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Lets Playyy, Find That Restriction!

ST240

By ST240,
in Engines & Forced Induction

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ST240 Enthusiast

ST240

Posted
Posted (edited)

Here's my ride:

RB25DE/30 Stock intake manifold/TB

Precision 6262 (~GT35R for arguments sake ;)) .58 T4 hot side

Tubular mani to 3" straight pipe

Same side entry vertical flow intercooler (omfg read below)

2,5" intake piping

z32

740cc

I'm having an issue where during a pull, nistune shows load index tapering off while boost is staying constant as well as timing/afrs. It is literally flat until 5500 rpm at which point it starts tapering off. It was REALLY bad, going from 75 to 61 with boost at 14 psi. l swapped my piece of shit horizonal flow, same side entry intercooler for a vertical flow one, and without chaning anything, a log showed it going from 86-75 with boost building to 16 psi. While this is a major improvement (the car feels awesome in comparison!) I still feel as if it is running out of puff at the top end. Could it be the stock intake mani/TB? I thought they were good to 300 rwkw and i dont think im there yet really. Has anyone else noticed this kind of thing?

Any info would be appreciated!

Edited by ST240
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ST240 Enthusiast

ST240

Posted
  • Author
Posted

Choked to the max. I'm guess your ignition timing is abnormally retarded too?

Can you buy replacement housings for precision turbos as easy as Garrett ones?

Running 16* at full boost so not too bad. Yeah you can, but i might be better off swapping the turbo all said and done. FFS. Just bought that thing.

You guys really think its the rear end in the turbo eh? Nothing intake related?

bubba Grand Master

bubba

Posted
Posted

Won't be stock intake mani and throttlebody unless you've left a rag in there somewhere. With the same head, RB20 manifold and throttlebody, pair of 2540's at 22psi we saw 365rwkw.

Not sure on exhaust housing specs of that turbo but I've had a small turbo on a 3L and when it choked up it nosed over hard (250-90hp in 1000rpm)

  • 2 weeks later...
Rolls Grand Master

Rolls

Posted
Posted (edited)

Must be really hard to starve off detonation with a tiny hot side like that. Does it make full boost by 2k and then dive over at 5k?

How physically big is the hotside though, is it comparable to a GT35 or is it bigger?

Edited by Rolls
  • 3 weeks later...
ST240 Enthusiast

ST240

Posted
  • Author
Posted (edited)

No my EGTs are fine. 800-900 in full boost?

No knock. No it spools by 3500-4000 and kinda dies off around 5500. The more i think about it the more i do agree with you guys that it is the hotside. The turbine is 62mm, which is also the size of the compressor. Which doesnt add up because it does feel kinda laggy somewhat. Unless my tune is way out.

Kinda wish id gone gt35r or gt30r. Not too much data on this turbo at all.

Edited by ST240
Rolls Grand Master

Rolls

Posted
Posted (edited)

The more i think about it the more i do agree with you guys that it is the hotside.

It is 100% the hotside, there is no doubt about it, eg what everyone in this thread has said.

Also 900c is very hot imo, should be more like 750c max. Change the rear of the turbo and it will fix all your issues, it is completely mismatched!

Edited by Rolls
R31Nismoid Grand Master

R31Nismoid

Posted
Posted

Yeah exactly, its getting so hot and causing so much stress you cant put any timing into it as a result. 700hp+ of turbo with a rear end capable of probably 400hp max... Not going to end well.

You need at least a .82 sized A/R, even a 1.06 would be fine (when talking GT35)

Will depend what options you have from precision but anything inbetween there as it also depends on how the A/R is calculated as the housings wont be the same as Garrett I'm assuming.

Maybe ask Precision what housing they have that is between a Garrett .82 and 1.06

That would probably be the best choice.

GTScotT Grand Master

GTScotT

Posted
Posted

What exhaust manifold have you got?

If its a cheapy maybe you can get it cheaply moded to T3 and get a Kando Dynamic T67. Cheap and easy way out, otherwise if its an expensive manifold I would try get another T4 frame turbo.

ST240 Enthusiast

ST240

Posted
  • Author
Posted (edited)

Thanks a ton guys.. Youve been a great help. I'll definately be looking into my options. Apparently I can get a twin scroll T4 .86 . Ive got a fairly good manifold on it; twinscroll but not true twin scroll with two gates ;). Ah its gotta be better than fully undivided :P .

Btw I am the tuner. And Ive been monitoring knock and there is none. But I am being conservative with my timing numbers.

Edited by ST240
Harey Veteran

Harey

Posted
Posted

My understanding is the twin scroll housings are roughly one size larger than the single scroll variants.

So 0.82 single scroll is roughly equal to 1.06 Twin Scroll.

So a 0.86 twin scroll might still be too small for a 3.0L, as it would equate to somewhere around a 0.7 single scroll which is too small.

I would recommend either a 0.8X single scroll or a 1.06 twinscroll.

ST240 Enthusiast

ST240

Posted
  • Author
Posted (edited)

Like i said I have a divided manifold with the appropriate runners in each port. What I lack is dual wastegates.

On a separate note, I was told by the precision guys that a .82 T3 open housing flows the same as between a .58 and .68 T4 open housing. Can anyone confirm? Sounds quite dubious.They also told me they dont offer any larger turbines because they "feel you should be moving to a 6265 if youre flowing that much". Lol. Guess the GT35R with a 1.06 is a silly idea then :rolleyes: .

Edited by ST240

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