DCR and Octane

[Dezertxj88]

In considering this .025" quench, have you taken into account piston rock at TDC? The piston is quite a bit much larger than a Honda's and will have larger number for piston rock.

I have, but again idk how much they will rock, and I don't have a true value on how much the rods will stretch. I haven't found solid numbers to say this or that..So again its like I'm going in blind..and as said I haven't made a final decision on pistons yet.
I do favor the 1.581 pin height KB 2229 because they have quite a bit of material to them...capable of a 30cc+ dish. They should hold up to more of a beating than the other KB's I was looking at..they were for the 4cyls. I think a 12cc dish and could only go down to like 16cc's..I think I'll end up with the 1.581 Kb 2229's because I will be turboing it, and all that material should take a beating if it detonates of anything..

[gonridnu]

I was looking into this as I want a good all around motor for all elevations for when I drive to CA and do the rubicon.

My static is 9.63 and dynamic 7.83 (by this sites calculator) with a .039ish quench and I run on 87 octane without noticeable detonation at about 5000 ft (knock sensor equipped Renix). Living at that altitude, we build engines to run on regular fuel here and then pray to the octane gods they run on pump premium if and when they ever end up living at sea level. The preferred static compression ratio for our altitude usually ends up being in the 9.3 - 9.8 range on a cast Iron head and 10.5 - 11 on an aluminum head, of course depending on application. The air is thin up there with the Rubicon being at 6000+ ft/ You could probably get away with 10.2 - 1 or even higher static compression on premium with a cast iron head so why be concerned about building around that parameter?

The pistons on a 4.6 application are almost a 4" bore. Piston rock has more to do with deck clearance than any other factor. I seriously doubt you can run a .025" quench on a cast cranked, stock blocked, skinny little stock 4.0 rodded, forged piston engine no matter how many times you slice silly putty. Honda B engines with aluminum blocks and heads, not to mention a short little stroke, short little rods, and a smallish bore are probably not the best comparison. I've built some race engines that showed very minor signs of piston to head contact and were still running so you can make some power by getting it close. You can also make a pile of pieces. I'm voting for a pile of pieces at .025"

[Gorillaxj]

Well to be honest, I plan on driving it there. Which will bring me down to about 2500ft or lower(from what I have read). The Rubicon isn't my fear, its getting there...I also like to go to Seattle wash.(sea level) If its a no go i can take other vehicles... I currently live at 5000 FT and in some low parts of moab drops to about 3800ft. still higher elevation, I just want a good all around motor, I don't want to worry about detonation. With my current setup I am at 8.2 DCR (9.6 SCR, just holding off to make sure everything is ok I have read its A-O-K on mid grade. I have read its high and should be ok on premium but may not be... I guess I am just being paranoid but want to build this once, and looking for people to give me the "go a head" , Mine is ok and I'm there or higher...

Just want to make sure I am within the safe limits... or when/if I will need octane booster. I plan on running a wide-band 02 to keep an active watch, and have all the upgraded cooling mods...

[Dezertxj88]

I was looking into this as I want a good all around motor for all elevations for when I drive to CA and do the rubicon.

My static is 9.63 and dynamic 7.83 (by this sites calculator) with a .039ish quench and I run on 87 octane without noticeable detonation at about 5000 ft (knock sensor equipped Renix). Living at that altitude, we build engines to run on regular fuel here and then pray to the octane gods they run on pump premium if and when they ever end up living at sea level. The preferred static compression ratio for our altitude usually ends up being in the 9.3 - 9.8 range on a cast Iron head and 10.5 - 11 on an aluminum head, of course depending on application. The air is thin up there with the Rubicon being at 6000+ ft/ You could probably get away with 10.2 - 1 or even higher static compression on premium with a cast iron head so why be concerned about building around that parameter?

