High compression 258s?

[danleym]

Hi all, first post here. First off- I don't have a CJ. But I have a AMC 258 i6 / 4.2l in my 1980 AMC Spirit that I'm in the process of building, and I figured the Jeep guys would have more info since no one really messes with 258s much in cars- seems most guys looking for any performance just drop a 401 in. Also, if it helps establish any Jeep cred, I have an 02 TJ, that has to count for something, right?

On to the question: has anyone here built a high compression AMC 258 i6 / 4.2l ? Upwards of 9:1? I'm wanting to run as high compression I can. Planning on all the right stuff- 4.0 head, Lunati 262 cam, .060 overbore. Gonna get the quench as tight as possible- probably .040. And adjust the dish on the pistons to get my final compression ratio. I'd like my dynamic compression ratio to be around 8.1:1 (higher if I can), and that puts me at around 9.7:1 static. I'm also at 9,000 feet of elevation.

So how high have you guys pushed compression on a AMC 258 i6 / 4.2l ? Even better if it's with a 4.0 head. What are the full build specs (or at least a dynamic compression ratio, too)? What fuel do you run? What altitude are you at?

Thanks!

 

[Kane]

Have you visited the Jeep Strokers forum website. Those guys live and breath modified AMC 258 i6 / 4.2l and 4.0 builds and will be able to give you good solid factual answers on what you want to achieve. I am sure a few people here have built their motors but that site will definitely have the right people to help you.

https://www.jeepstrokers.com/forum/

 

[gert]

welcome to the site, i have not personally built a high compression AMC 258 i6 / 4.2l motor but with the proper parts and planning air/fuel ratio im sure you will be fine, one note of great importance tho is to check your valve to piston clearance and to make sure your rocker wipes across the tip of the valve in the center before final assembly and dont run it lean when built, i would be greatly interested to see what you do

 

[Petescj]

Another good source:

Bulltear Pro-Parts Forum

 

[Patrick]

What would be the expected power increase? Sorry to answer a q with another q.

 

[gert]

What would be the expected power increase? Sorry to answer a q with another q.

I don’t know if he will get much horse power but I bet he will pick up some good torque


Sent from my iPhone using Tapatalk

 

[Patrick]

Hmmm, interesting. The much desired torque.

 

[danleym]

Thanks all!

I have been on the strokers website- and there's good info there, but mostly involving only a 4.0 block. With a AMC 258 i6 / 4.2l block and a 4.0 head, the diameter of the combustion chamber is slightly larger than the cylinder. Very similar, and I'm sure what works for the one is fine with the other, but I figure that lip could cause detonation earlier than you would find in a straight 4.0 block/head.

Hard to say on power gains. Not a lot of concrete data on high compression/big cam 258s. As a rule of thumb I've heard maybe 3% gain between 9:1 and 10:1- with no other changes. More than that, there just seem to be gains to be had across the board if going with a high lift/long duration cam, and corresponding improved intake and exhaust flow. Sounds like when done right they all build on each other. I wish I had the money to build an engine at 9:1 and dyno it, then swap the pistons to bump it to 10:1 and dyno that. Something tells me I won't talk Engine Masters into doing that episode though...

 

[Patrick]

Let me share some thoughts here. Mind you, I am not an engine builder, but I managed to successfully rebuild a GM 350 with mild to moderate specs.

We know that the compression ratio can easily be raised with one or a combination of these: overboring, stroking, and piston selection. Can’t this be done with a stock AMC 258 i6 / 4.2l ? I understand that the compression chamber and quench area might not be ideal for this goal, but, doesn’t the aftermarket make the right head? How about decking the head and using a thinner gasket? In my opinion, if these engines don’t respond favorably to these tricks, then you are probably better off considering reducing compression ratio and a turbo.

Here’s a couple cheap tricks for a better performing engine. Use gapless rings. They do reduce blow by and will attain more vacuum. Round off and buff all sharp edges on pistons and buff the flat cast surface on top. This will raise detonation temperature point.

 

[Breidhaupt]

Decking the block and reducing piston to deck clearance will help with bumping the compression ratio. Most AMC 258 i6 / 4.2l pistons sit .120 below the deck.

A good compression ratio calculator will help you figure out how far to deck the block to get the ratio you want. Most 4.0l heads are in the 60cc range, and AMC 258 i6 / 4.2l pistons are in the +16 cc range.