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Comp. Ratio w/ worked 4.0 head on .30 over 258?

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1982AMCConcord View Drop Down
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    Posted: Mar/23/2017 at 6:13am
Alright.. long story short... I ran across someone selling with a worked 4.0 head. It's all been machined and the shop quoted a 10.5:1 compression ratio to the guy... but that would be on top of a 4.0 block and not a 258. 

Can someone help me determine what the CR would be with this head on top of my 258?  

I have stock pistons and I don't think I can go that high of CR without eventually blowing a piston out. From my reading... the 4.0 has a flat top piston and I THINK the 258 has a lower compression dish piston. So I thought it MIGHT actually work out and thought I would ask for some insight or opinions about this combo here. 

My 258 is an 1982 model and has been bored .30 over during its last rebuild. 

Thanks.

John 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nodak68 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/24/2017 at 5:36am
John,

About any compression figure they give you is meaningless outside of the context of the engine you are putting the head on. I doubt you would get up to 10.5:1 based on head milling alone. I went into an app like Virtual Dyno and put the 4.0 stock chamber size (57 cc vs 62.5 ish for the 258) on a stock spec 258 with .030" overbore. That exercise resulted in bump of about .4 in compression (from 8.03 up to 8.43) all things being equal. That tells me that to get 10.5:1 via jus milling the head is unlikely.


To be sure you would really need to know what they did to the head, plus all the info about the motor you are putting it on. What pistons you would be running in your 258 (4.0 or 258 pistons have in the ballpark of 15cc dish), how much if any milling of your block, how much overbore (if any). If I assume your head has been milled .010, block decked .025, .030 overbore, .043 head gasket thickness, and a relatively stock style piston with about 15 cc of dish, I come up with a CR of about 9.0:1 using the Virtual Dyno app. With all that said, you really need to know ALL your motor specs and verify where you will end up for the particular combination.

Good Luck, and hope that helps...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Mar/24/2017 at 6:22am
Nodak... unfortunately... I don't really have any definite specs on my engine anymore. I probably did at one time... but it was rebuilt 19 years ago. It only has 30K miles on it since then. It has stock pistons. The only thing I specifically remember about the rebuild when asked what I wanted... I told them that I wanted a better cam. That is all I told them. When it was finished they told me they did a .30 overbore with a RV type cam and added stiffer valve springs. That's all I remember. 

As far as his cylinder head goes... stats are as follows... I just got them this morning.... sorry I didn't get them any sooner...

1996 0630 4.0 head

Port matched to an Offenhauser 4 barrel intake, flow tested, backcut valves, head decked for 10.5:1 compression, valve guides installed @1.710 @90lbs on the seat. .045 milled off head. 

Flowed 235.1 cfm @ .600

Crower spring kit, lifters, retainers and keepers 

Crower camshaft Level 4 hydraulic

https://www.crower.com/amc-199-258-6-cyl-64-83-compu-pro-hyd-cam-278-hdp.html



Edited by 1982AMCConcord - Mar/25/2017 at 9:31am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nodak68 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/24/2017 at 10:58am
I'm assuming you meant .045, not.45 milled off of head, as .45 is a lot (I think it would remove a good portion of the combustion chamber). Of it's .045, I'm putting chamber volume around 50cc at the low end ((highest compression), with stock-ish pistons and a .030 overbore my calcs put you in the 9.5:1 to 9.9:1 range, depending on whether or not your block was decked at all during the rebuild. 9.5 if roughly stock deck height, 9.9 if block milled .025 or thereabouts.

If you want to play with this more, recommend downloading the Virtual Dyno app. It's just a few bucks and allows you to run these calcs very easily.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nodak68 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/24/2017 at 11:54am
I'm assuming you meant .045, not.45 milled off of head, as .45 is a lot (I think it would remove a good portion of the combustion chamber). Of it's .045, I'm putting chamber volume around 50cc at the low end ((highest compression), with stock-ish pistons and a .030 overbore my calcs put you in the 9.5:1 to 9.9:1 range, depending on whether or not your block was decked at all during the rebuild. 9.5 if roughly stock deck height, 9.9 if block milled .025 or thereabouts.

