Rod to Stroke ratio

[WDE_BDY]

LeeRoy, if you think about the movement of the crank it always travels up and down the same amount but it also travels sideways by that amount. The longer the rod the less impact the sideways movement has on the pistons travel (remember the crank is moving in a circular fashion) so the piston starts acceleratring sooner (less dwell) so does not need as high a speed to travel the same distance. As has been pointed out above the average speed is the same but the peak is lower.

Callum

[Cameron_Datto]

Much more usable ,workable , Livable set of rod to stroke ratios are in the range of 1.85 down to 1.65 , higher ratios should be reserved for engines that operate at Consistently higher average engine speeds . while the lower ratios work best and are happier at lower engine speeds , and also when the engine must be strong through a broad speed range , the faster the piston velocities are across the top and bottom dead centres , the most significant DE-SENSITIZING effect upon valve timming ,because the effective valve opening and closing points fall within the the range of faster piston velocities,So giving a mothering a effect by being more tolerant and more forgiving to errors in the effective valve timming and hole in the gearbox ratios , A increase in Stroke is also a better De-sensitizer to valve timming than a increase in Piston Size if your going to increase your Displacement,

[YelloRolla]

I am going to risk showing how little I know of the subject here and jump in with a response without having read the reference links. I will say that there is a lot of conflicting info so far in this thread.
By using a longer rod - the piston will dwell at TDC (and therefore BDC) longer. Longer dwell at TDC will improve the engines combustion efficiency and the engine will be less knock prone (allowing lower octane fuels).
If you manage to achieve better combustion efficiency, then the ingition lead time will need to be reduced.

Now if the piston dwells at TDC and BDC longer (millisec/rev) then the time it spends between these points will be less - ie it still has the same stroke to cover but less time to do it in - therefore the acceleration of the piston (average speed/time) will become higher!?

I grant that the longer rod engine will not have the initial piston movement of the short rod and will therefore have less time to suck (poor terminology) on the intake charge. But the longer rod engine will be able to take advantage of higher intake velocities, be able to get away with less cam duration - all pointing to higher low-midrange torque.

[mic*]

First thing to get stuck in your head is that the instantaneous acceleration / decceleration of the piston changes constantly and dramatically.

The "dwell" at TDC and can be a misleading term to use. The crank is moving constantly so the piston is only at zero velocity instantaneously as it changes from upwards motion to downwards (TDC), so doesnt really "dwell" at all. The accel/deccel rate of the piston immediately before or after this instant of zero velocity is what we are calling dwell. A LOWER rate appears to dwell more at the turning point.

A longer rod will do this. A shorter rod will not because (stealing from above) a greater angle occurs between a piston and a short rod as the crank moves laterally away from the perpendicular centre line of the bore (following TDC). This means the piston must be pulled down further by the same travel in the crank.

Most interestingly, the same is NOT true of a short rod at BDC - which has not been mentioned so far. Because more angle has been created between the rod & piston, there are more degrees of arc in the rod from 90deg - 270deg (hope im making sense), which means more up-down motion is lost laterally through this part of rotation, and also that the arc will more closely match the arc of the crank (than does the long rod) and so will have lower instantaneous velocity/accel/deccel than will the longer rod.

[myne]

I've learned a fair bit from an oldskool perfectionist that's been messing with expensive engines since long ago.

Here's what he's said about engine geometry. http://www.theoldone.com/archive/motor-geometry.htm
Here's some more comments http://theoldone.com/archive/crv-b20...trokeratio.htm
And Here's one of the most insane|genius (you decide) buildups I've ever read : http://theoldone.com/archive/pro-sto...ing-engine.htm

Go through the whole archive, most of it disagrees with traditional thinking, so you're naturally going to reject it, but if the man is an idiot, why do people pay hundreds of k for one of his engines?

Worst case, it'll make you think a bit.

[ViPeR_NiPPleX]

The worst example of this has recently been (3 yrs. back) when NASCAR said they were going to place restrictions on compression ratios. I developed "hydralic connecting rods" which would extend in length as oil pressure built up with rpm. NASCAR's rules simply stated that the static compression ratio had to be less than a "certain" number. Now, static means "not running" to me, and my rods, when collapsed, made everything cc out to legal specs. But running, when the rods grew to a maximum of .125" and put the CR back where it "belongs" we made 100 hp more, and we didn't break any rules because they didn't say "running" CR.

That's genius!! Dynamic compression ratio, imagine that coupled with a turbo and computer controlling the oil pressure.... mmm, that coupled with vvtl-i of sorts!

[mic*]

Solenoid valves...

[kingmick]

That's genius!! Dynamic compression ratio, imagine that coupled with a turbo and computer controlling the oil pressure.... mmm, that coupled with vvtl-i of sorts!

racing is working out what you can get away with and or cheating!lmao