For a 461 or 468 BBC, with 4" stroke typically not going over 6600RPM, street/strip duty, mid-upper 11's in my Chevelle. What is the advantage to going with a .25" longer rod or a .4" longer rod over the stock lenght of 6.135"

I would guess a longer rod gives you a shorter stroke and means quicker rpm turn up. On the down side it will give you less cubic inches,

Originally posted by Bad Rat 414:
I would guess a longer rod gives you a shorter stroke and means quicker rpm turn up. On the down side it will give you less cubic inches, ??? A shorter crank would give me a shorter stroke. I am talking about the connecting rod length. Ie: changing the rod-stroke ratio.

A longer rod combined with a piston matched for it (higher pin location) will give you more torque. It won't like to rev like a short rod will, but torque is the name of the game graemlins/beers.gif

It changes the angle of the force that is applied through the rod to the crank throw, producing slightly more torque. It lessens piston skirt loading, allows a lighter piston, and lengthens the time the piston spends at top dead centre.

Other than the ability to run a lighter piston, the other effects are minimal depending on who you talk to.

Originally posted by Team140:
A longer rod combined with a piston matched for it (higher pin location) will give you more torque. It won't like to rev like a short rod will, but torque is the name of the game graemlins/beers.gif Actually, just the opposite.

Jim,
The long rod, in the this application will serve the simple purpose of making for a lighter rotating assembly.

Long rods require a shorter compression height making for a lighter piston, therefore a lighter rotating assembly.

Since lighter is typically better with rotating assemblies in drag racing applications, i'd say its worth while.

I seriously doubt that it will have an astounding effect on the characteristics of the motor overall.

I agree with both the last answers & feel the major benifit is you can get enough counterweight on the crank to balance the engine internally.

I have posted this before but here it is again :D

I'll probably catch all kinds of flack for this but here's my opinion.
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 eaiser. 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 probablly 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. :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 this year in a round table disscusion 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.

Imho and after reading and studying about this for quite a while, I suggest matching head ports, cam timing, carb. and intake and headers around a rod/stroke ratio of 1.7 to 1 b/c that seems to be the number that produces the most grunt in several given engine applications!!

5.0 Ford, and big Buick to name two..

pdq67

Originally posted by 10secBu:
Actually, just the opposite. Care to elaborate, and the reasoning behind it? Maybe I was raised around idiot engine builders.

Of all the engine builders I have talked with, a longer rod will produce torque sooner, but doesn't like high RPMs like a short rod motor will.

Originally posted by pdq67:
Imho and after reading and studying about this for quite a while, I suggest matching head ports, cam timing, carb. and intake and headers around a rod/stroke ratio of 1.7 to 1 b/c that seems to be the number that produces the most grunt in several given engine applications!!

5.0 Ford, and big Buick to name two..

pdq67 Or a 327 Chevy.... :D

Team:

I think you're confusing "rod length" with "stroke".

2 different things.

I think Wolfplace gave the best description of what it does. It makes the piston hang around TDC (and BDC also) longer than a short rod. This improves the motor's "thermal efficiency", which is how well it turns heat into motion. On the other hand, the shorter rod will get the piston moving away from the top at the start of the intake stroke more quickly; which helps cylinder fill, because the intake charge gets started moving into the cyl sooner and more vigorously.

It's one of those magazine-article-world things that doesn't carry over to real-world near as well as it sells magazines. It's about like worrying about "quench" in a 8½:1 motor with dished pistons: alot of noise over nothing.

In the game of extracting every last bit of power from a given combo changing rod length can be one factor to consider. But taken all by itself rod length has a miniscule effect on how the motor behaves.

It just depends on if you take into consideration the rest of the combo. Long rods dwell at TDC a tiny bit longer. That allows a very small reduction in ignition advance, so the piston has less pressure to fight against on it's way up the bore. If you were to couple that with a small combustion chamber that burns fast like some of the aftermarket designs or a nice closed chamber head, the reduction in required ignition advance could be even greater - resulting in a motor that makes a little more power at higher rpm. Also consider how rods effect DCR.

Rods alone won't change the character of an engine, but if taken as a whole they can be used to compliment the characteristics of a given engine combo. After all that's what it's about, making all the parts in an engine work together. Really though if you are going to spend a lot of time worrying about a particular aspect of an engine combination - I would spend that time thinking about the heads, because that's really where it all happens.

Originally posted by Motor Martyr:
Jim,
Long rods require a shorter compression height making for a lighter piston, therefore a lighter rotating assembly. Doesn't the longer rod weigh more than the shorter rod? So the piston is lighter, but the rod is heavier?? Not being a smart a$$, just trying to figure out the lighter rotating assembly theory by going to a longer (heavier) rod.

