I don't get it. If the piston accelerates/decelerates faster to/from tdc/bdc with a shorter rod due to the geometry of rod angles and the piston's length of travel (stroke) is equal, doesn't that necessarily mean that at some point piston speeds decrease with the shorter rod?
And since the middle of piston travel is where the intake valve is at max. lift, wouldn't higher piston speeds at this point make for more power?
And lastly, wouldn't more gradual acceleration/deceleration at tdc/bdc mean less stress on the rod/wristpin/piston which would allow for lighter pieces and all the benefits of less reciprocating weight?

 

Piston speed depends only on stroke and rpm, not rod length.

This may not be intuitive.

The maximum piston speed occurs with the crank throw 90 degrees to the bore centerline. The rod and piston are then moving in the bore at the velocity of the center of the crank pin; they can't move faster than the crank, nor can they move slower at that point. They then decelerate to zero at TDC and BDC. The rod length does change the shape of the velocity curve (and therefore the acceleration curve). There are some side loads which vary with the rod length, but not the maximum or the mean piston speed.

I don't agree that intake is necessarily at max lift at mid stroke.
It's closer to 60-70 degrees BBDC in most engines, somewhat past max piston speed point.

The longer the rod the more time the piston spends near TDC, the so-called "dwell time" as the rod goes over center.

Longer rods do give lower maximum g loading on the pistons at TDC, but not a lot. maybe 3%-4% from a 5.5 to a 6.5 inch rod on a SBC.

My $.02

 

I'm hesitant to get into this because I don't want to step on any toes, but here goes... The trick is that speed = (stroke x rpm)/6 is actually the "maximum average piston speed" or "average peak piston speed" (both are oxymorons). It's a decent rule of thimb, but not the most accurate representation of what actually goes on.

Let's make a better representation. Draw a scalene triangle. The longest side is the bore centerline, the shortest side is the crank arm, and the medium side is the rod. If we express the position of the piston (the length of the bore centerline side of the triangle) as a function of crankshaft angle, you have the piston position function. It's not a simple equation, and it only gets more complicated from there. The first derivative is the piston speed function, and the second derivative is the piston acceleration. (So calculus is useful after all!)

The other thing is that if the rod length is varied (within the constraints of reasonable piston compression heights) the true max piston speed doesn't vary much at all, and neither does the acceleration. So it doesn't seem that the R/S ratio in itself would not actually have a measurable effect. One result of the R/S ratio, as someone already mentioned, is connecting rod angularity, which is basically the angle between the connecting rod and the bore centerline. More angularity means more thrust-side wear (due to friction).

Now, there may be non-mathematical effects of long rods. It may be that a longer rod and a shorter piston is actually lighter. When the reciprocating mass is less, it takes less energy to change the direction of motion of the pistons and rods. (Remember that energy divided by time is power.) That affects not only ability to rev, but power output at constant rpms. (The pistons and rods have to change direction of motion even at constant rpm.) A lighter crank will only affect the ability to rev, because it has a tendency to stay at the same rpm (assuming neutral balance).

I think everyone had at least part of it right, and I hope this has cleared things up rather than making them more murky.

 

Hey, if the water were clear we wouldn't have any discussion.

Mean piston speed (stroke x rpm/6) is what is generally discussed as "piston speed" in most engine texts. The maximum speed the piston sees should be the velocity of the center of the crank pin, which is (stroke x rpm x PI/12) which is about 157% of mean piston speed.

As small as the differences in acceleraton are with rod length changes, I find it hard to accept the basic premise of this thread that port volume is related to R/S ratio. I agree with you, "So it doesn't seem that the R/S ratio in itself would not actually have a measurable effect."

 

Sorry to keep going on about this. I didn't mean to hijack this post, I'm just trying to learn something.
My reference to max lift at mid stroke was just a simplification to get to a point. I see how piston speed at "mid stroke" must be the same regardless of rod lenth.
It seems to me that a longer rod gives higher piston speeds before and after the mid stroke point, where valve lift is at or close to maximum, which would fill the cylinder better. That plus any weight advantage from lighter parts seems to make a convincing argument in favor of a longer rod/stroke ratio. Correct?

 

On the weight front, yeah that's the main reason I run longer rods, but if you could deck the snot out of the block then you could run lighter pistons and rods due to the shorter rod length.

The advantage of a longer rod in my eyes is the dwell time around TDC. It's got to help the flame front by keeping the air/fuel charge in a concentrated spot longer.

As for the part on piston speeds and where the valve is at max lift in relation to it. Remeber that the fluid of air doesn't react like mechanical things, it doesn't stop, start and accellerate, decellerate even close to what the mechanical parts of the motor. So in turn there is almost none to little effect on the incoming charge.

I don't doubt that there is something in this larger port size connection to the shorter rods. If guys have seen it, they have seen it. I just don't think there is a good explaination of it yet, which is why I don't buy it 100%. There has to be a reason for it, engines are just physics.

