12:1 @83mm vs 10.5:1 @85mm????

[atl3128]

Which one would produce the most torque a 12:1 std bore piston or a 85mm 10.5:1 piston? How would a megacycle .450 cam work with a 12:1 83mm or 10.5:1 85mm piston as far as low/mid range torque? Which would be the best combo?

Thanks

 

[jasonp]

The .450 is more of a mid to top end cam.. you loose quite a bit on the bottom.
The difference in bore sizes are that you do gain a little compression on the 85mm bore. Maybe a 1.5 hp, not really notable. With a 12:1 you will have more torque but have to run race gas. The JE piston can be made to have 11:1 pretty easy if not more and same goes for the 12:1.
If your going for all out torque then get a stroker, or go with 85mm 12:1, close the deck down and then run a megacycle mid range cam.

 

[atl3128]

The 12:1 piston is one that my step dad had in his warrior I beleive it is a JE piston he only runs 93 in it and it runs fine he said that on really hot days he uses some Octane booster to get rid of the ping. I have ridden the bike and it runs really strong i did not hear any pinging. I have always heard that 11:1 is the most you can run on air cooled engines before race gas. How is this possible?

 

[jasonp]

The 12:1 piston is one that my step dad had in his warrior I beleive it is a JE piston he only runs 93 in it and it runs fine he said that on really hot days he uses some Octane booster to get rid of the ping. I have ridden the bike and it runs really strong i did not hear any pinging. I have always heard that 11:1 is the most you can run on air cooled engines before race gas. How is this possible?

running the engine rich, jetting being off. Being higher up in altitude, different quality of gas. Running a big cam all stuff that is considered.
Personally after running mine around in the woods there is really no way I'd want to run any higher compression, the engine just runs really hot. (IMO)

 

[griff]

Which one would produce the most torque a 12:1 std bore piston or a 85mm 10.5:1 piston? How would a megacycle .450 cam work with a 12:1 83mm or 10.5:1 85mm piston as far as low/mid range torque? Which would be the best combo?

Thanks

Well, with IC theory, and stroke and cams being equal, the 10.5CR +2mm should yield more torque over a broader range.


As far as cams go, Jasons the man on whats what for the 'ole 350.

 

[dave76]

I don't think it really matters what static compression is, as we've descussed in detail it really comes down to cranking compression, and my quad runs great on 93 with 230 PSI of cranking compression. IMO the 12:1 piston somwhat compramizes a bit of power due to a large dome that would interfere with the flame front. Fellas???

 

[griff]

I don't think it really matters what static compression is, as we've descussed in detail it really comes down to cranking compression, and my quad runs great on 93 with 230 PSI of cranking compression. IMO the 12:1 piston somwhat compramizes a bit of power due to a large dome that would interfere with the flame front. Fellas???

Static compression always matters... thats where your baseline is established, and is typically used for determining the timing advance/retard envelope. From there, cranking compression can tell you a variety of other things, as you already know.

The large piston dome will have an effect on the flame propagation, heat transfer, and the overall process. However, it doesnt mean its a bad thing (it can be of course). Theres too many other factors - valve angle/location, shape and volume the head chamber, squish band, etc etc.

The entire combustion geometry must be taken into account. We'd need access to software that can do geometric modeling and simulation for combustion chambers, to see what changes have what specific effect. It could have a marked improvement, it could drop efficiency into the ground, or it could have no real effect at all.

 

[jasonp]

I don't think it really matters what static compression is, as we've descussed in detail it really comes down to cranking compression, and my quad runs great on 93 with 230 PSI of cranking compression. IMO the 12:1 piston somwhat compramizes a bit of power due to a large dome that would interfere with the flame front. Fellas???

Static compression and dynamic or cranking compression are good to know for engine building as far as making the most power for pump gas as other things as well. Basically the limit is right there at 220 psi for after that you starting to run right to the limit on what you can run for fuel as detonation will start becoming a factor. This being a air cooled engine the limit should come down as heat becomes a huge factor as there is no real way to get rid of it. There is really no real way to say at 220 psi this engine will no longer run on pump gas as there is so many factors involved... like where you live, quality of fuel, the way the engine was built, even casting marks in the head will have effects on this. But we can say right around there from what I've read is a good point to be at.. There is fact though that piston dome height over .100" you will start to run into combustion dynamics that will effect HP. For this small of an engine you most likely will not be able to tell a difference but when you are trying to get everything out of it you can then it is a little different ball game..