Hybrid turbo upgrade selection & general tuning question

[Pete_89T2]

Not sure if this is the right place to post this one, so mods feel free to move elsewhere as necessary. It's more of a general question, but since it applies to a 2nd gen, I put it here.

I have a nearly 100% stock '89T2 which is in need of a turbo rebuild. Car is meticulously maintained, and it's drivetrain has been super reliable. Only substantial engine-related mods to date are a downpipe to replace the pre-cat, a Borla cat-back exhaust, and a K&N airfilter, oil thermo-pellet mod & partial TB mod (retained thermowax fast idle). Upgraded radiator is on its way. The stock turbo will be out of the car this weekend, and I plan to ship it off to BNR for the rebuild job.

For the extra ~$150 or so, I'm thinking of going with the "stage 1" hybrid upgrade BNR offers, which adds to the stock rebuild work waste gate porting, a bigger compressor wheel (T04b V-trim) that fits within the stock compressor housing (housing is machined to fit), and they clip the stock turbine wheel a bit. All in all, BNR claims this stage 1 upgrade can support up to 300~320 rwhp with the right tuning & setup mods. My understanding is this upgraded turbo will flow a bit more air volume, but otherwise performs like the stock unit WRT spool up, at boost pressures consistent to the stock setup (7~8 lbs boost on an S5).

I realize to achieve the HP figures advertised, I'd have to tune the fuel mapping & spark to take advantage of the extra air volume this upgraded turbo can push and/or run it at higher boost pressures than stock, and I won't get there with the stock ECU. But I'm not chasing HP figures and I'm OK with the stock power for now.

So my question to all the turbo tuner gurus out there is if I run this stage 1 turbo out of the box, with the stock ECU & boost control setup intact, (i.e., limiting boost to the 7~8 lb range), will I run the risk of grenading my engine?

[FerociousP]

Most likely yes. You need the capability to run larger injectors. This can be anything from an RTEK to maybe an S-AFC. ***Were you running more than 8psi w/o and FCD?***

I run a stock turbo at around 11psi with an RTEK 2.0 with 750cc secondaries. If I got the BNR stage 1 the least I would to would be to go with 720cc primaries for safety sake. Anything more than that and I would want to go standalone, which isn't stock ECU and harness.

Rtek 2.0 upgrade is $450 plus a palm ($30ish) and gives you a ton of features for the stock ECU setup on budget and is plug-n-play. Injectors are around $100.

[Pete_89T2]

Originally Posted by FerociousP

Most likely yes. You need the capability to run larger injectors. This can be anything from an RTEK to maybe an S-AFC. ***Were you running more than 8psi w/o and FCD?***

Nope, I've been running the stock turbo at stock boost pressures, no FCD or any other mods to the stock ECU. My after market boost gauge has never read a *steady* pressure beyond 8 PSI with this turbo, but I've seen it spike up as high as 10~12 lbs for very brief instances.

With the stock ECU and boost control limits retained, I would assume boost pressure with the upgraded turbo would still be limited to stock levels - so I don't think I would ever be overboosting it. Also, this upgrade ports the wastegate, which in theory should make it less likely to overboost past the stock boost settings. The main concern I had is will the extra airflow volume that the stage 1 upgrade is capable of (at stock boost pressures) REQUIRE me to upgrade the fuel/EMS now, before adding the upgraded turbo, given that I'm not chasing after optimized HP numbers? Will running with the stock ECU (and its AFR mappings) cause me to go a bit lean under boost, which is possible given the additional airflow volume, and potentially grenade my engine with this turbo? From what I've read, the Mazda stock ECU mappings run pretty rich, and there may be enough of a factor of safety in there to accomodate a turbo that flows just a little more air than stock, at the same boost pressures.

[FerociousP]

Interesting. My experience and what I've heard is that the boost rises with a more free flowing exhaust due to less back pressure, and would need a FCD. But if yours isn't then you I would imagine you are fine. I run a full RB turboback exhaust and I hit 11-12 consistently and it bleeds back down to 8 with only the stock wastegate for control. It even will creep higher in 4th and 5th at certain RPMs. I am running a FMIC.

FWIW I would love to run a BNR stage 1 at 7-8psi. If my turbo ever has problems then I will probably do it. I'm not looking to make big power, just enough to get out of my own way.

[Pete_89T2]

Originally Posted by FerociousP

Interesting. My experience and what I've heard is that the boost rises with a more free flowing exhaust due to less back pressure, and would need a FCD. But if yours isn't then you I would imagine you are fine. I run a full RB turboback exhaust and I hit 11-12 consistently and it bleeds back down to 8 with only the stock wastegate for control. It even will creep higher in 4th and 5th at certain RPMs. I am running a FMIC.

The full RB turbo-back exhaust explains why you see higher boost pressures. I still have the stock cat, and probably always will need a cat to keep my car legally registered.

[djmtsu]

The problem with running with the factory ECU with the Stage 1 BNR is that you ARE going to hit the factory boost cut, which is very bad since it cuts fuel.

If you are going to go Stage 1, you might as well invest in at least the Rtek and larger secondaries, and get it tuned. Otherwise, stick to the standard rebuild, and get Brian to port the wastegate while he's in there. I would get an FCD as well for the extra insurance.

[FerociousP]

Originally Posted by Pete_89T2

The full RB turbo-back exhaust explains why you see higher boost pressures. I still have the stock cat, and probably always will need a cat to keep my car legally registered.

Yep. that'll do it. Either way, you are close to the factory cut which is why the FCD exists, so I just found it worth mentioning. To answer your question directly: A BNR stage 1 at even 7-8psi (below the stock boost cut) will have more flow, make more power, and put more demand on the stock fuel system than I would be comfortable with.



I have emissions as well, but in my travels I have aquired enough parts that I put the air pump back on and swap out the RB presilencer for a cat come emissions time. I also run the airpump outlet to the cat air pipe before I go to the emissions center. Then the airpump comes off and the presilencer goes right back under the car after I pass. The cost of a couple gaskets is worth it for a year of better exhaust. (If that seems weird, know that the ACV feeds airpump air to the cat as well as feeding more airpump air to the exhaust sleeves for emissions purposes. So, I am essentially doing the same thing... temporarily).

[88turboii]

if i were you and didnt want to spend a lot, you may get by with just throwing some 750cc injectors in the secondaries and call it a day. it will run a tad rich around the transition point, but the AFRs shoudl be pretty close to what you want under higher loads and prevent engine from popping. It wouldnt hurt to retard the CAS by 5 degrees too

[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.