Rod ratio and dwell time... doesn't make sense

[ptuomov]

About dwell this : taking it to extremes and if the above discussion about not sufficient burn time for a fast revving engine, dwell time on TDC could indeed be beneficial.

Rod length does not affect TDC dwell time in any meaningful way. It does impact maximum piston speed.

And piston acceleration, if the piston hits the crankshaft counterweight because of the too-short rod.

[MadBill]

..if the piston hits the crankshaft counterweight because of the too-short rod.

Was looking the other day inside a 8200 RPM 8.2" deck 380" SBF that went together with ~ 0.010" counterweight to pin boss clearance. Several of the latter showed pencil eraser-sized shiny spots...

[ptuomov]

..if the piston hits the crankshaft counterweight because of the too-short rod.

Was looking the other day inside a 8200 RPM 8.2" deck 380" SBF that went together with ~ 0.010" counterweight to pin boss clearance. Several of the latter showed pencil eraser-sized shiny spots...

Crankshaft scraper system integrated to the piston, didn’t see that one in the Mahle piston book!

[MadBill]

[cjperformance]

..if the piston hits the crankshaft counterweight because of the too-short rod.

Was looking the other day inside a 8200 RPM 8.2" deck 380" SBF that went together with ~ 0.010" counterweight to pin boss clearance. Several of the latter showed pencil eraser-sized shiny spots...

A dab of valve lapping paste on each counterweight should clear that up!

[MadBill]

Since it's been briefly turned as high as 8,500 a few times over five seasons, I figure it has self-clearanced by now...

[cjperformance]

Since it's been briefly turned as high as 8,500 a few times over five seasons, I figure it has self-clearanced by now...

Probably helps prevent excess rod compression at BDC !

[kimosabi]

Longer rods are more of a mechanical and angular benefit than just power. I'd go as far as saying piston sidethrust decrease for engine longevity is also a factor. You also get a lighter piston.

Power output is not all that makes a happy engine.

Have you calculated the change in angle ? You'd be surprised how little difference in makes as often you normally have a relatively small window that the rod length cAn fall into due to other constraints. It's fine to look at extreme differences but that's accademic in most cases

It was alluded to earlier but increased stroke more drastically negatively influences the things than a lower rod ratio. It so happens that almost always increased stroke brings lower rod ratio. most have trouble differentiating between the two.

Also the temperature and rpm has more effect on piston volumes at and around tdc due to change in rod length when you're talking under 1/2".

As another example on minute variation, the actual difference between a 3.75 stroke versus a 3.5 stroke. It's very little but have big differences in potential.

Engine building is about all the small things. A longer rod does have benefits but as always if the piston gets too short obviously you have to drop that plan.

[kimosabi]

Longer rods are more of a mechanical and angular benefit than just power. I'd go as far as saying piston sidethrust decrease for engine longevity is also a factor. You also get a lighter piston. Power output is not all that makes a happy engine.

Ok here’s a question. Suppose that the stroke is given and the deck height is given. Suppose further that the compression ratio requires a small shallow dish in there. What rod length and compression height should I pick if I want to maximize high rpm and high gas load reliability, understanding that a great many considerations are going into it.

For practical purposes, the 1980’s factory stock solution ended up with 56mm compression height, 150mm rod length with 78.9mm stroke and 100mm bore. What would be today’s solution with new piston technology and higher than stock rpms but with the same bore and stroke?

If you know you need high gas load reliability my opinion is the obvious answer should be go for the best piston height and stability. You can play with rod lengths and all that but if you have a rign pack stacked high you're not gonna take alot of gas load. I would compromise on whatever rod fits on a high gas load high power deal.

[digger]

Longer rods are more of a mechanical and angular benefit than just power. I'd go as far as saying piston sidethrust decrease for engine longevity is also a factor. You also get a lighter piston.

Power output is not all that makes a happy engine.

Have you calculated the change in angle ? You'd be surprised how little difference in makes as often you normally have a relatively small window that the rod length cAn fall into due to other constraints. It's fine to look at extreme differences but that's accademic in most cases

It was alluded to earlier but increased stroke more drastically negatively influences the things than a lower rod ratio. It so happens that almost always increased stroke brings lower rod ratio. most have trouble differentiating between the two.

Also the temperature and rpm has more effect on piston volumes at and around tdc due to change in rod length when you're talking under 1/2".

As another example on minute variation, the actual difference between a 3.75 stroke versus a 3.5 stroke. It's very little but have big differences in potential.

Engine building is about all the small things. A longer rod does have benefits but as always if the piston gets too short obviously you have to drop that plan.

1/4" stroke is not what I'd call a small change.

It's not about certain benefits it's about the nett benefits. Some perceived benefits may be real but there are negatives. An engine is a bunch of compromises it's about finding the right balance

[ptuomov]

Longer rods are more of a mechanical and angular benefit than just power. I'd go as far as saying piston sidethrust decrease for engine longevity is also a factor. You also get a lighter piston. Power output is not all that makes a happy engine.

Ok here’s a question. Suppose that the stroke is given and the deck height is given. Suppose further that the compression ratio requires a small shallow dish in there. What rod length and compression height should I pick if I want to maximize high rpm and high gas load reliability, understanding that a great many considerations are going into it.

For practical purposes, the 1980’s factory stock solution ended up with 56mm compression height, 150mm rod length with 78.9mm stroke and 100mm bore. What would be today’s solution with new piston technology and higher than stock rpms but with the same bore and stroke?

If you know you need high gas load reliability my opinion is the obvious answer should be go for the best piston height and stability. You can play with rod lengths and all that but if you have a rign pack stacked high you're not gonna take alot of gas load. I would compromise on whatever rod fits on a high gas load high power deal.

Why do you say high top ring placement means low stability? By my thinking the top land does nothing to stability. Right? It has to be about the ring widths, distance between the top and second ring, vertical distance between the first and second ring relative to the wrist pin bore center, and then skirt length.

Suppose that the top ring land width is given by the temperature limit of the top ring. And suppose that there’s a limit on the total piston height because of the counterweights and the deck height. To maximize durability under very high gas loads, where do you place the wristpin between the skirt bottom and the top ring groove? My guess would be somewhere about in the middle between the skirt midpoint and first ring land (between first and second ring) midpoint, but that’s just a guess. Does someone know the answer?

[kimosabi]

Have you calculated the change in angle ? You'd be surprised how little difference in makes as often you normally have a relatively small window that the rod length cAn fall into due to other constraints. It's fine to look at extreme differences but that's accademic in most cases

It was alluded to earlier but increased stroke more drastically negatively influences the things than a lower rod ratio. It so happens that almost always increased stroke brings lower rod ratio. most have trouble differentiating between the two.

Also the temperature and rpm has more effect on piston volumes at and around tdc due to change in rod length when you're talking under 1/2".

As another example on minute variation, the actual difference between a 3.75 stroke versus a 3.5 stroke. It's very little but have big differences in potential.

Engine building is about all the small things. A longer rod does have benefits but as always if the piston gets too short obviously you have to drop that plan.

1/4" stroke is not what I'd call a small change.

It's not about certain benefits it's about the nett benefits. Some perceived benefits may be real but there are negatives. An engine is a bunch of compromises it's about finding the right balance

It is a lot potentially for power and wear, but in real measurement it's not. Just like a longer rod, say 5.7 to 6" is not a lot as to measured angle to crank pin, but for sidethrust it makes a difference. You're not looking at drastic changes, like you say you're looking at net benefit. Rod ratio should be part of that in my honest opinion.