Difference between Light Duty and Heavy Duty 350?

[i8tokyo]

I've never really ran across this before, what is the difference between a 350 Light Duty vs. Heavy Duty? I show an LS9 engine code, but when you go to look up parts they give you an LD or HD choice and there are differences such as the plugs.. (Example: Ext Tip or not.) I haven't found any way to tell which motor I have at this point besides just knowing it's a 350. Any insight on this?

[75chev4x4]

Where are you looking for parts at? Is it 2 bolt 4 bolt mains it is meaning?

[i8tokyo]

Originally Posted by 75chev4x4

Where are you looking for parts at? Is it 2 bolt 4 bolt mains it is meaning?

Oddly enough, the spark plugs is where this came up. The HD plugs have the electrode sitting low into the plug where the light duty has the extended electrode. I've always just ran AC R45TS in my other 350 truck's. This one had Denso plugs in it that don't show to fit it in the first place. Just curious as to what the difference in 350's are.

[C-10 simplex]

Do you have the actual engine numbers?

[ray_mcavoy]

Is this for the 78 C-20 in your signature? If so, the light & heavy duty classification is for emissions purposes.

Starting around 1975 or so trucks over a certain GVW limit were considered "heavy duty" and didn't have to meet all of the same emissions standards as "light duty" trucks and passenger cars. That meant no catalytic converters, no EGR, and such for trucks in the "heavy duty" category.

For 1978, I believe the cutoff point was 6001 lbs GVW & higher except for California which was 8501 lbs & higher. So your C-20 is most likely in the heavy duty category.

[i8tokyo]

That would make sense. I was wondering if there was a head difference so I pulled the valve cover off to check the casting numbers. I guess this is the original motor, it has the 462624 heads (aka as crack-o-matics). So far no issues my way, but this does ramp up my efforts to get another enigine on the stand for next summer. Thank you all for sharing your thoughts. I was originally wondering why they would use the non-extended plugs on an HD motor. With 4.10's in it, 3,000 rpm on the hwy is the norm. I think it would have made more sense to have extended tip plugs for that application. But I just work on them, not design them.

Originally Posted by ray_mcavoy

Is this for the 78 C-20 in your signature? If so, the light & heavy duty classification is for emissions purposes.

Starting around 1975 or so trucks over a certain GVW limit were considered "heavy duty" and didn't have to meet all of the same emissions standards as "light duty" trucks and passenger cars. That meant no catalytic converters, no EGR, and such for trucks in the "heavy duty" category.

For 1978, I believe the cutoff point was 6001 lbs GVW & higher except for California which was 8501 lbs & higher. So your C-20 is most likely in the heavy duty category.

[Keith Seymore]

Compression ratio - HD had lower compression ratio for durability.

K

[str8axle55]

^^^ That is what I was thinking also, would explain the diff in plugs I would think.