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Old 11-03-2010, 09:27 AM
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need help finding the right pistons for my setup

Hey Nation,

Let me get straight to the point! I need help selecting the right pistons for my setup:

96 roller block- zero decked
062 Vortec heads-stock springs and valves-64cc
comp cam-xe250h valve lift .432/.444, duration@.050-206/212
std. crank, std. 5.7 rods
edelbrock 2116 vortec rpm intake
stock converter... 700r4 with shift kit, corvette servo
rear end.. stock.. 3.08 rear end.. (tires are about 30inches)..so I've been looking at a good rear end.. about 3.42 gears, 3.55 or 3.73. I shouldve started from the rearend from the beginning.. but I will take care of that when the funds come available for a good one.
I have a holley 600cfm carb... electric fuel pump..in which if you all have any ideas of a good electric fuel pump please pitch in. I had one go out on me after only a year.. so maybe it was the crappy brand I purchased.

So, pretty much I need help selecting a good set of pistons that will put me in the right compression class. I drive my 85 swb often, but not as a daily driver. I do live in a big city.. so highway miles are a must with a good attempt to have some good gas mileage. What should I aim for in my compression range.. and how should I obtain this? What head gasket..etc...your opinions and feedback are greatly appreciated. This will be my winter project.. giving me a reason to stay in the garage a little while longer!

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Old 11-03-2010, 09:59 AM
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What bore size? STD, .030, .040, .060?

I would go with something in the 18cc range with that cam, converter, and gearing. That should put you in the 9:1 CR range. A piston with a d-cup would be a better choice over dish.


18 cc

http://www.summitracing.com/search/M...?Ns=Rank%7cAsc
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Old 11-03-2010, 10:01 AM
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The block is a std bore...
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Old 11-03-2010, 10:07 AM
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These would be a good choice, IMO

http://www.summitracing.com/parts/UEM-KB142-STD/
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Old 11-03-2010, 10:10 AM
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Use a .040 gasket and you would be good to go.
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Old 11-03-2010, 10:13 AM
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Thanks for the advise on those pistons super beetle. You know what I mean about those highway miles down 45 from me! I'm 45 North of you in Dallas..
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Old 11-03-2010, 10:18 AM
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Most traveling in Texas is done on the freeway, that's for sure. Everything's a drive.
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Old 11-03-2010, 10:22 AM
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What grade gas do you plan to run?
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Old 11-03-2010, 10:26 AM
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I will be running maybe 89 to 93 octane... again.. will not be running down the track.. just an occasional run down the block.. and freeway!
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Old 11-03-2010, 10:38 AM
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In that case, you might want to consider 12 cc pistons. That'll bump your compression ratio to 9.5:1 and you need to run premium gas.
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Old 11-03-2010, 10:41 AM
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So with the 18cc.. I can run 87 octane.. but going with the 12cc.. the fuel needs to be premium?
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Old 11-03-2010, 11:01 AM
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Correct. With the 18cc piston your CR would be 9:1 and your DCR (dynamic compression ratio) would be at about 7.7:1, so you could more than likely run 87 octane. With the 12cc CR would be 9.4:1 and your DCR would be about 8.2:1. So you would need to run at least 89 octane. Theoretically.

http://www.classicinlines.com/CompressionRatio.asp
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Old 11-03-2010, 11:43 AM
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Quote:
Originally Posted by redfish211
Hey Nation,

Let me get straight to the point! I need help selecting the right pistons for my setup:

96 roller block- zero decked
062 Vortec heads-stock springs and valves-64cc
comp cam-xe250h valve lift .432/.444, duration@.050-206/212
std. crank, std. 5.7 rods
edelbrock 2116 vortec rpm intake
stock converter... 700r4 with shift kit, corvette servo
rear end.. stock.. 3.08 rear end.. (tires are about 30inches)..so I've been looking at a good rear end.. about 3.42 gears, 3.55 or 3.73. I shouldve started from the rearend from the beginning.. but I will take care of that when the funds come available for a good one.
I have a holley 600cfm carb... electric fuel pump..in which if you all have any ideas of a good electric fuel pump please pitch in. I had one go out on me after only a year.. so maybe it was the crappy brand I purchased.

So, pretty much I need help selecting a good set of pistons that will put me in the right compression class. I drive my 85 swb often, but not as a daily driver. I do live in a big city.. so highway miles are a must with a good attempt to have some good gas mileage. What should I aim for in my compression range.. and how should I obtain this? What head gasket..etc...your opinions and feedback are greatly appreciated. This will be my winter project.. giving me a reason to stay in the garage a little while longer!
There are two important decisions regarding combustion chamber floor and shape and compression ratio and certainly one more decision regarding strength.

