Hybrid turbo upgrade selection & general tuning question

[RETed]

Like everyone said, you still need to worry about the overboost fuel-cut programmed into the stock ECU, although it's been inconsistent on S5's.

Then, you'll need to address the additional fuel required for the larger turbo.

It's questionable how much boost the turbo will produce, even with the description you have given.

An upgrade fuel pump is highly recommended.
Also, rewire the fuel pump for best voltage - the stock wiring is problematic at this age.

Just to make things more efficienct, some kinda fuel control is needed to fine tune everything after all of the above has been added.


-Ted

[Pete_89T2]

Thanks guys, you've all confirmed what I suspected, but I was hoping I was wrong! Bottom line options are to: (1) Accept the stock rebuild and call it a day (and ask BNR if they can add the wastegate port while they are at it), or (2) Pony up the time & dollars to address fuel/spark management correctly and get the stage 1 turbo upgrade. I'll post another thread later to discuss option #2 in more detail, stay tuned.

Ted - I forgot to add the fuel pump rewire in my list of mods; did that one almost 10 years ago, but still have the original pump. Thanks for the reminder - I'll need to test and verify that my handywork is still up to snuff 10 years later! Also what did you mean regarding fuel cut being inconsistent in the S5's? Some S5 turbos hit fuel cut, others don't? Seems if the ECUs were that different across a production run, they should carry different part #'s but who knows what Mazda did? FWIW, I've never hit fuel cut with mine, but I wouldn't expect to given the very close to stock setup.

[RETed]

Overboost fuel-cut has been programmed in every stock ECU at approximately 1psi over stock levels.
This basically means just a little higher boost will trigger it.

Some stock S5's with stock ECU's have never been known to hit overboost fuel-cut even when obviously going way over stock boost levels.
(S4's, OTOH, will always hit overboost fuel cut at ~6.5psi.)
No FCD (fuel-cut defensor) or any other electronics that's molesting the stock boost sensor signal was obviously found...
We don't know why this is the case.
We can't seem to find a pattern with ECU part numbers either.
Some claim that if the car hits the overboost fuel-cut enough, it somehow disables the stock programming - I can't confirm nor deny any of this.
FCD's are cheap (i.e. Racing Beat), so it's really no excuse not to use one.
There's even a DYI version posted on the INet if you're good at handling electronics.


-Ted

[Pete_89T2]

Originally Posted by RETed

Overboost fuel-cut has been programmed in every stock ECU at approximately 1psi over stock levels.
This basically means just a little higher boost will trigger it.

Some stock S5's with stock ECU's have never been known to hit overboost fuel-cut even when obviously going way over stock boost levels.
(S4's, OTOH, will always hit overboost fuel cut at ~6.5psi.)
No FCD (fuel-cut defensor) or any other electronics that's molesting the stock boost sensor signal was obviously found...
We don't know why this is the case.
We can't seem to find a pattern with ECU part numbers either.
Some claim that if the car hits the overboost fuel-cut enough, it somehow disables the stock programming - I can't confirm nor deny any of this.
FCD's are cheap (i.e. Racing Beat), so it's really no excuse not to use one.
There's even a DYI version posted on the INet if you're good at handling electronics.


-Ted

Interesting, although I doubt the inconsistency is an intentional "feature" of the S5 ECU code as some would claim. Reasons being: (1) State of the art in OEM HW/SW technology probably wasn't up to the task back in the late '80s, and (2) Implementing it this way (i.e., disable fuel cut feature after "X" overboost events) runs counter to what a responsible company who warranties their product would do -- they would be opening themselves up to liability claims. If owner's were routinely overboosting their new FC's for whatever reason, and then the the fuel cut feature suddenly stopped intervening after "X" events, what would convince the dunderhead behind the wheel to return his FC to Mazda for service? If this is an intentional feature of the S5 ECU code, I think Mazda would have opted to enable the "limp home" mode after "X" overboost events, similar to what it does for electronic OMP faults.

My guess is that somewhere in the S5 production run, Mazda intentionally altered the ECU programming (overboost thresholds and/or fuel cut behavior) and didn't bother to change part numbers. Or if the ECU production & programming was farmed out to a subcontractor, the sub may have made the change unknown to Mazda.