I have read your efforts with a large GM car.
I gather a lot from what you have done. I found it very interesting about how you ended up keeping the 4.10 rear end gears...
Reading between the lines I figure your apox. 50MPG was done by a lot of engine off coasting....as you made a bunch of changes to allow that.
My clams of 35MPG in a 2000 Mercury Grand Marques was made with the engine fully engaged at all times. Driving on cruse control at 65MPH.
I plan on doing that with my Van. It just is too much bother and even technically illegal within most states as have lost of control of your car.
Other than that I find the lack of engine changes interesting, this is where I am putting major design changes first:
I have searched a dozen times for what I plan on building.
I cannot find any one is doing what I am going for:
A low rpm 383. My plan is for this motor to spend most of its life running at 1700 to 2200 RPMs ranges and operate from 500 to 4000. Everyone builds a Hot Rod version. i find it interesting that the Chevy 400 and the Olds 403 both had such torque curves, peeking at 2000RPMs.
I am trying to build as much torque as possible using a mild the 90 Caddy Cam, and plan on advancing it 4 degrees. I am much more interested in LOW RPM Torque than house power, Torque is better at low RPMs than HP which is made at higher gas using RPMs.
I have read in a number of books that for best highway performance and mileage you want everything geared to be running at as near the torque peek and your cruising speed, + or – 10 to 20%. So if my engine’s peek is 2000 RPMS I want my cruse speed to be within 200 to 400 of that RPM which on the low end would be from 1600 to 1800 and on the high end be 2200 to 2400. Every V8 I have tested gets it best MPG running under 2000 RPMs.
The combo I want is: a 400 crank machined for a 350. Stock 400 rods so I can keep my KB “D” shaped pistons. I am aware of the side loading on the bore, but as I do not plan on running over 5000 max, this is not a major problem. (This WAS custom 350, doing the 383 was too costly at the time.)
Cam will be a 1990 Caaddy Chevy stock cam which stock torque peek is 2000 to 2200. Lifters are stock 350 roller lifter set up.
Heads are 1970 early swirl port 192s, and other that major cleanup and 3 angled seats, springs tested and Perfect Circle Street Running (Oiling) valve seals, all rest left stock. I only plan of light grinding to ports to gasket match.
I will also run stock ratio roller tip rocker arms to lower the scuffing of the valves in the guilds.
I plan on feeding this with a 85 to 90s Camaro/Corvette TPI running MAF controls. I plan on having the stock Highway Mode turned on until I can get a special add on board to the PCM which allow greater tuning of both stock type tuning and fine tuning of the lean burn cruse settings, I will be able to super tune all operating systems.
Everything is built counter wise to common Hot Rodding standards. Stock heads, no opening the ports, no polishing them, (Everything I have read says the textured surfaces of stock ports HELP low RPM power, polished and opened (larger) ports are for HIGH RPM air flow and HARM lower RPM power) same for the heads. Low RPM cam that will not shift to higher RPM power curves, (the way most cams make more power, move the power curve up the RPM range)
Lastly as the Van has 3:43 rear end gears which helps move it off the line, but also runs in higher RPMs as highway speeds, I am thinking that a second overdrive added to a stock 4 speed auto with a OD gear can work and in fact greatly increase a stock car/van’s MPG, just by lowing the cruse RPMS from around 2600 RPMs to 1700 RPMs.
This seems to be the best of all worlds, stock gearing until the added Over Drive is engaged and the advantage of super highway gearing.
Plus under some conditions like upgrades I might find the act of being able to “Gear Spiting” as in using say the vans third gear and switching in the second over drive to get a good RPM about ½ of third and fourth.
As I have a couple of 50s B7W overdrives these will most likely only cost from a couple of hundreds to $500.00 low enough that there is a chance I will be able to pay off the cost in a couple of years thanks to the savings in fuel costs.
I did a lot of research and testing into MPG.
I tried HHO, special tuners called interrupt between the PCM and its sensors trying to fool the car to use less fuel. And a few other odd MPG devices.
The only thing I found that worked on my 2000 Mercury GM was leaning the A/F ratio to 16:1, on that car on the highway I was able to get 35MPG @ 65MPH, this was fully engaged motor running on cruse control full time.
But as I could not make the switch to a lean burn and back to normal it had a major loss of power. It was not a thing I could use daily.
The 80s Camaros TPI cars had such a set up. It was called at that time a Lean Burn Cruse setting, now it is called Highway Mode, what is did/does, was/is under light load, light throttle it automatically leaned the A/F mix, and with nearly any change in load and/or throttle smoothly switch out of Highway Mode to normal A/F mixes giving back full power.
This mode took a 20/25 MPG car to 30/35 MPG, just that little switch hidden inside the PCM.
And I did NOT make this up, I first learned about this on a site called Third Gen.org decades ago, here is a fast search showing some data about tuning highway mode as proof:
https://www.thirdgen.org/forums/diy-...servation.html
And a cure for NOX with this mode is to increase the EGR. This cools the chamber, lowing the NOX, allows for burning left over fuel normally take care by the Cats, displaces some of the incoming charge, and can help cut pumping loses by forcing the throttle to be opened more lowing the drag of engine vacuum.
There is also recover of engine oil vapors and gas tank gas vapors.
Now we come to the big questions. Will all of this work??
I will find out this spring.
Rich
Quote:
Originally Posted by funkhoss
I did some pretty extensive research on high-geared GM rear ends when I was working on my wagon. At one point, my wagon had a 2.14 rear, .73 overdrive (NV3500 transmission) and the factory 350 motor. It would do 55 MPH at just under 1000 RPM. At the time, I was using a heavy, 4.3 V6 flywheel, which helped with low RPM torque, but yes--the engine pulled that speed/RPM just fine. In fact, I would often cruise at 45 MPH and ~800 RPM.
The highest ratio that is available for the 8.5" 10-bolt in your G20 is 2.41. The 7.5" 10-bolt (which is what I put into my wagon) was available with 2.29 and 2.14 ratios, but that might not be heavy duty enough for your van, depending on how much you haul. (Astro vans had that rear, though, so they're tougher than you think. I don't know if an Astro van would swap into your G20 or not. They have the same wheel bolt pattern, but the differential is off-center and it may be a bit narrower.)
The only other option is a fairly rare 8.75" GM rear, with 12 bolts on the ring gear and 10 bolts on the cover, that came in some cars in the late 70's and early 80's. There was a 2.28 gearset for it, but that gearset only came in Cadillacs from 1977-1979. You could weld spring mounts onto one of those Cadillac rears and bolt it into your van, as it has the same wheel bolt pattern (5x5"), but it would be narrower than your factory rear and the axle shafts and outer bearings would be smaller/lighter duty. If you wanted a rear that was roughly the same width and the same strength (same shafts and outer bearings) as the factory rear in your van, you would need to find one of these 8.75" rears from a late 70's/early 80's B-body station wagon and swap in the 2.28" gearset from a 77-79 Cadillac. I seriously considered doing just that with my wagon, before I fully embraced P&G with EOC and gave up on extremely high gearing.
I hope this helps! Let me know if you have other questions about high-geared GM rears. Unfortunately I know very little about Ford rear ends. 
-Funkhoss |