differance in 258 and 304

[1986cj7]

Was wondering what the difference in horsepower and torque or between the AMC 258 i6 / 4.2l and AMC 304 ? I'm asking because I have a AMC 258 i6 / 4.2l in my jeep now and have a old AMC 304 in the barn both need to be rebuilt and wondering how much difference in power and torque. Also what would be the difference in swapping a 4.0 head on the AMC 258 i6 / 4.2l if I chose to rebuilt it into a stroker?

 

[jdcaples]

Was wondering what the difference in horsepower and torque or between the AMC 258 i6 / 4.2l and AMC 304 ? I'm asking because I have a AMC 258 i6 / 4.2l in my jeep now and have a old AMC 304 in the barn both need to be rebuilt and wondering how much difference in power and torque. Also what would be the difference in swapping a 4.0 head on the AMC 258 i6 / 4.2l if I chose to rebuilt it into a stroker?

I think this is what you're asking:

The highest HP and torque numbers for factory AMC 304 I think was
210 HP @ 4400 RPM & 300 FtLbs of torque @ 2600 rpm

The highest HP and torque numbers for factory AMC 258 i6 / 4.2l I think was
AMC 150 HP @ 3800 RPM and 240 FtLbs of torque @1800 rpm

Having said that, horsepower and torque are heavily influenced by component selection: heads, camshaft, pistons, compression ratio, engine bore (both about 3.75 inches wide) crankshaft (stroke) air induction, intake manifold, valve seat work, ignition system and about a dozen other factors.

I think a well-planned AMC 258 i6 / 4.2l build is able to "smoke" a mediocre AMC 304 build.

I'd spec the design for a the straight 6, AMC 258 i6 / 4.2l because it consumes less space under the hood than the AMC 304 V8; actually, I take that back. I don't know that for a fact. I do know for a fact I can reach all the spark plugs on my AMC 258 i6 / 4.2l easier than I can a v8 AMC 304 , and it takes two fewer.

Finally, I think most Jeep owners "recreate" in conditions where they want the most torque at the lowest RPMs.

-Jon

 

[Torxhead]

Other than what you get from the numbers that have been pointed out, I think having the extra two pistons makes for a smother drive especially making your way thru the rocks.

 

[mtnwhlr]

I think if you are going to do an engine swap I would go with the 4.0. It is the same size as a AMC 258 i6 / 4.2l and you get fuel injection. I think the 4.0 was the best engine Jeep used. And when people build a stroker they are using a AMC 258 i6 / 4.2l crank in a 4.0. So they are building a 4.0 stroker not a AMC 258 i6 / 4.2l stroker.

 

[ColoradoCJ7]

I have owned both and I would say the difference is night and day.

I love the AMC 304 (I have owned 2) and always felt the AMC 258 i6 / 4.2l was under powered (have owned 4).

Fuel injection is awesome when added on any of the motors mentioned AMC 258 i6 / 4.2l /AMC 304 or the newer 4.0l.

If you want more from a AMC 304 you can do carb/cam upgrades cheaply.

Just my two cents!

 

[Incommando]

Why limit yourself to a AMC 304 ? the 360 is dimensionally the same as a AMC 304 and the best years gets you 245 hp/365#'s of tq that even the largest stroked I6's have trouble making. That stock 360 would also run on regular fuel not the high test required of a stroker. The 360 also lacks many of the overheating and other longevity issues the owners of stroked I6's can get. The cost of the 360 swap would be a small fraction of the cost of stroking an I6 and retrofitting F.I..

When planning my build I actually bought a complete 4.0 drivetrain. However careful and realistic research showed me that for my needs a 360 swap was better. Sure I give up F.I. but the cost & increased power even at low RPM's more than make up for that. And the 360 bolted right in with zero issues and none of the expense and electrical nightmares that often pop up retrofitting F.I. engines into older vehicles.

If you want more from the 360 you can do carb/cam upgrades cheaply.

Some purist have stated they want to build a AMC 304 because "everyone builds a 360." Well unless someone is crawling into your engine compartment to look at the block casting no one would know that your AMC 304 was actually a 360. God knows hundreds of jeepers have been cheated by buying "360" CJ's that were really AMC 304 's.

 

[poppatello]

Why limit yourself to a AMC 304 ?

I'm going to take a wild guess and say because he already has the AMC 304 .

Remember that the year also determines what HP and torque you have coming from the engine. Early 70 AMC 304 's had more HP and torque then the latter half. It isn't much but every point counts.

