After 250 dyno sessions this article probably provides the definitive answer to something the late Smokey Yunick was passionate about, but I wont spoil the ending.
Read the pages from scan0001 (page 1).
Conrod length
Started by
Litre8
, Jun 18 2007 10:59 PM
25 replies to this topic
#1
Litre8
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Posted 18 June 2007 - 10:59 PM
This article was done by Circle Track mag a while ago now and it involved two SBC engine combinations (5.5" rod & 6.5" rod) that were tried with various head, cam and carb combos and then tuned for each combination.
After 250 dyno sessions this article probably provides the definitive answer to something the late Smokey Yunick was passionate about, but I wont spoil the ending.
Read the pages from scan0001 (page 1).
After 250 dyno sessions this article probably provides the definitive answer to something the late Smokey Yunick was passionate about, but I wont spoil the ending.
Read the pages from scan0001 (page 1).
#2
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Posted 18 June 2007 - 11:08 PM
Link no work for me
#3
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Posted 18 June 2007 - 11:15 PM
Link fixed..
#4
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Posted 19 June 2007 - 12:06 AM
very interesting read there mate! always good to read something technical like that and good to know that info too!
#5
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Posted 19 June 2007 - 12:40 AM
I'm divided on this one.
The shorter Conrod separated the men form the boys braking into Murray's corner, but the longer Conrod with the inclusion of the chase has added another dimension in recent years.
We're talking about Bathurst here..................are'nt we?
The shorter Conrod separated the men form the boys braking into Murray's corner, but the longer Conrod with the inclusion of the chase has added another dimension in recent years.
We're talking about Bathurst here..................are'nt we?
#6
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Posted 19 June 2007 - 01:09 AM
Smokey knows his stuff. 
#7
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Posted 19 June 2007 - 07:31 AM
i have read articals that are pretty conflicting 
#8
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Posted 19 June 2007 - 08:38 AM
Probably plenty of conflicting articles (& myths) but did they back it up with empirical results?
#9
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Posted 19 June 2007 - 09:39 AM
Hmm.........interesting stuff. The increase in performance seen by the longer rods appears to be due to just better intake dynamics and burn characteristics of the fuel , however, intake dynamics is also a function of the cam. Though the article mentions different cams were tried, how specifically were the cams designed to meet the needs of each setup? Was there anything about how the loads on the side of the bores would be larger with shorter rods? ie more friction.
How much improvement can be achieved by going to much longer rods, ie like twice as long(obviously the larger mass of the longer rods would be an issue )..............it would make a very tall motor
How much improvement can be achieved by going to much longer rods, ie like twice as long(obviously the larger mass of the longer rods would be an issue )..............it would make a very tall motor
#10
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Posted 19 June 2007 - 11:26 AM
you can build 100% identical engines and they wont produce exactly the same power.thats the problem.
#11
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Posted 19 June 2007 - 12:15 PM
True and that would be a problem IF the test was one just one combination and one dyno test.
#12
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Posted 19 June 2007 - 12:22 PM
very true but most of the articals i have read say 2-5% difference.that is not enough difference to convince me over two identical engines. in saying that i have just gone from short 400 rods to 6" ones. but the whole combo is different so i cant compare
#13
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Posted 19 June 2007 - 08:50 PM
Not a realistic test in my opinion. Normally you don't have the option to increase rod length by a whole inch. A lot of the power increase could be due the weight saving from the inch shorter piston and the rod/stroke ratios they use are well above the range of almost all engines.
Magazine tests are well known to exaggerate their results and this looks like no exeption. Plenty of tests have been done using more realistic rod lengths like 5.7" and 6.125" and no one has ever come up with anything near that sort of increase.
Magazine tests are well known to exaggerate their results and this looks like no exeption. Plenty of tests have been done using more realistic rod lengths like 5.7" and 6.125" and no one has ever come up with anything near that sort of increase.
#14
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Posted 20 June 2007 - 06:15 AM
Bodallafella, I have reservations about the results/test as stated above, however, I dont agree with your criticisms of it.Not a realistic test in my opinion. Normally you don't have the option to increase rod length by a whole inch. A lot of the power increase could be due the weight saving from the inch shorter piston and the rod/stroke ratios they use are well above the range of almost all engines.
Why isnt realistic?, they had real rods and pistons of different lengths.......or are you saying that the exercise is not applicable to most engine setups? I thought the idea was to show what could be achieved with using longer rods for outright performance, whether it could be done in every engine or being above the rod/stroke ratio of other engines being somewhat irrelevant, though I agree they haven't gone to great pains to point out that their setup is not typical of most street engines.
It does say in the article that the combined rod and piston mass was kept within a gram for both long and short rod combinations, though dynamically the loads on bearings and cylinder walls isnt the same due to the larger angles created by the shorter rods.
#15
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Posted 20 June 2007 - 06:37 PM
I missed the part where they said the weights were the same but I still think the test is flawed.
