Easiest way to boost performance on stock 258?

I've almost finished with my restoration on my '83 Scrambler. Now I am looking for simple/easy ways to boost the performance just a bit on the 'ol 4.2 258. Any easy ideas?

4.0L head swap, Mopar Injection, maybe TBI with timing control, turbo, or a supercharger. That's about the only way without going inside the engine.

I could be wrong, but other than making a stroker 4.7 (I think that is a 4.0 block with a 258 crank bore XX over?); the 4.0 head and injection are the only mods that produce solid gains....

The easiest/best performance gains for the drivetrain are in the axle ratios not the 258. What tire size and axle ratios do you have?

Also change out the pathetic ignition or at least do some upgrades to the current one such as the ford distributor cap, upgrade the ignition module and better wires.

With that said my motor build is this:
Comp 252h cam
Rebuilt 4.0 head with 3 angle valve job and all new springs
Cherokee header with mandrel bent 2.5" exhaust
HEI distributor out of an older Chevy inline 6 modified to work with the 258
Holley 2 barrel right now but I just got a Howell EFI kit but not installed yet
3.73 gears running 33 in tires
AX15 5 speed

All in all it it still a dog but much better then my old stock motor. The Scrambler is heavier than my CJ5 that had the stock motor and I can easily cruise in 5th with 75 little effort. The CJ5 I would have the gas pedal practically to the floor with a 4 speed. I hope the EFI helps some more because I will be pulling it next year for a bigger motor. I didn't expect a drag motor but I figure d I would get more power than I did.

I'm running 37's with 5.11 gears. I like where my gears and tires, etc, are…just looking for some possibly easy ways to give it a little more zip. Nothing major….way too deep into this jeep already!

Instead of all that money and still be a dog I dropped a 5.3l Vortec in mine. Even with all that power an only 30' tires it still was pretty much a dog. I changed the gears to 4:10 and wish I would have gone 4:88. It screams pretty good now but I'm still adding about another 100 Horses to it as soon as my cam and heads get here.
The first mod that should be made to a 258 engine is HEI ignition.

zr10054 said:

The first mod that should be made to a 258 engine is HEI ignition.

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THIS!!^^^^^^

When I was still running 258s that was the first thing I did when I got a new one, out came the factory stuff and in went a DUI setup, next I would put on a Holley 7448(personal preference, but at least something other than the carter) some I would need to swap jets and acc pump cams to get optimal performance, but mostly were fairly decent out of the box.

My answer was, instead of putting money into the 258, I swapped in an AMC 360. Maybe not the cheapest option in the short run, but it's a good long term solution. Of course, an LS motor might be a better and cheaper long term answer.

Unless you're lucky enough to have a good running BBD carb, a new carb and ignition upgrade helps a lot. MC2100 or Weber with either HEI or Teamrush are good and won't break the bank.

4.0 head and EFI would be next, but more $$$ and difficulty.

Junkyard HEI and MC2100 carb swap. Then start (smartly) pulling all kinds of vacuum line and wires out of that engine bay. Super simple and will run forever (as long as you aren't crawling at too crazy an angle.)

Anything more than that and just like folks are saying, you're putting too much money into something that would be better replaced.

That said, a solid 258 with a good HEI and MC2100 with good gearing and a smart driver will be plenty respectable up to maybe 33s as long as you're not coming from a V8 truck, muscle car type background with those sorts of expectations on power.

If I had a Jeep on 32s or below, this would be exactly where I'd be and I'd just drive the wheels off of it.

bigwalton said:

That said, a solid 258 with a good HEI and MC2100 with good gearing and a smart driver will be plenty respectable up to maybe 33s as long as you're not coming from a V8 truck, muscle car type background with those sorts of expectations on power.

If I had a Jeep on 32s or below, this would be exactly where I'd be and I'd just drive the wheels off of it.

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I hate to jump in to the middle of someone else's thread, but this is the 2nd/3rd time someone has stressed "right" or "good" gearing/tire combinations, and I'm curious...

I'm in the middle of installing MOPAR MPI on my 258... that should bring some welcome hp/reliability (amazing how much "crap" comes out of the engine bay), but I'm on the fence for next steps... I've got 31" tires and stock 2.73 gears. I'm leaning towards 3.73 and 33" -- which also means a small (I'm thinking 4") lift.

Does that meet your idea of "good gearing"? (No comment on the "smart driving" ).