The pistons on a 4.6 application are almost a 4" bore. Piston rock has more to do with deck clearance than any other factor. I seriously doubt you can run a .025" quench on a cast cranked, stock blocked, skinny little stock 4.0 rodded, forged piston engine no matter how many times you slice silly putty. Honda B engines with aluminum blocks and heads, not to mention a short little stroke, short little rods, and a smallish bore are probably not the best comparison. I've built some race engines that showed very minor signs of piston to head contact and were still running so you can make some power by getting it close. You can also make a pile of pieces. I'm voting for a pile of pieces at .025"

I'm not sure how me saying I've built Bseries and am used to tight tolerances, and checking everything over and over...relates to bseries are similar to Jeep 4.0's? I never made that comment!
Piston rock would be the determining factor, cause the rods aren't going to stretch that much, they won't be stretching much at 5200rpm. so having a .025 quench, I'd say at the very very most they'd stretch .02, and thats being generous. I'm willing to bet they won't even stretch that much, seeing as most cast steel rods stretch around .02-.03 around 7500rpm as stated, been tryed & tested... So I'd guess maybe .01 stretch around 5200rpm? Leaving .015 between the head and block..only thing to worry about then is piston rock. Again these are blind numbers because its never been tryed!
Just to do it I'm doing to shoot for a .025 quench..using the KB 3238. 1.592 pin height. I guess I'll be the one who either proves or disproves a .025 quench is possible, or if it'll be a "pile or parts".

My post isn't ment to be rude or sneering..I just think most go will the flow vs trying something new, and I appreciate all the thoughts, comments and concerns as its helpful in judging what to do and not do! But logic says it'll be possible, piston rock will be the only concern I have honestly! Rod stretch will not be an issue at our rpm range.

[gonridnu]

1 degree of piston rock is equivalent to approximately +/- .034" at the outer edge of the piston when measured on a 4" bore.

[gonridnu]

My post isn't ment to be rude or sneering..I just think most go will the flow vs trying something new, and I appreciate all the thoughts, comments and concerns as its helpful in judging what to do and not do! But logic says it'll be possible, piston rock will be the only concern I have honestly! Rod stretch will not be an issue at our rpm range.

That's the point, It's not trying something new. "Rule of thumbs" in engine building such as a .040" quench, .090"-100" of ex valve to piston clearance, .060" to coil bind, .00'1" of oil clearance per inch of journal, 10 lbs of oil pressure per 1000 RPM, ad nauseum exist for a reason. It is because by and large they have been proven to work over millions of engine builds over decades upon decades.

I first read that .040" quench number in an old small block Chevy engine book. I could be wrong but I believe it to be the one written by Smokey Yunick back in the late 60's early 70's. I had the pleasure of meeting him on a couple of occasions before his passing and to say he is really smart when it comes to motors is an understatement. In the Big Block Mopar B/RB Engine book by Mopar Performance, they recommend a .050" quench based on the larger big block Mopar bore diameter. Conversely, engines with smaller bores can utilize quench clearances lower than .040". Aluminum rod, block, and head engines all have an affect as well and can change the required deck clearance as can a forged vs. hypereutectic piston and the amount of skirt clearance machined into the block.

No one is saying that .040" rule, or any of the other rules, are an absolute number, however they are target numbers that should not be deviated from significantly and without good reason or proper engineering. The power and anti-detonation advantages do not significantly increase nor decrease as you move several thousandths plus or minus nominal, however the possibility of engine damage does increase exponentially as the piston to head clearance is reduced.

If you were to have posed the hypothetical question of how low can we go on a 4.0 I would have thrown my hat in the ring at .035" and maybe a little less if the skirt clearance was kept to a minimum, but why would you want an overheating prone off road/towing vehicle with super tight emissions skirt clearances just to tighten the quench which is going to yield minimal additional benefit anyway? You are proposing shaving 40% off of a proven rule of thumb. What is the thing I am missing that justifies the additional work to take a fairly simple non-race stroker build, with reconditioned stock uneven length rods no less, and build it to the ragged edge?

BTW ... I have no idea if a piston rocks a full degree in it's bore. The temperature the piston was at would determine the skirt clearance at the time. I also have no idea how much RPM has an effect as the forces of acceleration, deceleration, and combustion pressure exert force on the thrust side skirt. I do not know how much a stock 4.0 rotating assembly grows by and at what RPM Nor do I know the expansion rate of of the dome of a forged vs. cast piston.

But I do know that if you adhere to something resembling the .040" rule your pistons won't bounce off the cylinder head when you cold start it in the morning.

[Dezertxj88]

My post isn't ment to be rude or sneering..I just think most go will the flow vs trying something new, and I appreciate all the thoughts, comments and concerns as its helpful in judging what to do and not do! But logic says it'll be possible, piston rock will be the only concern I have honestly! Rod stretch will not be an issue at our rpm range.