Yes you can build a AMC 258 i6 / 4.2l or 4.0 stroker that will smoke a STOCK V8 (AMC 304 /360) any day. However, it's very expensive to do so. I can't say what the difference is going from a AMC 258 i6 / 4.2l to a AMC 304 or even 360 would be. I only have the AMC 150 in my Cj. I can say when I first got the jeep it flew like the wind. Now it's slower then a snail.

I also can't say how much a V8 rebuild would cost but can say I see a lot of threads where many are doing the rebuilds themselves. You save half or more on doing that.

It would be cheaper to just find someone that has already done this and needs money quick. They are out there and have zero or little miles on them. I purchase my rebuilt AMC 304 for 300 bucks! The PO stated it had less then 35,000 on it but I pulled the intake and oil pan and it looks like it has less then that.

I only went with the AMC 304 because it was the cheapest I could find. The only real issue is I have no idea what the rebuild consist of as the PO is the 2nd owner on the rebuild. Was the cam upgraded so fourth.Tired 360's in my area are going for 650 and up. Would I have gone with a 360? Cant say. I know I like to do things not many are doing. There is a huge price risk in doing that though.

I've heard many say that for the weight your not gaining much with a AMC 304 so your better off with a 360. I don't quite agree. Yes a 360 is more but a AMC 304 is more then a AMC 258 i6 / 4.2l . I've also heard folks running AMC 304 's saying they get much better gas mileage then those with 360's. So each engine has it's pros and cons over each other.

I would rebuild the AMC 304 . You already have it. Add a mild cam and any other small mods you can afford and I am almost positive you will be happy with the swap. There are other things to think about though. This is where I am now. Headers? Carb? Distributor? Ect. Good luck...

 

[1986cj7]

Thanks for all the opinions and options mentioned. Yes since I already have the AMC 304 is why I'm thinking about rebuilding it. One question. It is mentioned that the early 70's can make more hp-torque, where do I find the #'s on the block and heads of my AMC 304 to see what year it is? And any more info on the price of rebuilding a AMC 304 with mods, and AMC 258 i6 / 4.2l with mods aka 4.0 head and such? Thanks in advace.

 

[poppatello]

Thanks for all the opinions and options mentioned. Yes since I already have the AMC 304 is why I'm thinking about rebuilding it. One question. It is mentioned that the early 70's can make more hp-torque, where do I find the #'s on the block and heads of my AMC 304 to see what year it is? And any more info on the price of rebuilding a AMC 304 with mods, and AMC 258 i6 / 4.2l with mods aka 4.0 head and such? Thanks in advace.

If you still have the orignal valve covers there should be a tag on the front of the passenger side of the valve cover.

Look on page two of my post here. Half way down you will see a nice up close picture of my AMC 304 with the tag on the valve cover. It's painted over in the picture but I can read it on the stand.

http://www.jeep-cj.com/forums/f2/well-getting-304-a-14349/index2.html

I don't know how to decode it but that info tells you @least the year it was made.

I found my post on the JF site. This should help you a bit more.

http://www.jeepforum.com/forum/f8/304-engine-info-1389168/

 

[Incommando]

It is not that the later AMC 304 's can't be built to the exact same performance as the early ones it is just that the factory ratings differ. This is due mainly to tuning, bolt on
parts, and things that are changed in the rebuild anyway.

Remember that the biggest drop in factory power ratings had nothing to do with the engines. A new standardized rating of net results vs gross results is the culprit. People who say "well the 343 out does the 360" because of the factory ratings fail to realize that under the same rating criteria that statement would not be true.

 

[Old Dog]

V8's just sound so much sweater than 6's.

 

[1986cj7]

Incommando, what is the difference in torque #'s and hp between the RV cam and a mild cam, and what is the difference in the rpms at which these #'s kick in? And over all highway drive ability. Reason being is I drive about 50/50 highway and trail. I also have 4.10 gears in the axles.

 

[Incommando]

Those specs vary between the cam grind and manufacturer that you choose to use.

A pretty popular AMC V8 grind as used in jeeps is found on the Summit Racing 8600. Summit Racing Camshafts - SummitRacing.com

I chose a Comp Cam: COMP Cams 10-214-5 - COMP Cams Xtreme Energy Camshafts - Overview - SummitRacing.com It's main power band is 1,200-5,200 and I don't imagine being over 5,200 much.

