Theoretically, a 6" rod will give you more torque than a 5.7" rod.

Anyone know how much additional torque it's good for? (Assuming two otherwise identical 350's, one with 5.7 rods, the other with 6.0.)

Is it worth the additional expense on a hp/$ basis?

Any downside to using the longer rod?

Thanks!
Mike

You get to use a lighter piston. That is a good thing smile.gif

Theoretically, a 6" rod will give you more torque than a 5.7" rod.

Whose theory would that be?

David Vizard's

How does Vizard's theory compare to Stahl's or Draganowski's?

Originally posted by Epistuff:
You get to use a lighter piston. That is a good thing smile.gif Doesn't it also require the pin to be located higher up in the piston, removing a section of the ring land of the bottom ring?

Here's another view on long rods and their supposed benefits:

http://bellsouthpwp.net/k/_/k_jett/Images/longrod.jpg

I'm thinking a 2-3hp difference for a 500horse motor. You'll prolly see a bigger hp difference because of the lighter piston. For the reliability and longevity of a street motor I don't think a 6" rod in a 3.75" stroke or more small block is worth it. In a 350, 6" might be ok since the ring pack isn't up as high. For a full out race motor Smokey Yunick says to ..."stick the longest damn connecting rod in the engine that you can" hehe.

I'm going with a longer rod (6.535") in the new BBC for two main reasons...

1. the piston becomes lighter by a measurable margin

2. less side loading on the block which means longer cylinder wall and block life.

I doubt there's really performance gains in just a longer rod. But having a lighter rotating assembly is a definite plus.

SHORT CONNECTING RODS

Short connecting rods and small cylinder bore diameters (used in conjunction with a custom stroker crankshaft to restore displacement) are more able to resist detonation than longer rods and bigger bores. According to two-time champion Jon Kasse, the short rod yields very fast piston action at TDC and minimizes dwell time so the pistons get away from the chambers as quickly as possible. More time spent at TDC increases the chance that non-homogenized portions of the mixture will ignite on their own and rattle the motor. Smaller bores are advantageous because they reduce the distance the flame front has to travel and the smaller area also offers less opportunity for unwanted secondary flame fronts to develop. The small bore theory must not be taken to the extreme or valve shrouding becomes a larger issue. Canted valve cylinder head designs are more forgiving of small cylinder bores as they open the valves toward the center of the bore. Under the 2004 rules, only builders of Ford Cleveland style engines can take advantage of this fact.


Copied this from the Engine Masters in PHR

We ran a sb motor with 6.125 rods and custom diamond pistons, that thing was the torque monster from hell. It needed good fuel but it ran forever with regular maintenance. It was a wild motor to sit behind, i'm a firm believer of long rods and their benefits. Ralph

Originally posted by Epistuff:
You get to use a lighter piston. That is a good thing smile.gif I saw this mentioned a couple of times. Wouldn't the additional weight from the longer rod negligate the weight savings in the lighter pistons?

I originally saw the longer rod theory mentioned by Smokey. Usually, when Smokey talks he is right on. Is it possible he's wrong about this?

Anyone ever compare two otherwise identical engines on the dyno, one with regular, the other with longer rods?

Mike

77 Cruiser...Jim-

I wrote that. Strange seeing it here...LOL..!

Scott Parkhurst~!

Originally posted by Unclepennybags:
</font><blockquote>quote:</font><hr />Originally posted by Epistuff:
You get to use a lighter piston. That is a good thing smile.gif I saw this mentioned a couple of times. Wouldn't the additional weight from the longer rod negligate the weight savings in the lighter pistons?

I originally saw the longer rod theory mentioned by Smokey. Usually, when Smokey talks he is right on. Is it possible he's wrong about this?

Anyone ever compare two otherwise identical engines on the dyno, one with regular, the other with longer rods?

Mike </font>[/QUOTE]You have to sit down and compare piston weights and rod weights for each rod length. I did this comparison, but can't remember the exact weights & differences off the top of my head. The piston loses pretty good weight for each piston pin change for the longer rod. The rod itself as it gets longer doesn't gain that much in weight, so there's still a measurable new loss in bob weight. Plus, the piston is way at the end of the reciprocating mass and the losses in weight are more pronounced compared to weight gain of a rod which has it's weight more closely to the center of the crank by the rod throw. At least that is how envision it.

