figuring compression help needed.

Small Block Mopar Engine

  1. jeff schwarz

    jeff schwarz Well-Known Member

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    Can any of you guys help me figure my compression ratio with my combo?
    I have a 360 that is 20 over bore, the pistons are speed-pro h116cp, and they are .042 in the hole, the crank is 3.58 stroke, the heads are 68cc, and the gasket is .038 compressed, and thank you for the help.
    P.S. I called federal mogul 3 different times and got 3 different ratios.
    Thanks Jeff.
     
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    • flyfish

      flyfish C8H18+N2O = :-D

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      Compression Calculator

      This says 9.8:1....but that's assuming flat tops with no relief. This can be answer #4...can we get a 5? lol
       
      Last edited: Dec 6, 2019
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      • JBurch

        JBurch Well-Known Member

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        • Lefty71

          Lefty71 Well-Known Member

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          This is the best info i have ever found;

          Dynamic CR

          A bit of a tough read at first, but well worth it!
          ***EDIT*** I guess I should have mentioned. Once you figure your static compression, you'll be ready for bench racing. You'll need the cam specs to get into the math of dynamic compression before you buy anything for your build!
          HTH, Lefty71
           
          Last edited: Dec 7, 2019
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          • Max1196

            Max1196 Well-Known Member

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            9.22 ASSuming a gasket bore diameter of 4.12, What is your gasket diameter size?
             
          • RustyRatRod

            RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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            We have a great HOW TO section that has a detailed How To about measuring your compression yourself. You'll not find any online calculators that will be as accurate as doing the actual work yourself, plus, you get to learn something at the same time.
             
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            • jeff schwarz

              jeff schwarz Well-Known Member

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              my gasket bore diameter is 4.180, fel-pro 1008
               
            • dusterbing the peace

              dusterbing the peace FABO pyrite member

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              Another result of 9.22 for static compression.

              I used the mathulator here:
              Engine Compression Ratio (CR) Calculator

              Note that these calcs assume you're at standard conditions-sea level on a nice day. Post your cam specs and altitude and we can help with your dynamic compression.
               
            • jeff schwarz

              jeff schwarz Well-Known Member

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              cam specs are 268/272 duration 450/455 lift 228/231 @ 50 installed @ 108, altitude 700'
               
            • dusterbing the peace

              dusterbing the peace FABO pyrite member

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              Looks like your dynamic compression (with that little bit of altitude) is 8.44. You're set up quite nice for pump premiyummm and that cam oughtta be real nice on the street and have a nice lope with that 108. With an air gap and a little bit of converter (just an off the shelf, nothing custom) you should have fun...dunno what it's going in. You have gears/tire combo in mind?
               
            • jeff schwarz

              jeff schwarz Well-Known Member

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              All I have is the engine right now, I build it with all new mopar performance parts, kind of old school, took me 2 years to find it all, getting very hard to find, built it as outlined in mopars hustle stuff speed secrets for a 13.5 second car, I would like to put it in a 1974 dart sport, I would use a built 727 trans. with a low gear set and a 2500 stall converter, a 8.25 rear end with 3.55 gears and a sure grip, I think it would make a good street/strip car.
               
            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              P4452761 is a 110LSA hydro, so
              At 9.22 and in at 108,
              Static compression ratio of 9.22:1.
              Ica of 62*, 700ft

              Effective stroke is 2.84 inches.
              Your dynamic compression ratio is 7.52:1 .
              Your dynamic cranking pressure is 147.10 PSI.......................... 147
              V/P (Volume to Pressure Index) is 127..................................... 127
              But being a DC cam the the closing ramp is gonna be really long, and the Wallace calculator cannot correct for that.
              Here is what it might actually look like; I added 4* to the Ica, and changed nothing else
              Static compression ratio of 9.22:1.
              Ica of 66*, 700ft

              Effective stroke is 2.74 inches.
              Your dynamic compression ratio is 7.29:1 .
              Your dynamic cranking pressure is 141.07 PSI.......................... 141
              V/P (Volume to Pressure Index) is 118 ..................................... 118
              This is still not 100% accurate, just a guess.
              Those numbers mean the bottom end, below 3000/3500 is gonna feel like an old 2bbl smogger teen from say 1973
              So IMO, this combo does not have near enough compression to combo with a 2500TC and 3.55s........ on the street
               
            • jeff schwarz

              jeff schwarz Well-Known Member

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              But with a low gear set in the trans 1st and 2nd gear would really be 3.84 to 1 ratio 3rd would be 3.55 to 1 does that help?.
               
            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              I also get 9.22Scr allowing 5cc on the piston tops/ 9.8 without.
              Are you talking the A999 withe 2.74-1.54-100?