So right now I don't buy the intake port volume can increase on a optimized engine if I shorten the rod length.

I still think that the test has to be broken up in individual parts. You can't change more than one thing at a time to say that this effect is caused by this.

Bret

 

Ok,

So, if a shorter rod does not increase actual piston velocity from TDC and rod length has nothing to do with piston speed, than maybe the real reason for what I have seen is related to the dwell time.

Shorter rod, shorter dwell time. Begins moving sooner, and therefore creates the pressure differential sooner, and therefore the air gets moving sooner.

What I cannot understand from this, Is why I have seen shorter rods on a small cc, restricted intake application, Hurt power considerably. And a Long rod fixes this problem.

I know what I have tested and seen, And I know that a shorter rod coupled with a great set of heads will be beneficial. Now, it seems the actual reason for this may not be what I had thought, as I had always thought that a shorter rod created higher piston speeds.

Ok, wait a minute,

I think I figured this out, while typing.

The higher the RPM you are going to turn, the shorter the rod you want in the motor, the shorter the stroke, and the better the induction system. This way, at high rpm and sustained high rpm operation, the dwell time is less and therefore gets the new charge heading in the right direction at an earlier stage so that it is there when it is supposed to be, and/or as fast as it can be

The long rod would hurt an unrestricted high rpm engine due to the fact that it would posess the exact opposite attributes.

How does that sound to you Bret??

I am going to think about this some more....

-Nick

 

As far as some of the larger issues raised here, I have nothing to add. But one point I want to throw into the discussion: I have always used short rods in centrifugally SC motors designed for pump gas. My reasoning is that with less dwell time near TDC there will be less tendency toward detonation. Anyone care to comment on this theory? It does relate to the observations of "CompAirflow" about very efficient intake systems and short rods, if you think of a forced induction setup as a super efficient intake system.

Rich Krause

 

Wow, I never imagined that my question would lead to such a great discussion, but from reading some of the new replys, I haven't been in here since 5/2/03 some people think I was speaking of mean piston velocity instead of initial piston velocity as the piston starts to move away from tdc. Like everyone is saying a longer rod dwells longer at tdc where a shorter rod has already begun moving the piston. That's what I meant by shorter rod moving piston away from tdc faster than longer rod because of the longer rods greater dwell time. Sorry for any confusion on this, but keep going this is great!
My engine specifics are as follows in case anyone missed them 421 ci sbc 4.155 bore x 3.875 stroke x 5.85 rod x 1.237 piston compression height x 1.51 r/s ratio, AFR 220 race ready heads/ no extra porting.

 

Nick,

O.k. now that I thought about that a little, my thought is this.

Since it has less dwell and a slower accelleration from TDC it starts earlier and creates a pressure differential earlier. Since that is why the charge flows into the cylinder anways, there is a higher pressure above the valve and a lower presure below it, seems that the short rod starts that sooner. I would imagine duration has alot to do with this too. A restricted engine (say Winston Cup) has a max engine speed in the low 6000 range. Has much less duration then a 9-10K PST engine. So the point at which that valve is opening is ealier.

I think this is interesting because it seems to play with the valve opening point, which is secondary in importance to the Int closing point. Let's look at that area too. The piston is now coming up the bore earlier but at a slower rate.

Nick, shoot me a range of duration that you are working with on a PST engine say a ten deg range and a ICL neighborhood. That might get us closer to figuring out this problem.

There's something there, just have to know the why and how to see how to apply it other places.

Rich,

Since a blown application is a high VE application it's simliar in some ways and not in others. There's almost always a higher pressure situation above the valve for you so pressure differentials are not as critical. I think that this effect as well as intake manifold tuning are thrown out the window because of the constant positive pressure in the intake tract.

Short rods in your application make sense so you can get enough piston deck thickness. If you have too long of a rod the pin starts getting in the dish area and makes the piston weaker.

The shorter rod will dwell less, true but I don't know if that is going to lessen the likely hood of detonation. It seems that more dwell would need less ignition timing, since that more compact charge would burn faster, since it is more concentrated. Lower ignition advance is always what you do to stop knock. So seems that going along that line would make sense, maybe not.

Mr Z28,

I think you gotta get closer to the right ball park for head volume before you worry about the rods effect on it. We are not talking about a primary variable here, this is farther down the line than that. I would worry more about the intake manifold you are going to use first. On top of that, it seems that the short rod larger port theory works at very high RPM, in the range where street motors don't run.

Bret

 

"The maximum speed the piston sees should be the velocity of the center of the crank pin..." Yes, that is exactly correct, because the pin does not move with respect to the piston. "...which is (stroke x rpm x PI/12) which is about 157% of mean piston speed." Well, not quite--it'd be nice if it were that simple. It still depends on rod length. Draw the triangle and visualize how changing the rod length changes how the piston moves. Short rod: shorter dwell, higher acceleration from TDC and higher peak piston speed. Long rod: longer dwell, lower acceleration from TDC, lower peak piston speed. But all of these differences are extremely minute! The difference in dwell time (again, within the contstraints of reasonable rod lengths) is probably on the order of a few milliseconds. I can crunch some actual numbers later if anyone is interested.

