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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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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 9:53am
Originally posted by DaemonForce DaemonForce wrote:

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.

So tentatively.... if I got this head... I'd basically be at 10:1 and I could still run regular unleaded? Sounds like a win win right now... AND a lot cheaper then building the 360 I want to build. A lot less power too of course but good useable driveable power at this point must be much better than what I have now.. which isn't too bad but I know it could be better with this head and a better cam. I really hope the bottom end is good. Of course.. before I even bother... I am going to get a compression check and oil reading on the engine before I even bother doing anything.... because if for some reason the bottom end doesn't check out... then the 258 will be retired... 


Edited by 1982AMCConcord - Mar/25/2017 at 10:31am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amc67rogue Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 11:57am
Originally posted by 1982AMCConcord 1982AMCConcord wrote:



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...

                                                                         You did it again, Overbore should be .030 not .30 .
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Post Options Post Options   Thanks (0) Thanks(0)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 6:29pm
Originally posted by 1982AMCConcord 1982AMCConcord wrote:

Originally posted by DaemonForce DaemonForce wrote:

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.

So tentatively.... if I got this head... I'd basically be at 10:1 and I could still run regular unleaded? Sounds like a win win right now... AND a lot cheaper then building the 360 I want to build. A lot less power too of course but good useable driveable power at this point must be much better than what I have now.. which isn't too bad but I know it could be better with this head and a better cam. I really hope the bottom end is good. Of course.. before I even bother... I am going to get a compression check and oil reading on the engine before I even bother doing anything.... because if for some reason the bottom end doesn't check out... then the 258 will be retired... 
10.1 compression on regular?  itll ping, detonate like crazy..  I dont agree with DaemonForce.  but its your engine... not mine.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2017 at 8:56pm
this is not an area i have any experience in -- but doesn't cam (overlap) effectively lower compression ratio? the volume to volume ratio doesn't change of course but high overlap should lower compression effects. i assume the cam maker could tell you all about this.

if the head is off, why not simply cc it on the bench? if the piston is flat on top (even if it has small valve "eyes") you could calc the cylinder and headgasket volumes and either get very close outright, or at least verify/debunk other possibilities you have. nothing beats measurement.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Mar/28/2017 at 6:50am
Originally posted by tomj tomj wrote:

this is not an area i have any experience in -- but doesn't cam (overlap) effectively lower compression ratio? the volume to volume ratio doesn't change of course but high overlap should lower compression effects. i assume the cam maker could tell you all about this.

if the head is off, why not simply cc it on the bench? if the piston is flat on top (even if it has small valve "eyes") you could calc the cylinder and headgasket volumes and either get very close outright, or at least verify/debunk other possibilities you have. nothing beats measurement.


I totally agree. I wouldn't just slap it on without doing measurements and getting actual clearances but I just wondered if it would even be close to begin with. The guy said he was going to use it as a racing engine and I thought that would mean a really high compression to start with...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Mar/28/2017 at 6:52am
(Mistake post.)

Edited by 1982AMCConcord - Mar/28/2017 at 6:58am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Mar/28/2017 at 6:56am
Originally posted by amc67rogue amc67rogue wrote:

                                                                     You did it again, Overbore should be .030 not .30 .

Opps. Sorry about that. I'm not a machinist and I've only been learning about building these engines over the course of the last few years so I am not totally competent in stating the measurements correctly. Too bad the topic of the thread is locked now....
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