Thanks,
B²

This should probably be a PM but is this Chris Burson my Baylor little bro? If so, unbelievable reconnection.

b2esq said:

I hate to jump in to the middle of someone else's thread, but this is the 2nd/3rd time someone has stressed "right" or "good" gearing/tire combinations, and I'm curious...

I'm in the middle of installing MOPAR MPI on my 258... that should bring some welcome hp/reliability (amazing how much "crap" comes out of the engine bay), but I'm on the fence for next steps... I've got 31" tires and stock 2.73 gears. I'm leaning towards 3.73 and 33" -- which also means a small (I'm thinking 4") lift.

Does that meet your idea of "good gearing"? (No comment on the "smart driving" ).

Thanks,
B²

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Depends on transmission.....I have a t5 and 3.73s with 32's and I wish I have done 4.10s (or higher).

b2esq said:

I hate to jump in to the middle of someone else's thread, but this is the 2nd/3rd time someone has stressed "right" or "good" gearing/tire combinations, and I'm curious...

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Mainly, because there is some solid improvement to be made with matching gears to your average tire size. Upping the ratio (numerically) puts your engine in the higher part of the RPM/torque curve under all driving conditions. 31"=3.54/3.73/4.10, 33"=4.10/4.56, etc. Makes them scoot better and finding CJ people upgrading to heavy duty stuff and selling their re-geared stock axles is common. Then drops the 37" hammer. That's so much rolling resistance I would be hard pressed to squeeze enough "increased performance" out of a 258ci to be satisfied. Let alone any transmission short of a T-176 or better would be suspect to me. A nice 360/TH400 or T-18 would fit the bill for me.

CBurson said:

I'm running 37's with 5.11 gears. I like where my gears and tires, etc, are…just looking for some possibly easy ways to give it a little more zip. Nothing major….way too deep into this jeep already!

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With my CJ7, I tried it all....

I bought it with a tired (165K mi.) stock 258, BBD carburetor, CA emissions engine, 3.31 gears, T5 5-speed, and 32" tires.

I tried the nutter bypass on the ignition, and didn't notice any difference.

I switched to 4.10 gears, and obviously that made a difference... but you're happy with your gears, so that's a moot point.

I installed the Mopar MPI kit (and a new timing chain), and the driveability improved tremendously! But... not much noticeable difference in power.

I added a throttle-body spacer (which we all know is bunk), and it made no difference at all.

I rebuilt the engine, and added a Borla header. It sounded great! I really wanted to notice a difference... but it wasn't there.

Then, I installed a 4.0HO head... and it was amazing!

I never put it on a dyno, but the seat of my pants told me it was a good 25% power increase in the rpm range I was using. That engine pulled strong from 600-2500rpm!

I used to tell people that I was getting V8 power from my straight six... and I believed it!


HOWEVER, now I'm driving a CJ8 with an old, tired, carbureted 350 small block... and it would run circles around my previous 258 CJ7!

I took a ride in Randyzzz 5.3 Scrambler, and it's even better!


Sorry for rambling on...

To summarize, the best bang for your buck with the 258 is to add a 4.0HO head... but for similar cost and effort, you could swap in another 100 cubic inches (350 SBC), and be REALLY happy with the result.

There's no replacement for displacement....

Easy... cheap... guaranteed performance... and more fun than a barrel of monkeys!

Here another reality to this as well. My 258 that I did the previously mentioned work to is not very quick. I have a cj5 with a properly built chevy 350 and it pulls very hard all day long. Both vehicles around the town gas mileage are the same at about 13mpg. This is mostly 95% start and stop driving. I do not take any highway even to work. Both have the same gearing and use the same lift/tire combo. The 5 has a 4 speed while the 8 has a 5 speed. In my book the 350 is a good swap and may end up in my CJ8 this winter if I get the itch.

Kane said:

Here another reality to this as well. My 258 that I did the previously mentioned work to is not very quick. I have a cj5 with a properly built chevy 350 and it pulls very hard all day long. Both vehicles around the town gas mileage are the same at about 13mpg. This is mostly 95% start and stop driving. I do not take any highway even to work. Both have the same gearing and use the same lift/tire combo. The 5 has a 4 speed while the 8 has a 5 speed. In my book the 350 is a good swap and may end up in my CJ8 this winter if I get the itch.

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258 are not known for great gas mileage. When I swapped in my 5.0, I was actually getting 15% better highway mileage with the 8 cylinder than it did with the smaller 6 cylinder.

Supercharge it!

sent from my flaming samsung note7