expansion pipe anomoly

I have just measured my exhaust port in area by making a port map and then turning it into actual chordal area ie true 2d area. and i get an area equivalent to 25mm diameter. but using fos X comes out at 28.8mm and only if i reduce the rpm from my intended 10500 to 8500 will the formula make x = 25mm
can i simply substitute 25mm for x instead of the suggested 28.8mm?
i think i know the answer. but maybe my anomaly pipe behaved strange because not only was my blowdown too small but i had used 28mm as x making the pipe too large.

or maybe you should just enlarge your exit diameter ?
or even better, use Wobbly's formula of making the exhaust duct diameter optimal (90% of exhaust port area for single exhaust, and 75% for double/tripple ports) and start from there with your exhaust.
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there is no room for auxiliary exhaust ports. it is a single port as wide as i dare go or the duct will open up into the stud holes. i might have to try and relocate the studs and fill the holes so the passages to the aux ports can then be made.

I see now it is not mentionned in the text and maybe I wasn't clear enough.
you don't have to make auxiliary ports. you can aply the same rules for the duct as stated for a single exhaust port, just change the 0.75 (or 75%) of the cross sectional area at 1.5 time bore length to 0.9 (90%)

my ex port exit diameter has to be 25mm due to a threaded connector.
lets just pretend i am stuck at 25mm and thats the start diameter of the pipe. well after a 40mm duct. should i use 25mm as X?
or just use 28mm as per the calcs

aljaxon a écrit:

my ex port exit diameter has to be 25mm due to a threaded connector.
lets just pretend i am stuck at 25mm and thats the start diameter of the pipe. well after a 40mm duct. should i use 25mm as X?
or just use 28mm as per the calcs

Al, you shouldn't 'use' anything as X. You need to calculate X (it's straightforward enough) and start from there.
If you don't, you may as well neglect my exhaust concept alltogether.

X if i use your fomula is 28 as per the calc. my port area - equiv diameter of 25mm and the flange is also 25mm. and the duct is 25mm long but has to extend at 25mm for a total lengthe of 40mm. so i cant have the pipes physical start jump to 28mm?

aljaxon a écrit:

X if i use your fomula is 28 as per the calc. my port area - equiv diameter of 25mm and the flange is also 25mm. and the duct is 25mm long but has to extend at 25mm for a total lengthe of 40mm. so i cant have the pipes physical start jump to 28mm?

Five months ago I wrote

Frits Overmars a écrit:

The disadvantage of a short-stroke engine really shows up: 126° transfer timing may be good for maximum power at 10.000 rpm, but the blowdown angle.area from the 176° exhaust timing is only good for 3400 rpm... Assuming you are stuck with the 126° transfer timing, the best thing to do would be to raise the exhaust timing to no less than 200°, good for blowdown at 9250 rpm. This would give an X-value of 25,4 mm.

What exhaust timing en rpm did you use to end up with an X-value of 28 mm?

yeah things have moved on. i darent touch the porting on that other motor. its running well and goes over 100 kmh. it produces 10.8hp @9500

i have just finished building another identical motor 82cc but with a duration of 186 degrees and lower transfers 122
i think this will produce power slightly higher so am aiming for peak power at 10.000 rpm.
X= 28.2mm @10000 rpm

unfortunately the first 40mm of the duct/header must be 25mm due to limitations of the cylinder design.
i am thinking if either use X = 28 and just taper the header from its starting diameter at the flange
= 25mm to its 28 x 1.36 final dimension =38.1mm. and all the diameters based on 28mm or do i go with 25. what do you think is best?

I wrote that max.power at 10.000 rpm requires the 126° transfer timing that you already had, and 200° exhaust timing.  
Now you want to lower the transfer timing and still aim for 10.000 rpm. Go ahead and try it; you will learn something.

but frits something strange is happening. with 176 124 durations i get peak power at 9500 and the bike revs to 12k on the road with slightly too low gearing.  thats with a mis matched pipe with a peak of 11500.  i want to lower the transfer timing to give me more blowdown and raise the exhaust  to give me even more blowdown. its a single speed motor and i dont think 200 exhaust is a good idea. i need power at 5k rpm.

although my durations are wrong i think my areas might be askew and compensate for this so the motor works well. i might have lucked in so thats why i dont want to alter anything.

but the new motor i can change things.
if im getting peak power at 9500 and raise the exhaust duration 10 degrees it should peak far higher/ and so i will be able to lower the transfers a bit?

the motor is now running and im currently bedding the rings in for 40 miles. im using it with an off the shelf doppler streetcup pipe and it seems great so far. but i do want to make a pipe for it using your recommendations. if i go with 28mm i will have to dramatically alter the cylinder and my exhaust port only has an area equivalent to 25mm sq. thats using a port map and then calculating the real unfolded area. chordal. if i start at 28mm should my port area match that? or shpuld it be actually larger than the 28mm diam area? i see 90 and 75% of the are suggested for the duct wobbly etc. i have a sintgle exhaust port on this motor.

aljaxon a écrit:

the motor is now running and im currently bedding the rings in for 40 miles. im using it with an off the shelf doppler streetcup pipe and it seems great so far. but i do want to make a pipe  for it using your recommendations. if i go with 28mm i will have to dramatically alter the cylinder and my exhaust port only has an area equivalent to 25mm sq. thats using a port map and then calculating the real unfolded area. chordal.  if i start at 28mm should my port area match that? or shpuld it be actually larger than the 28mm diam area? i see 90 and 75% of the are suggested for the duct wobbly etc. i have a sintgle exhaust port on this motor.

