Inlet vs exhaust flow rates for turbos vs NA

[Ben Wilson]

Bear with me here, I'm trying to think something out.

Talking about heads here. In a normally aspirated motor, you don't need as much exhaust flow as inlet because you have a much larger pressure differential due to all the combustion, expansion etc. If you doubt what I'm saying, just look at the valve sizes..

When you start to talk about turbo cars, a couple of things change.

Firstly, you've got higher pressure in the plenum so you've got a higher pressure differential on the inlet.

Secondly, you've got the exhaust backpressure caused by the turbocharger, so the pressure differential across the exhaust port may be higer - but you've also got higher combustion pressure.

My question is - Is the pressure differential across the exhuast ports going to be higher on a turbo car than a NA car? Am I just doing my usual thing and overthinking it again?

[Ben Wilson]

Just got some numbers from a mate:
His EVO III had around an 86% ratio.
My silvertop 20v has around a 59% ratio.

Apparently the target ratio for NA motors is around 75% or so.

I'm thinking for a turbo 20v, some exhaust porting is in order.....

[nick.parker]

Ages ago I noted that that exhaust ports of a 16V 4AGE are smaller than the sum of the two exhaust valve open areas. Yet there exist Formula Atlantic valve to increase the valve diameter from 25.5mm to 27 or 27.5mm. My new extractors have a larger bore by 2mm or so than the stock manifold, and since my car is supercharged I was thinking about boring the ports a little. It looks like the water jacket is not a problem. I already run a larger exhaust cam than inlet cam.

Regards, Nick

[Enchanter]

Just got some numbers from a mate:
His EVO III had around an 86% ratio.
My silvertop 20v has around a 59% ratio.

Apparently the target ratio for NA motors is around 75% or so.

I'm thinking for a turbo 20v, some exhaust porting is in order.....

I would agree that the 20v would benefit more form exhaust porting than inlet porting. It does seem the EVO being a turbo has better flowing exhaust ports, however I'm sure I heard somewhere that the 2jz atmo exhaust ports are less restrictive than the turbo ports, the reason being that the higher velocity exhaust gasses would spool the turbos faster.
I might be wrong on that though, someone will chime in Im sure.

I always refer to this site for na stuff.
http://headerdesign.com/extras/engin...Cylinder_Heads

Cheers, Tim