Cams are discussed in this thread:
http://www.jeep-cj.com/forums/f2/camshaft-recommendations-14589/

Derf here made this chart: Grimm Jeeper

If you want some hands-on research you can go there and plug in your tranny, Transfer Case , axle gears, and tire size. Then go down and look at the various rpm's at a given speed and find what you actually hit in your usage . Now if you are leaving the above components alone you can then go to the website of the various cam manufacturers and pic the cam that meets your needs.

 

[poppatello]

Those specs vary between the cam grind and manufacturer that you choose to use.

A pretty popular AMC V8 grind as used in jeeps is found on the Summit Racing 8600. Summit Racing Camshafts - SummitRacing.com

I chose a Comp Cam: COMP Cams 10-214-5 - COMP Cams Xtreme Energy Camshafts - Overview - SummitRacing.com It's main power band is 1,200-5,200 and I don't imagine being over 5,200 much.

Cams are discussed in this thread:
http://www.jeep-cj.com/forums/f2/camshaft-recommendations-14589/

Derf here made this chart: Grimm Jeeper

If you want some hands-on research you can go there and plug in your tranny, Transfer Case , axle gears, and tire size. Then go down and look at the various rpm's at a given speed and find what you actually hit in your usage . Now if you are leaving the above components alone you can then go to the website of the various cam manufacturers and pic the cam that meets your needs.

Now that is some of the best info I have ever seen...

 

[1986cj7]

So I belive what I'm hearing is you can build a mod AMC 304 cheaper then a 4.0 stroker and have better results. I've also seen where most strokers need 91+ octane. This may be a stupid question, but I think it would also be able to put a 360 crank in a AMC 304 or is that going to far and wind up with the same issues a the 4.0 in having to use higher octane fuel. The reason I'm asking so many questions is, beside just rebuildingt a motor I'm not to versted in modifications that can be done to up grade a motor build besides a carb up grade and cam. I have been resherching a AMC 304 mod build on the internet but not sure what to belive as is the case on the internet.

 

[poppatello]

I would be interested in the AMC 304 info myself...

 

[1986cj7]

Here are some sites I've been reading on the AMC 304 stroker using a 390 crank.http://www.amcyclopedia.org/node/2136

 

[Tommy986s]

I installed the AMC 304 with T-150 in my Cj, Love the sound but for everyday driving and long highway runs I would probably opt for the 6 cyl. Gas mileage is a factor also, the V8 will drink more than the il6 as long as mods are comparable.

 

[jasonmark1993]

amc engine casting numbers

Should help you ID what you have.

An RV cam is supposed to give you the best torque for towing and such.

An engine is just an air pump so the old saying "there is no replacement for displacement" is true, everything being equal of course. I dont know much about the AMC 304 cylinder heads, but Im guessing they are small port and small valve. If you are going to bore and run new pistons, Ive read you can run a set of flat tops out of a 290 to raise the compression. The cam Incommando linked or a similar one would work great with the stock heads. Throw a four barrel intake and carb on there and you'd get a huge power and torque increase and still have a great idle.

If you want to get crazy you can port the heads or buy some Indy heads.

There are a couple AMC specific parts vendors. I think I saw the 290 pistons at American Parts Depot. Might give them a call.

 

[1986cj7]

amc engine casting numbers

Should help you ID what you have.

An RV cam is supposed to give you the best torque for towing and such.

An engine is just an air pump so the old saying "there is no replacement for displacement" is true, everything being equal of course. I don't know much about the AMC 304 cylinder heads, but I'm guessing they are small port and small valve. If you are going to bore and run new pistons, Ive read you can run a set of flat tops out of a 290 to raise the compression. The cam Incommando linked or a similar one would work great with the stock heads. Throw a four barrel intake and carb on there and you'd get a huge power and torque increase and still have a great idle.

If you want to get crazy you can port the heads or buy some Indy heads.

There are a couple AMC specific parts vendors. I think I saw the 290 pistons at American Parts Depot. Might give them a call.

Thanks for the link. I am thinking about porting the heads and valve job depending on cost. This is the big decision that I'm going to have to make, getting the most out of the motor weather it be the AMC 258 i6 / 4.2l stroker or the AMC 304 build or stroking it without breaking the bank. Also I do not have the original tag on the heads for the AMC 304 is there a set of # stamped into the heads them selfs? The only # on the block of the AMC 304 I could find were on there drivers side back by the fly wheel housing, are these the #'s I need or do I need to look Else where? Thanks in advance.