Penguinracer

Hi All,
my build is nearing completion but it has changed from being a 2.5 litre to a 104mm bore 2.7.

I specified a GTX3071R Gen 2 when the plan was for it to be a 2.5 litre & I think I'll start with that, but I've also got a GTX3576R Gen 2 as a second option (both with Teal 0.82 hot sides).
I also have a 1.06 for GTX30.

Any recommendations as to which is likely to be a better fit for road/ track car?

The pertinent featuress of the engine build are:

• MID Sleeves (104mm bore)
• SFR big valve N/A head
• SFR Intake with 70mm TB
• SFR Stage 1 headers/Cross-Over/ 3' Down pipe / 3" Test Pipe / 3" Borla system
• SFR Stage 2 I/C
• Wizard radiator
• Sebtrab 925 oil cooler
• A/C delete
• Web 274 cam (254 duration 0.480 lift)
• 8:1 Mahle custom pistons;
• Carillo rods
• LR Dry Sump system
• EMTRON KV8 ECU
• Injector Dynamics ID1300 injectors;
• SFR Stage 2 fuel system
• Bosch 044 lift pump & twin Bosch 044 pumps from 1.5 litre swirl pot
• EMTRON CDI Driver with COP
• Sequential Injection
• Sequential fuel injection
• Clewett Cam trigger
• TTV 60:1 tooth flywheel
• KEP Stage 1 PP / Organic CP

I figuring a bored 2.7 will be an incremental improvement on a 2.5 in terms of unshrouding the valves & promoting better breathing but not have the torque uplift a 2.8 litre "stroker" would offer. I like the shorter stroke for smoothness, revviness, reduced crank flex and to retain a semblance of originality.

Bear in mind, in developing this car, one of my criteria was that despite the modifications, it remain a "matching numbers" vehicle as I like that direct connection / lineage back to the factory build.

Tim

gruhsy

Try them both and report your findings

Thom

Gen 2 GTX compressors seem to flow more but also seem less efficient at peak than Gen 1 units. They are not worth the extra expense in my opinion.
I would not use a hotside any smaller than a GT35 with a short runner intake like the SFR's, which will open up the top end above 5k which is where the GT35 hotside will perform best, but I would not use anymore overlap than the standard 9R camshaft with a low static CR of 8:1.

ealoken

I went gtx3576g2 on my 2.5 build. the GTX30 will be to small in the big end.

jimbo1111

I used a gt35r on a 2.5 liter years ago with excellent results. imo you are going too large on the turbine housing. I wouldn't go larger than a .63 ar. The turbine wheel probably has a 61mm exducer and back pressure shouldn't be an issue. Of course its subjective relative to how much boost and type of fuel you are going to use. My suggestion is based on 1 to 1.2 bar boost and pump gas.

333pg333

I think running a 35 with a .63 housing is too imbalanced and asking for back pressure. There's many mitigating parts to a motor and usually not a '1 size fits all' situation. I'd take tips from Thom above as he has tested quite a few of what you're looking at Tim.

Penguinracer

Hi Thom, ealoken, jimbo & Patrick,
thanks for your advice.
I'll try out try out the GTX3576 & report my findings.
It would be really good to see back-pressure data on the Garrett turbos with various configurations of 951 engine.

Obviously going too far chasing reduced back-pressure could result in a laggy engine with a narrow power band at the top of the rpm range, but finding
that compromise between area under the curve & avoidance of power drop-off above 6K would be ideal for me.

Realistically, can see myself running up to 1.2 bar & 6.5K.

As usual, these boards are full of useful advice from those intrepid adventurers who have sallied forth & experimented bravely!

Thank you all.

Tim

ealoken

Welcome sir

im realy closing on getting my car ready, needs a dyno in August, Nurburgring in october, i will post backpressure and all dyno info

Erik