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This is for a 4x4 and I know there are some very sharp guys in this subject question I have on here. Its not gonna be a race engine. Its gonna be a mild build to run on pump gas. I have a set of fuelie's going on it and should be around 9 to 9.3 to 1 comp. Cam is 220/[email protected] 507lift 111 lobe sep hyd roller. The question is rod length. 5.7 or 6 inch?? Thanks!!!
 

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This is for a 4x4 and I know there are some very sharp guys in this subject question I have on here. Its not gonna be a race engine. Its gonna be a mild build to run on pump gas. I have a set of fuelie's going on it and should be around 9 to 9.3 to 1 comp. Cam is 220/[email protected] 507lift 111 lobe sep hyd roller. The question is rod length. 5.7 or 6 inch?? Thanks!!!
Are you going by a out of the book cam or are you working with a cam grinder for the best bang for your'e buck? If you are working with a cam grinder--ask him! M
 

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Village Idiot
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If I am buying new stuff I would go with 6.0” but if you already have the 5.7 rods I would use them. For a truck either would be fine.

Paul
 

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Are you going by a out of the book cam or are you working with a cam grinder for the best bang for your'e buck? If you are working with a cam grinder--ask him! M
Interesting response and one I had not ever thought about before. No doubt a longer rod in a 383 is a good thing, but I always thought of it more about side pressure on the piston skirts as to benefits of going longer. Help me understand what effects rod length would have on cam selection? Rod bolts hitting cam (should not matter long or short)? Would the change of dwell time at TDC/BDC have an effect?
 

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Village Idiot
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Interesting response and one I had not ever thought about before. No doubt a longer rod in a 383 is a good thing, but I always thought of it more about side pressure on the piston skirts as to benefits of going longer. Help me understand what effects rod length would have on cam selection? Rod bolts hitting cam (should not matter long or short)? Would the change of dwell time at TDC/BDC have an effect?
A shorter rod is going to pull harder earlier on the intake charge than a longer rod. Also, if your heads are a little on the big side for your combo then a shorter rod helps to crutch it. Now, those are the things your cam grinder are going to take into account to figure out what the optimum cam timing for the events will be.

Confession time, unless I am building a max effort race engine I don’t stress those smaller details. Especially in a street car or general duty engine. If you are class racing and you need every bit of help you can get then you better think about those details carefully.

There is a little more to it, but that should get the ball rolling to start firing the synapses.
Paul
 

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Interesting response and one I had not ever thought about before. No doubt a longer rod in a 383 is a good thing, but I always thought of it more about side pressure on the piston skirts as to benefits of going longer. Help me understand what effects rod length would have on cam selection? Rod bolts hitting cam (should not matter long or short)? Would the change of dwell time at TDC/BDC have an effect?
Yes it would. It has to do with rod length and rod angle in relation to both. BUT being a spec engine I don't know what is legal and what is not. A camshaft controls the world in this area? M
 

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steelcomp was here
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When you increase stroke, you always want to increase the rod length if possible especially if the heads are going to be on the small side. There is no reason to not run the 6" rod. There are several reasons to not run the 5.7. ;)
 

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Interesting response and one I had not ever thought about before. No doubt a longer rod in a 383 is a good thing, but I always thought of it more about side pressure on the piston skirts as to benefits of going longer. Help me understand what effects rod length would have on cam selection? Rod bolts hitting cam (should not matter long or short)? Would the change of dwell time at TDC/BDC have an effect?
Rod length, or more, rod ratio will change dwell time and also peak piston acceleration and where it occurs in regards to crankshaft degrees of rotation from TDC. Piston side loading and additional friction are also issues with the shorter rod. Going from a 5.7 to a 6" rod is a notable change in ratio and rod angle. Not insignificant.
 

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Village Idiot
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Rod length, or more, rod ratio will change dwell time and also peak piston acceleration and where it occurs in regards to crankshaft degrees of rotation from TDC. Piston side loading and additional friction are also issues with the shorter rod. Going from a 5.7 to a 6" rod is a notable change in ratio and rod angle. Not insignificant.
Scott, thanks for hopping in and sharing your thoughts as your posts are often thought provoking. So in this basic combo have you seen a measurable difference in bore or piston wear between the two rod ratios to warrant a serious rethink of using a 5.7 rod instead of a 6.0 or longer?

Paul
 

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steelcomp was here
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Scott, thanks for hopping in and sharing your thoughts as your posts are often thought provoking. So in this basic combo have you seen a measurable difference in bore or piston wear between the two rod ratios to warrant a serious rethink of using a 5.7 rod instead of a 6.0 or longer?

Paul
Not so concerned about wear as I am additional parasitic drag. Wear and friction usually go hand in hand...but not always. Bottom line for me, anything in the 1.6 - 1.7 ratio is going to be fine and if the difference is in that range, then I say use what parts you have. If there is a choice, I always recommend going with the longer rod combination. In the case of the 383, you actually lose a little ratio when going with the 6" rod. (1.62 compared to 1.6) so even with the 6" rod, you're barely maintaining the General's original design.
 

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Okay, I see where you are coming from. I generally like using as long of a rod as I can depending on the application. I was trying to put myself in the OP’s shoes building a 383 for his truck. For a budget build, if I already had a decent 5.7 rod I would use it. If I did not have rods already I would go with a 6.0 as piston options are endless for that combo. At that point I would be concerned with wear as I would need the engine to last. I figure a 5.7 rod is still much better than the factory 400 length rods. Anything over those is an improvement. :)sphss

It is good to see you pop back in here.

Paul
 

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Discussion Starter · #12 ·
Thanks everyone for the responses!! I was hoping Scott would pop in on this. I haven't got the rotating assy as of yet. I was thinking along the lines if you go up in stroke, the rod should go up with it to maintain the rod ratio as close as possible. But that said, I was also wondering if the 5.7 rod could be better in a 4x4 use engine that most likely won't get wound to the moon like a race engine does.
 

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I have a brand new set of 1142 MLS gaskets.
 
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