Author Topic: rod length  (Read 1032 times)

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Hog Head

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on: March 25, 2010, 04:12:23 am
Rod length is a subject of great discussion in air cooled VW engine circles and I something I delved into with the new 2332cc engine I am building now.

The same thoughts occur to me in the RE engine. Given that Carrillo custom make the rod it is possible to have a different conrod length. 
Would there not be advantages to a shorter rod, and using a different piston with the pin moved upward?
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ace.cafe

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Reply #1 on: March 25, 2010, 01:24:08 pm
I'm working on a package with Mondello for a different rod length option.

This is something that gets pretty complicated, pretty fast.

To boil it down, you need a longer rod to move the pin up in the piston, unless you shorten the barrel. And to use a shorter rod AND move the pin up in the piston, you'd have to do a huge shortening of the barrel.

Here's the basics. A long rod is better than a short rod. Smokey Yunick used to say to use the longest rod that you can physically get into the motor.
Long rod gives less side thrust force, dwells longer at TDC for making more power, and gives less stress on the crank and bearings.
The only thing that a short rod does better is get to peak torque thrust(rod at right-angle to the crank throw) earlier in the cycle. Oh, and it can weigh less too.

If we use a shorter rod, we need to shorten the barrel. The Enfield wrist-pin location is already very low on the piston, so it would do better to be higher up.
If you move the wrist pin up 3/8" and shorten the rod by 3/8", then you have to cut 3/4" off the barrel, probably losing a fin or more of cooling fin area. It's going to make the engine have different breathing characteristics which will affect your cam timing selections.

On the other hand, you could do what Mondello and I are working on, which is a longer rod. Move the pin up on the piston by about 1/4"-3/8", and shorten the piston skirt to suit, which lowers the mass of the piston, and also positions the pin in a geometrically better spot on the piston.
By lengthening the rod by the same amount that you move the pin up in the piston, you keep the same height at TDC and BDC, and don't have to shorten the barrel.
However, a longer rod is heavier, and with a steel rod, adding more steel rod length to allow removal of aluminum piston skirt isn't a real good trade.
So, we're looking at a longer rod made of aluminum, from a custom shop that make s top-of-the-line drag racing rods from aluminum.
The original Bullet design con-rod is aluminum, but the India-made ones aren't very  good metallurgy. A really top quality aluminum rod could do the job in our engines, even in a performance role, and keep mass minimized.
By using a longer aluminum rod, we don't increase rod mass much, and can carve some mass off the piston. It would be a lot lighter than the steel rod and a light piston. And it would have the longer rod advantages, although to be honest about it, it wouldn't change that very much because we already have a long rod.
Aluminum has its limits for fatigue, but if we don't over-stress the rod with insane rpms, and keep revs to our 6000rpm or even 6500rpm limits, it will last as long as the rest of the engine. Joe had a set of these rods in his street Oldsmobile 442, which had a stroke of over 4", and revved to 6500rpm, and they were in there for 50k miles, and were still in there when he sold the car. They might even still be going.

I have done some preliminary design work on this, and we can do it, and I can have the pin-height changed on the ACE piston to suit this new geometry.
I may be experimenting with this over the summer.
I have to buy a batch of rods and pistons for this, because they don't make just one. So, it's going to be a several thousand dollar experiment for me, and I have to budget that.

The current trend in shorter rods that we see today, are primarily resulting from the trend toward stroker motors which require a shorter rod for maintaining the same deck height in the engine with the longer stroke. It would still be preferable to have a longer rod in them, but they can't fit it in. What they do then is get the longer stroke crank they want, and get a piston with as high a pin location as they can, and then use the rod length that will fit them together. Usually a shorter rod results from that.
 
« Last Edit: March 25, 2010, 01:38:40 pm by ace.cafe »
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