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http://www.rustpuppy.org/rodstudy.htm

I was reading through this article and wanted to know if it was part of the endless number of monkeys clattering away on keyboards all day.

How do you feel about the long rod/short rod thing?

Is it worth the added expense to go to a longer rod in a typical hot rodder application?

Kev
 

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We had a long discussion on this subject a while back. If it doens't show up via a search then it got deleted. I don't feel like retyping a very long post but here's the gist of it:

Smokey Yunick made the original statement that you should "stuff the longest connecting rod in that will fit".

Bill Jenkins says certain engine combinations "like" a certain rod length.

I think this is closer to the truth and here's why:

If you take any concept to the extreme it should become obvious that it is or isn't linear.

For obvious reasons a 1" connecting rod is too short and a 7" connecting rod is too long (won't physically fit in a SBC). As far as long rods go it's possible to make the rod so long the piston CH has to be extremely short. This can create piston instability, rocking and ring seal goes to hell.

Since a rod that's at the outer limit of physical packaging restraints (long or short) begins enter the gray area of creating detremental problems there logically must be a length or range of lengths that are optimum.

Within that range there are reasons to run a certain rod length.

A small block Ford has a very short deck and must run a relatively short rod 5.090". Compared to a 5.0 rod the 5.565" 400 SBC "short" rod is "long".

I think I said before that the optimum rod length is the one that connects the optimum piston CH to the optimum stroke.
 

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bore vs stroke

I have a question about oversquare vs undersquare motors. Typically an oversquare motor (bore larger than stroke for example the 350ci chev) will have a higher rev capability and make more HP and less torque than an undersquare motor (stroke larger than bore for example the olds 455). Undersquare motors seem to be well suited to developing low rpm torque.

So with that being said what typically is the result of a motor with a bore and stroke equal to each other? (ie 4.250 bore x 4.250 stroke)
 

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What you are describing is a general popular observation that you may have seen on "the internet" but it's not the whole story by any means. What's left out is piston speed and valve area and BMEP and VE and a bunch of other variables that all effect engine operating characteristics.

I'll give you some examples: The D16A1 engine in my ice racing Integra has a bore of 75mm and a stroke of 90mm. That's very undersquare but it revs to 7,000 and makes power up high. The ZR1 LT-5 Corvette with the Lotus designed, Mercury Marine built "350" engine had a bore size about 3.9" and a 3.66" stroke. That revved higher and had more torque than a bigger bore, shorter stroke (4" x 3.48") Gen I LT-1 350. It also had more real horsepower at 375 SAE net than the old engine's 370 Gross rating.

So obviously being oversquare or undersquare is not all there is to how an engine runs. I'll have to explain later as to why. It's the weekend!
 

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Thanks Paul I look forward to hearing about it. Have a great weekend. I was sure there is more involved then just the relationship between the bore and stroke but I was just asking sorta a rule of thumb kinda question which I know you don't like because in reality there isn't any rules of thumb cause just as soon as you thnk you have one someone will show you something that blows it out of the water. :)
 

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Well, I forgot about this until I saw the current Hotrod magazine article on the subject. They shoot holes in old the Smokey Yunick theory about "using the longest rods that will fit" are best. I think I have some similar postings on that very subject. Hopefully Steve hasn't deleted them. (update: the older posts are gone).

I'll gather/merge what I can find and put this in Best of Tech.
 

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Another angle on 427 BBC vs 427 SBC - I have heard that the big advantage of a BBC is that it's easier to get heads that will keep it fed properly. The BBC stuff is physically bigger, so there's more room to work with (larger valves, better ports) , and it's easier to get better flow numbers. I'm not saying it can't be done, but all things being equal, it's easier to get good flow numbers out of BBC heads. And for a large-ish cube engine, you'll need it!
 

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I decided to use the longest rod (6.8") that would fit in my block with the stroke I was using because it would make internally balancing the engine easier (read less costly) and the longer the rod the less load seen on the thrust side of the piston.
 

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DriveWFO said:
Paul, thought you'd be the first to chime in on this one :) When comparing a short rod to a long rod, how much more time (relatively speaking) does the piston spend at TDC?
The whole rod length topic is like quick sand. You can't get through it easily.
I take a Holistic approach to engines. It's all connected together and you can't make simplistic statements about individual components.

