Thoughts on GT3071 vs GT2871

[Drax240z]

One of the first steps of my engine build this winter is to figure out what turbo I need to get. I'm hoping to get some insight from some of you folks, the application is autocross.

 

Engine: 2700cc, 8.3:1 CR, Intercooled, SDS, forged pistons

Target power: 300-325rwhp

Desired characteristics: Responsiveness!!!! I am desperately trying to stay away from anything that builds power any slower than the stock T3. I'm hoping that the increased efficiency of the GT series turbos combined with the BB will allow me to achieve better top end power, and at least as broad a torque band as before. I am more concerned with going too large than too small!!

 

I've been pouring over compressor maps, and have two turbo's that I think make sense, but I'd love some real world perspective from you guys.

 

1) GT2871R - looks like there are variants that will meet my power goals, and the idea of a smaller turbo has some appeal, even given the modifications necessary for the T25 flange.

 

2) GT3071R - A bit more room "to grow", but I've got a pretty good view of what I want out of this build, and 325rwhp is it... so is there any need to go this large?

 

I'd appreciate any first hand experience you guys could share.

[Drax240z]

I'd also be interested in hearing characteristics of other turbos in that size range... I keep hearing the GT35 as an option but it feels a little big for my rather modest power goals.

[jc052685]

I would say go with the 3071. Good chance you will want more later anyway. The 2871 is rated from 300 to 400 crank hp. If you are wanting 325 wheel I think that will be on the upper end of of thee turbos ability. You would be better off with the 3071 as you can make the same power on less manifold pressure. Will be easier on thee motor with less exhaust manifold preessure too. I have heard the 3071 and 3076 have similar spool to the smaller 2871.

 

I assume your motor is an over bored destroked L28? With only 2700cc you are not going to have thee spool time of a standard or larger L28.

[Drax240z]

Thanks for sharing your thoughts. I'm not overly concerned about "wanting more later" as this is later. I'm fully prepared to buy another new turbo and make the mods necessary if that arises. However, I've been driving this car for 10? years now, and for my uses (street/auto-X) there is a limit to practical power levels and I am more concerned with power delivery than the peak number.

 

The engine config I am looking at is only 63cc less than the standard L series turbo, and has a full 9 points more compression. (7.4:1 vs. 8.3:1) I expect it will spool as well as a stock turbo L28 (or maybe better) in the end.

[240hoke]

Seems to me like you could bump the compression up even farther for the power levels you are looking at. Taking it to 9 or 9.5 would give you better drive ability off boost and allow you to spool faster.

 

Good to have you back btw!

[Drax240z]

Thanks Austin! Nice to feel the excitement again when I look at the Z. I spent far too much time working on other peoples cars, sortof lost the appeal for me for a while. I see you are still building, even after completing one of the nicer Z's out there!

 

Point noted on the CR. I've had some detonation issues in the past and might be overly passive with my selection of CR.

[Supra510]

"I'm not overly concerned about "wanting more later""

 

Famous last words I'd error toward the larger turbo, but be careful re: your compression ratio unless you really, really think 325 rwhp is all you'll ever want. My prediction is, once you get used to 325, you'll want more. Too high compression ratio will cause you headaches down the line.

[Drax240z]

The thing is, even with more (peak) power I don't think it's likely to make me faster around the autocross track. I could use a little more at this point, but to be honest not a whole heck of a lot more than the 232rwhp I am making in the previous setup. Widening the power band would be more useful to me, as well as adding some area under the curve.

 

Famous last words, yes. But for the purpose I want this car to serve at this time it makes sense to me. I can always buy a bigger turbo later.

[thehelix112]

There are a lot of options for the 3071R. What were you thinking?

 

Also, the compressor on the 3071 has a slightly bigger inducer than the 2871, which gives you a few more lb/min. I think you'll be running out of steam maybe a few hundred rpm later with the 0.63 GT30 versus the GT28.

 

But I'd expect the GT28 to be at full steam not much past ~2000, with the GT30 with a 0.63 maybe a few hundred rpm behind.

 

If it were me I'd be looking at a few things:

 

1. The GT30 has quite a few more options for turbine housing which can let you tune some of its characteristics.

2. The cam profile you are thinking about or already have, and how you prefer to drive. If you prefer to drive and have a cam that supports it, maybe lean towards the `500rpm higher' option?

3. The GT28 is not much bigger than the stock turbo on a SR20DET.

 

Dave

[ktm]

You would be better off with the 3071 as you can make the same power on less manifold pressure.

 

That statement is not true. Manifold pressure is manifold pressure, regardless of the size of the turbo. Your fueling is tied directly to your manifold pressure and VE. The only way to get more power for a given manifold pressure is to increase your VE by intake and/or head work.

 

Boosting at 10 psi on the stock T3 and 10 psi on a T4 results in the same power, everything else considered. However, if you are pushing the limits of the smaller turbo, then you will see an horsepower gain by switching out a turbo to a larger unit.

[ktm]

As for the GT turbo series, I guess I need to read some more about the decreased spool characteristics of the BB turbos. I know that my T03/04E 50 trim, 0.63 A/R will spool to 22 psi by 3200-3500 rpm in 3rd and 4th gear; 15 psi comes around 2900 to 3100 rpm.

 

I am able to achieve this two ways: 1) springing the WG actuator and 2) my EMS controls a boost solenoid, which does not let the WG actuator see MAP until 15 psi.

[thehelix112]

ktm,

 

If it were possible to get a two turbochargers of different size (T3 and T4 in your example) with "everything else considered" being identical, I would agree with you, however, its not.

 

**General generalizations disclaimer goes here**

 

The larger turbo will have a larger compressor wheel which will be able to move the same amount of air with greater efficiency.

The larger turbo will have a larger turbine housing and turbine wheel which will reduce the back pressure on the exhaust valve and improve the pumping efficiency of the engine.

 

And, your fueling is not directly tied to manifold pressure and VE. Its tied to the mass of air the engine is capable of ingesting. Higher pressure means more mass, lower temperature (greater compressor efficiency/intercooling) also means more mass.

 

Dave

 

That statement is not true. Manifold pressure is manifold pressure, regardless of the size of the turbo. Your fueling is tied directly to your manifold pressure and VE. The only way to get more power for a given manifold pressure is to increase your VE by intake and/or head work.

 

Boosting at 10 psi on the stock T3 and 10 psi on a T4 results in the same power, everything else considered. However, if you are pushing the limits of the smaller turbo, then you will see an horsepower gain by switching out a turbo to a larger unit.

[ktm]

ktm,

 

If it were possible to get a two turbochargers of different size (T3 and T4 in your example) with "everything else considered" being identical, I would agree with you, however, its not.

 

**General generalizations disclaimer goes here**

 

The larger turbo will have a larger compressor wheel which will be able to move the same amount of air with greater efficiency.

The larger turbo will have a larger turbine housing and turbine wheel which will reduce the back pressure on the exhaust valve and improve the pumping efficiency of the engine.

 

And, your fueling is not directly tied to manifold pressure and VE. Its tied to the mass of air the engine is capable of ingesting. Higher pressure means more mass, lower temperature (greater compressor efficiency/intercooling) also means more mass.

 

Dave

 

Dave,

 

I was simplifying my explanation with regards to fuel requirements and expressing it in terms of tuning parameters. I know it is directly related to the mass the engine is capable of injesting. However, the mass of air is a function of MAP when talking about forced induction. Your turbo is external to the cylinder and is providing the air mass. The "efficiency" of the manifold and the head will allow more of the mass for a given boost pressure to reach the cylinder and not be lost due to losses (i.e., small diameter runners, sharp bends, reducers, etc.). I was simply trying to make a correction that simply swapping out a turbo does not necessarily result in more power for a given pressure. It may or may not, depending on where the other turbo is on its compressor map and where the new turbo is on its map.

 

I fully understand the differences in efficiency and should have qualified my statement. Greater efficiency, less work, less heat, more mass, more power.

 

The issue has quite a few variables and oversimplification leads to conversations like this one.

[jc052685]

ktm,

 

If it were possible to get a two turbochargers of different size (T3 and T4 in your example) with "everything else considered" being identical, I would agree with you, however, its not.

 

**General generalizations disclaimer goes here**

 

The larger turbo will have a larger compressor wheel which will be able to move the same amount of air with greater efficiency.

The larger turbo will have a larger turbine housing and turbine wheel which will reduce the back pressure on the exhaust valve and improve the pumping efficiency of the engine.

