I have an 86 cherokee 4x4 with the 2.8L V6 and a 5 speed in it. I'm wondering what some options are for getting more performance out of the motor or if I should just swap out the motor. If I should swap what would be the easiest to swap into it. I've heard that the GM 3.4L V6 will bolt right up.

Mike thise little guys run pretty good with the stock size tires.. If you have installed taller tires you may want to put gears in there instead of another engine.
Little can be done to improve the horsepower of smaller engines. The object should be to increase the rpm it runs at.
If taller tires are on there or in your future even a little larger engine will need to have some gears to keep it's rpm up.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

Mike,
Before anyone here tells you scrap your 2.8, I'll tell you I know a guy who owns 2 early small Cherokees, both with 2.8's and autos. One is a driver, and the other is heavily wheeled, totally built, running 36's. The secret is gearing. The lower the better. (numerically higher). It is true these motors had some dependabilty issues, but if your's is not a super high-mile engine, it should survive fine. Watch the oil pressure, these things had a tendency to wipe rod bearings.
The 3.4 is a viable swap, but you won't really gain that much horsepower. Gear it first, then go from there. It can (and has) been done.
Good luck.
Corona <img src="/forums/images/graemlins/cheers.gif" alt="" />

Corona the rod bearings go away from too tall gears... Lugging the engine is what puts the most strain on the rods.. I learned bout that back in 63/64.. I put 3 sets of rod bearings in a brand new 63 Ford pickup.. I was towing a mobile home with too tall of a gear.. I changed the gearing and that sucker went to town..
Although this is rarely mentioned here or elsewhere, running tall tires is the start of rod bearing failure.. changing the gear ratio will stop rod bearing wear..
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

Hey Jim,
What is your theory behind lugging the engine loading the rods differently than during normal operation? I am not doubting you, I am just curious for a little more of the theory behind it.

Thanks,

Sean

Ok... When the engine is running at low rpm and the throttle is wide open, AND the engine cannot GAIN rpm.. The engine gets the largest possible explosion trying to push the rod into the crankshaft.. BUT the crank is not speeding up.. due to the? hill? large tires? tall gearing? heavy tow load? So the rod is giving the most push and choking out the lubricating oil.. over and over again.. never having relief for cooling from a gush of cool oil into the bearing surfaces. Or a lessening of the throttle position which would allow less torque on the rod bearings.
I learned this with that new 63 Ford and a mobile home. Changing the gear ratio alowed the little engine to turn more rpm at less throttle.. which equaled less torque on the rod bearings.
New bearings were installed at 2500 miles, 4500 miles and then never again.. all the time towing the same mobile home.
Big Jim
<img src="/forums/images/graemlins/pfft.gif" alt="" />

Hey thanks for the info. My problem is the engine does have alot of miles, 235,000 or so. I do plan on going with some bigger tires but not real big, maybe 30" or so probably no bigger than 31" for now. I like to mostly trail ride and I'm not really worried about going as big as possible. I've found a Edelbrock cam and intake for the motor and I was thinking about rebuilding it and boring over 20 to 30 and also changing the gears now that you told about that. I may even get a new carb because it's stock now. Will these things be worth the money I'm spending because not counting the rebuild and bore we're talking about close to a grand. Should I just scrap this idea and go with a motor swap to get more power for my money?

I am going to have to think about this more, and draw up a couple free body diagrams, and do a few hand calculations. I agree that the engine has more load on it under those conditions, and it makes sense from a loading and resistance standpoint. And at WOT the vacuum advance will not be in play because there would be near zero vacuum, so detonation would not come into play, but when the fuel and air is exploding, the piston is at TDC so the force of the explosion should be relatively even across the piston face, and get transmitted straight into the connecting rod and crack and main bearing since everything is directly in line. Now as you move away from TDC you will start to load everything differently since its no longer in a straight line.

The part that is bugging me the most is what the force is to break the hydrodynamic wedge of oil between the rod bearing and the crank. This should be dependant on clearances and on oil pressure. I would question if maybe your 63 Ford would have lived under the same condition if it had tighter clearances between the rod and crank, or if it had higher oil pressure. Granted it seems like gearing was an issue too, but its still rolling around in the back of my head.

Out of curiosity, Jim, did you replace the rod bearings or did the shop? Did the crank get replaced too, or re-surfaced? Did you actually have a rod knocking at 2500, and 4500 miles?

Sean

Yes I had rod knocking. Yes the Ford shop did it free. NO nothing was changed except the rod bearings.. The mains were all ok each time.
The "straight line" thingy you speak of is bothering me a bit.. The rod is always sending or receiving pressure in a "straight line" from the crank.. granted they are not always direcrly in line but the pressure IS..
Oddly the piston pins never made noise.. As long as I owned that ride they were quiet.
I have slept a few times since 63 and cannot remember the gearing but as I think of it I am sure I went up (numerically) two ratios from stock...whatever that was.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

i see it like this (and granted not doing any calculations or nothing)
occasionally i've had the jeep stuck to the point where the engine just quit because the tourqe required to turn the wheels just wasn't there. this is the point where most people with more than a stock 258 break stuff, or, with a good driveline, get out of the situation. with gearing, the tourqe is multiplied, so the explosion in the cylinder can move the crank around easier. when lugging the engine, bigger boom in the cylinder wont make the tourqe the situation is calling for. hence the added pressure on the bearings.