Turbo comparison basics

[fnqcairns]

Can anyone have a stab at what would be the result of this hypothetical please?

A fairly standard car of around 4L capacity fitted with a Garrett t25 bush turbo 0.60 intake, 0.86 exhaust ...this turbo has been compared to the disco potato by a few.

Then another near identical Garrett T25 bush turbo is fitted and nothing else is changed except the intake is now 0.80 and the exhaust is now 0.64.

I know there could be more to it but at a basic level what sort of car performance in comparison would the driver notice he is now driving?

All thoughts most welcome.

cheers

[-Scott-]

I can't figure out how the inducer can be more than the exducer.

Are you sure you're not quoting intake & exhaust A/R ratios?

[ledgend80]

x2

[-Scott-]

If you're talking about A/R ratios, I would expect the change in exhaust A/R ratios to generate boost at lower rpm (the turbo will spin up faster), but the engine could run out of puff (become "strangled") at lower rpm too.

I can't get my head around intake A/Rs. I'll watch this with interest.

[fnqcairns]

I can't figure out how the inducer can be more than the exducer.

Are you sure you're not quoting intake & exhaust A/R ratios?

Yeah you are correct I will edit the post if it lets me, steep and slippery (obviously) learning curve lately.

[fnqcairns]

If you're talking about A/R ratios, I would expect the change in exhaust A/R ratios to generate boost at lower rpm (the turbo will spin up faster), but the engine could run out of puff (become "strangled") at lower rpm too.

I can't get my head around intake A/Rs. I'll watch this with interest.

yeah I to can sort of get my head around the exhaust side, but when different sized intakes are added to the equation it all goes to pot!

Been searching everywhere for a basic explanation but have got nowhere.

With hope the good people here will have the knowledge needed.

cheers

[anzac]

the second turbo will come on boost earlier because the smaller exh housing forces higher gas velocity. The larger intake will allow the turbo to flow more air, therefore the second turbo has a higher HP potential.

The really critical thing is what size wheels and their trim are fitted.

on a diesel, go the smaller exh housing because they don't rev high enough to really get the turbo spooled up.

example of this is:

My SR20DET silvia runs a GT3071R with a .86 exhaust housing. doesn't come onto full boost (20psi) until just over 3500rpm
Same turbo with smaller .64 housing and the car was on full boost by 2000rpm. Made slightly less power as the turbo restricted gas flow at higher rpm. Need to change to an external wastegate to solve that.

On a TD42, you want it on boost as early as possible otherwise you run out of revs before you run out of turbo.

[-Scott-]

the second turbo will come on boost earlier because the smaller exh housing forces higher gas velocity. The larger intake will allow the turbo to flow more air, therefore the second turbo has a higher HP potential.

Does the smaller exhaust A/R create higher back pressure at higher revs? Wouldn't this work against peak power?

[80's_delirious]

the second turbo will come on boost earlier because the smaller exh housing forces higher gas velocity. The larger intake will allow the turbo to flow more air, therefore the second turbo has a higher HP potential.

Does the smaller exhaust A/R create higher back pressure at higher revs? Wouldn't this work against peak power?

yes, hence . . .

Same turbo with smaller .64 housing and the car was on full boost by 2000rpm. Made slightly less power as the turbo restricted gas flow at higher rpm.

[KiwiBacon]

yeah I to can sort of get my head around the exhaust side, but when different sized intakes are added to the equation it all goes to pot!

Been searching everywhere for a basic explanation but have got nowhere.

With hope the good people here will have the knowledge needed.

cheers

Different intake A/R's are used to modify the compressor map. Usually you don't get a choice as the compressor housing A/R is matched to the wheel.

[-Scott-]

the second turbo will come on boost earlier because the smaller exh housing forces higher gas velocity. The larger intake will allow the turbo to flow more air, therefore the second turbo has a higher HP potential.

Does the smaller exhaust A/R create higher back pressure at higher revs? Wouldn't this work against peak power?

yes, hence . . .

Same turbo with smaller .64 housing and the car was on full boost by 2000rpm. Made slightly less power as the turbo restricted gas flow at higher rpm.

Yes. In his example, he was only modifying the exhaust housing, and made no mention of modifying the intake.

So the change mentioned in the OP could increase the top end through modifying the intake housing, but decrease the top end by restricting the exhaust. I'm wondering what would be the nett result? Will the free flowing intake provide sufficient benefits to overcome the restrictive exhaust?

[chimpboy]

So the change mentioned in the OP could increase the top end through modifying the intake housing, but decrease the top end by restricting the exhaust. I'm wondering what would be the nett result? Will the free flowing intake provide sufficient benefits to overcome the restrictive exhaust?

Is this where it starts coming down to all the other factors in the setup, and you can't really generalise?

[-Scott-]

So the change mentioned in the OP could increase the top end through modifying the intake housing, but decrease the top end by restricting the exhaust. I'm wondering what would be the nett result? Will the free flowing intake provide sufficient benefits to overcome the restrictive exhaust?

Is this where it starts coming down to all the other factors in the setup, and you can't really generalise?

Probably. I expect this is where you need to start looking at compressor maps. But, since I've never seen one, I'll bow out here.

[Bush65]

Changing A/R has more affect with the turbine housing, and much less with the compressor.

With the compressor most change is produced by changing the trim or impeller diameter (or both).

In general, when comparing compressor maps: inducer diameter has biggest influence on flow, exducer has biggest influence on boost pressure, smaller trim gives better efficiency. Compressor Trim = inducer^2/exducer^2 x 100

This is why for particular sizes (T25 in this case) you can have different A/R turbine housings (but not usually for compressor) and different size impellers and trim for compressors.