milling heads for comp ratio increase

Discussion in 'Small Block Mopar Engine' started by 70Hardtop, Apr 21, 2018.

  1. 70Hardtop

    70Hardtop Well-Known Member

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    Hi, I am sure this has been talked about before, can anyone guide me to where on this forum if possible? I can't seem to locate the information.

    The question is about how much to mill the head surfaces and inlet (intake) manifold for, say, one point extra CR. Is there a rule for LA engines that says, if you mill the heads this much, you will get this much CR increase? Also, if I mill the heads say 25 thou, is it also 25 thou off each intake surface? Or something different? My engine (273) is approx 8.5:1. I want to go to about 9.5:1. No other option is available for me at this time.

    My question is, what is the rule for this? What amount taken off the heads of an LA engine (in my case, a 273) will produce approx 1.0 point increase of compression? And what is corresponding amount required to take off the intake surfaces?

    Thanks!!
     
    Last edited: Apr 21, 2018
  2. famous bob

    famous bob mopar misfit

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    I don't know the answer , but they are diff. !
     
  3. toolmanmike

    toolmanmike FABO Staff Staff Member FABO Gold Member

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    You can, I did but just because I could. My heads were different part numbers (one was not original) and the porting and head machining didn't cost me much so I went for it. You should cc the heads and figure the compression ratio you have. I don't have an idea how much of a mill gets you "X" increase in compression. I would imagine it also depends on how big the chambers are. Compression is important and you don't want it too low. The wrong cam can bleed off compression and you end up in the 7's and no bottom end power. If you mill .030 off the head surface you will want to take .028 off the intake surface of the head. Here's just one thread here on the subject. I used the search feature and typed "273 milling" to get a list. **Milling .030 off '920 - 273 heads**
     
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    • 69_340_GTS

      69_340_GTS Well-Known Member

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      You always want to mill combustion chamber and intake surfaces of the heads. That way you don't wind up with a "custom" intake manifold that only fits one motor. Rule for SB Mopars is you take 95% of whatever you take off of the chamber surface off of the intake surface. For example, .030 off chamber surface and .0285 off intake surface.
       
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      • RustyRatRod

        RustyRatRod Just another dumbass. FABO Gold Member Technical Editor

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        That's a lot of money to spend for just a 3% power increase........IF you're lucky. Seems counter intuitive to me. I would just leave it alone.
         
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        • 67Dart273

          67Dart273 Well-Known Member

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          Let's say you have a set of heads that "need work" anyway.....Need a valve job, maybe a couple of valves, etc, plus the added expense of surfacing...........how close does that get to a new set of EQ heads?
           
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          • j par

            j par Well-hung Member

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            A good running 318 for $150 off Craigslist? If you weren't in Australia....
             
          • 70Hardtop

            70Hardtop Well-Known Member

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            Thanks for informative reply Mike. My 273 is all stock so it has the original CR of around 9:1 But because I have to use 318 head gaskets, which are slightly larger bore and so will reduce the CR to about 8.8. It is a 67 block but has the later 1968-71 318 heads which are open chamber 60cc. Valves: 1.78 / 1.5"

            It also had a hydraulic cam whereas the 1967 273 engines were supposed to be mechanical (and with 57cc chambers). I don't think it had ever been pulled down so it may have been an early example from the factory with the later fitments. However I am fitting a small solid cam (0.47 lift) and the 273 mechanical adjustable rockers etc to bring it back to what a 1967 273 should be. Also it is getting a Carter AFB (4 barrel) and M1 intake

            But I want to increase the CR to over 9:1, around 9.5+ would be good. Not too much as it doesn't have the hardened exhaust seats. But I will run an additive for that.

            I could put in stainless flat head valves which would give that increased CR (over the stock tulip/dished style valves) but the cost is too much. Buying the valves plus getting the seats cut for the 1.88 /1.6 valves. Best option is to mill the heads.

            It's an easy question to find the answer for, I will keep looking through the internet or call some engine shops. Just thought someone might know off the top of their head. If there was some basic rule of thumb about it.

