any point in going for a 2871 if a 2860rs is ok? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 09-06-2010, 01:52 PM   #1
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Default any point in going for a 2871 if a 2860rs is ok?

i see people on here mentioning going for a 2871 .82 a/r over the 2860rs .82a/r

iam aiming for 280 wbhp no more.so is a 2871 pointless then or will a 2871 make more power right accross the rev range up to 280 compared to a 2860?

i read in a magazine try to go for the smallest turbo you can to achieve your power output what do you all think?
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Old 09-06-2010, 02:23 PM   #2
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i read in a magazine try to go for the smallest turbo you can to achieve your power output what do you all think?
^This.
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Old 09-06-2010, 02:36 PM   #3
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also keep in mind that a huge turbo is not efficient at lower boost/power levels... a lot of times guys will say this turbo doesn't "wake up" until xx psi, etc. This is why you look at the compressor maps, plot them out with your goals in mind and figure out what is best. Having a little extra headroom is fine but don't overdo it... GT2860RS should be just fine for your goals- btw is what wbhp lol- do you mean wheel or brake hp, im assuming wheel
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Old 09-06-2010, 03:16 PM   #4
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Quote:
Originally Posted by aaronc7
also keep in mind that a huge turbo is not efficient at lower boost/power levels... a lot of times guys will say this turbo doesn't "wake up" until xx psi, etc. This is why you look at the compressor maps, plot them out with your goals in mind and figure out what is best. Having a little extra headroom is fine but don't overdo it... GT2860RS should be just fine for your goals- btw is what wbhp lol- do you mean wheel or brake hp, im assuming wheel
I think 280whp on a Mustang or DD is the max for a GT2860rs.
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Old 09-06-2010, 03:47 PM   #5
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for whp i usually go by dynojet numbers for comparison but that's just me, it seems like they are more consistent as there is less stuff for the operator to vary/mess with to throw off numbers. Mustangs usually read lower but I've seen shops where they read way higher than a dynojet even... due to miscalibration I am sure. But you are probably about right... typical mustang dyno 280 sounds about right.

I've never ran one but I thought the GT2860RS was good for about 350-360 bhp, especially with the .86 housing- either way he says "280 no more"... the 2860 seems like a good choice. I guess if in doubt go a little bigger and get the 2871. no idea if this guy is built or not but if so i would pick 2871 hands down
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Old 09-06-2010, 03:58 PM   #6
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bhp =/= whp
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Old 09-06-2010, 05:28 PM   #7
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yep i meant [email protected]
i only plan to fit m tuned rods hence dont want to go more than [email protected]rs for your replies.
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Old 09-06-2010, 06:09 PM   #8
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I haven't seen any air intake temperature comparisons but a biggeer turbo will flow more boowst at less psi which means lower AITs and less detonation
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Old 09-06-2010, 06:21 PM   #9
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/\ This.
more air at less pressure = cooler air
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Old 09-06-2010, 07:13 PM   #10
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Originally Posted by faeflora View Post
a biggeer turbo will flow more boowst at less psi which means lower AITs and less detonation
Within reason, yes.

As an example, a few weeks ago I was dooling around with turbo selections for the '99 engine which I still haven't gotten around to building. Did a bunch of compressor plots, based on the assumption of wanting to hit about 250-275 HP. Obviously there's a lot of uncertainty in the initial computations, you have to make some educated guesses about your engine's VE at various conditions, what sort of pressure ratio you're going to need to hit it, etc. And of course, I haven't even gotten to turbine maps (which I'm still trying to educate myself on.)

Here are a couple of example plots which I came up with. I plotted five points for each one, I think they were every thousand RPM from 3,000 to 7,000.


Surprisingly, the 2560 came out relatively well. We're definitely not in the sweet spot, and getting a little close to the choke line, though.





Now that's ok. More headroom, and spending a fair bit of time in the center of the island.





Again, looks pretty good.





Now we're getting a little too large. Moving dangerously close to the surge line, and never really dwelling in the sweet spot.




As above, this is just way too much compressor. At the lower RPMs, we're well under 70%. IOW, this big turbo is going to be making more heat than the smaller ones at the lower flow rates.


Of course, it's hard to be precise here. There's no substitute for experience, which by necessity is usually the experience of others.
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Old 09-06-2010, 07:57 PM   #11
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I think 280whp on a Mustang or DD is the max for a GT2860rs.
+1. Under 280, do the 2860. over 280, do the 2871.

If you even THINK you'll want 280+ someday, do the 2871, though. The spool hit is miniscule, but it will flow a solid 10lb/hr more than the 2860RS up top.
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Old 09-06-2010, 08:32 PM   #12
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Hustler, for your compressor map posting, check the different trim 2871's as well...

You posted a 48 trim but there is also the 56 trim; which in my research has had the same surge line, but much more headroom in the center island and a higher choke line.
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Old 09-07-2010, 11:08 AM   #13
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+1. Under 280, do the 2860. over 280, do the 2871.

If you even THINK you'll want 280+ someday, do the 2871, though. The spool hit is miniscule, but it will flow a solid 10lb/hr more than the 2860RS up top.
what 2871 spec do you recommend over 280?
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Old 09-07-2010, 12:34 PM   #14
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If you're going with the 2871 it seems like a no brainer to get the 56 trim compressor. all 3 maps attached
Attached Thumbnails
any point in going for a 2871 if a 2860rs is ok?-2860rs.jpg   any point in going for a 2871 if a 2860rs is ok?-48-trim.jpg   any point in going for a 2871 if a 2860rs is ok?-56-trim.jpg  
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Old 09-07-2010, 12:37 PM   #15
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correct, the 48 trim looks to be able to support lots of boost, but a smaller window of heat efficiency 37 lb/min vs 45 lb/min ~80BHP
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Old 09-07-2010, 04:48 PM   #16
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the gt2871r has a higher pressure ratio i learned whatever the pressure ratio is you subtract 1bar from it ie. 2 bar is actually 1 bar. so the 2871r runs at a higher pressure ratio than the 2860 but people on here say bigger turbos make more power at less boost iam confused.
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Old 09-07-2010, 04:59 PM   #17
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Quote:
Originally Posted by delcbr View Post
the gt2871r has a higher pressure ratio i learned whatever the pressure ratio is you subtract 1bar from it ie. 2 bar is actually 1 bar. so the 2871r runs at a higher pressure ratio than the 2860 but people on here say bigger turbos make more power at less boost iam confused.

keep reading... you're not even close to the point where you should start typing thing like this. Think about what you wrote..how can you apply what you wrote to say one is greater than the other?

if PR = Xbar - 1bar

then how can

2871 > 2860?

there is no logic behind your post.

Pressure ratio is the ratio of absolute boost pressure to atmospheric pressure.

12 psi (gauge) + 14.7 psi (atomospheric) = 26.7 psi absolute pressure in the manifold

26.7 psi(a) / 14.7 psi(a) = 1.82PR

If both turbos are pushing 15psi in the IM, then both have the same PR (all things being equal). a GT2554 and a GT3076 both running 15psi have the same PR of 2.0PR.

What they do not share is the same mass volume flow, the 3076 is able to compress and package much more air into the IM at 15psi vs. the little 2554.

Last edited by Braineack; 09-08-2010 at 09:39 AM.
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Old 09-07-2010, 07:35 PM   #18
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ok got you now thanks for explaining that.
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Old 09-08-2010, 09:38 AM   #19
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