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Post  EverySparePenny on September 1st 2019, 11:45 am

470" truck puller deal. 4.440 bore, 3.8 stroke, 10.3XX deck height, all OE iron. 800+hp, 16ish:1 compression, 8000-9000 rpm operating range for approximately 30 second passes.

What is the shortest recommended compression height? Custom dome piston obviously.

Thank you

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Post  cool40 on September 1st 2019, 3:45 pm

Whatever works with ring package without using supports.
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Post  427John on September 1st 2019, 4:14 pm

While I can't help you with advice I'm curious on your combination.Why the .050 destroke,is there a 470 limit on your class and thats what you had to do to get there with the bore you had?Also about your compression height question are you shooting for the shortest lightest piston possible with the longest rod?

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Post  EverySparePenny on September 1st 2019, 4:51 pm

427John wrote:While I can't help you with advice I'm curious on your combination.Why the .050 destroke,is there a 470 limit on your class and thats what you had to do to get there with the bore you had?Also about your compression height question are you shooting for the shortest lightest piston possible with the longest rod?



We have a 473" max cube limit. My block will need to go to .080 over. I am shooting for as short and light of piston as possible though also have an exceptionally long rod available to me.

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Post  Dave De on September 1st 2019, 10:18 pm

Running a compression height at 1.35" is about as low as you can go and not use support rails with a .990 pin diameter. This is going to require a 7.05 rod with a 10.300 deck height. If you are going custom pistons and a 7.00" rod then a 1.40" CH will go zero in the hole.
That is one long rod with the piston spending a long time at both ends.
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Post  EverySparePenny on September 2nd 2019, 9:14 am

Dave De wrote:Running a compression height at 1.35" is about as low as you can go and not use support rails with a .990 pin diameter. This is going to require a 7.05 rod with a 10.300 deck height. If you are going custom pistons and a 7.00" rod then a 1.40" CH will go zero in the hole.
That is one long rod with the piston spending a long time at both ends.

I don't think I am opposed to using support rails. Should I be?

What are the ill effects of using the 7"+ rod in this scenario?

The rods that I have access to, actually have a 1.031 pin size. I'm not sure if an aluminum rod can be bushed to .990 or not.

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Post  cool40 on September 2nd 2019, 9:33 am

You can get the pin size whatever you want with a custom piston. You can run a support rail they’re just another piece to worry with. The long rod only adds weight to the rotating assembly but you have no choice with the block. I’ve had good luck so far with a similar setup myself.
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Post  Lem Evans on September 2nd 2019, 9:51 am

EverySparePenny wrote:470" truck puller deal. 4.440 bore, 3.8 stroke, 10.3XX deck height, all OE iron.  800+hp, 16ish:1 compression, 8000-9000 rpm operating range for approximately 30 second passes.

What is the shortest recommended compression height?  Custom dome piston obviously.

Thank you

It'll depend some on how thick your ring pack is.

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Post  EverySparePenny on September 2nd 2019, 9:53 am

The 2 rod lengths we have on the shelf are 7.700 and 7.765. making compression height very narrow. The weight of this combo would still be lighter, I believe, than the 6.8 steel rod combination that is our other option.

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Post  EverySparePenny on September 2nd 2019, 10:02 am

As for ring package. I'd like to run the thinnest available for such combination.

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Post  rmcomprandy on September 2nd 2019, 10:29 am

Dave De wrote:Running a compression height at 1.35" is about as low as you can go and not use support rails with a .990 pin diameter. This is going to require a 7.05 rod with a 10.300 deck height. If you are going custom pistons and a 7.00" rod then a 1.40" CH will go zero in the hole.
That is one long rod with the piston spending a long time at both ends.

Actually Dave ... the longer the rod = the less time it spends near the cylinder bottom but, a lot longer time near the top.

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Post  cool40 on September 2nd 2019, 12:04 pm

7.7 rod ain’t gona happen. Don’t build it around a rod just because you have it. You should weigh them against the steel 6,8,you may be surprised.
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Post  Mark O'Neal on September 3rd 2019, 1:09 am

1.250 is about as short as I care to go, with rail supports (and there is no reason to avoid oil rail supports.) But even the 7.150 that you would need to run 1.250 is a total waste of effort. 7.100 rods are still available, from Molnar and others....if you really feel the need. The C/H would be about 1.290.

7.700 will work, but the pin will have to be in the deck of the piston with the top of the rod sticking out....that and you won't be able to run rings, no room for those. Would clear the crank very well, as an upside. That and pin engagement would be right at 100%.

As for where everything hangs out at top and bottom dead center..that doesn't matter.

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Post  EverySparePenny on September 3rd 2019, 8:34 am

Thank you for the responses guys. I will abort this thought and move onto more viable options.

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Post  gt350hr on September 3rd 2019, 11:14 am

Mark O'Neal wrote:1.250 is about as short as I care to go, with rail supports (and there is no reason to avoid oil rail supports.) But even the 7.150 that you would need to run 1.250 is a total waste of effort. 7.100 rods are still available, from Molnar and others....if you really feel the need. The C/H would be about 1.290.