            However I have found a little useful info which agrees with your post TMM. From an article by Paul Pitcher on how to port LA heads: "For any open chamber head, the chamber volume is reduced about 0.2cc for every 0.001" planed off the deck surface of the head. If more than 0.010" is removed from the deck surface, then the intake surface will need to be milled to allow the intake manifold to fit. Mill 0.0095" from each intake surface for every 0.010" milled from the deck surface of each head."

            So that has answered most of the question. I just need to to find the formula to work out CR from combustion chamber volume and maybe about 15 thou off the heads might be enough.
             
            Last edited: Apr 21, 2018
          • 70Hardtop

            70Hardtop Well-Known Member

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            TMM, I posted a reply but now I just read most of that thread you gave, very interesting and just what I wanted! Thanks again. I have all of the info I was wanting and can make a good decision.
             
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            • 70Hardtop

              70Hardtop Well-Known Member

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              Thanks - yes I just found that out, what you just said, in the post that TMM sent me. Very good info, much better idea to shave the heads both surfaces than a brand new M1 intake! And the 95% rule is what I was needing also
               
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              • 70Hardtop

                70Hardtop Well-Known Member

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                Well I have heard of 5-8% power increases with a one point CR increase. And it is not that expensive - it is actually the cheapest option I have. Around $120-$150 per head (Aust dollars). Compared to the other things you are thinking of - different pistons (plus then needs a rebore) $1000+. Different new cam: means new lifters also $600. New stainless valves as I already mentioned, means getting the seats recut...$650++...Getting the heads ported - big $$$ in Australia in my city $350 per head. So you can see that milling the heads is the best option - since I feel that a 9.5:1 CR will better suit the cam and 4BBL and M1 intake and headers I am using. It is only a budget rebuild, not wanting huge gains. I feel I should get a rating of 250-270 HP easily.

                The bottom end is already built. I had a small solid cam lying around for years so that was no cost. I had the solid lifters refaced years ago - Heads have had four valve guides done, that's all it needed plus those seats recut to suit. Then a good lap in and all valves seat and seal well.

                Thanks
                 
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                • 70Hardtop

                  70Hardtop Well-Known Member

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                  It's almost fair comment ...but only on the surface. First, I looked up EQ heads and I can only see them as a Magnum head. Meaning I would need to change to Magnum rocker gear . And even though they are cast in Australia, I can't see them anywhere here . I would need to get them from the States. They are iron heads so with freight and rocker assy I am looking at over US$1500 ! Are you serious? And all heads need to be checked over and fixed up - even big dollar units. That's more $$. Reading my own posts, I am doing a budget rebuild. Heads were fine, valves were all good except two, but found a couple of 318 ones that scrubbed up. Fixed four valve guides with K liners. Got them faced and total cost with the lapping job from a friend at $20 per hour was approx $300. I will also port match the heads to the intake myself - no charge $. If I get them milled it will be $260-300. So all up, around $600 max (US$470).
                   
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                  • 70Hardtop

                    70Hardtop Well-Known Member

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                    A good running 318 in Aust could be picked up for about $500. But most likely in another state. Our country is the same size as yours, and most stuff is over East. I am West coast. Shipping for that hunk of metal would run to about $300. Plus all the associated worries that go along with it. Is it as advertised? - lots of shonky crooks over here! How much extra $ would I have to spend on it? IMO the 273 is a sweeter and better engine than the 318 which we get here - a pissy 190 HP and cast crank. My 1967 273 I bought for $200 complete and running. I had a small solid cam and the adj rocker assy in my parts collection. It had a great forged crank that only needed a linish. It has light full floating rods and bores so good they cleaned up with a hone. And now mechanical cam 273's are getting very rare down under. It was no brainer - of course I want to use this fantastic little engine, not a common 318.
                     
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                    • d1970

                      d1970 TODAY IS A GOOD DAY TO HAVE A GOOD DAY

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                      what people don't know is that you should get the heads squared up with resurfacing equipment first. You can’t just set a head onto a fixture, clamp it down and start cutting. You have to line it up so the surface is parallel with the cutter head fore-and-aft, and side-to-side. Once you’ve done that you can set your depth of cut and proceed with a rough cut or finish cut. Getting heads milled the right way cost alot of money!!!
                       
                    • 69_340_GTS

                      69_340_GTS Well-Known Member

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                      There is an error in the above quote. I corrected it in red.You know what they say about those misplaced decimal points.
                       