Yeah, there have been rod length dyno comparasons. The differences seem to me to be within the repeatability factor of the dyno.

Awhile back a reputable builder built two 383 smallblock chevs, one with the 400 style short rods and the other with 5.7 rods and found no measurable difference- they were both +or- a couple of horsepower all the way throught the pulls. This story was in a magazine but can't remember which one. Joe Sherman might have been the builder. I often wondered why even the short rod motors like Ford's 302 work ok. Baffling stuff when you hear different opinions from so many knowledgeable people. Bob

Originally posted by Scotch:


I wrote that. Strange seeing it here...LOL..!

Scott Parkhurst~! Just seemed fitting at the time :cool:

OK, heres the numbers I dug up.

Eagle H-beam rod (bbc)
6.135" (stock length) 780 grams
6.385" (+.250" long) 785 grams
6.535" (+.400" long) 805 grams

JE dome piston (bbc 4.280" bore)
1.645" comp height (6.135" rod) 641 grams
1.395" comp height (6.385" rod) 631 grams
1.245" comp height (6.535" rod) 573 grams

6.135 setup = 780 + 641 = 1421 grams

6.385 setup = 785 + 631 = 1416 grams

6.535 setup = 805 + 573 = 1378 grams

Originally posted by 10secBu:
OK, heres the numbers I dug up.

Eagle H-beam rod (bbc)
6.135" (stock length) 780 grams
6.385" (+.250" long) 785 grams
6.535" (+.400" long) 805 grams

JE dome piston (bbc 4.280" bore)
1.645" comp height (6.135" rod) 641 grams
1.395" comp height (6.385" rod) 631 grams
1.245" comp height (6.535" rod) 573 grams

6.135 setup = 780 + 641 = 1421 grams

6.385 setup = 785 + 631 = 1416 grams

6.535 setup = 805 + 573 = 1378 grams I think the rotating weight is 2x rod. The lighter pistons will help. What we really should do is finder a lighter crankshaft! Man, the BBC cranks weigh a ton! I've got too much invested in my Lunati Racer Series to go down that road. Maybe on the next one ;)

Wouldn't the shorter rod, with less dwell time, fill the cylinder quicker since it accelerates faster?

Ken, I wasn't trying to calculate bob weight rather show that a longer rod/shorter piston height ISN'T heavier than a shorter rod/higher piston height combination.

Really, most of this stuff is trivial as most of us will never really see any performance difference in different rod lengths.

Originally posted by Fried_Guy:
Wouldn't the shorter rod, with less dwell time, fill the cylinder quicker since it accelerates faster? It may fill quicker, but may not fill more completely, especially at higher rpms.

just a rambling thought of mine...feel free to correct me ;)

Where's Mike Lewis when you need a real world experience answer :confused: :D

Originally posted by 10secBu:
Ken, I wasn't trying to calculate bob weight rather show that a longer rod/shorter piston height ISN'T heavier than a shorter rod/higher piston height combination.

Really, most of this stuff is trivial as most of us will never really see any performance difference in different rod lengths. Yes, I understand. I was just pointing out that there is not a 1:1 relationship between rod weight difference. IOW, if each rod is 20grams lighter you don't loose 160grams of rotating weight by using the lighter rods (I don't think so, anyways).

I'm no Mike Lewis but I've seen him answer about this before.

He always talks about a round-table discussion of Hi-Perf engine builders. I'll let him do the quoting when he chimes in, but this is what I remember:

Pick the crank you need to get your displacement, choose the piston you need to get your compression, find your the deck height you want, use a connecting rod that fills the space.

Also, Mike seems to like using the 6" rod on stroker small blocks to make internal balancing easier.

Mike, sorry if I got anything wrong.

I don't remember Visard ever saying that :confused: As I recall he said that shorter rods will make more TQ.

The reason shorter rods make more TQ is because of the greater rod angularity (leverage) on the crank pin. All in all I don't think the differences are very large between the two -- probably barely measureable.

There are advantages/disadvantages to both rod lengths depending on the application.

The long rod pistons & rod assemblies are lighter than the short rod.

The short rods motors have less clearance problems, less issues with room for the ring package.