7.700 will work, but the pin will have to be in the deck of the piston with the top of the rod sticking out....that and you won't be able to run rings, no room for those. Would clear the crank very well, as an upside. That and pin engagement would be right at 100%.

As for where everything hangs out at top and bottom dead center..that doesn't matter.

   All this from the guy who's company started the whole pin in the ring is an issue deal! My how times have changed.
7.100 is the practical limit for rod length. One thing to consider is the torque band rpm rises and becomes more narrow with the long rod than with the  6.7-6.8 combination which could be a problem as the weight comes forward on the sled. Matching piston speed to port flow ( affected by rod length ) is FAR more important than some minimal amount of time extra "burn time". Besides energy isn't being when the piston isn't moving.

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Post  Paul Kane on September 3rd 2019, 3:29 pm

A 7.1 steel rod will work fine and make piston selection easy.

If you want aluminum rods shorter than what you have then I have a set of Childs & Albert Series 200 7.25" rods. They will require a piston with a 1.15" pin location, which can be done....but the steel 7.1" rod & shelf piston (with a single change) is the easier way to go.
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Post  EverySparePenny on September 3rd 2019, 3:42 pm

Paul Kane wrote:A 7.1 steel rod will work fine and make piston selection easy.

If you want aluminum rods shorter than what you have then I have a set of Childs & Albert Series 200 7.25" rods. They will require a piston with a 1.15" pin location, which can be done....but the steel 7.1" rod & shelf piston (with a single change) is the easier way to go.

Thank you Paul. I believe I will stick with the easier selection after seeing these gentlemen's opinions and talking with a couple manufacturers. 7.1 or 6.8 seems to be the route to head.

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Post  Mark O'Neal on September 3rd 2019, 5:49 pm

gt350hr wrote:
Mark O'Neal wrote:1.250 is about as short as I care to go, with rail supports (and there is no reason to avoid oil rail supports.) But even the 7.150 that you would need to run 1.250 is a total waste of effort. 7.100 rods are still available, from Molnar and others....if you really feel the need. The C/H would be about 1.290.

7.700 will work, but the pin will have to be in the deck of the piston with the top of the rod sticking out....that and you won't be able to run rings, no room for those. Would clear the crank very well, as an upside. That and pin engagement would be right at 100%.

As for where everything hangs out at top and bottom dead center..that doesn't matter.

   All this from the guy who's company started the whole pin in the ring is an issue deal! My how times have changed.
7.100 is the practical limit for rod length. One thing to consider is the torque band rpm rises and becomes more narrow with the long rod than with the  6.7-6.8 combination which could be a problem as the weight comes forward on the sled. Matching piston speed to port flow ( affected by rod length ) is FAR more important than some minimal amount of time extra "burn time". Besides energy isn't being when the piston isn't moving.

I've been putting oil rings in pin hole since 1980. We made an issue of it on one piston, because, at the beginning of the stroker revolution, the kids kept leaving them out because they didn't know what they were for. (Ford guys....the Chevy guys were used to it)

But, what do I know.

As for rod length, here is Reher and Morrison's take, which I ascribe to:

We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I’ve seen dozens of magazine articles on supposedly “magic” connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued!

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn’t care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There’s no magic – a rod’s function is to connect the piston to the crankshaft. Period.

https://rehermorrison.com/tech-talk-10-by-the-book/

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Post  gt350hr on September 4th 2019, 2:01 pm

Typical drag race mentality. I will NEVER know as much as David Reher , NEVER. Unfortunately every engine isn't an NHRA Pro Stock engine There are at least a hundred dirt late model , sprint car , and road race engine builders that will disagree with David's opinion. An engine that runs from 8-10,500 rpm for 6.5 seconds and makes four closely split gear changes wouldn't be very sensitive to rod length. The statement is taken out of context similar to often quoted one from the late Smokey Yunick saying you always need to run the longest possible rod.

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Post  rmcomprandy on September 4th 2019, 7:59 pm

gt350hr wrote:Typical drag race mentality. I will NEVER know as much as David Reher , NEVER. Unfortunately every engine isn't an NHRA Pro Stock engine There are at least a hundred dirt late model , sprint car , and road race engine builders that will disagree with David's opinion. An engine that runs from 8-10,500 rpm for 6.5 seconds and makes four closely split gear changes wouldn't be very sensitive to rod length. The statement is taken out of context similar to often quoted one from the late Smokey Yunick saying you always need to run the longest possible rod.

So true ...
when any discussion begins about this, I'll just walk away because physics means nothing to most of these people.

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Post  Mark O'Neal on September 5th 2019, 12:21 am

Engine building is 99% opinion. You're entitled to yours, I presume I enjoy the same privilege.

The correct rod ratio is 8:1. 8 rods to 1 crankshaft.

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