73-87 Chevy _ GMC Trucks > Engine/Drivetrain

SM465 Mannerisms

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79 K20, 350, SM465.  Hey all, I haven't driven the SM465 very much.  Seems like there is an enormous jump from 1st to 2nd gear.  I run it up to 3500-4000 RPMs and it sounds like I'm on the backstretch at Daytona.  Then shift to second and it sounds like antique diesel tractor.  As long as I give it fairly light throttle after the 1-2 shift, it sounds normal and the RPMs slowly creep back up.  But if I give it much throttle - say half or more, it starts knocking.  Is this normal for the SM465?  Is the knocking sound detonation? 

It's a school bus transmission. Most people don't use 1st gear (Low) and there is no way for us to know what noise you are hearing without hearing it. If you are lugging the engine, it sure is possible.

What gear ratio does your truck have, and how big are your tires? If you're starting out in 1st with the SM465 which few folks do as VileZambonie pointed out, that suggests to me you're running 37's or larger.

Sorry...when I say first gear, I mean first gear not the low gear.  I am not sure what gears I have.  I have 35" tires.  I drove it some more last night and it helps quite a bit to rev the engine RPMs up some while letting out the clutch going into second gear.  Keeps it from lugging/knocking/pinging as bad. 

Based upon the info you've given it seems your tires are too large for your gears. Assuming the axles haven't already been changed, the tallest gears offered in a 1979 K20 are the 3.73:1, which should actually handle 35's without all of the drama as you are experiencing. But there are no guarantees on 40+ year old trucks. Gears such 3:40 or taller should only be found in 1/2 ton 6-lug axles and not 8-lug 3/4 tons, but those are the sort of gears that match your symptoms best.

I wonder now if all the the knocking might be an indication of either bad timing or more serious internal problems. I'd make sure your engine was in a proper state of tune first and continue diagnostics from there.


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