livemind

im in the process of building my motor slowly (ie gathering parts slowly) and am about to order a turbo mani from turbotim. i was originally thinking bottommount but im now settled on topmount. i have 9.0.1 pistons and will be ordering a cometic to bump it a tad more. i did utilize the search and looked for my answer all i could really find info on was the gt71, i was hoping without getting flamed too much that someone could take the time to help me figure the efficiency of the gt2876r with a .63 ar. i appreciate the help in advance. final hp goal is the 300 mark.

18psi

iTrader: (76)

You should easily be able to hit 300 with that setup, even 350 without much drama. Many here would advise against a thicker gasket, it doesn't work as well as it does on some other cars (I distinctly remember either Corky Bell or some other knowledgeable person saying something along those lines anyway).

y8s has a 2876, and many others (like savington) have 71's, and have nothing but great things to say about them. y8s made 250 on something ridiculously low like 10psi or something.

livemind

cool thanks il look into savington's and y8s's builds, im hoping the smaller ar will help somewhat significantly with building boost faster

18psi

iTrader: (76)

The smaller AR will help, the well flowing AF manifold/exhaust will help, and a good tune will help.

You need to post specifics for your build/setup in a build thread. That way people could weigh in with opinions and help you further.

Braineack

iTrader: (62)

(zack morris's phone)

Get the 71 not the 76

livemind

im sorry to sound really redundant, but i dont know how to read turbo maps, can you explain why getting the 71 with a bigger AR would be better than the 76 with a smaller AR? i am not totally knowledgeable on this by any means, but from what i gathered from reading getting the larger compressor with the smaller turbine would allow for decent spool time while leaving a little more head room for higher boost. maybe im backwards, please correct me if im wrong

Nagase

iTrader: (6)

You /really/ really need to do more research.

livemind

so after doing some more reading i found that indeed i was wrong to an extent. here is what i found:

A quick and dirty review of how a turbo works is essential as it is fundamental to understanding the tools we have to help us choose. A turbo is an air pump that’s powered by the energy contained in the engine's exhaust gas flow by spinning a turbine impeller wheel. That wheel rotates on a shaft that has a compressor wheel mounted to the other end that then also spins and forces more air into the engine's intake. It's the exhaust energy and turbine wheel that powers the compressor wheel to increase intake air pressure, and your boost controller that determines the amount of pressure (with the wastegate redirecting exhaust flow as required to prevent over-boosting). It's important to recognize that it's the compressor wheel that's in charge of reaching the desired boost pressure, and the turbine wheel’s job to spin it accordingly. When the turbine is struggling to do its job effectively the compressors ability to provide boost in a timely manner is compromised and we recognize this effect as turbo lag. When it's completely up to the challenge to power the compressor we recognize it as providing excellent throttle response.

In fact, our success in choosing the best turbo for our use rests solely on our ability to understand this relationship between turbine and compressor. And for our purposes of choosing among the GT line that relationship is primarily determined by (a) the relative diameters of those two wheels and (b) the aerodynamics of the turbine housing. The resulting performance is called Turbine Efficiency, and its measure is expressed as a percentage. A turbo whose turbine can efficiently power the compressor to produce quick spool and less restricted top end flow has a higher %, often close to or slightly exceeding 70%, while others are as low as 60%.

Here's what we're looking for in the Garrett specs:

(a) Garrett recommends a wheel diameter ratio range between 1.1:1 and 1.25:1 (compressor:turbine) to provide the best overall performance. As an example the GT28RS has a ratio of 1.1:1 (60mm/53.8mm) at the quickest spooling end of the range, and the GT3076R has 1.27:1 (76.2mm/60mm)…barely outside the other end of the range. The reason a large compressor wheel mated to a smallish wheel would not be able to spool as quickly is because a largish compressor wheel will need to turn slower to provide any given intake airflow than a smaller wheel would, and this in-turn forces the turbine wheel on the other end of the shaft to turn slower, and at speeds that it can’t operate as efficiently at. This is contrary to those that believe a comparatively small turbine wheel and housing will cause the largish compressor to spool more quickly. Dyno results confirm Garrett’s recommendations every time, while I have never seen evidence of a small turbine/large compressor spooling nearly as quickly.

Good examples to see this effect would be the GT28RS, GT2871R (or HKS GTRS), and GT2876R (or HKS GT2540R). All three share the identical turbine housing and wheel, but are mated with 60mm, 71mm, and 76 mm compressor wheels. The latter two compressor wheel diameters push the wheel ratio well outside of the recommended range to 1.32:1 and 1.45:1. Each larger compressor wheel causes a delay in spool of perhaps 750 rpm to ~17 psi and makes less top end power as well. The only way to make these wider spaced wheel combinations make more power is to significantly raise boost pressure. This however will not reduce lag, the restrictively small turbine wheel and housing will limit high rpm power as it reduces the entire engine’s VE, less ignition timing can be run at high rpm causing reduced power from the airflow, exhaust temps will be higher, and you’ll have to deal with all of the risks associated with higher boost levels. The solution is to follow Garrett’s recommendations whenever possible.

