the same rpm and why the curves cross always at 5250 rpm.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche