Choosing a turbine A/R Ratio

[TiredGXP]

Getting ready to order the turbo for a 3.1 iron head motor. Based on my research, it looks like a GT2871-743347-2 will be well matched to the amount of boost I plan to run. I'm trying to figure out what turbine A/R ratio to order.

The engine is going in an '85 Fiero w/Muncie 4-speed. Realistically, this vehicle will only be used for cruising around, and maybe the odd auto cross, so it will be living in the 2500 to 5500 rpm range with redline @6000.

Engine Specs
.75mm overbore 3.1
Sealed Power iron head hypereutetic pistons (expect ~8.8:1 CR after polishing combustion chambers)
Crane 2030 cam
1.6 roller tip rockers
head work limited to gasket match and bowl blend and general clean-up
Stainless steel valves
105# valve springs @ 1.7 inch installed height
Ported LIM (custom UIM being researched - I want to use the Sy/Ty TB)
W2A intercooler from Sy/Ty
7749 ecm, going to try to make $59 work on a 60V6 with DIS

Depending on how the tuning goes, I want to shoot for 12 pounds of boost.

Compressor specs:
71mm
56 trim
.6 A/R

I have to decide between a .64 A/R and .86 A/R turbine housing. I'm leaning towards the smaller A/R housing as it will allow the turbo to spool quicker, and provide boost closer to the bottom end of the expected rev range.

Will this housing be too restrictive to exhaust flow at higher RPM?

[RedZMonte]

I would go for the larger A/R a T28 turbo is awfull small. I think you will be happier with the results. You may also wanna consider going to a T3 based turbo.. still spools fast and will flow alot more... My TGP would fall on its face up in higher RPM's.. alot of guys with the TGP's go to a T3... But again if you stick with the GT28 i would opt for the larger A/R.

S

[TiredGXP]

Thanks for your comments.

Based on the compressor maps that I've seen for the GT2871 and various T3's, the turbo I'm planning on purchasing significantly outflows the T3 "Super 60". I expect that the heads/intake will be the major restriction, not the compressor.

Anyone else with comments/recommendations for a Turbine A/R for this application?

[robertisaar]

i have a T61 still sitting here from when i was getting ready to do my turbo build. the map matched my 3100 perfectly. i don't know about the iron heads though. the squirrel performance turbo calc is pretty detailed and seems accurate to me. might want to check that out.

[TiredGXP]

i have a T61 still sitting here from when i was getting ready to do my turbo build. the map matched my 3100 perfectly. i don't know about the iron heads though. the squirrel performance turbo calc is pretty detailed and seems accurate to me. might want to check that out.

I've used both the Squirrel Performance website and a spreadsheet I created based on the Garrett "turbo tech" formulas - virtually identical results in calculating pressure ratios and airflow in lb/min (spreadsheet takes into account intake losses, intercooler efficiency and altitude effects).

Based on assumed VE and BSFC at various RPM's, it looks like the iron head 3.1 will stay within the 75% efficiency island of the 2871 compressor throughout the entire power band - mostly due to the ghastly VE at > 5000 rpm.

Squirrel, and other sites, are great for comparing compressor maps. What I find really frustrating is that I haven't been able to find any really useful guidance on reading a turbine map. Hell, except for Garrett, you can't usually find turbine maps. I've been rereading Maximum Boost this evening, but Corky Bell also glosses over turbine and turbine A/R selection.

[robertisaar]

I've been rereading Maximum Boost this evening, but Corky Bell also glosses over turbine and turbine A/R selection.

trust me, i know, its sitting right in front of me. still worth the $35 i paid for it though. and i know aluminum head 3.1s are "ghastly" above 4500, i couldn't imagine a iron head at 5000.

and the reason why garrett is the only one with maps..... well, theres a reason why everyone likes garrett, its proven stuff.

[Joseph Upson]

I suspect the problem with turbine maps not being readily available is because there is so much variation in exhaust gas temperature and velocity depending on the engine and exhaust system. The compressor side is a lot more stable and consistent across engines but with exhaust temp rising and falling so much it's just not easy to depend on maps so readily unless your engine and exhaust is similar to the test mule the info was established on.

I believe the T28 series is to small, I ran a T3 .60/.63 on an iron head 3100 in my Fiero and could have used a little more turbine area. I was never successful with running it as high as 10 psi and didn't do a full investigation to find out if it was fuel related, or something else but I learned quickly that anything less than a T3 anything on a 2.8L and up is to small because I've turboed several motors.

Part of the benefit of the larger turbine housing is to help keep your fuel economy nearly unphased with reduced exhaust restriction. I averaged at least 25 mpg hwy with my high reving 4spd and low compression engine (iron heads, alumi head pistons) and that's probably the most impressive part about turbocharging, it allows you to have the best of both worlds, high power and high mpg.

[robertisaar]

Part of the benefit of the larger turbine housing is to help keep your fuel economy nearly unphased with reduced exhaust restriction. I averaged at least 25 mpg hwy with my high reving 4spd and low compression engine (iron heads, alumi head pistons) and that's probably the most impressive part about turbocharging, it allows you to have the best of both worlds, high power and high mpg.

as long as you don't have your foot into it, then of course... but, thats the problem, have a turbo, but not utilize its potential? not fun. i'm sure there are more civil members on the board than me, but i don't know if i could have a turbo installed and not get the the boost guage to go nuts.

