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Old 06-28-2007, 10:13 AM
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Rod length discussion, split from previous thread

Quote:
Originally Posted by burnt olds
do you own Exxon ,race gas or melt down
been there done that
the longer the rod the shorter the life , expecting to sink deep in pocket
your in the big league now, running a pro stock
motor that has to be torn down and gone through quite regular
no offense but longer rod with lower compression ratios that are more reasonable to street will not need to be rebuilt more often...6 inch rod on a 9.3:1 makes for some really nice mid range torque. longer rod only in increases piston dwell time and decreases the amount of "rock" or "slap" the piston has at the top and bottom of the cylinder. in effect providing a better rod ratio.
my 406 has a little over 120k on a 6 inch rod with 9.3:1 compression... i drive it ever where at 2700 rpm(67 mph)and alot faster if i want.
most race motors have the rods as long as they can get the pin to get the most effecient rod ratio.
the next one if a can do it may be a 6 1/8 length rod.

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Old 06-28-2007, 12:18 PM
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Quote:
Originally Posted by seriousracer
no offense but longer rod with lower compression ratios that are more reasonable to street will not need to be rebuilt more often...6 inch rod on a 9.3:1 makes for some really nice mid range torque. longer rod only in increases piston dwell time and decreases the amount of "rock" or "slap" the piston has at the top and bottom of the cylinder. in effect providing a better rod ratio.
my 406 has a little over 120k on a 6 inch rod with 9.3:1 compression... i drive it ever where at 2700 rpm(67 mph)and alot faster if i want.
most race motors have the rods as long as they can get the pin to get the most effecient rod ratio.
the next one if a can do it may be a 6 1/8 length rod.
Anytime I see this posted, I like to present the other side of it from a professional point of view, not based on my amateur experience. This was written by Ron Iskenderian, son of the famous "Camfather", Ed Iskenderian........

Tech Tip - 2005

Rod Lengths/Ratios: Much ado about almost nothing.

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.
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Old 06-28-2007, 02:21 PM
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well neat and some truth .. but to me the 65+ foot pounds i gained was huge for the price and well worth it in the fact that I needed it where it was at 1200 to 2800. slowing the piston at this speed helped with air charge into the cylinder..Mik Baker,BAKER enterprises,dick zabo shaker racing,jim gouged,high rev race engines, and i discussed this long before that artical was written. so i listened to a professionals about the gains and found what I wanted. and once again if you read what he said... if you increase your compression. cam or heads .. no offense but if i did this i would not have had streetable 406 with the amount of torque mine makes. not to mention towing with somehting that will need greater then 93 octane. so an increase in rod length is measurable (he did say this 5 to 7hp ) and was worth it to me. any gain is a gain,if you don't think that way than go race a spec road racing class or don't worry about degreeing your cam same thing... and some gains have adverse effects some don't this one doesn't.like he said i found the right point for my engine and what i want to do .i consulted with people who do what he does making sure i was not spending money i don't need to .. the next one is one to try but it may be to much....but my pros told this amture i might gain more at his rpm with a longer rod.
and please note i siad the most effeicent rod ratio not the biggest.
by the way i don't buy any magazines chasing the magic horsepower numbers....come ot think of it i have not bought magazines in 20 years..

let the flames begin...

Last edited by seriousracer; 06-28-2007 at 02:33 PM.
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Old 06-28-2007, 02:47 PM
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Quote:
Originally Posted by seriousracer
well neat and some truth .. but to me the 65+ foot pounds i gained was huge for the price and well worth it in the fact that I needed it where it was at 1200 to 2800. slowing the piston at this speed helped with air charge into the cylinder..Mik Baker,BAKER enterprises,dick zabo shaker racing,jim gouged,high rev race engines, and i discussed this long before that artical was written. so i listened to a professionals about the gains and found what I wanted. and once again if you read what he said... if you increase your compression. cam or heads .. no offense but if i did this i would not have had streetable 406 with the amount of torque mine makes. not to mention towing with somehting that will need greater then 93 octane. so an increase in rod length is measurable (he did say this 5 to 7hp ) and was worth it to me. any gain is a gain,if you don't think that way than go race a spec road racing class or don't worry about degreeing your cam same thing... and some gains have adverse effects some don't this one doesn't.like he said i found the right point for my engine and what i want to do .i consulted with people who do what he does making sure i was not spending money i don't need to .. the next one is one to try but it may be to much....but my pros told this amture i might gain more at his rpm with a longer rod.
and please note i siad the most effeicent rod ratio not the biggest.
by the way i don't buy any magazines chasing the magic horsepower numbers....come ot think of it i have not bought magazines in 20 years..

