What's fuel cut?
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>MY 20 PSI 3RD GEN VIDEO>>
A distant memory that's what
Turns out there's a lot of misinformation on what triggers the 3rd gen ecu.... Most people think there's just a hard 17.999 psi threshold that kills the party the instant the turbo tries to boost higher, but that's not true.
Before I go on:
>>>> Refer to the following at your own risk. Raising boost is a good way to damage your engine. I cannot be responsible for any damage YOU inflict on your ride using the information below. >>>>>
>Seriously, cracked ringlands, melted pistons, blown head gaskets, burned valves, shattered ceramic turbines, death, general unhappiness etc, can all occur when you defeat fuel cut! I'm just a dude sharing my experience modifying my car, attempt at your own risk!!>>
Also, In my opinion water injection is mandatory. The cooling effects of water in the combustion chamber acts as a knock suppressor that is critical when raising boost. In fact, go read a book on what goes on in a combustion chamber to really appreciate how awesome Water injection is. Otherwise, enjoy your cracked ring lands
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Water Injection Explanation Video>>
Anyway
The ecu is not just looking for a single voltage from the map sensor. Fuel cut is triggered by multiple variables including at least
throttle position, engine rpm, and rate of boost build, probably more
I realized this when my boost controller would hit and hold 19 psi at part throttle (think 3/4). Or when I could hit and hold 19 psi at WOT below 5500 rpm w/o issue. How could this be possible if 17.99 psi was a hard limit??
Here's an >
>example>> of what I think the effect of rpm is on the map sensor voltage required to trigger fuel cut:
The higher the rpm, the lower the boost pressure required to trip cut. Thus the key to preventing fuel cut from ever triggering, is to cap the map sensor just below the lowest threshold voltage during the worst case mix of engine rpm and throttle position. The best way to do this is with a map clamp.
Unfortunately there's a lot of hate for map clamps around here; the biggest complaint I've heard is that fuel cut would still seem to engage at random times... Well, that's just those other two variables kicking in to lower the threshold voltage. Again, the ecu is looking for a combination of parameters to trigger cut, you need to find the worst case scenario and adjust your clamp to cap juuust below it.
Here's what I use:
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>Supra-Store Boost-Cut defender Link
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This thing is basically a reverse engineered Greddy BCC down to the the wire color (you can even use the Greddy install manual if you want). Its obviously set up for a supra, but all you have to do is take it apart and rotate the adjustment screw counter clockwise to lower the cap voltage. It took me half an hour of adjusting the screw counter clockwise 1/4 turn at a time until I stopped hitting fuel cut under any circumstance. Easy.
I wired mine in right at the ECU (will update with wiring specifics later, too tired at the moment)
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>>>But what about air fuel ratios!!?>>>>
Watch the video, these cars run so rich that my afr needle pegs at 10 even at 20 psi... did I mention water methanol injection?
You may be wondering why the AFR hovers around 12 for a few moments in some of the pulls. It's because I was flooring it from around 2500 rpm; the st205 ECU leans out the mixture below 4000 rpm to help spool the turbo faster. After 4k the AFR collapses to 10 and stays there all the way to redline. Leaning out the mixture to help reduce lag is a pretty common strategy among OEMs.
For me there have been zero issues. once I set voltage I never hit cut again. ever.
I've also never had issues with knock, and I've done multiple hard track days in 90 degree heat. During some events the car got so hot that I could literally hear the coolant boiling in the turbo between sessions, even after 5 min of idling. Still no knock, or pulled timing, or anything.
This post has been edited by enderswift: Sep 21, 2016 - 12:06 PM
