longer connecting rod

wiseman79

Top Fueler
Dec 18, 2004
2,565
0
0
Central Virginia
www.performancetech.net
I used 5.7's in my 413. My understanding is that a longer rod remains at a closer to parallel angle to the bore/piston, therefore power is delivered to the crank better versus at a greater angle. Also I thought it decreased side loading the piston into the outside wall of the bore on the compression and exhaust stroke. However longer rods at a certain point bring the wrist pin into the ring area and requires design changes in the piston that may not be beneficial. Again, this is knowledge picked up from god knows where over the years but makes sense to me.

EDIT: oops, I didn't read all the posts carefully, I could have saved time and just said I agree with Squire Racing above. :D
 

JUAN W

Dragway Regular
i gain just 2 tenths with 6'' rods and .5 more compresion ratio
 
A

Anonymous

Guest
The problem I see most on the internet is the term *longer rod* like i posted above, what exactly is a long rod? In my world anything over a 1.7 ratio is considered long.

wiseman79, you said because the rod is straighter it is able to deliver the power better. What you are missing is as the rod gets longer it dwells at tdc longer so it is not pushing for that amount of time.


I'll get this moved
 
A

Anonymous

Guest
what?

JUAN W said:
i gain just 2 tenths with 6'' rods and .5 more compresion ratio

hey JuanW you sure it wasn't from new rings in the motor....i can't see gaining 2 tenths from just adding 6" rods......
 

JUAN W

Dragway Regular
Ok this is the story, my old combo was stock crank and rods,speed pro forged flattops with 2 valve relief, victor jr. an isky solid flattapet 254/[email protected] .547/.548 lift, canfield heads,750 dp, my best e.t with this combo was 11.31 in a cold night. Then i bought a set of lunati pistons for 5.7 with scat rods and crank same damper and flexplate and the first pass was 11.24 in a hot day, the mechanic installed 4 pistons the wrong way and the valves was hitting the pistons from the begining, i broke a piston in the second pass but i am sure that i could ran at least 11.10 in the same weather conditions than the stock rods, both engines was 10.8 cr.
Now (check my sig) i am running 11.40cr more gears, a little big tires and cam and the best e.t. in winter is 11.01, i ran a best of 11.20 last fryday but the summer is very hot here. I really dont see a big diference between the 3 combos.
 

JUAN W

Dragway Regular
Razor maybe the new rings help but in the stock rods pistons i was using childs & albert zero gap seconds rings.
 

CutlassRacer

MalibuRacing Junkie
Dec 18, 2004
5,402
0
0
Gainesville, FL
www.facebook.com
What do you more serious guys think about the advantage of a faster piston speed away from the intake valve as it opens? To me, it would seem that since the piston is going a little faster away from the opening intake valve, the draw of air into the cylinder would be better, possibly filling the cylinder better......
 
A

Anonymous

Guest
at slow speeds yes. but when things get going you need the slower speed. (does that make sense)

You make a good point and that's probably why the general used such a short rod in the first place. lot's of piston speed early with a big bore. Sounds like a lot of torque early.
 

Mistah

Frequent Racer
May 30, 2005
361
0
0
Maui, Hawaii
www.mistahrecords.com
Here I go jumping in.........
I became a believer after sitting down for a bite to eat at lunch about twenty years ago. I noticed this old man sitting alone toying with his calculator. After a few minutes he notices I am emersed in an article on how to control detonation on particular cylinders of a given racing engine while other cylinders were ok. We strike up a conversation, and he explains that he is a retired engineer from Germany. The article explained that it was common practice to physically reduce cylinder pressure by the tedious process of removing material from the piston of the offending cylinder and rebalancing the recyprocating assembly until detonation was eliminated in order to achieve this to produce a dynamically sound engine which yields more power throughout the RPM range despite having different compreesion ratios from cylinder to cylinder. He explained that pratice was antiquated, and he and his company had been utilizing individual cylinder timing to achieve the same thing. YES THIS WAS OVER TWENTY YEARS AGO. I am only starting to learn how to do this with my MSD programmable digital-7 now! He goes on to say they also have incorporated other methods to help keep detonation at bay like employing a Methanol/Ethanol - Gasoline mix(sound familiar?) variable timing, and longer connecting rods. I asked about the longer rods and this is what he said. You divide rod length by your stroke and you will get your rod ratio. A rod ratio at or near 2.0 is optimum. He mentioned some of the same benifits that Squire, Wiseman, and Bigtime were talking about, but he went on to say two things that caught my attention 1, the piston had a longer "dwell time" at BTDC, which could benifit cylinder filling(normally aspirated applications) and at TDC, which helps the engine require less advanced timing, allowing a more complete burn and reducing the potential for detonation. and 2. Optimum rod ratio allows your engine to utilize it's intended design potential by providing a platform to work off of.

