Head/compression ratio

[AlabamaBill]

How much can shave off a stock head, a early casting 686, with out getting into trouble with clearance issuses and water jackets. I like to up the compression a bit on the motor I fixing to rebuild. Any good jeep 242/258 resources for improved torque and performace out there. Just a newbie having hard time find somethings with this search engine.

Thanks
Bill

Is there any way to search engine to look for 3 character words or numerals?

 

[Dr. Dyno]

Since your engine has a short duration cam and relies on a knock sensor to control ping, I suggest you don't get carried away with increasing the CR. If you shave 0.010" off the head and use the Mopar 4529242 head gasket, the CR will rise to almost 9.1:1 and that's high enough.
If you want more performance ideas, go to my website.
______________________________
The sleeper: 1992 XJ Laredo - 4.6L Stroker - 5-speed manual
Rear Wheel Dyno - 210hp@4900rpm, 250lbft@3750rpm
Performance - 1/4 mile [email protected] in full street trim
Websites - Jeep 4.0 Performance, 4.6L Stroker Build-Up, Dino's Jeep Tricks

The sleeping: 1995 Mustang GT - 5.0L V8 - 4-speed auto
EGR/smog pump delete, gutted cats, 2.5" performance mufflers

 

[AlabamaBill]

I have thought a stroker but not this trip so speak. I have a CNC head from hesco I am saving for that build. This motor supposely had a bad head but since it laying in back seat, I am not sure what the lower end is like either. I was hoping to push CR to about 9.5 to 1 basically in stock. I was gonna get it back togather as cheap as possible but figure I up CR while head was off.

Thanks
Bill

 

[jacked]

shaving .030" and using the mopar gasket will yield approx 9.35 cr. must use 93 octane.

 

[BBeach]

Do you have to use 93? There's plenty of other engines out there running higher and only have to use 91 or so. Maybe its just the design of the jeep head or something? Its 8.8 stock, and thats fine on 87, i dont think that small jump is gonna make the difference between the two fuels.

 

[ChevelleSSLS6]

Do you have to use 93? There's plenty of other engines out there running higher and only have to use 91 or so. Maybe its just the design of the jeep head or something? Its 8.8 stock, and thats fine on 87, i dont think that small jump is gonna make the difference between the two fuels.

Once over 9.5 then higher octane fuels is likely necessary with iron heads. Or at least midgrade. I dunno about head design, but I do hear that much CR increase needs higher octane fuel. Then again, back the timing off a little and it'll be ok.

 

[BBeach]

Once over 9.5 then higher octane fuels is likely necessary with iron heads. Or at least midgrade. I dunno about head design, but I do hear that much CR increase needs higher octane fuel. Then again, back the timing off a little and it'll be ok.

I know but for 9.35 CR to require 93? It doesnt seem like its much higher than running 87 at 8.8, especially when you can proly get away with a little bit more out of 87, maybe 9:1 CR tops. But for that extra .35CR to need that much of a change doesnt make sense on a NA cast iron Jeep head.

 

[jeepinwi]

My stroker has about 9.25:1 CR and a wide .0955" quench. I can run 89 octane all day with ZERO pinging.

 

[Flash]

My stroker has about 9.25:1 CR and a wide .0955" quench. I can run 89 octane all day with ZERO pinging.

What elevation do you live at?


Flash.

 

[old_man]

You would be better to take some off the block instead of the head, to reduce the quench height.

 

[jeepinwi]

I'm at about 750 feet elevation. I agree that milling the block is wiser to reduce the quench. You don't want to go too thin, or you may run into piston to valve clearance issues.

 

[Flash]

I'm at about 750 feet elevation. I agree that milling the block is wiser to reduce the quench. You don't want to go too thin, or you may run into piston to valve clearance issues.

whether you shave the head, or the block to raise compression.........valve to piston clearance will get closer so make sure you check it before you assemble a valve bender!

Quench is hard for me to explane........barly understand my self, but here go's.............

Those that under stand it better, fill free to correct me!!!!!!!!

Quench is basically the squeeze clears between the head and the piston.
if you shave the head there is no difference but if you shave the block you are making the distance between the piston and the head surface closer.
I'm sure some one will come along and do a better job of explaining it!


Flash.


Flash.

 

[AlabamaBill]

Me still confused somewhat, I remember old dirt trackers want to deck the block of SBC so the piston was virtually even when TDC. Is there a min. quench for 4L and what the heck is it good for? Do you have valve intrusion below the head?...I am so confused of course that is normal. It seems a very high quench according the currnet defintion in the thread, would be the upper block space acting as part of the chamber head and that dont seem like a good ideal...so confused.

Bill

 

[Flash]

My stroker has about 9.25:1 CR and a wide .0955" quench. I can run 89 octane all day with ZERO pinging.

Hmmm 9.25:1 at 750 feet............maybe i could get away with 10.00:1(or at least 9.7) at 7000 feet...........Naaa Dynamic Compression Ratio would be to high with a RV cam.



flash.

 

[MudDawg]

Proper quench provides turbulance in the fuel air mixture...this helps the mix "blend" better....less lean and/or rich areas in the combustion chamber from uneven mixture....minimum quench should be .040" to avoid any contact between piston and combustion chamber.

It also helps to "center" the mixture in the combustion chamber to help with flame propogation near TDC. Fortunately, the plug is in a good open location so electrode shrouding is not an issue.

The mighty 2.5 runs at 11.3 CR.....It definately needs 93 octane...and if I feel like cranking open the blue bottle it gets 116 octane.

As far as the valves hitting the piston...not very likely at the lifts used in a 4.0 (typically in the .470 range) The valve timing would have to be WAY out of whack.

 

[Flash]

Proper quench provides turbulance in the fuel air mixture...this helps the mix "blend" better....less lean and/or rich areas in the combustion chamber from uneven mixture....minimum quench should be .040" to avoid any contact between piston and combustion chamber.

It also helps to "center" the mixture in the combustion chamber to help with flame propogation near TDC. Fortunately, the plug is in a good open location so electrode shrouding is not an issue.

The mighty 2.5 runs at 11.3 CR.....It definately needs 93 octane...and if I feel like cranking open the blue bottle it gets 116 octane.

As far as the valves hitting the piston...not very likely at the lifts used in a 4.0 (typically in the .470 range) The valve timing would have to be WAY out of whack.

11.3 :shocked: thats not factory compression is it?


Flash.

 

[AlabamaBill]

Ah, getting less confused now. How is quench measured, TDC to top Deck of block? or add gasket thickness so to get actuall bottom of head? Also min. is 40 then what is optima? I seeing those in .09s and I believe stock is a bit less then that. With All that said, should I mock up engine and take a measure to deck then have block decked to optima measurement correct? I hope that is right or I am still confused LOL. I hope I getting there!

Bill

 

[Flash]

Me still confused somewhat, I remember old dirt trackers want to deck the block of SBC so the piston was virtually even when TDC. Is there a min. quench for 4L and what the heck is it good for? Do you have valve intrusion below the head?...I am so confused of course that is normal. It seems a very high quench according the currnet defintion in the thread, would be the upper block space acting as part of the chamber head and that dont seem like a good ideal...so confused.

Bill

Also remember that depth of the push rod, in to the hyd lift, is vary.....vary important and whether you shave the head or deck the block, or both, this will affect the rocker arm (lifter depth) and will need to be adj to compensate for the push rod that is now to long!!!




Flash.

 

[AlabamaBill]

DOH!, had not thought about long push rods. My question can you find shorter push rods and how would you calculate the right lenght. Or could some of extra lenght be taken up adjusting the rocker arms, seems like somewhere in my ancient memory that was used to solve the problem but then again I have so much disassociate memory floating around, to many hits to heads. I vaguely remember you judge a good football practice by the number of peope got knocked out. If you could stick two amonia capsules up the nose and still no reaction that was a hit. Sadistic coaches aye?

Thanks
Bill

 

[Flash]

DOH!, had not thought about long push rods. My question can you find shorter push rods and how would you calculate the right lenght. Or could some of extra lenght be taken up adjusting the rocker arms, seems like somewhere in my ancient memory that was used to solve the problem but then again I have so much disassociate memory floating around, to many hits to heads. I vaguely remember you judge a good football practice by the number of peope got knocked out. If you could stick two amonia capsules up the nose and still no reaction that was a hit. Sadistic coaches aye?

Thanks
Bill

If your determined to get high compression then custom push rod would be reqired.............they make a shim, that you put under the rocker arms that raised up the rocker arm to compensate form the head/deck milling.

I thing that Hesco and Crane, probaly others sell them in a .030" .060" exc.


Flash.