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CNG info on older systems

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  • CNG info on older systems

    More CNG info.
    Comparing two older system conversions.
    Chevrolet 350 (tired engine) -- 105 lbs compression, 45 psi running compression (volumetric efficiency) engine running snap throttle 100 psi. Engine vacuum 15.5 inches at idle. Timing called for 6 degrees base, 14 degrees centrifugal and 16 degrees vacuum for total timing of 36 degrees at 2800 rpm.
    To drop HC to the lowest reading -- advanced base timing to 17 degrees. recurved distributor centrifugal advance to 18 degrees and used an adjustable vacuum advance & set to 14 degrees. Total advance 49 degrees on CNG for best performance and lowest HC on 5 gas.

    Chevrolet 350 -- 120 lbs compression, 50 psi engine running, engine running snap throttle 115 psi. Engine vacuum 16.5 inches at idle. Timing called for 8 degrees base, 15 degrees centrifugal and 16 degrees vacuum -- 39 degrees total advance at 3200 rpm.
    To drop HC to lowest reading -- advanced base to 15 degrees. recurved distributor for 20 degrees centrifugal, and stayed at 16 degrees vacuum --- total timing 51 degrees at 2500 rpm.

    The first engine still has a lack of power on cng, the second you can not tell the difference except WOT on a steep hill.
    It appears to me that the higher compression does a much better job using the fuel for power and economy. The timing advance curve needed to be changed on both for optimum performance, but the higher compression engine needed a little more advance.
    Those of you who have recurved a distributor know that grinding the weights and changing the springs is a SWAG anyhow, but I think I got them pretty close to their peak. One vacuum advance had to be changed to an adjustable (Mr Gasket) --- the other seemed to be just fine. I have also found that the longer the rod and the longer the stroke the less timing advance because of the rod angle -- you know the theory!
    For those who have worked on this stuff -- I have not done any readings to see how the engine temperature affects the quench area for emissions. What is the optimum operating temperature for an older engine on CNG? I am guessing we should use 195 degree t-stat but I have not experimented with it at all.
    Many of the vehicles needing CNG conversions in our area are older farmers work trucks. The gas prices are killing them & they are needing help.
    I am trying to share information I think someone could use, I would appreciate any recipocation from others who have been there & done that.
    Stan M.