Exhaust port height on Piston-port 250 ?

[John Orchard]

Hey Chaps  :-)

I purchased a ported barrel for a Yamaha DT1MX, I'm concerned that this porting could be too radical.

This is a piston-port 250 with a bore/stroke of 70x64, much the same as early KX250, TM/TS250, & CR250M.

The distance from the top of the cylinder to the top of the exhaust port is 36mm, stock on these cylinders is 39mm.

Does anyone have experience to say if this is in the ballpark?

Regards, John.

[pokey]

Good to be concerned. Raising the port  effects the port timing as your aware, This means the pipe needs to changed  so the returning charges from the rear cone can be timed to the port opening and closing. a Higher port also reduces the effective range or PB.

Plot the ports on a timing wheel or just a circle of paper with 360 degrees.  and look for 190. over that then your losing  unless you can increase rpm
Raising the port can increase HP IF it can do higher RPM as you lose torque but raised RPM cancels out the loss in power.

 In short, the pipe needs to be changed to suit the raised and changed port timing and the engine needs to be able to scream its head off.
 You could just suck it and see what it runs like with what youve got.

[John Orchard]

Thanks for confirming what l felt, the bike has a Bassani chamber (l think?), it now has a 15mm shorter header after l modded it to clear a high-mount front guard, maybe the short header & high port opening might work together? As you say , l will suck it and see :-) Stay tuned (so to speak ;-) ).

[Momus]

A standard YZ250D 70/64 had the exhaust port open 34.3 from the top. Harry Klemm (DG) ported those for MX to 33.4.

I have a 73 TM250 cylinder with the quite narrow bridged exhaust port opening at 36 mm. With a 32 mm carb and the primitive 2 transfer arrangement, opening at 52 mm down the hole, the bike is tractable.

[TT5 Matt]

I raised the exhaust port on my ts185a 1.5mm and it took a lot off the bottom end power to the point its just usable for puttering around my farm but great for circle work which is what I mainly use it for,i also cleaned up/dedagged the transfers and opened up the intake port to take a 32mm carb which yet to fit as it goes well on the std 26mm carb though I might try an er185 29mm oval bore carb next.as Pokey said suck it and see if its suit your riding style,take a few different spockets with you to get the power in the sweet spot.

[pokey]

Im leaning to think that bore was always meant for grass track. Bassani's usually have a long belly section  that are good for enduro. Im also thinking a shorter belly would be the way to go but  trying what you have costs nothing . Good luck with the build.

[John Orchard]

A standard YZ250D 70/64 had the exhaust port open 34.3 from the top. Harry Klemm (DG) ported those for MX to 33.4.

I have a 73 TM250 cylinder with the quite narrow bridged exhaust port opening at 36 mm. With a 32 mm carb and the primitive 2 transfer arrangement, opening at 52 mm down the hole, the bike is tractable.

Thanks Marcus, that makes me feel better knowing your TM is close, reed valve engines seem to run a higher ex port. My transfers are  51 mm from the top of cyl.

[crossedup2]

Hi All

Piston Ports aren't happy with high exhaust port heights. However, they can cope with some increase. Below are copies out of the 250 Yamaha Clymer. The reed valve Motocross version ( Enduro  not shown has a 37mm port height). The other two are shorter than your 36mm height.

The real trick with PP's is the intake. Earlier mods for the smaller bores (125 and 175) was to raise the bottom of the piston 1mm. I've raised the skirt by 2mm on my reasonably standard 360 with great results. Next piston I'll go to 6mm or maybe 8mm shorter skirt. On mine I basically made the skirt flat. When I go bigger I'll curve it similar to YZ pistons.

I've discussed with a friend how far you can go and he has gone as high as 16mm off the skirt on an old TS400. He recounted that the best results peaked at around 10 or 12mm.