let the flames begin...

Are you saying you got 65 ft-lbs more with 6.0" rods than with 5.7s? how did you measure?
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Old 06-28-2007, 03:18 PM
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64 cc heads on 406sb

get with it guys i'm not the one asking the question in this matter
go to top of page and send him your comments and results
i just tried to give him advice
don't need any myself

BURNT OLDS
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Old 06-28-2007, 03:31 PM
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yes in the rpm range i was looking for improvement i gained that . my stock 400 rods were 5.565 so the change was slightly bigger..
chassis dynos on diffrent occasions. the only diffrence being humidity and temp.. same dyno twice and one checking against numbers of the other.
no real gross engine hp gain though.
the 406 that was removed(bad thrust) was all stock rotating assembly except for je pistons.and the 406 that went in was with the htcc crank and 6" rods with a set of je pistons with a touch less compression 9.3:1 versus 9.5:1.. poured...not guessing.
everthing else including cam was reused( i said i was a tight wad.)comp cam split profile roller hydrulic. same degreeing on cam. same heads World product heads. same carb, intake, dist,msd,machine shop doing the machine work.

.
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Old 06-28-2007, 09:21 PM
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All I will say is you didn't chassis dyno your engine in the 1200 to 2800 RPM range. All anyone has to do is the math on what 65 ft/ls of torque increase is at that level to know that what you claim is absurd.

In this day and age only girlfriends car how long your rod is. Most engine builders don't.
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Old 06-29-2007, 05:00 AM
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Quote:
Originally Posted by seriousracer
well neat and some truth .. but to me the 65+ foot pounds i gained was huge for the price and well worth it in the fact that I needed it where it was at 1200 to 2800. slowing the piston at this speed helped with air charge into the cylinder..Mik Baker,BAKER enterprises,dick zabo shaker racing,jim gouged,high rev race engines, and i discussed this long before that artical was written. so i listened to a professionals about the gains and found what I wanted. and once again if you read what he said... if you increase your compression. cam or heads .. no offense but if i did this i would not have had streetable 406 with the amount of torque mine makes. not to mention towing with somehting that will need greater then 93 octane. so an increase in rod length is measurable (he did say this 5 to 7hp ) and was worth it to me. any gain is a gain,if you don't think that way than go race a spec road racing class or don't worry about degreeing your cam same thing... and some gains have adverse effects some don't this one doesn't.like he said i found the right point for my engine and what i want to do .i consulted with people who do what he does making sure i was not spending money i don't need to .. the next one is one to try but it may be to much....but my pros told this amture i might gain more at his rpm with a longer rod.
and please note i siad the most effeicent rod ratio not the biggest.
by the way i don't buy any magazines chasing the magic horsepower numbers....come ot think of it i have not bought magazines in 20 years..

let the flames begin...
I am going to have to call you out on this one...... Show me the before and after dyno sheets... also show me the dyno sheets after you changed back to the orginal rods to prove the standard.......It's called A-B-A testing...If you do not have that info available for us to see then what you have is called theory.....And like the rest of the "long rod" theory it's all un-proven.

There is no way you picked up 65ft lbs at any rpm range with just a rod swap... Something else changed along with it...Like compression????

