Rear mount turbo question

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hrforever

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196
Location
Melbourne, Australia
Well my original thought was to move everything around in the engine bay to make space for a single turbo and as I was going to be making the headers to suit I figured this would be the easiest avenue. BUT, the other day, as I removed the centre muffler to improve the exhaust acoustics and I thought what if I put the turbo where it was. There's heaps of room for the turbo, the return piping might be a challenge as will the oil return, but with the turbo down there, the engine bay can stay clean and uncluttered.

So here's what I've got and what's planned

95 SC400 motor
W58 gearbox (yes, I know the issues)
Making 4,2,1 headers (1 5/8" into 1 3/4" into 2" into single 2.5")
Maintain twin high flow cats
MSIII
Injector size increase
CoP's

My research has lead me to believe a turbo with an A/R around .84 would provide responsive boost and because of its rear mounting it would not cause to much concern for the smaller rodded motor or the gearbox (.96 might be easier again on the running gear but lag might become an issue??).

So having got this far, my question relates to T3 Vs T4, open Vs divided. As the turbo will be placed where the exhaust is a single 2.5" pipe, after cats but before any mufflers, which combination would be recommended and does the overall size of the turbo change dramatically depending on what's chosen?
 
Don't confuse lag with turbo threshold...
A 1.06 T3 GT35R spools quick on 4.0 engine..
A T04Z 66mm turbo with .84 T4 housing spools just as well..
Doesn't hit as hard but power comes on "about" the same rpm..As an example..
There are newer turbo's out now with billet wheels etc which out spool older designs..
Wheel trim comes into play.. Check with the people your buying turbo off...
I guess the split pulse theory doesn't quite work the same with rear mount ??
Being a V engine I would run each bank to each side..
6 of 1 and half dozen of the other theories...
 
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hrforever said:
My research has lead me to believe a turbo with an A/R around .84 would provide responsive boost and because of its rear mounting it would not cause to much concern for the smaller rodded motor or the gearbox (.96 might be easier again on the running gear but lag might become an issue??).

So having got this far, my question relates to T3 Vs T4, open Vs divided. As the turbo will be placed where the exhaust is a single 2.5" pipe, after cats but before any mufflers, which combination would be recommended and does the overall size of the turbo change dramatically depending on what's chosen?
Click to expand...

Firstly, the a/r number means nothing, till u include a turbine wheel size.
T3/t4 ? .. That is no really important till 750-800 hp worth of exhaust flow.
Being a rear mount the exhaust pulses would have decreased in intensity, and mass flow is more the important factor.
The ability of the turbo to process the exhaust gas is the turbine size... Where works to the best is the a/r housing size.
The 2.5 dia tube fits nicely into t4, and 242 cubes as a single entry turbo would work very good.
 
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Thanks for the reply's.

As this is not going to be an out and out tyre burner, drivability is the aim, only with the added oomph. So if the T4 fits the 2.5" exhaust nicely, will the open or divided be preferred. Additionally, some additional research tells me a T3 with 0.84A/R would have a smaller turbine wheel, and therefore housing, than a T4 with 0.84A/R. Would this be correct or am I mis-interrupting the specs?
 
As Ash said single entry will do fine..
Generally yes the t04 is larger..
Keep in mind you STILL have 4.0 of exhaust
flow with a little boost...
 
With the aim of extra drive ability , more turbine Pressures desirable to give the smile to the drivers face.....lol
The t3 is not usually bigger, with the availability of larger turbines into t3 housings, many more opinions are given.
Examples
1447 hp is logged from Twin 62 's on 347 cubes on an engine I'm aware of.
Also Precision sell a t3 900hp turbos, and turbonetics 800+ hp t3's
We put a set of twin t3 turbos on sbc, went 8.6 @ 3400 lbs ...foot braking at Calder park the other nite...av gas deal.
A drag car in USA I deal with, twin turbo has 2.5 pipes from the collector to t6 flange.
Only makes 3000hp, and has over 10 psi LESS back pressure to boost.
That's 1500 hp through 2.5 inch pipe.

So, the Turbine wheel is the restriction usually.

On a street car up here, we changed a turbine housing from a v8 combination to inline 6 to gain all round performance.
The number of pulses make a difference to the turbine wheel from engines of same cubes, but different number of cylinders.
But saying that, with your combination Being a rear mount, that sort of nullifies the pulses, but wheel still has to deal with mass flow.
Turbines react to heat, pressure, and less exstent density of gas flow.

Pick the turbine wheel size to suit the flow of hp desired....
Pick the housing a/r to dictate the response needed and engine rpm you want the engine to produce the power.
Pick the turbine foot family , that fits the exhaust piping on the build, and packaging into vehicle.

Tune the fuel system and boost to marry up the short falls between the 3 choices.
Happy boosting..
 
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I wouldn't use divided turbine housing. It restricts the exhaust flow. Power is 1 thing, but restricted exhaust flow will cause overheating, not to mention backpressure that hurts power.
 
Well, it looks like I need to finish the headers and get the exhaust operating again to best determine its flow. As I have not set any planned power target, I suppose I'll have to pick a figure and build towards that, keeping in mind the other things Ash has pointed out and the limitations of the motor/gearbox combination.
 
ash said:
Lol...
U always quote and suggest a to4z , .84AR for everything...;)
Click to expand...

Yea .. It's the smallest I would use on 4.0 though..I figure being rear mount the A.R could be a little smaller..
As EVERONE seems to worry about quick spool...
Besides .84 t04 housing would flow close to 1.06 T3..
Being that this turbo is roughly 100 h.p more than the GT35R.
Yep a fraction large.. These turbo's where built for 2J 3.0 engine..
For ultimate power you could go nuts..
Post suggestions guys..
 
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the 35R's are good but getting a bit old in the tooth. if budget allows, i'd be more inclined to go a Precision PT6262 with the billet comp wheel and new CEA turbine wheel, even the journal bearing version of this turbo spools up faster than the GT35R.

we took a GT35R T3 .82 rear housing off a mates 1JZ, put on a Precision PT6766 journal, T4 .68 was only a 200rpm difference in spool.
 
I'm thinking smaller
He is old running oem rods pistons etc
And it's rear mount

He only wants to turn it to 5500-6000
 
FoPar said:
we took a GT35R T3 .82 rear housing off a mates 1JZ, put on a Precision PT6766 journal, T4 .68 was only a 200rpm difference in spool.
Click to expand...

Exactly with the t3 t4 comparison....
It's the turbine wheel that's the restriction..
 
Keep in mind engine size..??
He's after lazy power ...
Yep lighter assembly will help plus there is
no water cooling on those Precision turbo's..
Price would be better too...
 
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