(b) The turbine housings are designed to maximize turbine efficiency. In some cases though a turbine housing will be made or modified to fit specific user applications like space constraints or the lack of suitable sized exhaust manifold turbine mounting flanges for some popular applications. This has led to small turbines modified to stuff in large wheels, large turbos with small turbines made to fit onto small exhaust manifold flanges, smallish turbos modified to fit onto large flanged manifolds, etc…and all of them have reduced the turbine’s efficiency to spool quickly and produce the strongest powerband. The impact of some twin scroll housings can’t be predicted because of their lack of turbine efficiency ratings by Garrett, but their impact will be seen in dyno results. In some of these cases the wheel ratio will appear to be ideal, but the modification to the turbine housing itself can negatively affect turbine efficiency. This is why it’s important to know the Garrett tested Turbine Efficiency % rating.

The various iterations of GT3071R is a good example of these variables. All models use the 71 mm compressor wheel, but some have a 56.5mm turbine wheel stuffed into a machined T28 turbine housing, some have the better matched 60mm turbine wheel fitted to a twin scroll housing of unknown efficiency, and the one that mates it with the 60 mm turbine wheel and T3 single scroll housing. The latter is surely the best of the bunch using Garrett’s specs and recommendations, and it’s very high efficiency rating of 72% and ideal wheel ratio of 1.18:1means that for this size of turbo you are unlikely to find anything that will out-spool or out-flow it. It also means that the similarly flow rated GT2871R models with less than ideal wheel ratio and as little as 60% turbine efficiency will not perform as well as the GT3071R at 72%. Some feel that the GT3071R versions that have been used on the 3S-GTE have been less than stellar performers, and this is consistent with Garrett’s specs, ratings, and the recommendations presented here. Let’s choose the best model and set some powerband records!

I’d recommend that you first use the Garrett compressor maps to identify the compressor that can flow your requirements (see Garrett’s Turbo Tech section for these calculations), and then to consider the wheel ratios and the turbine efficiency ratings found on their turbine maps as a guide to matching that compressor with a suitable turbine wheel and housing. While efficiency actually varies with flow rate, pressure ratio (think boost level), and turbine wheel rotation speed, the stated maximum efficiency rating is going to be quite comparable among all models within the GT line.

Now you need to choose the turbine housing AR. You’ll notice on the turbine maps that the efficiency curves are different for the various available turbine housing AR. These shows that the lower AR housings are more efficient at lower flow rates generated at lower engine rpm, while higher AR housings are more efficient at higher rpm flow rates. These housing options will allow you to choose between maximum low rpm spool and power at the cost of a little high rpm peak power, or maximum peak power at the cost of a little lower rpm performance, or something in-between if there’s a 3rd option. The lower AR made for a significantly stronger powerband overall on this setup, and I believe we will see this trend with each turbo model and engine setup…if peak power is your goal the higher AR will likely provide that every time.

Nagase

iTrader: (6)

Wrong to a rather large effect.

Sav even refers to higher AR in the EFR thread, as well as calls it 'the hustler effect' of running a smaller turbo with a bigger A/R to gain torque and HP.

livemind

so my question is which gt2871r is my best option for fastest spool and still hit my 300hp goal with efficiency

Nagase

iTrader: (6)

What do you mean 'with efficiency?'

You can hit 300hp on a 2560, or a 2860, or a 2871 or 45r.

livemind

off topic but why would garrett design a turbo that sits so far out of their recommended efficiency range? twin application maybe?

Nagase

iTrader: (6)

Recommended ranges are just general rules.

Different engines combine differently with different turbos.

Around here, there being no appreciable difference in spool between the 340hp 2860 and the 420hp 2871 is accepted... but talking to full race, that just blew their mind. It's a Miata thing, where on the map it makes airflow and at what velocities.

livemind

that makes complete sense, i am new to the miata ring and am trying not to bring my experience with past turbo cars with me, which .ar on the gt2871r would be better the .64 or the .86? i read up on sav's build and with the .86 he makes great results, how would the .64 compare?

Nagase

iTrader: (6)

The .64 would make slightly better spool, but with less torque overall

If you only want 300chp, you have no reason to go bigger than 2560.

livemind

sorry i didnt specify, its a 1.8 94 my plan is to overbuild my motor and shoot for my starting point to be 300whp but buy a turbo that would give me a little head room

Nagase

iTrader: (6)

A little headroom?

Be specific and honest with yourself.

Try to use actual numbers, it's very helpful.

livemind

specific and honest headroom would be after final tuning just sub350whp. my last car i had set up to run 345whp on the street and 370whp at the track, my goal is to keep the same type of set up in a car that is smaller lighter and handles better, and also needs less power. my whp # goal is Street:290whp Track:sub350whp

Nagase

iTrader: (6)

The 2560 is too small for that.

The next size up is the 2860... which isn't much different than the 2871.

The .86 A/R will give you better torque.

Just get the .86 2871, BEGI/FM/AF/ARTech manifold/DP/Exhaust, inconel studs.

With a 2871, .86 A/R, you could push just above 400whp, down the road.

Nagase

iTrader: (6)

Oh, also? Stick with an OEM head gasket. If you're on pump, stick with 8.6:1 or 9:1 pistons. If you're on E85... go up to 10-11.5:1. There's little point of doing anything in the middle.