6" v 5.7" rods
Thread Starter
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
6" v 5.7" rods
I know this topic has been beaten around before but I have lost my links and can't find anything in the search.
Pros and cons of difference lenght rods in 383 / 406?
Seem to remember something about angles and piston speeds?
Thanks
Pros and cons of difference lenght rods in 383 / 406?
Seem to remember something about angles and piston speeds?
Thanks
Race Director
Joined: Feb 2003
Posts: 18,484
Likes: 1
From: Everyday you must choose between the pain of discipline and the pain of regret. Fredericktown, OH
Originally Posted by rodj
I know this topic has been beaten around before but I have lost my links and can't find anything in the search.
Pros and cons of difference lenght rods in 383 / 406?
Seem to remember something about angles and piston speeds?
Thanks
Pros and cons of difference lenght rods in 383 / 406?
Seem to remember something about angles and piston speeds?
Thanks
Drifting
Joined: Feb 2002
Posts: 1,952
Likes: 2
From: Little Rock Arkansas
Long Rods- less thrust side wear and longer dwell at TDC. This means that more efficient combustion takes place requiring less ignition lead and octane.
Short Rods- Faster velocity means greater cylinder filling.
-Jeb
Short Rods- Faster velocity means greater cylinder filling.
-Jeb
Race Director
Joined: Sep 2000
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From: The Top of Utah
Originally Posted by LT-4 CE
However, with a 6" rod in a 400 block, you would like need to grind the rod bolts and/or go to a small base circle camshaft to clear. With Eagle 5.7" rods in my last 406 we did have to grind the rod bolts a bit.
RACE ON!!!
Race Director
Joined: May 2001
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I like to think of it as an intended usage question. If you're going for a high boost application or a nitrous application, stick with the shorter rod. You see the longer the rod the higher the wrist pin must be located in the piston. I believe with a 6 inch rod you start getting into the rings a lot more than you do with a 5.7 You also end up with less material in the piston. This might be lighter but likely also weaker.
You could always split the difference and install a 5.875 rod.
You could always split the difference and install a 5.875 rod.
Melting Slicks
Joined: May 2001
Posts: 3,270
Likes: 26
From: Winchester VA
Cruise-In V Veteran
Originally Posted by jburnett
Long Rods- less thrust side wear and longer dwell at TDC. This means that more efficient combustion takes place requiring less ignition lead and octane.
Short Rods- Faster velocity means greater cylinder filling.
-Jeb
Short Rods- Faster velocity means greater cylinder filling.
-Jeb
I was reading popular hot rodding's dyno shootout where they were limited to 409cid and 92 or 94 octane. Most used a shorter rod to prevent detonation as the piston spends less time at tdc (moving faster as you said). This was to maximize the usable compression ratio. What do you think, it seems to be in contrast with what your saying about on the longer rods octane/ignition. I say seems because...well, there a million things I could be missing.
Team Owner
Joined: Sep 2001
Posts: 62,143
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From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
I remember so many debates on this here before, and the main idea I got out of it was that if you were sticking with NA and not a Supercharger or juice, that it really doesnt matter which one you use.
If there is something specific you want out of the motor, like the ability to go to 8000rpm, or handle forced induction, or something like that, then you should say so before you get recommendations. But for a street or light race engine, it really wont hurt it to use either length.
If there is something specific you want out of the motor, like the ability to go to 8000rpm, or handle forced induction, or something like that, then you should say so before you get recommendations. But for a street or light race engine, it really wont hurt it to use either length.
Instructor
Joined: Aug 2004
Posts: 245
Likes: 0
From: Tampa FL
Originally Posted by jburnett
Long Rods- less thrust side wear and longer dwell at TDC. This means that more efficient combustion takes place requiring less ignition lead and octane.
Short Rods- Faster velocity means greater cylinder filling.
-Jeb
Short Rods- Faster velocity means greater cylinder filling.
-Jeb
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Race Director
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From: Everyday you must choose between the pain of discipline and the pain of regret. Fredericktown, OH
Originally Posted by CFI-EFI
The 400 block has the same crankshaft to camshaft centerline as all SBCs (Gen I and II at least), so the 400 isn't a special problem. Also the length of the rod make NO difference in rod to camshaft clearance. It is the configuration of the big end of the rod, and not the length that causes the interference.
RACE ON!!!
RACE ON!!!
Race Director
Joined: Feb 1999
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From: Indianapolis IN
I have a 406 and used 5.7 rods and did not grind anything on the rods or the block or the oil pan, I have a "reduced" base circle cam, I dont remember the diamater, I want to say .984"
Last edited by LD85; Jan 6, 2005 at 06:01 PM.
Race Director
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From: The Top of Utah
Originally Posted by LT-4 CE
Curious then, why was the stock 400 rod 5.565"? Also, in multiple 406s, I've used GM 5.7 and Eagel 5.7 and in each case I was told I would, and did have to, grind some on the rod bolts to clear the cam. Seems as I was told the rod length in the 400 had some clearance concerns with the cam. Not arguing, just clarifying.
