As I understand XViper, a 7lb. pully will work with the stock injectors, ECU, Comptech fuel pressure regulator, etc. What is the upper limit here? Is it 7 lbs., 8lbs., or 9lbs.? At what "pressure point" do we reach the limits of the stock injectors and Comptech supplied controls.

 

Yes, the 7 psi pulley is sold by Comptech and it requires nothing else but to turn up the fuel pressure. I can't say what the limit may be in terms of increasing boost.
It depends on the individual set-up. As Trip has discovered, 6 full turns has not done it for him, while 2 full turns was all that was needed on my friend's car. On the other hand, Trip's set-up is not entirely "stock" Comptech.

 

7psi is it on the stock injectors. At 7psi the stock injectors are well over 90% capacity.

 

Well, a duty cycle of "well over 90%" would be bad, but is that actually what you're saying? Perhaps you mean that the injectors are at 90% of what they can handle effectively? I don't mean to be picking nits, but I want to be sure that I'm understanding correctly.

Thanks XViper. I guess the 7 lb. Comptech pulley is what I've heard called "the high boost pulley." It seems that there are a lot of people running the Comptech kits at 7 PSI without any issues. Is that perception accurate?

To summarize, we're good to go up to 7 PSI, but beyond that airflow exceeds the capacity of the stock injectors. Going from the stock Comptech pulley to a 7 lb. pulley will produce about 20 more HP. Isn't that another way of saying that the stock injectors are good for about 320 RWHP, and no more? It wouldn't matter whether that extra 20 came from more boost or some other mod(s), right?

Thanks guys. Your patience is appreciated.

 

As fuel pressure increases, fuel volume decreases. assigning a HP# for stock injectors is dangerous without taking into account what the fuel pressure is at.

Sideways was over 350whp on stock injectors at near stock fuel pressure, using an EMS.

A Vortech FMU has a 10:1 rising rate which puts fuel pressure in the 120-125psi range at full boost. With pressure that high the injectors aren't metering fuel, they are being blown open. My car was too lean for comfort, and making just at 300whp. After swapping to 440cc RC's dropping the FMU, I was at the same WHP# with 55psi of fuel pressure.

IIRC, the comptech FMU is a 8:1 unit.

There is a limit to how far you can increase the fuel pressure and get acceptable air fuel ratios.

 

OK, SlowS2k, this seems to make sense, but let me continue to question so there aren't any gaps.

As I understand it, Comptech equipped cars run quite rich; I assume that Comptech does this because it is better to err a little on the rich side than to be too lean, but the cooling of a richer charge might also be a factor. Wouldn't the primary difference between Sideways getting 350 HP with an EMS and 320 HP with a Comptech and 7 PSI be that Sideways had better control and could run closer to stoichiometric?

There has to be a direct relationship between maximum fuel flow and maximum (possible) HP, but I don't think it matters when we're working with simple enrichment and have to run rich. IOW, wouldn't it be fair to say that using the simple controls that come with the SC kits we're limited to about 320 HP with the stock injectors (and about 350 with a standalone ECU and competent tuning)?

I find it hard to believe that the injectors work at all at 120 PSI.

 

Sideways car is a bit different than most, with a ported cylinder head and more than stock boost. His 350hp# was below redline from what I recall.

The EMS allows total control over injector pulse width, instead of relying on the FMU for additional fuel. The stock PCM has no idea the SC is installed, and isn't changing injector pulse width over what a stock NA car would need.

The lower rising rate of the Comptech FMU actually allows more fuel volume than the higher rate/pressure of the Vortech.

I've seen lean dyno sheet from both kits. For a street driven SC'd car that only sees a few seconds at WOT they can run many street miles without a problem. Long term WOT is another story.

 

I'm getting a real education here, and can't express how much I appreciate it.

I have some understanding of how ECU's work, including the stock and standalone replacements, but I could ask a ton of questions about them too. Right now though, I'm just trying to get a more complete understanding of the Comptech controls and their limits. I probably have a better feel for what is possible with larger injectors and a standalone ECU than I have for what is possible with the simple Comptech system (though you guys are quickly changing that).

About the fuel pressure ... At 8 PSI per PSI-boost and 6 PSI boost we're already looking at 48 PSI plus the stock pressure (which I seem to recall being forty something). That's nearly 90 PSI! Even more at the Vortech 7 PSI and 10 PSI ramp. !!! I thought typical injectors started getting into trouble above 70 PSI. Is that information out of date (or wrong)?

About "long term" ... That brings up another area that I've been wondering about. As soon as we can get wideband and EGT sensors installed I'll be doing data logging to keep an eye on things, but since you guys have already been down this path, you can tell me what I should expect. The information would probably be useful to some owners who won't be instrumenting their installations.

Fuel demands have got to be high as redline/full-boost is approached; much higher than stock. The stock fuel lines, fuel rail, etc., are all sized for much less flow. Obviously they can deliver enough fuel to meet short-term demands, but how long can they keep it up? For instance, is it unreasonable to expect a car with a stock Comptech kit to survive a "top end run" on a level road? Is that asking the motor to self destruct? What about track days? (I'm afraid to track the car before getting some additional instrumentation installed.) What, exactly, does "long term" mean?

Finally, and this is purely curiosity, why does the Comptech FMU allow more volume at a given pressure than the Vortech? Does their FMU restrict volume in some way?

 

All FMU's increase static fuel pressure, due to the extra plumbing in the return line even without boost applied to the diaphram.

There have been several Stock SC'd kit equipped cars that have seen track use, Hecash, Modifry and Asura are some Comptech users I can think of. Both of these guys work on there own cars, so you'd have to ask them if anything over the stock kit was done.

I remember another Vortech car that leaned out on the Autobahn after several minutes at WOT.

I don't have flow #'s on either FMU, only the rule that volume decreases as pressure increases. The FMU and standard fuel pressure regulator are the restriction in the return line which sets fuel pressure. You can't have pressure without some type of restriction.

 

I didn't know Modifry had a Comptech.

I'll have to pick his brain.