If you want to play with this more, recommend downloading the Virtual Dyno app. It's just a few bucks and allows you to run these calcs very easily.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 6:34am
What year is your 258? Stock compression varied over the years. A stock 4.0L has 8.7:1 compression. 82-86 Jeep and Eagle 258s had 9.2:1 stock, but only the 83 Concord had that compression. Utility Vehicles were classified differently for emissions controls than Passenger Cars -- the Eagle was classified as a UV (before the term SUV was used). Emissions controls either improved or standards changed for 1983, or AMC was granted a waiver due to low volume, so the Concord got the same compression as the Jeeps and Eagles -- but emission controls were likely different.

1982 car 258 was 8.2:1. 79-1981 is listed as 8.1:1, 78 and earlier 8.0:1. So unless you have one of the 82 and later models with 9.2:1 compression you're starting with a lot less. You can get in the ball park by simply subtracting 8.7 from 10.5 (1.8) and adding that to whatever compression your engine should have. So with a 8:1 engine you should have about 9.8:1.

Stiffer springs.... if you don't know what springs were used make sure you run off-road racing oil or some ZDDP additive. Comp springs are way too stiff for regular oil. Crane makes some for the AMC six that is right at the max you can run with regular oil, no additives on a flat tappet cam. Those are just over 300# per inch. Mopar Performance 4.0L springs are just under 300#, but still stiffer than stock. IIRC they are actually stock Magnum V-8 springs.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 9:31am
Originally posted by Nodak68 Nodak68 wrote:

I'm assuming you meant .045, not.45 milled off of head, as .45 is a lot (I think it would remove a good portion of the combustion chamber). Of it's .045, I'm putting chamber volume around 50cc at the low end ((highest compression), with stock-ish pistons and a .030 overbore my calcs put you in the 9.5:1 to 9.9:1 range, depending on whether or not your block was decked at all during the rebuild. 9.5 if roughly stock deck height, 9.9 if block milled .025 or thereabouts.

If you want to play with this more, recommend downloading the Virtual Dyno app. It's just a few bucks and allows you to run these calcs very easily.

Sorry.. misprint there... .045!!!! I fixed it! 

I really don't know if the block was decked... it was supposed to be a basic stock rebuild with an RV cam and a .30 overbore. That is all I remember. If block decking is part of standard rebuild then yes... if not then no...


Edited by 1982AMCConcord - Mar/25/2017 at 9:37am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 9:36am
Originally posted by farna farna wrote:

What year is your 258? Stock compression varied over the years. A stock 4.0L has 8.7:1 compression. 82-86 Jeep and Eagle 258s had 9.2:1 stock, but only the 83 Concord had that compression. Utility Vehicles were classified differently for emissions controls than Passenger Cars -- the Eagle was classified as a UV (before the term SUV was used). Emissions controls either improved or standards changed for 1983, or AMC was granted a waiver due to low volume, so the Concord got the same compression as the Jeeps and Eagles -- but emission controls were likely different.

1982 car 258 was 8.2:1. 79-1981 is listed as 8.1:1, 78 and earlier 8.0:1. So unless you have one of the 82 and later models with 9.2:1 compression you're starting with a lot less. You can get in the ball park by simply subtracting 8.7 from 10.5 (1.8) and adding that to whatever compression your engine should have. So with a 8:1 engine you should have about 9.8:1.

Stiffer springs.... if you don't know what springs were used make sure you run off-road racing oil or some ZDDP additive. Comp springs are way too stiff for regular oil. Crane makes some for the AMC six that is right at the max you can run with regular oil, no additives on a flat tappet cam. Those are just over 300# per inch. Mopar Performance 4.0L springs are just under 300#, but still stiffer than stock. IIRC they are actually stock Magnum V-8 springs.

It's an 1982 258 in a Concord. I run Mystic motor oil. It's got the correct starburst rating on the bottle for flat tappet cams. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 9:39am
So with a compression ratio so close to 10:1... could I even run this iron head on pump gas or would I have to use premium? 

I'm not about to start mixing race fuel into this 6 cylinder! haha.. although that would be cool.. at least for a little while!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaemonForce Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 9:49am
I've been doing it for years dude. 10.2:1 is kinda fair for a small L6 or V8. 10.5:1 is best on mid-gradeand 11:1 is starting to push it to where I consider premium or race fuels. My 258 ran 10.2:1 compression for 6+ years until the timing chain jumped a tooth. I'm still sorting out my 304 but it's in the same ~10.X area.
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