summerinmaine Posted August 24, 2008 Share Posted August 24, 2008 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? Link to comment Share on other sites More sharing options...
HOrnbrod Posted August 24, 2008 Share Posted August 24, 2008 You should ask this question over at the Jeep strokers forum. Good bunch there, they'll fix you up. :cheers: http://www.jeepstrokers.com/forum/index.php Link to comment Share on other sites More sharing options...
ejndssn Posted August 25, 2008 Share Posted August 25, 2008 x2 that is where i have started as well :brows: also dino web site has some additional infomation for ya http://www.jeep4.0performance.4mg.com/ check it out Link to comment Share on other sites More sharing options...
Eagle Posted August 25, 2008 Share Posted August 25, 2008 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. Link to comment Share on other sites More sharing options...
Geonovast Posted August 25, 2008 Share Posted August 25, 2008 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. Link to comment Share on other sites More sharing options...
Pete M Posted August 25, 2008 Share Posted August 25, 2008 Personally, if the engine runs well and produces good power, I'd leave it alone. Maybe use a thicker oil to boost pressure. Link to comment Share on other sites More sharing options...
Geonovast Posted August 25, 2008 Share Posted August 25, 2008 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? Link to comment Share on other sites More sharing options...
Eagle Posted August 25, 2008 Share Posted August 25, 2008 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. Link to comment Share on other sites More sharing options...
jtdesigns Posted August 25, 2008 Share Posted August 25, 2008 Do they make a thicker headgasket for the 4.0? Link to comment Share on other sites More sharing options...
summerinmaine Posted August 25, 2008 Author Share Posted August 25, 2008 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 Link to comment Share on other sites More sharing options...
Pete M Posted August 25, 2008 Share Posted August 25, 2008 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. Link to comment Share on other sites More sharing options...
Eagle Posted August 25, 2008 Share Posted August 25, 2008 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. Link to comment Share on other sites More sharing options...
summerinmaine Posted August 25, 2008 Author Share Posted August 25, 2008 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)? Link to comment Share on other sites More sharing options...
Geonovast Posted August 25, 2008 Share Posted August 25, 2008 For the D30 it would be cheaper and easier to just find a new axle and swap it. Try searching for a a new 44, but that you will probably have to have re-geared. Link to comment Share on other sites More sharing options...
Pete M Posted August 25, 2008 Share Posted August 25, 2008 I would confirm your gear ratio before getting too involved here. There's a small chance it was changed by a previous owner. Link to comment Share on other sites More sharing options...
summerinmaine Posted August 25, 2008 Author Share Posted August 25, 2008 I would confirm your gear ratio before getting too involved here. There's a small chance it was changed by a previous owner. I will do that, but even with the stock 235/75 tires 5th gear was pretty useless. I'd be amazed if they aren't 3.07s. Link to comment Share on other sites More sharing options...
reson46 Posted August 26, 2008 Share Posted August 26, 2008 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 Link to comment Share on other sites More sharing options...
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