Exhaust port questions.

Sujet: Exhaust port questions. Sam 9 Jan 2021 - 15:44

Hi all, could anyone comment on whether or not my thinking on my exhaust ports is correct or not? If It's wrong please say so, it's how we learn.

Exhaust port in the cylinder is trapezoidal in shape, 42.7 mm wide at the top (70% of bore, chordal measurement), port area is 11.10 sq/cm, my thinking is I should have a similar area at the port outlet i.e. 37.6 mm dia. The exhaust flange diameter would be 39.50mm dia (1.05 of port dia). Am I on the right line or way of course?

Look forward to your replies, thanks.

Sujet: Re: Exhaust port questions. Dim 10 Jan 2021 - 15:40

the "wobbly exhaust duct" :

[Vous devez être inscrit et connecté pour voir cette image]

not mentionned : for single exhaust ports, you need 90% instead of 75% port area (or the blow down area of the port if you know it).

Sujet: Re: Exhaust port questions. Dim 10 Jan 2021 - 19:35

In addition, you have a lot of info about exhaust ports in the "all you wanted to know about the Aprilia 125 RSA by Mr Jan Thiel and Frits Overmars" series of topics (I think we are at 5 parts now) in the 2 stroke section. It's a long way to read but believe me it's worth the effort.

Sujet: Re: Exhaust port questions. Lun 11 Jan 2021 - 10:57

It might be useful if you could provide some basic data about your engine, such as bore, stroke, rod length, is it piston port, reed or disc-valve induction how many transfer ports, and gears, 3-4-5-6 and so on? What is the intended use, is this a race engine, sprinter, fast road etc. The more information you can provide will ensure a more accurate and appropriate reply.
There is little point in trying to tune an engine with an exhaust port timing of a balls out racer having transfer timings of a sport roadster and inlet timing of a commuter bike! I`m sure you get the picture, all the best with your project.

cheers, Mel

Sujet: Re: Exhaust port questions. Lun 11 Jan 2021 - 12:43

he is only asking a question about the exhaust duct, so all those things are irrekevant.

Sujet: Re: Exhaust port questions. Lun 11 Jan 2021 - 13:06

You don`t really believe that do you?

Mel

Sujet: Re: Exhaust port questions. Lun 11 Jan 2021 - 17:00

look at Wobbly's duct. the only things you need are true exhaust port/blowdown area and the bore diameter.
it is relevant for all timings/rev's/gear's/.... it's purpose is to have 0.8mach in the duct for the pipe to work optimal. everything you mention is relevant for the rest of the tune of the engine, but not fr the duct. maybe for a WC-level the duct needs tobe changed slightly (allthough reading Wobbly's comments on the kiwi-biker forum I doubt he changes anything, or he keeps the final 1% of his secrets to himself, which I also doubt)

Sujet: Re: Exhaust port questions. Mer 13 Jan 2021 - 20:19

JanBros a écrit:: the "wobbly exhaust duct" :

[Vous devez être inscrit et connecté pour voir cette image]

not mentionned : for single exhaust ports, you need 90% instead of 75% port area (or the blow down area of the port if you know it).

That's very interesting, thanks.

Sujet: Re: Exhaust port questions. Lun 23 Oct 2023 - 18:43

i want to copy this wobbly duct idea for a single exhaust port cylinder but my duct is only 29mm long and the recommendation is for it to be 1.5 times the bore size. which in my case is 47mm x 1.5 = 70.5mm and then a spigot to bring it back to 100 percent of cyl wall port area which is 0.5 times bore =23.5mm long. so its 104mm until the flange proper. and start of header.

so with my existing cylinder duct only being 29mm should i have make the first 75mm of my pipes header to follow this convergence then divergence?? (29 plus 75 = 104 which is 2 x bore.)

Sujet: Re: Exhaust port questions. Lun 23 Oct 2023 - 23:27

yes
if you have a watercooled engine, you might as well make a duct through which also cooling water can flow, cooling the first part of your exhaust (where the fresh charge comes). cooling this part is good for power. cooler duct = cooler mixture and cooler mixture = more mixture back into the cylinder.

Sujet: Re: Exhaust port questions. Mar 24 Oct 2023 - 0:01

sadly its aircooled.
wouldnt a lower temperature affect the speed of the waves or pulses in the pipe making it seem longer?

Sujet: Re: Exhaust port questions. Mar 24 Oct 2023 - 22:11

aljaxon a écrit:: sadly its aircooled.
wouldnt a lower temperature affect the speed of the waves or pulses in the pipe making it seem longer?

yes, but than you can shorten it a bit and still gain.

Sujet: Re: Exhaust port questions. Mar 24 Oct 2023 - 23:48

ok lots to think about. thanks

Sujet: Re: Exhaust port questions. Jeu 2 Nov 2023 - 12:19

i made a "wobbly duct" using the first sections of my header to get the length - 2 x bore. i used 90% of my port area for the narrow diameter. i read i could have used my ports blowdown. according to your (JanBros) spreadsheet that works out at 51% of the port area. isnt this too small a constriction for the duct?
so which one is best to try?[Vous devez être inscrit et connecté pour voir cette image]

Sujet: Re: Exhaust port questions. Jeu 2 Nov 2023 - 16:34

for the Wobbly, I think you best stick to his values, so use the 90%

Sujet: Re: Exhaust port questions. Jeu 2 Nov 2023 - 20:33

JanBros a écrit:: for the Wobbly, I think you best stick to his values, so use the 90%

thanks Janbros. now to decide whos calcs to use Frits or Blair. or both. ive used both in the past. ie been reading the kiwi biker forum and there are lots of suggestions about percentages of header and to end of diffuser / tuned length as a percentage. 12 years ago i was told some "secret" percentages that all good pipes have and now it looks like its common knowledge. also there is a page in blairs book where amongst all the gobbledygook there are a couple of pages that i understand that also give his percentages.
all within a couple of percent of each other.

» [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked)
» [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)
» Difference in port timings
» HONDA RS125R Port timing differences
» [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)