How Much Can Heads Improve Compression???

Small Block Mopar Engine

  1. krazykuda

    krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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    How much impact can changing the heads make on compression???

    We're tinkering on the son's 72 Challenger. It's been passed through the family now from my brother, to me, to my brother, and now to my son... It's been in the family since 83 and has just over 99 k original miles on the odometer.

    Waaayyy long time ago, when my brother had it, he had the original 72 318 engine rebuild with 360 heads and 2.02 valves. I'm not sure if the engine has even been started for break-in yet.


    The heads were done when leaded gas was available and didn't have hardened seats in it when my son got it back from my brother. So we pulled the heads and had our head guy install hardened seats and "tune them in" a bit...

    Now we have the engine pulled and are going to assemble it again. We checked the compression using my patented "How to Check Compression" method in the How-To section as here:

    How to Check Compression


    Soooo... When we checked the compression on the 72 318 with 360 heads (I'm not sure if they are stock pistons or replacement, but I think stock), we ended up coming in around 7.6:1 :violent1: :wack:

    We would like to get it up higher so we have more efficiency out of our engine. It's just a street cruiser, nothing radical with lots of cam or compression. We do have the opportunity to put in a rainbow of different cams while we have it on the stand also, so we're looking to dial in the compression and pick a good cam out of our "stock" that I have "acquired"...

    We are going to put into my son's 72 Challenger Ralley, 318, 904, slapstick, dual scoop hood, 3.55 sure-grip 8 3/4" rear axle.


    Here's the car:

    Eng Pull A01 B.jpg

    Here are different stages of the engine pull:

    Eng Pull A07 B.jpg


    Eng Pull A14 B.jpg


    Eng Pull A17 B.jpg


    Here are some shots of the engine when we pulled it:

    Chall Eng A03 B.jpg


    Chall Eng A09 B.jpg

    Here it is on my handy-dandy rolling dolly:

    Chall Eng A12 B.jpg



    Here's the thread on how to make the dolly:

    How I Made My Engine Dolly
     
  2. Krooser

    Krooser Building Chinese Free Engines since 1959...

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    Machine the block to achieve a 'zero' deck and use a thinner head gasket. Unless you have about 9.5 or higher compression ratio leave those aftermarket cams in the box… they won't help.

    You may want to find a pair of 302/308 heads from an 80's 318. Mill 'em down. They have smaller swirl combustion chambers.

    I have a set of 360 J heads that have been angle milled and, with a flat top piston, we figure the compression will be around 12-1 which is just right for the engine we are building.
     
  3. krazykuda

    krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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    I have a "rainbow" of different small block heads on the shelf ready to go from my head guy.

    I have heads for the 340/360 ranging from 68 - 80... (and a few in-between...)

    But I did also find two series of 273/318 heads that look interesting....

    One set is from 68-72 casting and has 1.88" intakes, 1.6" exhaust, and ported.

    then I also found a set of 67 273 castings, also gone through by the head guy with 1.88" intake and 1.6" exhaust valves, and ported...

    Now I don't have a flow bench, or access to one, but I think the 273/318 heads with the porting and 1.88"/1.60" valves will be just fine on the engine, and also be able to get us better compression numbers. We are going to cc the heads in the next week, and I will post the results when I can get them and analyze them. My head guy is pretty good at porting, so I think the "smaller" heads will be close to a stock 360 1.88" head for flow or maybe a little better, so I don't think that I need to run 360 heads with 2.02" intakes - I'll save them for a future 360 build...

    So here are some pictures of "Option A" - 68-72 318 heads, ported, with 1.88" intake and 1.60" exhaust - black heads:

    68-72 SB A01 B.jpg

    68-72 SB A03 B.jpg


    68-72 SB A04 B.jpg


    68-72 SB A07 B.jpg



    Here's "Option B" - 67 273 heads, ported, with 1.88" intakes and 1.60" exhaust valves - "red heads" - gigity, gigity...

    67 SB A01 B.jpg


    67 SB A03 B.jpg


    67 SB A06 B.jpg


    67 SB A09 B.jpg


    67 SB A10 B.jpg
     
  4. krazykuda

    krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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    The point that I'm trying to do here is not to do any more work to the short block, it's been freshly rebuilt many years ago. I'm just going to check the torque on all the crank and rod bolts and try not to disassemble the short block. The short block is fresh and ready to go, I don't want to throw away money on upgrades.

    The gaskets that come in the rebuild kits are .065" thick and a little too much. We also ordered and now have a set of .040" head gaskets, which will take about 4 cc's out for us.

    I want to see if I can get close to 9.0 with just changing the gaskets and using the heads that I have available now.

    The 67 heads look promising with their small combustion chambers...

    We will be picking the cam later after we figure out which heads we are going to use. I do have some "mild" street cams that may be ok. I don't want anything too big if I don't have enough compression to back it up.... I'm thinking around 268° duration range.

    Then I have to mock it up and check the piston to valve clearance....

    Then assemble the engine and drop it in... :burnout:
     
  5. nm9stheham

    nm9stheham Well-Known Member

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    If I have my notes right, you're going to drop about 7-9 cc's going from the option A 675 to the option B 920 heads. (The former have 65-66 cc's typical and the latter in the upper 50's I do believe; anyone else. please chime up if I am wrong. Assumes nothing has been shaved, which may not be true....I would just CC the 2 heads to be sure.)

    Are these pure flat top 318 pistons with no valve reliefs, or 2 or 4 valve reliefs? Any pix of the piston tops? Can you provide the piston top below deck measurement?

    Guessing on pure flat tops with no valve reliefs, and a 1.74 compression height which puts the piston about .080" in the hole, the computed CR changes from 8.3:1 to 9.1 by changing the head chamber volume from 66 to 57 cc (head A to B), with a compressed gasket thickness of .032.

    Valve reliefs will drop those numbers so you need to help us on that....I'd CC those too. (Tilt the engine, use light oil in the burrette, Mazola, whatever...)And a piston deck height.

    The heads look pretty nice to me.
     
  6. mderoy340

    mderoy340 Well-Known Member

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    8920 head should be around 65cc. With a thin head gasket stick a 268 cam in it. A good converter and you will have a very nice street car.
     
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    • RustyRatRod

      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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      Option "C".

      a blower.
       
    • 1970Duster

      1970Duster Well-Known Member

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      Option D

      Twin Turbo
       
    • nm9stheham

      nm9stheham Well-Known Member

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    • toolmanmike

      toolmanmike Moderator Staff Member FABO Gold Member

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      Option B Those are some nice looking heads! I would have chosen 302 castings if it was a option. The 920 chambers are very close the the 302 chambers and the ported runners are as good or better than the 302's. You should raise compression a half point without pistons or a lot of decking. Or you could leave the compression low and do a Rusty Mod or 70 Duster mod! LOL
       
    • Krooser

      Krooser Building Chinese Free Engines since 1959...

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      Those little 273 heads are gems… they will work good but I's see if you can get a little more 'squeeze' out of them.
       
    • cdavis

      cdavis Well-Known Member

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      Wrong answer! I have a magnum 5.2 with 8.86 cr. Roller cam 210/220 @.050, 1.72 roller rockers that give me .538 lift. Ported magnum heads with 1.97 int valves. Engine dynoed 290 at the wheels. 3800 lb Dakota ran 13.27. So a non stock cam will work even with lower compression.
       
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      • 1970-dart

        1970-dart Well-Known Member

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        i should have loaded a set of those heads in my car when i was there lol


        :blob:
         
      • Krooser

        Krooser Building Chinese Free Engines since 1959...

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        I stand by my answer…
         
      • krazykuda

        krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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        Yes, 302's would be another good option, but I don't have any of those, especially finished.

        This is my son's car, and we're trying to get it running and driving. It would be nice to boost it, but that would cost $$$$,and we have other projects going and have to keep the costs under control. (Not to mention that Dyer's Blowers is only 20 minutes from here....)

        We have two garages full of Mopars that don't run right now, and we need to get some running.

        This engine was freshly rebuilt back in the mid 80's and hasn't had any miles since. I don't want to reinvent the wheel here and start from scratch, just try to get it optimized with the parts that I have on hand.

        I'm just trying to see if we can get the compression up to a more reasonable number. This just started out as pulling the 360 heads to get hardened seats installed. Then we decided to check the compression, and it was going to be low with those heads, so we are trying to see if we have something in our pile-o-parts that we can use (and I have a big pile of parts)...

