Kb944 dilemma, 9.0:1 CR or cut deck for 9.3:1 CR

[Retlaw01XJ]

Will the cost of decking give a worthwhile performance increase?
I need to confirm exact specs, but this is what I'm looking at:
2001 XJ 4.0, bored 0.020", using KB944 pistons, SCAT crank. 2004 0331 'tupy' head with polished chambers.... 58 cc before... will measure 'after' (maybe 60cc?). Ports are getting a clean-up and valve guide boss slimming, nothing major.
Planning on reusing stock '01 cam for now, head gasket = Victor, I measure 0.042" on the metal, 0.058" on the sealer beads.... hope it's 0.043" installed.

Initial calculations with 21 cc dished pistons 0.028" down give a CR around 9.0 with 0.071 quench.
If I cut the block 0.020", It'll boost CR to 9.3 with a 0.051 quench........Is that a significant change, or shouldn't I bother?
It'll be primarily street driven. 87-89 octane would be fine.
KB is showing a higher CR, which concerns me..... am I wrong?

I could have the bored block back on Friday. Was gonna do a mock-up this weekend to measure stuff.
Questions..... are my CR numbers close? Should I just have the shop cut the deck (now), or should I do a mock-up first to confirm?
.......and is the difference in CR and quench even worth worrying about?
Appreciate your insight!

[Missourian]

With the exception of my block being a different casting our builds are identical. I wanted 9:1 or lower so that I can run on 87 octane. I did not zero the deck, had it checked for square/flat. There is extra cyclinder height in the last years of production for the 4.0L blocks that gives you a lower CR (mine calculated to 8.96:1) unless you deck the block. Personally, I would leave it alone unless your block is out of square. You can also increase the volume of your head and again reduce your CR.

8.9:1 would be ideal and close to OEM specs for the 4.0L allowing you to run on 87 octane fuel without pinging.

[SilverXJ]

Static compression has no bearing on what octane you can run. Its the dynamic that matters. I've heard 8.5:1 for pump gas (91) but it differs.

[Retlaw01XJ]

There is extra cyclinder height in the last years of production for the 4.0L blocks that gives you a lower CR (mine calculated to 8.96:1) unless you deck the block.

Interesting. IIRC I was just under 0.030 deck clearance with the stock setup at teardown. I hear the KB 944's may be 0.020" below that.
I'll have to mock it up this weekend and measure EVERYTHING to know where I really am.
What threw me for a loop (initially) was that KB advertised the CR at 9.6:1

Static compression has no bearing on what octane you can run. Its the dynamic that matters. I've heard 8.5:1 for pump gas (91) but it differs.

Understood. Stock '01 cam at 8.97 static gives 7.4 DCR. Cut deck for 9.3 SCR results in DCR of 7.7.
Was also surprised to discover the Comp 232-4 cam has earlier intake closing that will raise the DCR.

I'll report back in a few days with my findings.

[Missourian]

Static compression has no bearing on what octane you can run. Its the dynamic that matters. I've heard 8.5:1 for pump gas (91) but it differs.

Thank you for that bit of information. There is a lot of information to be learned. No so different than figuring out the geometry of a suspension system and trying to build somthing that will perform well on and off the road after being lifted to clear larger tires.

[gradon]

Not that KB's math is right, but they spec a +.030" piston with a CC of 60ccs and a deck height of 9.433"(~.020" taken off the stock 9.450-9.456"). I would take ~.030" off to zero deck it if I ran them(but I won't since they don't work the Eagle 6.150"s I have).

[Retlaw01XJ]

Got my block back today (Monday). I only had enough time to drop in the Scat crank and it turns easily, so apparently it didn't get too bent during shipping! (The packaging is pathetic!)
I'll pop in the rods/pistons and measure the deck clearance Tuesday. Should be 0.285" if my research and manufactures specs are accurate.
I best check all 6 cylinders since Chris mentioned his rod lengths varied about 5 thou.

On Sunday, I cc'd my head with polished chambers and they range from 57.6 to 58.6 and one at 59.1cc. damn. Maybe if I have one long rod I can swap that one to the big chamber. Average computes to 58.3 cc.
Dropped head off at shop for sonic check ($20), valve job ($150) and resurfacing if it needs it ($70). Volumes could change....

I'm planning on a target of 0.005" to 0.010" deck clearance. That'll give me a CR of 9.4, a DCR of 7.75, and a quench of about 0.050". ...That's with the stock 2001 cam.
Sound safe?
If it runs well, I can swap in a healthier cam which will also raise the DCR to 8.0-8.2 range.... that could be premium fuel range (?) For now, I consider the stock cam a trial run to determine DCR and octane compatibility.

