I'm not awake enough yet for this but I wanted to tell Mistah good job for the explain. i couldn't get that thru the keyboard last night.
longer connecting rod
- Thread starter Twister79
- Start date
Reher-Morrison did some testing years ago with their BBC motors and varying rod lengths. The results: the rods were a great way to connect the pistons to the crankshaft. They found NO difference in their testing. But I'd imagine that you may see a difference if all is optimized especially in a smaller motor. I always ran a 5.54" rod in my motors, along with a good-sized intake port volume/cam profile. My thought was it would build some awfully good torque for throttle-stop racing. The only real benefit I see to longer rods is the obvious decrease in cylinder wall loading which can be very harsh with a longer stroke and shorter rod length, plus the lighter piston weight as mentioned already.
JUAN
i see a big difference in the speed pro pistons and the lunati pistons as in weight.....
JUAN W said:
i see a big difference in the speed pro pistons and the lunati pistons as in weight.....
Squire Racing said:
Thanks!
To further elaborate, long rods will not make horsepower by itself, but allow horsepower to be made more eficiently, sort of like nitrous oxide. Nitrous alone does not burn but allows more fuel to be put through the motor to make power. It's working in harmony where these gains are made.
Mistah, A few things you are missing here:
Connecting rods break for a variety of reasons, material fatigue, rod bolt fatigue, detonation, piston mean speed, piston weight, none of witch is related to the ratio.
Volkswagens don't blow rods as they make very little power (40-60hp) and the rods are beefier than a small block. (part of the reason they make no power).
Your analogy of two people running down a hill does not work as the runners are not connected to the ground, nor are they moving up and down.
If Smokey Unick had access to today's technology he probably would never had made a statement like that. The piston is the most important. I believe that the rod itself is the least of the problem in the engine. What you need to do is get the right piston, measure for your rod, then you can design your cam and plenum volume.
Individual cylinder timing has been around for years. Crane cams brought it to the main stream years ago (my race box was bought in 9 I even put one on my Harley in 99. but the problem is how many people actually have the knowledge to use it?
Either way if you built to identical engines and just changes the rods there would be no measurable power from the rod length alone.
Last but not least Rod lenght also has a lot to do with the induction method.
Connecting rods break for a variety of reasons, material fatigue, rod bolt fatigue, detonation, piston mean speed, piston weight, none of witch is related to the ratio.
Volkswagens don't blow rods as they make very little power (40-60hp) and the rods are beefier than a small block. (part of the reason they make no power).
Your analogy of two people running down a hill does not work as the runners are not connected to the ground, nor are they moving up and down.
If Smokey Unick had access to today's technology he probably would never had made a statement like that. The piston is the most important. I believe that the rod itself is the least of the problem in the engine. What you need to do is get the right piston, measure for your rod, then you can design your cam and plenum volume.
Individual cylinder timing has been around for years. Crane cams brought it to the main stream years ago (my race box was bought in 9 I even put one on my Harley in 99. but the problem is how many people actually have the knowledge to use it?
Either way if you built to identical engines and just changes the rods there would be no measurable power from the rod length alone.
Last but not least Rod lenght also has a lot to do with the induction method.
Also, as one side comment on Mistah's explanation, the length of the runner's legs in that example would closer parallel the stroke of the engine, not the rod length, and the speed at which their legs were moving would mimic RPM. The speed of the longer runners legs is displacing the same amount as the smaller runner, only because he's "revolving" at a slower rate 
The disadvantage of the long rod comes to play when dealing with motors and boost. The increased dwell time becomes a problem. In these apps you don't need to piston just hangin' out at the top or bottom or the bore. N2O may be the split some where between. And NA ,IHMO I say go with a long rod that does not compromise the piston and ring seal.
Robert brings up a good point with the boosted stuff. Reading his post directs my thought process towards the mixture motion around TDC and the increased dwell time achieved with longer rods.
Pro-stocker's have spent tons of time moving valves around in the cylinder to increase the squish/quench pad in their motors. transman pointed out the testing by Reher-Morrison (pro-stockers) on rod length. i wouldn't dare hypothisize their testing perameters but with these quench chamber advances and their extreame engine speeds maybe they are not concerned.
Take two motors one with .080" quench and a short rod. The other with .040" quench and a longer rod. Which one will have better mixture motion? I say the tight quench/longer rod. Tighter quench area coupled with the piston location on the longer rod (still talking 3.75 stroke/6"rod 1.6 R/S ratio) will greatly enhance detenation resistance. Which means more power per stroke, pushing that envelope.
I'm still sticking with Mistah's analogy of the runners. I hear what your saying Supe but the runners Hips are the crankshaft and his legs are the connecting rod......and yes that makes his feet the pistons :roll: None the less his feet are still in different places in relation to his hips.
BTW this is good stuff keep it going.
Pro-stocker's have spent tons of time moving valves around in the cylinder to increase the squish/quench pad in their motors. transman pointed out the testing by Reher-Morrison (pro-stockers) on rod length. i wouldn't dare hypothisize their testing perameters but with these quench chamber advances and their extreame engine speeds maybe they are not concerned.
