Is Using A Gt30 1.01A/r Housing On An Rb25 Crazy?

[JustinP]

I know usual wisdom states that when using a GT30 series turbo on an RB25 that you go with a 0.82 A/R rear (or 0.63 if you really want response). The 1.06 rears are better left to RB30's.

However I have heard some comments from people saying that if you have a proper twin scroll setup you can get away with the bigger rear turbine on a 2.5L or thereabouts engine, gaining a lot of top end performance with not much loss in response.

For my never ending build I had bought and installed an IW GTX3071R with 0.82 rear, mounted to the OEM exhaust manifold. I have changed directions yet again and I've decided to go with a high mount and externally wastegated setup. Therefore I will need to sell my rear housing and get a new one to suit. I will likely be going for a twin scroll 6boost manifold.

I was looking for a new turbine housing and noticed these newer Garrett models with a 1.01 A/R instead of 1.06 A/R:

http://www.atpturbo.com/mm5/merchant.mvc?Screen=PROD&Store_Code=tp&Product_Code=ATP-HSG-211&Category_Code=NGH

Given a twin scroll setup will improve response and given that 1.01 is slightly less of a jump compared to 1.06, is going for the bigger rear housing a silly idea?

The car will not be my daily driver or anything, so I'm not looking for fantastic response and would be happy having the additional power. But I don't want it to be so laggy its just stupid. I know the GTX's like more boost and will be running as much as it will take. The car is also running a bigger cams and forged / balanced bottom end, so happy to use higher revs if needed. I also will be running a WMI setup. And I'm running 86.5mm pistons, so there is a tiny bit more capacity there too

Thoughts on the idea?

Edited November 25, 2014 by JustinP

[Dale FZ1]

If the application is not road-only, or an occasional weekend toy, you will generally be running at elevated engine rpm (yes?).

There's a few unknowns regarding flow capacity of the split pulse housing vs a comparable single scroll design of same/similar nominal A/R. Possibly the 1.01 TS will be only moderately higher flowing than a 0.82 single scroll, so I'd suspect that a little more efficient scavenging at higher PR and rpm will give a bigger power number with little difference in response and power down low. That is without changing cams.

The cam change is really only going to effectively shift your torque curve to the right, maybe counteract the benefits of the TS.

Using anti knock/detonation strategies ie. E85 fuel or water injection should naturally lead to higher output, so shift the curve upwards.

I'd give it a go, you might be surprised how driveable the combination is. Your power output is going to be capped by the capacity of that little compressor, but good setup/tune should yield 325-340rwkW across a broad range.

I'd also recommend broadening your tastes and think about Borg Warner split pulse units rather than just change housings on your existing spec Garrett.

[JustinP]

Cheers, thanks for that. And yes elevated rpm will be more the norm I think I think I might give the 1.01 housing a go.

Im not against looking at other turbo's, and given I'm going to need to get everything fabricated anyway there would be no issue in changing. I will do a bit of research on the Borg Warner line.

If the application is not road-only, or an occasional weekend toy, you will generally be running at elevated engine rpm (yes?).

There's a few unknowns regarding flow capacity of the split pulse housing vs a comparable single scroll design of same/similar nominal A/R. Possibly the 1.01 TS will be only moderately higher flowing than a 0.82 single scroll, so I'd suspect that a little more efficient scavenging at higher PR and rpm will give a bigger power number with little difference in response and power down low. That is without changing cams.

The cam change is really only going to effectively shift your torque curve to the right, maybe counteract the benefits of the TS.

Using anti knock/detonation strategies ie. E85 fuel or water injection should naturally lead to higher output, so shift the curve upwards.

I'd give it a go, you might be surprised how driveable the combination is. Your power output is going to be capped by the capacity of that little compressor, but good setup/tune should yield 325-340rwkW across a broad range.

I'd also recommend broadening your tastes and think about Borg Warner split pulse units rather than just change housings on your existing spec Garrett.

[discopotato03]

There is a train of thought that "T3": twin scroll turbine housings don't quite do it as well as the T4 flanged ones even in the same AR size .

If staying with the GTX3071 it may not be an issue given the limits of the compressor side but Geoff Raicer is probably the one to ask .

A .

[Dale FZ1]

If there is one issue about the centre bearing housing I really dislike, it's that Garrett make it so compact (short) that getting spanners onto oil/water lines is very difficult. This is more evident once you start using bulkier split pulse housings that push over into that area. Recent experience with fitting up a GT3076 / 1.06 split pulse combination was memorable in that respect.

[scotty nm35]

But that housing wasn't designed by Garrett, it was cast in some Mexican garage. The one the OP linked looks ok to me.

Personally I would stick the .82 on the factory manifold, and stop wasting coin on manifolds and parts you don't need for that power level.

[AngryRB]

^^^ +1, or you could use the forged pistons in an RB30 block and go GT35, proven with no guess work..

[Dale FZ1]

^^ both valid points, although the OP already has built his bottom end as RB25 (pistons use different gudgeon pin height), and the Garrett branded casting does not look to be any less bulky in the critical areas.

I'd maintain the view that if he wants to go to the trouble/expense of split pulse then look further afield than Garrett and reap the benefits.

Edited November 26, 2014 by Dale FZ1

[discopotato03]

That BB centre section was based on the GT25 plain bearing housing . GT25 housings are more compact than GT30 and yes getting at some of the bolt heads can be interesting . Most people working on them often have "special tools" or normal ones reshaped with a bit of heat .

The next size up centre section is more like plain T3/T4 dimensions but they start in the GT37s etc .

Gotta run cheers A .

[JustinP]

Thanks for all the feedback and thoughts, much appreciated

After doing more research I really am tempted by some of the BW EFR series products (T4 twin scroll). The idea of having the BOV and boost control solenoid built into the compressor housing is appealing as it would reduce the complexity of my setup somewhat and I would need slightly less fabrication work done. It does look a bit ugly though!

Even the idea of running an IW version is growing on me, that would really simplify things for me. It just seems counter-intuitive to go down that path though if I will be running up around the 30psi mark.

Should I really consider an IW version over an external setup at those power / boost levels? Are they really able to flow enough and control boost well?

I'm also thinking I might bump up my power goal a little, would be nice to crack the 400rwkw mark The only thing is I think my fuel system will not be up to it.

Cheers

Justin