Originally posted by Team140:
</font><blockquote>quote:</font><hr />Originally posted by 10secBu:
Actually, just the opposite. Care to elaborate, and the reasoning behind it? Maybe I was raised around idiot engine builders.

Of all the engine builders I have talked with, a longer rod will produce torque sooner, but doesn't like high RPMs like a short rod motor will. </font>[/QUOTE]It's all about leverage on the crank. The most torque you can apply occurs when the force is applied at a tangent to the crank circle. If you've ever ridden a bike, you know that tilting your leg to create this condition on the pedal crank results in the bike accelerating faster. The inherent angularity of the short rod applies the most force to the crank at the top of the stroke, resulting in the most torque at this point. At some point wall friction, flame front physics, piston speed, etc. overcome the advantage of the leverage and the longer rod may work better. Draganowski's rod study shows the calculated difference in effective stroke and angles.

Originally posted by Epistuff:
</font><blockquote>quote:</font><hr />Originally posted by Motor Martyr:
Jim,
Long rods require a shorter compression height making for a lighter piston, therefore a lighter rotating assembly. Doesn't the longer rod weigh more than the shorter rod? So the piston is lighter, but the rod is heavier?? Not being a smart a$$, just trying to figure out the lighter rotating assembly theory by going to a longer (heavier) rod. </font>[/QUOTE]Yes, but its offset by a piston that weighs alot less then it would with a taller compression height.
The increase of rod weight is much offset by the decrease in piston weight.

here's an example:

the constant is the 4.00" stroke:

I'll use Crower BBC billet rods:
6.136" lenght: weight 850 grams

6.386" Lenght: Weight 870grams:

SRP closed chamber pistons:

1.645 compression height (for 6.136" rod): 638 grams

1.395 compression height (for 6.386" rod): 599 grams


6.136" package: 1488 grams total

6.386" package: 1469 grams total

As you can see the extra weight of the rod, is offset with weight removed from the piston.

Piston weight is more important, since the beam of the rod is closer to the crank then the piston removing weight from the piston will have a more positive effect than removing weight from the beam of the rod.

The weight difference really depends strongly on the specific combination of parts you use.

I went through this same exercise once recently, elsewhere; I picked Eagle H-beams (350, 5.7" vs 6") and JE pistons.... another popular, likely combo of parts. In that case, the longer rod ended up weighing about 35 grams more than the short rod.

This is one of those things where in a situation where you're rules-limited, and it's allowed, you'll make more power with (say) a 4.155" bore and 3.31" stroke with a 6.125" rod, than if you get the same 5.8 liters with a 4.060" bore and a 3.48" stroke and a 5.7" rod. Not by much, maybe; but more is more. Is it the right thing to do under those circumstances? Absolutely: if you don't, you automatically lose. Is it worth it on the street? Usually not. If there's no rules limiting other things, you can pick up that 0.8% (or whatever) total output some other, usually cheaper, way.

Originally posted by pdq67:
I suggest matching head ports, cam timing, carb. and intake and headers around a rod/stroke ratio of 1.7 to 1
pdq67 :eek: Wouldn't that be a 6.8" rod for a 4" stroke?

Good discussion going,...what do most of you guys run as far as rod sizes for a 4" stroke BBC?

Here is some interesting reading with pros and cons to both short and long rod engines and what components "supposedly" work best with each application&gt;&gt;&gt;&gt; Rod Ratio web site (http://victorylibrary.com/mopar/rod-tech-c.htm)

I should also add that i agree with Mike, in that Rod size is about the most minute thing in the engine combo, and way overdone.

6.8" rods are more then likely for tall deck motors.

select a piston..stroke and deck height...let the rod fit in between

I said 1.7 R/S ratio. It just seems to work "best" when everything else is sized for it..

And yes, everybody knows a BB has the poorest R/S ratio of all of the 440/454/455/460/472/500 BB's.

But it makes up for it in port and valve size b/c it rpm's better then the other stock motors, imho..

And yes, the tall deck truck block can be used to allow use of longer rods in the BB's if you want to put up with it's hassle factor..

Of all the GM Corporate motors, the 472 Cad. at 4.30" b x 4.06" s with a 6.75" long rod for an R/S ratio of 1.66 should be a real dandy of a motor after it's heads are reworked so the 2.19"/1.88" valves can be used and a CR. is picked so pump gas could be used..

It could use a forged crank and a better balancer too....

