How much boost?

[gkinder951]

How much boost can a stock 951 (944T) small block with a stock head take?

[aeronautica86]

I hear about 14psi pretty reliably...

[gkinder951]

It's gotta be higher than that. I run 21-23 consistently, I just wonder how far I can push that is I get an even bigger fuel delivery system. Or......should I?

[xveganxcowboyx]

I think the key word here is reliably. 21-23 is probably not something you can run daily and still be reliable. I certainly wouldn't turn it any higher than it is. If you want to run that high on stock internals I would recommend a boost controller with 2 stage boost and run lower daily and higher when you need it. I don't know all that much about 944s, but most turbo cars cannot be cranked that high without modifications.

[gkinder951]

Yeah, I understand and the last thing I want to do is blow an engine. Hoever, isn't the first thing to go the head gasket? So it's basically digital, it either can or can't handle it.

I have a boost controller (GReddy) and a few modifications (MAF, larger injectors, larger turbo, ported and polished heads), but the block is stock (no titanuim connecting rods and stuff). I suppose I'll crank it back a little 1.3 bar or so for daily driving. It's pretty hard for me to have it and not use it (still a kid at heart).

The car was set up by Technodyne Racing (now gone) in Arizona. Talking to Chris Cervelli (owner and guy who did the work), he said the only thing limiting the horsepower right now is the fuel delivery. I'd like to take it up to 400 rwhp or so (about 340 now), but am a little gun shy on ripping the thing apart because it can't take it.

I've seen places getting >500 hp out of the 951s. and am not sure how much they do to get that.

[xveganxcowboyx]

The HG is not always the first or only thing to go. It's entirely possible to lean out (that's where your fuel delivery could help) and fry the rings/seize the block. You could also crack the head or block, throw a rod or rod bearing, etc.... An improperly built high boost engine will also have a much shorter life.

You also have to consider the drive train. I think somebody on here has said around 300 hp is all the stock tranny can take. That's what I've heard about those engines as well. Granted I own a N/A so I haven't researched it all, but this is what I've heard or otherwise know about FI vehicles.

I warped the head then fried one FI car and threw a rod on another. I've had two and broke two and learned from my experiences. I ran boost beyond what the turbo was supposed to be good for on one of them and it greatly exaserbated the cars problems.

[gkinder951]

All good thoughts. I am also worried about the drive train, but it seems ok and I've had this setup for about 2.5 years, maybe 40K miles.

I had an N/A also before this and this car was setup when I bought it. Gotta admit, it is fun, fun, fun! I'll probably take your advise and back down on the boost to less than 20psi (shucks). I think I'd rather have it last than have it blast.

Is there a standard calculation for boost versus hp (providing no other limitation)?

[xveganxcowboyx]

In theory you double your HP from NA state at 1 bar. Exhaust restrictions and other factors make this not quite right, but you should get maybe between 60-80% gain at that pressure. Just a rough guess, but it should be relatively close.

Things like how hard the turbo is working to produce that flow also come into play. Also remember it's not just a peak HP goal. Opening up the HP band is just as important as raising numbers.

[gkinder951]

but opening up the HP band has more to do with how quickly it spools up than anything else in a turbo.

My car is set up with a straight 4 inch pipe to the muffler, no CAT. It spools up rather quick at about 3.5Krpm to 1.4. If the car is bone stock at the N/A rating (150hp or so) you would expect the stock turbo to have about 255hp (using 70%). With no additional limitation 1.5 bar would be 382hp and 357 hp at 1.4. That does sound about right. I suppose the multiplying factor drops with restrictions on exhaust and such so the stock number would work out to maybe 60% multplier.

[xveganxcowboyx]

Did they drop compression for the turbo? Also what kind of boost is it running stock? 10 psi or so? I don't think it's running 1 bar stock. That seems high.

[gkinder951]

No I don't think the compression was dropped. Can't see any reason why really.

I believe the 1 bar stock. I drove a stock 944T once and the Porsche guage railed at 2 bar. The Prosche guage shows 2 bar and is actually 1, 1 bar is equal to 0, according to my GReddy boost guage. That's kind of strange to me but in theory an N/A is actually 14.7psi (1 atmosphere) so I guess it's kind of how you look at it....either in atmospheres or boost which would be anything above 1 atmosphere (14.7 psi).

btw - bar is a metric measurement that is really 0.99 atmospheres, so it's more like 14.5 lbs/sq inch per bar.

[xveganxcowboyx]

The 2 bar actually being one makes sense, but does seem a little unusual.

1 bar of boost is high for stock settings. What is the stock HP of a 951?

I always wondered exactly what 1 bar was. I knew it was very close to atmospheric pressure, but not exactly. I always just use it as double atmospheric pressure when talking about boost.

[gkinder951]

The stock hp (I'm sure it's bhp) is 217 hp at 5800 rpm. The S series turbo was 247 hp.

So if the stock 944 is 150 hp at 1 atmosphere, 1 additional bar would calculate to 300 hp with no loss (2X). If losses and inefficiencies create a multiplying factor of say 1.6 (60%) then you'd get 240 bhp. Mine was dyno'd at 321 rwhp at 1.4 bar. Using the same equations (150 x 1.4 bar boost x 1.6 factor) it would be about 336 rwhp. Now mine was rear wheel horsepower and my understanding is that the drive train loss is about 10-15%, so that would equate to about 370 bph @ 1.4 bar and about 396 bhp at 1.5 bar boost.

I'm sure there is a lot more to the math than that, but it sounds pretty close for a simple method.

[xveganxcowboyx]

Yes, that sounds about right. For some reason I thought the turbo model had less power, but the math is about correct.

Drive train loss can't really be quantified in percent. It's usually a relatively constant number. Say 30, 40 or 50 HP. It may change a little bit as power goes up, but say a 100 HP car will lose 30 before the wheels. If that same car was built to 1000 HP it would not lose 300 to the drive train. Probably between 30 and 50 total.

And yes, RWD does lose a little more power to the wheels than front wheel drive, because the drive shaft is longer, thus more flex in the metal before the power gets to the wheels. I think the actual numbers on this are pretty negligible(sp?).

BTW, this whole post just makes me want a turbo model even more. I've decided the 3 things I'm really missing are the redesigned interior, the power I want and a LSD.

Anybody else on here have any info on boost? I know others know more about this car than I do.

[87Porsche951]

gkinder951,

1 atmosphere on 951 is no boost. 1 bar on the stock bauge is atmospheric pressure not actual boost pressure.

Its not realy the amount of boost the engien can handle its how much air the head can actaully flow.

Stock turbo is not gonna be able to make more than 18psi without runnning past it effienceny range and going more than 18psi is just gonna heat up the air to much.

Also all the engien failures/headgasket problems I have heard of where because of poor tuning and the engines detonating.