Off Topic - Question for any Turbo Experts...

[Danbob]

I know it's not EFI related, but this is the only forum I'm on that tends to give actual factual answers to technical questions,

So if you know your turbo theory, please answer:....


What is the effect of having a smaller compressor wheel, with a larger Trim?

For example I have a 2 compressor wheels, one is a T04E-46 trim wheel.

Inducer 50.7mm
Exducer 75mm
Height 25mm - From bottom of fin
Trim 46

And another compressor wheel
Inducer 48.2mm
Exducer 68.9mm
Height 24mm - From bottom of fin
Trim 49

As you can see the T04E-46 has a larger Inducer and Exducer diameter than the other wheel , however the Trim value is lower.

- What effect, in terms of performance and efficiency of the turbo, does this have?
What is the trade off between trim and diameter?

Thanks
Dan

[VLC-05L]

You want the wheel to match the compressor cover in any case. Putting a smaller wheel in a cover wont work properly as there will be a decent gap between the wheel and the wheel.

That aside, from my experience, the inducer size is what governs how much power the turbo can support, and the exducer size governs how much boost you will be able to efficiently run. That is, for fixed inducer size, a larger exducer will lead to better efficiency at higher boost levels.

The trim is just the ratio of the inducer size to the exducer size squared. The trim is actually the ratio of the area of the inducer and exducer circles, but mathematically simplifies to (in/ex)^2.

[Danbob]

Hi, Thanks for responding,

I have a compressor housing to match each wheel.

I understand your point regarding diameter, but surely as there always is, there must be a trade-off. Otherwise bigger inducer and exducer would always be better for every application.

I would expect the trade-off is either a higher surge limit, or more lag, but I’d like to understand which compressor attribute affects which characteristic - I know every compressor design will be subtly different, but there must be some basic guidelines?


i.e.
Larger inducer Diameter = Better ……………… but worse ……………….
Larger Exducer Diameter = Better ……………… but worse ……………….
Higher Trim = Better ……………… but worse ……………….
Effect of the number of blades?
Compressor fin height?

Since asking the question I’ve found a map for each comp wheel and found the T04E-46 is a better match for my engine. But for education I’d still like to fully understand.

[VLC-05L]

Well the only way to properly assess how well a turbo will be suited to your application is to look at the compressor map for it.

But going bigger will move the surge line to the right, but that might not be important at all if your engine is big enough, or rev hard enough, and therefore operate to the right of the surge line. Anti surge comp covers help as well, billet wheels help, but all this is reflected in the compressor map.