Of course, the 455 Buick is it with respect to being right on in the R/S ratio dept., imho...

pdq67

Well, after reading the info on that website I posted, I am really confused. According to their information, my present motor should have a single plane intake, large runner heads, large carb and tall gears. graemlins/sad.gif I am totally opposite of all of that now and I know it didn't like the single plane and big heads. I think I'll just stick with a 6.385 rod.

Originally posted by Epistuff:
Well, after reading the info on that website I posted, I am really confused. According to their information, my present motor should have a single plane intake, large runner heads, large carb and tall gears. graemlins/sad.gif I am totally opposite of all of that now and I know it didn't like the single plane and big heads. I think I'll just stick with a 6.385 rod. =
Jim,,
Excellent plan graemlins/thumbsup.gif
Just my humble opinion based on the importance of rod length that I posted previously in this thread though :D

Here's an interesting exercise:

Set-up an Excel spreadsheet to calculate piston position vs. crank angle for a typical 400 SBC using stock 5.565 rods vs. 5.7 rods.

I did this once (I have since lost the file), and I have found the difference to be so minor, it would probably make no difference in a real world set-up.

If you want to get fancy with your math, you could do some Calculus and look at the 1st derivitive with respect to time and establish piston velocity for a given rpm, the second derivitive will give piston acceleration. I used to know how to do this, but I am getting stupid as I get older. :D

Anyway, my conclusion was that I do not care about rod length, and I would have no problems using stock 5.565" rods in a street 400 SBC.

Probably the best example of a good balance of rod length is the 427 Chevy. The 3.76 stroke with a stock 6.135 rod yields 1.63 rod ratio and as far as I am concerned, there is nothing smoother than a Big Block 427. I was looking into this as well a few months ago. A 4" stroke motor will need a rod 1/2" longer to get the same rod ratio. The piston pin gets a lot higher and just doesn't make as much sense unless you are trying to fill up a tall deck block. Personally, if RPM's are the target, use the 427 crank and go for a big bore. I don't know anyone who has built one for street use, but I would bet a 4.5" bore x 3.76" stroke with good heads would produce one healthy 478CI engine that could pass for a 396.

Originally posted by novadude:
Here's an interesting exercise:

Set-up an Excel spreadsheet to calculate piston position vs. crank angle for a typical 400 SBC using stock 5.565 rods vs. 5.7 rods.

I did this once (I have since lost the file), and I have found the difference to be so minor, it would probably make no difference in a real world set-up.

If you want to get fancy with your math, you could do some Calculus and look at the 1st derivitive with respect to time and establish piston velocity for a given rpm, the second derivitive will give piston acceleration. I used to know how to do this, but I am getting stupid as I get older. :D

Anyway, my conclusion was that I do not care about rod length, and I would have no problems using stock 5.565" rods in a street 400 SBC. Could not agree more!!!

The rod length debate is interesting, and in all reality only about 1 in 10 people really state anything worth reading.

I wrote the excel spreadsheet you refer to in college years ago. The difference in piston position as a function of crank angle for 5.565" rods vs. 6" rods is insignificant in a 3.75" stroke sbc. The difference in velocity (the first derivative) is slightly greater. The difference in acceleration (the second derivative) is even greater, but still insignificant.

Reher Morrison stated something along the lines of "choose your piston and ring package as required and choose the rod length as required to connect the piston to the crank"

What this means supports what about three or four others have stated in this post: rod length is not a real important consideration in engine design.

I have a 5.565" rod 383 that runs so strong I am afraid to shift into second with my foot to the floor in fear the car will go in the ditch.

To hell with rod length!!!!! Spend your money on cylinder heads.

Epistuff, In a long rod big block Chevy (6.535) the rod will be about 19 grams heavier (Oliver billet) while the pistons,(J&E), are 72 grams lighter vs. (6.136) stock rod size with a 4.32 bore.

Hey, while everyone is on the subject ... I'll ask my question in regard to rod length.

It has always scared me to see the rod pin that high up into the ring pack. Personally it would seem there would be some detriment to having the pin broke through the middle of the oil ring ... Or, is this as miniscule as the rest of the rod length story?

Sherman did a comparison awhile back with a 383 chev, he dyno'd the identical engine combos except 1 had 5.56 rods and the other had 5.7 rods and it made no more than 2hp difference throughout.

Personal experience. We had a 468 std deck motor put the same parts in a tall block with .400 long rods. It made it a completely different motor. The torque band moved up in RPM to the point it would drive right through the trans brake. I read the Reher Morrison book about rod length but strange a friend had them build a std deck 565 and they used a .400 long rod. You do not go to the trouble of putting a .400 rod in that combo without a real good reason.