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Old January 3rd, 2022, 11:46 AM
Prowbar Prowbar is offline
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Join Date: Dec 2020
Location: Friesland, the Netherlands
Truck: 1965 GMC 1500, 478 V6, SM420
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Default Re: The fuel efficient GMC V6

Quote:
Originally Posted by turbobill View Post
This is my take.

The not so great fuel mileage in my opinion is mostly the result of the not so favorable "surface to volume" ratio of these very oversquare engines. Large bore engines tend to have greater surface area at TDC for combustion heat to be lost through. A helpful fix there is the heat barrier coatings.....for the piston heads, combustion chamber surfaces and at least the valve faces. This can be done to the stock pieces.

Next up is compression. The higher the compression ratio, the more the thermal efficiency (everything else being equal). Within the confines of stock parts, head and deck milling along with the thinnest possible head gasket. Other than that, custom or different pistons.

Engine friction.....friction coatings, low tension rings, smaller piston skirts, roller lifters, roller bearing rockers with roller tips, synthetic oil, but most important of all.............reduced engine RPM.

These engines make good low RPM torque so an axle ratio change, taller tires or overdrive transmission would probably be a lot cheaper than some of the custom parts/fabrication/modification required for different rods, Chevy pistons etc. Each ten percent increase in engine speed increases parasitic (friction) losses twenty percent. Cut RPM in half, engine (and anything turning with it) friction is reduced to 25% of it's previous value at twice the speed.
Great post. Coatings are definitely something to look into. High RPM is a killer indeed. One can say that the 3/4 ton trucks with their 4.11 (like mine) or 4.56 gear ratio are much too high for todays use. 3.07 or 3.23's would have been much better suited to fuel economy. I've ran some calculations and right now I'm at 1650 rpm while driving 60 km/h (~37 mph) which should be ideal. With 31 inch tires. A .78 overdrive like gear vendors raises that number to ~48 mph @ 1600 rpm. A 3.54 gear swap (lowest in the Dana 60) about 43 mph @ 1650 rpm.

But these overdrive units are overpriced...

And something like a NV4500 changes the shifter position, which I don't like.
TKO500/600 would be nice, with the multiple shifter locations, but then again much too expensive.

I would like to keep the SM420 and possibly add an aux. overdrive if I can come across one. Maybe a Laycock (which the GV is based off of) as found on Volvo's but I don't think they can handle the torque.
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