Im just about ready to get the 720 kit together and OMR and I chatted, I liked his ideals and asked if I could post his answer, he said yes! but only to real Answers and Logical dicussions on low range torque.
Velocity, flame travel and combustion chamber flow would be good technical information.


OMR:
The mathimatical difference between 11-1 and 13-1 is 4%.
Thus with your motor being roughly in the range of 60 - 65 HP, the increase would be 2.5 HP.
I have not seen big numbers with the pentroof head design. with high compression. unless you are running big duration cams. High domed pistons cause air flow turbulence and the only way to get around it is with big cams. Its the hot setup for drags but not for hot trails or duning.
Cylinder pressure comes from compression and ignition advance. I would run a lower compression by lowering the 13-1 piston by .040". This is done by adding 2 more base gaskets(total then 3).
13-1 piston has a dome size volume of 21cc. By lowering the piston .04", your new compression will be 11.5-1. With better flow the HP should be the same as 13-1.
To increase bottom, buy a Big Gun cdi that has 2 more degree's of advance and it is added at the beginning. Thus you would gain back bottom cylinder pressure.
Cams WEB:Custom Grind <-been asked not to list it.

 

What cam profile is that?

 

flame travel is important, and I agree that everything must work together in order for it to work. Drop the jug on a large domed piston and use some mild cams and the result is pretty much worthless (in my opinion). You'll tend to have a bunch on the bottom but absolutely no run-out at all. Cam's, PORT DESIGN/porting AND valves are huge when it comes to high compression. Add the wrong fuel in the mix and your also losing ground. It all works together....

With so many cam choices out there, I think the solid money is on sticking with a builder that has a package. Not so much a package deal, but a parts package that they know works, has been tested, and has proven it's self in the longevity arena. If you are spec'ing your own cams, just make sure you are using someone who knows what they are doing. NO CAM is perfect, and no PORTING is perfect... Just a lot that are better or worse than others.

 

STOCK CAM SPECS
0.389"lift
236deg. duration

 

That's an interesting cam grind.

 

Yeah Im very intrigued when I sat down with Steve and Lori at Web for the cam design, but again Im not racing this motor, and im of the economy sized rider and was looking for plain and simple torque grunt, but with what im reading i may drop the compression of the piston to 11 to 1 instead of the 13 to 1.

 

I may be outside the box as far as conventional wisdom is concerned but I don't think you can simply quantify the performance advantages of higher compression by using a percentage to determine the hp increase.

The higher compression and proper ignition advance will make more cylinder pressure and it kicks the piston over TDC much quicker and harder which means more force is applied to turning the crank shaft which means the motor is making more torque.

Again, hp is just a mathmatical equation based on Torque and RPM. A percentage unit in measuring the benefits of increased compression might get you close on the torque output increase but there are so many other variables that play into holding that curve out and what hp results from just the compression and how hp is effected by the changes the increased compression has on the efficiency of the other components.........

It also doesn't tell you about RPM acceleration. That is the main reason you want high compression in a race motor. The rev quicker. You can take a 80 hp motor that revs 20 percent quicker and compete with 96 hp motors.......all other variable being equal.

If I was only building a dune bike, I would run 11.5:1 compression in a big bore. Sounds like that is exactly what you are doing....................[img]i/expressions/face-icon-small-wink.gif[/img]

One other thing that wasen't mentioned is the huge effect porting has on cylinder pressure. After the piston kicks over TDC and is moving down on the intake stroke, porting greatly effects VE which will effect cylinder pressure. You can have the same compression piston but the motor that sucks more air/fuel into the motor on the intake stroke will have more cylinder pressure from BDC to ignition and from ignition to TDC.

Cams can ruin cylinder pressure if not matched to timing and fuel burn rate even if they have the lift to handle the volume of air your porting is affording. If they start to open the valves a little too early and bleed off cylinder pressure, all the VE and compression in the world won't help you when it is time to make power. This is why I don't like 109's..................compared to some of the newer technologies in cam grinds specifically for the DS motor.......

Great topic! Thanks for letting me share my $0.02[img]i/expressions/face-icon-small-smile.gif[/img]

 

Is that at all rpms or only at low rpm?

 

Which part are you referring to?

 

The bleeding off of cylinder pressure.