              In this case the 3.55x2.74 is a starter of 9.73 so that is fine, if the rest of the combo was up for it. Lets look into it;
              With a VP of just 118, this pretty stinking low; and the 2500TC is gonna keep the engine in the marshmallow zone for quite a while
              That combo of yours has the potential to make about 320 hp at 5200, but the very low pressure is gonna steal about 5 or 6 % so that brings the 320 down to maybe 300 even.It also has the potential to make about 360 ftlbs at 3700,maybe more, but again the pressure is not there so I might expect 330. And while these numbers don't look that bad, the problem is the rapid fall off below the torque peak due to the low pressure. Like I said, the power down there is gonna be about like an all stock low-compression LA318 which comes in at about 138psi/ 120 VP.
              >So, say your engine is capable at WOT to deliver 240ftlbs at your stall of 2500 rpm; best guess.
              By the time this gets to the rear axle, and with NOT subtracting for gear-train loss, it has been multiplied to 2330 ftlbs plus whatever TM is in the TC. So at zero mph this could be as much as 4000, so the tires will spin, you don't even need full throttle. But as soon as they do, the TM will fall back to say 2600... at WOT. So if the tires are already spinning, then this is enough to sustain the spin still at WOT. But now with the tires spinning, and the speed increasing the rpm begins to rise, and you can rev it to whatever you want, and yur having fun, right. Ok so then the tires hook and you run outta gear and have to shift .That 268*cam will power peak at about 5200, and the drop-off with a 110 cam means you have at least 300/400 more rpm on the plateau, so call the shift rpm 5600, and this occurs at ~44 mph.
              At the shift the rpm will drop to 3150,and guess what happens next. Your Tm is now maybe 280 ftlbs x 1.54x3.55x1.05 =1607 into the rear axles,much too low to continue spinning, and actually much too low to do much of anything..... so yur run is over.
              280 ftlbs at 3150 is 168hp. I may have underestimated your specific torque number but it hardly matters because to continue accelerating briskly you would need about 2000 to 2200ftlbs back there,not 1607, and that would be 360ftlbs at the crank. Now yur combo has the potential to do that, just not in it's current configuration
              But lets put that all aside cuz not everyone cares about the zero to 60 ET.
              >Let's see what happens at 32 mph Kickdown..
              So say you are just cruising around in second at 32=2200 at zero slip, sounds great. Hammer time; you floor it, and the trans goes into 2.74 low gear and the rpm jumps up to 4100, so that's looking good, you are about 400 rpm past the torque peak which I estimated to be 330@3700, but your torque curve is so flat lets say it's still 330; which at 4100,is actually almost 260 hp. Ok so 330x2.74x3.55x 1.05 in the TC comes to 3370ftlbs, which is plenty to have fun with.
              Maybe I spoke too soon. Oh wait, we're back in first gear,lol. None the less, at 32 mph, you are having fun.
              > how about hiway passing?
              ok you get stuck behind grandpa doing 58 in a 65. Down goes the pedal, up comes the rpm, to 4150, you find ~260 hp, and? Yeah not very impressive, but you manage to scoot by.
              Ok so we're hitting 2 outta 4 now. Take off is fine, KD at 32 is fine, second gear at WOT is sortof a bust, and passing is ho-hum.
              > How about Third gear? ok yur gonna take second to 5800 @81mph. On the shift the Rs drop to 3750, and that's right on the Torque peak, great but the power might only be 235hp, so you ain't going anywhere very fast. The estimated power peak of 5200comes at 112, but you'll never get there with this combo. Expect possibly 100mph@3500pounds@4600, maybe that's too generous.
              > how about fuel mileage?
              Well 65=2870 at zero slip, which is doable.The power stroke is extracting for 112* by the spec, so hey the cam is not your enemy. But with a VP possibly as low as 118, and pressure possibly lower than 141psi,this engine is not very efficient
              > how about starting?
              Yeah that will work ok, but it will never spring to life; expect it to start sorta like an old 318. I suggest a mini starter; it's less embarrassing.

              OK so let's review;
              Take off is fine, KD at 32 is fine, second gear at WOT is sortof a bust, and passing is ho-hum.
              Fuel economy sucks, starts ok
              Forget third gear
              Well that is altogether better than I expected.
              Should burn the crapiest gas you can imagine
              Looking back, I mighta overestimated the 300hp
              But there's no way I would screw it together for myself like that.
               
            • Lefty71

              Lefty71 Well-Known Member

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            • RustyRatRod

              RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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              Jeff, you need to realize all these numbers thrown at you and the ones you are providing are useless UNLESS you have measured and verified them YOURSELF. It's a very simple process to do and will eliminate any guesswork whatsoever. Chrysler had a WIDE tolerance for machine work, so deck heights, chamber volumes and the like will probably not be as specified. You must measure them for an accurate outcome, otherwise you have no idea exactly where you are. I don't care how many forum gurus tell you that you have "X compression ratio" unless they've just come from your place measuring everything, they haven't a clue.
               
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              • Lefty71

                Lefty71 Well-Known Member

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                On the other hand, he has said he already owns most everything, so you gotta assume he's going to bolt it together. I'm with you tho, for me the learning myself is a strong part of the hobby (i'm not a mechanic, nor a machinist). HTH, Lefty71
                 
              • RustyRatRod

                RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                What does "owning most everything" have to do with anything here? Not bein a smaartass, but help me out here. I "own everything" with my slant six build, but I am still measuring everything out for proper final machine work and figuring exact compression, not going by "rated and listed" numbers, but going by "actual measured" numbers. The two are never the same. Ever. I'm not trying to be an ass here. But unless the OP takes graduated, real measurements, he is only guessing at his final outcome.
                 
              • jeff schwarz

                jeff schwarz Well-Known Member

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                The engine is finished, all the specs. I listed at the start of this thread are verified by the machine shop, the low gear set in the trans. that I plan on using is a 727 with 2.74 to 1 1st. gear, 1.74 to 1 2nd. gear, and 1 to 1 3rd. gear, AJ would that help?.
                 
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                • jeff schwarz

                  jeff schwarz Well-Known Member

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                  I should also add in my combo I did not use the heads that mopar listed, mine are 974 castings that are ported with 1.88 and 1.60 stainless valves, but I did not have them flow tested, no one around here had a flow bench.
                   
                • Lefty71

                  Lefty71 Well-Known Member

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                  Nothing. I think we agree 100% about the way to do a motor.
                   
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