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Old 10-14-2012, 10:44 PM
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5.7's or 6 in rods

Planning on building a new drag racing motor probaly a 383 but on a tight budget and i'm wondering if there is a big power difference between a motor with 5.7 rods and on with 6in.planning on a 30 or 40 over bore with around a 280 comp solid lift cam and procomp aluminuim heads and flat top pistons.something tha will run on pump gas.going 5.7 is definatly cheaper but is the 6in gonna blow it out of the water.

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Old 10-15-2012, 12:36 AM
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For a moderate engine 5.7 is good.how long of stroke? how much horse power? cheap and racing do not go in the same conversation
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Old 10-15-2012, 07:00 AM
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remember the longer the rod the harder it is on cylinder wear....
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Old 10-15-2012, 07:27 AM
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the 6" won't "blow it out of the water" but there is a slight hp advantage as well as a slight durability advantage. In most cases the cost difference between the two is very small.
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Old 10-15-2012, 10:26 AM
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Please read Tech Tip 2005 here.....it was written by Ron Iskenderian, son of Ed Iskenderian, the "Camfather". When I don't fully understand something, I try to find out what the experts and engineers have to say about it. Ron addresses in this Tech Tip tutorial the fact that a 6" rod in a SBC 383 puts the wrist pin up into the oil ring, adding complexity with additional parts to keep the oil control rails from rotating and snagging in the pin hole. In my opinion, it's just "something else to go wrong".
ISKY Racing Cams - Do It Right. Race with the Legend. Camshafts, Connecting Rods, Valve Springs, Lifters

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Old 10-15-2012, 10:37 AM
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Please read Tech Tip 2005 here.....it was written by Ron Iskenderian, son of Ed Iskenderian, the "Camfather". When I don't fully understand something, I try to find out what the experts and engineers have to say about it. Ron addresses in this Tech Tip tutorial the fact that a 6" rod in a SBC 383 puts the wrist pin up into the oil ring, adding complexity with additional parts to keep the oil control rails from rotating and snagging in the pin hole. In my opinion, it's just "something else to go wrong".
ISKY Racing Cams - Do It Right. Race with the Legend. Camshafts, Connecting Rods, Valve Springs, Lifters
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Old 10-15-2012, 10:56 AM
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In other words......low budget = K.I.S.S.

By the way, welcome back tech.
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Old 10-15-2012, 11:34 AM
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We try to put 6" rods in most 3 3/4" stroke engines. Getting the rod/stroke ratio "up" is important. JR, that's exactly backwards. A SHORTER rod has a "steeper" rod angle (the relationship of the the rod to the crank at "full swing"). This puts additional "load" on the piston as it pushes "up" the wall.

Longer rods reduce that angle, as said. They also allow the piston to "dwell" (where the pistons stops and changes directions) at TDC and BDC an additional degree or two (depending on the length change). On the intake stroke, this allows better "filling" and more cylinder pressure on the "power" stroke.

We've done experiments with circle track engines over the years. The 6" rod versions tend to do "better" at higher revs and "constant' revs (long straights, wide turns). The "short rod" engines seem to make more low-end and "come of the corner" harder.

We've been using oil ring "spacers" for years. Zero issues. EVER...

FWIW

Jim
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Old 10-15-2012, 11:37 AM
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Read my response in the "basics" forum addressing this issue.

Jim
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Old 10-15-2012, 12:25 PM
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Quote:
Originally Posted by Mr. P-Body View Post
Read my response in the "basics" forum addressing this issue.

Jim
Ummm, how about a link Jim?
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Old 10-15-2012, 02:05 PM
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Quote:
Originally Posted by chris78 View Post
Planning on building a new drag racing motor probaly a 383 but on a tight budget and i'm wondering if there is a big power difference between a motor with 5.7 rods and on with 6in.planning on a 30 or 40 over bore with around a 280 comp solid lift cam and procomp aluminuim heads and flat top pistons.something tha will run on pump gas.going 5.7 is definatly cheaper but is the 6in gonna blow it out of the water.
There is no big power difference between the two. In many cases there's barely any difference unless the engine combo was designed from the bottom up to take advantage of the 6" rod's charecteristics. The best thing that can be said for the 6" rod in a moderate build is there's less side loading. The downside is possibly cost and slightly more complexity.