Couple of items to mention:
- shop had to grind down the small end of the rods for piston clearance (add $60). I'll have to ask if the final weights match, or were even checked.
- Looks like the 2001 cam specs may be wrong in the FSM (and a web chart)..... spec'd as 109 deg lobe separation and 114 deg intake centerline..... I believe that's backwards, unless they installed the cam way retarded.

BTW, today is my 30th day of Jeep ownership! Talk about a crash course in engine builds! I'm learning fast... you guys rock!

[Cheromaniac]

- Looks like the 2001 cam specs may be wrong in the FSM (and a web chart)..... spec'd as 109 deg lobe separation and 114 deg intake centerline..... I believe that's backwards, unless they installed the cam way retarded.

Not backwards. According to the valve opening/closing events that I found on at least two different sites, the LSA and ICA calculated at 107.3 and 114.3 degrees respectively, meaning the cam is installed 7 degrees retarded.
The stock '91-'95 cam has an LSA of 112 and an ICA of 120 degrees so that's installed 8 degrees retarded. The late intake valve opening/closing events in both cases reduces the DCR and allows a stock HO 4.0 to run on 87 octane without needing a knock sensor.

[Retlaw01XJ]

Not backwards. According to the valve opening/closing events that I found on at least two different sites, the LSA and ICA calculated at 107.3 and 114.3 degrees respectively, meaning the cam is installed 7 degrees retarded.

The CR calculator on this site shows the late cam with a 114 Lobe Separation Angle..... so is that incorrect? The discrepancy has me confused. Eventually I'll profile my cam and know for sure exactly what it's specs are.

Peculiar results on my deck clearance measurements. I'm getting approx 0.036" at the front of the cylinder, and 0.030" at the rear of the cylinder..... on all 6 cylinders. I am measuring over the pin, piston rock will only vary the measurement by 0.002".
I flipped one piston/rod around backwards, and the piston is tilted the other way.
The most reasonable explanation I can come up with is that the rods were not reconditioned properly and are not plumb and square. I tried measuring the piston-pin-to-piston-top and it seems pretty close front-to back.
Any other explanation?
Sigh.... off to the machine shop again tomorrow.

[Retlaw01XJ]

the rods were not reconditioned properly and are not plumb and square.

Did some searching and the correct term is 'connecting rod parallelism'..... meaning the axis of the big end must be parallel to the axis of the small end.
Tough to find an actual spec, but I saw 0.001" per 5" a few times.
FSM shows 0.002" per inch as the limit for 'bend'. Per inch of what??? length???.....0.002 x 6.125'= 0.01225"....that's a LOT of bend!
Confused.....Hope to get some answers at the shop on Wednesday.....

[Busey]

your crankshaft is not parallel to the deck. Mine was the same way . I mocked up the pistons after the cylinders were bored but before final decking. After measuring deck clearance, I had .005 more clearance at the front. I measured each cylinder and the clearance decreased evenly to the rear, so i knew it wasn't the connecting rods. I gave my machinist my clearance readings and he decked the block parallel to the crank. After final assembly I rechecked deck clearance and it is now parallel, with just a slight variance which is most likely in the rods.

[Retlaw01XJ]

The variation is nearly the same for each cylinder.
Front of cylinders measure .039, .037, .036, .036 , .036, .036
Rears of cylinders measure .032, .029, .027, .030, .030, .031
....so the deck is fairly flat front-to rear. But each cylinder shows a .005 to .009 variation front-to back.
The issue appears to be with each piston or rod, and each of the 6 are approx the same amount off!

Head guy at the machine shop was out today, but I spoke to one of the machinists. He's somewhat perplexed by the situation as well.
First step will most likely be to check rods and pistons for plumb, square, parallelism, perpendicularity, or whatever!
I'm guessing either the rod parallelism is out, or the piston machining is off.
I just hope the fix is easy and cheap!

[Busey]

I understand what you are saying now. The small end of the rods must have been bent when they pressed the old piston pins out. That sucks ! I hope they can fix it.

[Cheromaniac]

The CR calculator on this site shows the late cam with a 114 Lobe Separation Angle..... so is that incorrect? The discrepancy has me confused. Eventually I'll profile my cam and know for sure exactly what it's specs are.

That would be a great idea. At least I'll know if the specs that I have in my spreadsheet at http://www.angelfire.com/my/fan/Jeep4.0Camshafts.htm are correct.

[Retlaw01XJ]

Quick update....
Picked up everything from the machine shop today.
Being conservative, block was only cut 0.020" for a deck clearance about 0.13" and quench near 0.056".
CR computes to 9.3, about 7.7 DCR with stock cam, , or just over 8 with a Comp cam.

No definite answer on why the pistons appeared to be tilted 0.006" towards the rear when in the block. He did show me the piston/rod alignment fixture and results indicate the piston will be square in the bore and not tilted like I feared.

Only 33 days into this build... hope to have it installed in under 45 days, and finalized by 60 days.