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Overboost problem (1.5JZGTE)

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Hello everybody,

Today I had my S14 on the dyno, but during the first big pulse we had big overboost.

Let me explain it and give you my setup:

Trubo : Garrett G35-900 twinscroll A/R 1.06

Wastegate : Tial MVR44mm single wastegate

AAC : BC272° with tomei cam gears

Head : OEM, only BC spring and retainers

Engine VE : 9.67:1

The problem was even without any springs in the wastegate I was still building 1.83 Bar of boost... :(

I think my wastegate is too small or maybe the Turbine housing A/R is too small.

the main point now is that I would not like too loose to much power band by mounting a bigger A/R and I'm actually not sure it will correct the problem at all.

That's why I'm asking your help and would love to take your experience and knowledge,

I thank you in forward for your answer and wish you a good day,

Kind regards,

Séverin Stoeckli

Are you sure your manifold / boost pressure sensor is calibrated correctly? Can you confirm with the pressure sensor on the dyno?

Was your boost pressure 1.8 bar (ie, above atmospheric pressure), or your manifold absolute pressure 1.8 bar?

Hi David,

Thanks for your reply 👍

Yes, everything was correctly calibrated.

The Dyno and the ecu were reading exactly the same pressure.

The boost is 1.8bar above atmospheric, I'm not taking the atmospheric pressure in the reading.

HI

The best way to test it. Remove the wastegate completely than do test run, if the turbo still boostcreep. The manifold design is not good.

Post picture of your manifold design.

Hi,

We tried to remove everything in the wastegate and the boost did fall a little bit (1.70bar), but we didn't run it without the wastegate at all.

I've thought about this, but the manifold looks actually really good for me (see the picture).

Don't you think it could only be the wastegate or the single wastegate design that makes it overboost like this?

Attached Files

The manifold design looks very good for power/torque- but I think the chap means the way the waste-gate is plumbed in. In this case, the way it's designed there will be much less bleeding off of the gases into the waste-gate because of the 90 degree angle on the rear - worse, the front has two 90 degree angles.

If it's redesigned with two 'gates, both directly onto their collectors, the 90 degree angles will be less important. Better would be using curved bleed tubes, placed for an easier transition into the 'gates.

Chipping in...

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Hi guys,

Thanks a lot for your replies 🙏

Let me update a bit the investigation.

The manifold design seems to be good, but the holes made in the manifold where the tubes goes to the wastegate were smaller and misaligned, that meens the flow was really restricted and the gases couldn't flow correctly.

I've cut everything, grind the holes and now I have to make new tubes for the wastegate.

Do you guys think the MVR 44mm is enough or should I go bigger? Maybe a 60mm WG would be better?

My boost goal is 1.7bar but I want to be able to regulate from 1bar.

You guys were right thinking of a problem from the manifold itself 👍

I would suggest dual 44's, that way you can optimise each, rather than compromise at least one collector's bleed-off. The smaller sizes may be easier to locate than the bigger single, too.

The reason I suggested a curved pipe, earlier, is that it/they would flow well from the manifold if tangental, and possibly make the placement of the actual gates easier than using a straight pipe angled off.

You could probably go twin 38mm, but if you want to be sure, you can go twin 44mm (plus you already have one!). A single 60mm would probably be fine as well. As it's a divided manifold, I think it would be beneficial to run 2x WG so there is no "crosstalk" before the turbine.

Hello to everybody,

After a few investigation, I decided to go with 2 TIAL 44mm wastegate.

Indeed with a twinscroll manifold it's always better to go with 2 wastegate instead of one, but at first I was a little bit afraid about space with 2 wastegate ;).

I'm gonna use 48mm pipes for the feed and 42.4mm pipes for the dump. Unfortunately, for the dump pipe, I couldn't access to 45mm pipes in a short time. But I'm sure the little difference in diameter, will not create any problem for the gas flow and create any restriction. What do you guys think?

I put you a picture here with how we're gonna route the wastegates to the manifold.

Attached Files

with the two gates there is a dedicated bypass for each scroll so you should see an improvement.

One thing I've had with a 2JZ before was not enough timing which created enough heat to generate more boost than expected, adding in some ignition advance cooled the EGTs and lowered the boost levels

Hi Chris,

Yes exactly it would definitely improve the flow and should correct my problem.

Yeah we tried to put more advance and I also worked a bit on the cam timing to help. It lowered the boost but not enough. We went from about 1.83bar to 1.70bar, and always with the wastegate totally empty of all the stuff inside it.

We ended up to the conclusion of maybe a problem with the manifold or the housing A/R. By cutting the WG tubes of the manifold I found that the holes in the manifold were smaller than the WG tubes.

I'm actually rebuilding this part of the manifold and as soon we put the car back on the dyno, I will give you some feedback on how it worked ;)

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