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

Quote:

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.......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?

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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.

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.

I try to use the longest rods I can on stroker motors, as you already know longer is better,right? Ask any women about long rods in there favorite stroker motors

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.

That's good news. I know this subject did cause a bit of a stir when posted some time ago, just couldn't find the post.

Another side bar I was concerned about was the pin being in the oil ring groove on the pistons. I think with the compression distance (1.350"), I will end up with, I will be pretty safe.

This is a custom Diamond piston for a 4.125" stroke and 6.200" rod........

1.322" CH

A longer rod will hinder cylinder fill due to the slower movement from TDC , the advantage being on the power stroke where it will 'capture' cylinder pressure for a longer period of time , bit like clenching a fire cracker as opposed to resting it in you're palm.

Anyhow , compression height is more important than rod length.

nice looking piston. Don't worry about the pin being in the ring land, with the oil ring support it really is not a big deal.
With the cylinder fill/rod length issue, I thought the shorter rods helped port velocity when using heads with a large port window compared to engine size/RPM, but the longer rods helped when using smaller port heads, and at higher RPMs?
I think in this case, the length difference would be too small to notice any difference.

I won't know for sure until I get my block back from have the valley modified for the roller lifters however, a quick calculation yeilds approx 1.340" CD which may just miss the oil ring groove anyway.

So if the difference in using the 6.250" rods compared to the 6.125" is negligable, may as well stick with the 6.250" as the pistons will be little lighter too??

Will the 1.340" CD piston still be as durable for street applications as lets say, the 1.460" CD piston with the 6.125" rod?

Nice looking piston.....what size ring pack does it use 1/16 etc?

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.

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.

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.