Connecting rod length theroy......

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Connecting rod length theroy......

Post  Tennessee Bullitt on February 19th 2013, 8:44 pm

This rod question tends to mirror the brand-x small block. Having a 460 with the factory 3.85 stroke, and no patricular setup, it can be mild to wild. Having the 3.85 crank with the 6.605 rods, how would that combo compare to a 3.85 stroke with a 6.8 rod length? I ask because I was on the computer looking around and found pistons made for the 6.8 rod on the 3.85 crank and led me to the whole deal where SBC guys swear by running the 6.0 rods vs the 5.7 rods. Other than the piston staying at TDC longer and building slight more compression for the burn. Does it move the TQ curve up higher or give a better TQ curve like the chevy guys always talk about? Wonder how it would do in a BB application.
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Re: Connecting rod length theroy......

Post  466cj on February 19th 2013, 9:36 pm

Rod ratio will be 1.76 for the 6.8 rod vs. 1.71 for stock 460 rod/stroke. Difference is longer rod will have more dwell time around TDC and not push the piston into the cylinder wall as much. Real world answer is don't waste you time or money. People got hung up on long rods years ago when Smokey Yunick said to use the longest rod that would fit. You will never notice the difference. BTW a 429 has a 1.84 rod ratio and a 454 Chebby has a 1.53 ratio which is about the same as a BBF with a 4.5" stroke and 6.8" rod at 1.51.

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Re: Connecting rod length theroy......

Post  Tennessee Bullitt on February 19th 2013, 9:52 pm

I was thinking the same Steve. I can't figure out why it would be put on the table now instead of years ago. Still sounds cool to have it done in a BB application. I still like my stroker setups.
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Re: Connecting rod length theroy......

Post  maverick on February 19th 2013, 10:34 pm

Some details here... www.rustpuppy.org Scroll down to "Rick's study of connecting rod length". Sheds a little light...
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Re: Connecting rod length theroy......

Post  Paul Kane on February 19th 2013, 10:52 pm

When I see inquiries on discussion forums about "6.7 vs 6.8" con rods in a 4.5 stroke 385 Series, I find the subject essentially moot since it is not as siginficant as most people make it out to be--certainly not from a rod ratio stand point in that application. I also feel that in most (but not all) cases the subject is overthought even in regards to the "D9TE block or not" cases as well.

But in the case of the "long rod 460" with the KB855 or KB856 piston, we are now talking about several changes simultaneously, including:

  • ~0.200" increase in rod length over OEM
  • 0.300" less journal diameter on the crankshaft journal
  • a lighter piston with the 6.8 rod than a comparable forged 460 flat top piston for the stock length rod
Depending on the engine application and usage, some people might feel that the above-noted changes collectively do make a difference in certain applications.

Paul


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Re: Connecting rod length theroy......

Post  DanH on February 20th 2013, 2:43 am

on a restricted engine , be it carb, plate , head or manifold/combo of them -the longer rods seems to be best when higher rpm is needed.

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Re: Connecting rod length theroy......

Post  dfree383 on February 20th 2013, 9:39 am

some times shorter rods can go faster down the track than longer rods...... but whos counting.
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Re: Connecting rod length theroy......

Post  rmcomprandy on February 20th 2013, 11:46 am

DanH wrote:on a restricted engine , be it carb, plate , head or manifold/combo of them -the longer rods seems to be best when higher rpm is needed.

Unfortunately ... everything in his report is about the short block; (he did leave out "dwell amount at BDC", though).

The minor effects the rod ratio has on air velocity, required port volume and valve timing events as well as the valve lift cycle is much more important for making power than anything you do within the short block; (as long as you are using a GOOD short block).

Use whatever rod fits your entire application the best and forget about its length...

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Re: Connecting rod length theroy......

Post  LivermoreDave on February 21st 2013, 7:30 am

Like most have said, don't complicate the math, usually what fits will work. I have read that with higher piston speed, a cylinder can continue to pull on the intake tract after BDC, shortly before the pistons starts to climb and a lighter rod may offer a bit of engine acceleration in certain applications with all else considered, all assets of a shorter rod.

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Re: Connecting rod length theroy......

Post  DanH on February 22nd 2013, 12:41 pm


any one ever check what NHRA ProStockers a Nascar Cup engines use ?

the 1.8 area or better

built with spec for what its used for

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Re: Connecting rod length theroy......

Post  jc10000rpm on February 23rd 2013, 12:19 am

nhra pro stockers in a whole different world...... look at horsepower per cubic inch

nascar whole different world........... look at sustained rpms

not trying to argue any certain point here at all, just saying i dont think we should compare the applications

example: iv seen short rod deals do real well when trying to avoid detonation, based on the other design constraints of the build

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Re: Connecting rod length theroy......

Post  DanH on February 23rd 2013, 2:00 am

jc10000rpm wrote:nhra pro stockers in a whole different world...... look at horsepower per cubic inch

nascar whole different world........... look at sustained rpms

not trying to argue any certain point here at all, just saying i dont think we should compare the applications

example: iv seen short rod deals do real well when trying to avoid detonation, based on the other design constraints of the build

"application" what do Pro Stockers do different than a bracket racer or a quick xx racer ?

cup car's they do use brakes , then have to climb back up to rpm

btw . whats the spread of hp on a 472 cube BBF? all BBF 472 engines the same HP? No. then whyhp to cubes compare

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Re: Connecting rod length theroy......

