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Minimalist Stroker


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My 4.0L has about 300K miles on it, and the compression is about 155-160# in all cylinders (pretty good, right?), but the oil pressure sinks from 40-45# when cold down to 10# or less when hot on idle. It runs at about 20-25# at running speeds. Also, I suspect my rear main is TU, as I lose oil out the back (not a lot, but noticeable).

 

If I'm going to do any kind of rebuild to address the oil pressure/rear main, I'd like a bit more power as well (as 5th gear is virtually useless with my camper rig).

 

My grand scheme at some point is a stroker built for torque, but that would mean a new gearbox and transfer case to handle the increased power. However, for ease of troubleshooting/repair, I don't mind staying with the Renix system, since all my wheeling is down in Baja.

 

It occurred to me that I might be able to get some benefits while staying on the cheap side, by installing a stroker crank/new bearings/rear main. This should increase the torque without stressing the gearbox too much, right?

 

My question: Can I do a crank swap without changing the conrods, and thus do it as a mini bottom end rebuild? If so, what gains (if any) should I expect in the torque/power department?

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My question: Can I do a crank swap without changing the conrods, and thus do it as a mini bottom end rebuild? If so, what gains (if any) should I expect in the torque/power department?

No, you cannot.

 

There are two ways to build a stroker out of the 4.0L engine. One is to use the 258/4.2L crank with the matching 258/4.2L rods. That gets the piston-to-deck height close to correct, but because of minor differences, it actually puts a stock (or stock oversize replacement) piston slightly too close to the head, resulting in high compression and ping. The usual "cure" for that is a cam with more overlap to bleed off some of the excess compression pressure.

 

The other approach is to use the stroker crank with the 4.0L connecting rods. If you think about it, you'll quickly realize that the longer stroke with the same rods will put the piston above the top of the block at TDC. The stroker engines that use the 4.0L rods (which are stronger than the 258 rods) all use custom pistons with the wrist pin set higher in the piston so as to drop it lower in the bore at TDC.

 

Neither approach will be easy for you if you're trying for a quick cure on the oil pressure. I'd suggest just dropping the pan and replacing the main bearings and rear main seal, then closing it up and keep on driving while you collect the parts and the $$$ to build a real stroker.

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There are two ways to build a stroker out of the 4.0L engine. One is to use the 258/4.2L crank with the matching 258/4.2L rods. That gets the piston-to-deck height close to correct, but because of minor differences, it actually puts a stock (or stock oversize replacement) piston slightly too close to the head, resulting in high compression and ping. The usual "cure" for that is a cam with more overlap to bleed off some of the excess compression pressure.

 

I'd only seen that using the 258 rods actually put the pistons lower in the cylinder, and and you could deck the block by 0.020" to get some more compression.

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I'd only seen that using the 258 rods actually put the pistons lower in the cylinder, and and you could deck the block by 0.020" to get some more compression.

I may be remembering incorrectly, but I used to spend a lot of time on the stroker e-group in the days before I decided it wasn't worth the money and aggravation (for me) to build a stroker. There was a lot of discussion about buying one particular brand of aftermarket piston, or trying to find a source in Mexico for VAM pistons, to get slugs with enough meat on top to dish them out to reduce compression. The other approach some people took was to deck the block in the hope that achieving a near-ideal quench height would offset the increased compression. However, actual results with that approach varied, and the people who had the best results were those who also used aftermarket cams with lots of overlap. That resulted in a high mathematical compression ratio, but lower actual compression pressure due to the loss of some compression before the exhaust valve closed.

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Alright...he's got a 4.0 MJ with a 5 speed. 31 inch tires. Camper.

 

If he's sitting on 3.07s, that alone would explain the power loss.

 

What gearing do you have?

 

3.07

 

 

Fixing that would most definitely take priority if it were me. 3.07s bring on the suck. Even 3.55s would feel superior. 4.10s would be better.

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3.07s and 31s are rather sucky even without a camper. That's how my '88 was when I bought it. I put in a set of 3.73s, but only because I already had them here (they had been destined for the XJ, with the intention of running 30x9.50 tires) so I used what I had. If I had been buying gears specifically to run with 31s, I would have chosen 4.10s.

 

The 3.73s with 31x10.50s have the exact same overall final drive ratio (RPMs to MPH) as stock tires with 3.55 gears. To routinely carry a camper 4.10s would be far better.

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Alright...he's got a 4.0 MJ with a 5 speed. 31 inch tires. Camper.

 

If he's sitting on 3.07s, that alone would explain the power loss.

 

What gearing do you have?

 

3.07

 

 

Fixing that would most definitely take priority if it were me. 3.07s bring on the suck. Even 3.55s would feel superior. 4.10s would be better.

 

I had thought of re-gearing at one point, but was given to understand that the cost of two new gear sets (D30 and D44), plus the skilled labor to install, was too pricey to be worthwhile.

 

Any idea what the gears would cost? Does it make sense to get used? Can they come from XJ or other Jeeps (or other makes)?

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I concur with the suggestions of lower gears. But, before you tear apart the engine, you may want to make sure the oil pressure is too low. For the 4.0L it is normal to have low oil pressure at idle when hot. If I remember correctly somewhere around 12 psi is normal when idling hot.

 

Willy

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