New build from central Maine

[caveman]

Hi all
I've been lurking here for a couple of years now, Trying to get my little brain around all the numbers that you guys throw around.
I bought a 93 grand cherokee donator vehicle last year and pulled the engine yesterday. Plan on putting it my YJ-7 by next spring.(YJ-7=88 YJ with a CJ nose.)
Plans are to build it at a reasonable price. The more i don't have to do to the engine,"Machine work" The more mods i can put into the engine."camshaft, header and new intake manifold. I plan to run a motorcraft carb to keep it simple.
I also have a donor engine out of a late 70 CJ-7 for the crank, rods and front pulleys.
Thanks to everybody for all the information that i've already got from this forum and for all the questions i might have in the future.
Sam

[caveman]

I got my donor 4.0 engine tore down and everything looks good inside. Somebody's been there since new but Everything is standard. Maybe just a ring job. A lot of carbon.
The donor 4.2 crank and rods are from a 78 CJ. A little surface rust from sitting, But i think that they'll clean up ok. I like the idea of the extra counter weights.
I hope to get it to the machine shop sometime this week to get the stock pistons swaped and the crank turned, block honed, cam bearings installed, new freeze plugs and everything checked for straight.
Cam choice is a comp 68-201-4 Its got a longer duration to bring the dcr down so i can run pump gas, Does anybody see any problem with this choice or a better option?

[JeepFamily]

I can't speak specificaly about your cam choice but the rest looks fine. One note in using stock machining a.k.a. decking, stock 258 rods and crank, you should have no problem runing pump gas. Your CR should be around 8.8 to 9.

[caveman]

OK thanks. I thought i read somewhere that the CR would be around 9.5 without relieving it somehow. Are your numbers with a stock cam? It would be great if it is.

[JeepFamily]

The CR I gave was the static number. Go to the 'Stroker Compression Ratio Calculator' on this site to see how engine machining, piston and cam impacts engine parameters including static and dynamic CR.

[caveman]

I've played with that calculator a couple of times and either i don't have my numbers right or i don't understand all i know about it.
I took the engine to the machine shop yesterday to get the millwork done on it. Going to have a valve job with a three angle grind and true up the head and block, Block bored .060 over, new pistons pressed on my rods. Rods trued up and rebored, Crank ground, New cam bearings and new freeze out plugs.
List of parts includes
93 head and block
78 crank and rods
.060 over stock pistons
Comp K68-201-4 cam kit
APP head, main and rod bolts
Borla look alike exhaust header
Clifford high port carburator intake 47-4520

[caveman]

I've spent some time trying to figure the compresion ratio calculater out and i think i've got it figured out. I've come up with 10.23:1 static and 8.73:1 dynamic. If my figures are right i'll be golden with pump gas.

[JeepFamily]

Not knowing how much material you are removing from the deck or head I used factory tolerances including 58 cc head and got 9.97 static. Given your quench height is "0.73 and your elevation you may experience detonation issues?

[caveman]

I don't know how much is going to be taken off either. Just enough to square things up. Elevation here isn't very high. Only 400 hundred feet above sea level and i don't mind using supreme gas. It's not a daily driver. Just a weekend trail rider/ crawler.
I got the same 9.97:1 static compression ratio and when i put in my elevation and cam profile i got 8.48:1 dynamic ratio. Is that going to be enough so i don't have a big problem with pinging?

[caveman]

I just went out to my garage and measured from top of piston to top ring and came up with .332. I put it into the calculater and it brought the dynamic compresion ratio up a little to 8.5:1. The pistons i measured are what i believe to be stock and the new ones are just stock oversize.

[Muad'Dib]

I can say with a certain level of confidence that 8.5DCR is about the Highest you want to go with pump gas (Premium). Especially if your quench is high.

I have a small Quench (my HG thickness.. 0 decked) and my DCR is 8.47. When I travel over to the coast or the over the Cascades (>500ft) then sometimes (not always) I will have a very slight ping. Its really light and Octane booster makes it go away. I haven't been over there in awhile though.

[JeepFamily]

There are some relatively easy things you can do while the engine is apart to stack the deck in your favor. Make sure there are no sharp edges inside the combustion chamber. A little polishing here and there will go along way. Pay attention to the spark plug you use including rounding sharp edges on the ground electrode. Use the thicker 0.051" Felpro head gasket instead of the performance 0.043" unit. This will reduce your CR by about 0.2. Anyway, hope this helps.

[caveman]

Thanks for the info. I ordered the complete mopar gasket set but will get the felpro head gasket for sure. I'll check for sharp edges on the block and head and get that taken care of as soon as i get them back. As for the spark plugs, I'm reading that i should use one heat range cooler than stock.

[Muad'Dib]

There are some relatively easy things you can do while the engine is apart to stack the deck in your favor. Make sure there are no sharp edges inside the combustion chamber. A little polishing here and there will go along way. Pay attention to the spark plug you use including rounding sharp edges on the ground electrode. Use the thicker 0.051" Felpro head gasket instead of the performance 0.043" unit. This will reduce your CR by about 0.2. Anyway, hope this helps.

The thicker HG will reduce CR, but will also raise Quench. I would favor the tighter Quench vs the .2 Compression reduction.

[caveman]

The thicker HG will reduce CR, but will also raise Quench. I would favor the tighter Quench vs the .2 Compression reduction.

Now i'm confused again. Can you explain why you favor a tighter quench vs the compresion reduction? Or point me to something that will explain why.
I have no idea how much these small differences in numbers, either way, will affect the performance and driveability on pump gas.