"race spec(?)" tuning advice needed(timings...cento)
Posted: Wed Dec 19, 2012 3:50 am
Hi all! I'm building an engine for a cento(perhaps that's my first mistake!! 
) and i'm a bit stumped on which way to go. I have a ton of scooterboy friends, but no one ever seems to know what the f*ck i'm talking about, so i'm hoping someone here may be able to help!
I've already bored the barrel to 56.4 to accept a tzr piston. Head reprofiled to suit. Crank was trued and tig'd. I plan on building it this weekend, but not sure on the timings i should start with. Everyone says "4 speed gearbox", but they're harder to come by and out of principle and simplicity, i wanna stay with the 3 speed box. Tony cassidy's visualizer shows some ok top speeds if i can get it to rev!
Right off the start, the piston on untouched ports gives an intake timing of 170. Transfers show at 112, and exhaust 135. That's 11 degrees of blowdown. I plan on starting with a mikuni vm24, and this spring i'll build 4 expansion chambers for it....one designed for 5,000rpm, 6k, 7k, and 8k. For old school road racing, i don't plan on going reed valve unless i have to. I really want to stay piston ported. Perhaps 2 or 3 boost ports depending on what i can fit in there.
My question is exhaust timing. 135 is short. I don't want to wreck the barrel by going too far, so i plan on grinding out the piston for experiments. 135 is short though! 11 degree blowdown won't let it rev very far. I feel i should do some port opening on the barrel first. Maybe start at 150 degrees? That gives 19 blowdown. Is 150 too high of a starting point for what i'm building ya think? And how is(should) that 170 intake effect the rest of the engine? What would you do?
) and i'm a bit stumped on which way to go. I have a ton of scooterboy friends, but no one ever seems to know what the f*ck i'm talking about, so i'm hoping someone here may be able to help!I've already bored the barrel to 56.4 to accept a tzr piston. Head reprofiled to suit. Crank was trued and tig'd. I plan on building it this weekend, but not sure on the timings i should start with. Everyone says "4 speed gearbox", but they're harder to come by and out of principle and simplicity, i wanna stay with the 3 speed box. Tony cassidy's visualizer shows some ok top speeds if i can get it to rev!
Right off the start, the piston on untouched ports gives an intake timing of 170. Transfers show at 112, and exhaust 135. That's 11 degrees of blowdown. I plan on starting with a mikuni vm24, and this spring i'll build 4 expansion chambers for it....one designed for 5,000rpm, 6k, 7k, and 8k. For old school road racing, i don't plan on going reed valve unless i have to. I really want to stay piston ported. Perhaps 2 or 3 boost ports depending on what i can fit in there.
My question is exhaust timing. 135 is short. I don't want to wreck the barrel by going too far, so i plan on grinding out the piston for experiments. 135 is short though! 11 degree blowdown won't let it rev very far. I feel i should do some port opening on the barrel first. Maybe start at 150 degrees? That gives 19 blowdown. Is 150 too high of a starting point for what i'm building ya think? And how is(should) that 170 intake effect the rest of the engine? What would you do?
crank and gearbox wise, you cant use a 4 speed box with the cento crank you will have to find a starstream crank and gear box. there are front sprockets availble now to make things better. take a look on the