E/I flow ratio with boost
Turbo Mini Forums

Calling head flow experts -

From my knowledge of naturally aspirated engines, I know that an ideal ratio of exhaust flow to intake flow is considered to be about 80%. Higher exhaust flows than 80% of intake flow unbalance the engine toward intake side limitation, while exhaust flow lower than 80% of intake flow leads to exhaust side choking. OK so far.

The ratio has been determined empirically through a lot of dyno work over the years, and is explained by the intake being driven by lower pressure than the exhaust in a real engine, though both sides are flowed at 28" of water pressure on the flow bench.

Now when you start boosting the intake, clearly you are driving the intake side of the engine with more pressure than in the naturally aspirated case. Therefore it seems to me to follow that you want more exhaust flow relative to the intake flow under flow bench conditions. You would want exhaust flow to be 90% or maybe even 100% of the intake side flow. It's also true that we are increasing the pressure driving the exhaust side since we are combusting more mixture, so maybe these effects balance out.

I searched and could not find such a discussion on the forum here, so if you point me to an old topic please be gentle. On the web I found only one reference to this kind of effect - some dune buggy folks were saying that on their turbo VW engines they make the exhaust valve the same size as the intake valve and they see better performance and also the engine runs cooler.

Anyone tried this?

Edited by fortfun on 14th Oct, 2007.

1275 with Back Door Turbo

Found this from Vizard:

http://books.google.com/books?id=mm8oM7_Ou...cd=2&cad=legacy

Hope the link works. Anyway, he is saying that 75% E/I ratio is usually ideal for natually aspirated, and for 15psi of boost you want to go up to an 80% to 85% E/I ratio.

What do you guys target on the flow bench?

I bought a new-old-stock cylinder head casting, and would like to plan out what to do with it to get maximum output.

1275 with Back Door Turbo

Another view on the subject:

http://www.pumaracing.co.uk/Exvalve.htm

This one claims that the extra cylinder pressure on blowdown makes everything balance out and you should use the same E/I ratio as for naturally aspirated.
Further, the Puma site says 32mm is all the exhaust size you ever need for 350cc cylinders.

1275 with Back Door Turbo

Dont forget camshaft comes into it.

Add longer duration on the exhaust side, and its quite easy to make it flow more....in a roundabout way.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

my turbo head has 33mm inlet and 33mm exhaust valves. seems to work ok.

The use of 28" water gives a benchmark for comparison purposes and is OK for NA engines where you have atmospheric pressure filling the cylinders and a scavenging exhaust helping evacuate them.

28" of water equates to approximately 1.0psi. Very often the flowrates recorded on the bench are far in excess of what actually ocurrs, hence, actual pressure loss through the ports is far less.

In a moderately boosted engine, say 10psi, the effects of big ports are far less significant because in percentage terms the loss of 1.0psi through the system is easily made up by the turbocharger.

If there is a penalty, then it will be a higher exhaust pressure that may affect the volumetric efficiency (VE) of the engine.

In NA tuning, you need to drop the back pressure to achieve a high VE. This is done by optimising the port and exhaust system size.

With turbos, there will be a restriction to flow caused by the turbocharger that far exceeds any head loss purely attributable to head ports.

Ultimately the turbo converts all the pressure energy in the exhaust into velocity energy in the turbine nozzle. For every change of state there is an energy loss.

I'm not convinced that slowing down the exhaust flow with bigger ports is beneficial as the turbo only has to speed it back up again.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

my head had 32mm inlet and 32mm exhaust but valve seat recesion forced the change to 33/33. its also been ported alot and with this my C/R is 8:1

originaly the head was done by Vmax and ended up in the hands of JF who i purchased it off.

i'm sorting another head which will retain the beak in the chamber and have standard MG metro valves. it will also be ported and give a C/R of 8.3:1

i've also been trying out a standard MG metro head giving me a C:R of 8.8:1 and i have to say at 11psi it goes as well as the Vmax head at 18psi but this i was putting down to the C/R

Excellent. Do a back to back test, that's ALWAYS very interesting.

If you face valve seat recession, why not fit unleaded inserts? Possibly too late now

to do a back to back test i would really need to have all the heads with the same C/R then see what boost each can take and judge performance differences for the same boost and max boost with say a G meter

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will