258 combinations

[rr41mag]

I'm doing some mods to my AMC 258 i6 / 4.2l . I presently have headers and a dui distr. Also have a 4bbl intake. What cam and 4bbl would go good with this combo?

 

[Lumpy Grits]

Remember, that is a tractor engine that has a 4500 rpm 'redline'.
Have to still SMOG mine-So beware, if that's the case.
LG

 

[elwood blues]

Remember, that is a tractor engine that has a 4500 rpm 'redline'.
Have to still SMOG mine-So beware, if that's the case.
LG

Will you stop freaking telling people the AMC straight 6 is a "tractor engine"? It was never used in farm implements, as a matter of fact up until it was replaced by the 4.0L in 1987, it was basically the same engine Nash designed in the 1940's, heck even the 4.0L was designed with a lot of off the shelf parts for the AMC 258 i6 / 4.2l . It is just like every other straight 6 engine ever developed, even compared to the last of the Ford 4.9L 300 straight 6, who many say is the best straight 6 ever designed, which redlines at 5600RPM, or the very last manufactured straight 6, the Chevy Atlas LL8, or Vortec 4200, and it redlines at 6000RPM.

I'm doing some mods to my 258. I presently have headers and a dui distr. Also have a 4bbl intake. What cam and 4bbl would go good with this combo?

COMP, no questions asked COMP Cams: 6 Cyl, 199-258, 4.0L. Summit has kits https://www.summitracing.com/search...798+400114&SortBy=Default&SortOrder=Ascending

Even with a COMP cam, and headers and a 4bbl intake, I don't think you could ever need more than a 600cfm carb. Holley used to make off-road carburetor, you could also go with a marine carburetor, it's got the rollover protection already built in.

 

[Tinman22]

A 390cfm Holley works well. At 100% ve. A AMC 258 i6 / 4.2l needs 373 cfm good luck with that 100% thing your more than likely around 80-90 % ve so you would need around 310 cfm. Set up properly with center hung floats it pretty good off road too. We set it up with center hung floats because it sits on an offenhauser intake sideways. We run a crower 44247 cam .518/.527. Gm 305 domed pistons. It makes decent power with plenty of torque and runs out well past 5000 rpm. Kinda have to agree with lumpy on the 4500 thing with stock motors. Torque (a couple of hundred ft/lbs) peaks at around 2000 rpm and hp (120 crank?) between 4000-4800 every one we have kinda falls on its face at around 4300 or so. 300’s weren’t much better the bulk of them (1978-1986) made between 100 to 120 hp at 3400 ish rpm and 220 ft/lbs at a screaming 1600 rpm. Compared to say a ford 6000 tractor pretty spunky really (around 70 hp flywheel) and 180 ft/lbs I think?? The earlier 258’s heavy block and crank really do pull like (Tractors) due to internal rotational mass and a heavy flywheel.

 

[Ultrunner]

Good afternoon all, new to the CJ site and to the forum; and just getting starting with my ‘86 CJ7 rebuild. I am preparing for my engine rebuild and wanted to pool the crowd with respect to performance modifications.

I have read and researched the Clifford 6=8 mod and the Mopar EFI for the AMC 258 i6 / 4.2l .

Is one better than the other? Can they be used in tandem? Is there a better option to boost performance in the AMC 258?

For my purposes this engine will meet my needs. No heavy duty 4-wheeling etc... just semi-daily driving and fun weekend car to take to running trails and camping. Thanks and sorry for the long post, but I know you guys are the experts.

 

[Torxhead]

The Mopar f.i. kit is the best you can get. Headers help also, but you will get high underhood temps from them and a fresh air intake would benefit from that.

 

[Ultrunner]

Thanks Torxhead for the reply.

I am in a position where I am going to have to completely replace my engine. I read some things about 258s reconditioned as "stroker" engines. Does anyone know anything about these in a CJ7 ? Is it worth going this route? It seems I can do this for close to the same price as a baseline AMC 258 i6 / 4.2l that I would then modify with a 4.0L head and fuel injection.

Would it be better to simply get a crate 258 and add a FI modification and forego the stroker option? Again, this is going to be a semi-daily driver with some light trails occasionally. Thanks for all the advice! Cheers.

 

[Tinman22]

I would take a look at some of the less involved fi options. Mopar’s kit is really good but for the almost $3000.00 dollar buy in by the time your turn key is a bit salty. Thats almost as much as our class 3700 truck with a holley dominator efi system. Holley has a 2300 2bbl master kit for about $1000,fitech has a 2bbl unit for a bit less ($800) but you would need pump,hose,ect. Both would work on a AMC 258 i6 / 4.2l . With an offenhauser dual plane, hei conversion,moderate cam and a slight bump in compression would get you into the mid 200’s with a strong torque curve.

 

[mtnwhlr]

Thanks Torxhead for the reply.

I am in a position where I am going to have to completely replace my engine. I read some things about 258s reconditioned as "stroker" engines. Does anyone know anything about these in a CJ7? Is it worth going this route? It seems I can do this for close to the same price as a baseline 258 that I would then modify with a 4.0L head and fuel injection.

The stroker motor is a 4.0 with a AMC 258 i6 / 4.2l crank. witch is a great motor.

 

[Ultrunner]

I would take a look at some of the less involved fi options. Mopar’s kit is really good but for the almost $3000.00 dollar buy in by the time your turn key is a bit salty. Thats almost as much as our class 3700 truck with a holley dominator efi system. Holley has a 2300 2bbl master kit for about $1000,fitech has a 2bbl unit for a bit less ($800) but you would need pump,hose,ect. Both would work on a 4.2. With an offenhauser dual plane, hei conversion,moderate cam and a slight bump in compression would get you into the mid 200’s with a strong torque curve.

Yeah, I was looking at the FI options and thought, "man that MOPAR is SO much more expensive, is it really worth it?" Thanks for offering a few other options. Whether I go the stroker route or not, I am planning on installing headers and looking into the Offenhauser dual plane. Once I get going I'll add reporting on progress.

 

[Torxhead]

Ultrunner, I think you should have started your own post. It makes it easier for someone using the search feature for a particular topic. Anyway, from what I have researched the Mopar fi kit is worth it if you want the power. It was designed by Chrysler for use on the AMC 258 i6 / 4.2l . It also requires premium gas and is 50 state legal for smog requirements. Other kits are available which are simpler, cheaper, better gas mileage, and can use regular gas, but still offer more power than a carbed setup. They will also not stall on a steep incline. Headers offer more power, but that hot air under the hood will also get fed into the induction unless you have a fresh air intake. I have heard of excellent results with the 4.0 head and crank.

 

[Ultrunner]

Ultrunner, I think you should have started your own post. It makes it easier for someone using the search feature for a particular topic. Anyway, from what I have researched the Mopar fi kit is worth it if you want the power. It was designed by Chrysler for use on the 258. It also requires premium gas and is 50 state legal for smog requirements. Other kits are available which are simpler, cheaper, better gas mileage, and can use regular gas, but still offer more power than a carbed setup. They will also not stall on a steep incline. Headers offer more power, but that hot air under the hood will also get fed into the induction unless you have a fresh air intake. I have heard of excellent results with the 4.0 head and crank.

Torxhead, thanks again. I completely understand a separate thread would have been helpful for future searches. I wasn't sure of the protocol being new to the forum; not a good excuse I know, nonetheless, should I make any further inquiries, I'll move everything to a new thread to include what was already discussed here.

Cheers!