att:DEF and others re smaller wheel for 2871

[nisspug]

Hi. I'm new here and found the site via DEF and reading his excellent article over on bimmerforums on reading compressor maps.

I have a redtop SR20DET in a Peugeot 505 and recently bought a rebuilt GT2871 52 trim/.64. Then I read an article over on boostcruising where the author stated that the 2871 is basically a mismatched turbo and that the 71mm comp wheel is too big for the 53.8mm exhaust wheel and slows the spool of the exh wheel too much.

He mentions Garrett's recommended comp/exh wheel ratio of between 1.1:1 and 1.25:1 and points out that the 2871 has a wheel ratio of 1.32:1. Going by this formula the max comp wheel size that would suit a 53.8 exh wheel would be a 67.25mm wheel.

Sorry for being long-winded but I'd like DEF's and other's opinions of the idea of changing the 71mm comp wheel on the 2871 for a 67mm Garrett GT wheel. I tried to source a 69mm wheel as on the HKS2535 but have been told they are not available. I've posted my calculations for the SR20 below using DEF's calculator and drawn them onto the flow charts of the above wheels as well as the GT28RS disco.

I should say that I don't want to go to cams at the moment and only use the car on the street and am more interested in response than max hp but as the 67mm wheel flows 40lb it should be able to flow 400hp or so. From the maps would the 67mm wheel be a good midway point between the disco's response and the 2871's topend?

[Your Mom]

from what i'm told you would need to balance the whole assembly if you did change a wheel or else you experiment you fail pretty quickly. i'm guessing the only way to do that is send it to garrett.

[nisspug]

Yes I would def get it fitted and balanced by a turbo shop.

[Def]

IMO, you're splitting hairs when you start talking about shaving a few mm from a compressor wheel. Does it make a difference? Yes. Is it really noticeable when you're driving the car? Probably not so much.

It doesn't sound like this is an "all out build", and what you're talking about doing isn't cheap on top if a $$$$ turbo.

I run a GT2876R .64 because I got it dirt cheap used. Even bigger wheel mismatch than the 2871. You know what? They really don't feel all that different from the driver's seat. It rocks out just fine on track or the street.

When you get to where your build is "maxxed out," then you can worry about the small details.

What are you doing for tuning, as that will make a way bigger difference than a few mm difference on the compressor wheel dia.

[nisspug]

For tuning I have a Nistune USB chip ready to go in and I'll get an experienced local tuner to dyno tune it.

For supporting mods I have the usual 3inch exhaust, Z32 AFM, Porsche fuel pump, PWR 500x300x68 front mount etc. Still looking around for injectors and I'll probably go for 550s as this should be enough for the 2871 or '2867'. (I got scammed buying some 740cc HKS injectors from a seller in Europe which never turned up but luckily paypal reimbursed me.)

My thinking was why not get it right from the start? The HKS2535 is reputed to be an excellent turbo. The 67mm wheel will cost me $140 to buy and another $100 to install and then I can sell the 71mm wheel for around $100 so it wont end up costing me much.

How does the 67mm's compressor map look to you for the SR? Is it an improvement on the 71mm map as far a midrange torque goes?

[Def]

I would stay with the 71 mm wheel - end of story. You will want more power in the future, give yourself room to grow and don't worry about the gnat's ass sized details.

There is nothing "right" about the 67 mm wheel other than it supposedly follows some rule of thumb better than the 71 mm wheel by a small amount. Like I said, there are way more important things to concentrate on with the car(suspension, chassis, brakes etc).

You seem pretty much dead set on spending more money for the 67 mm wheel, so I hope it gives you what you're expecting.

[turtl631]

Check out codyace's dynos with .64 2871r...what could you really improve? Spool and max power won't get much better on a pump gas SR.

[nisspug]

Fair enough-thanks for your opinion. Will have to give it some more thought. Yes I've seen codyace's dyno and it looks great but there's nothing to say it coudn't be improved on. From memory he's also had the exh mani flowed. But I asked for opinons and got them-thanks again.

[Z28ricer]

You couldnt just slap the 67mm wheel in anyway, the cutout in the bearing housing has a machined recess that the wheel sits in, the housing has to match the major diameter of the compressor wheel.

