Expansion chamber problems

[NickMcDonald]

Howdy. I've started calculations for a pair of fairly basic single stage diffuser expansion chambers for my gt125 twin, and ive run into trouble. I subtract the accumulative lengths of diffusers, baffles, header pipes etc from the tuned length of the pipe, in an attempt to work out the length of the parallel walled center section. According to my very possibly flawed maths, this center section will work out at 475mm?! All the examples given in Graham Bells 2 stroke tuning book, regardless of cylinder size, vary very little and are all around the 130mm mark.

Now I've checked and double checked my calculations, and this is the number i keep coming up with! 475mm! Surely this cant be correct? Could it? Someone enlighten my plebeian mind. Thanks.

[Yow Ling]

Howdy. I've started calculations for a pair of fairly basic single stage diffuser expansion chambers for my gt125 twin, and ive run into trouble. I subtract the accumulative lengths of diffusers, baffles, header pipes etc from the tuned length of the pipe, in an attempt to work out the length of the parallel walled center section. According to my very possibly flawed maths, this center section will work out at 475mm?! All the examples given in Graham Bells 2 stroke tuning book, regardless of cylinder size, vary very little and are all around the 130mm mark.

Now I've checked and double checked my calculations, and this is the number i keep coming up with! 475mm! Surely this cant be correct? Could it? Someone enlighten my plebeian mind. Thanks.

Yes it cant be correct, but nobody can help you unless you post the measurments annd calculations , not just the wrong answer

[NickMcDonald]

So Im definitely wrong? Thats all I need to know! Back to the drawing board for me....Ill try sort out where Im messing up. Cheers!

[koba]

So Im definitely wrong? Thats all I need to know! Back to the drawing board for me....Ill try sort out where Im messing up. Cheers!

Are you aiming for stupidly low rpm? Or std exhaust port height?

I'm no expert but I'd suggest an absolute minimum of 190 deg export duration to get any sort of usable result.

[koba]

Yes it cant be correct, but nobody can help you unless you post the measurments and calculations , not just the wrong answer

So Im definitely wrong? Thats all I need to know! Back to the drawing board for me....Ill try sort out where Im messing up. Cheers!

Read Yow Ling's post again, perhaps some emboldening will help.

[davebullet]

I'm interested in you re-exhausting your GT125. I also have one and my main issue is not around trying to eek out power or cut down on weight (won't make any difference to me on the track) - but with the low factory exhausts on the suzuki GT125 frame - getting the nylon for the track to extend past the exhausts - hence something higher up / shorter might be needed.....

[NickMcDonald]

The exhaust port duration is standard, at 183 degrees. The exhaust PORT, the gap in the cylinder bore, is approximately 24mm. I say approximately as I dont have the correct tool for measuring it super accurately, and as this is really just a practice exhaust, im not too fussed. The exhaust flange face, where the header pipe bolts onto, is 28mm diameter. According to my workshop manual, these motors peak out at 10,000rpm. These are the figures im working with. Im going through Graham Bells book and using that as my guide, rather than using the online pipe calculator.

[koba]

The exhaust port duration is standard, at 183 degrees. The exhaust PORT, the gap in the cylinder bore, is approximately 24mm. I say approximately as I dont have the correct tool for measuring it super accurately, and as this is really just a practice exhaust, im not too fussed. The exhaust flange face, where the header pipe bolts onto, is 28mm diameter. According to my workshop manual, these motors peak out at 10,000rpm. These are the figures im working with. Im going through Graham Bells book and using that as my guide, rather than using the online pipe calculator.

Good to read that you are reading the book.
Give it a crack with what you come up with and then read and learn more.
It would be great if it was easy to follow a book and come up with all the answers but sadly it just ain't that simple BUT you will certainly make something better than standard on an engine like that.

If you want to do HEAPS of accurate measuring I can chuck the info through the program I have and see what comes out the other end.

Make sure you check out the ESE thread, tons of incredibly useful info in there.

[koba]

For the record by the Bell maths we have:

183x42545
_________

10,000

= 782mm tuned length to centre of baffle.

[NickMcDonald]

.....how the hell did I end up with just over a metre of tuned length then? Fark. So it seems i must have fudged up the first, and simplest calculation. Bluuugghh.

[TZ350]

For the record by the Bell maths we have:
183x42545/10,000 = 782mm tuned length to centre of baffle.

.....how the hell did I end up with just over a metre of tuned length then? Fark. So it seems i must have fudged up the first, and simplest calculation. Bluuugghh.

You may not be as wrong as you think, the "True" Tuned Length starts at the piston not the exhaust flange so the pipe itself is not as long as the true tuned length. And you have to be carfull when compairing different Tuned Lengths as some like Bell measure it to the mid point of the baffel cone others like Blair, Wob, Frits measure it to the end of the baffel cone so a difference of 80-120mm is very possible between the different ways of calculating it and then changes in the average exhaust gas temperature used in the calculation can make 20-100mm difference too.

[NickMcDonald]

Dredging up this thread again. I've been through the calcs to build this expansion chamber and the parallel section is very long at 350mm but when I think about it in context and proportions it doesn't seem to bad. I thought I'd put the calc up to see if there is there is anything I may be doing wrong or may have not taken into account.

The calculation is based off graham bell's performance tuning book.

The engine is a Suzuki GT125 twin so 62.5cc cyclinders
Exhaust port is 24mm
Header inside dia 29mm

Tuned length:
(183 x 42545) / 10,000 = 778mm

Header length:
24mm x 9 = 219mm. 9 being taken from the table

Single stage diffuser length:
(60-29/2) x 8.7769 = 136mm

8.7769 is the cotangent of 6.5* which is taken from the table in the book
60 is 2.5 times 24mm (port size)
29mm is the stock pipe dia which I'm using for simplicity

Baffle length:
(60/2) x 5.1446 = 154mm

5.1446 is the cotangent of 11* which is taken from the table in the book

Reflection point is 154/2 = 77mm

Parallel section length = 778.6 - 216 - 136 - 77 = 349.6mm

Now that kind of seems alright when considering the proportions and it being a 50cc but 350mm is very long. Maybe are the dimensions slightly skewed because the ports are stock, it's 50cc and I'm used to seeing large port 250cc chambers?

[Linmag]

Dredging up this thread again. I've been through the calcs to build this expansion chamber and the parallel section is very long at 350mm but when I think about it in context and proportions it doesn't seem to bad. I thought I'd put the calc up to see if there is there is anything I may be doing wrong or may have not taken into account.

The calculation is based off graham bell's performance tuning book.

The engine is a Suzuki GT125 twin so 62.5cc cyclinders
Exhaust port is 24mm
Header inside dia 29mm

Tuned length:
(183 x 42545) / 10,000 = 778mm

Header length:
24mm x 9 = 219mm. 9 being taken from the table

Single stage diffuser length:
(60-29/2) x 8.7769 = 136mm

8.7769 is the cotangent of 6.5* which is taken from the table in the book
60 is 2.5 times 24mm (port size)
29mm is the stock pipe dia which I'm using for simplicity

Baffle length:
(60/2) x 5.1446 = 154mm

5.1446 is the cotangent of 11* which is taken from the table in the book

Reflection point is 154/2 = 77mm

Parallel section length = 778.6 - 216 - 136 - 77 = 349.6mm

Now that kind of seems alright when considering the proportions and it being a 50cc but 350mm is very long. Maybe are the dimensions slightly skewed because the ports are stock, it's 50cc and I'm used to seeing large port 250cc chambers?

Howdid it go? Did you build your chambers and tested them?

I'm asking since I also are looking to do the same project.

/ Magnus in Sweden