Max CR with 87 octane

[Jim K in PA]

as for pistons there are a lot of choices for custom out there in a reasonable price range for what the KB's are going for.

Bulltear is another name if have seen and spoke with months ago. Last time I spoke with someone from there they said they had there slugs on backorder. There foreign supplier was backlogged and they said there was not going to be any till something mid year (which is approaching). They get there slugs and machine them down some to there specs <$500 last i saw.

Brian

FYI - I just took a look at the Bulltear site, and they no longer list any pistons for the Jeep inline six.

[Jim K in PA]

This discussion prompted me to do some comparative runs using the CR calculator above. I am moving closer to finalizing my stroker plan, and the need to run it on less than 89 octane fuel, at sea level, heavily loaded, is forcing some parts choices to achieve that goal.

Given the fact that the stock Jeep 4.0 is generally known to run just fine on 87 (or lower) octane at sea level (in my experience with my '05 LJ at least), I compared the three 4.0 cams and came up with the following.

Static CR for all three is the same at 8.7:1

Dynamic CR was found to be:

Stock ’05-‘06 4.0
Advertised duration: 264
Dynamic compression: 7.06

Stock ’96-‘04 4.0
Advertised duration: 253.3
Dynamic compression: 7.47

Stock ’91-’95 4.0
Advertised duration: 270
Dynamic compression: 6.82

Now for the dangerous part. "Logic" would suggest that I would want to build a stroker aiming for a DCR of around 7.5:1, which is what the '96-'04 series of engines ran (and the longest run of any cam in the entire 4.0 series).

I did the calcs, and I came up with a combination that should make the engine happy running on 87 octane fuel. Again, using the site calculator, a long rod stroker with 0.30 over IC944 pistons with the 21cc dish, zero decked block, 0.043 head gasket, and the '91-'95 stock cam results in a DCR of 7.65:1. Advancing the cam +4 degrees would raise it to 7.89, and retarding it would drop it to 7.4.

This sounds like a very reasonable, tune-able combination (if you use an adjustable cam gear), with no need for custom pistons.

Opinions? What did I miss?

[Root Moose]

Jim, Sounds like you are further along than I am to starting building.

I don't know if it will work in the real world but that is the same mental gymnastics and numbers I came to as well.

My current thinking is that it may be prudent to compare what cams are available for different scenarios... if it pings retard the cam or if it makes sense change the cam. Advance till it pings and then back off two degrees to deal with heat soak when being worked in the field... more?

There is also tuning the PCM to make it work as well. My gut says that if you were to look closer at the DCR of the engine that matches your generation of PCM that may be a decent place to start. the next question is whether to bump/lower the DCR and compensate with the cam if required.

I'm convinced the base programming in the stock PCM leaves a lot to be desired. There's got to be power gains and advantages of higher DCR available by adjusting the tuning.

I am building the engine for an XJ. It may be worth looking at the configuration of those engines (if different) to see if there are different DCRs. [edit: nvm, looks like the XJ is identical to the TJ, et al.]


I dunno... just my free form thoughts over the first cup of coffee of the day.

- Chris

[Cheromaniac]

I did the calcs, and I came up with a combination that should make the engine happy running on 87 octane fuel. Again, using the site calculator, a long rod stroker with 0.30 over IC944 pistons with the 21cc dish, zero decked block, 0.043 head gasket, and the '91-'95 stock cam results in a DCR of 7.65:1. Advancing the cam +4 degrees would raise it to 7.89, and retarding it would drop it to 7.4.

This sounds like a very reasonable, tune-able combination (if you use an adjustable cam gear), with no need for custom pistons.

Opinions? What did I miss?

Sounds like you've got it pretty well sussed out.

[Jim K in PA]

Chris - I am getting there. I will be speaking with a couple of machine shops in the next month or so to determine where I am going to have my work done. Before I start ordering parts, I am attempting to narrow down where I want to go with this engine. My "arbitrary" goal is a torque curve as flat as Florida and as thick as a Kardashian's ass. I would like to see north of 320+ lb-ft from 1500-4500RPM. I don't really care where the HP peak lands. This is a truck engine for my purposes. To give you a reference point, for torque and durability, I would put in an IH 345 if that made any practical sense at all . . .

All the PCM tuning in the world will not correct an incorrect engine build. For sure it will be immensely helpful in getting the most from the combination, but timing is timing, and AFR is AFR. I have an '05 LJ with the NGC controller, and FlyinRyan will likely be assisting me with the tune.

[Root Moose]

Sounds like we are very much on the same page and trying to accomplish the same thing. You've read my reliability thread. I too want to get the mechanical configuration "right" on the first go. I just wish there was more research available for determining the optimal DCR on these tractor engines.

I'm considering casting about to look for a used `97-`01 block next week. I have a ACH 0331 head sitting on the work bench already and a CJ7 crank still installed in a CJ7 carcass. Seriously thinking about getting a Scat crank though... zero fatigue cycles has its appeal. I still need to determine the exact interchange for the block with the other platforms - just not enough time in the day.

I was about to switch platforms after our last trip. I just don't have enough engine for the Jeep to do what I am asking. But looking at the power/tork (and weight!) numbers on the other options (WJ HO, TLC UZJ100) I've decided to stick with the XJ for now and seriously look at doing a stroker. Lighter platform with the same engine numbers ought to work even better and I can "keep" the money I've sunk into the Jeep already. That's the theory anyway. A built WJ will likely be 500+ heavier than my built XJ, UZJ will be 6000+ lbs or almost 2000 lbs heavier than my XJ.

