Turbo Calcs

[oondy]

I'm trying to work out the pressure ratio and CFM(lb/min) so that I can buy my turbo for my GQ LWB TD42.

I found these two simple equations:

Pressure Ratio: Pr = (14.7 + BOOST) / 14.7 (-1 for pressure drop at turbo intake)

and

CFM = ( Litres * RPM * VE * Pr) / 5660

for the VE (Volumetric Efficiency) I used 80%, is this too high, too low?

I have calculated that for 8 psi boost, I get a Pr = 1.66 and CFM @ 4200 RPM (MAX CFM NEEDED) = 385 CFM = ~27lb/min

I also calculated the lb/min for around 2000rpm = 14lb/minute

do these numbers seem right?

Its just that I was set on the GT28RS then I plotted these points on the compressor map and the point Pr=1.66 @ 27lb/min is pretty much on the choke line of the map.



Considering that I rarely drive to the redline - and especially once the turbo is on i won't need to - is this not much of a concern?

looking at the compressor map of the GT28R, the Pr=1.66 @ 27lb/min is right in the sweet spot of the map......



...or maybe i' just have NFI what I'm looking for.

are there similar maps for the turbine side of things?

cheers

OONDY

[KiwiBacon]

Your calculations seem quite good.
But you haven't accounted for the heating of the air which the turbo causes.
I don't put VE in the calcs, since it's a constant I prefer to add it at the end.
Since I didn't put in the VE and you didn't modify your calcs for the heated air, our results are the same.

My spreadsheet with 4.2L engine displacement, 8psi boost and 65% compressor efficiency puts it at 14.3 lb/min. Almost exactly what you got. But your VE will lower this air consumption further.
The density ratio is 1.28, so you'll get 28% more air than your engine without a turbo.

I'd recommend a T28 with a .64 exhaust A/R. The compressor options won't matter a lot, as you've found with the two compressor maps already.

Yes there are turbine flow maps, but they're quite different. Basically a line showing mass flow and pressure ratio. You can use them to compare a known turbine against an unknown but it's quite difficult to use them from scratch.

[oondy]

Cheers KB.

I'm not after huge performance, just the general power up, say 120-130kW @ flywheel (~160HP).

Just trying to decide on the turbo, the rest I can sort out pretty easy - manifold and ducting is not an issue, just gotta sort out oil & water lines and that should be about it.

cheers

OONDY

[KiwiBacon]

Cheers KB.

I'm not after huge performance, just the general power up, say 120-130kW @ flywheel (~160HP).

Just trying to decide on the turbo, the rest I can sort out pretty easy - manifold and ducting is not an issue, just gotta sort out oil & water lines and that should be about it.

cheers

OONDY

Shouldn't be too much of a problem. A mates father put a T3 turbo on his safari 4.2 and with a bit of tweaking it put out 180hp at the rear wheels.
That was with no intercooler, I'm not sure how much boost but it wasn't severe.

A T28 will be a great start, a ball bearing one (GT28) will be even better.

[oondy]

whats the difference between the GT28 and the GT28RS then if they're both BB turbos?

but then maybe me asking that question would clarify the fact that I have NFI what I'm doing!!??

cheers

OONDY

[KiwiBacon]

whats the difference between the GT28 and the GT28RS then if they're both BB turbos?

but then maybe me asking that question would clarify the fact that I have NFI what I'm doing!!??

cheers

OONDY

I think it needs to have a R suffix to be a ball bearing turbo. The RS is the disco potato which used to have a special "super awesome" compressor wheel design. It may also come with a larger turbine housing, so check that. Diesels boost sooner and work best with the smallest housing that won't choke the engine.

I suspect the old "super awesome" compressor wheel design is now throughout the GT turbo range.

[oondy]

Thats why I was thinking the GT28RS with the 0.86 turbine Housing.

cheers

OONDY

[KiwiBacon]

Thats why I was thinking the GT28RS with the 0.86 turbine Housing.

cheers

OONDY

What's the rev limit on those?
A T25 with 0.48 A/R turbine housing doesn't bottleneck on a 3.9L that revs to 3600.

[oondy]

I guess the hardest part to calculate would be the size of the turbine Housing.
I spoke to the guys from HPIAB and they said they'd used a 0.86 GT28RS on a 4.0L Diesel before with great success. but then Ray from hall turbos said it would be too small.

ho knows realy.

cheers

OONDY

[KiwiBacon]

I guess the hardest part to calculate would be the size of the turbine Housing.
I spoke to the guys from HPIAB and they said they'd used a 0.86 GT28RS on a 4.0L Diesel before with great success. but then Ray from hall turbos said it would be too small.

ho knows realy.

cheers

OONDY

If you're lucky, the 0.64 and the 0.86 housing will be completely interchangable. If you buy the parts from a decent turbo shop they should let you swap until you're happy.

The difference in rpm to get boost between then will be the same as the % size difference, in this case about 35%.
I'd start with the smaller one, accentuate your engines low end torque with boost at low rpms, better fuel economy and better drivability.

You've only got 300cc more capacity than my engine, there's no way that going up 1 turbo frame and 3 exhaust turbine sizes can be "too small".

[oondy]

well, it sounds like you know what you're talking about so i'll take your word for it.....0.64 HSG it is......that is if i can swap it if need be.

cheers

OONDY

[Patroler]

whats the difference between the GT28 and the GT28RS then if they're both BB turbos?

but then maybe me asking that question would clarify the fact that I have NFI what I'm doing!!??

cheers

OONDY

I think it needs to have a R suffix to be a ball bearing turbo. The RS is the disco potato which used to have a special "super awesome" compressor wheel design. It may also come with a larger turbine housing, so check that. Diesels boost sooner and work best with the smallest housing that won't choke the engine.

I suspect the old "super awesome" compressor wheel design is now throughout the GT turbo range.

If you have a look at the website below you can see the difference between the GT28R and the GT28RS, the main thing you notice is the housing on the GT28R has the bolt on flange style (good to fit to stock sr20 and ca18 style applications) whereas the RS has the hose clampable style compressor hsg.
http://www.atpturbo.com/Merchant2/merch ... y_Code=GRT

Here is another turbo calculator site i found, you can enter various rpm points and various other things then select a turbo from the list and it plots your points onto the compressor map
http://www.squirrelpf.com/turbocalc/ind ... =Calculate

I punched in a few rough values and the GT28RS or the one below it - GT28R-2 came up pretty good..

[Dzltec]

What power do you want out of the vehicle? We have had succes with smaller bb turbos as well as larger ones. It all depends on what you want. Do you want low down grunt or high power. This will decide your turbo for you.

Once a housing has been used you wont be able to send it back, also cost up a housing, I dont think they are cheap.


Andy

[oondy]

I'm not after huge performance or earth shattering dyno results.

I'm just after the same sort of power as if I were to go and buy an off the shelf kit, I think most of them say 120-130kW at the flywheel, thats all I'm after - maybe a little more, depending on how easy it handles this increase.

My GQ has done 260,000km so I won't be pushing it too far at all, just want it to be better on the highway for hills, overtaking and economy - I'm sick of putting my foot almost flat to pull 100-110km/h.

cheers

OONDY

[KiwiBacon]

Once a housing has been used you wont be able to send it back, also cost up a housing, I dont think they are cheap.

Andy

For a new one definitely. But a shop with a decent turbo graveyard may have used housings you can trial and swap.

[me3@neuralfibre.com]

TO28 is std fitment to Nissan Silvia amongst other things. Cheap 2nd hand. I paid $150 for mine. (different application, same turbo)

Paul