Thread: Of Rods And Myths

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As I was reading through a bunch of forums, I keep coming across rod myths and misconceptions, and mass confussion I'm going to try to help out and others should share their knowledge also.

1. Rod length strokes or adds cubic inches: TOTALLY WRONG
Rod length has nothing to do with stroking or cubic inches
the original 400 had 5.565 rods. You can use 6" rods in the
same engine and still have a 400. The part that rod length
plays on a engine is thrust and angle, also dwell but we'll
get to that. When GM went to the 400, instead of changing
the pin bore (compression height) they shortened the rods
which really wasn't a good idea because of the thrust and
angle at which the rod worked put a stress on the bore
and wore it out in no time. You need a good rod to stroke
ratio. The longer the rod the less stress and more effient
it is. With long strokes and standard deck heights, rod
lenght all too often get compromised. Example--A stock 400
has a ration of 1.48:1 and it wears bores quick. If you were
to stroke a chevy block to a 4" stroke and use a 6" rod, you
would only have a L/R ratio of 1.50:1 which is still short.
But you can't add any more rod length because your left
with a compression height (distance from the centerline of
the wrist pin to the crown of the piston) of about one inch
and three rings have to fit in that area. In fact on that
setup the piston pin is into the oil ring. A 350 with a stock
5.7 rod has a L/R of 1.64:1. Always use the longest rod
possible.


2. A long rod engine reduces detonation. WRONG
A long rod actually has more dwell time at TDC and
Therefore the air fuel mixture is warmed up longer, giving
the charge a better chance to detonate. A short rod engine
doesn't get a chance to dwell because of its acute angle and
is jerked around.


3. Longer rods give more compression. WRONG
The longer rod doesn't push the piston up the bore any
further, just the pin location changes.


4. A short rod engine makes more low end torque. NEVER PROVEN
Even if it does, bore wear off sets it.

ON A FINAL NOTE:

ONLY BORE SIZE AND CRANK THROW ADDS CUBES.

Originally posted by camaro_fever68
As I was reading through a bunch of forums, I keep coming across rod myths and misconceptions, and mass confussion I'm going to try to help out and others should share their knowledge also.

1. Rod length strokes or adds cubic inches: TOTALLY WRONG
Rod length has nothing to do with stroking or cubic inches
the original 400 had 5.565 rods. You can use 6" rods in the
same engine and still have a 400. The part that rod length
plays on a engine is thrust and angle, also dwell but we'll
get to that. When GM went to the 400, instead of changing
the pin bore (compression height) they shortened the rods
which really wasn't a good idea because of the thrust and
angle at which the rod worked put a stress on the bore
and wore it out in no time. You need a good rod to stroke
ratio. The longer the rod the less stress and more effient
it is. With long strokes and standard deck heights, rod
lenght all too often get compromised. Example--A stock 400
has a ration of 1.48:1 and it wears bores quick. If you were
to stroke a chevy block to a 4" stroke and use a 6" rod, you
would only have a L/R ratio of 1.50:1 which is still short.
But you can't add any more rod length because your left
with a compression height (distance from the centerline of
the wrist pin to the crown of the piston) of about one inch
and three rings have to fit in that area. In fact on that
setup the piston pin is into the oil ring. A 350 with a stock
5.7 rod has a L/R of 1.64:1. Always use the longest rod
possible.


2. A long rod engine reduces detonation. WRONG
A long rod actually has more dwell time at TDC and
Therefore the air fuel mixture is warmed up longer, giving
the charge a better chance to detonate. A short rod engine
doesn't get a chance to dwell because of its acute angle and
is jerked around.


3. Longer rods give more compression. WRONG
The longer rod doesn't push the piston up the bore any
further, just the pin location changes.


4. A short rod engine makes more low end torque. NEVER PROVEN
Even if it does, bore wear off sets it.

ON A FINAL NOTE:

ONLY BORE SIZE AND CRANK THROW ADDS CUBES.

This is a very good post.Time and time again people are confused about this.Everyone knows how I dislike the 400 blocks.If someone wants to build one run at least a 5.7 rod or better yet try the 6.0.An engine that runs a longer rod will have a better ring life and ring seal because the loading of the cylinders is reduced with the longer rod.I also believe with the piston dwelling long at T.D.C. you are able to light off the fuel and air mixture more efficiently.

Hi Ray, Erik. I would add one thing more if its OK. The racetrack and the street are 2 different realitys. Taking a race car onto the street requirea all kind of compromises, and trying to run a street car against purpose built racecars is like a 1 legged man at a [well u get the drift]. We all want to think we have a special kind of car for our own bragging rites, and thats a big part of the fun, as long as we keep it in perspective. Race engines work best on the track and any attempt to put them on the street compromises all the engineering genius.

this is only true to a point .the long rod will make the pistons shorter. pin is hirer up and with some stroke the pistons get verry short and the skirts to .so this works on the rings and on the walls so on a race engine rod speed is the eschew short rod can be made to run good and piston companies can re work skirts design or tapper trunning may help to so if not careful you end up were you started .working the rings and wallls and the pistons not stable in the bore. i look at it what will it takes to hook the crank to the pistons . is this a good CH can i get a goood ring pack on the pistons how much will i have from top of the pistons to 1 ring groove?? i have had custom pistons made for longer rods and see test done on this to apoint this may be true but you have to ask what are you going to do with the engine??