Your 250 may liven up a bit with a piston chop. Good luck with it.

https://drive.google.com/file/d/1AiuHeBvMsuHsA2KMC1CFZmyxqCY-ww27/view?usp=sharing
https://drive.google.com/file/d/1_C-GPcuuQudDSoMGfeKMhK84L3zzgdit/view?usp=sharing


Peter B

[Lozza]

Hey Chaps  :-)

I purchased a ported barrel for a Yamaha DT1MX, I'm concerned that this porting could be too radical.

This is a piston-port 250 with a bore/stroke of 70x64, much the same as early KX250, TM/TS250, & CR250M.

The distance from the top of the cylinder to the top of the exhaust port is 36mm, stock on these cylinders is 39mm.

Does anyone have experience to say if this is in the ballpark?

Regards, John.

Whats the rod length?

[John Orchard]

Hey Chaps  :-)

I purchased a ported barrel for a Yamaha DT1MX, I'm concerned that this porting could be too radical.

This is a piston-port 250 with a bore/stroke of 70x64, much the same as early KX250, TM/TS250, & CR250M.

The distance from the top of the cylinder to the top of the exhaust port is 36mm, stock on these cylinders is 39mm.

Does anyone have experience to say if this is in the ballpark?

Regards, John.

Whats the rod length?

The short rod (earlier model had a 5mm longer rod), 125mm centre to centre.

[John Orchard]

Good to be concerned. Raising the port  effects the port timing as your aware, This means the pipe needs to changed  so the returning charges from the rear cone can be timed to the port opening and closing. a Higher port also reduces the effective range or PB.

Plot the ports on a timing wheel or just a circle of paper with 360 degrees.  and look for 190. over that then your losing  unless you can increase rpm
Raising the port can increase HP IF it can do higher RPM as you lose torque but raised RPM cancels out the loss in power.

 In short, the pipe needs to be changed to suit the raised and changed port timing and the engine needs to be able to scream its head off.
 You could just suck it and see what it runs like with what youve got.

How are my calculations?

Given that a piston moving from TDC to BDC moves through 180 degrees of crank rotation.

A DT1 has a total piston stroke of 64mm. So in 64mm of piston stroke the crank rotates 180 degrees.

180 degrees divided by 64 = 2.8125 crank degrees per 1mm of piston stroke.

So, the exhaust port opens at 39mm ATDC which is 109.6875 crank degrees, 109.6875 subtracted from 180 leaves 70.3125 of open exhaust port BBDC, plus the same number of degrees ABDC (70.3125) is a total exhaust port open time of 140.325 (70.3125 x 2) crank degrees?

[sleepy]

Calculations not good. You forgot about the conrod moving in an arc at one end. Do you self a simple pencil drawing of a 64mm circle and then add the conrod 125 or 130mm depending on what you use. The small end is always straight up and down from the center of the circle and the big end follows the circle. Using a protractor and a compass you should be able to work it out.

[John Orchard]

Calculations not good. You forgot about the conrod moving in an arc at one end. Do you self a simple pencil drawing of a 64mm circle and then add the conrod 125 or 130mm depending on what you use. The small end is always straight up and down from the center of the circle and the big end follows the circle. Using a protractor and a compass you should be able to work it out.

Ah of course !!!!  Der what a dick I am  8-)

[John Orchard]

Found this online calculator, whadya think?

http://www.torqsoft.net/piston-position.html

[sleepy]

Good for working out ignition timing angles ect but give it a go should work. I worked out the math formula for doing the same a little while back, staying awake for most of my math classes wasn't a waist after all.

Found the post from a while back with my formula. The computer program will just use a similar formula as it basis making it much easier.

A bit of maths to work out the grey matter.

R= rod length
S= half stroke length
TA= timing angle

R+S- sq root(R sq-(sin TA x S)sq) - cos TA x S = BTDC

Sorry I don't know how to get the square or square root symbol while typing.
Try it out with a stroke of 48mm and a rod of 90mm with a timing of 14deg and you should get .903mm. You will have to find your own sin & cos values.