[2000Si]

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. <Source>


<Picture Source>





<Picture Source>


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.

[fallstar01]

great write up. Definitly helpful although keep in mind most mustangs redline somewhere between 5500 and 7000 rpms however it sounds like the smaller trim wont do as much. Is there a perfect turbo for a low revving motor with alot of displacement exhaust?

[2000Si]

That's for you to decide. The write up shows you how to figure out the proper A/R and trim necessary to get a good, fast spooling turbo to help the low end torque grunt.

The part about the high revving was because it was written about Honda's. You can disregard that, unless you wanna build a 4.6 with an 8k redline and turbo it. lol

[e3sean]

Originally Posted by 2000Si 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. <Source> <Picture Source> <Picture Source> 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.

bump i wanted to bring this back up it has been most helpful

[hannes_slanec]

@2000SI


Thanks Sir!


Very nice and interesting.

My knowlegde about turbos is limited and altough your explaination is very exact I still have problems to determine what size of turbo I should take.

I build a 68 Impala with 540cid BBC with Dart 320 alu-heads and want to go with a twin turbo setup.

I have enough torque through the huge displacement but I would like to have additional power in the mid to high rpm - range.

From your explaination the 48 A/R housing would be the right one since my engine got redline at 7000rpm but I will rev it only up to 6500-6800maximum rpm. I have around 9,6:1 compression and threrefor I will not go higher than 6 - maximum 9psi. I have bought 2-1/4" upsweep dragster headers and will convert them to nice turbo headers.

What is your recommedation for a turbo, is a pair of T4´s to small for 6-9psi?

Also I would like to keep down with the weight since the BBC got enough weight already and I have been told that a bigger turbo got around 50pounds, is that really true.

Thanks Hannes

.

[banned]

good info, but your a/r suggestions are well off... Im not even an expert, but I can tell you that a 76mm w/ .96 will make boost around 3200

[SpeedSicks]

Originally Posted by banned good info, but your a/r suggestions are well off... Im not even an expert, but I can tell you that a 76mm w/ .96 will make boost around 3200

that depends on the motor that the turbo is linked to, mind you..

[r.barn]

Picking A/R comes down to spool time vs. top end power band.
You need to decide where you want your power band and plan accordingly from
the start.

Power band should not only decide A/R, but cam selection, heads, intake
compression ratio, rear gears ..... there's a lot to this and no magical
method or perfect right or wrong answer.


I'll add my bit to the sticky.


Using and reading Compressor Maps

First we need engine air requirements.

( cid X .5 X Ve X rpm ) / 1728 = cfm
( 14.7 + psi ) / 14.7 = P.R. (press. ratio)

P.R x CFM = boosted CFM

boosted CFM / 13 = lbs/min

So we have a 302 with Ve of 80% and 6,000rpm power band
wanting to run 15psi.

( 302 X .5 X .80 X 6,000 ) / 1728 = 420cfm
( 14.7 + 15 ) / 14.7 = 2.02 PR.

420 X 2.02 = 848 cfm boosted
848 / 13 = 65.2 lbs/min

65 lbs/min and 2.02 PR are the numbers needed to plot
out on your compressor map.

You also need to do the calculation at the rpm that full
boost will first be reached and make sure it does not cross the
surge limit line to the left at the map.

You want your plotted points to remain inside the map and as
close to the center as possible, while using smallest turbo that will
meet your goals. For twins just divide the turbo load in half.

Good link for turbo and s/c compressor maps.
Same calculations can be used on an s/c compressor map too.
Turbo and Supercharger Maps