Chamber Improvements For This Head aka. Where to go from here?

[Claviger]

Thank you all in advance. Recently I was forced to tear down a healthy motor due to a timing chain that exited the premises at only 15,000 miles of use! That's been fixed and hopefully resolved as a potential failure point.

I am looking to improve power on this project.
4" bore
3.71" stroke
8500 RPM redline
40mm intake valves
33mm exhaust valves
Motorcycle engine

currently making 1.56 lb-ft/ci and 126hp/liter

This is with a straight 45 degree valve seat single angle and the chambers seen below.
- No room for bigger cams.
- Intake length is well tuned and provides about 5lb-ft improvement over 1" long velocity stacks across the entire torque curve with an identical peak power.
- Exhaust is well tuned to provide effective scavenging.
- Dry Sump.
- Crank case is scavenged to a negative pressure using the exhaust.
Aside from cleaning up the casting flash in the chamber and putting a 60 degree cut behind the 45, what would you suggest to improve flow?

Gentle cleaning done only.

Chamber Pics.jpg

Sorry for the crappy piston pics.

Pistons.jpg

New Dyno.jpg

[ptuomov]

What fuel?

[Claviger]

Oh sorry, I left some useful info out.

92 pump
12.5 static compression
Intake Cam at .04" lift at 66 degrees ABDC
21 degrees timing at full throttle from 7000 onward, more added mothing, 24 degrees didn't produce any knock so set at 21 and left the margin.

[David Redszus]

I would leave the valves alone and pay attention to port and inlet runners to smooth out the lumpy torque curve
and raise rpm.

And work on mixture tuning. A better fuel would also help.

[Claviger]

Thanks for the reply David, appreciate the thoughts.

Definitely will have to see what can be done in the runners beyond what's already been done. I know the bowl can benefit from a little smoothing, there's a tiny lip, but still a lip on the back side of the seat where it meets the bowl on the short side.

Regarding RPM and Fueling, we tried leaner and richer mixes, the engine likes being just under 13:1, either direction was a fall off, so we went with what the engine wanted.

[bill jones]

---how did you determine the sparkplug ground strap orientations?

[Claviger]

No effort was put into their orientation, they are facing the direction they end up at when torqued properly.

I had thought to orient them in a particular manner but was discouraged and told it makes no difference in a dual plug head.

If that's something that will likely make a difference I'll happily invest the little bit of time to do so.

[gunt]

Not sure based on the graph why the suggestion of more rpm came , it is clearly falling if you cant move timing , make it so , the hums in the center i'd have them sanded flush towards the intake side if not both , plugs would have to be tested but might be worth the effort , but i'd say it would have to show something ,
work a lot on the throttles