the 2871 has the same hot-side as the gt2860rs super turbo.

GT2860RS with 0.86 A/R
vs
GT2871 with 0.64 A/R

I'd need to see the full turbine maps. Basically look at the compressors, for say 3500 RPM and 7.5 psi, read out the RPM and efficiency. Calculate the shaft power and engine horsepower. Then, look at the turbine maps and determine if that exhaust flow at that horsepower can deliver that required shaft power at that shaft RPM. Whichever has more excess shaft power will spool quicker at 3500 RPM. Repeat this at 0, 5, 10 psi; at whatever psi there is no excess shaft power from the turbine, will be the max boost at that RPM.

P.S. my gut feel is the 2871 with 0.64 would spool quicker, but a 2860 with 0.64 would be even quicker.

The reason for that is the small A/R would generate more shaft power at lower exhaust flow rates, even when comparing the 2 compressors (and thus at different shaft RPMs).

what if I let you pick one of two conditions? all used with the same spark table OR all tuned to MBT by whatever means necessary--like a super low comp motor or water injection or 100 octane fuel.

you can't really argue the point about MBT without actually putting the .64 on and tuning it because even though it might seem to preclude running near MBT, none of us has data to prove it. so lets assume we can cheat that portion by other means in choice 2.

All those turbos share the same hotside wheel. 53.8mm 76 trim. here's the garrett excuse for a map.

http://www.turbobygarrett.com/turbob...&064turb_e.jpg

I'm with Hustler on this one. I'm switching to a bigger A/R turbine because I almost exclusively track my car and I would rather have lower underhood temps and lower engine load than 400rpm earlier spool.

Bigger A/R = less restriction = less overall heat output by the turbine = lower underhood temps = happier track car..... at least that's what "Maximum Boost" taught me.

circular logic.. hustler got his turbo originally from Corky who wrote that book :)

Well, I'm new here, so I'm not going to jump in the fray...but I do have a question:

Where are the pre and post turbine pressure measurements? That is what dictates whether a turbine A/R is too "big" or too "small."

Turbine pressure ratios dictate spool. Not EGT's, although EGT's and mass air flow dictate how much energy there is to be extracted from the exhaust.

I am running the .64 on my 2860 for my street/track car. I did that beacsue I am ultimately not looking to make 375 hp, just around 300-340. Even then, the car will be a handful on the track.

I don't feel that in the range that I will be running, with respect to power, that the heat will cause a significant enough difference to merit a larger A/R. If and when I push it further, then it may come into play more. At that point I will move up to an .86 2871, IF I need more than 325hp on the track for some retarted reason. These assumptions were made on my limited knowledge of how all of this crap works, so if my logic is wrong please call me out on it.

Garrett agrees with that line of thought too:
TurboByGarrett.com - Turbo Tech102

I may have fallen victim to my own idiocity. With my smaller .64 ar I am not hitting a brick wall for power. It may be the turbo or may not, but i have a feeling it is just because I thought I was doing the right thing for response, but it punished me severely in overall power. I should have gone with a .86 2871 to begin with.

Links to dyno sheet?

Sorry, I looked.

Details on how Y8's is a mouthy-------? Seriously, sentence #1 and #2 contradict the last sentence.

Not to cause a fuss, but I don't understand the third sentence with all the buts. Actually, it may have been infinitely more less clear if you had buts instead of periods between every statement. I am just saying that I had to read it twice and still don't know what it says. I think the first and last sentence make perfect sense though. Sorry for my idiocitiness.

:drama: Continue

These are really good threads, just the right amount of discussion vs. friendly shit-talking.

This thread, and about 1/2 dozen more like it have cemented my choice of the .64 Disco as the turbo of choice for a 300whp street-car.

What I really want is a few more people to start exploring the 300whp range on stock 99-00 motors and gather datapoints. So far, there are only a few guys who do it.

S4, exhintake, adj cam-gears, Adaptronic, 3" exhaust... I CAN'T WAIT FOR CRUISE TO BE OVER!!!

BTW this is what full turbine maps look like so one would be able to predict spoolup. Compare to what y8s posted above from Garrett:


http://upload.wikimedia.org/wikipedi...urbinechic.gif


And a subset of the above

http://www.turbodriven.com/images/tu...rinciple_g.gif

Edited for binge drinking post crapping stupidity.

Sorry, I need to clean up that last sentence. I really didn't read through it before I posted. What I was saying is that I thought the trade off for ultimate power for quicker response would be better than what it was. I am now feeling the punishment for not doing my homework and assuming my logic is flawless.

This turbo would really be great for a daily car.

There is a lot of assuming and bs in this thread. Making decisions without some sort of data and analysis means you all are tuning blind...

What are your pre and post turbine pressure measurements? What are your compressor efficiencies? What is your pre-turbo pressure to manifold pressure ratio? What data are you using to make your assumptions?

you of all people should know how important it is to question the current dogma. that's what I'm doing.

you mean you should have chosen the 2871 .86 at first instead of trying the 2860 .64 for a street car and hoping for better spool with the latter?