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I am building a 489 it is also a 10.2 Block,

I have been trying to figure out compression ratios and I have been using the www.rosspistons.com calculator to do so.

My question when I look at pistons in jegs, summit or on flatlanderracing.com some are for a 6.135, 6.385, and 6.535 Rods. This is the same way with cranks too. Have these just been pre balanced or is there a difference between the piston for a 6.385 rod and a 6.135 rod.

I am asking because rosspistons calculator give you the the net dome cc needed to achieve a particular compression ratio. SRP makes say a 30 cc dome pistons for a 6.135 rod and then a 26cc dome for a 6.385 rod. If I wanted a little more compression could I use the 30cc piston with a set of 6.535 rod or do I have to use only pistons that are for 6.535 rods?

Thanks for all the help Mike
 

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Think about a block....

There's a specific distance between the centerline of the crank, and the deck surface. Call it "height" for lack of a better word.

The "top" of the piston must come up to the top of the deck, more or less.

Therefore, the sum of ½ the stroke, plus the rod length, plus the piston "compression height" (distance from wrist pin CL to "top"), plus the "deck clearance" (.025" or so stock, usually more in "rebuilder" pistons like TRWs or cast or hypers, ideally .000"), must equal the "height" of the block.

For a given block and stroke, if the rod is longer, the wrist pin bore is drilled higher up in the piston. Obviously this can't be changed after manufacture. If you use a piston with the bore too low (too much compression height), the piston will come up out of the block and hit the head. Too little CH, and the piston won't make it to the top, and you'll end up with SUPER low compression. Both of these outcomes are considered BAD.

Rod length doesn't affect the crank, except that for VERY long strokes, some minimum rod length is required. Reason for that is the counterweight. Think about what those are... at each rod throw, the rods are on one "side" of the crank CL, so the counterweight must be on the other. Meaning, when the piston is at BDC, the counterweight is pointed straight up. Meaning, there has to be enough space BELOW the bottom of the piston, to fit a large enough counterweight, to counterbalance the mass of the rods and pistons. Meaning, the rod has to be long enough, for the pin bore to be up high enough, for the bottom of the piston to be out of the way of the CW. It's possible to get away with a little less than that; the factory did that with the 400 SB and the 454, by using "external" balance. This only works up to a point though, and "internal" balance is always preferred, if possible.

However, when you move the pin bore upwards to accomodate a longer rod, sooner or later it reaches the rings. Clearly this is a compromise in the ring package design. The rings have to be narrower, closer together, and/or closer to the "top" of the piston. Sometimes a little spacer support thing with ring grooves can be used, to go in the end of the pin bore and hold the rings. You would need to make sure that whatever combo of stroke, rod length, and deck height of the block you use, allow a sufficiently robust ring set. Not much fun to use a 30-lap ring package on a motor that's supposed to last for 50,000 miles.... tends to lead to disappointment.

Companies like Ross, JE/SRP, Wiseco, Arias, Venolia, etc. ("real" racing piston mfrs) will make you a piston of ANY size dome, ANY bore, ANY pin height, ANY ring package, and so forth, subject to the physical constraints of a piston in general. I wouldn't get all hung up on just what's listed in the catalog. If you want something not listed, call and ask, or fill out their form for a quote. Talk to your racing machinist first though, and make sure you're not missing something in the design of your motor, that will cause trouble somehow.
 

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Think about a block....

There's a specific distance between the centerline of the crank, and the deck surface. Call it "height" for lack of a better word.

The "top" of the piston must come up to the top of the deck, more or less.

Therefore, the sum of ½ the stroke, plus the rod length, plus the piston "compression height" (distance from wrist pin CL to "top"), plus the "deck clearance" (.025" or so stock, usually more in "rebuilder" pistons like TRWs or cast or hypers, ideally .000"), must equal the "height" of the block.

For a given block and stroke, if the rod is longer, the wrist pin bore is drilled higher up in the piston. Obviously this can't be changed after manufacture. If you use a piston with the bore too low (too much compression height), the piston will come up out of the block and hit the head. Too little CH, and the piston won't make it to the top, and you'll end up with SUPER low compression. Both of these outcomes are considered BAD.

Rod length doesn't affect the crank, except that for VERY long strokes, some minimum rod length is required. Reason for that is the counterweight. Think about what those are... at each rod throw, the rods are on one "side" of the crank CL, so the counterweight must be on the other. Meaning, when the piston is at BDC, the counterweight is pointed straight up. Meaning, there has to be enough space BELOW the bottom of the piston, to fit a large enough counterweight, to counterbalance the mass of the rods and pistons. Meaning, the rod has to be long enough, for the pin bore to be up high enough, for the bottom of the piston to be out of the way of the CW. It's possible to get away with a little less than that; the factory did that with the 400 SB and the 454, by using "external" balance. This only works up to a point though, and "internal" balance is always preferred, if possible.

However, when you move the pin bore upwards to accomodate a longer rod, sooner or later it reaches the rings. Clearly this is a compromise in the ring package design. The rings have to be narrower, closer together, and/or closer to the "top" of the piston. Sometimes a little spacer support thing with ring grooves can be used, to go in the end of the pin bore and hold the rings. You would need to make sure that whatever combo of stroke, rod length, and deck height of the block you use, allow a sufficiently robust ring set. Not much fun to use a 30-lap ring package on a motor that's supposed to last for 50,000 miles.... tends to lead to disappointment.

Companies like Ross, JE/SRP, Wiseco, Arias, Venolia, etc. ("real" racing piston mfrs) will make you a piston of ANY size dome, ANY bore, ANY pin height, ANY ring package, and so forth, subject to the physical constraints of a piston in general. I wouldn't get all hung up on just what's listed in the catalog. If you want something not listed, call and ask, or fill out their form for a quote. Talk to your racing machinist first though, and make sure you're not missing something in the design of your motor, that will cause trouble somehow.
Very nice explanation.
 
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