They try to apply their results to all engines but by using such ridiculous rod/stroke ratios and exaggerating the difference between the rod lengths they have acheived results which don't apply to ANY engine
The claim that a longer rod will make more power in any engine is not true, there are plenty of combos that make more power with shorter rods and the difference either way is never more than a few HP.
This test was done to back up claims they made in a previous issue. Do you really think they would print results that prove them wrong? They print whatever they think will sell the most magazines. The article looks like it was written about 20 years ago and still no-one else has acheived similar results.
As for Smokey Yunick, the guy was full of crap and was famous for fudging test results and cheating. If you don't believe me, look up the adiabatic engine. Every single claim made regarding that engine has since been proven false and nobody (including Smokey) was ever able to repeat the results.
They try to apply their results to all engines but by using such ridiculous rod/stroke ratios and exaggerating the difference between the rod lengths they have acheived results which don't apply to ANY engine
The claim that a longer rod will make more power in any engine is not true, there are plenty of combos that make more power with shorter rods and the difference either way is never more than a few HP.
This test was done to back up claims they made in a previous issue. Do you really think they would print results that prove them wrong? They print whatever they think will sell the most magazines. The article looks like it was written about 20 years ago and still no-one else has acheived similar results.
As for Smokey Yunick, the guy was full of crap and was famous for fudging test results and cheating. If you don't believe me, look up the adiabatic engine. Every single claim made regarding that engine has since been proven false and nobody (including Smokey) was ever able to repeat the results.
Edited by bodallafella, 20 June 2007 - 06:38 PM.
#16
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Posted 20 June 2007 - 06:57 PM
As for Smokey Yunick, the guy was full of crap and was famous for fudging test results and cheating.
Bodallafella... wash your mouth out......
There are many seasoned engine builders around who would lynch you for that statement!
Cheers Greg..
#17
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Posted 20 June 2007 - 07:09 PM
Get your lynching sticks out! I stand by my comments and my mouth remains unwashed. Smokey knew his stuff but he died with most of that knowledge. The information that he shared was designed to keep him ahead of the competition.
Edited by bodallafella, 20 June 2007 - 07:10 PM.
#18
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Posted 20 June 2007 - 07:32 PM
Exactly.They try to apply their results to all engines but by using such ridiculous rod/stroke ratios and exaggerating the difference between the rod lengths they have acheived results which don't apply to ANY engine.
SBC engines normally use 5.70 or 6.00 rod lengths and dyno testing goes both ways. This test really was a waste, no-one would use either rod length.
CURRENT thinking suggests the long rod deal to be a benefit as it makes for a lighter piston and therefore a lighter rotating assembly. The money spent on the special piston could be saved and spent somewhere it would really make a difference. Yes a long rod has slightly less friction, but a short rod pulls away from TDC faster, drawing in a bigger charge.
David Reher (Reher Morrison Racing Engines) said it best. The connecting rods only job is to connect the piston to the crank. Length has very little effect on HP produced. In fact now deck heights for SBC are dropping to 8.500" in serious competition(ie current NASCAR). What this says to me is that the weight saving from lopping a half inch off the decks is far more beneficial than fitting a long rod.
JMHO
#19
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Posted 20 June 2007 - 10:32 PM
So accepting claims/"thinking"/statements with no test results to substantiate it is fine but imperical results achieved after investing 10 days effort and 250 dyno runs should be dismissed?
#20
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Posted 20 June 2007 - 11:55 PM
I think Dave Reher's opinion carries a lot more authority than 250 misguided dyno pulls but it's not just him. Shorter rod lengths and deck heights have become the norm in most forms of racing from V8 supercars to Pro Stock and no one is reporting any power loss after thousands of tests. The same applies to street engines. People used to put 6" rods in everything but now most have realised that the supposed gains are just not there.
Edited by bodallafella, 20 June 2007 - 11:55 PM.
#21
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Posted 21 June 2007 - 08:54 AM
Dave Rehier has a "barrow" to push as well. Don't forget that either.
The argument for lower deck height is one of chassis balance and a lower centre of Gravity.The current thinking is "that if we sacrafice 10hp by using a 1" lower deck height and a shorter rod then we can get the top of the engine 1" lower in the chassis" This has come about because most top level teams now do their engine building in house. so they think of the car as a package more now, rather than the old days when it was a chassis that a power train got dropped into to go racing.
BUT I THOUGHT... this was about more /less power with longer rods????
NOT why are current teams using shorter deck heights to change the CofG of the race car.......
TWO totally different things.
My opinion is that a longer rod will most often make more Torque than a shorter rod.
A shorter rod/deck height will give a car a lower CofG making it turn better or able to have a lower bonnet height, a therefore better aero.
The balance point for race car teams is though... At what cost to weight and chassis balance/ engine power?
Don't confuse the 2 points they are totally different questions.
Cheers Greg..
