Stumble under hard accel

[MoparStephen]

I have just got my 2.2L Turbo II running in my 87 Daytona Shelby Z. It has a ton of mods:

• +40 injectors
• T3/T4 hybrid turbo, .50A/R with 3" swingvalve
• TU cast exhaust header
• FWD Performance F3 cam (272/262 duration at .006 tappett lift on the intake/exhaust, 460/460 gross valve lift on the intake/exhaust, centerline 114) installed 4 degrees advanced
• Wiseco forged pistons +.020
• 255 lb/hr fuel pump
• 3 inch exhaust, no cat
• AFPR - 30PSI at idle, 50PSI under boost
• stock throttle body
• stock intercooler
• FWD Performance cal (BoostButton - I'll be placing an order with you ASAFP)

The motor idles nicely (I was expecting a bit of a lope to be honest) with 15 in/Hg. I get 135PSI static compression, and have my base ignition timing set at 12 degrees. I am currently running a manual boost controller set to 11PSI. I plan to eventually up this to 14.5PSI (based on advice from Chris at Turbos Unleashed) My EVO wideband shows 14.5 to 14.7 at idle, and my spark plugs look good. The motor has about 350 miles on it.

The engine is pretty slow to build boost and under WOT in 1st gear it has a bit of a stumble or miss around 3500RPM. When this happened, my wideband was reading between 10.5 and 11 so I am pretty sure I am getting enough fuel. Once it gets past this, it will spin the tires to 5000RPM and do the same in 2nd gear. I have not taken the motor above 5000 yet due to the low mileage. Unfortunately, it doesn't always stumble, but it also never pulls as hard as I think it should.

Any ideas what I should try first?

Stephen

[BadAssPerformance]

Sounds like a nice Z build! Sluggish could be cam timing or ignition timing (cal?)

[Ondonti]

Need more information. Wheelspin doesn't help dianose anything.

If you are in 3rd or 4th gear and not spinning and you feel problems but AFR is solid, then you are probably suffering from spark blowout. That is if all your timing etc is good.

[MoparStephen]

The AFR is fine in 3rd and up. Full throttle and full boost it stays between 10.5 and 11.5. I am running a slightly larger than stock coil, since my stock coil had a pretty big dent in it.

From the replies here, it sounds like the calibration I have is not good. I just received my TL866 burner and have a few EPROMS on the way.

One quick question though - is there a recommended base timing to set on these modified motors? I am currently at 12 degrees, but only because that's where my stock motor was. I do not know how much "range" the logic modules have. I do seem to be getting some spark knock under full throttle, and this is with only 11 PSI and 993 octane.

[MoparStephen]

Major necro bump here, but I didn't want to start a new thread. Apologies if this is not the right thing to do.

Anyhow - I have a few years on the motor now, and am breaking in a new transmission with quaife which I just assembled this winter. I am now running the 8.5mm Aurora wires and new coil. Boost is around 11.5PSI. Posted a video of the stumble I get. I am 99% sure the final issue I am having is that I am running far too rich when under full throttle. Apologies for the bad video - the gf was holding my cell phone and wasn't expecting to be thrown around the car. I'll get a Go-Pro soon



I have wideband on there. The video was ----, but it got good audio. The gauge readings were:
Boost is set at a very low 11.5PSI
AFR when first breakup showed 11.2, but immediately dropped and staid at 10.0 which is the gauge min
Fuel pressure is around 42PSI. I run the +40% which are rated at 55PSI, but go lower PSI to lean them out some.

When steady state cruising, I stay around 14.4 and can average 30MPG at 75MPH. My plugs aren't covered in carbon nor are they wet. Mid-throttle everything is as it should be, and by middle of 3rd gear I can go full throttle without really any issues.

I've been chasing this issue for over 2 years now. I think I need to get some dyno time so I can have good full monitoring of everything. I created a new tune this evening and will be testing it shortly. I turned on anti-lag retard and also hopefully cut the fuel back quite a large amount. If this doesn't do it, I'll have to start looking at ignition related issues. The car runs perfectly through 6000RPM when not under boost (ran it with the top IC hose disconnected to verify it wasn't a valve spring or lifter issue).

[glhs727]

Just out of curiosity, why such low boost? most hybrids don't even reach their peak of efficiency until the upper boost levels, and so I could see a big turbo car not having good drivability when only running 11 psi of boost vs. 18 psi or even 25 psi.

[MoparStephen]

Just out of curiosity, why such low boost? most hybrids don't even reach their peak of efficiency until the upper boost levels, and so I could see a big turbo car not having good drivability when only running 11 psi of boost vs. 18 psi or even 25 psi.

I figured I'd try turning it up once I could get it running OK, and also have traction. Even at low boost and with a quaife, it rips the tires loose.

[BadAssPerformance]

random thoughts...

FWD Performance F3 cam (272/262 duration at .006 tappett lift on the intake/exhaust, 460/460 gross valve lift on the intake/exhaust, centerline 114) installed 4 degrees advanced

Probably doesn't affect stumble, but why so advanced on the cam?

[*]AFPR - 30PSI at idle, 50PSI under boost

AFPR should be 1 for 1 so is the 30PSI with vacuum connected? Check with vacuum disconnected

The engine is pretty slow to build boost and under WOT in 1st gear it has a bit of a stumble or miss around 3500RPM. When this happened, my wideband was reading between 10.5 and 11 so I am pretty sure I am getting enough fuel. Once it gets past this, it will spin the tires to 5000RPM and do the same in 2nd gear. I have not taken the motor above 5000 yet due to the low mileage. Unfortunately, it doesn't always stumble, but it also never pulls as hard as I think it should.

Slow boost build could be many things. When does full boost happen?

if the boost is laggy and you get a spike the boost rate of change might just dump extra fuel and choke it for a second. Can you roll into throttle (instead of floor it) and prevent stumble?

What is AFR at steady state WOT at 5k rpm with no stumble?

for 3500rpm stumble check
-plug gap
-hall effect pickup
-grounds

[MoparStephen]

Thanks for the tips - due to weather and work, earliest I'll even be taking the car outside is this Saturday.

RE - the cam advance. I'm an old school V8 guy, and normally with a cam that aggressive with the RPM range I would want, I'd go a few degrees advance. I have not touched the centerline since installing the motor. I am looking into some local dynos to see what I can do about having the car tested. It's too dangerous and non-productive trying to "street tune" it. Oh yeah - forgot. Cold cranking compression test averages 135PSI.

[going4speed]

why such a low fuel pressure?

[MoparStephen]

I scaled my calibration to use 42PSI. I'm 42PSI at idle, so from my planned 11 to 14PSI of boost I should be 53PSI, around where the injectors would max out.

[going4speed]

threw me off in the first post it says 30psi