connecting rod length

Discussion in 'PSI Superchargers Tech Questions' started by Slayer, Jan 21, 2005.

  1. Slayer

    Slayer Member

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    I am building a 400 in hemi for IHRA top dragster and quick eights. The rules allow only an 8-71 high helix blower thus the small cubes. I currently have a TFX 96 standard deck with a 4.28 bore. With a 3.5 stroke what rod length should I use or should I go to a .5 short deck block? If so, what rod length? Thanks
     
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  2. Randy G.

    Randy G. Top Alcohol

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    You need to find someone to physically take a look at what you've got and help you. Pistons/pin location and diameter, valve and deck clearance, compression ratio, head gasket thickness among other things need to be considered besides just your deck height. No one can tell you on an internet message board what rod legnth you need without a lot of information. Then you should pour the whole thing to make sure you've accomplished what you're after.

    Good luck.
     
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  3. Slayer

    Slayer Member

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    Thanks Randy. My current motor has a 4.5 stroke with 11.9 compression using a .090 head gasket. I could just change crankshafts to the shorter stroke and make up the difference in the rod length but is a .750 longer rod too long? Too short of a rod side loads the piston. What about too long of a rod if there is such a thing? By too long I mean with piston to head and valve clearences corrected. Is it better to run a long rod in the taller block or a shorter rod in a .500 shorter block?
     
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  4. nitrohawk

    nitrohawk New Member

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    I and some other people are running rods in the range of 7.450 to 7.470. with the 3.5 stroke. However this is with fat heads. The best way to get correct rod length I have found is to cut an old rod in half and put a
    3/8th rod between the two halves with a left hand thread in one half and right hane in the other.
    Put the rod in the motor with a piston and adjust the length with your head gsk and head on the block until you get the desired cc's at top dead ctr. that will give you the compression you want.
    Tack weld a 3/8 nut in the middle and it can be adjusted in the motor. The other simpler way is to call your piston manf. and they can give you the rod length needed for the compression you want using their piston.
     
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  5. Randy G.

    Randy G. Top Alcohol

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    There are a lot of theories on correct rod ratios, but I've seen really good runs in our class by guys who run tall or short deck blocks.

    I would be more concerned about getting the rod length correct and running a thinner head gasket than a .090 (like .040 to .070 range) with the clearance and compression ratio you're looking for. If you have access to a machine shop, you could whittle up an adjustable length rod out of an old rod and play with it until you are completely satisfied. That would simplify the math a bit for you. I wouldn't assume that your estimate of a .750 long rod is going to work.
     
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  6. Alkydrag

    Alkydrag Sr. Dragster

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    The problem with running a rod to stroke ratio that is more than say 1.8:1 is that when ratios are higher than this, the dwell time of the piston at TDC is so long that it can actually hinder cylinder head flow, ie, air movement into the cylinder stops, creating reversion in the intake manifold. The byproduct of this is high boost numbers. Remember, boost is what's in the manifold, not the cylinder. Now you have an unhappy motor. Your combination creates a problem in itself. You have no choice but to have a rod length of 7.000" or longer with a .500" short deck. My suggestion is to make a rod length of 7.015", compression height of 1.455" this would make for a very safe piston and use the short deck block. This is assuming your using a fathead. If your using a bigger chambered head like a Veney or Dart, then that changes things drastically. You can run the fathead at zero deck, but with the other heads, they are always out of the deck at TDC. Sorry if this drags on, but I do this for a living Monday thru Friday.
     
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