OZ951

Yes at modest power levels. As HP goes up you might want to consider a higher volume fuel pump.

Matt H

Mike - what was your concern? Having it run rich all the time?

Oz- what do you consider modest? I will probably replace it come February regardless.
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Mike S

Generally, if you go too large on the injectors you get poor off-idle performance or poor atomization at low rpm's. I'm not sure if this can be dialed out or not. I'm assuming it's less of an issue on the 944 turbo's since I've seen quite a few people run 72lb injectors without problems. I just decided to be a little cautious and use the 55lb'ers since I would never see more than 400HP.

Matt H

I SERIOUSLY doubt I will see 400 on this motor. On the next one I will for sure. Would be nice to use some parts from this one! I have my doubts this car will make 350RWHP. The motor has 195 on it but the internals looked very good. I saw no reason to mess with it right now, with the intention of going to 2.8 or larger in the next year or so. The impending purchase of a 993/996 may make that wait a little longer though.
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Russ Murphy

Lumpy idle quality potentially with big injectors running at low duty cycles (less than 1% with my 72 lb'ers). Heck, cruising on the highway the duty cycle is only 3-5%. I'd say go with the 55-ish lb/hr injectors in your case.

Tomas L

The pressure drop over the injectors is still 2.5 bar and their flow capacity is accordingly. The increase in fuel pressure from boost only has effect on the fuel pump's capacity, not the injectors.

Tomas

TonyG

Russ Murphy


You have a lumpy idle with 72lbs injectors?

My idle is very smooth with the 72lbs injectors and a a much larger cam than stock.

TonyG

Pauerman

How much fuel pressure is in the rail @ 15lbs of boost with a 3bar regulator?

The injector sizing worksheet on the RC Engineering site is a pretty cool tool for deciding on what size to use. The only variable I'm not sure about is the rail pressure while on boost.

What is the typical % loss from crankshaft HP to RWHP? I was guessing at 15% - is that accurate?

Hope this helps

Vic

http://www.rceng.com/technical.htm

Waterguy

Vic, thats a useful website. However, they are fairly conservative on the B.S.F.C. values they recommend. If you plug in 951S injectors (31 lb/hr @ 36 psi) and their recommended BSFC value of 0.6 for turbos, you will find that the injectors require 120% duty cycle at stock boost and horsepower. I have back-calculated from a couple of data points that I have and estimate that the BSFC value for our engines should be about 0.45.

By the way, as Thomas L pointed out, the fuel pressure rises linearly with the boost pressure and doesn't affect flow through the injector in a turbocharged engine. Just use a value of 2.5 bar (36 psi) or 3 bar (43.5 psi) as fuel rail pressure in the equation, depending on which pressure regulator you are running (stock or 3 bar.) Use a BSFC of 0.45.

Commonly accepted driveline loss seems to be 15%.

sean.

Maybe a little off topic. Can you use 55lb injectors with stock chips? Or do you need something to control duty cycle at idle? I'm worried about lumpy idle and out-of-wack a/f ratios.

Laust Pedersen

I am reading this thread with interest, since I’ll need to upgrade very soon, but I am not familiar with some detailed engine controls for the 944T, so therefore this question:

Is the injection timing “true sequential” (one “squirt” per cylinder per combustion) or is it running in “batch mode”?
If it is true sequential, then I would be less concerned about too short pulse width at idle and be more comfortable with an injector flow rate overkill.

Laust

TonyG

Laust Pedersen


It's gang-bang... er... batch fire.

TonyG

Ben Z.

I don't think I am understanding everything correctly wrt to how the change in fuel pressure from boost doesn't effect the injectors(Tomas L's post). Anybody happen to know a good source to gain a better understanding of this and related fuel injection info? I am thoroughly confused.

Thanks.

OZ951

Bosch has a book on their fuel injection systems that is very informative. It covers the stock system used in 944s.

Russ Murphy

TonyG,
No, actually my car idles pretty well mostly. It's just a little inconsistent - changes with engine temp. Heck, what do I expect, it has no air idle control. The way the SDS works makes it a little difficult to have really small changes in MAP/FUEL values in the idle range and bigger injectors can make that even more of a challenge depending on how you've configured the range of your MAP/FUEL values. At each pressure site you enter a
value between 1 and 255, so if you start with 1 at -19 in/hg and you go to 2 at -18 in/hg you've just doubled the MAP/FUEL value, vs. starting at 10 and raising it by 1 (a ten percent increase). If you start too high you run out of "headroom" for your high boost values. Really more of an issue in my particular situation.