Re: The fuel efficient GMC V6
 

Hmm yeah makes sense.... I don't have a $8000 budget either... But an overdrive is a good idea. If you can achieve 1600 RPMs at cruising speeds you're taking advantage of the engine.

Hey a guy needs something to fantasize about...

Re: The fuel efficient GMC V6
 

Don't reckon I'd make a good presenter :helpsign:

Re: The fuel efficient GMC V6
 

My all original '66 1000 with a 305E got 15.5 Highway right after I "super tuned" it.

I was pretty satisfied with that.

Re: The fuel efficient GMC V6
 

Hello Bill,

If I may ask, what do you mean with a super tune?
Also, what gear ratio did the rear axle have?

Re: The fuel efficient GMC V6
 

Plugs, Points, Condensor, Wires, Air Filter, Cleaned and Re-adjusted the Carb to the highest vacuum, set the Timing a few degrees advanced. Also ran a straight-thru Muffler for less back pressure.

The Rear was stock, original. I never really checked on it, but they were probably 3.73's.

Re: The fuel efficient GMC V6
 

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.

Re: The fuel efficient GMC V6
 

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.

Re: The fuel efficient GMC V6
 

I don't know what your budget is but the Ranger box might be what you're looking for. It fits between the bell housing and transmission so all it does is handle engine torque. It is available in either under or overdrive however they are about nonexistant at the moment.

The biggest downside is that you have to move the SM420 back so there must still be room for the shifter, and the transfer case lever in the more rearward position.

Another option is finding the GMC V6 to Chevy automatic parts and using a TH700R4 or TH2004R behind it.

Re: The fuel efficient GMC V6
 

Great idea the Ranger box. However, that will probably mean I'll have to give up the 3rd seat as the shifter is set back towards the bench.

Automatic is an idea. Not my first choice though.

Finding overdrive here in the EU is much more difficult. First I need to drive my truck some, just passed the first inspection :teehee:

Re: The fuel efficient GMC V6
 

When I still had the 305D in my 62 1500 with 318 three speed and .456 rear gears I got 15 mpg on trips averaging 60 mph, couldn’t hardly believe it but it did. Now I have a 401 magnum with a 650 Edelbrock EVS2 and a 319OD trans and am getting 12 on trips. Pipes are pretty black and I think it’s a little rich. I put a bung in the exhaust and acquired a air fuel ratio device ,but have to wait till spring and the salt is off the roads to try. Has anyone tried this approach or tried fuel injection. Dave