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Joined: Sat Aug 22, 2015 2:20 am
Posts: 69
Agreed, Ill flow it and have a bit of a poke around in the port to see what going on and get some reference measurements, but I think spending long hours getting exhaust port flow just right on the bench isn't beneficial for the reasons I mentioned above. Just my 2c though, I'm sure others have different experiences.

Wed Jan 04, 2017 4:20 pm

Joined: Sat Feb 27, 2010 12:50 am
Posts: 451
Location: Pennsylvania
Maybe this will help you with your exhaust Flow Test on your flow bench
This was written By 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.

Wed Jan 04, 2017 8:28 pm

Joined: Sat Aug 22, 2015 2:20 am
Posts: 69
I'm not disagreeing with Darin Morgan, but just because you have achieved good flow properties on the flowbench doesn't mean that the behaviour will be the same when you change the gas properties. Is it better than nothing? Of course it is and if it works it works, and taking note of the properties on the bench is still a good way to document your development for future reference. But it's a bit like wet flow testing, we do it (if we have the equipment...) if we can because it's one step closer to simulating the actual fluid mix in the port, hot flow testing is just a bit impractical though. I guess what I'm trying to say is that any development on the exhaust side requires a bit more extrapolation to transfer behaviour to the running conditions seen by the port, and extrapolation usually leads to all sorts of disasters and errors.

In a nutshell...exhaust flow is complicated, and I think even Darin Morgan would agree with that.

I'll have to try set up some CFD examples and change only the fluid temperature to confirm if I have a point or I'm full of s^&t. :)

Wed Jan 04, 2017 10:34 pm

Joined: Mon Feb 23, 2015 6:10 am
Posts: 269
Location: Northern NSW, on the wrong side of the Great Divide.
From everything I have read, and from what I have seen others do (my own experience is limited so I'm not professing to be any sort of expert), any indication of improved exhaust flow will show up as an improvement in engine efficiency as long as all other variables take advantage of of the improvement. You can have a great flowing exhaust but it wont show anything if the intake is a dud or the cam doesn't work with it. As I said before a flowbench is just a tool. Understanding the tool, the results it provides and being able to build an engine around them is where people who have experience shine.

Every job is a self portrait of the person who does it.
Autograph your work with excellence.

Wed Jan 04, 2017 11:17 pm

Joined: Sat Feb 27, 2010 12:50 am
Posts: 451
Location: Pennsylvania
From what I have been reading and searching about
exhaust ports it does come back to What
OwenMM wrote just because you have achieved good flow properties on the flowbench doesn't mean that the behaviour

It would be very interesting to see what you come up with OwenMM
when you use your CFD software will you be able set the pressure to
500 PSI and temp to 1300 º ?? :)

Here is the read link I found on the internet :)

For Got to add this link darn it. :oops: ... low-bench/

Fri Jan 06, 2017 7:45 pm

Joined: Sat Aug 22, 2015 2:20 am
Posts: 69
I sat down to set up this in CFX last week but I didn't realise my licence had expired. Hopefully I'll get Ansys back up and running soon but I can set the fluid properties and boundaries to pretty much anything you can think of. I don't have the time, knowledge or computing power to set up a really good transient port simulation unfortunately though.

Wed Jan 11, 2017 7:11 am
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