why is A/R so important?

[1.8]

sorry for the stupid question. can't search for A/R since it's too small of a word. can anyone tell me? i keep reading on here and everyone ask about a/r.

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[mrbsponge]

AR 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......

but put a nozzle on it an the pinwheel will spin like mad................

but there is issues, with the nozzle on the end, you lose volume but gain pressure. with the nozzle off you gain volume but lack pressure and cant turn the pinwheel as much..............

real simple, on small displacement motors, a smaller AR is nicer, on larger obviously larger due to exhaust volume.

a larger AR will spool later and provide a higher power band, if you motor is capable of reaching the RPMS it should be used in.

just my 2 centS

[Bailhatch]

that's about right^. I think one thing many people miss is that a small turbine like a T25 with a large A/R like .80 is nothing compared to a stage 5 t3 turbine in a .63 A/R. Same applies to compressors. The A/R tells you nothing about the wheel size, which is the primary factor in how much it flows, spool characteristics, and power level.

[Chip]

To add onto mrbsponges analogy, take a garden hose open ended and blow it through a pinwheel, turns slow. Put a small nozzle on it and flow it through the pinwheel and the pinwheel goes faster. Increase the size of the garden hose from 3/4 of an inch to 2 inches, and triple the pressure. The nozzle is going to be very restrictive and back up the garden hose. The open hose is going to flow well, and allow all the water through the pinwheel. The A/R ratio basically chooses your operating RPM range of a motor, the more flow(RPM's) you're trying to push, the larger the A/R ratio you need/want. 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, with 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.

[Bailhatch]

Originally Posted by HybridKOOP

The A/R is simply a ratio between the compressor or turbine wheel, and it's respective housing..

No, has nothing to do with the wheel. It is a ratio between the cross sectional area of the housing (where the gas flows) and how far it is from the center. it could be determined without any wheel at all.

[SiKid86]

Good stuff here. so t3/t4 hybrids have smaller exaust sides and larger intake sides so the exaust will spool faster thus making the intake side spool faster.. Right?

[1.8]

i love u guys. thanks for the info.

would i be safe with .63 on a fully built LS?

[1.8]

opps. double post.

[XDEep]

http://www.supracentral.com/content.php?article.57

[mrbsponge]

nice find!!!!!!!

[Bailhatch]

Originally Posted by 1.8

i love u guys. thanks for the info.

would i be safe with .63 on a fully built LS?

what size wheel? what turbo? ASSuming that A/R is so important is kind of a problem if you want to discuss turbo sizing.

[fidelity]

yea what mrbsponge said

[1.8]

SC61

[Bailhatch]

hp goals =?

[1.8]

i really want 300-350hp

right now, i have a built LS head by JG.

gonna be running stock block for now but will be getting 84mm sleeves with pistons, rods, etc.

[mrbsponge]

what is max rpms turned????

[1.8]

Originally Posted by mrbsponge

what is max rpms turned????

i havn't decided yet. what do u recommend for my setup?

[Ri5e]

Originally Posted by 1.8

i really want 300-350hp

right now, i have a built LS head by JG.

gonna be running stock block for now but will be getting 84mm sleeves with pistons, rods, etc.

Do you want to make 300-350HP with a smaller turbo+high boost or bigger turbo + low boost?

What do you want to use the car for, auto crossing or straight line drag?

[1.8]

Originally Posted by TurboSmart

Do you want to make 300-350HP with a smaller turbo+high boost or bigger turbo + low boost?

What do you want to use the car for, auto crossing or straight line drag?

i want the sc61 (would u consider that, big?) & use it for straight line drag.

[mrbsponge]

the main problem you will run into is running out of rpms, the sc61 is mainly an upper rpms turbo, where as the LS motor has more torque lower, and you would want to exploit that even more, i would choose cams tha would function till about 8400rpms, and with you having a 2.0 84 mm, it will be even better. see sometimes the problem is there isnt a spot between the .63 and .48 if they had like a .55 or so you would be golden. i suggest a t3/t4 60-1 in .63 ar on both sides

[1.8]

how's the GT35 compared to the SC61? would it be for higher rpms too?

[mrbsponge]

the gt35 is an awesome turbo but has to be operated at 13psi or higher to be efficient, and keep out of compressor sugre range, at 14psi on my friends b16 he made 385hp!!!!! i was impressed. i run a t3t4 57 trim .63 ar, i run 14.5 psi and made 326 plus i use a wet 20 shot nitrous kit, the NOS gave me 63 hp on forced induction, i love it. i am putting that motor in a hatch right now instead of a coupe and am going to be running 18-20psi and the same 20 shot, hoping for 10s

[Dee]

Here the exlanation I posted last year.

[1.8]

those are some nice numbers. since i got the A/R down, can u guys explain the "trim?"

i'm a noob when it comes to turbo.

[Dee]

I showed how to calculate trim in my old post. Trim is basically the difference in the exducer and inducer of an exhaust wheel. Trim has the similar effect of exhaust housing size,. It will determine when your boost curve starts, how broad it is and when it falls off.

formula for determining trim
[(minor wheel diameter)x(minor wheel diameter) / (major wheel diameter)x (major wheel diameter) ] x 100= trim


T04E 60
[(2.290)(2.290)/(2.950)(2.950)] x 100= trim
(5.2441/8.7025) x 100= trim
.6026 x 100 = trim
60= trim


If you try to run a high trim large wheel on an engine with limited exhaust energy and low boost you will end up with very little boost in the usable HP range.

Extreme example: D15B motor boosting 8psi with an straight T04E (AR.60 compressor, O trim in an AR.84 housing. This engine will start to boost at 6500rpm . That turbo on a non-ported RX-7 at 14psi of boost will net 320~380whp depending on the dp/exhaust, injectors and tuner. On a B18C at 18psi this setup will need a 9K rpm limit to ensure you use all of the boost range but the compressor housing will be the limiting factor. Yes, the straight T4 housing seems to be large but a T3 Stage V is actually larger. Stuff that T4 blade into a T3 housing and you will cure some of the spool up belly-aches associated with the straight T4 setup. Yes, I'm getting offtrack now. The O trim blade has a slightly large inducer (major diameter) but a smaller exducer (minor diameter) than the T3 stage V. Because of it's larger inducer the T4 O trim blade's hp curve will be peaky, narrow and fall off abruptly. The T3 stage V blade would start boosting earlier, produce a wider HP range and not fall off as abruptly.

Eventhough most exhaust wheels are advertised with an alphabetical or numerical identifier (N,O,P, Stage III, Stage V, etc) there is an actual trim number (in decimal form) associated with the blade. Many people confuse AR and trim when talking about turbos. You have to really study the blade instead of just reading the trim levels. The trim level may seem right but the actual dimensions of the blade could be completely the opposite.


Oh yeah... for the Frankensteins out there: Please do NOT use a setup intended for RX-7 13B/RE's on your SOHC grocery getter


Exdep -- it's better to use minor/major diameter instead of inducer/exducer since the inducer/exducer of the compressor and exhaust wheels are opposite.

compressor:
inducer = minor diameter
exucer= major diamter

exhaust:
inducer = major diameter
exducer = minor diameter


Modified by Dee at 11:43 PM 10/16/2004