Curently I am working on my cylinder , the timing numbers are based of depth of top edge measurements and known deck height (it's easy to see where the ring topped out at and measure from the top of top ring to top of piston. It isn't perfect but it's close and my cuts are made conservative leaving the final few thousandths or not to a blue stone or 80 grit flap wheel or even back to the carbide if more than 10 or 15 thousandths need to come off.
My exhaust port has been raised and sits around 89 to 90 degrees atdc giving me 180-182 exhaust duration I plan in running this to 87 or 88 atdc and 184-186 duration
Since I felt like even 90 degrees for exhaust and the stock 119.5 for transfers giving 29.5 degrees of blowdown might be a bit too much. I decided to pull the blowdown in a little . Once I made a short file that allowed me to hand cut the transfers I decided that I might be able to go a little further.
Currently my primary transfers are at 118 atdc giving me 124 transfer duration, with my exhaust port at 90° atdc , the blowdown remains the same as stock at 28.
Higher blowdown numbers leave more time for fuel/air mix to short circuit out the exhsust at lower rpm, and also give more time for exhaust to clear out of the cylinder at higher rpm basically making its part in the symphony prefer higher rpm.
My plan was to move the exhaust to 88 atdc 184 duration or maybe more
116 atdc 124 dur 28 blow dow main transfers and
118 atdc 122 dur 30 blowdown on the secondsries, as well as raise the boost port the same number of degrees as the exhaust port.
Which should put it at 122°for boost port.
I will have to measure to see where I am at on the transfers, and exhaust at the moment , but I am along ways away from 190-195/130 or such so I can still go either way as far as the numbers go.
To me having the mains open a little earlier would seem to be the right idea as they would have good velocity opening first and force mixture toward the back to help scavenging and getting the exhaust out, also staying open a little longer making the last fuel in have a little more velocity as well as being .
The biggest delima I am having is trying to basically guess at what transfer timing will go well with 184-188 duration for exhaust, on my Toomey piped blaster. Keep in mind that I really don't have a means to adjust the transfer angles.
I can follow guidelines for a standard 2 stroke which the blaster isn't unless we are talking 70's tech, I can figure I only want to raise peak power rpm by maybe 500 to 1000. I know I don't want to kill the midrange because that's were my boys spend alot of their time "for now" , because the both get a face full of grin when she starts to crackle and they know it'll rip if they give it to it.
I am not asking for any secrets, not trying to build a bike to compete on the circuits. Just want to know if I am barking up the right tree with around 29 duration to match my 184-188 exhaust dur. target. And maybe a hint as to if I were to spread it across the transfers, if I am headed the right direction?
Took me hours of reading and study, as well as hours of looking, cutting, filing, sanding.....think talk rinse repeat.
My exhaust port has been raised and sits around 89 to 90 degrees atdc giving me 180-182 exhaust duration I plan in running this to 87 or 88 atdc and 184-186 duration
Since I felt like even 90 degrees for exhaust and the stock 119.5 for transfers giving 29.5 degrees of blowdown might be a bit too much. I decided to pull the blowdown in a little . Once I made a short file that allowed me to hand cut the transfers I decided that I might be able to go a little further.
Currently my primary transfers are at 118 atdc giving me 124 transfer duration, with my exhaust port at 90° atdc , the blowdown remains the same as stock at 28.
Higher blowdown numbers leave more time for fuel/air mix to short circuit out the exhsust at lower rpm, and also give more time for exhaust to clear out of the cylinder at higher rpm basically making its part in the symphony prefer higher rpm.
My plan was to move the exhaust to 88 atdc 184 duration or maybe more
116 atdc 124 dur 28 blow dow main transfers and
118 atdc 122 dur 30 blowdown on the secondsries, as well as raise the boost port the same number of degrees as the exhaust port.
Which should put it at 122°for boost port.
I will have to measure to see where I am at on the transfers, and exhaust at the moment , but I am along ways away from 190-195/130 or such so I can still go either way as far as the numbers go.
To me having the mains open a little earlier would seem to be the right idea as they would have good velocity opening first and force mixture toward the back to help scavenging and getting the exhaust out, also staying open a little longer making the last fuel in have a little more velocity as well as being .
The biggest delima I am having is trying to basically guess at what transfer timing will go well with 184-188 duration for exhaust, on my Toomey piped blaster. Keep in mind that I really don't have a means to adjust the transfer angles.
I can follow guidelines for a standard 2 stroke which the blaster isn't unless we are talking 70's tech, I can figure I only want to raise peak power rpm by maybe 500 to 1000. I know I don't want to kill the midrange because that's were my boys spend alot of their time "for now" , because the both get a face full of grin when she starts to crackle and they know it'll rip if they give it to it.
I am not asking for any secrets, not trying to build a bike to compete on the circuits. Just want to know if I am barking up the right tree with around 29 duration to match my 184-188 exhaust dur. target. And maybe a hint as to if I were to spread it across the transfers, if I am headed the right direction?
Took me hours of reading and study, as well as hours of looking, cutting, filing, sanding.....think talk rinse repeat.
Last edited:
