My buddy just had his engine bored at a machine shop figuring the old tired 4.0 would come back to life and then some. After the rebuild he was supposed to take it easy, which he did. After the "break in" period......absolutely no power gain whatsoever. He also had a new cam from comp cams installed. I'm now thinking its the transmission? Can anybody give me/us some insight? Thanks!!!!!
Hey thanks for the reply. I think it was a .40 overbore, cant remember. Basically the jeep had absolutely no power before and he figured it was pissed out (the cone air filter he had was not filtering worth **** either). The machine shop told him to expect 300 horsepower after all said and done which surprised the **** out of me, but I didnt really know. After the rebuild it is the same as before, and it was expected to atleast be better than before ya know?
300 HP from a overbore. ..... Whatever that guy is smoking is illegal. The 4.0 is a strong motor but not a powerhouse I have a stroker in my YJ and do not build that kind of horsepower. Tell us about the jeep. Transmission type, tire size, gear ratio. Year of manufacture. Cam number. Perhaps we can make some intelligent sugestions. Unlike the 300 hp overbore.
99TJ 4.0 Auto, Lifted and Locked with a lot of Junk
94 yj 4.6 Stroker, 14Bolt, HP44, 101 wheelbase
I brake for Junk Yards
The 300 was a mistake, he said he probably heard 30 and added a 0...haha.
Anyway....the thing hardly has any power, and tonight I found out it has very slow throttle response. A quick foot to the floor in neutral brings the rpms up to a couple grand. Maybe that tells us a little more? Help is much appreciated, thanks!!
all they did was a stock rebuild with a cam, your not gonna feel a hell of alot of difference from before, that cams not crazy enough to do much, youll just a get a little more torque and maybe a very slight lope to the idle. boring a motor out doesnt give you any noticible power. i dont know who keeps tell people this. decking the head would bring the compression up a bit but 15 its not enoguh to bring the motors already low compression up to anything good. maybe the shop didnt degree the cam up strait up?
full time tech for chevy, part time trail rider
95 Ford Lightning (sold), 09 Caddy CTS-V, 04 Buell Firebolt XB12R w/true flame paint, 00 harley XL chopped, 81 Trans Am, 85 K5 Jimmy 4 inch w/35s, 92 cherokee
10,000 original miles
4.5 inch RC lift
32 11.5 bfg muds
If it started out making good power and it sluffed off to what it is now, then a couple things could have happened...
1-timing jumped on the camshaft
2-the lobes on the camshaft wore out/improper assembly from rebuilder
Pull your valve cover and check to see that all valves are opening and closing the way they should be (you should see nearly .5" of lift on the push rod). If you do, then check your valve timing.
Also, poor ignition timing will give slow throttle response as well.... Double check your timing...
What's your baseline for comparison? What was the compression in the cylinders before the rebuild? After?
Was it put on a dyno before or after the rebuild? The butt makes a lousy dyno, it won't calibrate at all. The expectations were far greater than what was achieved and that makes the butt dyno feel even worse.
Keep in mind that free advice can frequently be worth less than you pay for it.