Opening chambers vs dishing pistons?

[Desertjr]

Nosigma aka John just posted he had 62cc chambers with .010 milled. I need to make up about 5cc somewhere, be it pistons or chambers. I personally think 5 but ill let you guys tell me your opinion. Do I open the chambers & save me 100 bucks or dish my pistons out the extra 4-5cc?
My last head I did the chambers and wasnt all that happy with how the chambers came out & I accidentally opened up the 6th and final chamber I did about .012 past the gasket so we started on a new head.


My motor specs have been posted, but I'm at sea level in blazing hot SoCal and visit Death Valley aka hell on earth which has 120* temps at times I believe.
Specs are 4.2l my style stroker
-.012 deck height (above deck)
.043 head gasket
.0299 quench
Stock 96 cam, advanced it 4*
15.3 piston dish
57.7 cc chambers currently
3.935/3.500 bore & stroke


Compression is currently 9.76 & Dcr 8.6:1 with this setup above. I NEED it to run happily on 87 because certain places I visit while wheeling only carry 87 such as small 1 shop towns ect. I've seen a build with worse quench than mine and 9.6:1 static 8.2:1 Dcr run on 87octane happily, noted here.
http://www.jeepforum.com/forum/f19/87-o ... ld-913883/

I'm not sure what his valve overlap with the comp 232 compared to mine? Camp 232 vs stock 96 cam? According to the Cam sheet its 27 vs 41.6? Is this correct?
I'm basically trying to find out how much I'd need to lower my compression to happily live on 87 octane. Is 9.76:1 static & 8.6:1 dynamic too much with such a tight quench? Maybe I should open up the chambers just a cc or 2 to drop it a wee bit to 9.6:1/8.4:1?

Help me out a bit here guys?

[Cheromaniac]

If you've already assembled the engine, I suggest you carry a bottle of Octane Boost in the Jeep for those occasions when the only fuel available is catpiss.
Otherwise it's easier to gain a few cc from the chambers. If you have a head gasket, scribe the gasket bore margins on the head so that you don't cut too far. A light grind and polish should be enough.

[Desertjr]

If you've already assembled the engine, I suggest you carry a bottle of Octane Boost in the Jeep for those occasions when the only fuel available is catpiss.
Otherwise it's easier to gain a few cc from the chambers. If you have a head gasket, scribe the gasket bore margins on the head so that you don't cut too far. A light grind and polish should be enough.

Not assembled yet, the head was the last piece in which I decided to do it over because the mistake I made. Dino what do you think would be a "safe" compression ratio to shoot for? How much should I drop it? Also does the stock 96 cam have more overlap than the comp 232?
I'm guessing if I add 2-3cc to the head it'll be enough? The 4.6l build I linked it holding on 87 with 9.4/8.2 compression, on a comp 232 with a .051 quench. Ill have a .029 quench with a oem 96 cam advanced 4*. What compression should I be looking for?
Should I opted for the. .051 gasket and run a .038 quench?

[Cheromaniac]

The stock '96 cam has a later intake valve opening than the Comp 232 so it should yield a lower DCR and allow the engine to run on lower octane fuel. Advancing the cam increases the DCR and the engine's octane requirement so I think you should install the cam straight up.

[SilverXJ]

I seriously doubt you will run 87 with 8.2 DCR. He either doesn't know what ping sounds like or is running stupid rich.

[TR1Hemi]

68cc is possible with the head.

[Desertjr]

68cc is possible with the head.

Bingo, and Chris my motors not running yet. I'm trying to figure it out before I get it back together. Or were you referring to the 4.6l build I linked?
Installing the cam straight up drops me .2 points on Dcr. Opening the chambers to 62cc gives me 9.35:1/8:1. What do you guys think of that? I don't want any questions if it'll run on 87, and I don't want to have to resort to octane booster. Prefer to just have a good solid normal bullet proof motor.

[SilverXJ]

I was referring to where you mentioned his numbers for the link.

[Desertjr]

I was referring to where you mentioned his numbers for the link.

Aww ok, so his couldn't be running on 87 with a 8.2 Dcr because his quench? His overlap? All the above? He's got videos on YouTube. tuned via a SCT programmer I guess? Took a few tunes to get it right according to him.
http://m.youtube.com/watch?v=CRRlSbiX0Rk

Obviously I can "make" mine run on 87.0 by playing with plug heat ranges & running it rich, but I'd prefer not to. Mine anyway of the week will run on 91 and be tuned for it. But there's times on the trail you borrow gas from another jeeper, or stations in the middle of no where only carry 87. So its hot to be live able, safely on 87. My apexi safc II will switch between 2 maps at the click of a button. 1 map for 91oct, the other for 87oct.

What's your opinion here chris? My specs were posted above, if you've got some time you wanna play with the numbers & see what you can come up with? With my quench height, I can do a .043 or .051 gasket, either or gives me my desired .040 or under. So that's a variable, I'd prefer to not dish the pistons which are 15.3cc..I'd rather pick up the extra in the head.
What's a safe cr for me? Death Valley is actually below sea level from memory and it gets HOT there. I wanna have my jeep run correctly while there, if it does it there, it'll do it anywhere.
I just don't wat it to even be a worry in my mind if it'll live on 87oct or not happily without any tricks.

[Desertjr]

68cc is possible with the head.

Are you not worried about the shape of the chamber at all? I thought the offset shape of the chamber by the spark plug was to promote swirl? Yours is completely gone?
The provision opposite the spark plug wasnt in Hescos aluminum head, so I'm cutting that out. Same goes for the actual "ramp" or "angle" of the chamber walls. My last head I ended up dropping that angle down some, the walls weren't as steep as a stock chamber. Good or bad?
I also shaped the radius of my chamber quite a bit. On the side of the head where the chamber starts to open up around the edges of the valves. for am easier reference, you could say around 10 o'clock, 2 o'clock, 4 o'clock & 8 o'clock position. I shaped/opened that area a bit, made it more open/wider and smoother radius vs the stock closer/sharper radius. Any advice on this?

The general rule here seems to be don't change the port shape, yet slowly more info is coming out on the chambers and what people are doing to theirs. How they are picking up size and what they are doing. My valves gain me 1cc all together. I'd like to get around 64cc chambers. Currently 57.7-57.2 all untouched, with my valves its 58.5 roughly.

Also since I'm slightly bigger valves, sbc sized, 1.95/1.54. Should I be looking into hardened seats or? I've seen a few cases of sunken valves and feared it may be from no having hardened seats with larger valves?

[SilverXJ]

Aww ok, so his couldn't be running on 87 with a 8.2 Dcr because his quench? His overlap? All the above? He's got videos on YouTube. tuned via a SCT programmer I guess?

Pulled a bunch of timing, running rich who knows what he is doing. 8.5:1 DCR is the general limit for Premium pump gas on an iron headed engine.

What's your opinion here chris? My specs were posted above, if you've got some time you wanna play with the numbers & see what you can come up with? With my quench height, I can do a .043 or .051 gasket, either or gives me my desired .040 or under. So that's a variable, I'd prefer to not dish the pistons which are 15.3cc..I'd rather pick up the extra in the head.
What's a safe cr for me? Death Valley is actually below sea level from memory and it gets HOT there. I wanna have my jeep run correctly while there, if it does it there, it'll do it anywhere.
I just don't wat it to even be a worry in my mind if it'll live on 87oct or not happily without any tricks.

Your numbers check out, but check your quench math. .043-.012= .031", slightly off, but not enough to matter.

For example: On one engine I had a 8.42:1 DCR. It would ping on anything but premium. On my current engine, with a DCR of 8.03:1 I can run midgrade, being slightly richer than I would like (12.3:1).

[Desertjr]

Aww ok, so his couldn't be running on 87 with a 8.2 Dcr because his quench? His overlap? All the above? He's got videos on YouTube. tuned via a SCT programmer I guess?

Pulled a bunch of timing, running rich who knows what he is doing. 8.5:1 DCR is the general limit for Premium pump gas on an iron headed engine.

What's your opinion here chris? My specs were posted above, if you've got some time you wanna play with the numbers & see what you can come up with? With my quench height, I can do a .043 or .051 gasket, either or gives me my desired .040 or under. So that's a variable, I'd prefer to not dish the pistons which are 15.3cc..I'd rather pick up the extra in the head.
What's a safe cr for me? Death Valley is actually below sea level from memory and it gets HOT there. I wanna have my jeep run correctly while there, if it does it there, it'll do it anywhere.
I just don't wat it to even be a worry in my mind if it'll live on 87oct or not happily without any tricks.

Your numbers check out, but check your quench math. .043-.012= .031", slightly off, but not enough to matter.

For example: On one engine I had a 8.42:1 DCR. It would ping on anything but premium. On my current engine, with a DCR of 8.03:1 I can run midgrade, being slightly richer than I would like (12.3:1).

I think I had my quench at .029 because the highest piston is .014 outta the hole. But on average .012. What do you think of a .051 gasket giving me a .038 quench, and 63cc chambers?
Gives 9.1 static & 7.98 dynamic. Could do 64cc and drop it to 9.0/7.9 with the same .038 quench?

[TR1Hemi]

I have a .060 over block, I didnt touch the quench pad at all, and with a zero deck height and .045 ballpark quench, I should have plenty of mixing. The extra cc's are worth more than the swirl shape of the unmodded chambers. I suppose if i was worried about every absolute HP and MPG, I would have just dished the pistons. But this is something I could do myself. The machinist agreed, cheaper, and accomplishes the goal of no ping. This was accomplished by scribing the MLS gasket on the head, the laying the chamber back from the spark plug hole outward, feeling/looking for high spots, and taking them down, and making all the transitions/radius smooth. Amazingly the chambers ended up all the same...dont know how I did that...lol.

BTW, I went through a boatload of dremel bits, between the chambers and ports, so it wasnt exactly free...lol.

[Desertjr]

I have a .060 over block, I didnt touch the quench pad at all, and with a zero deck height and .045 ballpark quench, I should have plenty of mixing. The extra cc's are worth more than the swirl shape of the unmodded chambers. I suppose if i was worried about every absolute HP and MPG, I would have just dished the pistons. But this is something I could do myself. The machinist agreed, cheaper, and accomplishes the goal of no ping. This was accomplished by scribing the MLS gasket on the head, the laying the chamber back from the spark plug hole outward, feeling/looking for high spots, and taking them down, and making all the transitions/radius smooth. Amazingly the chambers ended up all the same...dont know how I did that...lol.

BTW, I went through a boatload of dremel bits, between the chambers and ports, so it wasnt exactly free...lol.

Oh I'm with ya, a easy 100 bucks in stones and papers and 1 carbide bur down the drain on the first head. On your head it appears around 3-5 o'clock & 9-7 o'clock there's a valley so to speak shaped around the valve? Is this what I'm seeing or my eyes playing games? Any tips on what you've done? It appears you just made the chamber on the quench side flat vs curved? And the opened up and made the ramp/angle slightly less on the spark plug side?
Of course deshrouding valves and such as well.

[TR1Hemi]

Yep thats pretty much it. I am at 2800' so this thing should run on the 85 octane cougar piss...lol...and still be fine on 87 when I head down do 600'. It may not work out, but at least everyone knows the metal is there to do it! BTW, this was a 720 head off of eBay.