Originally posted by pat mccarthy
this is only true to a point .the long rod will make the pistons shorter. pin is hirer up and with some stroke the pistons get verry short and the skirts to .so this works on the rings and on the walls so on a race engine rod speed is the eschew short rod can be made to run good and piston companies can re work skirts design or tapper trunning may help to so if not careful you end up were you started .working the rings and wallls and the pistons not stable in the bore. i look at it what will it takes to hook the crank to the pistons . is this a good CH can i get a goood ring pack on the pistons how much will i have from top of the pistons to 1 ring groove?? i have had custom pistons made for longer rods and see test done on this to apoint this may be true but you have to ask what are you going to do with the engine??

I agree with you Pat, that's why I chose to build the 420 instead of the 434. I had to find a compromise between rod length and compression height that would hold up on the street.
I didn't like the way the wrist pin went into the oil ring. Be OK on a race engine along with gas ported rings etc. etc. They only have to last a few runs down the quarter. So all in all its all a compromise and you must choose wisely how your engine is to be used and build it accordingly. All too often I see hotrodders trying to take a shortcut to power and over cam, over head, and spend a lot of money to go slower than where they started. I had a friend that knows it all and I watched him take a good running mustang with a 302, running mid=11s and spend 5k on it to make it run 12-0s

Originally posted by Ives Bradley
Hi Ray, Erik. I would add one thing more if its OK. The racetrack and the street are 2 different realitys. Taking a race car onto the street requirea all kind of compromises, and trying to run a street car against purpose built racecars is like a 1 legged man at a [well u get the drift]. We all want to think we have a special kind of car for our own bragging rites, and thats a big part of the fun, as long as we keep it in perspective. Race engines work best on the track and any attempt to put them on the street compromises all the engineering genius.

I would hardly call running a 5.565 rod in a 400 (engineering genius)This was only laziness on the engineering departments part.Hot Rodding is all about modifications of any and all kinds.

Originally posted by pat mccarthy
this is only true to a point .the long rod will make the pistons shorter. pin is hirer up and with some stroke the pistons get verry short and the skirts to .so this works on the rings and on the walls so on a race engine rod speed is the eschew short rod can be made to run good and piston companies can re work skirts design or tapper trunning may help to so if not careful you end up were you started .working the rings and wallls and the pistons not stable in the bore. i look at it what will it takes to hook the crank to the pistons . is this a good CH can i get a goood ring pack on the pistons how much will i have from top of the pistons to 1 ring groove?? i have had custom pistons made for longer rods and see test done on this to apoint this may be true but you have to ask what are you going to do with the engine??

Are you saying that as the rod gets longer your piston gets so small that it starts to cock more in its bore?? I can see that happening........but also if you go with a short rod won't your crank throw hit the piston? I didn't think about that piston wobble that can happen, but it sure makes me glad that I opted for the 420 instead of the 427 with it one inch CH

I think for a street small block with a standard deck height, a 3.875 stroke and 6" rod should be max for durability........Just another thought.

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Originally posted by camaro_fever68
Are you saying that as the rod gets longer your piston gets so small that it starts to cock more in its bore?? I can see that happening........but also if you go with a short rod won't your crank throw hit the piston? I didn't think about that piston wobble that can happen, but it sure makes me glad that I opted for the 420 instead of the 427 with it one inch CH

I think for a street small block with a standard deck height, a 3.875 stroke and 6" rod should be max for durability........Just another thought.

yes that is what i am trying to say the piston pin is now thru the oil ring pack and there is no skirt and the top part of the piston is 0.030 smaller and the pin is now up to the top and not much skirt and this will not make it better. this was a comment on rods and how i see it .a lot of pistons for longer rods are made for racing and not street use so they do not have much skirt less drag . that is why i do not think longer rods do not all ways make things better .

I think you guys have done your homework well. You get a cookie.

Check this Dave.

I had a race engine or two with the oil rings in the pin hole. Works great on a race track, probably not a good idea for the street. Would seem the longer the rod the better, to the point of not having the oil ring in the pin hole.

What about ring gap on a long rod motor?? A guy around here preaches no ZGS rings on long rod motors....any logic to that???

Originally posted by Dave Severson
I had a race engine or two with the oil rings in the pin hole. Works great on a race track, probably not a good idea for the street. Would seem the longer the rod the better, to the point of not having the oil ring in the pin hole.

What about ring gap on a long rod motor?? A guy around here preaches no ZGS rings on long rod motors....any logic to that???

The longer the rod the better till the point of being in the oil ring is good.

On the zero gap rings, seems as if that's not nearly as good idea as once thought. When I ordered my Ross pistons, I was advised to stay away from them and go with file fit. Here's something I found on Federal Mogul's web:

http://www.federal-mogul.com/newsroo...ng.pdf#search='zero%20gap%20'

Thanks Fever, saved that one two. The advise you got is the same I got when I was talking pistons with a Wiesco rep.....suppose there's something to it????? Guess I'll go with what the manufacturers say. Thanks for the link

Every engine that I have built for circle track that is for a I.M.C.A. modified or U.S.M.T.S. engine has at least a 6 inch rod in it.The Dart block which is 400+ cubes will use a 6.125 rod.The benifits of using a long rod have been proven to us on the dyno and also ring wear,cylinder wear,and piston skirt wear has been greatly reduced.