Long Rod vs Short Rod
[QUOTE=03whitegsr;10136621]The rod ratio has very little impact on RPM capability with regards to piston/rod forces, "all things being equal." We are talking 50-100 RPM difference at the 10,000 RPM level between the 150mm rod and 156mm rod, it's nothing.
Things aren't equal though and the long rod setup can have a lighter piston which reduces wrist pin forces. While the long rod ends up heavier compared to the short rod, the net effect CAN BE a lighter rotating assembly.
My opinion, but if it is an engine that's going to see actual competition, the best bet is simply using shelf-stock parts so that when you blow them up, you can replace them with little down time. If you start buying rods and pistons in non-standard sizes, you have the potential to waste an entire season waiting for new parts. If that's not a concern, then you might as well optimize if you are willing to spend the extra.
It does seem the longer dwell time at TDC can cause problems on low-octane fuels though...[/QUOTE]
I agree the change is far to small to sweat it. keeping it simple and using mass produced parts is far better idea.
Things aren't equal though and the long rod setup can have a lighter piston which reduces wrist pin forces. While the long rod ends up heavier compared to the short rod, the net effect CAN BE a lighter rotating assembly.
My opinion, but if it is an engine that's going to see actual competition, the best bet is simply using shelf-stock parts so that when you blow them up, you can replace them with little down time. If you start buying rods and pistons in non-standard sizes, you have the potential to waste an entire season waiting for new parts. If that's not a concern, then you might as well optimize if you are willing to spend the extra.
It does seem the longer dwell time at TDC can cause problems on low-octane fuels though...[/QUOTE]
I agree the change is far to small to sweat it. keeping it simple and using mass produced parts is far better idea.
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The rod ratio has very little impact on RPM capability with regards to piston/rod forces, "all things being equal." We are talking 50-100 RPM difference at the 10,000 RPM level between the 150mm rod and 156mm rod, it's nothing.
Things aren't equal though and the long rod setup can have a lighter piston which reduces wrist pin forces. While the long rod ends up heavier compared to the short rod, the net effect CAN BE a lighter rotating assembly.
My opinion, but if it is an engine that's going to see actual competition, the best bet is simply using shelf-stock parts so that when you blow them up, you can replace them with little down time. If you start buying rods and pistons in non-standard sizes, you have the potential to waste an entire season waiting for new parts. If that's not a concern, then you might as well optimize if you are willing to spend the extra.
It does seem the longer dwell time at TDC can cause problems on low-octane fuels though...
Things aren't equal though and the long rod setup can have a lighter piston which reduces wrist pin forces. While the long rod ends up heavier compared to the short rod, the net effect CAN BE a lighter rotating assembly.
My opinion, but if it is an engine that's going to see actual competition, the best bet is simply using shelf-stock parts so that when you blow them up, you can replace them with little down time. If you start buying rods and pistons in non-standard sizes, you have the potential to waste an entire season waiting for new parts. If that's not a concern, then you might as well optimize if you are willing to spend the extra.
It does seem the longer dwell time at TDC can cause problems on low-octane fuels though...
So by "cause problems" I presume you mean detonation, what are the chances of this on 98 octane. Will a good tune more or less eliminate detonation or just reduce the probability of it happening. So from what I gather.
The pros of the long rod setup are:
-better load and wear on bearings and piston side wall
-reduced engine vibration (harmonics)
-maybe better VE due to longer dwell time at TDC
-Increased rev capability with greater reliability vs a standard rod at the same RPM. [quote] John Bradly.
Cons:
-may cause detonation on lower octane fuels due to longer dwell time at TDC
-down time due to not being of the shelf parts if something fails
Any one care to add.
By "problem" I mean the added dwell can require you to use less timing advance to avoid detonation. Less timing will lead to less burn time and lower average cylinder pressures during the power stroke.
As always, it is a balance between one benefit and another.
I don't think the difference between 150mm and 156mm is enough to matter though on a 88mm stroke and even less so on a 100mm stroke. Keep in mind, those Honda B-series motors all have like 1.6:1 rod stroke ratios and still manage to rev to 11,000 RPM and make power there.
As always, it is a balance between one benefit and another.
I don't think the difference between 150mm and 156mm is enough to matter though on a 88mm stroke and even less so on a 100mm stroke. Keep in mind, those Honda B-series motors all have like 1.6:1 rod stroke ratios and still manage to rev to 11,000 RPM and make power there.
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By "problem" I mean the added dwell can require you to use less timing advance to avoid detonation. Less timing will lead to less burn time and lower average cylinder pressures during the power stroke.
As always, it is a balance between one benefit and another.
I don't think the difference between 150mm and 156mm is enough to matter though on a 88mm stroke and even less so on a 100mm stroke. Keep in mind, those Honda B-series motors all have like 1.6:1 rod stroke ratios and still manage to rev to 11,000 RPM and make power there.
As always, it is a balance between one benefit and another.
I don't think the difference between 150mm and 156mm is enough to matter though on a 88mm stroke and even less so on a 100mm stroke. Keep in mind, those Honda B-series motors all have like 1.6:1 rod stroke ratios and still manage to rev to 11,000 RPM and make power there.
