horsepower and engine mods

horsepower and engine mods

justinfonder

Jeeper
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Location
Fort Pierce, FL
Vehicle(s)
1981 CJ5, 258 w/ clifford intake and holley 4bbl/T-176/D300. 32x11.50s
I'm fairly new to this forum and to the Jeep world all together. I've owned my CJ for about 18 months or so now. I come from the world of old school hot rodding and i've noticed that there is not much emphasis on horsepower and engine mods for the CJ. I've read that the AMC 258 i6 / 4.2l was only rated at around 100hp. To me, this is a problem that must be addressed.

granted, that 100hp is just a number, as I would have put my CJ at closer to AMC 150 based on how it performs. The AMC 258 i6 / 4.2l is a torquey motor and I think its capable of so much more than what they give us stock. So for my question (which i pray for the old hot rodder in me starts a very long thread)

What have yall done to your AMC 258 i6 / 4.2l 's for engine mods?

I have a stock rebuilt AMC 258 i6 / 4.2l with a 4bbl clifford intake and holley carb.
In about 2 weeks it will also have an HEI distributor, hedman header, and 2 1/2" exhaust coming out in front of the rear tire. muffler only, no cat.
 
wow, CJ, thanks for that link. Glad I read that before I bought the header for my AMC 258 i6 / 4.2l head. looks like i'll be calling the nearest parts yard for a 4.0 head first. I already know where this is going. I'm going to tell the wife i'm going out to change the oil, she's gonna come out 4 hours and a 12 pack later to the engine dangling from a hoist and my garage scattered with parts. it's what we do:chug:
 
your new head needs to have a three angle valve job and some of us lile to have the extra water jacket holes welded but the JB weld treatment is just as popular. Do a trial fir of the intake and exhaust before mounting the head, you have to do a little creative die grinder work before the ports all line up. Actually I like to assemble the head, intake and header and torque it before putting it on the block but I never pulled the engine to swap the head. The reason being that some of the intake/exhaust bolts are a bit of a challange to reach with a torque wrench and socket.:D
I replaced the manifold bolts with alloy steel studs, it makes assembly a lot easier when it aligns itself.:D
I also fabbed new fender washers from slices of a 1 1/2" shaft that I drilled a hole in the center of. Most of the new bolted connections need bigger washers than the originals.:cool:
 
I got a noticeable difference by adding on a Howell fuel injection system, plus a reduction in gas mileage, other fuel injection kits are also available.The distributor cap is way to small and can get sparks jumping around inside cutting back on performance, solving this issue is done by an inexprnsive "Team Rush Upgrade" which a larger distributor cap is added or an hei distributor installed. The CJ electrical system uses the body,frame, and engine to return the electricity back to the battery. Connections can get corroded after all these years and using copper wire to bypass the highly resistive steel and cast iron can help out. The final gear ratios are quite a bit on the high side and can stand improvement there depending on how you use it. After you have that thing for about 20 years or so you should have a good idea how to set it up. The main thing is to just have fun with it.
 
plus a reduction in gas mileage??:cool:



I got a noticeable difference by adding on a Howell fuel injection system, plus a reduction in gas mileage, other fuel injection kits are also available.The distributor cap is way to small and can get sparks jumping around inside cutting back on performance, solving this issue is done by an inexprnsive "Team Rush Upgrade" which a larger distributor cap is added or an hei distributor installed. The CJ electrical system uses the body,frame, and engine to return the electricity back to the battery. Connections can get corroded after all these years and using copper wire to bypass the highly resistive steel and cast iron can help out. The final gear ratios are quite a bit on the high side and can stand improvement there depending on how you use it. After you have that thing for about 20 years or so you should have a good idea how to set it up. The main thing is to just have fun with it.
 
