Swapping in a 258 in place of a 304 - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 03-20-2001, 11:19 PM
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Swapping in a 258 in place of a 304

I currently have a '78 CJ-5 w/ a 304 backed by a 3-speed...the T-150 if I remember correctly. I have an '80 CJ-5 w/ a 258 and a 4-speed (not sure...maybe a T-176) available to me as a parts donor. I like my 304, but it guzzles an enormous amount of gas, and the 3-speed really cranks up the RPM's at road speeds. The mileage on the 304 is unknown, but it's been around for a while, leaks a bit, and is generally starting to show it's age. The donor 258 has 88,000 total, and was rebuilt about 4,000 miles ago. It runs nicely, and has the 4-speed behind it. In addition, it HAS to be easier on gas than the V-8 w/ a 3 spd. What I'd like to do is do an engine/tranny swap. My assumption on the process would be that I would need to drop the respective engines, trannys, and transfer cases, and swap. I haven't looked at the mounts yet, but as far as I can tell I can swap both the engine and frame mounts. The frame on my '78 has additional sets of holes in the frame behind the mounts, indicating it's a simple swap. My question is what am I missing here? What am I not accounting for? Would I swap the driveshafts as well? I would think there wouldn't be any issues w/ having the 4-speed fit....the shifters would be in roughly the same place....the mounts should be swappable, as far as I can guess. I assume I might need to swap clutch and shift linkages. My big wonder is what I do about the wiring. Will I need to worry about hooking it up, or will it be all interchangable. I'm not terribly experienced w/ electrical stuff. Any answers or suggestions regarding this whole procedure I'm about to undertake would be greatly appreciated. Thanks,

Ryan
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post #2 of (permalink) Old 03-20-2001, 11:26 PM
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Re: Swapping in a 258 in place of a 304

I can only offer a little help.
The mounts will swap no problem.
The fuel lines come up on opposite sides so you will have to work that issue as well.
The electrical I don't have a clue. When I dropped the 360 I did a painless kit at the same time and yanked the entire harness without thinking. Completely rebuilt the engine wiring harness and it wsan't that tough with all the help I got here.

K-16
post #3 of (permalink) Old 03-20-2001, 11:50 PM
 
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Re: Swapping in a 258 in place of a 304

It looks like you have your part numbers right but I might add that the V-8 has a D20 transfer case and the 1980 I-6 should have the 'shortie' D300 TC.
They should both have the same bellhousing.
The front and rear axels are the same type so it looks like a complete engine, trannie, transfer case and drive shafts ... plus maybe the skit plates ? will swap right out.
The ignition wiring in my 1977 was basically just a coil wire from an 'ignition on' circuit. I can't recall if an '80 was computer controlled or not ... bears checking ...

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post #4 of (permalink) Old 03-20-2001, 11:56 PM
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Re: Swapping in a 258 in place of a 304

Your road rpm won't change, unless you are putting in a overdrive, 3 spd or 4 spd are both 1:1 in high gear.


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post #5 of (permalink) Old 03-21-2001, 03:47 AM
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Re: Swapping in a 258 in place of a 304

I really don't think you will see that much of an increase in mileage, but that is just me thinking. Other than that I can't really help you. Your are a few years ahead of me in the JEEP evolution, but I bet we can solve any problems you run into.

Swavy00

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post #6 of (permalink) Old 03-21-2001, 08:55 AM
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Re: Swapping in a 258 in place of a 304

if you have accesss to an entire donor jeep then should be no problem whatever you run into.
you can just go back and get the driveshafts or wiring, plus you have the donor to look at as a blueprint.
leave your computer on and dive in
use this board and no worries mate
good luck and keep us informed
bob

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post #7 of (permalink) Old 03-21-2001, 12:01 PM
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Re: Swapping in a 258 in place of a 304

All good info so far. The '80 CJ5 should not be computerized. That didn't start until '82 I believe and wiring should be identical for both. Some wire routing and length changing might be necessary. As Dirtrod said the RPM's at road speed won't change. The 3spd and 4spd have the same gear ratio in high gear. Try driving that 304 like you have an egg under the throttle pedal and see if it helps. The 258 would be better, but to what degree? At highway speed it's really the aerodynamics that are killing the mileage and the 258 may take a little more throttle than the 304 does.

'75 CJ5,258 w/Howell EFI,T18a,4.27's,33's,On-board air,Warn 8274
post #8 of (permalink) Old 03-21-2001, 06:21 PM
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Re: Swapping in a 258 in place of a 304

You will also have to change the throttle cable and the exhaust. The 304 throttle cable is too short and the exhaust problems are obvious. Use the oil and cooling senders from the 304 and the gauges will work properly.
These are some things I ran into on my swap from a straight 6 to a V8.

Mizu
post #9 of (permalink) Old 03-21-2001, 07:53 PM
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Re: Swapping in a 258 in place of a 304

rmueller23,
If you end up doing the swap would you be interested in selling your V8 mounts?? I would sure like to buy them if you do.
As for your electrical question, everything will swap without any trouble, your 80 CJ-5 doesn't have a computer so no problems there. But your 78 will most likely have a Prestolite ignition system which would be good to change, if the wiring harness is in good shape on the 80 CJ-5 then I would swap all that stuff too so you don't have to figure out how to change it over to the MotorCraft ignition.

Peanut

post #10 of (permalink) Old 03-21-2001, 07:58 PM
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Re: Swapping in a 258 in place of a 304

the gas milage.. will not be that much of a diffence... you might see 15 miles to the gallon out of that 258.. if its in decent shape.. really good shape with some upgrades you might see 19... or alittle more.. but just swapping these engines.. i dont see it being worth it just for mpg..... if there is something else driving this.. than ok..

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