That's the point, It's not trying something new. "Rule of thumbs" in engine building such as a .040" quench, .090"-100" of ex valve to piston clearance, .060" to coil bind, .00'1" of oil clearance per inch of journal, 10 lbs of oil pressure per 1000 RPM, ad nauseum exist for a reason. It is because by and large they have been proven to work over millions of engine builds over decades upon decades.

I first read that .040" quench number in an old small block Chevy engine book. I could be wrong but I believe it to be the one written by Smokey Yunick back in the late 60's early 70's. I had the pleasure of meeting him on a couple of occasions before his passing and to say he is really smart when it comes to motors is an understatement. In the Big Block Mopar B/RB Engine book by Mopar Performance, they recommend a .050" quench based on the larger big block Mopar bore diameter. Conversely, engines with smaller bores can utilize quench clearances lower than .040". Aluminum rod, block, and head engines all have an affect as well and can change the required deck clearance as can a forged vs. hypereutectic piston and the amount of skirt clearance machined into the block.

No one is saying that .040" rule, or any of the other rules, are an absolute number, however they are target numbers that should not be deviated from significantly and without good reason or proper engineering. The power and anti-detonation advantages do not significantly increase nor decrease as you move several thousandths plus or minus nominal, however the possibility of engine damage does increase exponentially as the piston to head clearance is reduced.

If you were to have posed the hypothetical question of how low can we go on a 4.0 I would have thrown my hat in the ring at .035" and maybe a little less if the skirt clearance was kept to a minimum, but why would you want an overheating prone off road/towing vehicle with super tight emissions skirt clearances just to tighten the quench which is going to yield minimal additional benefit anyway? You are proposing shaving 40% off of a proven rule of thumb. What is the thing I am missing that justifies the additional work to take a fairly simple non-race stroker build, with reconditioned stock uneven length rods no less, and build it to the ragged edge?

BTW ... I have no idea if a piston rocks a full degree in it's bore. The temperature the piston was at would determine the skirt clearance at the time. I also have no idea how much RPM has an effect as the forces of acceleration, deceleration, and combustion pressure exert force on the thrust side skirt. I do not know how much a stock 4.0 rotating assembly grows by and at what RPM Nor do I know the expansion rate of of the dome of a forged vs. cast piston.

But I do know that if you adhere to something resembling the .040" rule your pistons won't bounce off the cylinder head when you cold start it in the morning.

I guess I can disgress. For me it wasn't about more work..just claying the motor. The only thing that would change is what piston I buy. A 1.581 vs a 1.592..thats why it was considered, because the 1.592 pin height pistons are cheaper, and require more extra cost or machine work.. I guess I just want to do it because no one else has..to be different I guess lol. But I do agree its not the smartest thing to do..I've also read over and over, that .035-.040 is where you want and no more.
We are going to dry assemble tomorrow to get an idea of where I'll be at with deck height over all, using the stock pistons. Like I said I do favor the 1.581 pin height pistons because they are much thicker, thus should take more abuse! We will see where I'm at and go from there.

[Gorillaxj]

Well to be honest, I plan on driving it there. Which will bring me down to about 2500ft or lower(from what I have read). The Rubicon isn't my fear, its getting there...I also like to go to Seattle wash.(sea level) If its a no go i can take other vehicles... I currently live at 5000 FT and in some low parts of moab drops to about 3800ft. still higher elevation, I just want a good all around motor, I don't want to worry about detonation. With my current setup I am at 8.2 DCR (9.6 SCR, just holding off to make sure everything is ok I have read its A-O-K on mid grade. I have read its high and should be ok on premium but may not be... I guess I am just being paranoid but want to build this once, and looking for people to give me the "go a head" , Mine is ok and I'm there or higher...

Just want to make sure I am within the safe limits... or when/if I will need octane booster. I plan on running a wide-band 02 to keep an active watch, and have all the upgraded cooling mods...

