ursus

Piston velocity is mainly dependant on stroke not rod length. Using the formula I have tabelized velocities for dif stroke and rod lengths at dif crank angles. note at max velocity increasing stroke from 4" to 4.25 " increases piston velocity by 6 feet per second while increasing rod length from 6.135 to 6.7 using the same stroke only changes max velocity by 0.6 fps, the difference is even les at 5 degrees of rotation close to TDC. Wear is caused largely by increased rod angle and side force, that being said a longer rod is indeed generally preferable

mcollinstn

ursus,

it would be cool to see a graph or chart of piston ACCELERATION and DECELERATION using the combinations on your chart. I can't help but think that the short rod/long rod difference in decel/accel would be significant. Obviously, max accel and max decel will occur at a different crank rotation for a long rod vs short rod motor - that's why a graph would be cool. Whaddaya think?

My calculus is a little rusty. See what you can come up with.

ursus

I'll give it a try next time I can't sleep at 4am but an initial look tells me acceleration dif is not that great either, eg for a 4" crank 6.135 rod between 1 and 11 degrees the piston accelerates from 2 fps to 22 fps for a total of 20fps in 10/360 of a revolution at 5000 rpm about 60000f/s2.

The 6.7 inch rod it accelerates from 1.98 to 20.5 fps in the same time so the acceleration is 19.5 fps in 10/360 , about 58500 f/s2

This is just avg accel of the 1st 10 or so degrees of crank otation not instantaneous but the instantaneous will be of a similar percentage diference, 60000 vs 58500

P.S. sorry to those of you for one of those more math than you can stomach posts, but sometimes you have to do the math to see what is true vs myth,

eg

myth: there is a big dif in piston velocity on a long versus short rod

fact: there is a big dif in piston velocity on a long versus short stroke

myth: a short rod can increase piston velocity so much it can outrun the flame front.

fact: the flame front traveling at close to the speed of sound 700+fps is never in danger of being out run by the piston which has peak velocities in the 100fps range and as low as 2.02 or 1.98 fps close to TDC long vs short rod

Dave F

Ursus,

In general I agree with what you're saying. I should have concentrated more on the angle thing. I simply gave my opinion to the max for practicallity, and now I will add that I agree with the group that for longevity, available choices in combinations and those type of things, 4.25 is the BEST choice.

As has been said, a 4.5 stroke can be put in a short deck, however, the pivital word I'd like to use is "practical".
To put it simply, angles and piston thrust are not in a good position when trying to do this.
It has also been said that those combos do not seem to hold up to the test of endurance, and I agree, for the above reason.

My opologies to the group for not giving a full explanation in my first post.
Thanks for the further explanation Ursus, and others

DAVE

Dave F

what the hell is an opology anyway?

I'm starting to type like Puder