The fuel efficient GMC V6
 

Here's something I've been tinkering with for a while. I've been reading that old post from James Cleary and he mentions outstanding fuel economy from an engineering standpoint. Well, as we know, these V6's are anything but fuel-efficient.

Which got me wondering, is it possible to improve the mileage on these V6's? I believe so, by reducing the internal friction that these have. The main advantage we already have is 2 cylinders less than a V8, meaning less friction.

Some ideas I had: (which apply to a 305E)
1. Custom (longer) connecting rods and the use of 4.250" big block Chevy pistons with thin, low tension, piston rings. Modern materials make this possible. This would reduce the massive friction of the thick piston rings and the large piston skirts.
The longer connecting rods are needed because of the higher wrist pin location.

Does anything know what length the stock rods are, and what the deck height is?

2. Another advantage is the compression gain, with a flattop or even a slight dome design. Up to 9.5-10:1 static. Maybe even higher, given the rod ratio.

The disadvantage would be to have everything rebalanced.

Of course, the mains and rod diameters are larger on a V6 than a comparable engine, adding to the friction, which is a given. But I have a hunch this (different piston) will improve the efficiency of the engine significantly.

Reducing additional friction could be done by the switch to a roller camshaft.

Another idea is to change the intake manifold, but I'm not sure how good or bad these are. Optimized and equal fuel distribution to all cylinders is the idea.

I'm no engine builder, just a student on the subject, but I'd love to hear your ideas. It is not my idea to write off the base design, just some thoughts to improve on it.

Re: The fuel efficient GMC V6
 

I can't help, but if someone were to design and build a piston that helped with mileage and efficiency, and was a drop-in replacement for ours, I'd buy 12 of them for the two engines I have.

Re: The fuel efficient GMC V6
 

That is a great idea; omit the whole BB chevy piston and longer rod and design a piston with the same weight and wrist pin height, which uses the thinner piston rings, ups the compression by removing the dish, and reduces friction by reducing the large piston skirts...

Also, different diameters could be designed and manufactured for the 351, 401, and 478 engines. This would also do away with the 3 compression ring design which definitely adds to the internal friction.

Re: The fuel efficient GMC V6
 

Stock rod length: 7.17825" C-C;
Piston Deck Height 0.000"
Not for sure on the Piston Dished Volume but I believe it is around 10CC.

Plans for my 305:
To increase the compression ratio I am planning on offset grind the crankshaft to increase the stroke, going from stock journal size to 0.030" undersize. Then mill the head of the piston (will be 0.030" oversize) to get it back to a zero deck height, this will remove most of the dished from the head of the piston.

Also in the works I have been flow testing the heads to improve the flow. Been getting mixed results from this. Still in progress. I will be targeting high swirl and all the flow I can get. Stock head has no swirl and badly shrouded valves.

I had already modified the intake manifold to be equal to the Magnum engine and had installed the Holley 2 barrels carb. I need to schedule time on the chassis dyno, this will show the change from the totally stock dyno reading.

Re: The fuel efficient GMC V6
 

Thanks James. What would the compression ratio be after you've performed the modifications? How much displacement do you gain?

Looking at a picture, indeed, those valves are shrouded pretty bad. Should be better on the larger bore engines, if they have larger chambers?

Custom pistons are expensive, but do you see any gains in my idea? Maybe it is possible to modify a standard piston by machining to up the compression and reduce friction.

Re: The fuel efficient GMC V6
 

After all of the modifications, compression ratio would be around 8.75 (stock is 7.5:1). Displacement should be around 310 ci.

Gains and expenses in your idea, with custom made parts anything possible.

Re: The fuel efficient GMC V6
 

Some where there is a thread that I read where someone turn the rod journals and used connecting rods and I believe the piston too out of a Hemi. The engine was able to turn 5,000 rpm.

I have personal seen at a swap meet where there was a V12 that used a crank trigger and individual coils for each spark plug for the ignition.

Re: The fuel efficient GMC V6
 

Maybe just spend that risky $8000 development cash on gas for a stock 305E. The difference between 12 MPG and 15 MPG is about 5 cents per mile.

A 700r4 and a 3.08 differential is another option.

What would be the reason for reconstructing a 305E instead or installing GM: 5.3 V8 @ 21 MPG highway...or other modern motor. Lots of good used motors out there if you want a restomod style GMC.

Maybe this is just a "what if" post. Not sure. This kind of talk on 6066Club is messing with Mother Nature and Father Time.

(I am a closet V8 guy)

Re: The fuel efficient GMC V6
 

Another possibility is to get a big time sponsorship from a TV show about fixing up rare old pickup trucks with a large parts budget, for your custom pistons with plasma ceramic coated piston tops, anti-friction coatings on the piston skirts, low tension rings, custom grind cam, roller cam followers, roller rocker arms, and today there's no reason NOT to run EFI, except for the cost of the aftermarket system.

Another thing that hurts these 60 year old trucks is they are so heavy. Everything is heavier than the same part on a new truck.

Hope you guys have success in achieving your goal!

Re: The fuel efficient GMC V6
 

Haha! now that's the ticket! Call up ol' Garage Squad or someone and hover over them so they don't just throw crap in there and call it done like a lot of these shows do.

I can't believe out of all our members, that no one is independently wealthy enough to finance something like this. Where's Jay Leno when you need him? :helpsign:

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

Re: The fuel efficient GMC V6
 

I've heard of one guy using a throttle body on a 305 V6 but he only posted one picture.
http://67-72chevytrucks.com/vboard/s...d.php?t=486316 Posts 15 and 17 is about the TBI.

Most EFI is just a glorified TBI unit. This is great when the intake manifold does its job of feeding an even air/gas mixture to all cylinders. But I'm not sure it actually does. Difficult to find out other than running the engine with an O2 sensor in each exhaust outlet to find the A/F ratio of each cylinder.

An air-fuel ratio gauge is definitely invaluable when tuning the carburetor. I'd be interested in your finds!