Safe rpm?

Street Ryderz

Well-Known Member
Oct 14, 2016
Torque production is moderated by blowdown angle and percentage. 16-22° is good for a wide output. 22-28° for narrow powerband at the top.16-18% 8-10k range, 18-19.5% really needs a strong pipe but will output torque at 8k-13k. 28°+/ 19.5%+ will need alot of work. High blowdown usability will be effected by anything not being in tune. Pipe, square exhausts ports at the header without taper, transfer port shape/roof. Anything that effects the balance of pressure will be the key here in letting the most amount of mixture to be trapped. Vibration only happens outside of a window of rpm range it was designed for. As long it is balanced and the parts hold any RPM is safe. Stock engine parts honestly 8k+ is bad and the needle bearing is usually not the greatest.
Show me a pipe that carries for 5k ? Scooter pipes have the broadest band width and at best will yeild a 4k pull,dirt bike pipes are more commenly only a 2k width maybe 3k if highly tweaked and that's right at the top.Blow down does not determine where the pipe hit's nor is it a true factor of torque production as you put it.Blow down allows the the exhaust to excape the cylinder before allowing the next fresh charge in and the duration of that period determines how much or little aid the excaping pressure has in pulling that fresh charge.if it's too long there is little to no aid,if it's too short it pulls out more than it should leaving the charge short both of witch will sacrifice torque. Below the pipe where the return is mistimed proper blow down would be very short duration and open later to be optimal but being fixed we have to find the happy medium,this is why dirt bikes use a power valve this gives them varible exhaust timming and there for the blow down timming also changes through out the rpm range that the valve operates in witch retains the best cylinder fill in that range and that's what is making power and torque!