One for turbo experts / turbine theory

[Thom]

Hello,

Looking for answers regarding comparing two turbos that use the same compressor but different turbines, the GTX3076R and the GTX3576R.

The GT35 turbine wheel measures 68mm, the GT30 60mm.

Here are the respective turbine maps, each showing flow capacity with the different housing sizes available.





From comparing these maps, can we say that :

1/ A GTX3076R 1.06 will, at peak, flow the same as a GTX3576R 0.82, since both hotsides flow a peak of ~27 lbs/min?

2/ Since the GT35 turbine wheel is larger, should a GTX3576R 0.82 spool slower than a GTX3076R 1.06?

3/ If a GTX3076R 1.06 flows the same and spools quicker than a GTX3576R 0.82, is there a point at all in using a GTX3576R with either the 0.63 or 0.82 GT35 turbine housings?

Thanks.

Edited by Thom on Tuesday 8th April 09:07

[Thom]

Engine is a 3L spinning to 7k rpm with 1.2 bar of boost, currently showing 1.3 bar of turbine inlet pressure (static) up to 6k then increases to 1.7 bar at 7000 rpm. With a GTX3076R 0.82.
The increase in TIP corresponds with a slightly falling torque curve past 6k rpm as shown on the rollers.

[Thom]

I have already bought the 1.06 housing and plan to fit it soon. It's also what appeared to me as the logical next step considering how turbine inlet pressure evolves with engine speed. I don't care much about losing a couple of hundreds rpm in spool since the spool is indeed already more than very good.
Got a 272/272° seat-to-seat single camshaft with 48° of overlap and have played with retarding cam timing but torque is still slightly dropping off past 6k, so I'm quite sure the larger turbine housing is in order.

But then my original questions remain - how would, for example, a GTX3576R 0.82 compare with a GTX3076R 1.06? These two hotsides flow a peak of 27 lbs/min, but is the compressor ever going to reach the limit of the larger turbine if it's going to spin slower to flow the same?

Edited by Thom on Wednesday 9th April 10:35

[Thom]

So I replaced the GT30 0.82 with the 1.06 housing and must say I am well chuffed with the results. Lost perhaps 400 rpm in spool but definitely feel nice little gains up top.
I'm yet to measure back pressure again but from the shift in the usable rpm band I would guess it may be hardly any higher than boost up to 7k rpm.

Anyway, back on topic :

stevieturbo said:

Just because they flow the same, doesnt mean each will have the same amount of drive energy to actually spin the compressor wheel

chuntington101 said:

Steve couldn't the 35r turbine be more efficent though, this reducing the drive presure required? Would that result in faster spool?

Compared with the smaller GT30 turbine wheel, shouldn't the larger GT35 turbine wheel, once it's "up to speed", transmit more inertial energy to the compressor than the smaller GT30 turbine wheel would?
Shouldn't this result, with the GT35 turbine, in slower initial spool from low revs but also in better boost response when going through the gears under full load?

A bit like switching from the smallest to the largest ring gear on the pedal side of a bike...?

Edited by Thom on Tuesday 20th May 11:22

[Thom]

My set up is nothing to write home about. It's a an old 4 cyl SOHC engine using an antiquated 2V head design. Head flow is so poor that it's been difficult for me to get more than 405 hp / 401 ft.lbs so far. That was with the 0.82 housing though, and I feel like I picked perhaps 10-20 hp up top with the 1.06 housing. I plan to increase boost as I have not been able to get anywhere near the knock threshold since I fitted this 272° camshaft, even after wildly increasing ignition timing.
Wish I could rev it well past 7k, but these old lumps tend to go pop with the original oiling system when revved too high, and I probably reach the flow limits of the head somewhere around 6k rpm anyway.

Essentially trying to make ends meet with a basically old engine design, where traditional turbo sizing does not seem to apply.

[Thom]

Just thought I would post an update as I replaced the GTX3076R 1.06 with a GTX3576R 0.82.

So the larger turbo spools slightly later than the previous one, but by the time it does the engine feels like it's making more torque pretty much everywhere.

Even if levels of turbine inlet pressure never suggested that the smaller GT30 turbine was on its legs, I can say the larger turbo flows quite a bit more than the previous one according to adjustments needed in the ignition timing, fuel and wastegate duty cycle tables. Didn't get the chance to dyno the car again but I would guess I picked perhaps ~30hp.

My understanding is that the GT30 turbine does not get to spin this compressor at the speeds required to let it flow all it can, at least on my engine.

If we compare both compressor maps, we see that efficiency islands are much narrower with the GT30 turbine than with the GT35, which, now that I have tried both turbos, suggest to me that the GT35 spins slower than the GT30 for the same compressor flow. Inferior acceleration of the compressor makes for wider efficiency islands, if that makes sense...?

Also, when the larger turbo is into boost, it feels like it spins quite a bit more frantically than the smaller one, suggesting that the larger turbine accumulates inertia with more ease than the smaller one. More "drive energy" as mentioned in post #2.