 

And, your fueling is not directly tied to manifold pressure and VE. Its tied to the mass of air the engine is capable of ingesting. Higher pressure means more mass, lower temperature (greater compressor efficiency/intercooling) also means more mass.

 

Dave

 

Thank you for helping me not look like an idiot to others:mrgreen:

 

Turbo Efficiency - if turbo A can produce 12psi of manifold pressure with 16psi exhaust pressure and turbo B can produce 12psi of manifild pressure on 10psi of exhaust pressure than tubo B will make more HP with thee same amount of manifild pressure. Thus one turbo can infact make more HP on the same manifold pressure.

[ktm]

Thank you for helping me not look like an idiot to others:mrgreen:

 

Turbo Efficiency - if turbo A can produce 12psi of manifold pressure with 16psi exhaust pressure and turbo B can produce 12psi of manifild pressure on 10psi of exhaust pressure than tubo B will make more HP with thee same amount of manifild pressure. Thus one turbo can infact make more HP on the same manifold pressure.

 

I see what you are saying now. As I indicated above, it is a problem of over-simplification. There are too many variables to make such sweeping statements.

[Clifton]

I would go with the 30 series. May as well run the stock one over the 28. There is a guy running a 30 on an SR and is more than happy with the response. It is a track car though, not autoX. He says it comes on fast and never has lag. I understand you not wanting lag for AX, it sucks. I would look for dynos of similar sized engines with each turbo and see where the torque comes on at. It would give you a good idea of the boost curve.

 

 

 

As for 10 psi being 10 psi T3 or T4. A larger turbo doesn't have to have a larger compressor. Assuming one is matching the compressor wheel to the engine HP, it would be the same wheel T4 or not. If you want 400hp you need a wheel capable of 400 hp. So the compressor wheels would flow the same regardless of T3 or T4 flange. This leaves it only to the hot side. You can go to a stg 5 t3 and get close to a T4 P trim exducer but the inducer is still small. The T4 will has a larger turbine inducer giving more leverage. Even if you went on the small side ( O trim) the T4 wheel is the winner for hp. Less back pressure makes more power. It's hard to make 350+ rwhp with 15 psi on a T3 flanged turbo. Easy on a T4 with the same cold side. Less back pressure FTW.

[Drax240z]

As for the GT turbo series, I guess I need to read some more about the decreased spool characteristics of the BB turbos. I know that my T03/04E 50 trim, 0.63 A/R will spool to 22 psi by 3200-3500 rpm in 3rd and 4th gear; 15 psi comes around 2900 to 3100 rpm.

 

But I'd expect the GT28 to be at full steam not much past ~2000, with the GT30 with a 0.63 maybe a few hundred rpm behind.

 

As subjective as these kind of statements can be (and I know all the disclaimers that go along with them) they are quite helpful to me. If this car wasn't going to be used for autocross the T3/T04E 50 trim 0.63A/R was high on my list as good bang for the buck. KTM, what RPM do you start seeing boost by?

[thehelix112]

FWIW, my GT3582R with 0.82 housing, stock 7.4:1 CR L28ET sees positive pressure by 3000 in 1st, full boost (20psi) by 4500.

 

Dave

[Afshin]

I also run a T03/04E 50 trim, 0.63 A/R with SDS fuel spark management.

Will it does spool to good psi at reasonable rpm, it is really slow before that. In other words it start spooling late but then ramps up quickly. This is in comparison to my previous T3/T4 with a stock turbine. SO boost come on about 300 rpm later, but feels like its' off by 500 or more rpm. Using this configuration for open track events, i find the lag a little excessive and modulation of power in the 2500-3200 rpm range a little tricky.

As such the car is much faster on the street, but not on smaller more technical tracks. This problem would only be amplified for an ATX set up.

Of note, a main problem with my set up is that I'm still on stock cam and the mismatch with the turbo is quite noticeable so as the turbo comes to life by 2000 rpm later the cam chokes it (I have 3" downpipe, good intercooler....). I can feel that the turbo has so much more room to go on the topend as opposed to my previous one which did not want to go past 15 psi as effortlessly.

In short, I agree with your thought process and would not lose sight of sacrificing top end power for a wider power band for any car aimed at autocross and would keep linearity of spool up in mind (to allow for proper throttle modulation) as well as what psi can be achieved at a certain rpm. In general, I would avoid larger turbine set ups more so than larger compressors (I wish I had spent the extra money for a BB turbo).