The argument for lower deck height is one of chassis balance and a lower centre of Gravity.The current thinking is "that if we sacrafice 10hp by using a 1" lower deck height and a shorter rod then we can get the top of the engine 1" lower in the chassis" This has come about because most top level teams now do their engine building in house. so they think of the car as a package more now, rather than the old days when it was a chassis that a power train got dropped into to go racing.
BUT I THOUGHT... this was about more /less power with longer rods????
NOT why are current teams using shorter deck heights to change the CofG of the race car.......
TWO totally different things.
My opinion is that a longer rod will most often make more Torque than a shorter rod.
A shorter rod/deck height will give a car a lower CofG making it turn better or able to have a lower bonnet height, a therefore better aero.
The balance point for race car teams is though... At what cost to weight and chassis balance/ engine power?
Don't confuse the 2 points they are totally different questions.
Cheers Greg..
#22
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Posted 21 June 2007 - 06:48 PM
We are talking about rod length but deck height is closely related. The weight saving from a lower deck height is a good thing but the real advantage is that a lower deck gives you a stiffer block and allows you to design a better manifold and produce much more power than you can lose from the shorter rod.
The effects of rod lengths are worth discussing because it gets you thinking about engine dynamics but people get carried away. When it comes to designing an engine combo, the importance of rod length should be way down on your list of priorities. There's nothing wrong with using a high rod/stroke ratio but you should never compromise on things like piston design, stroke or deck height to acheive it.
The effects of rod lengths are worth discussing because it gets you thinking about engine dynamics but people get carried away. When it comes to designing an engine combo, the importance of rod length should be way down on your list of priorities. There's nothing wrong with using a high rod/stroke ratio but you should never compromise on things like piston design, stroke or deck height to acheive it.
#23
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Posted 21 June 2007 - 07:24 PM
^ Totally agree......
1st prority always has to go to airflow.
somewhere down the list after you have optimised everything else is consideration to engine dynamics and frictional loss (rod length).
One particular type of engine that does benefit greatly from absurdly long rods is the "restrictor" engines that the Nascar guys run on the ovals. With only a 2 barrel carb to feed the engine the engine designers are looking for maximum dwell time at TDC, to help increase airflow. Some of the rod lengths they use are out to 6.5".
These are extreme examples though of an engine that is useless below 6000rpm, and when i say useless..... I mean almost undrivable. Look at the spread on the gearratio's they use at the oval tracks and it will give you an idea of just how "top end only" these things are.
Cheers Greg...
1st prority always has to go to airflow.
somewhere down the list after you have optimised everything else is consideration to engine dynamics and frictional loss (rod length).
One particular type of engine that does benefit greatly from absurdly long rods is the "restrictor" engines that the Nascar guys run on the ovals. With only a 2 barrel carb to feed the engine the engine designers are looking for maximum dwell time at TDC, to help increase airflow. Some of the rod lengths they use are out to 6.5".
These are extreme examples though of an engine that is useless below 6000rpm, and when i say useless..... I mean almost undrivable. Look at the spread on the gearratio's they use at the oval tracks and it will give you an idea of just how "top end only" these things are.
Cheers Greg...
#24
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Posted 22 June 2007 - 02:46 PM
There would be a lot of work involved in reducing deck heights which would include custom manifolds or spacers. so for the mr average torana engine builder would you use standard rods/pistons or is the long rod worth the extra money, isnt that the main question?
is it true that a long rod dwells at TDC for slightly longer creating a better burn. as it does this it also takes longer for it to reach its maximum speed on its way back down the bore at which time the valves are fully open which gives better flow?
david Vizard also recommends long rods, he seems to know what he is doing whith chevs anyway
is it true that a long rod dwells at TDC for slightly longer creating a better burn. as it does this it also takes longer for it to reach its maximum speed on its way back down the bore at which time the valves are fully open which gives better flow?
david Vizard also recommends long rods, he seems to know what he is doing whith chevs anyway
#25
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Posted 22 June 2007 - 08:52 PM
Very interesting information guys! And to be honest, a large amount is over my head!
In a street engine, a longer rod combinaion would be advantageous for the decreased side loadings on the pistons/bores? Would this be of greater concern than the piston velocity that is altered by the rod ratio?
When we rebuilt my engine, we went for a 5.7" rod for a couple of reasons, but mainly it was because we had the parts.. We could have gone for a 6" rod, but i was advised that the gains simply werent worth the added headaches and costs associated.
Cheers and thanks for the brain food!
In a street engine, a longer rod combinaion would be advantageous for the decreased side loadings on the pistons/bores? Would this be of greater concern than the piston velocity that is altered by the rod ratio?
When we rebuilt my engine, we went for a 5.7" rod for a couple of reasons, but mainly it was because we had the parts.. We could have gone for a 6" rod, but i was advised that the gains simply werent worth the added headaches and costs associated.
Cheers and thanks for the brain food!
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