Turbos: A/R Ratios

[Dieselboy]

I ran across a brnad new International 4700 series truck the other day which had a DT466, and cast into the impeller housing of the turbo was the A/R ratio; .58! There was no wastegate either.

So .81 and .84 a/r ratios will spool up very quickly on our engines that range from 379 to 395 cubes. And this engine had a .58 with 466 cubes. One would think that a larger engine would have a higher ratio since there is more exhaust to move the impeller, yet this particular example is just the opposite.

I know there's more to it, so what am I missing.

Grape, Doc, GMCTD?

[gmctd]

Those numbers are ratio of nozzle area to dimension from center of nozzle area to center of turine shaft - Area/Radius

The diameter of the scroll housings are classed at industry standards of T-1, T-2, T-3, T-4, etc to T-8 for automotive use.
The classifications are for engine displacement and flow rate.

A 3.0 liter V-6 running max 6000rpm will need a considerably smaller turhbo than a 3.0 liter V6 turning 18000rpm.

Also applies to 400cuin Diesel at 3500rpm or a small-block Chev turning 7500rpm.

The smallest T-1 can have the same 1.0 A/R as the largest T-8, tho dimensions are drastically different.
Within each class, wheel diameters and scrolls can be mixed and matched for optimum tuning to engine displacement, torque, horsepower, rpm power bands, etc.

The GM-8 is modified T-3 equiv - the DT-466 turbo is probably a T-5 or T-6, could even be a T-8 with those ratios.

A\R of 1.0 the nozzle directs flow to the outside of the turbine wheel - more power but less rpm - can drive larger compressor wheel, where exhaust energy, volume and heat, is high.

A/R of 0.5 the nozzle directs flow toward center of wheel, where less available exhaust energy creates high rpm for required Boost.

Increase nozzle area to reduce velocity and turbine rpm
Increase radius to reduce turbine rpm

Increase A/R to compensate for overboost, decrease A/R to increase Boost.

Within each classification.

Increase\decrease classification does the same.

Size does matter! :cool:

[ 06-04-2004, 09:23 AM: Message edited by: gmctd ]

[grape]

Was the exhaust side a .58 or the intake? That sounds like a common T04E inlet housing A/R. They have a 2.5" air intake, they are a touch smaller physically than the T04b stuff.

[Dieselboy]

The exhaust side was .58. This does help explain why that turbo sings just off idle.