Turbo Hp Ratings

[Primordial]

I was wondering if someone could explain the HP rating on some turbos? I dont really understand how this number could be derived?

Eg. I have an Apexi AX53B70-P25 which is rated at 380HP.

[TTT]

they work it off LBS of air it can flow at given psi of boost.

that one of yours flows 38LBS/min of air at what ever boost they rate it at.

cool?

[Primordial]

Ah ok, yeah I see exactly what you mean. So this means that an engine has to flow exactly X amount of air to make X amount of horsepower, so therefore a turbo that flows 38LBS/Min = approximately the amount of air required to create 380HP of power?

[discopotato03]

That theory makes the broad assumption that the manifold/turbo can pass enough exhaust gas while running on boost . If it cant the exhaust manifold or turbine inlet pressure can go ballistic and eat your pistons/valves/head gasket .

Cheers A .

[Warpspeed]

The way Garrett and most other turbo manufacturers rate there turbos is as follows.

They run up the compressor until the pressure ratio is exactly 2.0, that is exactly 14.7 psi boost. They then steadily increase the airflow up past the efficiency peak until the adiabatic efficiency drops back to 60%.

You can do it yourself by eyeballing a flow map. Draw a horizontal line at 2.0 pressure ratio, and note where it crosses the 60% island on the map in the choke region (to the right). Then drop a vertical line down to indicate flow.

Engine power is assumed to be one Hp for each 1.5 CFM at that operating condition.

It is just one standardised operating point on the whole flow map to very roughly compare one compressors flow capacity to another compressors flow capacity. An actual real engine may operate a long way from both 14.7 psi boost, and the 60% efficiency point on the map. But it is still a pretty good average operating point to choose.

The exhaust turbine flow, or back pressure, does not come into this at all, but is also extremely important for the engine.

[Primordial]

Ohhh of course.. Thats what I was going to say next!

Awesome description. I see exactly what you mean. Thanks for the answer mate!

[Busky2k]

I dont think turbos are rated at a PR of 2.0. I've seen most of them just rate their turbo's hp at the maxium flow, regardless of the PR. A good example is the GT2871R, who's hp ratings are judged at a higher PR than 2.0

[discopotato03]

Actually you will find that most compressor maps are graduated in pressure ratio vs the mass of the air pumped in lbs/min . The problem with measuring airflow in cfm is that it gives no clue as to the air temperature and therefore density .

Because of the mores law factor the maximum choke flow is usually considered to be what the compressor can dish out and horsepower worked from ~ 10-11 Hp per pound of air .

This really is a nonsensical way to rate a turbochargers power or I should say mass airflow potential . It makes the huge assumption that the turbine can get the compressor/housing combination to perform as it did on the test flow rig . All too often it can't so the result is not an accurate real world achievable one .

Cheers A .

[Warpspeed]

Both you guys are both perfectly right of course, but the method I described is the official one used by Garrett.

The only way to make any sense out of any of this, is to get a compressor map and plot where you are.