Just a thought!

Degrees are degrees regardless of stroke. A longer stroke(or a bigger bore) creates a stronger signal in the intake tract and therefore greater velocity in the ports. This causes port saturation at a lower rpm level and drives the HP and torque peaks to a lower rpm point.
I am far from an expert on this issue there are members that could explain this much better. But I thought I'd give it a shot.

 

I think that this topic has been touched on before, but anyhow.... I'm thinking that a stroker motor (4.5 for instance) will be pulling down in the cyl. longer than a 3.85 stroke? right? So the same cam from 460 put into a 545 would possibly not use the increase in time to BDC? This is why a high rpm cam for a 460 would make the same power at a lower rpm in a 545?

I think Im on the right track here, but please enlighten me if I have any misconceptions.

Thanks guys

Casey

You are on the right track...

For every 50 cubic inches you add in displacement you need to add about 12 to 16 degrees of additional intake duration in order to maintain the same rpm peaks for HP and torque. Has to do with intake valve closing point relative to the entire length of the stroke.

If you utilise the same set of cylinder heads on the larger engine you should also increase overlap to help fill the cylinders better via a tighter lobe sep until you hit duration at .050" lift numbers above 270 degrees.

It is all about time vs total port flow. If you have a given amount of port capability you will need more time to fill the additional displacement and you achieve this with added duration and overlap.

This is a simplification but hopefully helpful...



8)

 

Not to go off on a tangent, but I recently came across something relating to varying rod length (for a given stroke) and it's affect on engine dynamics, saying: "Another consideration is rod length and it's effect on the engine's output. Changing the rod length will affect the piston location in the bore relative to cam timing everywhere except at TDC and BDC thus changing the dynamics of the engine." I hadn't really thought of that before, and I can't really understand it! Seems like that's all made up for in compression height?

 

Oh no, not the rod length thing again! :lol: While it's true that rod length does affect dwell times and piston speeds, it's soooo minor that it makes no tangible difference in a typical engine. The easiest way to think of it is in extremes. When you draw it out it becomes clear. The long rod will put the piston near the top and bottom of stroke for more crankshaft rotational degrees than a short one. However, the piston moves faster near TDC than BDC. The reason is that when approaching TDC, the big end of the rod is not only moving up with the crank journal, but also moving toward the cylinder centerline. As it goes past TDC, the rod is not only being pulled down, but also away from the centerline. Approaching BDC, the big end of the rod is moving down, but toward the centerline of the cylinder. This slows the piston down. After BDC the crank is pushing the rod up, but the big end is swinging away from centerline. That also relatively slows the piston. The longer the rod, the more this acceleration difference is amplified. BUT, its effect in an engine of normal proportions is VERY minor when comparing say a 6.605" rod to a 6.8" rod.

Now, to really cook your noodle, a long rod will increase BOTH bdc and tdc dwell times in comparison to a short rod. If there was no rod at all, the piston speed would be constant throughout the crank revolutions (of course that's impossible to build an engine of that sort). EDIT: Impossible to build a typical four stroke engine as we know it with that design.

 

It does "seem" like the difference in varing rod lengths is small, but think about how many actual explosions (power strokes) you would have in an 1/8 mile pass at say 5000 rpm's and 7500 rpm's. I think the more explosions you have in that 1/8 mile the faster the car will go, whether its from increased cam duration or a lever advantage from a longer rod, its all about the combo and how it works together. You really don't have that much time to make more explosions and it gets less for every tenth you speed up so you need every advantage you can get.

 

the number of firings is the same. Each cylinder will fire every other revolution of the crank no matter what rod length is. The way to get more explosions is either add more cylinders, run a two stroke engine, or spin more rpms... all of which are proven to make more horsepower. Now, since the long rod DOES increase dwell times around both TDC and BDC, the piston must move relatively faster at the mid stroke points to make up for those slower times. Soooo, that is where the added torque of a long rod motor comes from. I've heard it over and over again that the torque increase is due to the added combustion pressure that builds as the piston holds near TDC for a longer time with the long rod, but I don't think that's it. I don't think it's all of it, at the very least. If the piston must move fast during mid stroke to turn the crank less degrees than it did at TDC, that means you have mechanical torque multiplication. That piston moving at high speed as it gets 1/3-1/2 the distance down the bore while the big end of the rod is moving away from cylinder centerline but also downward is, at least as far as I think, what makes the torque increase more than the added pressure that builds with longer dwell near TDC. Still, the effect is relatively minor for most purposes. If you're trying to squeeze the last little bit of power out of a motor and you aren't concerned with piston design or piston speed (which I think deserve much more notice than rod length) then by all means, stick the longest rod you can in there. A longer rod should also require slightly better gas for detonation resistance if everything else is equal. The short rod does get the piston out of peak cylinder pressure quicker and therefore there is less opportunity for detonation to set in.

 

I know the # of firings is not changed by rod length. I am referring to gained efficiency by changing the rod length and increasing the actual # of rpms in a fixed amount of time. If, say you pick up a tenth of a second from a longer rod and pick up 150-200 rpm to do it there is less "time" to make the actual single engine firings , power strokes (whatever you want to call them). You have to look at "when" the piston is pushing the rod and "when" the rod is pushing the piston. The efficiency gain might be small , but its making that crank easier to turn by leverage whether its squeezing that intake charge or on the power stroke. Thats my story and I am sticking to it :lol:

 

A smart fella posted the formulas for determining piston dwell here a few years ago, and it was really cool to see the math and lose the conjecture.

IIRC, the result was a long rod increasing dwell at TDC and decreasing dwell at BDC, and a short rod decreasing dwell at TDC and increasing dwell at BDC.

I would love to see the formulas again, if anyone that knows cares to post it up.

Greg