Big Joe - rod/stroke b.s.

But wouldn't that just be changing the angle of the engine from current to something more akin to 60* or whatever...
It kind of just changes the figuring of HP to a different engine design angle wise.

Pardon my engineering mind if this sounds a bit too technical. I've probably posted this before too.

I wrote a simple program years ago using the laws of sines and cosines to evaluate piston motion for a 331 using 5.155 289 length rods vs a 331 with 5.4 rods. I used this tool to help decide to build a 5.155 rod 331.

Results i've seen from the output of the program illustrate the differences below:

Longer rod:

1. Decelerates more at TDC and BDC thus reducing the G forces when the piston changes direction
2. Does spend more time at TDC like folks have mentioned.
3. Accelerates more between TDC and BDC.
4. #1 provides an advantage for high rpm operation by reducing rod stress
5. #2 may be advantageous at high rpm during combustion

Shorter rod:

1. Is further down the bore. The longer rod piston does not catch up with the shorter rod piston until just before BDC.
2. Is further up the bore. The longer rod piston does not catch up with the shorter rod piston until just before TDC.
3. Both 1 and 2 may be advantageous in cylinder filling and cylinder evacuation.


General comments:

1. It takes significant differences in rod length to realize significant differences in motion. The difference between a 5.155 rod 331 and a 5.4 rod 331 is .012 at 90 degrees...less everywhere else. A 1/2" difference in rod length produces .024 difference max and 1" difference in rod length produces .044 difference max. This is saying the shorter rod engine has the piston down the bore and up the bore more than the longer rod piston by the amount indicated at 90 degrees...less everywhere else.
2. The longer rod engine has less rod angle and less friction. The difference in side loading between 1 or 2 degrees, say 17 and 18 degrees or 17 and 19 degrees is 5.5% and 11%, respectively. A piston with a larger skirt will help to spread this load out more.
3. A longer rod engine uses a shorter piston (assuming the block deck height is the same). Advantages are less reciprocating weight, less friction and less rod stress. Disadvantages are more piston rocking.

I only saw one technical flay in blkfrds post.

#2. Does spend more time at TDC like folks have mentioned.

Well, "folks" and you are slightly incorrect in this statement.

To be correct 'technically' the pistons in a rotating assy, regardless of rod length, spend exactly the same amount of time at TDC if the engines are running at the same rpm. (TDC is measured in distance and position rather than in length or period of time)

The one piston (short or long stroke) cannot stay at the point of TDC any longer than another piston (short or long) as the stop/go motion of the transition from BTC to TDC to PTC(past top center), will be nearly instantaneous in each circumstance.

So you're right that the long rod has more time in the upper quadrant due to it's slowing as it reaches the apex then the slower acceleration as it departs,(thus creating less stress on the wrist pin and other moving parts) than the shorter rod, but it's always at the point of TCD for no longer than that specific Degree of rotation which is almost an immeasurable amount as far as on a time line is concerned.

And with the ignition occurring at a position BTC, my entire statement is irrelevant to this post as far as practicalities are concerned.

Did I screw up again or did I do good?

Originally Posted by FEandGoingBroke

I only saw one technical flay in blkfrds post.



Well, "folks" and you are slightly incorrect in this statement.

To be correct 'technically' the pistons in a rotating assy, regardless of rod length, spend exactly the same amount of time at TDC if the engines are running at the same rpm. (TDC is measured in distance and position rather than in length or period of time)

The one piston (short or long stroke) cannot stay at the point of TDC any longer than another piston (short or long) as the stop/go motion of the transition from BTC to TDC to PTC(past top center), will be nearly instantaneous in each circumstance.

So you're right that the long rod has more time in the upper quadrant due to it's slowing as it reaches the apex then the slower acceleration as it departs,(thus creating less stress on the wrist pin and other moving parts) than the shorter rod, but it's always at the point of TCD for no longer than that specific Degree of rotation which is almost an immeasurable amount as far as on a time line is concerned.

And with the ignition occurring at a position BTC, my entire statement is irrelevant to this post as far as practicalities are concerned.

Did I screw up again or did I do good?

Oh no...a technical flay!!

I meant that it dwells near TDC longer, but man oh man it ain't much. Look at this extreme example.

I checked it using 1" rod length difference (6.155 rod 331 vs 5.155 rod 331). How much longer does the longer rod piston stay within 0.5 inches of TDC?...about 25 microseconds at 6000 rpm...about the time it takes for the crank to turn another 1 degree. That seems like a whoop dee doo to me...technical jargon for BFD.

For someone to say that longer rod length makes the piston dwell longer at TDC is a weak argument kinda like saying that Obama doesn't use a teleprompter for words like "of" and "the".