The long rods create less side loading

The short rods have higher piston acceleration rates (good for low end, bad for high RPM)

The short rods have more leverage on the crank pin, marginally more low end TQ

Long rods have better ring sealing at high RPM due to slower piston acceleration rates (ring flutter)

The short rods move the piston through TDC faster and are in theory more detonation resistant.

In summary, short rods are good for engines that will stay &lt;6500, 6" rods better for higher RPM engines. Either rod will work, there isn;t a huge difference in most applications IMO

So there you have it . . . my opinion on rod length ;)

Here's a link to more info (http://www.grapeaperacing.com/GrapeApeRacing/tech/connectingrods.pdf). It's a PDF file so you'll need Acrobat Reader.

Not again :rolleyes:
Here is a "short" little post to chew on,, or up as you desire :D

On balancing & weight, for a V8 it is 50% reciprocating & 100% rotating.
This means it is the weight of the lower end of two rods, bearings & the weight of 1 piston, rings, pin, locks, & the upper half of the rod plus an amount for oil which some use & some don't, I use 5 gms.

The major forces on the rod are piston & attendant parts weight on overlap at TDC.
The lighter you can make these parts the happier your engine will be & the further (and faster) it will RPM before very bad things happen assuming you do not sacrifice needed strength for weight.

On rod length, here are a few of both my thoughts along with a few other people a lot smarter than I that I have posted before about a dozen or so times ;)


Here is my take on rods, I posted this on another forum a while back in response to losing power in a 400 small block from using a "short" rod:

Short answer, Basically you are not losing anything.
My preference is a 6" rod simply because it makes internal balancing much easier but power wise don't expect any "magical" gains with a longer rod.
There are arguments for both short & long rods & in my opinion the best rod is the one that is long enough to connect the piston & crank together & leave you enough room for internal balancing.
Their is a minimal gain in regards to cylinder wall loading which gets way overblown in a longer rod but the shorter rod is less sensitive to intake reversion given the same cam as the piston dwells longer around BDC.
It also moves away from TDC SLIGHTLY faster which tends to help crappy heads or restricted intake stuff.

Here is a little more I have posted before for a little more info some of which is also above :D

I'll probably catch all kinds of flack for this but here's my opinion.
First let me say I normally prefer a longer rod but,,,,,
Rod length is one of the most over rated, over discussed parts of engine building.
I favor a rod long enough in strokers to be able to clear the needed counter weight as it normally makes balancing easier. If the rod is too short you can't get enough counterweight on the crank to balance it.
If you were to take the crank angles differences between say a 5.7 & 6" rod & overlaid them on a graph you probably wouldn't be able to differentiate between the two lines on the graph!

In a longer rod engine the piston dwells longer around TDC & less around BDC which can be argued both ways.
If it is moving away from TDC slower it isn't putting as much power into turning the crank but it gives the charge more time to build pressure.

Ok, if it's a "short" rod it is moving away from TDC faster it will put more power into turning the crank sooner & should tend to help crappy heads or restricted intake stuff. :confused:

My basic opinion of rod length is if you don't have a good reason to change that actually makes sense,,,, leave it alone.

This was summed up real good at the Superflow Advanced Engine Technology Conference a few years ago in a round table discussion with some of the very best in the business of engine design in NASCAR.
This ain't a direct quote but it went something like this:
You decide on a stroke, design a piston to fit your needs, measure the deck height of the block & then make something to hook them together ;)
That was pretty much the consensus on the importance of rod length in overall engine design.

Originally posted by Pat Kelley:
Here's a link to more info (http://www.grapeaperacing.com/GrapeApeRacing/tech/connectingrods.pdf). It's a PDF file so you'll need Acrobat Reader. This article states that "less rod angle also gives better average leverage on the crank for a longer period of time". Would I interpret from this that he means that if the rod was straight up and down all the time it would have maximum leverage? It seems to me that a shorter rod would have more leverage on the crank since it approaches a 90° rod/crank angle higher up in the bore when the pressure on the piston is higher.

You guys are making a big deal out of this long rod short rod stuff.....the only benefit is the lighter piston hanging on the end of the longer rod...in an aluminum rod engine this is beneficial,think of it in terms of mass trying to pull apart the rod :cool: