Help Needed, Engine Guru's/Builders Enter Please?

C

casual

NAXJA Forum User
Location
high dezert
Hey guys I'm doing an OEM rebuild on a JY motor I just picked up for dirt cheap. Its going to be a daily driver that will see some desert trails.
So it will be pretty mild & stock. I plan on a small cam & valvetrain upgrades, .20 or.30 overbore, slight CR increase. The motor was at the machine shop & got honed. But cly. 6 still has scoring. So my .20 or .30 overbore plans are now useful!
If anyone can help me pick the right set of pistons plz?
I'm looking for budget .20 or .30 over pistons & rings! I'd like to get a CR of 8.8-9.0:1 on 87oct. DAILY!
Heres the 2 sets I'm looking at. I'm not sure of the difference, besides of the piston ring thickness? Which would be better?
They are both .30 over!
http://www.summitracing.com/parts/SLP-H825CP30/
http://www.summitracing.com/parts/SLP-H802CP30/

And final question is should I just go .60 over? I was thinking I might as well go .60 over with this motor to get a little more power?
Once this motors build and dropped in I'll be stroking the motor thats in my jeep now. So no I won't stroke the one being built currently.

1: If I go .30 over which pistons out of the 2 are best? Or any other suggetions?
2: Anybody see an issue with going .60 over & not stroking it?
3: And if its ok/smart to go .60 over which pistons would be best?
 
I've been running the H825cp30s in my stroker for 2+ years. I see no problem with going .060" over--some will say that if you have an issue in the future, you won't be able to re-bore the cylinder, but there are people with .080"-.125" overbores with custom pistons on their strokers(get the block sonic checked for over .060"). The pistons cost the same and going .060" over will give a little more power and get rid of more of the shelf that the top of the block makes in the combustion chamber(so unshrouds the valves some). You can use either the 802s or 825s--one is metric and one is standard--just make sure you get the correct rings. Deck clearance will go from being .0215" to .0315" cause the pin height is .010" less on the Sealed Powers. Personally, I'd get the machinist to remove .020-.030" from the block and use the thinner .043/.044" head gasket instead of the stock .051", but that'd put you at 9.1:1 SCR(.020" cut) and 9.3:1 SCR(.030" cut). The better quench should make the high compression still work fine on 87 octane. As for cams, the stock valve springs are good for up to .44" lifts, but not .45" lifts(I'm running the mopar 30ab cam and had to get the bigger P5249464 springs), so take that into account when you choose.
 
gardon your a whealth of info lol. If you don't mind me picking your brain for a few, I'd sure like to get some more info/help.
If you want we can either talk here or pm's.

Where does your motor run at mostly? Where does it makes it power?
Think you can help me put a setup together that would suit my needs? Everywhere else i ask seems to be dead or opinionated.
 
Octane is related to ping or detonation as you probably know, but compression ratio isn't the only thing that determines when it happens. I run 10.4 compression on the cheapest gas available without ping. The secret is the quench height and or cam. I run the H802CP pistons.
 
My peak torque is probably around 3200-3300 and it pulls hard to redline(even though it makes a racket). I tend to do 70-80mph on the beltway and do not mind keeping the rpms in the 2500-3300 sweet spot. I shift gears in the high 2ks-3k range and usually don't let the revs go below 2K. Might as well keep everyone involved and keep the thread going cause I still consider myself a novice when it comes to engine building(working on the 2nd this spring: a 4.9l roller-stroker that'll see boost down the road).
 
I wasn't aware you could run that much compression & not knock or ping lol. I've build a few motors, including a bored 1.9L B18CR. Its running 12.3:1 on 91oct daily & runs high 10's in the quarter in a honda hatch with full interior. I'm proud of that especially since I'm only 19.
Anyway I'll guess I'll get every one opinion on what I should do with my motor as long as no one says stroke it lol. Please give your opinion on how I should build this motor.

I plan to daily drive this motor and need it to last at least 200k & it needs to run on 87oct with no problems. It will see some mud but nothing serious, as well as some fast desert trails where I'll flog the hell out of it!
With that in mind, plz give your opinions on what I should do.
 
Ok, I searched your posts and you stated in one that you have a 96. Im going to assume this engine is going in that jeep.
First off, you have the engine all the way apart, why not stroke it?

Now that i got that off my chest if it were me this is how my build would go for a non stroked jeep engine with a scored cylinder.

Bore block .60
Get these pistons http://www.summitracing.com/parts/SLP-H802CP60/
Find a stud girdle off a newer jeep engine and retrofit it to fit your engine.
If Im trying to keep it on a budget I'd keep the factory cam in, if I could spend some extra cash then I'd get a comp cams 231
Then id get one of these throttle bodies. http://strokedjeep.com/tbody.html
Run a good air filter
Pick a header
Once you decide what lift to run for pre-running and have it installed and can cycle it, Check for interference with your down tube after the header and if clearance isnt an issue then have a exhaust shop make you a new down tube (2.5"). I believe Borla sells a header that is certified with a downtube.
Make sure your cat is 2.5" in/out
Id run a Spin Tech muffler (2.5" in/out)
If It was in my budget Id run a set of Hesco's roller rockers, they are expensive, but adjustable. Yella Terra also makes a set of roller rockers, but I dont believe they are adjustable.
While the engine is being built and the head is off Id port match it with my intake manifold and grind off any left over casting inside. Get a 3 angle valve job.
use a mopar performance .043 head gasket.
Wrap the intake manifold with a heatshield insulation
then of course Do a full tune up (cap, rotor, plugs, wires, fuel filter)
make sure your tps is adjusted properly
use a set of precision auto injectors
 