If it was me I would run 6" rods. That said, there are many thousands of 3-3/4" stroke SBC engines using 5.7" rods. Some are real animals, too! But if you can get a deal on a quality set of 5.7" rods and pistons to match, I sure wouldn't hesitate to use them.

IMO, a correctly matched combo of parts for the job the engine is to do, good dynamic balance, splayed 4-bolt caps, quality rod and crank forgings, quality fasteners, and careful attention to details during assembly (to name a few things) are more important than that added 0.3" rod length.
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Old 10-15-2012, 02:55 PM
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TI, the OP has two identical threads going on this. Here and: 5.7's or 6 in rods
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Old 10-15-2012, 05:41 PM
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Quote:
Originally Posted by cobalt327 View Post
TI, the OP has two identical threads going on this. Here and: 5.7's or 6 in rods
OP, your other thread has been merged here, don't post the same issue in different forums.
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Old 10-15-2012, 06:23 PM
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Quote:
Originally Posted by chris78 View Post
Planning on building a new drag racing motor probaly a 383 but on a tight budget and i'm wondering if there is a big power difference between a motor with 5.7 rods and on with 6in.planning on a 30 or 40 over bore with around a 280 comp solid lift cam and procomp aluminuim heads and flat top pistons.something tha will run on pump gas.going 5.7 is definatly cheaper but is the 6in gonna blow it out of the water.
For the same type rod there is little to no cost difference. While I'm not hard over on the 6 inch rod as there is not a great deal of benefit, the benefit is there and is minimally measurable at the crankshaft on an engine dyno vis-a-vis the same setup with a 5.7 inch rod. So for a race engine it does put a small advantage in your corner and race engines that seperate the winners from everybody else is paying attention to the small advantages. Everybody can run the same big parts of whomper-stomper cam and super-duper intake, an MSD distributor with sparks so bright they shine through the cast iron, and a carb so big it sucks in the neighors dog and kids. So the seperation to the winners circle is really the little details more than this big stuff everybody else has.

So of any place to save money on a race engine or anyother engine isn't in the block and bottom end. If you bolt the good stuff to an insufficnetly strong bottom end when it fails it will take your good stuff with it. When on a budget you're a lot better off to put your emphisis on the bottom end to keep the engine together, enjoy the competition even if your not winning you're learning and piecemeal the top end out as the budget improves.

I don't recommend getting crazy with the boring bar .030 maybe .040 at the most. Since about 1974 these modern castings can be pretty thin and when you're pushing on the motor you need all the strength you can retain. Where a sixty over block may run a long time on the street it will let you down on the track, if it doesn't out right fail it will at least allow enough cylinder wall movement that the ring seal gets upset and allows excess blow by which is horsepower lost to the crankcase.

Since if you're racing on sanctioned tracks you'll probably need a good quality SFI approved damper regradless of the crankshaft. This lets you consider a 5140 forging which can be had for a lot less than a 4340. The 5140 material is a bit stiff making subject to failure without a good damper, but since you'll need a good damper even if running a 4340 crank, you can save a few bucks to put against better machining and a set of 4340 rods. For rod selection I'd need to know more of what you expect from this engine and balance. There is no doubt that for a race engine internal balance is far superior to external balance. Starting with a forged crankshaft you've got a better chance of acheiving that because the material is more dense than a casting which often puts getting the stroker motor balanced when care is taken in the selection of rods and pistons to keep the weight down. However, this adds to the cost of these pieces but that can be balanced against the cost of mallory metal to balance the shaft. So this is a place where you need to study just what you need to get to where you want to go.

So a large question get to be where do you want to go and how much budget do you have to get there? How much risk of the engine are you will to or can you afford to take?

Bogie
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Old 10-15-2012, 07:41 PM
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The trend towards long rod technology can be traced back to Smokey Yunick saying you put the longest damm rod you can find. John Lingerfleter I remember when he used to re-machine Buick rods for the 5.85 length.They are now well past the 6" rod size. You look at some of the newer design engines all with longer rods. So yes there are advantages. Oh Howards no longer offers a 5.7 stroker crank by it's self.
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