[timg]

For the 2871R, I would use the larger turbine housing. It has a fairly small turbine even for a 2 liter 4 cylinder.

The GT3071R has the same compressor, but a larger turbine. It would have a bit more lag, but make more power. The GT3071R is available with either a T2x turbine housing or a T3 turbine housing. The T3 housing would be much more interchangeable with conventional and larger turbos, but the T2x housing would be interchangeable with the GT2871R and other smaller turbos.

Either should be a decent choice for your goals, but I would prefer the GT30 series over the T-2-small series.

Tim

[TiredGXP]

I see what you guys are saying.

Now that I've taken a closer look at the turbines on the various options, I see what you mean about the 2871 having a small turbine wheel.

It looks like even the GT3582 would provide 74-76% efficiency in the anticipated power band of this engine. Seems to be a 68mm turbine wheel on this unit, which is in the range of some of the smaller T4 turbines.

Hmmm, a couple of hundred more for either the 3071 or 3582 and also need an external wastegate for a couple hundred more...oh well.

[Joseph Upson]

Fuel economy under normal driving can be adversely affected when the turbine housing is on the small side. Of course you're not going to install it and not use it. It just helps for exhaust to flow as if the turbo is not there when not in boost mode.

[RedZMonte]

I havnt looked at what your motor would map out at but i am still a fan of the GT3271. Great turbo.. efficent and a wide map.. Not ball bearing and avalible in a twin scroll housing and much more affordable then a ball bearing.

http://www.turbobygarrett.com/turbobygarrett/catelog/Turbochargers/GT32/GT3271_452203_1.htm

[TiredGXP]

I havnt looked at what your motor would map out at but i am still a fan of the GT3271. Great turbo.. efficent and a wide map.. Not ball bearing and avalible in a twin scroll housing and much more affordable then a ball bearing.

FWIW, here's the inputs I'm working with using the Squirrel Performance Calculator:

VE numbers are a guess, but based on a N/A BSFC of .49, these numbers calculated out to 140 HP @4800 and 166 lb-ft or torque @ 3400. Close enough to the figures GM rated the engine at to keep working with. For purposes of turbo calcs, I went conservative with the BSFC and upped it to .55 which, I suppose, is the same as saying efficiency decreases by 12% by going turbo.

Here's a selection of compressor maps charted out based on these inputs:

It looks like any of the three produce acceptable results, with the 50 trim T04E as the most efficient of the three, all appear capable of producing maximum boost @2600 without surging. The question is which turbine configuration will spin that compressor up without creating excessive back pressure at higher rpm.

All things considered, I'll probably go with the T04E. Found a source with a water cooled center section, ball bearings and 65mm 70 trim .63 A/R turbine.



Just for the hell of it, if you crank out the airflow numbers at various rpm's and assume 9-10 hp produced per pound of airflow, you come up with these "calculated" hp and torque graphs. Fun to play with , but not necessarily any relationship to reality.

[ForcedFirebird]

FWIW, here's the inputs I'm working with using the Squirrel Performance Calculator:
[ATTACH]2371[/ATTACH]
VE numbers are a guess, but based on a N/A BSFC of .49, these numbers calculated out to 140 HP @4800 and 166 lb-ft or torque @ 3400. Close enough to the figures GM rated the engine at to keep working with. For purposes of turbo calcs, I went conservative with the BSFC and upped it to .55 which, I suppose, is the same as saying efficiency decreases by 12% by going turbo.

Here's a selection of compressor maps charted out based on these inputs:
[ATTACH]2372[/ATTACH][ATTACH]2373[/ATTACH][ATTACH]2374[/ATTACH]
It looks like any of the three produce acceptable results, with the 50 trim T04E as the most efficient of the three, all appear capable of producing maximum boost @2600 without surging. The question is which turbine configuration will spin that compressor up without creating excessive back pressure at higher rpm.

All things considered, I'll probably go with the T04E. Found a source with a water cooled center section, ball bearings and 65mm 70 trim .63 A/R turbine.



Just for the hell of it, if you crank out the airflow numbers at various rpm's and assume 9-10 hp produced per pound of airflow, you come up with these "calculated" hp and torque graphs. Fun to play with , but not necessarily any relationship to reality.
[ATTACH]2375[/ATTACH]

Bar-none...

Based on experience with boost and 60 degree motors, the t04e 46 to 60 trims are awesome on these motors. For the iron heads they will spool a little later, and on he aluminum heads they will be in 12+ psi range before 3k RPM. Forget t25 housing as they will choke ALL of them.

Loner's car ran a best of 12.9sec/113mph using a 60trim t04e .50A/R compressor and .63 A/R turbine, spinning at least 100' with drag radials and a 1994 Chevy Corsica FWD ~3200lbs.

[TiredGXP]

Bar-none...

Loner's car ran a best of 12.9sec/113mph using a 60trim t04e .50A/R compressor and .63 A/R turbine, spinning at least 100' with drag radials and a 1994 Chevy Corsica FWD ~3200lbs.

I'm hoping this will have similar acceleration to my LS4 powered Grand Prix, but without the massive torque and understeer....mid to low 13's would be nice