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Author Topic:   rods
tforsythe
Member
posted November 04, 2004 08:19 PM
could anyone tell me if there is any advantage on a 6"rod over a5.7 or any disadvantage this is in a 355 with a two barrel


racer17j
Member
posted November 04, 2004 10:09 PM
with the longer rods the piston stays at tdc longer


sleepy 1h
Member
posted November 05, 2004 02:07 AM
With the longer rod you take away cylinder wall pressure witch in turn reduces wear on the piston skirts,cylinder walls and rings. Also on the dyno produces a little more torque. sleepy


sixwillwin
Member
posted November 05, 2004 06:33 AM
Below is copied from Isky cams website, this short vs. long rod gets so old, everyone has a different opinion, mine is get a longer rod if you have ALL your other ducks in a row, they are many many better engine improvements to make first..........

"Rod Lengths/Ratios: Much ado about almost nothing.

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid *** ." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype."

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The Joy of Six website


waltonjr1
Member
posted November 05, 2004 07:17 AM
Good info six!


Ego Racing
Member
posted November 05, 2004 07:38 AM
I agree but you must also remember: Isky makes cams not rods.


dirtbuster
Member
posted November 05, 2004 08:19 AM
Keep in mind that longer rods not only reduce loads on the cylinder walls but also reduce bending stress on the rods themsleves. In a street car or even a hobby type class it might not make much difference but in a race engine(mods on up) I think the advantage of reduced angularity is definetly worth it, just to save wear on parts.

[This message has been edited by dirtbuster (edited November 05, 2004).]

Dixon
Member
posted November 05, 2004 08:46 AM
I'm not so sure that a longer rod reduces bending stress. Granted, the pressure against the cylinder wall is less, but you've got a longer "fulcrum" which may actually increase the stress. Just a thought...


dirtbuster
Member
posted November 05, 2004 10:15 AM
That is true about a longer rod if not designed properly to take into account the added length. When I said bending stress i meant in the fact that the force was applied more vertically throughout the stroke as compared to the shorter rod.


stockcar5
Member
posted November 05, 2004 11:15 AM
longet rods also mean lighter pistons.
im sure if long rods were such a poor upgrade then there would be no reason why nextgel cup engines use alot of 6.2"-6.4" rods.


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www.geocities.com/dirtstockcar5

[This message has been edited by True Blue (edited November 09, 2004).]

outlawstock17
Member
posted November 05, 2004 12:01 PM
who cares what some freako cam grinder said.......smokey yunick said, "put the longest rod that will fit in it and go racing".



waltonjr1
Member
posted November 05, 2004 12:57 PM
I think his main point was that there are plenty other things to do that will give you greater performance gains than a slightly longer rod....He's probally right.



dirtbuster
Member
posted November 05, 2004 01:40 PM
Performance is one thing. Performance and durability is another. Sure the longer rod may only make 5 more HP but it will reduce wear on the cylinder walls, pistons etc. Allowing you to make that HP longer without having to rebuild. Its like the difference between cast and forged crank. Neither is necessarily going to make more power but one will sure last longer.
I wouldnt necessarily go out and buy a longer set of rods and then replace the pistons to match just to do it. But when buildign from scratch there is no reason not to go with the longer rod. At least in my opinion.


0six
Member
posted November 05, 2004 04:38 PM
setup wins more races than power.
case in point i started the year out with a 355 (plenty of power) half way through the season i hung a rod out the side of it. i didnt have a good backup motor so i put in a fresh 305 out of a friends car. after the next race we started working on the cars handeling and was in the front with my little 305. i didnt win any races with it cause it slung a rod through the oil pan. im not saying to run a 305 but i am saying a descent, durable engine and a good handeling car will win more races than an ill handeling car and a gorilla motor. you spend your money getting 5,6,7, or maybe 8 horses, ill spend my money on shocks, springs and tires and ill out run ya!

just my 2 cents

waltonjr1
Member
posted November 05, 2004 05:00 PM
Thats true but the guys with the gorilla motor and good set up will beat you every week.



sixwillwin
Member
posted November 05, 2004 07:45 PM
geez...i guess outlawstock17 doesnt like Isky cams!! I was just providing info, who the h*ll said Smokey Yunick was the end all of knowledge anyway, sorry I tried to contribute to the question........ and Ego is right, Isky doesnt make or sell rods, so they have nothing to gain or lose, just their findings....and if you would read it more carefully, it says there are greater gains to be made elsewhere in the engine.

