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  #1 (permalink)  
Old 05-04-2005, 08:29 PM
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5.7" or 6" rod

I finally got the "o.k" a min. ago from cnc motorsports I want to excange my pistons and rod's for a 6" instead of the stock 5.7

do you guy's think i'd be better off to exchange 'em for the 6" rods and pistons instead of keeping the 5.7" ones i have now



J

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Old 05-04-2005, 08:33 PM
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I don't like the fact that the 6" rods make it so the oil rings are up into the piston pins. I'd rather have the reliability of the 5.7".
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Old 05-04-2005, 09:17 PM
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From a power standpoint there is no difference.
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Old 05-04-2005, 09:32 PM
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Pardon me??? No difference. Why would they change something like 5.7" stock rods to 6", 6.150" etc.

What would be the point?? I know theres a difference, I would just like to here why you think there is "no difference" power wise.


bonuts
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Old 05-04-2005, 10:02 PM
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We have had many discussions on here about the 6" vs 5.7". MOst people on here will agree on one part of the battle and that is the 6" rod is better for endurance but not power. I believe the 6" rod is better for both power and endurance.
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Old 05-04-2005, 10:23 PM
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anybody ever heard of smokey yunick?smokey says use the longest rod possible.
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Old 05-04-2005, 11:11 PM
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I figured that would rouse the crowd. Here is my basis for saying that.

Invest $40,000+ in a dyno, build motors with every combination of bore, stroke, rod ratio you can think of to come up with 350 CI, using 305, 307, 350 and 400 blocks and you'll see, it's not going to make a difference.

Another option, which I found very well written, is to go pick up the latest Hot Rod. In that issue is an EXCELLENT article on bore/stroke relationships that also discusses the rod length issue. Although you may raise the issue that the motors tested were big blocks the theory and relationships remain very valid. The article does go into some depth relating directly to the small block at the NASCAR level as well. Very much worth the read. One of the few articles I have ever read in a magazine I could directly relate to in the real world.

Many of the testing theories they conducted in that article we have run time and time again searching for that "wonder combination". So far rod length is not where the money is going. The measurable power is with the flow bench, we have 2 now, porting, lots of time with Dema Elgin at Elgin Cams, intake work, carburetors and fuel. We are also investing time on the dyno with vac pumps. Want to gain about 20 lb/ft of torque through the mid and top RPM range on a 380 HP motor. Run a Moroso vac pump at 15" of depression. Make sure your oil pressure is good though as that will fall 14 lbs with the pump hooked up. That's a mod that appears to be worth power.

At the level almost ALL folks are at with street motors it's just no worth worrying about. There are WAY more important areas to worry about. The cylinder head, camshaft, and intake to name three biggies. Correct tuning of the carburetor and ignition advance curve is more important that rod length. When you read the article in Hot Rod they mention the amount of time they spent tuning these motors for best power. On the dyno it's not unusual to spend days on the tuneup to get it optimized.

Oil control is another area worth spending time on to retain HP.

Yes, I have heard of Smokey. I used to see him every year at the Performance Racing Industry (PRI) Show until his death. A very nice guy with a pleasant wife. He gave great presentations. He was very deaf the last year or two of his life by the way. Why he pushed Prolong products at the end I'll never know. Must have been good money.

As for endurance, really can't say. All I know is you see lots of short rod motors with lots of miles on them that still run strong.

Pin height is a valid concern, although on a 6" small block not a show stopper. I have never seen a ring problem with this combination. Getting the pin height up too high though with too long of a rod can cause issues though with the pin itself, seizing in the bore. Heat into the pin has been an issue with a lot of very long rod combinations. Special pin coating have been developed as well as lube tube feeds drilled into the rods for pin oiling to combat this problem. Many builders have quite the long rod bandwagon simply due to this reliability issue. This is not street related but race related.
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Old 05-04-2005, 11:20 PM
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i agree with rick. THe VERY minute horsepower difference you're going to make (coupled with a smaller low end torque number that a long rod produces, something most people are unaware of...) its not worth it to play the long rod game unless you're really in the game for every last bit you can get, like in a high dollar motor. Using really long rods on a street motor is like putting titanium valves in a street 350. Sure, its nice, and yeah, it might help a little, but there are better ways to spend that big money. Get the picture?

K
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Old 05-05-2005, 07:02 AM
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rods

Quote:
Originally Posted by Rick WI
From a power standpoint there is no difference.
WOW for the first time in history i am not the only one that has figured this out!!!!!! Thanks Rick!!!!!!

This has been discussed in depth many times before on this as well as other forums and i always ask from the person that tells me how good the longer rod engine is to "show me your proof" and up to and including today no one has ever shown proof of there theroy........ Most of the people that i have discussed this with before have plagiarized there ideas from some other write up about long connecting rods, usually if you trace it far enough it comes back to a magazine deal that was sponsered by someone that sells connecting rods!!!!!!!!

If you go to reher and morrisons and isky web sites they have some good explinations about all the theorys surrounding the longer connecting rod.....Oh yea they sell engine with longer rod beacuse that is what the customer demands, plus it really does not hurt anything... it just does not help either....

Think about this..... the location of the piston in relation ship to tdc or bdc between a 5.7" rod and a 6.0" rod in a 3.75 stroke engine is never more then .017" difference........ How much additional HP can .017" make??? 1 percent,,,1/2 percent????? None?????

As for smokey.... that statement in the book is taken out of context.... He just touts using the longest rod you can,,, but does not say anything about the benifits of using a longer rod.... Plus it's just his opinion...

