FindMe

JP... yes this is good debate/learning material, but I think we may have got off track. You are speaking of average piston speed, and ursus is measuring the amount of distance traveled by the piston in a given time. It may help if we are going to talk about piston Velocity, if everyone was on the same page and knew the definition of Velocity so we aren't comparing apples to pistons... lol

ve·loc·i·ty
(vEl¼s“¹-t¶) n., pl. ve·loc·i·ties. 1. Abbr. vel. Rapidity or speed of motion; swiftness. 2. Abbr. V Physics. A vector quantity whose magnitude is a body's speed and whose direction is the body's direction of motion. 3.a. The rate of speed of action or occurrence. b. The rate at which money changes hands in an economy.

We all have valid arguements of sorts, but if we are going to talk about the effects of rod length changing the rate of piston Velocity as it is defined, then those same simple physics still will confirm that the average speed, amount of distance travelled in a given time or how far it travels does not apply to anything to do with the Velocity of the piston. While stroking an engine will change the events of the rotating assembly, the reason for adding rod length is not to gain or lose piston speed for cubic inches, but to change the rate and timing of a pistons velocity on a given cubic inch motor. Bigger is always better, but increasing the rod length does nothing to give you more cubic inches or displacement. Simple physics.... it changes the timing of a fixed event in a given engine, taking stress off the rods, side load off the pistons, and adding to the power event BTDC. A 454 is a 454, a 502 is a 502, a 350 is a 350.... simple physics..... ~I think~

RumRunner

When I was talking about piston velocity I was not talking about it's up and down movement rather it's time at TDC, & BDC will change with rod length (given everything else is equal) Plus the longer rod will change your side load on the wrist pins and piston walls

blown1500

RumRunner, your exactly right. My point is, velocity is synnomous with speed, not acceleration or change in velocity and my statement stands.

jpclear

Confusing the issue will not change physics. Average (and over 90% of terminal) velocity of a piston at a given/fixed rpm is completely dependent on the distance of the center of the crank pin from the center of the crank (that would be 1/2 total stroke). We need to end this before we get stupid (which obiviously we are not) and mess up everyone else. Catch you all on another thread cause I don't think we're helping anyone out here anymore . Later --- Jer

FindMe

BINGO RUMRUNNER At least we are on the same page...

THAT is exactly what changes by altering rod length, and we can pull different aspects of a relative event into it and argue it's correct, or this changes that, but nothing anyone but you and myself said can change those properties. And Blown 1500 Im not going to argue with you and what you say may be true, but it has nothing to do with the specifics of changing rod length without changing the stroke,...PERIOD It is a timing event relative to TDC and BDC, and my physics work fine when I don't try to compare things that are not related, and stay on topic..... still some good info came out in this thread, and thanks to all for sharing...

ursus

1st of all I said the piston will travel a greater distance in the bore because it is on a crank with a longer stroke, 4.25 inches vs 4.00" and that the length of the rod is does not matter to this. At 0 degrees the piston pin and hence the piston will be 4" or 4.25 inches away from the point where it was when the crank was at 180 depending on the cranks stroke, period, rod lenth does not affect this in any way shape or form.

2nd, Velocity = distance / time

if you travel , 4.25 inches versus 4" in the same ammount of time, because the rpm is the same, the velocity increases, it does not get any simpler

ursus

Now to answer the original question
1. no you can't run more stroke just because of rod length.

1/2 Stroke + rod lgth + comp height = deck height of block

so if you change stroke you must adjust either rod length or compression height ( the distance between the center of the piston pin and the top of the piston) or both.

2. Although it can no you don't change compression ratio with rod length, if you lengthen the rod say 6.135" to 6.385" and change nothing else the piston would crash 1/4 inch through the head, see part 1. If you change the rod from say 6.385 to 6.135 and change nothing else compresion ratio would indeed be lowered from say 9:1 to 5:1

3. So what does dif length do? a longer rod will be at less of an angle when the crank is at 90 degrees maybe 18 degrees instead of 17 degrees. This will mean less side loading on the piston in the bore which is desirable, less drag=more power and less wear. To use a longer rod the the compresson heght of the piston must be decreased the same ammount though which is not desireable, less land are between rings = less reliability. So there is an optimum compromise for a given deck height.

blown1500

Ursus, you have the right idea. Dwell time at TDC increases as rod/stroke ratio increases also. This is a good thing for performance. Smokey Yunick said to max out everything then put in the longest rod you can use. A boat motor is a puller motor and if you check rod length in industrial applications, they are usually very long, indeed. Short rods do make better response at part throtle, low rpm-not usually a priorty in our world. The dyno and real world has shown me that the longer rod, even if you have to do tricks to the oil ring is worth the effort. Use your own judgement. We like to run 6.8" rods in a 4.25" stroke engine-better power, economy, and longevity-even though most builders seem to prefer 6.385" rods for this application.

ursus

Yes

Using a standard 9.8" deck block and a 4.25 " stroke builders have basically 2 choices, 6.135" rods and 1.52" comp height pistons or 6.385 rods and 1.27" compresion height pistons the short rod is a little steep the geometry is pushed to 19.45 degrees, with 17 or less usually being considered optimal. The 6.385" rod however only leaves 1.27" deck height which is not much room for rings and they sometimes have sealing/reliability problems, this is why when building a 540cid it is preferable to use a tall block, 10.2 or more

A 10.2 inch block allows 6.535" rods a 1.52" comp height and 18.3 degree max rod angle geometry. Using 6.8 " rods in a tall block (10.2") you get a nice 17.4 degree max rod angle geometry but you are back to 1.27" pistons probably not the optimum but it is a compromise eiter way

Mr Gadgets

Ok.. very interesting.. But if you had the choice of building a long rod motor or a stroker motor.. which would make more power or better suit the marine enviroment?
I took things to the extreme and ened up with a 1.88 rod ratio in my BBC... I have not been able to run the combination to it's limits to see if I made the right choice..

FindMe...... concerning the timing issue.. how much timing should I run wiht such a ratio.. are we talking a great deal of reduction? or maybe a few degrees??
That poses another question.. how does everyone determine max timing for a motor combination.. without a dyno.. my boat is my dyno.

Thanks..

Dick