924Board.org

[Rick MacLaren]

I hear ya Vaughan...I found an oil leak from one of the lines that heads from the turbo area...>

[-nick]

woah big guy. i'm just trying to help. you _can't_ base whether you're faster than another car on chassis dyno's.

first of all:
wrx = 18.4lb/hp (based on your 168 rwhp)
you = 19.9lb/hp (based on your 141 rwhp)

wrx 0-60 = anywhere from 5.8 to 6.3 depending on who you read.
you 0-60 = ?
stock US 931 = 7.5sec
euro 931 = 7.1sec
carrera gt = 6.8sec

the carrera gt puts out 210hp, say 168rwhp with 20% driveline loss, that's 27 more hp than you and it's still at least half a second slower.

*no you will not take it off the line to 60mph*

wrx top speed = 145mph
931 top speed = 140mph

the wrx hits redline (8000rpm) at top speed in 5th, meaning it has shorter ratio gears than the 931.

*ie, it will be faster than you in any situation. -or maybe i'm wrong and the oxygen down here in reality is a little thinner than wherever you are.

this 200hp with 20psi boost on your 931 you here is probably a flywheel hp estimate. you're not going to make 60 more hp at the rear wheels with 5psi more boost.

these are very simple numbers. since you couldn't figure these out i thought you probably didn't know how a dyno works either (most people don't these posts are supposed to be informative not inflammatory). you could have kept this a friendly info exchange which is what i was attempting.

bye bye
-nick

[data]

I would not advise anything over 11lbs of boost without an intercooler, unless you wait until winter and run when it is below 50 deg. with 100 octane fuel. A 931 with 11 lbs of boost non intercooled would run the quarter between 15.7 and 15.0 dependent on traction. the difference between stock and
boost and 11 lbs is significant in torque.Also the stock k26 will not hold anything above 16 lbs to redline. At 15 lbs of boost and alot of modifications to head, intake exhaust , intercooler . cams etc i saw 275hp to the rear wheels. with the stock k-26 ( the larger early one). My next dyno run will be at the highier boost pressure of 21lbs but with race gas not 93 pump gas.
At this boost level the car runs as strong
as 930 turbos ( stock) and i don't believe
the wrx would be a problem.

[-nick]

i misread Rick's first post. i was going by his tag line which says "1980 Porsche 931 C6H12O6 injected 15 PSI" thinking he's running 15lbs of boost. within the post it says the stock 7lbs. (what does this tag line mean?)

i was wondering why your rwhp seemed so light if you were running 15 lbs. Data's numbers seem right on for an additional 8lbs. (and the mods to make it work).

the porsche924.co.uk site mentions getting +20bhp from an additional 1lb of boost. this is probably a good reference.

'just trying to be informative'
-nick

[larso]

[924 turbo]

[larso]

so does the 931, if you have the 6600 RPM rev limiter/

[924 turbo]

In a purely theoretical basis, the US-spec '80 931 should make approximately 200hp (not RWHP) at 15psi. The addition of an intercooler would be a big bump in efficiency. As far as I know, the Carrera GT engine is basically identical to the 931 engine, except for higher compression pistons (8.5:1), the turbo, and the intercooler. Adding an intercooler (such as a 951 or other front-mount) should be even more efficient than the factory intercooler.

The Carrera GT would make approximately 240hp at the same 15psi as the '80 US spec turbo.

Disclaimer: My calculations are based on relative efficiencies of the engines, calculated with displacement and boost pressure relative to horsepower. These are, of course, purely theoretical and not based on any form of reality, not bounded by physics, and, not suprisingly, completely in conflict with the basic laws of thermodynamics.

[924 turbo]

[larso]

correction, 80 931 you only have to do one shif. Not sure about the 81-82.

That brings up my next question.
What is the rev limit on the 82/81 931s, and how many shifts do u have to do.

[data]

The US spec 931 biggest problem was the EPA gearing in the transmission. For better gearing use a 951 gearbox ( this is what i'm using) or even a 944 gearbox. This will be as big an improvement as turning the boost
up. On the stock bottom end i would believe
( after 15 years of modifying these engines)
400 hp is possible with out modifications.
My block has been o-ringed because i had headgaskit problems over 15 psi. But i believe that was due to the stock ignition /cis system. Now with a full engine management system i have not seen that problem again but also the block is o-ringed.
The key to big torque and hp is a bigger turbo the later k-26 where goood for no lag but killed top end power . try and find a 79-81 style k-26 it has the same hot hosing but smaller inlet so at 15 psi it flowed a lot less, but spooled a lot faster. These turbo's heat up the intake air rapidly and would be detrimental to engine life not to intercool the car. Now get rid of the gearbox and at 15 psi you would be able to run down that WRX. Also the stock cis system will not fuel the car at anything above 15psi and this again will hurt the engine. On the GTS engines Porsche used a v-8 fuel distributor to fuel that engine ( that is how much more fuel the car would need at 17 psi).

[larso]

And the only disadvantage with the 931 tranny would be handling, and that they are probably hard to find...

The difference between the 79-80 and the 81-82 turbos is that the later one is a little smaller correct? SO it spools up faster but doesn't let as much quantity in...now most people have told me that they don't notice any size difference in the two turbos when they take them apart and look at them, what gives?

[data]

ON the later 81-82 k-26 turbos the exhaust hot housing is the same but the inlet is smaller with a different size and shape impeller. These early turbo where good for poss. 240 hp max.

[Rick MacLaren]

Data that's good information, Thank you.

Jon excellent help. Thanks. It's good that you bracketed the answer within 15 and 20 PSI. I think I can get 200 HP at the rear wheels...no cat, no smog pump, dropped some weight recently. But before taking a challenge from a WRX I'm gonna make sure I have 20 PSI.

Nick: You said "...or maybe i'm wrong and the oxygen down here in reality is a little thinner than wherever you are..."

I would agree with that assessment Nick...but I guess atop Mount Olympus it's hard, without oxygen getting to your cerebral cortex, to transmit the wealth of your knowledge to us mere mortals. Still, you got the RWHP wrong on both cars (doh!), you couldn't figure out how I might vary my boost across driving conditions (doh!) and then lectured to us on the deeper 'meaning' of dyno statistic (rolling eyes as if a teen is educating an adult male on the finer points of seduction). Well done lad!
_______________
1980 Porsche 931 C6H12O6 injected 15 PSI
1979 Porsche 924

[ This Message was edited by: Rick MacLaren on 2001-08-08 03:21 ]

[AppleBit]

Formula to figure out boost changes:

=(horsepower for stock boost / ((stock turbo boost / 14.7)+1) * static compression) * ((new turbo boost / 14.7)+1) * static compression

Easy enough for ya?

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