Undersquare vs. Oversquare Engines

Discussion in 'Mopar Performance Issues' started by ramcharger, Jan 8, 2009.

  1. ramcharger

    ramcharger Dismember

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    This is just a theory as I have not the funds to build a couple of engines just for comparative reasons. But here are two theoretical questions in regards to LA strokers vs. B block big blocks set up with the same or very close to the same displacement:

    1. Let's take a well built LA stroker with a 4.040 bore giving us a hair over 408 ci and a 400 B block bored .040 over giving us a hair under 408 ci. Let's top these engines with a pair of well ported Eddy heads (same amount of time put into both), keep the comp ratio at a sane 10.5:1 and stab in a cam that is identical. Lets say a solid roller cam 250 dur @ .050 and .600 lift ground on a 108 CL. Top it off with a single plane manifold and a 850 cfm carb.

    Which engine will give us the most power and at what rpm? I would think that the B block would far out power the LA stroker due to the larger valves, ports and bore that would unshroud the valves and less friction from side loading due to the longer connecting rod (6.638 vs. 6.123).

    2. Now, lets do nothing but reduce the size of the cam to 230 dur @ .050 and .540 lift but still a solid roller and switch to a dual plane manifold but keep everything else identical. Will the LA stroker provide more torque than the B? Will it be at a lower rpm? What about the area under the curve? Would the B block have a higher HP peak due to the bigger ports, valves, etc?

    Any and all responses are appreciated.
     
  2. crackedback

    crackedback FABO Gold Member FABO Gold Member

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    Stroke is going to be the big difference maker here, LA = 4" B = 3.38"

    I'm guessing, the LA combo will make more torque down low in both instances, the B will make more peaky HP up top.
     
  3. 340sFastback

    340sFastback Well-Known Member

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    And how much lighter is the LA engine going to be compared to the B engine.
     
  4. Petzschler

    Petzschler →Welcome back←

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    niether example would be an oversquare engine.
     
  5. ramcharger

    ramcharger Dismember

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    The 400 B engine is oversquare, the bore is larger than the stroke. The LA stroker is pretty much square in the configuration that I gave. it would be undersquare @ 426 with a 4.125 stroke.
     
  6. ramcharger

    ramcharger Dismember

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    Good question!
     
  7. 72bluNblu

    72bluNblu FABO Gold Member FABO Gold Member

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    360's are around 550 lbs, a 400 is about 620 lbs, so the BB is about 70 pounds heavier. Those #'s are supposedly fully dressed engines with stock parts.
     
  8. MOPEkidD-3

    MOPEkidD-3 Torsional Member

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    Like crackedback said, the B engine will make more HP and have a higher redline, while the LA stroker will have more low-end torque. I think it would be awesome if someone built a stock-stroke 400 with 11.5:1 compression, ported aluminum heads, and a big solid (or solid roller) cam to rev to 7500+ RPM. Wouldn't be too great at the strip, but it would be a MONSTER for road racing. Kind of a poor man's Hemi, but much lighter and a little more efficient at lower RPM.
     
    • Thanks! Thanks! x 1
    • Petzschler

      Petzschler →Welcome back←

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      thanks for the correction. but you said 408" on both so......
      a better comparison would be 408 mopar smallblock vs 408 chevy smallblock.
       
    • ramcharger

      ramcharger Dismember

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      Or even a stroker LA vs. a Ford 400M at exactly the same bore and stroke. Of course that would negate my under/over hypothesis. IIRC 3 or 4 400Ms placed in the top 10 in the last engine masters challenge. One of them won it! I kept it Mopar only as this is a mopar site, lol.
       
    • rumblefish360

      rumblefish360 so close yet so far away

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      Not to mention that it's a big block vs small block question of similar dispalcements. Not a brand M vs C vs F.

      The issues of these engines or diffences are in the hardware and not the general build of speced parts, though they have a play due to the engines configurations.

      Short stroke vs long stroke
      big valve vs small valves
      big bore vs small bore.
       
    • Petzschler

      Petzschler →Welcome back←

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      the most IDEAL engine would be the longest rod combined with a stroke under 3.5" (providing the piston pin dosn't go to far into the rings). you want rod ratio's in the 1.8-2.0:1 range. combine that with a bore larger then 4inches. those types of engines breath deep, and rev high.. the ticket you need for high horsepower and flat torque curves on a naturally asperated engine.
      cheers
       
    • ramcharger

      ramcharger Dismember

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    • moper

      moper FABO Gold Member FABO Gold Member

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      I think the issues are apples to oranges... By limiting heads to one brand and style, and camshaft, and compression.. and you eliminate maximizing each combo with the available parts for them. The stroke is the key in both cases.