Post  rmcomprandy on February 23rd 2013, 2:08 pm

DanH wrote:
jc10000rpm wrote:nhra pro stockers in a whole different world...... look at horsepower per cubic inch

nascar whole different world........... look at sustained rpms

not trying to argue any certain point here at all, just saying i dont think we should compare the applications

example: iv seen short rod deals do real well when trying to avoid detonation, based on the other design constraints of the build

"application" what do Pro Stockers do different than a bracket racer or a quick xx racer ?

cup car's they do use brakes , then have to climb back up to rpm

btw . whats the spread of hp on a 472 cube BBF? all BBF 472 engines the same HP? No. then whyhp to cubes compare

Being that you like asking impertinent questions then I'll ask you one.
How many bracket racers use an engine with such a narrow power band that they NEED a 5 speed manual transmission and are only 2,400 pounds maximum with a door car...?

A different world, indeed.

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Re: Connecting rod length theroy......

Post  DanH on February 23rd 2013, 8:10 pm

rmcomprandy wrote:
DanH wrote:
jc10000rpm wrote:nhra pro stockers in a whole different world...... look at horsepower per cubic inch

nascar whole different world........... look at sustained rpms

not trying to argue any certain point here at all, just saying i dont think we should compare the applications

example: iv seen short rod deals do real well when trying to avoid detonation, based on the other design constraints of the build

"application" what do Pro Stockers do different than a bracket racer or a quick xx racer ?

cup car's they do use brakes , then have to climb back up to rpm

btw . whats the spread of hp on a 472 cube BBF? all BBF 472 engines the same HP? No. then whyhp to cubes compare

Being that you like asking impertinent questions then I'll ask you one.
How many bracket racers use an engine with such a narrow power band that they NEED a 5 speed manual transmission and are only 2,400 pounds maximum with a door car...?

A different world, indeed.

would of help if you defined the narrow power band in rpm .

there are two local small block bracket racers that do .
now if you can tell me how many bracket racers there are in the world .

now tell me how many times they shift at NASCAR short tracks per lap .
none is the answer

"impertinent" - time for you to get thicker skin

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Re: Connecting rod length theroy......

Post  rmcomprandy on February 23rd 2013, 10:00 pm

As my PM's have scolded me ... I should know better than to feed the trolls. silent

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Re: Connecting rod length theroy......

Post  maverick on February 23rd 2013, 10:11 pm

rmcomprandy wrote:As my PM's have scolded me ... I should know better than to feed the trolls. silent

Laughing Laughing Laughing
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Rod Length - Don't Get Spun Up

Post  mkopmani on February 23rd 2013, 10:16 pm

Randy's right on rod length - it's waaay overdone. If you don't believe us, do some research and look at what David Reher has to say about it, and what Curtis Leaverton (Desk Top Dynos - Dynomation) says. Keep these things in mind:

A. Any additional "dwell" benefit for pressure is near TDC, where the crank slider mechanisim is at its lowest efficiency.
B. Modern cylinder heads flow a lot better than the old factory junk.
C. Look at Pro Stock HP levels/ci and the deck heights and rod ratios - case closed.
D. Combustion chamber and efficiency burn rate have more effect on pressure decay.
D. Smokey was right about a lot of things and probably what he said then about long rods, but because of "B" things change.


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Re: Connecting rod length theroy......

Post  samiam on February 24th 2013, 2:08 am

So then the Perfect set-up would be a 429 crank and stock rod length, and largest bore possible ,,,and a shorter deck ,,, right ????


Last edited by samiam on February 24th 2013, 2:10 am; edited 1 time in total (Reason for editing : forgot to mention)
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Re: Connecting rod length theroy......

Post  dfree383 on February 24th 2013, 3:32 am

samiam wrote:So then the Perfect set-up would be a 429 crank and stock rod length, and largest bore possible ,,,and a shorter deck ,,, right ????

I didn't realize every situation had the same perfect solution....... What works on a pro stock car may not be the best on a well pump........
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Re: Connecting rod length theroy......

Post  DanH on February 24th 2013, 6:59 am

rmcomprandy wrote:As my PM's have scolded me ... I should know better than to feed the trolls. silent
you mean your not eating anymore?

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Re: Connecting rod length theroy......

Post  DanH on February 24th 2013, 7:02 am

mkopmani wrote:Randy's right on rod length - it's waaay overdone. If you don't believe us, do some research and look at what David Reher has to say about it, and what Curtis Leaverton (Desk Top Dynos - Dynomation) says. Keep these things in mind:

A. Any additional "dwell" benefit for pressure is near TDC, where the crank slider mechanisim is at its lowest efficiency.
B. Modern cylinder heads flow a lot better than the old factory junk.
C. Look at Pro Stock HP levels/ci and the deck heights and rod ratios - case closed.
D. Combustion chamber and efficiency burn rate have more effect on pressure decay.
D. Smokey was right about a lot of things and probably what he said then about long rods, but because of "B" things change.


-Mike

(C) what is the rod ratio of the NHRA Pro Stiockers?

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Re: Connecting rod length theroy......

Post  Outlaw5.0 on February 25th 2013, 9:05 pm

NHRA prostock is in the 1.70 area, give or take depending on the bore and stroke.

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Re: Connecting rod length theroy......

Post  DILLIGASDAVE on February 25th 2013, 9:37 pm

So a 351M & 429 both have a decent R/S ratio in stock form.
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Re: Connecting rod length theroy......

Post  Outlaw5.0 on February 25th 2013, 10:10 pm

Anything above 1.50 is good in my opinion.

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Re: Connecting rod length theroy......

Post  whatbumper on February 25th 2013, 11:38 pm

I've always been told to pick a crank you like and a piston you like and connect the two.

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