exhaust port size?

[bc]

How important is the size of the exhaust port? Basically if you have the flow that you are looking for but the port and valve seem small for the size of the engine how much does it matter? Is there a point where now matter what the exhaust flows more power will not be made until the port it self is enlarged?

[panic]

Widmer has used supersonic exhaust ports on the B429 IIRC. Evidently the scavenging is so much better that the higher pumping loss is eliminated.

[LilRacr]

You know I have always thought about the same thing. I have always shaped the port to what I think will have good velocity and then flowed it to see how it looked. When I get the port to look like I want it usually flows plenty to keep up with in take side.

I would like to hear other experiences on too large or too small an exhaust port, or valve for that matter.

[bc]

Anyone have any opinions on this?

[learner]

I have never messed with the sonic exhaust ports, however, i have a copy of dynomation and i have inputted information based on information that Widmere has given on his website about the ports. All i can say is that dynomation shows exactly what Widmere says about the power improvements and the change in shape of the torque curve. It doesn't prove anything but i do believe in a small or relatively small ex port. The exact size obviously depends on many factors including the shape of the port.

[crazycuda]

This brings up a good question. Volume or air flow. Heres how I look at it, exhaust is a burnt gas aka no liquid so therefore it will have less volume and/or mass vrs the intake a/f mixture. One of the old porters I used to be friends with said that if you had 25cc for every 100 that was good. In other words if you had a 200cc intake runner and a 50 cc exhaust runner as a base line you were doing good. Then you just open the exhaust port to the cfm you wanted it to flow.

[panic]

"it will have less volume and/or mass vrs the intake"
???
MUST have the same mass. Conservation of matter law.
Volume is larger, since there are no droplets present and temperature is easily 1000 degrees higher.

[cboggs]

exhaust is a burnt gas aka no liquid

Isn't water a by product of the combustion cycle?

so wouldn't that mean that there is some "liquid" in the exhaust flow?

Curtis

[Ron E]

exhaust is a burnt gas aka no liquid

Isn't water a by product of the combustion cycle?

so wouldn't that mean that there is some "liquid" in the exhaust flow?

Curtis

Curtis....and on my 2.3 fords, wet flow in the exhaust is critical.

[Darin Morgan]

My personal opinion and from what I have learned about exhaust ports I have to say that small super fast exhaust ports make more power over larger high flowing exhaust ports except in the case of full exhaust systems such as Nextel cup engines. For some reason they like a slightly larger exhaust port but no where even close to what I would call large. Large and Small are ambiguous. In my book anything over about 110% of the valve area is large and anything under 105% of the valve area is very small but the exit velocity seems to play a role here as well. I try to adhere to the 105-108% in our pro Stock engines and it seems that I am not alone in my theory because many of the top notch heads I have seen are about the same or within about 2%. Another very big thing to consider in the tuning of exhaust ports is there sound or should I say the lack of sound. How smooth an exhaust port sounds and how quietly it can move the air are both very serious factors to consider. As the valve opens the sound of the ports should smooth up and get increasingly silent. The loudest portion of the exhaust flow on the bench is from .200 to .400 after that they should go increasingly silent with every lift increment. I have had exhaust ports that actually cracked and popped like fire crackers! With a little seat blending and chamber work I managed to smooth up the flow, gained a measly 2 cfm average and gained 26 horsepower and it still was not correct because the port was to big. The hardest thing I do is try and fix exhaust ports that are screwed up. Its much easier to fix intake ports!
Like an intake port, an exhaust port can be made to flow a great deal of air, Just make it big.

Some rules I live by.

(1) Exit area = 105-110 % of the valve.

(2) Exit air speed at a minimum of 300 and a max of 330 ft/sec mean.

(3) Smooth silent flow by at least .400 lift and absolutely by .500 lift.



On another note, I am not sold on the theory that the flow in the exhaust port goes Sonic. Anyone here care to prove this theory?

[crazycuda]

"it will have less volume and/or mass vrs the intake"
???
MUST have the same mass. Conservation of matter law.
Volume is larger, since there are no droplets present and temperature is easily 1000 degrees higher.

Panic , let me make sure i understand you correctly, your saying the exhaust volume is larger since it is a burnt heated gas not a liquid.
if that is true then everything i learned is wrong and and all exhaust ports are way to small

cboggs, im sorry for not including water in the burnt mixture. I was using the refrence as a burnt gas not a liquid vapor mix like the incoming a/f charge.