      #1....The LA RPMs will feed the 4" crank engine to 6500 with a std CNC program netting 270-280cfm. The suggested cam is smaller in terms or duration at .050 than I'd use. A solid roller would be more like 265-270°@.050 at .600 lift, and with taking out for lash and the nasty pushrod angles, that lift at the valve is closer to .560. Also, carb is on the smallish side. I think performance wise 520-540 at 6300 is close. I built a 416 very similar to that that made 540 with a dual plane and 950 carb, using a smaller solid roller cam. The torque peak will be higher than mine tho. I'd think 470-490tq @ 43-4400rpm would be what I'd expect.
      The B wedge with it's shorter stroke and a CNC head in the 310 area will be undercarbed but not by much. But it will make it's power higher in the rpm band. I think the numbers would be similar, but slightly lower even at peak. I'd be looking for 510-525hp at 6800, with torque in the higher rpms too. I'd be looking for peak torque in the 5K area with a peak around 470 pound feet. To get things better, I'd run a higher static ratio, closer to 11.5 and a single plane intake for this one, plus a carb in the 950 area.
      I have a program that uses a bunch of data points to approximate frictional losses for an engine... Interestingly, the hp it estimates being lost by the 4" is higher, but at 6600, the B wedge has the same hp loss as the 4" LA does at 6K.
       
    • Petzschler

      Petzschler →Welcome back←

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      care to share that program? nice read btw...
       
    • Petzschler

      Petzschler →Welcome back←

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      look at the bore/stroke ~ rod ratios on ANY formula one engine.
      cheers
       
    • dgc333

      dgc333 Well-Known Member

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      FWIW, one of the car rags built to identical BB Chevy 454 motors except for one with a low rod ratio and one with a high ratio just to see what the difference would be. They made almost identical power, as close as any two engines can be.
       
    • moper

      moper FABO Gold Member FABO Gold Member

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      "Most guys take the stroke, the block, and the piston design they want, and simply use the longest rod that connects them." I think that's a line from one of Smokey Yunick's books and it fits. F1 has absolutely zero to do with any drag racing engine, and less with any street engine. They are far beyond anything we'd run, in size and rpm range and as such, the tiniest advantage needs to be exploited by them. Rod ratios for what we do mean very little, but for them can be 10s of horsepower or another 400rpm. (at 19,000+rpm I guess another few hundred would be a big deal...)

      The program is Larry Meaux's Pipemax. It's a cool deal, originally written for designing headers. It assumes a lot for it's calculations for horsepower and torque, but for port dynamics and sizing it is very good. The down side is, nothing is plotted. It's just lines of data based on a single rpm so if you're not well rested, it can be hard to compile what you need...lol.
       
    • Petzschler

      Petzschler →Welcome back←

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      Moper,
      the reference to F1 engines is that they go totally against todays thinking of what the ideal bore/stroke-rod ratios should be. i just thought it was interesting! 750+hp and only 250-300ft-lbs of torque will lay waste to most cars on the 1/4mile lol.
      cheers

      dgc333,
      then they couldn't have used a long enought rod with a shorter piston..otherwise it woulda been significant. you wouldn't happen to have a link would you? or specs on what they did?
      cheers
       
    • FISHBREATH

      FISHBREATH Well-Known Member

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      How much does an F1 car weigh?
       
    • moper

      moper FABO Gold Member FABO Gold Member

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      They are truely amazing. As is a top fuel car. So will a military jet on take off. I still dont see the comparison.

      All of those share almost nothing in common with most cars, including production models. I guess I'm old fashioned in that I like things to stay relevant...lol.
       
    • Petzschler

      Petzschler →Welcome back←

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      less the my ex-girlfriend!
       
    • MOPEkidD-3

      MOPEkidD-3 Torsional Member

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      Well, with a weight of less than 1500 lbs, you don't need more than 300 lb-ft to accelerate to 60 in less than 2 seconds.
       
    • 6pk2goDemon

      6pk2goDemon Mopar Mod Staff Member FABO Gold Member

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      :toothy10: Would love to contribute to this thread, but I have no freakin idea what yall are talking about, but always willing to learn......:-D

      Pretty technical...LOL
       
    • camd64

      camd64 Remember Landy

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      Im learning on this one too. of course it took about six times of reading through it to start getting it all straight.