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Race Director
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From: Everyday you must choose between the pain of discipline and the pain of regret. Fredericktown, OH
Originally Posted by CFI-EFI
I didn't design the 400 engine and I can't tell you the reasoning for the short rod. It may have been to keep the pin lower in the piston to make room for the rings and ring location they wanted. The reason a 400 rod clears the cam and the 5.7 doesn't, has nothing to do with the center to center distance on the rod. Think about it. Why would it? Take a close look at the hole the bolt goes through in the fork end (main body) of the rod. You will notice that the length of the hole is shorter in the 5.565" (400) rod than in the 5.7" rod. That moves the bolt head closer to the part line between the fork and the cap. It also moves the bolt head further from the camshaft. Check the parts books and you'll find that the 400 rod bolt is unique...it's shorter than the others. THAT is why a 5.7" rod may hit the cam on a 3.75" stroke engine, where the 5.565" rod doesn't. NOT the center to center length. Aftermarket and "stroker rods" with the bolt head at the other end, and where the bolt is threaded into the fork end of the rod DO clear the cam on a 3.75" stroke engine. Simple, eh?
RACE ON!!!
RACE ON!!!
Last edited by LT-4 CE; Jan 6, 2005 at 08:44 PM.
Race Director
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From: The Top of Utah
Originally Posted by LT-4 CE
I don't disagree with what you are saying, but cam/rod bolt clearance is, from the way I understand it, a by product of the rod length.
This, as I understand it, positions the rod bolts closer to the cam shaft, thus creating at least a portion of the clearance issue.
This, as I understand it, positions the rod bolts closer to the cam shaft, thus creating at least a portion of the clearance issue.
Whatever!
RACE ON!!!
the shorter rod will produce faster piston speed which helps fight against detonation because it spends less time at top dead center
Race Director
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From: Everyday you must choose between the pain of discipline and the pain of regret. Fredericktown, OH
Originally Posted by CFI-EFI
So if you mind was already made up... Why did you ask???
Whatever!
RACE ON!!!
Whatever!
RACE ON!!!
Uhhh, since when is asking a follow up question considered a bad thing. I have an understanding of the situation that differs from yours. All I was trying to do was explain my rationale and get your insights relative to what you understand to be the reasons here. If everyone knew everything there is to know about this stuff, there would be no reason for a forum and we all build pro stock motors for a living. All I can do is share what the engine builders I've worked with told me as I built various motors for my race car. Perhaps I mis-understood or perhaps they didn't communicate this piece to me very well. Either way, I am sorry you got the wrong impression of my follow-response. I am a firm believer in healthy "discussion" on the forum, it's the only way we can all gain more knowledge.
Melting Slicks
Joined: Jan 2000
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From: NW Chicago Suburbs
Originally Posted by LT-4 CE
Uhhh, since when is asking a follow up question considered a bad thing....I am a firm believer in healthy "discussion" on the forum, it's the only way we can all gain more knowledge.
To some people it's more important to win the argument than it is to be correct.
Stay away from 1 3/4 headers or you'll really get a ration!
Race Director
Joined: May 2001
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Originally Posted by silver & red CE
I agree 100%. I'm an engineer at a semiconductor company, working in the technical end for 20 years. Its been my experience that folks with the weakest agument use rude condesending comments as a method of silencing any open discussion, thereby protecting their fragile egos against being pointed out that they don't know what they're taking about.
To some people it's more important to win the argument than it is to be correct.
Stay away from 1 3/4 headers or you'll really get a ration!
To some people it's more important to win the argument than it is to be correct.
Stay away from 1 3/4 headers or you'll really get a ration!
There is no need to ever be rude here, it just detracts from your credability. Oh, and I'm a firm believer in 1 3/4 headers over 1 5/8 
Race Director
Joined: Sep 2000
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From: The Top of Utah
Originally Posted by LT-4 CE
Uhhh, since when is asking a follow up question considered a bad thing.
Originally Posted by LT-4 CE
I don't disagree with what you are saying, but cam/rod bolt clearance is, from the way I understand it, a by product of the rod length. When you run a longer rod, the relative postion of the rod in the motor (in therms of it angle to the crank and piston - since the crank and cylinder bore are a fixed location) is slightly different when you change the lenght of the rod. This, as I understand it, positions the rod bolts closer to the cam shaft, thus creating at least a portion of the clearance issue. That certainly does not discount the design difference you referenced as well.
This exact question troubled and mystified me some years ago. The best answers I could get were vague and similar to what it appears you have been told. I wasn't satisfied without a more concrete answer. No one mentioned the different big end of the rod or the rod bolts. I don't think this is common knowledge. So I did some research. My response to you was the result of that research. The difference in the design of the big end of the rod made more sense to me, than the slightly different angle created by the difference in rod length.
Originally Posted by LT-4 CE
I am a firm believer in healthy "discussion" on the forum, it's the only way we can all gain more knowledge.
*I* personally, have not, concretely, PROVED the EXACT cause of the difference in clearances. Maybe what we ought to do, is saw a couple of rods a part, weld up a 5.565" rod with a 5.7 big end and a 5.7" rod with a 5.565 big end, install them in a block with a camshaft, and check the clearances. Any bets??? Now THAT is an opening for discussion.
RACE ON!!!