I built 2 406 sbc about the same time.. They were almost twins. same pistons in both. One was a 5.7 rod and one was 6.0 rod. The guy with the 5.7 rod engine ran the crap out of it, just beat it like a 3 year old in wal-mart.... The 6.0 rod engine was taken much better care of.. Both were with-in 25 hp of each outer.... When i got them back to freshin the pistons in the 5.7 engine looked like new... In the 6.0 engine they were all but trashed...
The shorter skirt reduces the load carrying abilities of the piston and that is why the 6.0 rod pistons looked so bad IMO

Keith
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Old 05-05-2005, 08:43 AM
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I do agree that this is "all out stuff". Not worth the worry on the street.

On the many side by side comparisons, I do not agree and dont believe I've seen a valid side by side made yet.

Now this is only my opinion and I am only throwing it out for you guys to ponder for a moment.

For a valid side by side to take place, you couldn't simply pull the 5.7 rods out and install 6.0s and complete the test with the expectation that anything will change. The reason for my theory is this.

1). Your all out cam is set up to make a particular valve event happen with a particular piston event. Thus, having changed piston event without regard for the rest of the combination (valve event among others) will never increase hp or torque.

2) As miniscule as .017 is, if a guy were able to blow down a cylinder sooner based on more work performed earlier in the stroke, that would be a measurable benefit to exhaust flow and the possible result/benefit would be a less diluted intake charge.

3) Ditto on the intake side. Any change in piston speed or event will dictate a change to lobe profile to attain maximum hp & torque.

Now whenever I talk with the cam grinder about a maximum effort combination, one of the many questions regarding the combination is always rod length. I dont believe this particular question is based on small talk and believe there is a reason for him asking everytime.

I do agree with the longer skirt yeilding less rock and would recommend it for a motor that was intended for fun on the street for a long time. However, for racing I believe the increased weight of the piston would hinder performance.

I believe anything you change on an all out combination without regard to the rest of the combination will result in less performance not more.
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Old 05-05-2005, 09:13 AM
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Rods

56MAYNARD,

Some good stuff in there.... I can understand the need for a cam change in reguards to the rod length to gain the most advantage or for a fair test. I got to this point before with a very knowledgeable builder and when i asked for a actual amount in the two comparisons. He was like....well..... uh..... maybe 1 percent??? But never really sounded sure if it was that much... I just don't think the data is out there for hard numbers. I would guess the people that spent the time on the R&d for this issue just never found anything???? Or else decided to keep it to there self.....

The thing i don't understand totally yet is... If you talk to the cam grinder and he asked you about rod length, Then he sends you a cam based on that. The cam is still installed at the same location in reguards to the valve events. ( there is not one installed location for a 5.7" rod engine and another for a 6.0" rod engine) Or another way of saying it a cam for a 5.7 rod engine and a 6.0 rod engine will still have the intake closing at say for example 42 deg abdc. So your only changing the other 3 events for this cam example. So if you used the same cam in both engines and made the intake closing event correct for each engine would it really make a measurable difference with the other 3 events?????? Not a statement because i really do not know it's a question....

I was told, but i am not sure if it's true that GM spent countless $$$$$$ on the nascar engine program. Starting with a rod that was like 5.2" long and went up to 6.2" long. Changing cams along the way to get the most benifit from the rods they ran. They came to the conclusion that it' a none issue??? Again not sure if it's true..

I love talking about the techincal side of engine building like this.... All bouncing ideas off of each other. I would guess the typ of person to ask this question would never be to the point where he is at the all out end in his engine program....

I do agree with you on the skirt and weight deal on the piston for a street / race engine. I think many people with the street engine think this typ of thing will gain them some big increase in HP because they saw in on the net some place..... But your right it all has to do with intended usage of the engine...

Keith
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Old 05-05-2005, 09:38 AM
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Maynard, In the Hot Rod article they did tune for the rods with the camshaft, there is a lengthy discussion about it along with the lash specs they settled on.

They also had a discussion with DEI on the long rod issue and they claim significant increases with the long rod. Significant is a maximum of 3 HP.
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Old 05-05-2005, 11:38 AM
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Here's my humble, most un-scientific opinion.

A connecting rods job is exactly what it's name implies. If it does that job reliably then yes, it makes more horsepower than one that doesn't. I could care less how long or short it is as long as it stays put.

I've never used a dyno, I occasionally use a flow bench to quantify porting results. I build engines and run the snot out of them until they break. I've built and rebuilt engines with long and short rods and I have NEVER seen a difference in E.T. or MPH as a direct result of the use of a longer or shorter rod in any engine.

Larry
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Old 05-05-2005, 12:08 PM
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You want a side by side comparison? Call Bill at Hendrens Racing Engines
828-286-0780. He's an engine builder that does Circle track, marine, drag, salt flats, and others. He makes a 1000's of engines. As far a SB2.2 stuff, he's got you covered, for they even build those too.
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Old 05-05-2005, 12:18 PM
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data

Quote:
Originally Posted by johnsongrass1
You want a side by side comparison? Call Bill at Hendrens Racing Engines
828-286-0780. He's an engine builder that does Circle track, marine, drag, salt flats, and others. He makes a 1000's of engines. As far a SB2.2 stuff, he's got you covered, for they even build those too.
Any chance of you getting some of this data and posting it??? I am sure he would not want all of us calling him and asking the same questions....

Keith
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