That being said, I think you're splitting hairs and probably worrying about nothing anyway, plus the only time I can remember seeing that same statement was on a guys page who didnt seem to know WTF he's talking about, other statements like the S14 turbo only being good for something like 270whp, much larger turbos only being capable of marginally more, etc...

[Def]

You couldnt just slap the 67mm wheel in anyway, the cutout in the bearing housing has a machined recess that the wheel sits in, the housing has to match the major diameter of the compressor wheel.

That being said, I think you're splitting hairs and probably worrying about nothing anyway, plus the only time I can remember seeing that same statement was on a guys page who didnt seem to know WTF he's talking about, other statements like the S14 turbo only being good for something like 270whp, much larger turbos only being capable of marginally more, etc...

This is also a factor - but I didn't want to get into specifics since there is more to it than just "bolting on a new wheel and balancing it." It'd only work if the housing has the exact right inducer and the same profile as the other wheel(very very doubtful). Any gap created here will absolutely kill compressor efficiency. See cheap eBay turbos for a good example of this, and motorsports stuff like the TR30 for how you can gain efficiency closing this down(at the expense of more life as the CHRA bearings wear).

[nisspug]

This is the article and I dont think he mentions the T28 at all so you're thinking about someone else:

http://www.boostcruising.com/forums/...07267&hl=turbo

His ideas about the 3071 being a better matched turbo than the 2871 seem to be born out by the current popularity of the T3 3071 over the 2871 (at least here in Oz) so I don't think he's just some crackpot.

I never planned to just 'slap on' a comp wheel of any description. Please don't assume I'm a total noob to turbos, cars or the SR-just new here. I bought the 2871 without a comp cover and so I bought a std S15 T28 comp cover cheap which has to be machined for the 71mm wheel anyway (got quoted $250 for a new 2871 comp cover over here in Australia) so it won't be an added expense to machine it for the 67.

I guess I hoped to have a discussion based on the comp maps and maybe learn a bit more about them.

[Def]

Garrett doesn't release real turbine maps, do the compressor maps are what they are.

Not sure what you really wanted to talk about. Yes, the smaller comp wheel will not make as much power as the 71 mm wheel. Will it reach lower massflows at a given PR with the same RPM? Probably. It will also require more RPMs for the same power potential. Does it match up better to the 53.8 mm NS111 turbine wheel? Maybe a little bit, but no one knows for sure.

The important question is will that few percentage points of extra turbine efficiency and little more compressor efficiency really make a tangible difference in how the car feels? No, it really won't.

People get "target fixation" om certain numbers or efficiency percentages without realizing there is a "big picture" that it all had to fit into. It's just not likely to do much more than limit total power capacity on the engine and give you an infinitesimal spool gain and weaker top end.

See the GT2860R for a more extreme example of this.

[nisspug]

I thought compressor maps were a more useful and exact tool than you seem to be saying they are. Sounds like its alot more of a suck-it-and-see situation. The 2871 map shows a peak efficiency at around 6000rpm at 22psi whereas the 3267's peak is around 5000rpm. I would have thought this would be quite a big effect on how the car makes power and drives but maybe I'm wrong.

Anyway not sure which way I'll end up going but if I decide to use the 67 I'll post up a dyno. Thanks for your input.

[Def]

A few percentage points don't make much difference - so the math. Yes, 76% sounds way better than 72%, but in the end you only increased efficiency 5%. You also have a turbine that affects things MORE than the compressor, and Garrett doesn't give out detailed turbine maps. Which is why I say there's no way to do all the math... because well, there's no way to do all the math when not everything is given.

Wheel inertia plays a larger role than a few points of compressor efficiency, but it's still not that huge. You're paying attention to compressor maps too much.

[VIDAL BABBOON]

Is the car on the road and running ?

Im reading this and thinking that you are doing all the math before fitting etc ?

I have the larger of the 2871's installed and i have a fully built and flowed engine so my opinion of the turbo is going to be no use to you at all unfortunately!

It sounds like you are worrying about nothing to be honest, if you are looking to get the most out of it then you need to be looking more towards the engine and increasing gas volume and flow.