Author Topic: Piston options  (Read 612 times)

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MakoML

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Piston options
« on: October 13, 2010, 06:26:38 PM »
Hey guys,  assuming the machine shop calls and says my toasted 97' TJ's  block cleaned up at .030 over,  I need to find some pistons for it.   Is there any reason for the stock dish type, i.e. valve interference?   Has anyone gone with a cast flat top and if so what was the CR raised to, and model and cost of pistons?   

I am just looking at doing the cheap and simple things to bump it up power wise if it can be re-built.  I have talked to Clifford about their 2.5L 264H efi cam kit,  which is not too bad priced at $350 for cam, springs, retainters, guides, seals, push rods and lifters.  I think it is a lot more cam than the mopar ones everyone wants to find.  I have a friend that put the mopar cam in a YJ and it screamed so I'd expect the Clifford to do likewise.

So I am looking into pistons now.  Anyone with an educated guess or actual experience feel free to share!

Offline sharpxmen

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Re: Piston options
« Reply #1 on: October 13, 2010, 08:37:41 PM »
the pistons have a D shape dish to improve mixture burn. If you go with a flat top piston the resulting CR will be 11.1 with a 1.580 compression height piston or 11.5 with a 1.592 comp height piston - way too much unless you want to run racing fuel. If you go with Speed Pro H802CP30 or H825CP30 or Sealed Power 677P30 or 677CP30 you'll have a 9.4:1 CR (these are 30 over) - if you want more CR take some off the cyl head and go with a performance cyl head gasket (thinner than stock).
 How do i know that, the stock Dish volume is 14cc, the chamber volume is 50cc - Since i am trying to lower my CR to about 8:1 i made an excel spreadsheet to calculate the CR based on bore, stroke, connecting rod length, piston compression height, dish volume, chamber volume and cyl head gasket thickness. If i put Dish volume 0 (zero for flat top piston) those are the numbers it comes up with.

KB Silvolite pistons 2229/2229C or 3241HC/3242HC come to 9.6:1 CR for the 30 over pistons according to my spreadsheet but I cannot guarantee that's the case (i found somewhere the dish volume at 11.5cc with a comp height of 1.581 and 13.3cc with the comp height of 1.592 for the latter but i don't know if it's accurate). for these pistons CR at 30 over is 11.1 and 11.5 respectively so still in the same ballpark but the final numbers are dependent on the compression height as well which i don't know what it is since you didn't mention what piston you'd like to use

don't know about the cam, there is very little info on their site (i am guessing 264 duration and it only specifies 474 lift which you're not going to get with the stock stamped rockers, more like 445) - I could not find the overlap and Lobe separation, for comparison the stock Jeep is 270 duration, 112 LSA, 46 deg overlap and 424 lift (that is theoretical lift, you get 400 measured valve travel with the stock rockers)

hope this helps. 
'95 YJ, NSG370 6spd / Hurst shifter, Dana 300 + 4:1 Doubler / tri-stick, Custom skid, Super D35 / Auburn LSD / 4.88, 35x12.5x15 BFG KM2, 64mm t/b, 1.7 RollerRockers, MkVIII e-fan, Dual Diaph Booster
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MakoML

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Re: Piston options
« Reply #2 on: October 17, 2010, 08:57:05 AM »
That does help a bunch,  I'll start looking at those pistons you mentioned.   Still have no word from the machine shop on the block so I have time to look into the pistons.