Lets dispose of strength first, forged for high output applications and or an engine that has detonation issues and or a blower or nitrous application. These are situations where not only is the power output high, therefore, the loading on the piston but also the possibility of extraordinarily high temperatures. To corral high loads and or temps a forged piston is necessary. My general rule of thumb is crossing the 1.15 horsepower per cubic inch barrier and up get forged, below that point hyper castings.

The piston makes up the bottom of the combustion chamber and needs to be shaped appropriately to mange the flame front and control overall Static Compression Ratio (SCR) in order to optimize the performance of the selected grade of fuel. To that end the Dynamic Compression Ratio (DCR) needs to be computed. This is nothing more than the SCR recomputed for the effective stroke loss that is a function of when the intake valve closes in crank degrees and how far up the cylinder wall the piston has traveled at that degree of valve closing. This is not linear to stroke, it is a trig function calculation that includes the rod length. The Keith Black site has a calculator:
http://www.kb-silvolite.com/calc.php?action=comp2 . For cast iron heads you want an SCR in the high sevens to lower eights to one. For aluminum heads this needs to be from the lower to mid eights to one for a normally aspirated or nitrous engine. For a blower these need to be backed down about a ratio.

To get the most energy out of the mixture the engine burns, the compression ratio, squish/quench characteristics, ignition timing, mixture ratio, engine and induction temperatures can be played with to maximize power under the detonation limit. Since you're running Vortec heads, the head side of the combustion chamber is already will optimized. But the shape of the piston crown has much to do with optimizing the compression ratios for the fuel octane. To that end a flat top piston is the ideal shape but not always the ideal compression ratio with a tight chamber head like the Vortec. The next best solution is the D shaped dish piston, this lets you dial in the compression ratio while keeping the flat tops ideal surface shape under the the head's squish/quench step. A tight closing step to the piston surface SQUISHES the mixture on the far side of the chamber to the spark plug side on the compression stroke. This process stirs the mixture well and forces it in front of the plug increasing the molecular density that results in an easier mixture to ignite and one that burns faster thus reducing the need for excessive spark advance and gets the burn over before detonation can initiate itself. The second event as these surfaces are close together is that the far side of the chamber has a lot of surface area to the volume. This QUENCHES the late burn heat out of the mixture reducing its tendency to self ignite ahead of the flame front which reduces detonation. The optimum clearance on the squish/quench deck to piston crown is .040 to .060 inch the lesser figure is more effective. Round dish pistons just can't bring this closure distance tight enough over sufficient area to be effective. This dimension includes the cumulative distances of the piston crown below the head deck and the head gasket thickness. One needs to carefully know the pin to crown distance of the piston, there are many rebulider pistons out there with a lower compression distance to restore factory like clearances with zero decked blocks. For hot rodding you want a standard crown distance piston with decked blocks in order to keep the DCR and SCR numbers where they can be controlled with the D dish volume. A little high DCR/SCR can be controlled by polishing the chamber and piston crown or selecting a different thickness gasket. A piston that sits too low in the block creates much bigger problems. An unfortunate but rather typical event happens when low crown rebuilder pistons are installed to an undecked block, than in the case of an SBC for example the piston crown instead of being .025 inch down is now .050 inch. Short of buying new pistons and or milling the deck, there is no inexpensive fix for this error, so double, double watch out for this.

Gearing also comes into play a heavy, high geared vehicle puts more load onto the engine which will cause the engine to be more detonation sensitive. Detonation thresholds are also easier to manage with a manual transmission than an automatic because of the simple expediency of being able to change the gear ratio the engine is working thru. Not that you can't change gears with an automatic but this ability is more limited by the operator because of the transmission's internal programming which can and does over ride manual selections unless the valve body or computer program is reworked.

Bogie
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Old 11-03-2010, 11:54 AM
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Thanks for the advise Bogie, very insightful information from all of you. So Bogie, what pistons would you purchase and gasket in order to run 87 to 89 octane. I'm not trying to use premium.. because I drive it a lot.. and gas is not getting any cheaper (plus the wife is 8weeks ) so.. I will take your advise with a spoon full of sugar!
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Old 11-03-2010, 05:59 PM
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This is another piston you could consider. It will put you right at 9.5:1 and it doesn`t require ring gapping. This is for the .030 oversize, but you can research back to standard.
http://www.summitracing.com/parts/STL-H670CP30/
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