Thread: Boost Spike
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Old 09-22-2013, 07:51 PM   #99
Seeking 9s on M6...
PMJ's Avatar
Drives: 2012 2SS/RS LS3 aka "BlackWidow"
Join Date: May 2012
Location: Houston, TX
Posts: 397
Originally Posted by ayousef View Post
I concluded that ill forget half of the stuff I wanna say by tomorrow so im typing this now.

- Disabling transient fueling MIGHT cause a change, transient fueling is basically a fuel adder/subtractor based on throttle position changes whether you are getting onto the throttle or off of it, and its purpose is to estimate the amount of fuel injected that will NOT end up in the combustion chamber yet stick on the intake port wall and evaporate from heat (simple definition). There is a master disable switch which could turn this off for some experimenting.

- Regardless of what tuners say I always put my wideband after the x-pipe or in the y-pipe if applicable, yes putting it that far isn't that accurate for part throttle and idle which I never use my wideband for but its spot on for WOT infact a reading at the collector could be 0.2:1 richer than having it far back, so its conservative.

- The FPCM is an electronic device that controls the fuel supply by changing the pulsewidth signal to the pump, when fuel demand increases it starts pulsing the pump quicker etc... The problem with this is the small delay between the ECU sensing an increase pump speed demand, increasing the pump speed, flowing the fuel to the injectors which is why you can clearly see a fuel pressure dip when you floor it. I was taking the injector delta thing you got on your log too seriously but it looks like its way off!

Injector pressure delta is basically (fuel pressure - manifold pressure), if manifold pressure is below barometric pressure (vacuum) then use a minus so the injector essentially becomes "bigger" and vice versa. Now the problem I see here is when you're going WOT, fuel pressure drops momentarily to about 45psi and you're instantly making 17.5psi of boost, so your injector pressure delta is about 27.5psi which is very bad itself. This is exactly why we have 1:1 boost referenced regulators because say on my car with a base fuel pressure of 60psi, the pressure delta is ALWAYS 60psi etc...

- On a Whipple I guess you're bound to seeing one side being richer than the other, I would like to think its because one rotor spins 66% faster than the other one but im not sure. Instead of moving the wideband just log narrowband readings from both cylinder banks because AFR is dipping past 14.6 so the narrowbands show a lean reading as well.

- Your IATs went crazy high on the last pull in the last screenshot however increased steadily on the WOT run before the last one, I think it has something to do with the detonation cause it really spiked. I think you should richen the mixture a bit or reduce spark advance just to be safe, make the IAT vs spark table more aggressive on higher IATs.

- So my conclusion here is restrict the bypass valve in the blower like Ed suggested, slow down the throttle transition rate and if you still have the fuel pressure drop at the WOT stab you can use it to your advantage, you should do a few runs and measure the fuel pressure delta at this very spot then goto the Injector vs flowrate table in the tune and at the same pressure delta spots REDUCE the injector size, this will trick the ECU to think the injector is smaller and force it to command higher duty cycle for this short moment only and your problem might as well be gone!

This is all theoretical of course >.<

believe it or not I did one 2-3 shift in my ZL1 and saw a lean spike as well, I haven't duplicated it yet however since im fixing my fuel heating issue first.
I'll give the transient fuel disabled a try tonight (I hope).

Yes, from what I've read and heard, the SS ECU does not acknowledge boost (despite the 3 bar MAP), so inj delta is nothing more than fuel pressure when in boost.

I don't think I buy the comment on the whipple pushing more boost on one side than the is all connected, so pressure should be distributed relatively equally...nice try though

The detonation definitely raises IATs quick. I was running race gas above, so I don't pull any timing below 200* on my race tune (even w/o detonation, running the whipple on a 2.75" pulley in Houston results in very high IATS). My street tune is more conservative...

Slowing the throttle definitely works to reduce the boost spike...when it works like it's supposed to...the spike to 50%+, drop 30%, spike back to 100% irritated me greatly... I think the jet Ted recommended will have the same effect mechanically instead of if it works, i'm going to move the throttle back to 100% and try to keep the damn computer out of it!

So I just looked at my friends log...I ran his car twice the last time I was at the track (his 2 fastest runs in his car ever ). He has a similar build with a Maggie vs. my whipple. He is running less boost, but I had this problem at 9# of boost as well. He doesn't have a wide band, and looking at his fuel pressure, he is still on a stock SS fuel pump (it dropped to 46# by the end of the pull & 38# on the stab). Based on the narrowband, he had what I would guess is a lean blip...but nowhere near the spike I have.
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