Let me go out on a limb to say that the optimum point of peak combustion pressure can be determined experimentally. This is not the spark lead degrees but the crank degree where the combustion pressure is highest. This is generally around 8-14 degrees after TDC. The correct spark advance keeps the peak pressure occuring at this experimentally determined point.

As rpm increases the mixture must be lit progressively earlier. This change in spark advance is the advance "curve" (it's really a slope).

Spark advance is not without consequences.

Lighting the mixture BTDC causes pressure on the accending piston. This is "negative" torque. The pressure after TDC is positive torque. The difference between the two is what you get at the crank.

Because rod length has an effect on the combustion volume at a particular crank degree it has an effect on the pressure at that degree. Again, I can't emphasize enough that everything has to be considered. The best spark timing for any combination is best determined experimentally.
 

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Doesn't rod ratio effect how much time the piston spends at TDC?

No. Why would it? Top Dead Center refers to a very specific and very small event when the piston is at the apex of its travel. Altering the rod/stroke ratio will not change this. Rick Draganowski has an excellent study of the physical aspects of changing rod length:

Rod Length Study

Note that the time spent within .250" of TDC varies only slightly, less than 1/2 degree from 5.565 rod to a 6" rod using a 3.75" stroke.
 

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Here I go dipping my toe in the quick sand.

I've seen that internet study but Dragonowski only focuses on the trigonometry. He uses an infinitely long rod for his study. How about an infinitely short rod also? The dynamic effects on the PV (Pressure vs Volume) curve are much more important than the trigonometry.

I've mentioned in previous discussions that too long of a rod can have negative performance aspects just as too short of a rod can.

Even the longest rod that can physically fit in a block would have other consequences such as the piston CH would be so short the piston could rock in the bore and lose compression from ring seal. A real short rod would require a piston with a large CH and would be heavy.
Therefore the optimum rod length is somewhere between too long and too short. To help you visualize the concept, imagine a 4" or a 6.5" connecting rod in a 350. Both extremes would be counter productive to performance. I seriously doubt that either would perform as good as a 5.7" rod.

If you plotted out the performance gains from all the rod lengths between extreme short and extreme long you get a parabola curve.
The variation near the peak would be small and the near the extremes it would be greater.

Since there is a peak (even if it's very small) to the parabola there has to be an optimum rod length (even if the difference is very small).
 

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DriveWFO said:
Can you set the spark advance for each individual cylinder rather than just for the #1 cylinder? What's the typical delta for all 8 cylinders with regards to this?
Yes, Nascar racers do this now. MSD even makes a distributor that allows cylinder specific spark advance. There's more to individual tuning of cylinders but that's getting into top secrets.

DriveWFO said:
As far as building a stout bottom-end is concerned, say for a 383, what stroke and rod length combination could safely spin the most RPMs??? The one with the slowest piston acceleration???
Uhhh...is this so you can say that "my 383 revs higher than yours?"
Stroke has more influence on piston speed than rod length. Rod length determines CH ( or vice versa) which influences piston weight.
Since the 4.4" bore spacing of a small block precludes a 4.5" piston with a 3" crank then the next shortest stroke that would still give you 383 with the max 4.2" bore is 3.45"

For practicality using a 350 crank in a Motown block bored to 4.185 would give you the best bang. A 6" rod would give you a light weight piston w/ 1.285" CH @ zero deck. The 4.185" bore will require a custom piston.

Take my advice. Lusting after ever higher rpm is the path to financial ruin.
Your money would be more wisely spent on more cubes. If you want screaming high rpm buy a Honda S2000 or better yet a crotch rocket motorcycle.
 

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Originally Posted by Mike Goble
The time the piston spends at TDC is exactly the same for all lengths.



64PRONOVA said:
Not exactly the case. The longer the rod, the longer the piston dwells around TDC.
Did I say that the piston dwelled longer around TDC? TDC is a singular point that is unaffected by rod length or stroke. It occurs when the piston is exactly at its zenith, not just somewhere in the area.
With a 3.75" stroke and a 6" rod, the piston is .3249" down at 30 degrees of crank rotation. Change the rod to a 5.7" rod and it's .3288" down at the same 30 degrees. That's a difference of .0039", less than 4 thousandths.
 
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