924Board.org

[gegge]

Agree, the stock cam is adequate for the 931. But the whole point is to get more power (hp) and 200lbs/ft@5000rpm will give you a faster car compared to 200lbs/ft@3500rpm. If your powerband is 3500-6500 rpm you should try to optimize the torque in that spectrum.

Remember that power is a function of torque multiplied with rpm.

I was actaully speaking of the hotside, I belive #4 is good for response but a true bottleneck. The S1 has a larger #6 turbinehousing, so did the GT. The S2/GT coldside is capable of more than 200hp but the efficiency drops below 65%. It is actually a very good idea to get a larger compressorwheel and get it to work in th 70-75% region since you don´t have an IC to cool the charge - efficiency is important here. The V3 is probably a K2670 (951 hybrid), and V1 and V2 equals K2664 and K2667 wheels: http://www.356-911.com/post1974/moderntuning/porsche924turbo.htm

I think it is possible to get minimium 200 reliable hp without an intercooler at stock boostlevels.

I run a 268° cam on my stock S2 931, the head is ported and the car is 300lbs lighter that stock. But I think the piston rings are bad and I have nothing to compare with. (The new engine will have a GTS turbo and a GT intercooler. Phase II will have a modified intake and 944 IC.)
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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

[Raceboy]

For a street car, max torque on 3500 rpm is better than max torque on 5000 rpm.

But as for the original post of this thread, bigger turbine is better for him because he gest better top end (= more power with the same boost) and less backpressure (= less danger of detonation because of less exhaust gases left in combustion chamber and thus lowering combustion temps).

Obviously bigger compressor is also better as gegge mentioned because it operates easily on higher amounts of air and boost.
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'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

[lordam]

Wow, what a response! Shows the true value of this board.
To do justice to the respondants, I thought I should leave this thread with a summary which would read something like this:

The FIA rules constrain modifications to the standard car to OEM options like theLSD (option #220).

The FIA rules leave freedom in cam (and the cam is not defined in the 931 homologation papers I have #660).

A lot of suggestions were made for preventing meltdown, and detonation. The use of high(est) possible octane fuel makes good sense. Even with restrictions to using pump fuel, AUS fuels are up to 98 octane. There were modifications to water cooling systems (some changes allowed to radiator, the others I am not sure), oil cooling and water injection (not on). Two things that are possible is getting rid of the heat by insulating exhausts and coating manifolds. Another option that this triggers is to move the injections into the inlet manifold, and use the fuel for cooling - legal if I use the same injectors- I think!

Suggestions were also made encouraging a meltdown by chasing power. I was planning shift points at 5800, so the rev range in 2nd gear is 3270-5800, 3rd gear is 3870-5800 and 4th gear 4340-5800. Unfortunately this leaves me half way between the 5000rpm (run out of puf with standard cam-hotly contested) and 6500rpm for a 268 cam. Very tempted to leave the cam change until the followers need changing.

I think the modifications to the turbo (#6 hotside, K2667 cold side) look interesting as these bits existed in various forms of the 931, and so are legal changes. And the turbo changes need to be combined with lowering the restrictions in the system by porting heads, and polishing the internals of all the inlet manifolds, and the inside of the turbo (which I have done).

And switching off the alternator on WOT is cool.

Thanks

John

[ideola]

Talking about fuels, have you considered E85? It's pump gas, and offers 105-110 octane. The trick will be figuring out how to legally modify the CIS to deliver enough of it, and also to prevent deterioration of seals. If you could figure this out, it would definitely improve the knock resistance, thereby allowing higher boost without intercooling.
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

And just one more tidbit. . .Steve at Integral Cams (if you can consider him an expert with personal experience in cam profiles) opined that a modern car would never have a cam designed as the NA cam was drawn up pre-1976. . .but as always, we will all draw our own conclusions based on all the info we can collect.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ideola]

If it were a modern car, it wouldn't have an 8v head either.
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

Well, we aggressively update those things which are within practicality, and live with the rest, don't we, Dan?


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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ideola]

I'm wondering what Steve would design for scratch for these cars? When I talked to him, he urged me not to go with the Stage II or III cams. But I bet one of those options would work nicely for lordam.
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

I thought the toofah cams they sold were inhouse designs?
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ideola]

Based on the info posted here, they appear to me to be progressive profiles based on the original design (but what do I know, I'm no expert with this stuff). At the bottom of the page, Steve does note that they can do "Full race profiles are available on a custom design basis."
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

[morghen]

if it beats a 951S than its at least as good as today's sport coupes.
a sports car that does 0-100 in under 6 seconds and 1/4 mile in under 14 is FAST TODAY.
VW Golf GTI does NOT do this nor does the SEAT CUPRA or any fast other hot hatches/sport coupes...924 may be old but with boost it kicks ass even today.
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Supercharger and EFI kits
https://www.the924.com

[lordam]

Re-reading Corky Bell "Maximum boost", I came across a quote that confirms the suggestions above about the turbo: "absence of intercooler means the design has to get maximum efficiency near peak power or maximum rpm, so the compressor has induced the lowest possible thermal load" pg37.

So the logic of reducing the input temperature by changing the compressor side looks good (byusing a bigger compressor wheel). So does the logic of reducing the backpressure on the turbine (hot) side by increasing the flow rates through the turbine (by using a bigger turbine). The effect might be to delay the onset of boost, but I would avoid falling compressor efficiency (and rising thermal load) in the operating rev range (3500 to 5800 rpm). Looks good in theory

So I have questions about the hot side and the cold side of the turbo:

(a) does anyone know if the 931 compressor housing (from say K26-2660 or K26-2664) can be machined out to fit a K26-2670 wheel, and if so, do they have the profile for machining the compressor housing?

(b) I am facinated that the turbine wheel and shaft are shared across the 931, GT/GTS, 951 etc. The difference between the 931 (K26-4.10 and K26-6.10), GT (K26-6.10) and GTS(K26-8.10) seems to be in the turbine housing. Can someone comfirm that the K26-8.10 from the GTS has a 3 bolt flange connecting with the exhaust manifold?

I did a search here to see if anyone had posted their experience of driving a GTS setup on the road (that is a 931 with K26-2670GA8.10). One European post from about 2001 said that with this setup the low end made racing difficult in tight corners. Does this suggest that the K26-8.10 is overkill for road racing?

John
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1981 931 ROW RHD Australian delivery, Bilsteins, LSD, adj coilovers, #350/550lb/in front, 27mm TB, adj rear sway bar, Targa setup

[gegge]

I own a K26 from the GTS and confirm the three bolt flange. I intend to build a 300hp engine during the winter but will use a #6 turbine housing instead of the #8. I think the #8 is too large except for top-end power and will hurt response. The Porsche built Audi RS2 with stock 315hp had a #6 so I asume the housing can handle the flow. I belive that you will benefit from a larger compressorwheel like 2664 to 2670 to improve efficency and thermal management.

Some time ago I bought a modified K26 with a 3064 compressorwheel from a 951S. That stock 951 compressor housing was machined and my intention then was to machine the stock 931 compressor housing and use the watercooled 951S bearing and axle as well. I can be done but is a bit complicated because of the shape of the 931 housing. I was given the advice to contact the local R/C club that flies jet-planes. They often build jet engines of automotive turbos.

The GTS used the same exhaust manifold as the S2 turbo according the GTS catalog.
_________________
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]