Intake and exhaust flow ratio

c2xejk · Post by **c2xejk** » Thu Jun 15, 2006 11:59 am

I am trying to understand the balance between intake and exhaust flow...

The head I am looking at right now as a E/I flow ratio of 0.56 (stock.) Which based on some searching is very low... Now that I have ported the head, the ratio is 0.60. Better, but still low I think.

I have been wondering, how much improving exhaust flow will help overall performance? How much do you watch the ratio vs, just improving things... What is the "ideal" ratio?

If I were to just increase the size of the intake or exhaust valves, which would give me better results? ie. Is there a real benifit to going +1mm with the exhaust valve only?

Thanks

SStrokerAce · Post by **SStrokerAce** » Thu Jun 15, 2006 12:24 pm

First off it depends on the application at hand. If it's a street motor then yes a higher E/I % will help since it will help with the cam selection and cutting down on exhaust duration to improve midrange and low end TQ. Most NA racing applications have relatively low E/I% ratios due to a lot of things. Mostly that the motors are intake limited so more intake valve will allow more cross sectional area thru the port and usually more gross CFM as well. The exhaust side of things on the other hand isin't always evaluated on more CFM flow the better, that doesn't always make the most power. The thing about these situations is that lots of modifications are acceptable and within the budget for the motor, mainly because they need to be competitive.

I'm guessing your setup is a small displacement motor? What are we looking at here? You are on the other side of the situation with a cylinder head with weak exhaust flow relative to what the majority of us work on with 2 valve american V8 heads, but the same principals apply.

Bret

buddy rawls · Post by **buddy rawls** » Thu Jun 15, 2006 4:01 pm

Dont get too wrapped up in the E/I relationships of a head (alone) being tested on a flow bench. It is meaningless unless the full inlet path and exhuast path is tested. Or at least the dominant path's cross-section. head and intake and carb, head and full length exhaust primary tube.

what I have found is that too low of E/I ratio and the valve events require too much and too early exhaust events, hurting the capability of the motor by limiting cylinder pressure. Sticking to the .7-.9 range provides a good/forgiving/tunable relationship. If the head (alone) is showing very low E/I, then it may be advantageous to restrict the inlet somewhat, to recover some of the relationship. Or do what you can to increase exhaust flow. But remember, it goes far beyond the 3" of exhaust port on the head. The overall path is what you need to be watching.

With muffled street motors it is not uncommon to achieve some very respectable power out of E/I ranges in the .60 (total system ratio), possibly even a tad lower. But you really have to play the game of dynamic compression versus valve events. The motor can reach a limit. With very low E/I, tuning is going to do best with very mild valve events and reasonably high intake flow capability versus the displaced cylinder volume. also static compression increases are helpful.

Lastly, sometimes what you have is what you have, and you deal with it and do the best you can.

c2xejk · Post by **c2xejk** » Thu Jun 15, 2006 6:14 pm

Some of the specifics of the application:

Street engine
Chrysler (Mitsubishi) 3L V6
2-valve/cyl
Intake-43mm
Exhaust-35mm
Will be used with a 5-sp manual transmission

MadBill · Post by **MadBill** » Thu Jun 15, 2006 6:57 pm

The higher the compression ratio (or rather the expansion ratio), the more you can bias flow to the intake for power, since the rapid pressure drop means using an early exhaust valve opening to combat the lower exhaust flow rate costs little due to the low residual pressure.

Joe Mendelis · Post by **Joe Mendelis** » Thu Jun 15, 2006 7:48 pm

SStrokerAce wrote:midrange and low end TQ.

BRET!! I told you only squirrels say "Low End Torque". You know it's true

c2xejk · Post by **c2xejk** » Fri Jun 16, 2006 7:16 am

How does turbocharging effect what you want the ratio to be?

RT · Post by RT » Fri Jun 16, 2006 3:18 pm

I wonder if it is the same head we have in this country for the Magna, which is a shockingly bad, the head looks like it was squashed flat and is probably 2.5 inches high(no SSR), the exhaust port is bigger than the exh valve and is bigger than the inlet port.

There was a big post about required exh flow for turbos, the jury is still out but it seams you don't need any more flow than for a N/A motor, maybe the higher end ie 80%

SStrokerAce · Post by **SStrokerAce** » Fri Jun 16, 2006 3:29 pm

Joe Mendelis wrote:
SStrokerAce wrote:midrange and low end TQ.
BRET!! I told you only squirrels say "Low End Torque". You know it's true

You know squirrel speak pretty well too!

blown265 · Post by **blown265** » Fri Jun 16, 2006 10:45 pm

Midrange and low end torque is sacrificed with 3/4 race cams. Just as well we mopar guys have purple shafts......

shawn · Post by **shawn** » Fri Jun 16, 2006 11:01 pm

Don't you get that from fiddling with 'em too much?
Shawn

Midrange and low end torque is sacrificed with 3/4 race cams. Just as well we mopar guys have purple shafts......

Post by **ClassKing** » Sat Jun 17, 2006 3:32 am

So...it's ok to say, "more torque at a lower rpm?"

Joe Mendelis · Post by **Joe Mendelis** » Sat Jun 17, 2006 11:38 am

ClassKing wrote:So...it's ok to say, "more torque at a lower rpm?"

I like to say more power at a low rpm and more torque at high rpm. The squirrels get confused when you say that because they only look at torque at low rpm because the number is bigger. Then all the sudden above 5252 horsepower is the magic unit and torque isn't talked about as much

rfl28 · Post by **rfl28** » Sat Jun 17, 2006 3:00 pm

"squirrel" beater !

MadBill · Post by **MadBill** » Sat Jun 17, 2006 5:20 pm

Hmmm...
Looks like it's time to disable the automatic Topic Reply Notification function on this one.