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Discussion Starter #1
I tried searching around but didnt find much data on the topic.

Obviously a smaller AR is going to spool quicker and make more power tq down low and a bigger AR should make more power up top.

The question is, what will be faster?
When I bought my turbo I was sold the bigger .96 a/r so it wouldnt fall off up top.

I did find one comparison which shows a .68 vs a .96 and honestly it seemed like the .68 made way more power and tq mid range with very little loss up top where the .96 made a bit more power.

Anyone have some HARD data?

1/4 mile times?

Dyno sheets?

My car is a street car so im seriously debating whether or not I should drop down to a .81 a/r or even a .68 a/r and how much they might hurt power up top.
 

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That all depends on your combo, how much hp you want, ci size, and rpms. If you have an engine that spins to 7k, a 70mm with a .68 ar will have major back pressre issues but if you have two of the same on same engine, will support enough to go in the single digits. Turbo size and a/r depends all on your engine and goals.
 

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Discussion Starter #3 (Edited)
stock 4.6 281 cu in

ported heads, eddy victor manifold, and some blower cams.

On the dyno the car peaked HP at about 6200~ RPMS on low boost 8-9 psi so I would probably get away with revving it to 6700 rpms or so with that setup but most likely 6500 rpm shift points would be the norm for me.

I assume the power curve wouldnt change when I turn the boost up but was just entertaining the idea of a smaller AR and since its a street car I really wouldnt have any problems giving up some power up top for a 400-500 rpm quicker spool and a big jump in mid range power....just not sure how much a .81 vs a .96 would help
 

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The bigger ar. should be fine at the track as long as your shifting a a little after peak power. Letting the rpms not drop below where you dump the clutch.
 

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I tried searching around but didnt find much data on the topic.

Obviously a smaller AR is going to spool quicker and make more power tq down low and a bigger AR should make more power up top.

The question is, what will be faster?
When I bought my turbo I was sold the bigger .96 a/r so it wouldnt fall off up top.

I did find one comparison which shows a .68 vs a .96 and honestly it seemed like the .68 made way more power and tq mid range with very little loss up top where the .96 made a bit more power.

Anyone have some HARD data?

1/4 mile times?

Dyno sheets?

My car is a street car so im seriously debating whether or not I should drop down to a .81 a/r or even a .68 a/r and how much they might hurt power up top.
I think what a lot of it boils down to is weather or not you'll go beyond the turbo's efficiency range.... if your power goals are small enough for a .68 to handle theres no benefit in upgrading, keep it small with a quick spool time.

I'm fighting this dilemma myself, people are recommending a .81 or .96 but my motor is a stockish 302, I think I'm gonna try a .68 personally.

I wouldn't mind finding a little more info myself.
 

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Discussion Starter #10 (Edited)
Yea, I guess it depends on your setup and how much boost you plan on running but I would imagine the smaller a/r would be better as long as the turbo is still in its efficiency range up top.


I can't imagine a 96 ar making more power on 7 psi vs a 68
 

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Then try to imagine more...

Much more to worry about than this vs that at 7psi (blanket statements). Exhaust port flow, header diameter, turbine, and downpipe have to be dealt with also for the whole package to work. Not to mention the all important cylinder head and camshaft choices.

Also on a street car I dont like smaller a/r's...quicker spool = more spin usually.
 

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Then try to imagine more...

Much more to worry about than this vs that at 7psi (blanket statements). Exhaust port flow, header diameter, turbine, and downpipe have to be dealt with also for the whole package to work. Not to mention the all important cylinder head and camshaft choices.

Also on a street car I dont like smaller a/r's...quicker spool = more spin usually.
Don't mean to hi-jack, but do you guys think a GT45 76mm with a .96 would spool before like 4k on a stock headed 302?

I have plans to go 351, etc. down the road... buying a turbo once would be cool.
 

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Don't mean to hi-jack, but do you guys think a GT45 76mm with a .96 would spool before like 4k on a stock headed 302?

I have plans to go 351, etc. down the road... buying a turbo once would be cool.
What turbine wheel?

What trans? What converter, who is doing the tune up?
 

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What turbine wheel?

What trans? What converter, who is doing the tune up?
He is watching his daughter but this is the GT45 turbo he was looking at.


Compressor Size: Inlet = 4 3/16", Inducer = 68.7MM, Exducer = 97.8 MM, Trim = 69,A/R = 0.66, Intake (ID / OD) = 2.25" / 3.25"

Turbine: Outlet = 4", Turbo flange = T4, Downpipe flange = 3.5" V-band, Inducer = 87.4 MM, Exducer = 77 MM,Trim = 92, A/R = 1.05



-The 0.68 AR GT45 specs are:
4" Inlet Diameter
3 1/2" V-Band Outlet
Wet Floating Bearing Type
T4 Flange
Oil Cooled
Inlet: M10*1.5, Outlet: 2 M8
Max Power Rating: 650HP

Compressor Wheel
.70 A/R Cold Side
Ind: 51.99mm
Exd: 81.74mm
Trim: 40

Turbine Wheel
3 1/2 V-Band .68 A/R Hot Side
Ind: 73.42mm
Exd: 57.97mm

Current stock t5 but will be swapped to built auto later. Yes we know the t5 will explode with boost but that's coming!

Mainly just concerned about too big of a turbo for a stock e7 headed 302 with an E cam. For intakes he has gt40ps if it matters.
 

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He is watching his daughter but this is the GT45 turbo he was looking at.


Compressor Size: Inlet = 4 3/16", Inducer = 68.7MM, Exducer = 97.8 MM, Trim = 69,A/R = 0.66, Intake (ID / OD) = 2.25" / 3.25"

Turbine: Outlet = 4", Turbo flange = T4, Downpipe flange = 3.5" V-band, Inducer = 87.4 MM, Exducer = 77 MM,Trim = 92, A/R = 1.05



-The 0.68 AR GT45 specs are:
4" Inlet Diameter
3 1/2" V-Band Outlet
Wet Floating Bearing Type
T4 Flange
Oil Cooled
Inlet: M10*1.5, Outlet: 2 M8
Max Power Rating: 650HP

Compressor Wheel
.70 A/R Cold Side
Ind: 51.99mm
Exd: 81.74mm
Trim: 40

Turbine Wheel
3 1/2 V-Band .68 A/R Hot Side
Ind: 73.42mm
Exd: 57.97mm

Current stock t5 but will be swapped to built auto later. Yes we know the t5 will explode with boost but that's coming!

Mainly just concerned about too big of a turbo for a stock e7 headed 302 with an E cam. For intakes he has gt40ps if it matters.
what he said!
guys on turbo forums seem to think a .68 will spool super fast which could be good for the t-5 but i want some top end steam, i'd stop short of 500hp with this motor for sure.
 

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some food for thought... EGTs

for some reason not lot of people run a EGT gauge and there's LOT to learn from having one
 
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