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The Joy of Six website


outlawstock17
Member
posted November 05, 2004 08:28 PM
nobody said smokey yunick was the "end of all knowledge", but david vizard's recent dyno testing proved smokey's long rod theory was right.

.....and i'm building an engine with an isky cam right now....not the cam they recommended though!

2nd2none
Member
posted November 05, 2004 11:59 PM
quote:
and Ego is right, Isky doesnt make or sell rods[/B]

Isky doesnt sell rods, they sell cams that work with the correct rod/stroke length.

you can not build the same cam engine with different rod/stroke lengths. The more rod there is the more duration on exhaust you need.



chomme
Member
posted November 06, 2004 05:48 AM
The benefit of the lighter piston far outweighs the actual length increase of the rod. The fact that the author of that doc. didn't mention the lighter piston part tells me one of two things. 1.) The whole things a half truth 2.) The author's an idiot. I'm betting #1 and there's alot of other items that were left out to express a personal belief.

I've run stock 5.7 rods and 6.0 rods in a 355. The 5.7 rod motor was easier to drive (revved slower), but the 6.0 motor revved harder and came off the corners a lot better. Associated parts (intake, heads, cam, headers, carb) were either the same or similar items, so I'm going with the 6.0s or longer if I can make them fit.

streetstock82
Member
posted November 08, 2004 10:57 PM
Just a thought to muster,
I run a 6.0 inch rod 383 chevy that was used in a super late model it's first season. It cost around $8000.00 to build. It ran upwards of 8800 rpm all season long and alot of races. It required minimal service and made 615 hp on the dyno. Granted, alot of things come into play with my results, but i think they were pretty good all in all.(P.S It has a reeds Mechanical roller cam in it. Not an ISKY.)


sdhnc29
Member
posted November 09, 2004 05:58 AM
chomme, Not to be a smart *** .........but Isky Tool Room Spring's are made by PSI for Isky.

I'm just curious who wrote the little article on the Isky web site?

Steve

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Hendren Racing Engines
Rutherfordton , NC
(828)286-0780


engineman
Member
posted November 09, 2004 07:23 AM
Ok I cant stand it anymore......I have done this over and over again with rod ratio. The one thing I can say for sure is, there is NO right answer. The right rod ratio for any engine depends on a lot of variables. I have run 5.565 up to a 6.250 rod in a 350, and have good results with both. However, if I was going to side with anyone...it would have to be with Smokey. In my experience longer rods make more power & have less cylinder wear...on the dyno & on the race track, BUT the "kink" in this hard earned information was a few years back when
one of my asphalt motors was getting it's **** kicked by the Fords we were running with...and the only thing I could figure is that it had something to do with the shorter rod they were running, they were pulling us hard off the corner,and I tried EVERYTHING to fix this problem, and in
desperation I built a short rod 350 (5.565) as an experiment...and he started winning again, However when we took it to the dyno, the short rod motor made a bit less power then our longer rod engines....I still don't have a solid answer....Maybe other car changes we made did it...Maybe my driver got better, I'll never know for sure.

I do agree with Ed Isky on a few points, the first is that I have NEVER seen a real "BIG" HP or TQ gain with a longer rod, maybe 15 HP is the most(on the dyno). And I think that there are 1000 things you should worry about before you worry about rod length.

However....if you have to buy a new set of rods and have the choice to make, I would go with longer rods.

For the most part....from what I have I have seen at the track, and heard from drivers I build motors for, plus what I have seen on the dyno...and what I have seen when I take the engines part....LONGER RODS PREVAIL 90% OF THE TIME!!!!!!!!!!! JMHO.