I did think of one thing though: the maximum angularity can vary by as much as 2-3°. Higher angle between the rod and the bore means more of the force of combustion is directed radially out from the crank centerline, so that force is simply wasted (resulting in lower power output).

Conventional wisdom says that to maximize VE (at the expense of wear) you should have a R/S ratio of around 1.4:1. A numerically higher R/S ratio is supposed to be less prone to detonation. Again, I doubt there's a measurable difference. A "good compromise" is said to be 1.6:1.

Just FYI, restrictor plate engines actually max out around 9000 rpm, and the internals are lightened like crazy. That's why they break so often.

 

Well, SStrokerAce said low 6's, you say 9000, but I think 7500 is closer to what's currently running in Cup restricted engines. 9000+ is unrestricted rpm range. At least that's what what the telemetry and some folks I talk with admit to. RPM on restricted engines is creepin' up.

I really don't recall many Cup restrictor engines dumping the bottom ends recently. My thought is that the valve motions are so severe, even at 7500, that "dropping a valve" is more common.

With a 300 rpm range at Daytona and Talledaga, IMO rotating inertia isn't a huge problem. Friction is the problem, so smaller bearings are used along with short strokes for lower surface velocities of all the rubbing parts. Lightweight pistons and rods keep the g loads, and therefore the bearing loads down. Friction losses and pumping losses go up with rpm, but so does hp if you can maintain the torque, which is limited by the restricted airflow. It's a tough compromise; gain a little flow, lose a little friction and get a little more torque at a higher rpm. That's more power at the flywheel.

My $.02

 

Nick, it looks like you are somebody whose input is valuable, and I'm sure we can all learn from you. I am happy to see you here, and I hope you don't mind if I pick your brain. I sit up at night (literally) thinking about just these subjects, and all kinds of things go through my head.. Unfortunately I haven't the budget to test my theories, and I rarely encounter people who 1) have the knowledge I am after and 2) are willing to share

Aside from that, I am curious about the statement that restrictor plate engines use a big bore/short stroke combination. This statement is counterintuitive for me, as it seems backwards. I don't have scientific information here, just speculating.. It seems to me that if two engines are the same CID, and one has a long stroke, it seems this engine would have more time to draw air through the restriction, and hence, more potential. By contrast, the big bore engine would seem to want more air for each degree of rotation, and looks to me it would be RPM limited... which isn't good for obvious reasons. The only bit of informations I have that I didn't pull out of thin air is the used stuff on ebay. I search through ebay often, checking out the used Cup stuff. Sure enough, I've found a few ads for long stroke cranks that were used at restrictor plate races. Of course, I don't know how old those parts are... could be very outdated.

Also on the subject of restricted motors, it seems that a long stroke engine would tend to make it's power at a relatively lower RPM range, which appears to be the setup for a limited cfm induction system, since a high rpm engine would appear to need more induction

My intuition tells me that a short rod, long stroke, short deck, small bore, large csa engine would lend itself well to restricted induction...

Lets say you were building a 360" or less engine that was limited to the restrictor plate setup. If'n you don't mind, what bore/stroke/csa/rod length/bench curves would you do, and what would you expect the tq/hp curves to look like?

I am asking to learn, and would love your input here I build quite a few performance street engines, but I don't know anybody that can teach me these things. The shop that does our machine work is among the best in KC, and even he really doesn't get into the cutting edges stuff... so your experience is very valuable to me.

Thanks, Matt

 

Also, big bore/short stroke is supposed to enhance cylinder filling over the same ci with a small bore/long stroke. But, more stroke means more torque. Everything's a compromise.

 

Maximum piston speed occurs at around 74-75 degrees ATDC for most standard SBC components (5.7-6.25 rods and 3.5-4.0 strokes).

As Novaman described, it is for the following reason. At about 70-80 degress ATDC, the crank pin is moving downward very rapidly, although not as fast as it does at 90 degrees ATDC. But, at 70-80 degrees, the rod angularity is getting worse. Even though the physical length of the rod is not changing, the vertical (height) component of the rod is always changing. At this point in the stroke, the vertical component of the rod is getting smaller at a rate that is faster than the change in velocity of the crank pin. In other words, since the rod is getting vertically shorter, it is ADDING to the velocity component of the crank pin, making the piston actually move faster than the vertical component of the crank pin.

Just some numbers, but with a 3.48" stroke, 6.00" rod, rotating at 9,000 rpm, the maximum piston speed is 8,539 fpm at 74 degrees ATDC, and the piston speed at 90 degrees ATDC is 8,177 fpm. That's about 4% slower than the max.

Shane