Quench, DCR, and Octane questions

[Gristleheart]

I want to be able to run 87 octane all the time (and not because worried about saving a few bucks per fill-up).
You can't do this if DCR is too high.
Reducing quench helps reduce the problems of having too high DCR.

But chasing a tight quench seems to complicate things. Decking the block, determining new pushrod length, additional dishing of the pistons, cam timing, etc.

It seems to me if I were willing to accept a quench that is roughly stock, it would simplify things trying to hit my octane/DCR goal (under 8.0:1).

How important is having a tight quench if I'm keeping DCR somewhere between 7.7 to 8.0?

I care more about bottom-end torque than horsepower. Based on what I've learned so far, I am considering getting a Mopar Purple cam (240 or 248).

How much performance would I be leaving on the table if I don't deck the block, add only a half point of DCR or less, and keep quench roughly stock? And are there any other downsides to going this route?

Thanks

[Cheromaniac]

Here's my modified poor man's stroker recipe:

Jeep 4.2L 3.895" stroke crank
Jeep 4.0L 6.125" rods
Keith Black UEM-IC944-020 forged pistons
9.25:1 CR
Stock 4.0 camshaft
Ported HO 1.91"/1.50" cylinder head
Mopar/Victor 0.043" head gasket
0.070" quench height
Ford 24lb/hr injectors for '87-'95 engines, Accel 24lb/hr injectors for '96-'04 engines, '98 Chevy LS1 25.2lb/hr injectors for '05-'06 engines
248hp @ 4900rpm, 306lbft @ 3250rpm ('87-'95 camshaft)
245hp @ 4800rpm, 315lbft @ 3000rpm ('96-'04 camshaft)

Just shave 0.020" from the block to reduce the quench to 0.050" and raise the SCR to 9.6:1, add shorter pushrods (Crower 9.600"), and upgrade the cam to something better than stock. The Lunati Voodoo #63500 or CompCams #68-231-4 would do nicely and give you a wide spread of torque, while the Mopar Performance grinds are IMO too mild for a performance stroker build-up.

[Gristleheart]

Thank you for sharing that recipe, and cam recommendations.

I am hoping to get answers to my specific questions, though, to improve my understanding of this stuff.

[Defcon92]

To throw it out their, i think the mopar purple 229 is a better choice from a reliability stand point. I posted a graph someone had made on the comp 232 vs the 231 vs the 229 vs a stock cam. The 229 & the 231 comp are within a few hp & torque of each other. BUT the 229 has less lift & duration which is nicer on valvetrain & overall life.


I use the 229 in my stroker. However its a 4.2l mini setup. My quench is .038. My Dcr is 7.9:1. I expect to run on 87 just fine, however it will run on 89 or 91 at all times unless on a trail or in the booneys with no other choice. I've had better mileage on 91 than 87 while stock with exhaust and intake work. I am also in so cal with 100*+ temps so idk if that plays into it or not.


I would shoot for a tighter quench & try to keep your Dcr is check by using different valves or opening up the combustion chambers ect. I used ls1 valves which are dished so they add about 1.8 cc I think. I also enlarged the chambers as mauch as safely possible. Ended up at 66cc. Hesco cuts out the "spark assist" thing in the chambers. It's directly across from the spark plug. I saved that out ect. Just followed some advice Benny gave me and when it runs I'm sure I'll be extremely happy.

[Cheromaniac]

Thank you for sharing that recipe, and cam recommendations.

I am hoping to get answers to my specific questions, though, to improve my understanding of this stuff.

Since your goal is to run 87 octane all the time, you'll need to do the following:

1. Set quench tight i.e not more than 0.050".
2. Keep DCR not higher than 7.5:1.
3. Use colder plugs than stock e.g. Champion RC10LYC.
4. Polish combustion chambers and piston dishes, feather sharp edges.
5. Make sure cooling system's up to scratch.
6. Make sure engine doesn't run lean.

[Jim K in PA]

What Cheromaniac said. You might want to peruse this recent thread, and search the forum. This is a popular topic, with some subjective elements.

Some good discussion and links: http://www.jeepstrokers.com/forum/viewt ... =15&t=4110

Learn the details, and use the site's compression ratio calculator linked above to do comparison analyses.

You also my want to consider waiting for the new head from Edelbrock. That may be a bit of a game changer, depending on the price.

[Gristleheart]

Hi Jim K,

I've seen that and many other threads. I've also searched this site many, many time, both through the built-in search feature and via google. I've played with the (very helpful) calculator countless times. I've even been a member of this forum longer than you have.

Based on my searches here and elsewhere, I still have questions:
The importance of quench is not made clear. Why did the engine not come with sub-50 quench from the factory if that number is so important?
Why should I worry about reducing quench if I'm going to stay with a(n almost) stock DCR? I thought the purpose of reducing quench was to be able to increase DCR without detonation.
How much does increased compression contribute to performance?

For a slow trail-driver - not tearing through the mud or flying up dunes - do I want peak torque at 3000 RPM? Wouldn't I want it lower?

I'm not aware of any head from Edelbrock.