Take two motors one with .080" quench and a short rod. The other with .040" quench and a longer rod. Which one will have better mixture motion? I say the tight quench/longer rod. Tighter quench area coupled with the piston location on the longer rod (still talking 3.75 stroke/6"rod 1.6 R/S ratio) will greatly enhance detenation resistance. Which means more power per stroke, pushing that envelope.
I'm still sticking with Mistah's analogy of the runners. I hear what your saying Supe but the runners Hips are the crankshaft and his legs are the connecting rod......and yes that makes his feet the pistons :roll: None the less his feet are still in different places in relation to his hips.
BTW this is good stuff keep it going.
rods
I would ask everybody interested in this subject to read this tech article..
http://www.iskycams.com/techinfo_index.html
You might have to go through it a time or two but it will really open your eyes about rod length..
Also take a few minutes and build a working model out of paper, make one rod 10" long and another rod 2" long and you will soon see about half of the theorys about rod length are wrong... You will need to make the rods that much different because if you do a 5.7" and a 6.0" rod you will never see it...
Pay attention to the chart at the piston locations between the 5.7" rod and the 6.0" rod...
There is alot of talk about crank angles,side loads and so on... If you look at the chart you will soon see the true amount.....In real numbers... Then you can decide for yourself if a piston that is an additional .020" down in the bore can really make 50hp difference,,,or even 10 hp????
One thing i have found over the last 3 or so years of this fight is that not one single person has had proof about rod length making or loosing HP... not one!!!!! The only way you can prove it is with A-B-A testing,, there is no other way.. if you change anything other then rod length it is not a true test....
I talked to a guy that raced a big block mopar RED, he had spent thousands of dollars testing rod lengths, He raced this car 1000's of passes over the years and he could tell you exactly what the car would et per every track condition... He told me he had every rod,cam,head combo on the car and that the length of the rod made zero difference in the et of the car...
Gm spent a bunch of money on this subject with the cup teams. They started with like a 5.2" rod and went up to a 6.4" rod and never found more then a few hp....they determined rod length was a non -issue...
In my own shop the closest thing i have as a test ..... I built 2 406 engines about the same time. one was a 6.0" rod engine and the other was a 5.7" engine.. The guy with the 6.0" rod engine took real good care of it, never over heated it, never detonated it... The guy with the 5.7" rod engine was a freak with his... Had it over 230 a few times, beat the crap out of it everytime he drove it and so on.... I freshened both of them. the pistons in the 6.0" rod engine were about beat,in the 5.7" rod engine they looked like new(same brand of pistons) So in a street engine there is really something to be said for a longer skirt and keeping the pin out of the oil ring.....
Keith
I would ask everybody interested in this subject to read this tech article..
http://www.iskycams.com/techinfo_index.html
You might have to go through it a time or two but it will really open your eyes about rod length..
Also take a few minutes and build a working model out of paper, make one rod 10" long and another rod 2" long and you will soon see about half of the theorys about rod length are wrong... You will need to make the rods that much different because if you do a 5.7" and a 6.0" rod you will never see it...
Pay attention to the chart at the piston locations between the 5.7" rod and the 6.0" rod...
There is alot of talk about crank angles,side loads and so on... If you look at the chart you will soon see the true amount.....In real numbers... Then you can decide for yourself if a piston that is an additional .020" down in the bore can really make 50hp difference,,,or even 10 hp????
One thing i have found over the last 3 or so years of this fight is that not one single person has had proof about rod length making or loosing HP... not one!!!!! The only way you can prove it is with A-B-A testing,, there is no other way.. if you change anything other then rod length it is not a true test....
I talked to a guy that raced a big block mopar RED, he had spent thousands of dollars testing rod lengths, He raced this car 1000's of passes over the years and he could tell you exactly what the car would et per every track condition... He told me he had every rod,cam,head combo on the car and that the length of the rod made zero difference in the et of the car...
Gm spent a bunch of money on this subject with the cup teams. They started with like a 5.2" rod and went up to a 6.4" rod and never found more then a few hp....they determined rod length was a non -issue...
In my own shop the closest thing i have as a test ..... I built 2 406 engines about the same time. one was a 6.0" rod engine and the other was a 5.7" engine.. The guy with the 6.0" rod engine took real good care of it, never over heated it, never detonated it... The guy with the 5.7" rod engine was a freak with his... Had it over 230 a few times, beat the crap out of it everytime he drove it and so on.... I freshened both of them. the pistons in the 6.0" rod engine were about beat,in the 5.7" rod engine they looked like new(same brand of pistons) So in a street engine there is really something to be said for a longer skirt and keeping the pin out of the oil ring.....
Keith
Re: rods
And the thing about that is that you probably never WILL see an ABA test with any results, because 1) someone would have to have a custom piston machined that is HEAVIER since the longer rod's off the shelf piston would always be lighter, and 2nd, if there is ANY difference, you'd probably have to have that motor tuned to Pro Stock levels to have any measurable result.
K-Star automotive said:
And the thing about that is that you probably never WILL see an ABA test with any results, because 1) someone would have to have a custom piston machined that is HEAVIER since the longer rod's off the shelf piston would always be lighter, and 2nd, if there is ANY difference, you'd probably have to have that motor tuned to Pro Stock levels to have any measurable result.
