Hello everyone. Lets discuss the following compressor/turbine flow maps.
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Its from a GT2871 if that holds any weigth.
What I want to understand is why the compressor seems to flow 2-2.5 times as much as the turbine can flow.
Are you supposed to flow half your exhaust out the wastegate ?

Loosely speaking, I seem to be hitting a wall in airflow through my turbo/engine. Once this wall is hit, the intake pressure remains constant but the turbine inlet pressure(backpressure) rises 2-3 times past intake manifold pressure. This means the turbine can't flow enough; or so I understand. If hard numbers are needed, let me know. I am trying to understand how to match a turbos' compressor to its turbine and to the engine/airflow target.

 

I'm not smart enough to give you the answer, but if you haven't played with Borg Warner's matchbot go check that out. Itll calculate waste gate flow, and back or drive pressure for a given turbine.

 

Dumb luck, shade tree experience, and experimentation!

Matich it for what is the question? Peak efficiency? Because that makes for a suuuper lazy response time and massive turbo.

What size engine? RPM range? Intended use? What are the major an minor diameters of each wheel?

 

Engine size = 2.6L, 7500 rpm revlimit, street use, turbine major diameter 54mm, minor 47mm, turbine housing A/R 0.64.
Hits a brick at 30lb/min intake air, pretty much from 5000 rpm to redline there is no increase in airflow despite boost pressure keeping still and the exhaust backpressure increasing to 20-30psi.

 

What kind of boost? If you try to run more, will it? how are you measuring 30lb a min? Sure MAF isn't maxed? What are the compressor specs?

If you cant get any more boost and BP climbs with no power, its tapped. Sounds like a relatively small unit for your cubic inches/RPM. Might look into something along the gt3582 range. The china units have been pretty stout in my experience. Hard to beat for the cost. My billet 64mm gt3582s were $250 shipped. The cast 61mm versions are 140-160 shipped.

 

No, as soon as target boost is met, the boost doesn't seem to spike then drop. When i say boost i mean above ambient pressure.
I am measuring the mass airflow via a MAF sensor, and not its not maxed. Its defined up to 46lb/min.
The compressor is 51mm/71mm.
Here is a log of mine to understand my concern about the turbine not flowing enough and bottlenecking the system.
map_press = boost above ambient, intake side just before the engine block;
emap_press = pressure above ambient between the engine and the turbine;
abs_eng_load = <100 means NA operation, under vacuum, >100 means boost area where 150% is about 7-8 psi boost. #ad

 

Some fluctuation in boost is due to my electronic boost controlers' PID parameters not being dialed in.

 

From general experience I can tell you a 51/47 turbo is too small for your cubic inches/RPM. Some better designs will go farther than a similar sized wheel, but there's only so much power they can handle. My 3.8 at less RPM was likely flowing similar. It really depends if want response or eyeball sucking HP. Pick a power goal, then pick your turbo. IMO that one is tapped out, and then some. I prefer big power and sacrifice response as I race autos in drag race situations. I realize that's not for everyone. I'd want a BORG S366 or similar if I was going to run a brand name turbo on something around your cubes/RPM. But if you just want a decent jump in power without sacrificing much response, a $160 shipped GT3582 clone would do it IMO.

 

Always start with the compressor.
You have 7 or 8 psi boost, that's a pressure ratio of 1.5 and airflow is 30 lb/min. Plot that on the compressor map and you see you are right at the edge of the efficiency map. You need a bigger turbo.

 

Boost doesnt tapper off, its the airflow.
Although not ideal, at 30lb/min at 1.5 pressure ratio I am still on the map, and I could be on 41-42lb/min at 1.8 pressure ratio. The compressor is not the issue here.

 

Because the compressor is as the name implies, compressing the gas. The turbine is an expander, it expands the gas as it passes over the turbine.

The wall you're hitting is turbine flow. You can't move more air through it than you are right now. You simply can't get more RPM out of it. To get more RPM out of the turbine you have to be able to move gas through it. You can't move more than you are right now so it will only spin so fast. Since it's tied to the compressor you're limited on how much air it can flow as a result. You can change to a larger A/R housing to get some more head room, but really with those cubes and RPM you're out of turbo.

 

But questions still remain; why does the manufacturer claim a HP number that is unobtainable ? And why do others seem to be able to get a lot more HP from their engines because I assume at 30lb/min you are not making more than 300 hp...? Or are you?
Then again, my original question remains unexplained. Why are small turbines matched to big compressors ? What comes in must come out, right ?

 

The manufacturers HP claim is likely attainable on a much smaller motor. Put that turbo on a 1.6 and It will likely hit its target. This is VERY common. You can't rate Turbos HP on every engine. Its why they have compressor maps. That turbo is too small for your cubes/RPM/flow. Drop your RPm down to 5k and you can likely squeak more flow out of it. Put a smaller cam or a top end that flows less and it will make more boost. MAF sensors are also not always spot on accuracy wise. you could easily be flowing more/less that its reporting. You'd need to know the turbo shaft speed to get the whole story. But again... that's a tiny turbo. Why not just replace with something more suited to your motor and power goal?

 

Why is it that a 76 GTS can make 1200whp on a 3.0L Supra, but tops out at 850 on a 6.0L LS engine? Same reason your engine is maxed out right now. You've got a turbo sized for your engine to produce gobs of torque, but not gobs of HP. It's just mismatched. I don't need to look at a compressor map to know that your 2.6L turning 7500 needs more turbo than a 2871.

 

At 3900 rpm and 22.5 lb/min, your wastegate is dumping 5.5 lb/min and the compressor is already well to the right of the peak efficiency island. (compressor too small).
To make power at 6,500 rpm you need to flow 22.5 x 6,500/3,900 = 37.5 lb/min - way off the map - you need a bigger turbo.
Your compressor hits the edge of the map at 30 lb/min. As you try to increase flow (move to the right on the map) the compressor efficiency drops rapidly and the shaft speed increases rapidly. The increased shaft speed increases turbine backpressure. You need a bigger turbo.

 

Yes. The compressor map slopes up and to the right. At high boost it flows a lot more and flow = power.

 

I WOULD DISAGREE I did the job for Ford Engineering for 39 years (Turbo Senior Engineer/Ford Research) and that turbo is a terrible match for what you are trying to do at 1.5 Pressure Ratio. As was suggested go the the Borg Warner site and learn what a proper selection would look like. You have a terrible compressor selection for that 1.5 pressure ratio.

Tom V.

 

Allright then. Lets see this new combo. Its the best I could fit given the exhaust manifold and downpipe.
The operating points have been plotted already. The issue still remains, as in the turbine flowing half the compressor. Is this any better than what I have now ?
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We don't know your power goals.

 

350 crank hp, or flow 40-42lb/min and what comes of it is enough 4 me.