I am rebuilding my 2000 Cherokee 4.0 with .060" overbore pistons and roughly was wondering ABOUT how much more displacement this creates AND what the newer (higher) compression ratio may do for engine performance. Anything else I should consider doing along these lines? I am going with ALL new bearings and newer head (2003) since mine had cracked. Gonna do the fuel injector upgrade later,.and already removed the oil filter adapter. Any recommendations,..especially those who have experience in this area? Thanks

 

well if anything going that big you will loose compression unless you get a dome top pistion or something of the sorts. you can have the block and head decked to gain back some of your compression. with .60 over your probaly looking at 4.2L instead of the 4.0 but if your going that far i would make it a 4.7L stroker which intails getting a 258 crank and .30 over but your .60 over so it might be a little bit stronger but not by much mabye changing it to like a 4.8L

 

ADDITION:

you are also gunna want to be very careful becuase going .60 over is the limit on just about any engine before you start getting warpped cylinders becuase the cylinder wall is really thin and really close to the water jackets. at this point your really close to catastrophic engine failure. but will last if taken care of

 

I believe it is going to be about 8 or so more cubes. An over bore isn't going to raise cubes as much as a stroker crank. Unless you are going to exclusively run high octane I wouldn't try to raise compression. The 4.0 has a relatively low compression radio (IIRC about 8.8:1) if you go more like 10:1 you may need premium fuel and/or retard the timing. If you go more than 10 you will need premium and in some cases even retarding the timing. Pump gas is crap.... especially the stuff that comes with ethanol.... which is pretty much everywhere...

 

.060 is a pretty stiff bore. Why suck a poke?

 

X2

 

OK I must be confused here because I thought that COMPRESSION RATIO is the ratio of CYLINDER VOLUME AT BOTTOM DEAD CENTER/CYLINDER VOLUME AT TDC. In other words it compares the maximum cylinder volume when the piston is fully withdrawn (at bottom of stroke) with the minimum volume when the piston is at full compression (top of stroke). If this is true then overboring only INCREASES the bottom-stroke volume and doesn't effect the TOP STROKE volume. This should RAISE the COMPRESSION RATIO,..right motorheads!

 

Boring INCREASES compression.
Same stroke, moving more air.

Ron

 

Depends what pistons you are using, the head gasket thickness and combustion chamber volume. Since you are changing so many variables bore diameter is one of the least contributing factors to compression ratio. But all things being equal changing JUST the bore diameter would indeed increase the compression radio.

 

When I was young and stupid, I had Chevy 350 block and was given a set of 327 forged pistons. The block was 30 over but the pistons 40 over. We re-bored the block to the new pistons. Then scrambled to find a crank that would fit. Block was slightly off so we cut the deck to get it correct. Heads were already tighter than stock (186 camel hump) but needed to be cut also..............................

Long story short we wound up with 10.3 to 1 compression ration. I made it live on the street by running Premium Fuel, 4 core radiator and believe it or not, re-installed the EGR valve! Ran a solid 100mph thru the quarter, so at 3800 lbs, it was putting a solid 300 horses to the rear tires.

All kidding aside it took a solid 9 months of tinkering to get the spark knock down to acceptable level.

Ron

 

[quote=jronald;1027837]Boring INCREASES compression.
Same stroke, moving more air.

 

depends on pistons

 

Boring will increase displacement but if everything else remains the same(IE: piston dish and combustion chamber volume) compression will drop. Compression is determined by the area above the piston at top dead center and boring the cylinder will make this area larger. The piston does not go all the way to the top of the cylinder and the rings are below the top of the cylinder this does increase the upper cylinder volume. You will need a piston with less dish or machine the head to keep compression the same or increase compression. The bad thing is I have not found many piston options for the 4.0 so machining the head is the best way I have found.

 

With 0.030" you are at 4.1 with 060" you are at 4.2

You can control the CR with the deck clearance and the head gasket. There are some thicker gaskets as well.

If you over bore the cylinder, you anyway need new oistons, gaskets, bearings then the question isr right, why not to get the 258 crank as well and realy ave an big advantage out of it.

Check my signature for detaild 4.7 build up

 

thank you thats what i was saying