964-C2

How do you know when you have reached the "efficiency limit" of the turbo (should not raise the boost pressure more)?
Is checking backpressure a good way? Shouldn't be more than 2:1???
Thanks for any input..

Xaerran

The easiest way might be to look at the map, if the turbo has one... what type of turbo are you wondering about? some folks on here may just "know" based on the turbo.

964-C2

I do not know the spec of the turbo, so I was wondering if there is a way to find a turbos "limit" in general.

Xaerran

I am not trying to talk down, but without some basic info on the turbo, it may be hard for folks to help... you need to look at exhaust and compressor housing size, the A/R and the compressor wheels. The type of bearings and cooling method can also play a small factor.

is this what you are looking for?

Paulyy

pretty simply when the turbo will not hold any more boost and not make any more power.

if you have a compressor map. (most turbos should have one)
get one of these.
https://www.turbobygarrett.com/turbo...structions.pdf

eman930

Whatch your intake air temps, if they start getting really hot and your not making anymore power even when you turn the boost up, then your out of your effience on that turbo

JacRyann

Exactly! The efficiency lines determine how much the air will be heated up. I'd say anything lower than 60% is off the island.

Calculate what the outlet temp would be for that efficiency and datalog outlet temps. You'll actually notice that the turbo doesn't actually "max out" at max-boost in the mid-range. It will actually be off the map in the upper-RPMs when boost is actually lower and dropping.

For example, K26/8 @ 20psi in mid-range 3500rpm is fine, but @ 6000rpm and 18psi, it's off the island and heating up air more than before. That's partly why the torque curve takes a nosedive on these cars. Replacing with larger turbo results in lower temps and more power at the same boost-levels as before.

Voith

This turbo calculator is nice:

http://www.turbos.bwauto.com/aftermarket/matchbot.aspx

Kris H

I started to compile some maps i came across here:

https://rennlist.com/forums/944-turb...ga-thread.html

You can see for yourself the exact parameters of quite a few chargers..

hp18racer

Anybody who's data logging, care to post turbo out temperature logs vs boost along with details of your motor/exhaust/turbo?

964-C2

Thanks for all answers. What about backpressure, will that tell me antydning?

JacRyann

Backpressure is an indicator of turbine-effiicency vs. compressor-efficiency which you originally asked about. The turbine has to develop a certain amount of power to spin the compressor-wheel up to a certain PR. If a lot of power is required from a small turbine, it will generate greater pressure-difference between the turbine inlet vs. outlet in order to generate that power.

So a small turbine wheel & housing may require a pressure-difference of 25psi (27psi in -> 2si out) in order to spin an X-sized compressor-wheel to 15psi. Now a larger turbine wheel & housing may only require only 15psi difference to generate the same flow from the compressor. At a cost of more lag in the low-end. It comes down to matching the power-target with the turbine and compressor wheels & housing to arrive at the most efficient balance of those parameters.

TurboTommy

Yep; that's all good explanation
Having said that, therefore, answering the OP's question: an abrupt rise in back pressure in the higher RPMs COULD be an indication of of the compressor side reaching it's "efficiency limit".
Because if the compressor is too small (running out of efficiency), the rotating group has to dramatically increase its' speed in order to keep up with the engine flow demands at constant boost , higher RPMs (which would mean an ever higher power requirement from the hotside, like JacRyann said).
However, the back pressure test is not the ultimate indication of turbo efficiency, because high back pressure could just mean the hotside is smallish.

Humboldtgrin

If your running 16 psi your exhaust should see 16 psi back-pressure. If it read 17 on the intake and 20 on the exhaust then you have meat the limit of boost at 16 for that turbo If your exhaust has more pressure difference then the intake you run to "reversion". That's where the hot exhaust gasses are trying to go back into the cylinder because the intake pressure is not overcoming the exhaust. Then you get a "popped head gasket" or "rod out the side of your block". I talked to Mike Lindsey about this for some time. So I bought a back pressure kit, I know not to over boost my engine.

Paulyy

That really doesn't make sense. especially to compressor efficiency.