Thanks

[Cheromaniac]

do I want peak torque at 3000 RPM? Wouldn't I want it lower?

My stroker produces peak torque at 3400rpm, but it's already producing 90% of peak torque at only 1500rpm and 82% of peak torque at 1000rpm.

[Defcon92]

For a slow trail-driver - not tearing through the mud or flying up dunes - do I want peak torque at 3000 RPM? Wouldn't I want it lower?
Thanks

You want the torque band to load up early, peak where ever & carry as long as possible.
As Dino said peak isn't everything. Under the curve matters! 75% or so of torque by 12-1500rpm would be great! If you on can carry pak or close to or through 3800 rpm you'll have a very fun jeep.

[Cheromaniac]

At higher rpm my stroker's still making 90% of peak TQ at 4500rpm but by 5000rpm it's dropped to 79% of the peak.

[Gristleheart]

What sort of performance difference is there going to be between, say, a Mopar Purple cam that is no degrees of advance, and a Lunati cam that has 4 degrees of advance?

[Defcon92]

What sort of performance difference is there going to be between, say, a Mopar Purple cam that is no degrees of advance, and a Lunati cam that has 4 degrees of advance?

Cant say but Ill leave this here. I posted it once before aftering finding it on this forum when I did research on the mopar cams.


You can clearly see a stock 96+ cam advanced 4* actually beats the comp 232 in low end torque by quite a bit. Horsepower to actually. They even out around 3000-4500rpm. Then the comp 232 takes a very slight edge to 5000rpm. I don't know about you but adding the cost of valve springs & such onto a build over 5hp & trq @ 4500-5000rpm seems silly. It also looses power where most people want it over the oem cam.
The 229 fairs very well against the oh so popular Comp 231 "torque grind". The comp has much more lift (.462/.485) than the mopar 229 (.440/.440) which means a higher ramp rate ect, which leads to a louder valve train & harsher wear conditions. Overall it helped me choose the Mopar as well as the Mopars "wide lobes". The comp 231 does beat out the mopar 229 by about 8-12 hp & 8-10 ftlbs in the upper rpm range according to the graft. Not enough to interest me into its cons! It also has some of the highest failure rates for the threads on the web.
Now the mopar 230ab seems to be a "fountain of youth" so to speak. Its discontinued but everyone here or anywhere I've seen seem to say it was the choice of many when it was around. Gradon here runs one actually. I chose the Mopar 229AB cam based off that graph & a few other factors such as wide lobes vs skinny lobes of other aftermarket cams which tend to have a higher chance of failure I've read. I also choose to upgrade to 1.7 roller rockers for more lift out of the same cam. This puts me closer to the Mopar 230Ab specs which was my goal.
Its very mild at idle & the rockers are so quite I can here my injectors pulsing & spraying over the rockers which is quite nice I must say. Also should boost mileage a bit. The "lope" most expect is no where to be heard of. Its got a very nice burble, solid smooth idle. You can tell its not stock due to the exhaust pressure its pushing out (more compression) but you cant tell its cammed type of thing. My buddy's stock 4.0 open header sounds like it has more lope than what my stroker has open header. I attribute that to the cam. The mopar 229 can also be run on good stock 4.0 springs or stock replacement 4.0 springs. I run stock ls1 springs in my stroker.


All in all what I can only comment on is what I know first hand. The Comp 231 in a stock 4.0 & a mopar 229 in my stroker. The mopar is a single pattern which in theory leads to better torque. I was always told like this, the dual pattern cams came around to help sub-par exhaust setups ect. The longer exhaust lifter & duration allowed more exhaust out. In-turn helping out a weak system whether it be chambers, runners, headers ect. I was always told by the older guys, "it also blows your torque right out with the burnt gasses". While a single pattern cam would close sooner, haver higher cylinder pressures & helped build torque because of that. But you would need a decent exhaust setup to preform as well as a dual pattern. I choose the single pattern, hopefully it'll par better than a dual pattern cam for mileage & power below 3000rpm or so while cruising. The idle is much smoother in this stroker to, but again not fair this thing is balanced out beyond all means. Took alot of extra time in that alone.
Driving both now I conclude the low end torque of the 4.2L mini stroker with 9.2:1 compression kills anything the jeep 4.0 with a comp 231 could produce. Its not a fair comparison because I've now got a built motor compared to a stock high mileage one. This thing pulls harder 100% of the way and doesn't give up even at redline.

I can say just off the specs the Lunati 63501 should out preform the mopar 229 from 3000-5500rpm. Its built for that though. In a bigger stroker 4.6+ its probably what your after. The smaller Mopar may end up choking that big of a motor on the top end because it'll need to breathe more. Youd probably give up 10-15hp & torque over the mopar. Although the mopar should preform better below 3000rpm. 1 other thing to think about is the dynamic compression with either cam. The mopar should definitely be higher in DCR & cylinder pressure because its a single pattern with less over lap, again thats what also helps it build the low end power.
If I do another motor in my time I'll be looking for a clifford 2081 & advance it a few degrees or or hesco RVOB6 and advance it 4*. Most like the RVOB6 but almost all say to advance it 4* as well.