I'm a jeep noob too and come from the same world as you. Sorry I didn't save the links but I've seen some of those 1.3L Suzukis with 4:1 xfer cases and 5+:1 diffs crawl up and over some crazy stuff with ease. Not sure but I think only around 80HP. My Limited Jeep knowledge leads me to the conclusion that I only need power to cruise at freeway speed. On the trail it's not really required.
 
for the most part I agree with you. this is a game of gears but a couple of extra horses does not hurt. This is doubly true if you have any intention of driving on the street as well as off road. IMO once you go with 4.58s and 35 inch tires it may not be the best choice for your daily driver.:D


I'm a jeep noob too and come from the same world as you. Sorry I didn't save the links but I've seen some of those 1.3L Suzukis with 4:1 xfer cases and 5+:1 diffs crawl up and over some crazy stuff with ease. Not sure but I think only around 80HP. My Limited Jeep knowledge leads me to the conclusion that I only need power to cruise at freeway speed. On the trail it's not really required.
 
Hopping up a Jeep motor....my favorite subject. :cool:

The AMC 258 i6 / 4.2l benefits greatly from the 4.0 head swap...but there are subtle things that can be done in conjunction to emphasize the performance this swap offers.

As was already mentioned, a better ignition system is the way to go with or without a headswap...The two general swaps are a Chevy HEI system and a Ford TFI upgrade. These are both very simple and relatively cheap stock upgrades that will add power and torque for the simple fact that the stock Jeep system is weak in all aspects.

You have two options really for fuel delivery (3 if you count propane)...a fuel injection system either aftermarket or utilizing a 4.0 stock system, or keeping it carbed. Staying carbed and using the 4.0 head opens a whole world up of carb upgrades, from 2bbl up to 4bbl. The new head can now handle the extra flow that the stock head can not. Clifford Industries has built an entire market based on the 4.0 headswap...and they are your friend :D

Now lets talk dirty....

If you really want to wake up the AMC 258 i6 / 4.2l with a 4.0 head, there are some things you can do that will give you upwards of 300hp and ungodly torque. These are upgrades I incorporated into my AMC 258 i6 / 4.2l build:

Pistons> Flat pistons will build up your compression ratio and we know what that does for power. I gained an 11:1 compression ratio with aftermarket pistons alone.

Cam> To properly utilize the extra flow the head offers, a longer duration cam is the best way to get the fuel mixture where it needs to be fast. I went with a Comp 262 duration cam which is about the top end you really want to go without going to larger Chevy valves. At that point, a 272 duration cam is ideal. Also think aftermarket valve springs but be careful unless you go with roller rockers. The stock valvetrain can handle only so much.

P&P/valve grind> A simple port and polish on the 4.0 head emphasizes the already good flow as you probably already know, and it doesn't hurt here. a 5-angle grind is a great investment...I got talked into this and was pleased.

From there, its figuring out how to best provide fuel and exhaust. I went with a Clifford intake, 500cfm 4bbl Edelbrock, and used XJ headers through a glasspack...no Cat. I completely desmogged this engine and used a TFI ignition. Very simple, easy to tune, and a breeze to trouble shoot.

As I mentioned, you can exceed the 300hp mark using a deeper cam, machining in Chevy valves, and compensating the valvetrain to handle it. With 7 main bearings, the bottom end can handle it just fine. But it's a lot of money and I stayed away from that level of upgrade.

The downside to all this drool-tech is that the AMC 258 i6 / 4.2l no matter what you do to it, will still have a redline of 4500 RPM...and at that range, a AMC 258 i6 / 4.2l sounds like it is coming apart.
 
The funny thing about all the mods to gain horse power to an engine is where the torque is in the power band.
Jeeps are not high speed vehicles, the place they need the power the most is very low down in the power band. That is why the 4.0 was considered one of the best engines ever put into a Jeep, it delivered 80 percent of it's torque at idle.
When adding mods for torque to a Jeep engine it is very important to keep a close eye on the torque curve. I know a lot of Jeep owners have bemoaned the fact their Jeep does not have the high end of a Vette, but then at idle they can pull a stump. Guess it is just what you want.:chug:
 

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