Al, I am not interested in your exhaust port area; that is Wobblys approach, not mine. And I don't recommend using port maps
If you insist on aiming for maximum power at 10.000 rpm, better make sure that both your blowdown angle.area and your transfer angle.area are up to the job. It will make all the difference between wishful thinking and engine development

Je ne pratique pas le deux temps, mais le raisonnement est intéressant à suivre….

C'est même très philosophique. Le côté technicien de Frits balaie toutes les certitudes qu'on peut avoir : entre s'accrocher à une idée que l'on pense juste et la réalité de la physique, chacun peut choisir son camp.

i thought i had posted a reply but its either disappeared or im losing my marbles.

ill be honest and admit i havent worked out my angle.areas i was hoping that would take care of itself and i was hoping if the motor is working well then a pipe made to your specs would compliment it.

looks like i will have to spend some time working it out. but when ive done that if my cylinder duct has a terminal id of 25mm can i taper the header from that 25mm to its D1 = 1.36 * X end diameter 1.36 * 28 = 38.1mm? of do i have to use 25mm as X?

aljaxon a écrit:

ill be honest and admit i havent worked out my angle.areas  i was hoping that would take care of itself...
looks like i will have to spend some time working it out. but when ive done that if my cylinder ...

Al, I will be glad to help you but you need to spend some of your time before you ask me to spend more of mine.

when calculating the transfer angle.area do i use the area of the port regardless of angle of flow or work out using its effective area. ie a 40mm wide port that flows at 30 degrees towards the rear has an effective width of 34mm

i am assuming thats the case.
i need some advice on the best way to map out my ports

use this :
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Frits Overmars a écrit:

aljaxon a écrit:

ill be honest and admit i havent worked out my angle.areas  i was hoping that would take care of itself...
looks like i will have to spend some time working it out. but when ive done that if my cylinder ...

Al, I will be glad to help you but you need to spend some of your time before you ask me to spend more of mine.

JanBros a écrit:

use this :
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Thanks janbros, ive already played with that and found it complicated. plus i read this you wrote

"you'll never be able to have enough blowdown-time.area with a single exhaust port, or if you'd try you'd end up with rediculous exhaust timing. compare it with a four-stroke using only 1 exhaust valve vs 2 : it's impossible to exit the same amount of exhaust gas in the same time through a smaller hole"

i only have a single exhaust port. so am i wasting my time?


i will have another look at it. on reading the second doc there are some words missing
you wrote:-

You enter the actual (not chordal) portwidth from from the portmap
(only for the a-symmetrical ports, you enter the width from ,,, --------

and then the words are missing. i dont think it matters in my case. i have 3 pairs of transfer ports and they are all rectangular. i call them main aux and boost. but i will have to call them abc and measure their centre lines to the centre of the exhaust port and NOT the centre line of the cylinder. which will always be the radius of the bore.

if i have any questions shall i ask the on this thread or your dedicated thread?

it should have been "You enter the actual (not chordal) port width from the portmap (only for the a-symmetrical ports, you enter the width from the "center location" to the port’s edge)."

discuss the spreadsheet in the designated topic please, might also be usefull to others

JanBros a écrit:

it should have been "You enter the actual (not chordal) port width from the portmap (only for the a-symmetrical ports, you enter the width from the "center location" to the port’s edge)."

discuss the spreadsheet in the designated topic please, might also be usefull to others

will do. cheers
as long as you dont think i am wasting my time with only having a single exhaust port. plus my transfer ducts or tunnels are far from the ideal teapot arms.

why should you be wasting your time because of a single exh port ?
My moped mx Honda Sky also only have a single exhaust.

JanBros a écrit:

why should you be wasting your time because of a single exh port ?
My moped mx Honda Sky also only have a single exhaust.

its just because the spreadsheet seems orientated towards highly tuned two strokes aprilia rsa 125/250 and you mention kr1s multi exhaust ports and like i said earlier
you said on the dedicated thread "you'll never be able to have enough blowdown-time.area with a single exhaust port, or if you'd try you'd end up with rediculous exhaust timing"

i dont want ridiculous exhaust timing because mine is a single gear moped with clutch bite point at 5k. so am i wasting my time?