Boost and AFR failsafe question

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95ponymobile
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Boost and AFR failsafe question

Post by 95ponymobile »

I have a buddy that has a Lexus with a 2jz turbocharged putting down over 700 to the wheels. On another forum recently, he posted a pic of a melted piston. I asked if he had some sort of failsafe in place in case AFR spiked like disabling the boost controller or a timing/rev cut and his tuner spoke up saying the AFR wouldn't have changed enough to trip a failsafe. It looks like to me, especially in boost with increased fuel flow, this would work fine. What are some of your expert opinions on this? Even if for some reason the AFR wouldn't change you could still trigger a failsafe off inj duty cycle or something to that affect or am I wrong on this. I see no way the AFR couldn't change if a cylinder went lean, especially in boost.
Matt Cramer
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Re: Boost and AFR failsafe question

Post by Matt Cramer »

O2 sensor do have a bit of a delayed reaction time. Failsafes are nice to have, but depending on the problem, some other strategy like a fuel pressure failsafe might react faster.
Matt Cramer -1966 Dodge Dart slant six running on MS3X
Drew442
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Re: Boost and AFR failsafe question

Post by Drew442 »

Yeah it would depend on how the failure occurred and under what conditions.

If it was a single cylinder issue at very high rpm there was probably no indicator that could be read to trigger a failsafe in time.

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Laminar
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Re: Boost and AFR failsafe question

Post by Laminar »

I've had a couple of my LS coils go bad on my engine, and just a few weeks ago the car developed a misfire due to a burned spark plug wire.

Image

I have a wideband on each bank about 2 feet downstream of the exhaust ports, and as soon as the cylinder started misfiring, I saw AFRs go from ~14 to ~17 on the affected bank. One bad cylinder in four is enough to make a noticeable difference. One cylinder in six would be less of a difference, but it should still be noticeable, no? If the engine is well-tuned in boost, you could set a decently tight AFR safety setup that would pull out quickly, right?
ol boy
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Re: Boost and AFR failsafe question

Post by ol boy »

For cylinder to cylinder comparison you'd want 6 egt probes. Those also have a bit of delay, even more than WBO2 but it would tell you quickly if one or several holes aren't performing well. If one hole is 150* higher or lower than the other 5. Maybe the intake manifold favors one or a set of holes better than the others. You wouldn't know if there was an AFR/egt spread until you test for it. Could be +- 0.5 AFR between all hole which still averages out to 11.5 under boost, so 11.0 on the low and 12.0 on the high. Not all injectors flow exactly the same either. By chance you end up with a slightly lower fuel flow on a slightly higher air flow cylinder. Head cooling will determine which holes can withstand the most heat and for how long before detonation sets in. At 700HP any slight variation across the holes could let the smoke out.

On the GM LS stuff #7 runs the hottest so most will put the highest flowing injector in that spot. The lowest flowing injectors in the front and stagger them back to #7.

My carb to EFI converted intake favors the inner 4 holes quite a bit. I've added 11ish% more fuel to the inners to even out the mixture which resulted in better idle and alot less wiggle delta on the AFR gauge.
306 SBFord, Torquer II EFI intake, 60 lbs injectors, 8 LS2 coils, VS Racing 7668 turbo, 4R70W, MS3x fw1.4 w/built in trans controller.
95ponymobile
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Re: Boost and AFR failsafe question

Post by 95ponymobile »

Thanks for all the input guys. Appreciated
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