The advantages are the reduced friction, better rod angle, reduced rod stress (which means less crank stress too), lighter piston....don't tell my 331 that though! It takes 7000 rpm blasts often. Thank god for rev limiters!

Originally Posted by blkfrd

The advantages are the reduced friction, better rod angle, reduced rod stress (which means less crank stress too), lighter piston....don't tell my 331 that though! It takes 7000 rpm blasts often. Thank god for rev limiters!

Most excellent! Thank You.

Tracy,
first- excellent post looking at the numbers your right- not much there, except frictional difference...

question again for you-
thoughts on offset wristpins to straighten the rod angle further on the powerstroke- and after that one, whats your thoughts about shifting the crank centerline/bore centerline instead of offsetting wristpins- would still straighten the rod angle on the power stroke, without rocking the piston(forget for now it might cost a bizillion dollars- just wonder thoughts on this approach...)

thanks for any insight-

Tim

Originally Posted by ford4v429

Tracy,
first- excellent post looking at the numbers your right- not much there, except frictional difference...

question again for you-
thoughts on offset wristpins to straighten the rod angle further on the powerstroke- and after that one, whats your thoughts about shifting the crank centerline/bore centerline instead of offsetting wristpins- would still straighten the rod angle on the power stroke, without rocking the piston(forget for now it might cost a bizillion dollars- just wonder thoughts on this approach...)

thanks for any insight-

Tim

Haven't really considered any of that very much. I'd like to modify my program to see the effects.

After some more consideration, I don't think I should be too quick to shrug off the benefits of piston motion due to a longer rod. In the real world, 25 microseconds may be significant when considering the speed at which the fuel burns. This may allow ignition timing to be retarded some.

On the other hand, for a shorter rod piston, seeing that the piston is lower in the bore on the intake stroke and higher in the bore on the exhaust stroke may also have some real world benefits. Maybe one can say that lower rod / stroke ratios are better for engines that have poor or moderately flowing heads.

There are advantages and disadvantages to just about everything and this is no different. If I didn't have to work for a living, i'd like to spend more time building engines to test a lot of this stuff...as if that hasn't been done before, right?...i'd like to see the results if it has.

HOW is a shorter rod "further up the hole"?? The stroke is a constant. The rod length is a constant. The centerline of the piston-pin hole WILL be "lower" at BDC with a short rod than a long rod, but it will also be lower at TDC.

The top of the piston will end up in the same location no matter the rod length (assuming the piston is designed for that rod length).

Here is a good example of length/stroke variances - compare the old Pontiac 400 and 455 motors. The bore is almost the same (4.12 vs 4.15), the big difference is the stroke (3.75 vs 4.21). Both use the same lenght connecting rods, 6.625" (LONG, huh?). 1.77 l/s ratio vs. 1.56. With a REALLY good cam selection, a 400 will make very similar hp (but a little higher rpm) to a 455 - that is with CR and all other variables held constant. But, 455's tend to be more tolerant of bad octane, and are not nearly as "picky" about the cam specs.

Originally Posted by L n L

HOW is a shorter rod "further up the hole"?? The stroke is a constant. The rod length is a constant. The centerline of the piston-pin hole WILL be "lower" at BDC with a short rod than a long rod, but it will also be lower at TDC.

The top of the piston will end up in the same location no matter the rod length (assuming the piston is designed for that rod length).

Here is a good example of length/stroke variances - compare the old Pontiac 400 and 455 motors. The bore is almost the same (4.12 vs 4.15), the big difference is the stroke (3.75 vs 4.21). Both use the same lenght connecting rods, 6.625" (LONG, huh?). 1.77 l/s ratio vs. 1.56. With a REALLY good cam selection, a 400 will make very similar hp (but a little higher rpm) to a 455 - that is with CR and all other variables held constant. But, 455's tend to be more tolerant of bad octane, and are not nearly as "picky" about the cam specs.

Because the geometry is different. This is a comparison. The shorter rod piston is further down the hole that the longer rod piston for the same amount of crank rotation (90 degrees is where the difference is max). By the time the crank turns another 90 degrees or BDC, the longer rod piston has caught up to the shorter rod piston and they are both at BDC. The same is true for when the piston is coming up from BDC...it is further up the hole for the same amount of crank rotation.

This is inline with the statement that longer rod pistons dwell near TDC more. The shorter rod piston has to be further down the hole for this to be a true statement.

The longer rod (higher rod/stroke ratio) does indeed dwell the piston near TDC longer.
This longer dwell time IS good for engines with restrictive cylinder heads.
However, nowadays large cross secton, high flowing cylinder heads are available, so the longer rods are not needed. Some of these heads are large enough, they benefit from a SHORTER rod that sweeps faster across TDC and gets the charge moving more quickly.