Have you given any thought to balance / blueprint yet? For our usage balance especially... I haven't gotten there yet.

[Jim K in PA]

I did the calcs, and I came up with a combination that should make the engine happy running on 87 octane fuel. Again, using the site calculator, a long rod stroker with 0.30 over IC944 pistons with the 21cc dish, zero decked block, 0.043 head gasket, and the '91-'95 stock cam results in a DCR of 7.65:1. Advancing the cam +4 degrees would raise it to 7.89, and retarding it would drop it to 7.4.

This sounds like a very reasonable, tune-able combination (if you use an adjustable cam gear), with no need for custom pistons.

Opinions? What did I miss?

Sounds like you've got it pretty well sussed out.

Hmmm. I'm disappointed. I was itching for a good debate . . .

Just to round out the points that led me to this particular combination choice, I considered the fact that the '91-'95 cam has the longest duration AND the highest lift of the OEM cams. LSA is also the highest (so should have plenty of vacuum at idle for MAP sensitivity). With the stock engine, DCR with this cam is the lowest of all three. But with the combination of stroker parts I listed, it actually plays out almost right where it should be for a street driven truck engine (in my opinion).

With respect to piston choices, I agree with comments elsewhere that KB missed the boat with the IC944 piston. It should have had a pin height that would essentially zero deck the block, and I think a 25cc dish would be about perfect. But, the block can be clipped for less than the added cost of custom pistons, and the 4cc of desired dish difference does not justify going to custom pistons either (again, in my opinion).

The stock valve train can also be used with this combination, with the possible exception of push rods. This cam is also dirt cheap. I got a Melling version from Amazon for under $100 delivered.

My plan is to install this cam straight up. I will degree it to make sure I have it where it needs to be.

[johnj92131]

I have also been playing with the Stroker Compression Calc...The renix cam has an even lower DCR at 6.36 Has the same lift and duration. The renix engine seems to have the torque peak lower than any of the other factory 4.0 cams. Shouldn't the lower DCR permit a higher SCR and still run on 87?

[Root Moose]

Jim, you planning on doing roller rockers?

[Jim K in PA]

Have you given any thought to balance / blueprint yet? For our usage balance especially... I haven't gotten there yet.

Balancing of the rotating assembly will be done. Per the other balancing thread, I will have the crank zero balanced, and I will weight match the rods and pistons. Smoothness +durability+power = happy Jim.

Jim, you planning on doing roller rockers?

Nope. Absolutely no need for RRs with such mild cams in my opinion, unless you want the bling. I may spring for the Hesco HD stamped rockers, but honestly, there is no reason to even do that. The stock OEM rockers rarely fail, and there is no measurable performance gain to be had with rollers unless you are increasing lift and spring pressure. I also want to be able to replace a rocker with a store bought piece if need be when I am thousands of miles from home.

[Jim K in PA]

I have also been playing with the Stroker Compression Calc...The renix cam has an even lower DCR at 6.36 Has the same lift and duration. The renix engine seems to have the torque peak lower than any of the other factory 4.0 cams. Shouldn't the lower DCR permit a higher SCR and still run on 87?

John - as you said the Renix cam has the exact same specs as the '91-'95 cam but was installed at -8 degrees, which is why the DCR is lower in the stock engine. That is also why I refer to only three stock cams, even though the Renix is listed as a 4th.

Retarding the cam will drop DCR, and will allow the engine to run on lower octane fuel, but at the expense of power.

[Jim K in PA]

Just to keep this from becoming a Jim-hijacked thread, here is a link to an interesting treatise on SCR, DCR, cam timing, etc., and is focused on Ford inline sixes. They are a push rod, non-cross-flow headed engine like ours, so their data probably has some relevance since they are just as out-dated as our inlines.

http://classicinlines.com/CompressionRatio.asp

The punch line is that the recommended DCR for use with Regular/Mid-grade (87-89 octane) is 7.0-7.7:1 according to their analyses.

[Root Moose]

Good find. I say build for DCR =~ 7.5, whatever easy combination of parts achieves that, tune with the cam and PCM.

Might be leaving a bit on the table but as long as it is dead reliable I'm ok with that.

[Jim K in PA]

Good find. I say build for DCR =~ 7.5, whatever easy combination of parts achieves that, tune with the cam and PCM.

Might be leaving a bit on the table but as long as it is dead reliable I'm ok with that.

Yep - agreed, and pretty much what I said above. Running the '91-'95 cam will take me just over that, but we are starting to split hairs here. If I get a little ping under load, I've got six gears, so I can drop down and get it out of the sensitive zone. I could use the 0.051 stock gasket and get it down to ~7.5, but that much quench might take me right back to the same conditions. I'll keep the quench to a minimum and run the cam straight up with 7.65:1 DCR. I should be good to go. Of course, this is based on the assumption that I am going no more than 0.30" over bore.

Now to find a machine shop . . .

[365Jeep]

Static CR for all three is the same at 8.7:1
...
Opinions? What did I miss?

I love mental gymnastics! ...so much to learn.

I have been going back and forth between the calculator here and Dyno2003. I have been looking for a DCR of 7.9 (0.045 quench), so I'm playing with the dishing to get my DCR where I want it, and I'm letting the SCR go where it may for a given cam. Then, I'll take the SCR and plug into Dyno2003 to look at the HP and TQ numbers. It's unfortunate you can't see TQ under 2000 rpms.

Maybe a DCR of 7.5 would be safer...