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Some SBC ideas/questions for people "in the know"

Discussion in 'The Garage' started by big_truxx, Mar 7, 2005.

  1. big_truxx

    big_truxx 1/2 ton status

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    I know a good deal about the SBC. I am self-taught so I do not know it all, yet anyway. :D Anyone know where to get a reasonable 3" stroke crank? Also 6.5" con rods? In case you are wondering I am considering making a 302 with long rods for a possible road racing motor in the future. I know a 383 built off a 350 would be the cheapest way to power being more cubes but a 302 with long rods would be more horses at rpm better suited for sustaining my speed. I just looked @ summit and was annoyed with the new layout. Maybe I just need to get used to it but I had an easier time finding items before. :dunno: Plus I didn't see what I wanted there.
     
  2. sled_dog

    sled_dog 1 ton status

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    The new summit layout blows. Anyway, I've considered this before. If I were to do it this is what I'd do:

    Aftermarket 400 bore block with 350 mains.
    Custom forged 3" stroke crank
    Forged 6" rods(can't say I've heard of anyone running 6.5)
    Forged Pistons
    REALLY good balance
    SFI flexplate and balancer
    ARP everything

    Should rev out to like 9K I'd say, and with less piston speed than a 350 at like 8000. I can't find stupid formula for piston speed, I think its "stroke x RPM / rod length. But I really can't remember.

    Crankshaft would likely have to be custom. I've never seen a 3" stroke crank for sale aftermarket. Call places like Eagle or Lunati I'm sure they will cut one for the right $$.

    Oh by the way my 400 block(bored to 4.165) setup would be an even higher revving 327 ;)
     
  3. beater_k20

    beater_k20 Banned

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    a 3 stroke, with a 6.5" rod would put you at a 1" pin height, which is invading oil ring territory.

    FYI, if you use a 1 pc RMS block, get ahold of a L99 4.3 V8 crank (92-94 Caprice, Buick Roadmaster, looks just like an LT1) and the 5.94" rods out of the same engine, you can then run off the shelf 5.7 pistons (1.56" pin height), while taking advantage of the longer rods. there wont be any noticable distance between the 5.94 and 6" rods. the 5.94 rods are powdered metal, just like the rods in the LT1s and the GM Performance Parts catalog, and are supposedly stronger than the old GM "pink" rods.

    another alternative would be to use a 6.25" rod with a piston designed for a 5.7 with 6" rods (1.26" pin height). this combination would put the top of the piston .010" over the supposed 9" deckheight of the SBC. most production blocks that i have seen are anywhere from 9.20-9.25", so just mill off the extra meat on the deck, and run a standard FelPro head gasket and have a near perfect quench distance. before purchasing any parts for this combination i would have the deck height of your engine checked to make sure you have enough to machine down to the proper height.
     
  4. beater_k20

    beater_k20 Banned

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    a 400 block bored to 4.155", a large journal 327 crank and spacer main beaings (commonly used to destroke a 400 to 377ci) would net a 353ci displacement. this combo would be fairly easy to come up with using faily common parts as well. the 3.25" stroke crank could be found in 68-69 327s, and it isnt rare to find a steel crank from the factory. then use 6.125" rods found at a circle track supplier, and off the shelf 400/6" rod pistons, would also put the piston up .010" in the bore, with a little decking this wouldnt be an issue.

    another thing to remember is that the compression ratio with the shorter stroke will be lower than a longer stroke engine.
     
  5. gravdigr

    gravdigr 1/2 ton status

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    I may be wrong here but don't you want shorter rods for higher rpm? I believe I read somewhere that short rods are better for high horsepower at high RPM due to the lower reciprocating mass where longer rods were better for low end torque due to more leverage on the crank.
     
  6. smalltruckbigcid

    smalltruckbigcid 1/2 ton status

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    Don't overthink the rod ratio formula....the Reher-Morrison pro stock bbc at 500 inches has a ratio of something like 1.48 and it goes 9500rpm all the time.
    Your biggest problem will be getting the heads to flow and have enough velocity built up during the short window of pull on the intake runner from 70 degrees atdc to about 70degrees bbdc, I say short because this is the area of stroke when piston speed is the highest and pulling the hardest on the intake runner. After 70degrees bbdc the piston starts slowing down and the intake charge will start running into the piston top and slowing the incoming charge, thats why the volumetric effiency is almost always less than 100% on NA'ed engines. The faster you get the charge moving and the longer you pull on it the higher your VE ratio will be, in other words if you build momentum in the charge it will keep filling the cylinder even with the piston going back up, which drives the VE up and makes more power. A 3.0 inch stroke engine will exert the most pull (highest piston speed) on the intake for about 1.6 inches of piston travel, the rest of the time the piston is either speeding up (coming away from tdc) or slowing down (approaching bdc).
    Personally I would suggest a 4.155 bore and 3.5 inch stroke with a 6 inch rod, the bigger bore will give good pull (vacuum) and the stroke length lots of time for it to pull on the intake charge, making a good VE ratio with a rod ratio of 1.71.
    The biggest challenge with endurance racing engines isn't the bottom end its the valvetrain...when a nascar engine goes late in the race outside of oiling problems its usually a valvetrain problem (broken spring or valve), so if you want to turn serious RPM's get ready to spend some serious money.
    Good luck and go fast
    George
     
  7. beater_k20

    beater_k20 Banned

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    i think the point was to be different, and this is just your run of the mill 377 with 6" rods.
     
  8. smalltruckbigcid

    smalltruckbigcid 1/2 ton status

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    True but I would not want to see him get screwed 'cuz 6.5 inch rods will be custom made $$$$$, but possibly a set of NASCAR rods, used 6.2 set would work? Which with a 3.0 stroke would mean a pin height of 1.3 approx. A callies crank would run about $1800 plus rods for $400-$500 (used but possibly less if you get lucky) pistons are about $700 for this setup plus bearing,balancers and flywheel and machining you'll have $4000. in the bottom end alone. I know a couple of guys that are running 3.25 stroke drag motors and getting 775hp from the dyno, but those have sheetmetal manifolds and a power curve that is very peaky and a ton of work in the heads. In a road race car those engines would be a huge dog....not enough torque off the corners IMO, but they would run fast on long straightaways....
    George
     
  9. AkMudr

    AkMudr 1/2 ton status

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    I felt lost after the change..almsot typed in www.jegs.com:confused:
     
  10. ntsqd

    ntsqd 1/2 ton status

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    Look up the series of dyno articles Circle Track did on long rods vs. short rods. What came out of that was that rod length had very little to do with peak HP or Torque. What rod length did change was the HP & Torque curves. Short rods made for an engine that was very 'peaky' (Think 2 stroke). Long rods made for very flat curves. Advantage of flat curves is that it's harder to be in the wrong gear. Disadvantage of long rods is that the piston dwells around TDC more so detonation becomes more of a concern.

    I can make no quality claims in either direction, but I thot I saw that Speed-o-Motive.com had a 3" stroke SBC crank.
     

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