My plans were to stroke my engine before, and I still have those plans. Except not this motor lol. I have 2 full motors, the one I'm building now is a 94. The motor I'm going to stroke is in my jeep being daily driven right now. But the motor in my jeep now is burning oil & starting to smoke so I just wanna get a strong rebuild, & take my time on the storker to come.
I want 2 different motor setups. My plan is to have this motor in for about a year until my storker is done. And in case the stroker ever goes down then I'll have my backup. Plus I'll have both options that way, big bore & stroker!:thumbup:
Norcalchris what you posted is basicly what I'm plaing to use, heres what I've come up with & posted on jeepstrokers.com last night..

I'll be using the Mopar(.043) HG. A Comp 68-231-4 cam & Sealed Power H802CP (.60 overbore) unless I can get away with more compression & use Keith Black 3241HC/3242HC (.60 overbore) pistons.
Head & block WILL NOT be decked. They will remain at factory specs!
PLEASE FEEL OUT/CORRECT ANY WRONG OR MISSING NUMBERS!

bore: 3.935
stroke:3.413
combustion camber:58cc (Factory Spec: I don't know if this changes when I change pistons or head gasket?)
deck clearance:.0215 (Factory Spec)
gasket thickness:.043
Piston Ring Height: (Didn't know what to put here?)
Piston to Bore Clearance: (Didn't know what to put here?)
gasket bore:4"
connection rod length:6.123
cam duration: 250*
cam LSA:111
degrees advance/retard:4

With these specs I've come up with:
249.04 CI---4.08L----8.89SCR----7.72DCR----.0645 Quench----52 intake closing valve angel.

Does this sound right? Are my calculations correct? Is 8.89 SCR/7.72 DCR safe on 87oct? Could I use Keith Black 3241HC/3242HC pistons since they have the same compression distance & such? Only difference would be there dish & the KB pistons would bump my CR up nicely.
These would bump my SCR to 9.06 & my DCR to 7.86! Would that be safe on 87oct. I'm 2800-3000ft. ABOVE sea level.

So everybody what do you think? Should I use H802CP pistons or Keith Black 3241HC/3242HC pistons. Please don't state on opinion, I'd like to build it on facts. I'm trying to be good horsepower with just a .60 overbore, higher comrpession ratio, & a cam. & still run on 87oct.
 
Last edited:
KB pistons have a 13.3 dish, the Sealed power is 15.1!
So I'd have a higher compression ratio with KB pistons.
From building N/A motors before I was always taught to use flat top or dome pistons in N/A builds. So I'm favoring the KB pistons for less dish. Both are within a couple bucks of each other. Both have coated piston skits as well.
My only concern is how much compression can I run without pinging on 87oct.

I just can't see running 91oct. daily & getting crappy gas mileage! And I mean it's a jeep not a performance car lol. I'm coming from a 400+ hp is300 so I'm trying to stay with cheaper gas lol.
 
You'll be able to run 87 with either of them--especially at your high altitude. Keep in mind that each one of these pistons have different pin heights, so the deck height, quench height, and compression will be different. A shorter pin height with smaller dish piston could have the same compression as a taller pin height with bigger dish piston. Coated skirts are good(the cp), hypereutectic alloy is too(the h).
 
The stock pin height is 1.602" and deck clearance is .0215". When you use a piston with a pin height of 1.592", the deck clearance increases .010" to .0315", the quench increases, and the compression decreases. Using the .043/.044" gasket will get rid of .007/.008" of that .010" quench you gained. I didn't mention it earlier, but if you do get the block cut, you should get a push-rod length checker and determine what now shorter pushrods you would need to keep the geometry correct. I use the old calculator on the stroker site: http://www.editgrid.com/user/brewmenn/stroker.new?savebar=0 cause it lets you enter in the pin heights.
 
gradon thanks.
What do you think power out put would be if you could estimate it?

Comp 231-4
Zero deck block
.60 over & stock stroke
.43 head gasket
9.3:1SCR & 7.92:1DCR
 
I was thinking 220ish as well(on the conservative side). Get a freer flowing intake, bored tb and exhaust and you'll take advantage of the cam's extra lift and make more power. Compared to stock, your static compression went up half a point, but the dynamic compression went up almost a full point, so that'll be a significant change.
 
The less you cut from the block the more reliable your motor will be. The ZH825CP30 pistons are "light-duty truck" rated and use a thicker (more reliable) ring. A 60-over bore might make a few hp more than a 30-over at the PEAK of the power range but you'll only know it's there if you dyno it. Increased compression rarely makes more power without using better fuel or fuel delivery modifications. The last set of "Keith Black" (name used under license) pistons I saw were a hidious joke; diameters varied ± 0.002 and the surface finish was very poor.
If you need reliability keep it stock. If you want power .. go big or go home :D.
 
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