Athena 68mm porting

[Ollies930]

Before this turns into an Athena bashing thread, I do understand that CPI cylinders are miles ahead of the Athena cylinders. However, I am trying to build a competitive vintage race bike (RZ350) and monoblock cylinders are not allowed. Therefore I am stuck with stock sleeved or Athena cylinders. I chose to go with the Athena 68mm kit due to the triple exhaust ports and the Nikasil plating.

The rules allow up to 450cc so I am using a 7mm stroker crank to get me to 443cc. When I ported the cylinders, I found that where the ports already were, I pretty much just opened the ports up downward to where the piston sits at BTC and with the piston popping out 2.5mm at TDC the intake ended up almost 131 degrees and with opening the center exhaust upward 1mm just so it did not expose the secondary exhaust ports at the same time I ended up with 198 degrees.

With these numbers in mind and not wanting to pop the pistons out of the bores any more than that, there was no shaping of the radii on the top of the intake ports. From the factory they appear to be nearly 90 degree turn into the cylinders, but there is a very sharp turn in on the large radius.

Would there be any benefit to put epoxy on the long side radius to help the air turn into the cylinder, or is there no measurable benefit to be had?

[trickedcarbine]

Weld the exterior for more meat

[registered user]

From the factory they appear to be nearly 90 degree turn into the cylinders, but there is a very sharp turn in on the large radius.

Would there be any benefit to put epoxy on the long side radius to help the air turn into the cylinder, or is there no measurable benefit to be had?

i dont understand what your trying to say here. can you explain it better or show some kind of pic what your describing ?

[trickedcarbine]

Think he's referring to the hard 90 they make in the transfers before the flow gets to the window and enters the cyl.

[Ollies930]

Thanks trickedcarbine, that is exactly what I was trying to explain.

As for welding, with the amount of filler needed to make a decent shaped port, I am afraid it would completely distort the bore. Am I incorrect?

[registered user]

ok i see what hes saying. transfers go straight up and turn sharp into the cylinder. no there aint a whole lot you can do about it. cylinders are to close together to do much. depending what upward angle your wanting them to enter the cylinder at, you could probly use some epoxy but i dont think it will make a hell of alot of difference because the lower portion of the tunell is still going to be mostly straight up but ya with some clever epoxy work you could probly lessen the sharp turn into the cylinder. this is idealy what you would want but unfortunatly with the cylinders so close together youll never get even close to a large sweeping radius like this

 

[trickedcarbine]

Thing is, it's probably not worth it to chase it down. Epoxy would definitely let you shape the port, but at the expense of volume. Just have to think, is changing the shape gonna help enough to make up for reduced transfer volume?

[registered user]

with the amount of filler needed to make a decent shaped port

you wont be able to make a decent shaped tunell no matter what you do. spread the cylinders a few inches then you might be able to do something worth while

[sleeper06]

You bought a 4 mil kit and are putting a 7 mil in just make sure you check your degree your transfer duration if untouched in a bone stock cylinder they can get pretty close to 130* make sure you adjust your blow down numbers properly so it will effectively pull gear and string out nicely

[Ollies930]

The kit I bought is actually for a stock stroke, as far as I know there is no other Athena kit available. For the port timing to be reasonable, the pistons have to stick out 2.5mm above bore. That way I ended up with 130.5-131 on the intake transfers.