------------------
Mark's Racing Engines

(903)883-0196

www.northtexasracing.com

engineman@northtexasracing.com


tforsythe
Member
posted November 13, 2004 10:09 AM
hey thanks for all the input and i think i'am going to build a 6" motor


awkwardjeff
Member
posted November 14, 2004 03:41 AM
I'm not sure if the paragraph below is your thoughts or part of the artical, but let me assure you.....a 5.5 rod doesn't have a longer dwell time at BDC. that is the benefit of running the longer rod. To me the benefit of running a longer rod is NOT that it stays on top or bottom longer in terms of crankshaft rotation. The beneift is the piston speed. because the longer rod has stayed on top and bottom longer it travles faster in the middle of the stroke when cam is at of near max lift.....when comparing degrees of crankshaft rotation with shorter rods. In a short rod motor the piston will move say 20 degrees of crankshaft rotation to travle from .050 down the bore to TDC BACK TO .050 DOWN THE BORE......WITH A LONG ROD IT WILL TAKE 28 DEGREES OF CRANKCHAFT ROTATION TO TRALVE THE SAME DISTANCE.......SAME WITH BDC......this is why the piston is travle FASTER in the middle of the stroke.......create a better velosity to pull air through the intake into the combustion chamber. to me that is the beneift of a long rod. This is also the smae reason the torque curve is moved up in the high RPM range, and bottom end power is sacrficed in a long rod motor.


from your post "In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!"

In closing, did the chart mentioned in the artical change cams???? I'm sure not....they tried to compare apples to apples. But if you change something like ROD length you should also change cam profile to maximize to benefit....... You wouldn't put a BIGGER head on the motor and just run it would you??? No you would need to change cams to take advantage of the new bigger head you just installed.

ISKY is a good company, and you are a good person for posting the artical, but all information can be tainted to sell someone's idea, or thought........



sixwillwin
Member
posted November 14, 2004 04:38 PM
awkward and others, I only posted the Isky stuff for info for the origianl posters question. Evidently it came across like I thought it was Golden and proven, it was only <<<< INFO >>>> to look at. I couldnt care less what size rod anybody uses, I have used several long rod motors in the past......

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The Joy of Six website


awkwardjeff
Member
posted November 15, 2004 04:43 AM
I hope what I posted didn't sound like I was trying to come down on you......that wasn't my intent in any way. I wasn't sure, and you didn't clear up if that paragraph I quoted was your thoughts or from the Isky artical...... and it doesn't matter.

My real point of the post was the LONGER DWELL TIME ON BOTTOM DEAD CENTER WITH A SHORTER ROD.........that is false....the opposite is true.,.........the longer rod has loger dwell time.....and in my opinion is the benefit of the longer rod.

I was trying to clear that up for you and everyone else.......thats all..... you posted something trying to help the original post but your post contained FALSE information. It wasn't theory, or open to debate.........it was a math problem that had the wrong answer, and my intent was to show the correct answer to the math problem.

You have always posted good information, and my hope is this post doesn't prevent you from doing so in the future. I respect your posts, and your thoughts, keep up the good work.

sixwillwin
Member
posted November 15, 2004 06:42 AM
Thx for the reply awkwardjeff, great info as always, you are correct, that any misinformation shouold be cleared up. Well, time to get the Farmall M ready to push snow as it getting cold here in the north. later sixwillwin

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The Joy of Six website


engineman
Member
posted November 15, 2004 07:27 AM
I'm going to go try a few things,with different rod lengths and the degree wheel I'll let yall know what I find in a day or two. see which one dwells longer at the bottom.

[This message has been edited by engineman (edited November 15, 2004).]

jracinp
Member
posted November 15, 2004 10:33 AM
been following this topic very interesting and have good info from all of ya

so now tell me where else that I need to look into to get some juice outta my S B C

my rules are

stock up to 30 over
no humped heads
iron manifolds
4 bbl gm /quad or roch -must use a stock type air cleaner (I use 1 from a 502 caddy )its the largest inlet I can find

1 outlet 2-1/2 max single exhaust system- must exit behind driver- stock type muffler (I use a V force and techs havent questioned it)
no camber
open rear only-
70 series tires radials- 7" rims no offset
no wieght rules

stock appearing /replacement springs


some info for my ride
78 riviera chevy 350
th 350 trans- rear gear /273/-80 inch tire roll

3585 total wieght sorry dont have the splits
lf-1150 rf-1250
lr-200 rr-225

battery is behind seat - cell set 1/3 right side 2/3 left

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racing is way too cool gave up golf for it

http://chasinracin.com/cgi-bin/showpage.pl?ID=on_my_6