Budget Rebuild With Extra Punch - Pistons?

[BlueShift]

Hey... I'm a Newbie Supreme now. Cool

While I was waiting for my heads at the machine shop, I figured I'd go ahead and try the re-ring I was talking bout in the "maintainence" section. Long story short - there's corrosion from water that ate a couple thousandths into the bore of cylinder number 4. It isn't coming out, even after semi-cautiously wailing on it with the deglaze hone, which probably isn't a good idea anyways. Looks like it's time to open the wallet and get a rebore/hone done. I think that means new pistons, too. Having never done this before, I have a couple questions on this as well if you guys would be kind enough to fill me in:

- Any boring will require new pistons, yes?
- As far as pistons go, what options do I have? I have a couple ideas (see below), but I'd like to know what's been done.
- What does it cost to have a bore/hone done, ballpark? Is there anything else that would be a really good idea to have done as well?

I found that stocker pistons don't cost much (25-50 apiece depending on source), and I have a few questions about some specs I've found:

Sealed Power, for a 91 Olds Cutlass Supreme:

H813CP: 3.6220 EARLY or REPLACES LATE USED w/E548K RING SET

COMP2)1.50mm;OIL1)4.00mm
COMP DIST;1.458";PIN DIA:.9054
DOMED HEAD:.040" HIGH - w/4 VALVE RELIEFS

Sealed Power, for a 96 Olds Cutlass Supreme:

H684CP 3.6220 LATE or REPLACES EARLY USED w/E928K RING SET
COMP1)1.20mm(1)1.50mm;OIL1)3.00mm
COMP DIST:1.464";PIN DIA:.9054"
DOMED HEAD .040" HIGH w/4 VALVE RELIEFS

I have a 91' motor from an automatic. What caught my eye is that the "early" piston has a '.040 dome'. Looking at the piston sitting in front of me, it looks dead flat except for 4 valve reliefs. Is Sealed Power # H813CP a Manual spec piston? Furthermore, the "Late" style piston has a compression height that is .006" higher than the early style piston, and also has a .040" dome. It says "replaces early" in there, if I change to the 1.20mm late style rings.

Big question: Can I slap in a set of oversize H684CP domed pistons for the 96' year engine into my 91' motor for a little more performance? Will I end up having to use 92 octane or even higher with a light cut taken from the heads, the extra compression height, and the dome? Will this work without nailing valves? Does the motor become interference if I do it? If I can't use later style pistons, are the H813CP early style pistons with a dome higher compression than stock automatic spec pistons? I ask these questions because I'm sure somebody has looked up replacement pistons and knows from experience whether this will work or not. I figure if I'm going to rebore/hone and get new pistons, that I might as well get a little more punch out of it. This is a performance motor, after all!

Secondary question: Not to sound like a broken record, or a n00b whiner about gas prices and stuff... but does anybody have a general idea at what compression ratio one would need 89 and then 92 octane gas? I don't have the faintest clue what these motors like to drink under various compression ratios and such.

You guys know your shit. Thanks again for the help.

[BlueShift]

I realize that I end up invariably having an ADD moment trying to read through all that myself. Sorry.

In short, can I install pistons with the .040" dome in my 91' Auto without crunching valves? I found some from Sealed Power (see above) that are made for this motor, but my pistons don't have any dome that I can see. Is this piston a manual spec piston? I've heard that nobody makes manual spec pistons anymore.

Can I install the pistons for the 96+ motor, with .006" higher compression height, and .040" dome into my 91" auto spec motor for more compression without crunching anything? Are the 96 chambers different? Will I need to run Premium?

I'm sure somebody has done this before.

[Doubt_Incarnate]

look up the specs for the 96 motor and see what its CR is and that should tell you what you'll end up with, shouldnt be a problem, if there is any increase in CR its not going to be a big deal, no valve problemsa, no excessivly expensive gas.

i think you have the compresion heights witched around, if i remeber i looked into pistons once and i thought the newwer ones had a shorted compression distance, but also a dome, so i figured it equaled out to the same and the old ones.

I dont have a tdc motor, but i like to know what pstons are avalable.

[BlueShift]

Well, the 96 is supposed to be like 9.7:1 right? Somebody told me that the 96+ has like cloverleaf shaped combustion chambers that have a smaller volume, while the earlier style have "hemi" style chambers (I think they're actually "pent roof" style chambers). Either way, the piston specs I posted up above are right from the Sealed Power division of the Federal-Mogul website. Higher compression height = higher CR due to fuel/air getting crunched into a smaller volume, right? So if both have the (same) .040" dome... the one with the higher compression height should yield more compression, I figure.

[Doubt_Incarnate]

ok, go for it. i just double checked my numbers and youre right. 9.7:1 isnt a big deal, alot of new cars run that and on regular, because gas is high ive been getting away with regular on 10:1 in my ls1. some people get awat 11+:1 on regular or mid grade and someone has 13:1 or so on premium. mostly just need to make sure it stays cool.

[LQ1]

If you want to raise compression, I would try and raise it by shaving the heads, maybe find a slightly smaller head gasket first. Try and raise the compression as high as you can before going to domed pistons. The flatter the top of the piston, the more equal the explosion pressure can cover the piston providing for that extra push in power.

[BlueShift]

Yeah, I was/still am going to do the head deck, maybe .010" or so. Maybe I should lay off a little if I get the higher CH/domed piston. However, my bores are toast so I need a rebore and new pistons anyways. I figure that if they were good for the manual spec early motors, and even better for the later year motors (for tq/hp), then it should be good for me too. I was worried about valve clearance, but it clearly says in a couple places that the late style piston will replace the early style.

I also have my suspicions that I might end up with more valve clearance if I used domed pistons (the valve reliefs must still allow so much clearance) vs shaving the head. Or I might do both - if the 96+ has smaller chambers, so the.040" dome, .006" higher compression height, .005" or .010" head deck, and .5mm overbore might land me right where I need to go with the earlier heads, CR wise though less than it'd yield in a 96. Rather be on the safe side if I have to pick.

You sound like you know your stuff, and I would agree that a flat top has better thermodynamic efficiency and such (more surface area bleeds off more heat and effects flame spread, right?) but wouldn't the sacrifice be worth it?

[LQ1]

If you are able to bore out the cylinders .060", and shave off .010" you can bring your compression up to 10:1. That is a pretty good number and seems like something you might be looking for, so finding a set of flat top pistons .060" larger around would provide a pretty good benefit. Your valve/piston clearence will still be that of about .072", and quench will remain the same at .047". If you can find yourself a thinner head gasket by say .010" of an inch, itll bump the compression slightly, but the bigger gain in power will come from the .037" quench area.