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                      • JBurch

                        JBurch Well-Known Member

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                        The surface to volume ratio when milling heads is as follows: on a small chamber 273/318 head, for every .0053 inches milled off the chamber face will reduce the volume 1 CC, for every .010 inch removed from the chamber face, it is recommended to remove .0095 from the intake face. Milling info from "How to Hot Rod Small Block Mopars" by Larry Shepard. Heads can be milled up to .060. Loose rule of thumb is, 10cc removed equals 1 point of compression.

                        Hope this helps
                         
                        Last edited: Apr 21, 2018
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                        • 69_340_GTS

                          69_340_GTS Well-Known Member

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                          Again, somebody is posting/copying incorrect info. Your .095 above should be .0095
                           
                        • JBurch

                          JBurch Well-Known Member

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                          My bad
                           
                        • aaronk785

                          aaronk785 Well-Known Member

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                          Id go .060 and .057. That might get u 9.5 with a .028 gasket. The mopar performance book is full of incorrect info. They say shaving the heads .060 on a later 8.2 440 will raise the compression ratio to 9.8. Not even close I would guess maybe 8.75. Doesn't make much difference if your .125 in the hole. Its a big of joke as picking a cam by lift.
                           
                        • j par

                          j par Well-hung Member

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                          That's why I said if you were here. I bought a 318 last year with the video of it running from a very nice guy that is actually a member here for 50 bucks. I milled some 360 heads to bring back the compression of the 318 heads. But I gained Flo. I milled them 50 thousandths which cost a hundred fifty bucks. Didn't have to mill or touch anything else. It did a 14.4 in the quarter mile with a passenger in my Duster. My wife 66 Barracuda had a 273 with the adjustable rockers. I actually used a set of adjustable rockers from a 273 for my stroker motor. I have Hughes rollers now. My point was more towards displacement vs. expense. Kind of like a 318 would be an easy Improvement all the way around. Or I always think about how a 360 takes up the same space as a 273? How you could completely do every angle at making power with a 273 and Along Comes stock 360 that fits in the same exact spot with more horsepower.
                          I was actually hell-bent on making the fastest 318 ever made, but come to find out it was cheaper to stroke a 360 than a 318.
                          Not arguing what you need/want just food for thought.
                           
                        • Krooser

                          Krooser Reform School Graduate

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                          I had a set of J heads redone at a local shop...new stainless 2.02 valves, 5 angle, teflon seals, angle milled (.080 cut) etc. Cost $718.00.... $3.00 more than a set of 993 Brand X heads for my kids b mod.
                           
                        • 70Hardtop

                          70Hardtop Well-Known Member

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                          Thanks - yes, my error. I just fixed that. Well I did put in the right numbers at first but then went back and removed a zero, must have been seeing double haha.
                           
                        • Krooser

                          Krooser Reform School Graduate

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                          Heck just cut down the 360 crank to fit the 318 block...redneck stroker!
                           
                        • 70Hardtop

                          70Hardtop Well-Known Member

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                          Sure does help - That's great info JB, exactly what I was looking for in a concise paragraph. Along with Tool Man's thread, it's all great. Had to think about these numbers though. I am sure these are for closed chamber heads - you say 'small' chamber, which are '64 to 67 year heads (57cc - there were two different casting numbers in the 273 heads). Then they went to open chamber (60cc) which are what mine are unfortunately. By those numbers, to get mine back to 57cc, would need only about 16 thou milled off (3 x 0.0053). But I think I would need more if mine are open chamber. If mine are around 8.8:1 now and wanting 1.0 extra, I think a full 50 thou would be necessary. I am even considering making my own gaskets to suit, out of sheet copper. A metal place just up the road can supply 1/2mm (0.020") copper for this. Sounds like a rather big job though! But I have new set of hole punches.

                          According to many people, and some here I have read on this forum, these are just junk 318 heads, nothing special. Maybe true, but they're all I have with my budget - I have another 318 and two 340s all needing cash thrown at them also.
                           
                        • j par

                          j par Well-hung Member

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                          Now this is where I'm going to have some trouble making sense? You have two 340's and a 318 and 273 and you want to spend the money on the 273?
                           
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