Think about it. Drag race type inductions, heads, camshafts, and headers are designed to make power by moving the max amount of air/fuel possible through the engine in the shortest ammount of time. The three most common ways to achieve this is supercharging/turbocharging, nitrous, and high RPM. High torque applications utilize a different aproach by overbuliding the application (i.e. 632ci mountain motors, usually employing 2 and 3 stage kits) to accomplish the same results. While it's the upper half that receive's all the attention, the bottem half is running out of steam well before realizing the full potential of the upper half. So as a result we see component failure and correct this by installing even stronger parts to compensate. The more power we dial in, the stronger the bottem end needs to be. You gotta ask yourself SELF! "Why is the rods trying to exit the crankcase as we increase horsepower while rev limits, static load (vehicle weight), and distance/duration (1/4 mile) remain the same?"
Here's an example:
Take two people (Keep it "G" guys), one tall with long legs, and the other short with short legs. Let's use them to represent our "long rod" and "short rod" in this example.
Now lets take them to the top of a hill and establish a given distance (like the 1/4 mile) of say 100 ft. . The for the puposes of this example the race will be going in the downhill direction to focus on the top speed aspect (where most bottem end failures occur). Soon after the race begins, the short guy's leg speed begins to "top-out" and results in the guy falling down (rod failure) because the increased demand is just too much for him to maintain. The taller guy on the other hand, continues to gain in speed and completes the course without incident. Now if you would shorten the hill to maybe 15 ft., I'm sure the shorter guy will have an increased usable operating range to utilize any performance benifits that he may have over the tall guy. You just won't win races after a face plant before realizing your engine's full potential. Cramming all the billet, mega forged, unobtanium rods with super light bob weight kryptonite crankshaft/hocky puck pistons in there is just not enough, and is a band aide fix at best.

Now I know there's gonna be alot of flack about this subject, I've got it amongst the guys around here. Smokey Yunick says to "STUFF THE BIGGEST ROD THAT YOU CAN POSSIBLY FIT IN THERE!". David Reher says "IT DOES NOT MAKE ONE OUNCE OF A DIFFERENCE, RODS JUST CONNECT THE PISTON TO THE CRANK."

Here's my .02.......You increase the stroke you mess with the rod ratio.
Poor rod ratios= Kaboom. High end parts and parts cycling just keeps you going a little longer, that's all. Don't cheat your azz out of fart.....fix the problem, not treat the symptoms. Still not convinced? Go check out the the WWII war birds......LONG ROD.

Turns out the old man was an engineer at Volkswagen. Wish I could've talked to him a little more.......when's the last time you heard of rod failure on a Volkswagen?

Aloha,
Mistah
 

10secBu

MalibuRacing Junkie
May 21, 2003
4,284
0
36
Westminster, MD
Some very good discussion here guys. While the long/short connecting rod debate has controvery on both sides, one issue is pretty clear to me at least.

A longer rod allows for a shorter/lighter piston. When I did this 461 shortblock two years ago, I researched rod length/weight vs different piston compression height/weight. It was clear that a longer rod/short piston made for a lighter bob weight. This was the major reason I went with a 6.535" rod instead of the factory 6.135" rod length.

Does it make any difference in performance? Dunno, but the lighter weight did make it easier for me to rationalize my decision. :lol:
 

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