        I have more engines laying around that need to be built for future projects, so I don't want to invest in the cost of supercharging or turbo. The more you modify, the more problems that arise, the easier it breaks.... KISS (Keep It Simple Stupid) is the motto I like to stick with....
         
      • RustyRatRod

        RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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        Karl, you know dang well you caint make an engine run better with better heads. You're startin to sound like that kook guy from Utah. IQ what's his name? lol
         
      • j par

        j par Well-hung Member

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        Option E , leave compression on the low side, will work good on cheap gas. Get running and out of shop. Low compression will work great with a shot of nitrous that can be added later
         
      • krazykuda

        krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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        It can still work on cheap pump gas in the 8's or low 9's for compression. I don't like below 8 if I can help it...

        It's going to be a naturally aspirated 4 bbl for cruising the streets. The bottom end is basically a stock rebuild, so I don't want to put too much stress on it with nitrous or boosting. We want street cruising and durability out of this build...

        The higher compression will make it more efficient. Here is a 12 minute explanation why, but at the end it boils down to efficiency is directly related to compression. Here's the physics and thermodynamics of the otto cycle explained (which is how engines work). Follow along and open your book to page 32:

         
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        • IQ52

          IQ52 Well-Known Member

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          From Idaho.................Idaho!
           
        • rumblefish360

          rumblefish360 I have escaped the evil Empire State! FABO Gold Member

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          Kraykuda; have read enough of your posts to choose between the two available heads for better compression and considering that the rest of the car is a 904/3.55 cruiser, the best way to go IMO is with the 273 heads that should be revisted, checked out, and installed after you cc the head and re calculate the ratio with a possible milling of the heads to get your 9.0-1.

          Flow testing the head and re porting it maybe an option to maximize the combination. The heads port windows and flow rates could possibly be improved over what you have now to allow better performance and effeicency out of the mill. On other 273/318 heads, the port windows have been able to be opened up to 340/360 size with good flowing ports. While the small engine head will not port out to whopping numbers, it will flow better (more) than a stock 340/360 head and maintain excellent velocity.

          If your not going back into the engine, then the head is the way to go.

          I would NOT try to zero deck that engine with the low compression slugs inside there now. The problems that develop with heavily milled heads and heavily deck blocks come into play to cause headaches and havoc. If you need a high compression engine, it starts with proper pistons followed incunjunction with the cylinder heads and then head gaskets.

          I didn't see what came you have available, but I would error on the small side with the above intended drivetrain combo. To be honest with you, the replacement upgrade 340 Purple cam performs really well with the intended combo. New modern cams should provide a better driver and running engine.

          In terms of duration @.050, I would not go past 230 but stay under that number.
           
        • JBurch

          JBurch FABO Gold Member FABO Gold Member

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          Instead of trying to guess what heads to use, first measure where the piston is at TDC, write it down, if the pistons have eyebrows, id the piston volume of the eyebrows is a catalog spec, write it down, get a measure of chamber volume of the heads you have to choose from, write it down.

          Figure out what you want for a compression ratio when you are done, calculate, using the information you have written down what your possibilities are with chamber volumes and gasket thicknesses; then you'll know how much you can incress your compression ratio.
           
        • Ben Drinkin

          Ben Drinkin Hey bartender! FABO Gold Member

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          I say just slap any set on there. Get it running and out of the garage. Move on to the next one. Then work your way back and im0rove on the 'running' cars.
           
        • krazykuda

          krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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          I'd rather build it right the first time. :D

          I don't accomplish anything if I half-assed slap it together and plan to do it again later... I will then have that left on my "to-do" list. :violent1:

          I want to cross it off and not feel like I'm trying to kick water up hill... :banghead:
           
        • krazykuda

          krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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          Ok: here are some of the specs:

          Piston drop is .087" down - flat top I can get pictures later.

          Volume above piston at TDC: 17 cc


          Bore = 3.91 in
          Stroke = 3.31 in

          Swept Volume: 651.29 cc

          "Kit" head gasket volume = 12.4 cc

          360 head volume Casting # 3671587 = 69 cc

          Clearance Volume with 360 heads = 98.4
          Compression with 360 heads = 7.62
           
        • txstang84

          txstang84 Well-Known Member

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          :happy1:
           
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