Check this camshaft mfgs opinion out

http://www.iskycams.com/techtips.html#2005





[Modified by Mover, 4:43 PM 10/22/2002]

 

Anyone else come across more info ?

 

Right, so our piston magically has a different velocity depending on whether it's at the top or bottom of the stroke? Whatever.

A lot of what he says makes sense, but most of it isn't applicable to the degree of r/s ratio adjustment that we're usually discussing. And in a Honda engine, you're rarely going to be able to adjust rod length without changing the stroke.

And in his example of going from a 5.7" rod to a 6" rod, the r/s ratio increases to 1.72 from 1.64. Of course you're not going to see much difference with that. And let's not forget, "high-rpm" for a small block Chevy is 6,000 rpms. More dwell time simply isn't as necessary.

There was an editorial in SCC fairly recently that discussed r/s ratios (probably one of Dave Coleman's technobabbles). If I remember correctly, his opinion was that it's not the all-important statistic to be concerned with, but nor is it something to be ignored completely.

 

Absolutely it does, and its math, not magic. Here's graphical evidence, plotting both a 1:1 and 2:1 r/s ratio on an otherwise identical setup motor (exaggerated to more clearly illustrate thepoint).



Now I agree with what you are saying about it being an overblown issue, but piston dwell times and their attendant velocities are of some small concern when designing and tuning an engine. Besides this has was discussed just last week here.


Ps- It depends on whose building the small block Chevy and what for as to whether it redlines near 6000 RPM. It's not really all that hard to build an 8000 RPM small block these days.


[Modified by texan, 9:19 PM 10/25/2002]

 

I thought a combination of rod length+wrist pin location along with the piston being used & pistons wrist pin placement controlled the dwell time, no?

 

Oh I know, I totally get the dwell time differences between differing r/s ratios - but he's saying that the dwell time at TDC will be different than the dwell time at BDC with the same r/s ratio.

Isn't that what he's saying? Or am I misunderstanding a double-negative or something? After re-reading it 5 or 6 times, I'm no longer sure.

 

oh........math

 

Daemione- You read it right the first time, a given lower r/s ratio (assuming equal stroke) will equate to a faster piston speed away from TDC, and conversly a slower piston speed away from BDC. This is shown in the graph I posted earlier (the left most point where both green and gray plots reach 0 velocity is BDC, the other is TDC). But that does nothing to explain why, so here goes using as little math as posible.

If stroke translated to purely linear movement, rod length would make no difference to piston travel at all. For our 3.5" stroke, any given movment would translate directly to piston movement. However stroke is not linear, it's rotational. Hence angles, and those damn dirty angle muck everything up. Rod angularity is the specific culprit here, so we must take rod angularity into account when plotting piston position relative to crankshaft rotation. So to keep this short (too late ), we'll look at what defines piston movement only after BDC and TDC, and not worry about approach speeds.

To calculate piston location after BDC at a given crank angle, we take the positive (upward) rod journal movement minus any rod angularity. The reason we must subtract rod angularity is that some of the rod's length is eaten up by the angle, meaning the piston hasn't traveled as far upwards as it would have assuming no rod angularity. So in this example it's clear to see that the lower the rod angularity, the FARTHER the piston actually travels up the bore for a given amount of crank rotation. Such is shown perfectly in the graph posted earlier in both piston location and velocity.

However if we want to calculate piston location after TDC, we take the negative (downward) rod journal movement and ADD any rod angularity. The reason is the same as before, rod angularity eats up some of the rod's length relative to no angular movement, only here that causes the piston to move FUTHER DOWN THE BORE. Same exact reasoning, but relative to piston location and velocity it functions in reverse.

Katman- Moving the pin up or down but keeping the rod length the same will do nothing. However, offsetting the piston pin laterally will change location because it alters rod angularity. Offset the pin so that rod angularity is decreased ATDC (a not unheard of practice in engine building) will cause the piston movement to behave exactly as it does with a longer rod, including lessening thrust loads on the piston rings by sideloading. BUT, doing this will increase rod angularity ABDC and cause an even more marked dwell at the bottom of the cylinder (including higher side loads). At first glance this is actually a better practice than longer rods because you get both better dwell ABDC (which is a good thing for a few reasons) and get to keep the dwell ATDC while lowering side loading during the most heavily stressed portion of a piston's life; directly after combustion (plus it's way easier to do than increasing deck height). However some people argue that the extra side loading on the up stroke is just as bad for engine longetivity and ring seal as less during the down stroke, because they maintain that piston speed is the principle loading factor and not the pressure of combustion. I am not an engineer and as such have no idea which is right, but in my mind offsetting the piston pin seems like a really good idea.

Hope this helps explain things, peace.


[Modified by texan, 9:26 PM 10/23/2002]

 

Hmmm, still having a hard time wrapping my head around how a crankshaft with a constant circular movement will mean different piston properties at the top & bottom of the stroke. I see it on the graph, and I trust your knowledge, but I still can't picture exactly why in order to understand. Frustrating.

Any convenient diagrams you have handy that might clear up my confusion? I feel like I'm only missing one key realization before it all makes sense.

Thanks for your input . . . .

 

Texan, you are correct in your assumptions. I have found (in testing not theory) that a .050 offset increases performance more than .120 rod length, this is not true of all engines but ones that do pertain to this board. I wish there was a magic rod/ratio because it would make my job easier, but it has to be matched to usable RPM's, cam duration and intake and ex. geometrys. This can only be done with R&D and the results are not that big.

 

Ah I see, thanks

 

So taking a look at these, hopefully you'll start to get the idea. The first image illustrates the waxing and waning of rod angularity based upon crank angle, which when paired with the following equation and visual reference should make my point...

As angle (a) becomes larger due to increased rod journal distance from the crank centerline, the fixed length of the rod (C) forces piston vertical position (A) to become shorter to keep the triangle of connection points intact. In other words, as rod angularity increases, the piston moves downward, as angularity decreases, the piston moves upward. Applying this train of thought to illustration 1 shows that in the green areas the piston's movement induced by rod angularity is upwards, whereas it is downwards in the blue area. Now if rod length were infinite this wouldn't occur (you'd get pure sinusoidal motion based upon rod journal distance plotted about the crank centerline), but since it is quite finite in real engines some angularity and therefore induced piston movement will occur; and remember that's entirely apart from the rod journal position and it's attendent piston movement.

Put all of that together and hopefully, if I've properly explained myself, you'll see the results. The greater the rod angularity (or you could say the lower the r/s ratio), the bigger the difference between piston dwell times at BDC vs. TDC. BDC dwell gets longer and longer, while TDC dwell becomes shorter and shorter. So more angularity = greater difference in dwell times. Did this work for you Daemione? Hope this helps, peace.

[Modified by texan, 6:23 PM 10/24/2002]


[Modified by texan, 9:19 PM 10/25/2002]

 

It is true that the rod ratio really does not have much of an effect on power output. The main advantage of a good rod ratio is improved reliability. A good rod ratio greatly reduces internal stresses and engine wear. Especially at high RPMs.

 

Its the stroke that has a more direct effect on piston speed (as demonstrated above) and internal stresses. Just ask the con rod bolts or the guys below

https://honda-tech.com/zerothread?id=316401&page=3




[Modified by Mover, 9:50 AM 10/28/2002]