[OldSStroker]

"it will have less volume and/or mass vrs the intake"
???
MUST have the same mass. Conservation of matter law.
Volume is larger, since there are no droplets present and temperature is easily 1000 degrees higher.

Panic , let me make sure i understand you correctly, your saying the exhaust volume is larger since it is a burnt heated gas not a liquid.
if that is true then everything i learned is wrong and and all exhaust ports are way to small

cboggs, im sorry for not including water in the burnt mixture. I was using the refrence as a burnt gas not a liquid vapor mix like the incoming a/f charge.

There way more VOLUME in the 1200F+ exhaust gas leaving the head than intake mixture. The temp causes the expansion. The exhaust port velocity is a lot higher than the intake. The pressure is also higher than the intake. So, higher pressure and higher velocity gets the higher volume out thru the relatively small exhaust port.

The water in the 1200F exhaust is a gas (aka vater vapor), not a liquid. Not until it cools below 212F or so does it condense back into liquid. At low power settings (cruise/idle, etc) this temp may be reached prior to the tailpipe outlet so liquid water drips out. Probably not so in high rpm max power situations.

Flow in the exhaust port is not "wet flow" meaning a mixture of liquid and gas. It's just relatively hot gases going like hell, at least initially. As the gas cools the volume decreases so the flow (volume/time) decreases. Same mass of course, so the mass flow remains the same.

[hotrod]

On another note, I am not sold on the theory that the flow in the exhaust port goes Sonic. Anyone here care to prove this theory?

I think there is a logical proof for it --- not sure anyone has the pressure data in the port and cylinder to create a mathmatical proof. If you look at the requirements for supersonic flow ( critical flow nozzles ) the flow cannot go supersonic until the pressure ratio across the orfice exceeds 1.89:1 (for air).

Likewise if the pressure ratio is greater than 1.89:1 the flow must reach sonic speeds at the throat.

The key question is the shape of the nozzle downstream of the narrowest point of the orfice. To go supersonic that portion of the nozzle has to be a diverging orfice commonly called a DeLaval nozzle. A good example of this nozzle shape in its ideal form would be a rocket engine exit bell.

http://en.wikipedia.org/wiki/De_Laval_nozzle

I think it would be pretty clear that the pressures inside the cylinder would be more than 1.89x the header pressure (especially in an open header system). In the book "Scientific Design of Exhaust and Intake Systems" on page 78 that at exhaust valve opening there were cylinder pressures of 70 psi.

So at least in the early part of the blow down process the flow has to hit the speed of sound at the valve seat or as the valve lifts farther the sonic flow would transition to the throat of the bowl. If the port shape from the throat is of diverging shape and does not cause flow separation, I think its a pretty good bet that for at least the first .200 - .300 of exhaust valve lift, you have the potential for supersonic flow.

This report may have the key info buried in it, but I have not had time to read it in depth, and make sense of all the math.

http://naca.central.cranfield.ac.uk/rep ... rt-786.pdf

This report has some hints of some very useful info but I have not found how to access the referenced "illustrations" Perhaps they are available only to members. ---- anyone know how to access these ?
http://members.eaa.org/home/flight_reports/epg4.html

Larry

[maxracesoftware]

This report has some hints of some very useful info but I have not found how to access the referenced "illustrations" Perhaps they are available only to members. ---- anyone know how to access these ?
http://members.eaa.org/home/flight_reports/epg4.html

Larry

http://members.eaa.org/home/flight_reports/index.html


http://members.eaa.org/home/flight_reports/epg1.html

http://members.eaa.org/home/flight_reports/epg2.html

http://members.eaa.org/home/flight_reports/epg3.html

http://members.eaa.org/home/flight_reports/epg4.html



Larry....what you are looking for is in these 2 .PDF Files (Pics/illustrations)

http://www.cafefoundation.org/aprs/epg.pdf

http://www.cafefoundation.org/aprs/EPG%20PART%20IV.pdf

[maxracesoftware]

Larry, One more .PDF file/link

this is more what you are looking for

http://www.okcc.com/PDF/Choked%20Flow%2 ... 0pg.48.pdf