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Turbo size, A/R and Trim explained
taken from another forum

A/R Explanation
A/R is the rated volumetric efficiency of a turbos 2 sections, so to speak. Imagine if you have a garden hose spraying water out at a pinwheel
with the hose open ended, the pinwheel spins okay. Put a nozzle on it an the pinwheel will spin like mad.

There are issues with the nozzle on the end, you lose volume but gain pressure. With the nozzle off you gain volume, but lose pressure and you can't turn the pinwheel as much.

Simply put, on small displacement engines a smaller A/R is better, on large displacement engine a larger A/R is better due to exhaust volume.

A larger A/R will spool later and provide a higher power band, if your engine is capable of reaching the RPMS it should be used in.

You can't cross compare different types of housings and wheels, but if you have a typical T3/TO4E 57 trim with a stage 3 exhaust wheel and a .48 A/R housing, it might have a powerband of 3000-7000, with the .63 it might be 4000-8000, and with a .82 A/R housing it might be 5000-9000. If you have headwork and cams that stop pulling at 8000 RPM's, it's smart to run the .63 A/R housing. If you have a fully ported head and huge cams that will make power till 9000, the .82 A/R housing would be a better choice.

[Image: arratio.jpg]

[Image: arhousing.jpg]

[Image: basics_trim_turbine_formula2.gif]
[Image: inducerexducer.jpg]

This rx7 link ( I provided tells you how to calculate the trim of a compressor wheel. Just a little fyi when calculating trims, you can calculate it in inches or mm it doesn't matter. Trim is just the ratio of inducer/exducer. The trim will come out the same whether you calculate inches or mm. The link also tells you how a higher or lower trim number determines where the compressor is the most efficient. Also check out the previously provided Garrett link to look at the products menu and the tech menu. You can use the products->turbochargers menu to calculate the trim of the compressor wheels. Use the formula in the rx7 club site or Garrett site to calculate the trim.

Found this whilst trawling the internet to understand what how to obtain low end boost from a turbo.
Also useful in working out whether a turbo is decent for what you want, afterall, most people say the T2 is the best!
Diablo Hdi Dturbo and 205 1.9 project - it lives!
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