Keith
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Old 06-29-2007, 08:47 AM
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Quote:
Originally Posted by k-star
I am going to have to call you out on this one...... Show me the before and after dyno sheets... also show me the dyno sheets after you changed back to the orginal rods to prove the standard.......It's called A-B-A testing...If you do not have that info available for us to see then what you have is called theory.....And like the rest of the "long rod" theory it's all un-proven.

There is no way you picked up 65ft lbs at any rpm range with just a rod swap... Something else changed along with it...Like compression????

Keith
It was a chassis dyno, so the difference could have been a change in fans, fan belt tension, tire pressure, etc.

tom
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Old 06-29-2007, 09:31 AM
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In a racing engine, Smokey Yunick always said "run the longest connecting rod you can possibly fit into the engine" As for street or light drag engines I don`t think he cared how long they were. He also said the engine had to be set up to take advantage of the longer rod or there would be no changes as the longer rod changes the breathing characteristics of the engine since the changes in piston speed, cylinder feeding and etc. I think he spoke of the cam and the heads. Even so in this case, if the combo was already set up for the longer rod, and by no means am I calling anyone a liar here, It`s still hard to believe it would pick up that much torque with just a rod change. Smokey always believed in the longer dwell time at TDC, and discribed it as there was more time with the longer dwell time as the mixture would be burning, but the piston isn`t moving yet, so the mixture would be at it`s full energy of combustion when the piston began to move down on the power stroke and in theory this made more power. By no means here am I pulling a david vizard or a smokey yunick, I`m only repeating what I`ve read. by no means do I believe everything I`ve read. But even so Smokey has been dead for quite a few years, I wouldn`t dare call him a liar either, as he proved what he was capable of and what his ablities were, his accomplishments spoke for themselves. Under another article, I read that most professional builders feel the best rod ratio is 1.8:1 to 1.85:1 for a acceleration engine. Again, just a opinion, by no means do I want or wish to open any can of worms nor get technical about the whole thing. I just purchased a set of 5.940 lenth PM rods from a L99 4.3 V8 engine used in the 90`s for a 327 I`m building, It`ll put the rod ratio at 1.82:1. I plan to further study smokey`s theory on this and what pieces to use. The guy I bought them from is located in atlanta, and since I have to go to atlanta tomorrow to see my nephew run his 20 laps at the nascar track in a real nascar, I`m going to meet with the gent and pick the rods up, he told me the same thing over the phone as he works for a engine shop up there, he said "nascar guys, most all the pro`s will tell you, run the longest rod you can fit in a racing engine"
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Old 06-29-2007, 11:02 AM
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rods

Double,,,Most of the "Long rod" is better discusions come back to the smokey book...

Very few people have ever really tested the theory...I think his exact words did not say that longer rods make more power/last longer etc...
-----------------------------------------------------------------------
But at any rate it's technology from the 70's.... Like Rick stated, engine builders that moved into the next century with engine building and practices have found that rods are down about 100th on the list of power making parts...

I talked to a builder that raced a RED big block mopar about this issue. He told me over the 20 plus years of racing that he has tried every available rod combo for his engine and never found the car any faster.... No matter what he changed to go along with it...And he made it clear he tried everything..

On the professional engine builders site the power of longer rods has been put to rest.... There is some times debate over durability issues but even those have been hashed out with out total resolution...

Keith
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Old 06-29-2007, 12:27 PM
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Quote:
Originally Posted by k-star
I am going to have to call you out on this one...... Show me the before and after dyno sheets... also show me the dyno sheets after you changed back to the orginal rods to prove the standard.......It's called A-B-A testing...If you do not have that info available for us to see then what you have is called theory.....And like the rest of the "long rod" theory it's all un-proven.

There is no way you picked up 65ft lbs at any rpm range with just a rod swap... Something else changed along with it...Like compression????

Keith
i knew someone was gonna ask that when i posted it .. i looked around and still will keep looking around not sure where they are at(moved since than)and it was 6 years ago.i should have them...so i understand with out proof it is a doubt..
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