my boost port numbers

here is my boost port specs....... did i screw my head up?

all numbers in inches

I used a .75 ball nosed end mill in a manual machining center we have at work

one port, .710 wide
1.725 long (.112 higher than the stock side ports)

raise the exhaust port .08

so what do you guys think?

well I took it for a spin today........I dont have a tach, and Im still running stock springs, so i would hold the brake and let the clutch get hot to let it slip more, and it still doesnt slip like I need it to......so im gonna have to wait untill my 1.5's from bat get here( thanks chris). but it does rev harder than it did.......think im gonna be limited by my stock reeds now though......

so far my mods are as follows,
9 fins removed from flywheel,
3 inches revmoved from header and added 3 to chamber
and now port work above.....
will give a better review tomrow when I take it to a big open area and let it rev till she revs no more......

i don,t think much of using a ball end tip when maching a cylinder port, it is wise to use a tree shaped tip as the flow will work much better.

Sorry, I’m in metric. Let me just convert your numbers...

Quote:

Originally Posted by Nashport_Native all numbers in inches

You used a 19mm Ballnose

Port = 18mm wide and is 2.8mm higher than the transfers.

And you raised the exhaust 2mm.


I think the exhaust will be much better now, regarding timing.

Boost port: the roof angle is ok if maybe a bit steep and the width is good, so shape wise its good.
BUT its duration if waaaay to long compared to the transfers. On this engine a 2 degree time (normal) difference in boost and transfer equates to about 0.4mm, so you’re way past that. Must have more like a 15 degree difference. The raised exhaust will help towards correcting the blowdown time though. It’s probably loosing a lot of good fresh mixture out the exhaust.
Really I’d say it has very bad flow characteristics now. You say it still works??

so what #'s do you recommend for a good functioning boost port, i want to put one in before I re-assemble with my bbk/fcsc but I'm looking for some trusted #'s on the design of it.

I can't give any tried and tested numbers as my engine is still a work in progress.
But make it the same height as the transfers, as wide as you can and still fit betweeen ring gaps of the piston, by 2mm into the cylinder wall - anymore and you risk breaking through.
And also put a window in the piston to get good flow up the port Joba good'n