View Full Version : Smokey Yunick on Rod Length (from old site)

08-20-2004, 03:11 PM
Registered User
Posts: 264
(6/9/04 10:06 pm)
Reply SmokeyYunick On rod length.. addition to an older post
In an older post the subject went astray into rod length and rod to stroke ratio.
I made an off handed comment That I should throw away some of my books that state longer rods are better. Or something to that effect. And I got a post back stating..

"I don't think that any of those authors ever claimed that rod length is a major factor in engine output.
The pharsing I have always heard is the long rods tend to make more power than short rods. No one ever says that number is a lot."

I just need to clear this up a bit. This next little bit is copied straight from Smokey Yunicks "Power Secrets"

Page 36
Before we go on,we might as well look at the subject of rod length a little more. There are few 'secrets' to discover about connecting rods, but there is one thing every raceing engine builder must understand; it is essential to use the longest connecting rod you can possibly fit in the engine!
***Note from me essentialy our engines ARE racing engines if you consider the time this book was writen and the horse power numbers they where acheiving are close to what we are running on the street now***
Rod length is tied to the stroke length of the crank shaft, and the ratio between the two is a significant factor in engine design. for example, if the rod 6.0 inches long and the stroke is 3.0 inches, the ratiobetween the two is 2;1. This comparison is often called the "rod ratio". Historicaly, the desighners of american v8 cumbustion engines have viewed rod ratios of about 1.65;1 to 1.75;1 as ideal for all-aroound engine performance. however, I feel the rod ratio in a racing engineshould be much higher. **read note from me***
All of the Dyno testing i have done indicates that a racing engine always produces more torque and horse power when the rod ratio is increased beyond 1.75;1. In fact in any conventional engine with a stroke length of approximatly 2.5 to 3.5 inches, I beleive there will be a noteable increase in performance as the rod ratio is increased - all the way up to ratios as high as 3.0;1...

Ok I cant type no more but I cannot dissagre with someone with the knowlege of a Smokey Yunick I dont care what some guy from Isky cams claims.

Just needed to clear something up for some that read that thread..

Smokey also speaks about 356 and less cube motors beacuse of racing rules. I firmly beleive that our engines are closely related to endurance engines because of sustained freeway speeds and us needing to get 70k miles out of them while running them hard from day one( after breakin)

Registered User
Posts: 331
(6/10/04 1:42 am)
Reply interesting
I always thought longer rods made more torque. Now, with out type of engines would it be possible to make it so the the bore is very large and the stroke very short so that it could rev high and make the power that way? What is the largest bore:stroke ratio that could be achieved with say a chevy, a ford, and a pontiac engine?

manuel scettri

Unregistered User
(6/10/04 6:45 am)
Reply try this.....hee, hee.
Take a 400SB and overbore it .030 or maybe .040, then stuff a DZ302 crank in it. The DZ crank has a 3" stroke and with the use of Clevite thick-wall bearings, you can do this easily. As far as rods go, you can (hee,hee) machine down 4.9 Ford rods which are forged to begin with and are far stronger than typical 5.7 rods, anyway. Their rod length is 6.209". JE makes custom pistons with higher wrist-pin locations for just such a motor. Then, you can run 11:1 on 87-octane without detonation. So, you really can have your cake and eat it too in terms of high compression with pump gas. A motor like this with nice heads (215cc) and a hydraulic cam of around 520-540 lift will easily get you 500-550 HP on the cheap-relatively speaking. Long rod motors are almost like cheating-they shouldn't work as well as they do, but they do! -G

Registered User
Posts: 234
(6/10/04 8:10 am)
Reply .
I am not disputing Smokey... and he basically is saying what I had claimed earlier.
Rod length does make a difference, but its not huge.

If you are building a custom rotating assy (which pretty much all race engines were in his day) and you are not interfering with piston integrety than you should use the longest rod you can. If the cost and strength is the same than a longer rod is better... slightly

I like the Isky article because instead of just making claims he shows data. You can track the piston movement and see EXACTLY what the varried rod length is doing to piston travel.