Well I ordered some 677cp's for my build. with 17.5cc piston dish, I should be at 9.52:1 SCR,and 8.19:1 DCR based on our calculator here(at sea level), with 57cc head chamber, 0.030 Deck Clearance, 4.2l crank+rods, 0.043" HG, 3.895 bore + 3.895 stroke(square), comp cam 86-232-4 cam. 0.73 quench (listing so you know where my numbers come from)

AT 5000 ft (with my elevation correction)
Static compression ratio of 9.52:1.
Effective stroke is 3.29 inches.
Your dynamic compression ratio is 7.20:1 .
Your dynamic cranking pressure is 138.55 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 7.20 :1.
V/P (Volume to Pressure Index) is 98

At sea level
Static compression ratio of 9.52:1.
Effective stroke is 3.29 inches.
Your dynamic compression ratio is 8.20:1 .
Your dynamic cranking pressure is 164.14 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.20 :1.
V/P (Volume to Pressure Index) is 116

So with a 7.2:1 active DCR at my elevation I am no longer worried about it as thats very low LOL. however form what I have been gathering 8.0-8.5 is the MAX for premium pump gas(with being "safe") and can even ping below that depending on quench height. So when at low elevations I will watch my wide-band and listen for detonation/ping and use octane booster to be safe. while 7.5-8.0 is the sweet spot for power using pump gas. Take into consideration that All this in internet info.., I am no scientist. I would love to be corrected if I am wrong.

Calculator I used which also matches our calculator for DCR at sea level....
http://www.wallaceracing.com/dynamic-cr.php

Any opinions?

[Cheromaniac]

I'm voting for a pile of pieces at .025"

If the pistons contact the head the best case scenario is damaged pistons, damaged head, and bent connecting rods. Worst case scenario you'll have the aforementioned plus a piston wedge in the cylinder bore, crack the block, cylinder fill up with coolant, hydrolock the motor, and wave your hard-earned $$$ goodbye.
It just isn't worth the risk going for that tight a quench.

[Dezertxj88]

Ok dry mock up complete..wanted to get a feel for how much deck cleaernce there will be. The block is untouched, stock bore and all currently, head shaved .008, stock pistons were above the deck by .018-.021. Is it normal to get off readings? Which means a 1.592 would be .013 above the deck. Giving me a .028 quench. The 1.581 pin height pistons would be .005 above deck giving me .036 quench. This is using a 505 metal head gasket...041 compressed height.

I'll be using the KB 2229 with there 16cc dish, giving me a 9.25SCR and 6.75DCR. That should drop slightly more because I didn't take the piston ring height into account yet. I'd say it'll just right for 8psi of boost.
Chose to go with the KB2229's over the others because they are much thicker, incase detonation encure. And because I don't want to risk my motor..2grand would be a hard pill to swallow lol. I'm not sure how much the pistons rock..I feel safe with the rod stretch, but piston rock may have been the killer for me..I will never know.

[herbiehind]

you could run a thicker gasket. you may be on to something. with a thicker gasket , factory is 51 right? , that'll get you above 40 . many would prefer this . no decking the block!! very nice .

[Dezertxj88]

you could run a thicker gasket. you may be on to something. with a thicker gasket , factory is 51 right? , that'll get you above 40 . many would prefer this . no decking the block!! very nice .

I could run a thicker gaset and a higher pin height but It'd be the same situation..all that would allow is to use the 1.591 pin height pistons which are like 128 shipped, while the 1.581 pin heights are around 210. '
I'm running the 505 head gasket becaues its metal and will hold up to the boost better vs a stock or replacement.

[SilverXJ]

The 505 gasket is a stock replacement. Most of the head gaskets for the 4.0L are now MLS gaskets.

[Dezertxj88]

The 505 gasket is a stock replacement. Most of the head gaskets for the 4.0L are now MLS gaskets.

Really? I think it was Zack, told me is a 3 layer metal head gasket.. .041 compressed. Said it was a special made head gasket for boosted applications, "tested to 20psi".
What about their metal intake/exhaust manifold gasket, they any good? All the ones i get are a cardboard type material..with metal rings around the exhaust ports.

[SilverXJ]

Yeah, thats just a stock one.. probably .043" compressed to, if checked correctly. There is probably some part number on it.

I have only seen a steel manifold gasket for the 0331 head. The others are either plain graphite/composite (looks like a greasy metal) or the cardboard looking stuff with the metal rings. Its actually not cardboard, I believe its graphite based as well, but not sure about that.