Hello guys
we have resolved the creep issue we talked about here (i'm gonna also post there whats the problem was for future reference)
https://www.miataturbo.net/diy-turbo...tching-108982/

for whom dont want to read all the back story, the turbo itself was the wrong turbo. We are just at our first steps in the turbo world, as we do mainly N/A miata build, and we trusted the wrong turbo builder shop which make us lose money and time.
Finally we managed to get our hands on a TD04 subaru (subaru p/n 8M04-400-541) and we had done a flange adapter to match it to the 4-bolt T25 exhaust flange.
A lot of dirt work after - to adapt all the small things (downpipe, WG bracket, inlet pipe, airbox etc etc)- and the car start, works with no problem, and we resolved the creep issue - we get something like 0.2 bar (2,9 psi) @7000 rpm so good enought to control our boost target pressure (0.6 bar / 8,7 psi)

We're good, we say to ourselves,
let's go on a dyno and get the car tuned by respectable tuner.
Here the problems arises: the car has too much torque at too low rpms, and we did not reach the power levels we expect / everybody seems easily reach
The car obviusly is a blast to drive like this on our mountains road, but these torque levels at these rpms can harm seriusly the engine.
We really dont want to give the customer a timebomb. And above all, if we do not understand, we do not grow up...and we really really want to understand the problem.


TL;DR
I attach a small review of the car and the dyno result:
NBFL 1.8 w/ VVT, stock engine
Homemade 2.5" downpipe
Wrong kraken T25 exhaust manifold with adapter to 3-bolt subaru pattern
Very short airbox to turbo piping
Very short turbo to TB/manifold piping (watercooled IC)
No IAT issues on the dyno. The hottest dyno IAT levels recorder was like 49°C (120 far )
ME442 ecu
380cc yellow rx8 injectors
Blow off valve
mechanical (pneumatic) actuated wastegate, adjusted to 0.6 bar / 8,7psi.
The car hold the pressure good until the end. We tried to go up with the pressure (0.9 bar) but the only thing we got is 5-6 hp at the same rpms as before and 1 kgf to the same rpms as before


Just remember we're speaking about Nm and BHP, not RWHP






How the heck everybody seems to just throw a TD04 subaru at these cars and get the max torque at around 4.500 rpm and 230-240 BHP around 6500 rpm?

 

Is there a log of a dyno pull? To me, it looks like a small turbo. Peak torque and peak boost happening around 3000, then tapering off, with a flat line of power up top, but I'd like to look at the boost curve.

 

We did not have log of map on dyno
I can do some pull logged for you
As a long time lurker i recognize your expertise and appreciate you spending some time.

270 nm are around 199 ft/lbs. It this torque level enought safe @3000 rpm in your opinion? The car Is a blast on hills and very interesting to drive, altought 200 bhp are... Not so much.

We used the commonly praised TD04 from Subaru. How it can be so restrictive in our case only?

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It's a turbo that likes to blow up often here in the states, not sure where you heard it's reliable. It's not as bad as their transmissions and head gaskets, but it's up there.

Take a log of a pull and post it and your tune here, we can take a look.

around 200ft/lbs is rod bending territory, you're definitely on the lower side though.

 

I will do
In Europe Is commonly praised as cheap, available, cheap to rebuild and effettive.
It Is Indeed all of this but in our case, not so effective.

 

I attach the log file and base VE and base advance
Ideally we are aiming for shifting torque upward of around 1500 rpm (now the maximum torque is 27 kgfm @3000 rpm...unsafe)
The wastegate actuator was set to 0.5-0.6 psi.

 

What makes you say tq at 3000 rpm is unsafe?

 

Original engine, stock. VVT.
270 nm (200 ft lbs), but rpm are waay too damn low

And above all we want to understand how everybody gets nice high rpm bhp and us not.

 

People have been throwing the equivalent of 200 across-the-pond lb-ft at 3000rpm on stock rods for years.

I suspect your answers are found on the respective compressor and turbine flow maps for your turbo.

What is your idle vac?

 

Around 0.4 kPa absolute

 

That can't be right, and can you be more precise?

Looking for a number between 25-32 kpa absolute.

Please also provide idle afr, spark advance, RPM

 

Think he means 0,4 bar.

Why not pull a few ° timing out below 4k, if scared?

 

If he means 0.4 bar he needs to check his mechanical (cam) timing, paying special attention to the locations of the Z/I/E marks on the ex gear at TDC.

 

We will check timing just in case
However the car was up to spec perfectly prior the turbo works, but in a miata is like a top much easy job to skip on

 

With that ex cam gear, make sure the dowel pin is in the correct slot. That cam gear can be mounted in 3 different slots.

 

We do a lot of n/a miata, no problem ever but i guess i will check, nobody Is perfect

 

Timing checked. It is correct.
At idle the car makes around 35-40 kpa absolute, it doesent seems so off to me.

 

What is your idle RPM, AFR, spark advance? 35-40kpa (hot) would be a big red flag for me.


Edit: did you take a picture of the cam gears at TDC? Did you confirm the correct position of the dowel on the ex cam?

 

I will tomorrow, now its like 2 am lol.
​​​​​​The car idle stabile at 900-950 rpm and drive well.

 

Idle vac is a great way to evaluate how well an engine is running. It is basically a repeatable dyno. It is very difficult to use drivability as a good indicator because it is both subjective and low output is compensated for automatically with the throttle.

To put it into perspective, I would expect a healthy BP with VVT to idle at 28kpa +/- 4kpa when hot with no accessories. At 35-40 kpa you are requiring ~33% more air (and efficiently combusted fuel) to produce the same amount of horsepower. This is a strong indicator that something is wrong.

You have several other observations (unusual boost onset, low power, excessive boost creep, lack of expected power gains) that can all be related to an overly advanced exhaust cam. The overlap is so great that you pollute the charge, combust in the manifold, and have massive reversion under power.

All of this can also be caused by a significant restriction in the exhaust path as well, so if you have a catalytic converter, it might be worth an inspection.

seriously though, post a compressor map for that turbo.