This originally came up because the Desktop dyno does not have a provision for rod length (the least of its short-comings).
I can tell you this - on the Performance Trends Engine Analizer (the industry standard engine simulator) there are provisions for rod length in the program. Varrying rod length (within reason) typically does not effect the power curve by more than +/- 2HP

Registered User
Posts: 90
(6/10/04 9:55 pm)
Reply Re: .
Check this out, this is right along this topic:

Jerry Stahl's input on rod length (http://www.stahlheaders.com/Lit_Rod%20Length.htm)

Registered User
Posts: 265
(6/11/04 12:59 am)
Reply Re: SmokeyYunick On rod length.. addition to an older post
Well we have only proven 1 thing everyone has their own opinion..lol There is only one solusion AGREE TO DISAGREE. I would really love to know some Nascar short track set ups and long track set ups to see what they run.

Registered User
Posts: 143
(6/25/04 2:50 am)
Another Opinion: David Reher On Rod Length
Copied from the Reher-Morrison website (http://www.rehermorrison.com/). There is a very cool tech articles section.

"I've seen dozens of magazine articles on supposedly "magic" connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued!

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period."

Unregistered User
(6/25/04 11:50 am)
Reply well....
For a street engine, being able to run pump gas with 11:1 and no detonation/power drop-off tells a different story than the rod length being un-important. -G

Unregistered User
(6/25/04 3:01 pm)
Reply long rods and 400's
now i'm no 400 expert,but they seem to have a problem with long rods and side wall loading/bore cracking.i talked to 2 differnet engine builders (after i destroyed 2 identical motors in 1 year) and they both said that the longer rods have a habit of putting more outward force on the outter wall of the bore causing block failure. to get around this you can fill the stock block or use an a/m block with thicker walls . i am planning another 406 build with a similar setup you mentioned above/327 crank/ford rods with a set of older sportsman heads 2.08/1.65,210 ports just gasket matched and flashing cleanup and a BIG hyd roller. wheres that just bring it guy he plays around with 400's?

Registered User
Posts: 211
(6/26/04 4:37 pm)
Reply controlled test

Have you built the EXACT same engine, except for rods and pistons, to test whether your knock resistance is really a result of rod length? I mean everything: Heads, cam, intake, exhaust, valve size, rockers, gasket thickness, plugs...

Registered User
Posts: 124
(6/26/04 7:19 pm)
Reply rod length
For coming off turn 2 or turn 4 at Daytona, long rods might get you a jump out of the turn. I will grant you the slight increase in torque you get because the piston spends more time around TDC allowing cylinder pressure to build up a little more before the piston starts moving down the bore on the power stroke. Great for extended hi rpm racing with hi octane gasoline.
But for a hi performance street engine that you have spent long hours and long $$$ on getting it to make big power with pump gas the absolute last thing you want is the piston hanging around at TDC and spiking the cylinder pressure. In this case a shorter rod that gets that piston moving down the bore ASAP on the power stroke is a much better choice.
And talk to any top line gasoline drag race engine builder and when asked 'how long the rods should be?' the most common answer is long enough to connect the piston to the crank. Engines built to gulp large amounts of NOS are usually built using short rods as well, again because of the cylinder pressure issue.

Registered User
Posts: 33
(7/11/04 11:44 am)
Reply Rod length
I think the rod length arguement could go on forever. There are certain advantages to each depending on the specific use of the engine.

I'm using long rods ( 6.8 ) on my BBC, not because of any incremental gains in torque or horespower, but because of emissions. The longer dwell at TDC allows more complete combustion, especially in the crevice volume areas such as the piston to head quench, and the volume between the piston and cylinder above the top ring land. GM went to longer rods with it's LS1 variants, and that's one of the reasons.

And yes, I do have to get pass the sniffer test on a yearly basis.


Unregistered User
(7/12/04 9:45 am)
Reply oops, just now seen ya!
Sorry, Walapus-I just caught that post! No, I have not done back-to-back tests to confirm that being the reason for no detonation. I change a lot of things each time to come up with a different combination-I don't have the resources to do my own "shootout" if you will. -G