255 walbro w/o afpr causing bucking at high psi?

[kazzam]

This is a question for those who have experience with AFPRs.
I have a 255 walbro fuel pump and no AFPR. When the boost is below ~20 psi, everything runs great. But when the boost starts to get above that, say 22psi, my car starts to hesitate for a brief second and then when the boost falls off a little, it is normal. Now i know that with high boost, the spark could be blown out which would cause the same thing. My plugs are gapped to .026 and are 1 step colder ngks with magnacore plug wires.

This happends when I just hit boost at ~3700rpm and until it falls off a little at around 4500rpm.

Is it possible that my spark is indeed being blown out? If so, what kind of boost can the stock ignition coil handle (i know it is alot but please reasure me before spending more money on an ignition sys)?
OR
Is it possible that when my boost spikes to ~22psi, my fuel presure gets astronomical without the AFPR and dumps way too much fuel in the chambers and that causes the hesitation?

Please, an expert opinion would be helpful.
PS: I will be ordering an AFPR soon but not sure about ignition sys.

 

[donmagicjuan]

If you could, please post your ECU+ files for a 3rd gear WOT capture both at <20 psi and at 22 psi or wherever the problem occurs. That will give me a better idea of what's going on.

Running a 255 pump without a regulator will cause you to run extremely rich at idle and low load conditions, but shouldn't affect your fuel system when at WOT, especially at high boost/high RPM. Still, as you probably know, it is very much in your best interest to replace the regulator ASAP.

 

[kazzam]

If you could, please post your ECU+ files for a 3rd gear WOT capture both at <20 psi and at 22 psi or wherever the problem occurs. That will give me a better idea of what's going on.

Running a 255 pump without a regulator will cause you to run extremely rich at idle and low load conditions, but shouldn't affect your fuel system when at WOT, especially at high boost/high RPM. Still, as you probably know, it is very much in your best interest to replace the regulator ASAP.

I dont' think i will be able to post any logs anytime soon. Perhaps in a few weeks when my car is registered again and insured. For now and until it gets a little cooler here in Florida, it is parked in the driveway.

Could you answer my question on a theoretical basis until i could get those logs? for instance, to what power level or psi have stock 2g coils been used without problems? Spark plugs are only about 1.5 years old but have less then 3k on em.
ALso, could a boost leak at high psi make my car hesitate like that (too much fuel, not enough air...i haven't done a boost leak test in a year or so) - would prabably be a good idea to do that huh?!
Please tell me, what would you be looking for in a log on ecu+ to determine if my spark is blowing out..
Let me know if any of this was helpful.

 

[andymoraitis]

Running a 255 pump without a regulator will cause you to run extremely rich at idle and low load conditions, but shouldn't affect your fuel system when at WOT, especially at high boost/high RPM

There's no way to guarantee a 1:1 rising rate under boost without an AFPR so there will always be inconistency in the tune with respect to AFR. As such the motor can go lean.

 

[fulanititoo8198]

I'll just throw this in... I've been having similar problems. and it defenitely has to do with the fuel pressure. I have a 255 with a fpr and any adjustment in the fp makes a huge difference in how the car runs. Last night I was doing 18lbs and backfired like hell. So careful just wait for your regulator it should fix your problem.

 

[donmagicjuan]

There's no way to guarantee a 1:1 rising rate under boost without an AFPR so there will always be inconistency in the tune with respect to AFR. As such the motor can go lean.

That is a very good point, Andy. However, it always made sense to me that the departure from linearity occurs at the lower end of the curve, specifically in the region where there exists a vacuum in the manifold, corresponding to the time when the regulator is being overrun. I would think that as long as the regulator was not being overrun (at WOT), it should still satisfactorily maintain the correct D/P across the injectors. Am I missing something here?

(BTW, this statement in no way supports the use of a larger than stock fuel pump without an AFPR.)

kazzam, I was going to look at your ECU+ logs because I'm not convinced you have a spark extinguishing problem. I have a very similar setup, and I see zero ignition problems all the way to 26 psi. Also, if the problem were severe enough, I would think your sputtering would be accompanied by a CEL.

 

[andymoraitis]

I fully agree that the variance to base fuel pressure will be greater at lower RPM's, but what I'm alluding to is the fact that there's no real way to know the variance at WOT. It could be rising at .8:1 or 1.5:1 and there's no way to tell unless fuel pressure was logged from inside the car and observed. What concerns me is that even if the FPR was overrun at lower RPM's how do we know that there's a stabilization point where everything equals out and thereby rises at 1:1 through out the remainder of the RPM range? I've only tested variance at idle and can't say what the difference would be as the RPM's climb, but it's very possible to run lean or remain pig rich. Hell, we both know either one is undesirable.

Also, I know you'd never advocate running a 255 without an AFPR. No worries there.

 

[donmagicjuan]

I can see what you mean now, and I'll agree with it. Any irregularity would definitely cause some issues with consistency in the tune.

My little disclaimer was more for everyone else. I just didn't want to be misinterpreted.

 

[andymoraitis]

I doubt anyone would misinterpret your information considering your scope of knowledge. I think what we need to focus on is that it's an unknown. If someone had the means to actually test variance through the RPM range and get us close to a real rising rate figure, it might only apply on their tune and wouldn't be applicable as a standardized measurement of rising rate variance with no AFPR. That's what makes it tough to pin down.

Know what I mean Vern?

Either way, you did add a disclaimer and you'd have to be an idiot to miss it.

 

[kazzam]

Thanks!!! All that info helped tremandously. As soon as i get the afpr on there, i'll post back and let everyone know how it worked out.

 

[donmagicjuan]

Good, I hope that ends up being the only cause of your problems. Sorry to run out on you fellas-- the wife had some other plans for me.

I definitely see where you're coming from, Andy, and I agree that an overrun regulator will make for some unpredictable fuel pressures. It would be interesting to see how pressure behaves over the entire RPM range, though. Unfortunately, I like my car a little too much to experiment with that.

 

[andymoraitis]

I definitely see where you're coming from, Andy, and I agree that an overrun regulator will make for some unpredictable fuel pressures. It would be interesting to see how pressure behaves over the entire RPM range, though. Unfortunately, I like my car a little too much to experiment with that.

Me too Don. The last thing I need is to have another motor go south, especially when you consider the money that's in the bottom end of this sucker.

 

[Kevin Jewer]

I've actually found stock regulators to be MORE reliable under WOT conditions that AFPRs. One problem is the very common leak at the top of the aeromotive unit that everyone (incuding me) uses causing non linear rise in fuel pressure. Make sure you seal that 1/6th NPT fitting! They tap it too deep. The other is the crappyness of aftermarket fuel pressure guages causing people to set incorrect base pressure. I dealt with both of these problems simultaneaously, and until I figured out what was going on, it was not fun The stocker just plain works. At WOT anyway.

 

[andymoraitis]

The stocker just plain works. At WOT anyway.

I'm just going to stop talking....

 

[Kevin Jewer]

Maybe I just had good luck with them, who knows.