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Old 12-26-2012, 05:34 PM
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I wanted to go back through this thread and make some observations and comments about questions that have been raised......

Originally Posted by ChevroletSS View Post
What is the purpose of 6 inch rods compared to 5.7 inch?? Are 6inchers what are in 400sbc?? Im trying to figure out what rods Im gonna go with. Do you need shorter pistons with 6 inch rods???
There are a couple of advantages to using 6 inch rods versus 5.7 inch rods.
1. Less side-loading of the piston skirt against the cylinder wall would seem to me to lessen friction and allow more horsepressure at the flywheel.
2. A longer rod will require a shorter piston compression height, allowing more room for the counterweights of a stroker crank to clear the underside of the piston at bottom dead center. The more counterweight you can use, the less Mallory Metal you will have to use to balance the reciprocating assembly. (Mallory is expensive to buy and expensive to drill the counterweights and insert the heavy metal).

The main disadvantage to longer rods, in my opinion, has been the fact that the wrist pin gets pushed up into the oil ring space and you must use another rail to prevent snagging of the oil ring ends in the wrist pin hole. It just smacks of a band-aid fix to me and adds the complexity of more parts to go wrong in the long run.

Lately though, some piston manufacturers have stepped up to the plate with a concentrated ring stack that allows the wrist pin to be clear of the oil ring groove. I'm stoked over that and no longer have any issues with using a 6 inch rod in a SBC build. If I were to build a 383 tomorrow, I'd probably use forged Probes or other quality pistons that feature a concentrated ring pack along with a Scat cast steel crank and 6 inch Scat I-beam rod. David Vizard has taken 550+ hp out of the Scat cranks and rods for years now with not a single failure according to what he writes. Good enough for me.

400 small block Chevys came from the factory with a very short rod, 5.565 inch. I'm sure the GM engineers took side-loading into consideration when they designed the motor, but its intended use in station wagon and large family sedans (low rpm operation) made it worthwhile to use the shorter rods to clear the cam and oil pan rail.

Originally Posted by ChevroletSS View Post
But what is the difference in the rods besides length???
6 inch rods will dwell the piston longer at the top of the stroke, while dwelling it for a shorter period of time at the bottom of the stroke. Some folks feel they can get an advantage of dwell time at the top of the stroke to allow the mixture to burn a little longer and make more cylinder pressure. Other folks will tell you that there is no free lunch and that if the piston dwells longer at the top of the stroke, then it must, of necessity, dwell for a shorter period of time at the bottom of the stroke, decreasing cylinder fill time before the intake valve closes.

As I stated above, the main advantage of a longer rod for me, at this point, is to make it easier and cheaper to internally balance the motor and to reduce side-scuffing of the piston skirt against the cylinder wall.

Originally Posted by ChevroletSS View Post
I think Im gonna go with an assembly internally balanced with 5.7 rods. Its hard to pull the trigger and throw money at this cause I just wanna make sure I get what is best.
I would advise internally balanced with 6 inch rods. It'll be easier to balance.
Now, you may come back to me and say that the assembly you're purchasing is already balanced. BULLHOCKEY. Get all your parts together and take them to the machine shop to be balanced. TRUST NOBODY.

Originally Posted by ChevroletSS View Post
Also what's the difference in 5140,4260,8740, and 4340 rods and can I go longer than 6 inch and I what about h beam, I beam, or a beam. Floating pin or pressed fit pin. Does it matter if its shot peened or polished and shot peened finished. Straight or tapered.....
Different alloys of steel are used by different manufacturers depending on load, cost, machinability, etc.

Properties of 5140 steel.....
eFunda: Properties of Alloy Steel AISI 5140

I don't think there is a 4260 alloy....

Properties of 8740 steel....
eFunda: Metal Alloys Keyword Search Results

Properties of 4340 steel....
eFunda: Metal Alloys Keyword Search Results
4340 Alloy Steel Alternative | Pennsylvania Steel Corporation :: Flexor
Tool Steel, Alloy Steel, Powdered Metal - 4340 Alloy Steel

Longer than 6 inch? Yeah, there are SBC rods available up to 6.5 inches, but I suspect the custom pistons to use them would be very expensive. Personally, I don't know because I have never had the need for 6.5 rods because I have never built a 1,000 hp SBC.

I-beam rods will work fine in most any motor that any of us put together for street use or occasional drag strip blasts and generally are less expensive than H-beam rods, which will hold together better in high-hp applications.

Pressed pins are the norm for street/strip motors. Floating pins might be a better choice where the motor will be disassembled frequently, like in a circle track motor or Competition class drag racing motor. And, the lock rings used for floating pins adds complexity to the mix, something else to go wrong (like popping out of the pin bore or breaking and wrecking the piston and cylinder, for instance).

Shot peening will add roughly 100% strength to the resistance to cracking. Cracks generally form on the outer surface of a part rather than further inside the casting or forging. Peening closes the "skin" of the material and discourages the formation of cracks. Polishing is another way to help prevent the formation of cracks in the outer skin of parts. First you smooth and polish the part, then you shot peen the part.

Originally Posted by ChevroletSS View Post
What's better, dished or flat top pistons.
It's not a case of one being better than the other, it's a case of using whichever design you need to bring your combination together. Personally, I think a guy would be better off centering his build around a flat-top piston just for the flame propogation and squish properties involved, but that's just a personal feeling. Adjust the chamber volume to arrive at the proper static compression that you're shooting for and the proper camshaft intake closing point to arrive at the dynamic compression ratio that you're shooting for. COMBINATION, COMBINATION, COMBINATION.

Originally Posted by ChevroletSS View Post
If it has 50 less no biggie. Just knowing I got good parts capable of making those numbers are good for me. I dont even have a dyno anyway so it not like Ill actually know which sucks and I dont know anyone who has one
I like your attitude. I know that when I was playing with the DynoSim program, many times I spent several hours just changing cams and altering the installed positions of them to gain another 5 or 10 hp. For instance, it's pretty easy to pick up 10-15 hp with 1.6 rockers over the standard 1.5 rockers, particularly with good heads. Many times you can pick up another 10-15 hp by advancing or retarding the camshaft. Don't misunderstand, I would not automatically go with 1.6 rockers, but many times, if you have the correct camshaft timing in the first place, the extra valve lift will put you over the top. My very best advice to you would be to invest in a DynoSim program and play with it. 150 bucks will get you the latest software with an 8000 camshaft file to pick from. You'll be able to tweak components and see what will work best for you before shelling out the shekels. I have no doubt that with the software, you could mirror the results that were achieved by the folks who put the combination together and tuned it for AFR.
I very highly recommend the following 2 products.....
DynoSim5 Main Page
CamDisk8™ CamFile Library Main Page

Originally Posted by ChevroletSS View Post
It doesnt make sense to me that you can bye brand new main caps for a block and get them line honed to work but you cant take used main caps and make them work.
Aftermarket caps have more meat on them, making it easier to make a round hole. OEM caps will take a lot of labor time to match up and maybe your machinist figures that you will not want to pay the labor time for him to do the matching up. You can cut into the main saddle only so far, even though there are short chains available for the timing set. As AP72 mentioned, if you're gonna pop for aftermarket caps, go with 4-bolt splayed caps.

Last edited by techinspector1; 12-26-2012 at 05:47 PM.
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