WHICH ROD LENGTH
#938819
02/27/11 07:24 AM
02/27/11 07:24 AM
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Joined: Nov 2006
Posts: 370 Queensland Australia
moparmacka
OP
enthusiast
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OP
enthusiast
Joined: Nov 2006
Posts: 370
Queensland Australia
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In an another post I asked about the compression height of a piston for my SB stroker with 4.000" stroke and 6.250" rod. The crank has Chev 2.100" B/E and 340 mains. I have since spoken to the guy I am buying the 6.250" rods from and he said if it easier he will swap them for a 6.125". Apart from the vast off the shelf pistons to suit the 6.125" rod, is there any great advantage (rod ratio etc) in swapping to a 6.250" rod, considering this will not be an all out race engine? I would still have to go a custom piston to obtain the correct comp ratio anyway (aiming for 10.8-11.0 with E/brocks. Most inverted domes are 9.5-10 and flat tops yeild 11.5-12 static comp ratio). Thanks Macka
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Re: WHICH ROD LENGTH
[Re: moparmacka]
#938820
02/27/11 07:47 AM
02/27/11 07:47 AM
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Joined: Jul 2006
Posts: 4,595 On the south side of Nowhere
S/ST 3040
master
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master
Joined: Jul 2006
Posts: 4,595
On the south side of Nowhere
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Quote:
.......is there any great advantage (rod ratio etc) in swapping to a 6.250" rod, considering this will not be an all out race engine?
No. Even in a complete race application, the difference would be negligable. However, since you say a custom piston is required either way, I'd do it. I usually build what I want, when I'm paying for it.
IIRC, Don Gentry(B3422W5) used a 6.250" rod in his 4" engine. I'm sure, it's been done many times. There is no downside to using it in your case.......Cost being the only factor.
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Re: WHICH ROD LENGTH
[Re: moparmacka]
#938821
02/27/11 08:13 AM
02/27/11 08:13 AM
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Joined: May 2008
Posts: 5,399 Aurora, Colorado
451Mopar
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master
Joined: May 2008
Posts: 5,399
Aurora, Colorado
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I doubt you'll notice any difference with a 2% change in rod length. In most normally asperated engines the preference is to use a longer rod. I think the reasoning is the long rod keeps the piston near TDC (Dwell) for a longer time than a short rod. On the intake stroke this allows the cam to open the intake valve further before the piston reaches maximum velocity. On the compression/ignition stroke it gives the fuel longer to burn with the piston near TDC creating more cylinder pressure. The longer rod would have less angle offset so it should put less side load on the cylinder walls.
Last edited by 451Mopar; 02/27/11 08:14 AM.
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Re: WHICH ROD LENGTH
[Re: Cab_Burge]
#938823
02/27/11 04:13 PM
02/27/11 04:13 PM
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Joined: Feb 2009
Posts: 1,532 off the grid
340B5
pro stock
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pro stock
Joined: Feb 2009
Posts: 1,532
off the grid
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A long rod has more of a straight down push@ 90* ATDC. Try peddling a bicycle uphill standing on the peddals vs sitting on the seat. The longer rod will make better mid range torque.
Yeah, it's got a smallblock.
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Re: WHICH ROD LENGTH
[Re: moparmacka]
#938830
02/28/11 05:55 AM
02/28/11 05:55 AM
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Joined: Jul 2006
Posts: 4,595 On the south side of Nowhere
S/ST 3040
master
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master
Joined: Jul 2006
Posts: 4,595
On the south side of Nowhere
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That piston has a very small dome to offset valve reliefs and yields 13:1 453 grams w/out pin Ring pack is 1/16" x 1/16" x 3/16" Has been running since 2008. Changing rod lengths will have no effect on port velocity. That is determined by port size and engine displacement and RPM.
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Re: WHICH ROD LENGTH
[Re: 602heavy]
#938831
02/28/11 04:17 PM
02/28/11 04:17 PM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,206
New York
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X2: a minimum safe distance for the ring package is step #1, rod length is whatever you have left. Don't try to use the same CD as another engine, the spaces needed differ quite a bit: 1. actual ring thicknesses (1mm vs. 1/16" etc.). 2. crown down-distance for ring #1 (must be greater if nitrous or blown). 3. minimum crown thickness to prevent valve relief from getting too thin at the bottom. This varies with valve size, placement, stem angle and lift. 4. how long is the skirt? If you have 2" of skirt below the pin (assuming it clears the CW and doesn't hang out of the bore 30° B/A BDC), the CD can be even shorter.
Almost any 1.400" is safe. At 1.300", some. At 1.250" you want a .927" (SBC) pin etc. Below that it's not going to last very long.
Boffin Emeritus
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Re: WHICH ROD LENGTH
[Re: polyspheric]
#938832
02/28/11 05:17 PM
02/28/11 05:17 PM
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Joined: Jan 2003
Posts: 9,996 Frostbitefalls MN (Rocky&Bullw...
gregsdart
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master
Joined: Jan 2003
Posts: 9,996
Frostbitefalls MN (Rocky&Bullw...
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X3: a minimum safe distance for the ring package is step #1, rod length is whatever you have left. Another strong consideration, and maybe ranking first, is off the shelf availability. If you can, get the most components (especially pistons) that you can in case you have a problem. This is not a race motor I assume, and the pros argue the benefits of rod length. The possible gains being small, i wouldn't worry about it much.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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