<VV> Fuel Economy

[Brent Covey]

Hi Bob!

I deeply appreciate the compliment-

My experience suggests these numbers are fairly readily obtainable in some circumstances with a broken in stock Corvair. I do not think they would be difficult to obtain in anyone elses car if the cars tuned fairly well and jetted appropriately. Some areas with reformulated fuels will never come close to the mpg figures formerly possible on the older fuels that had greater thermal energy- all my vehicles have lost some of thier original fuel efficiency since the middle 1980's as the fuels have evolved, and can take a strong downturn in areas with very tepid fuel blends, such as SoCal. I have had to revise float settings and jetting in many in cmpensation as well. All my vehicles are subject to annual emissions tests and I have a fair idea of the state of tune year to year. All the figures I mention have been converted from Imperial gallons to US Gallons using the 6/5ths formula. Just the seat of the pants feel is the California Reformulated gas seems to cost about 3 jet sizes worth of power output. Traffic, temperature, grades, altitude, driving habits and cruise speed all have some impact- nobody gets good economy in any vehicle at 75 mph. I keep close track of fuel economy in all my vehicles in a log book and can cite the variances for most circumstances.

Speaking in generalities, Powerglides do seem to get better economy for most people in most driving than sticks- I have converted a few Corvairs from Powerglide to stick or vice versa and without other modification the Powerglide using the original engine in the same car usually has better highway economy by a noticable margin in non 140 cars. They also seem to reach higher top speeds using the same engine. I imagine the main advantages of Powerglide is it has less drag at highway speeds maintaining high clutch pressure than churning the 80W in the manual trans, and it is better at maintaining engine speed in a responsive zone with smaller accelerator pump motions than a stick car requires. Powerglide also tends to moderate driving styles, a thoughtless driver will have his excesses curbed with Powerglide somewhat. Coasting with a closed throttle on a Powerglide doesnt drag fuel thru the idle circuit as much and they have less engine braking by automatically selecting a high gear the instant throttle pressure is reduced. 

140's are a wild card, most 140 Powerglide cars I have owned have had 3839891 cams (the early RN engine, early '65 design) and got deplorable city fuel economy in the 15 mpg zone in town, where 110 Powerglide 3.55 was pretty consistently in the 18-19 mpg range in normal mixed city driving. 140 Powerglide with the manual trans cam has considerably worse economy than any 110 HP or 140/MT combination, the city traffic penalty is circa 20-30%. I get very similar mileage in my 1981 Fleetwood Brougham (4400 lbs, 2.41 axle, 368 V8 140 net HP with V8-6-4) to what I got in most of my 140 Powerglide '65's. That is poor mileage for a Corvair indeed, especially considering the Cadillac is nearly as snappy a car for acceleration. I think the 140 MT distributors have a poorly matched centrifugal advance curve that forces the initial lead to be retarded too far and this may be what comprimises the economy somewhat, relative to the 110HP. Using the 95 HP distributor thats all out at 4200 might improve economy significantly in 140 RB/RM/RN engines with 3839891. The original 140 PG distibutor thats 18 @ 3200 seems a very good match for Powerglide equipped 140 engines using 3839891 however. The late Milton Binon of Ontario CA quoted an overall average 28+ mpg on his Datsun fuel injected '68 140 Powerglide car he drove to the '87 CORSA convention, when I discussed it with him, so theres room for improvement apparently in the world of 140/Powerglide fuel economy....

The 3.27 does not seem to have a noticable effect on economy with the 110 engine with Powerglide- my mileage did not seem to change conspicuously but quietness at high speeds was much improved and in my area, British Columbia, we have very long mountain passes and being able to force a downshift to low in the 50-55 mph zone to get past a semi is a nice advantage over a 3.55 car that upshifts around 48 mph at WOT. Performance in mountain driving seems about par with either axle ratio, both pull 6-7% grades quite well at 55.

My experiences with 140 HP, again in generalities, is they do not return the degree of economy that 110's do, even in similar cars. I have a '66 140-4-3.55 Monza converttible that has had its mileage closely recorded since 1972, that had consistently had returned 22-24 mpg in highway driving at 60-70 mph. This car hasnt been in service since 1990 so given the new oxygenated blend fuels I'd expect the economy to be around the 22 end of that scale now. I have little experience with its economy at more modest speeds like 50-60. I have very consistently had 25-27 mpg out of nearly every 110/Powerglide engine late closed car I have ever owned at 55-65 speeds. It is possible to hit 28 occasionally under very favourable circumstances-no wind, high altitude, 55-58mph, moderate weather-(This is using GM assembled engines, 3839891 cams, 51-53 jets, Powerglide, bias belted or 185-80 radial ply tires and either axle ratio.) I rarely exceed 100 km/h which is 63 mph. Fuels (summertime anyhow) in Canada generally use MMT and not MTBE or ethanol, which may make them more potent than some US reformulated blends. I would not anticipate the economy penalty for a reformulated fuel would be more than 10% in any event.

I'd driven Corvairs exclusively for a 15 year period, had 38 so far, and theres been one in our family garage since 1972 and feel the general results I have seen are readily obtainable for the average person under similar circumstances. My cars all had GM assembled engines, very careful attention to optimum jetting for the altitude (usually somewhere around a 51-53), HV enrichment carbs,  a correctly matched (stock part number, best match of the ones on hand) vacuum advance unit and a stock distibutor. On my 110 engines advancing the timing 5 degrees will produce light ping at any rpm and throttle opening, pretty much, its just behind the point of detonation in most driving, the factory 110HP advance curve is a very perfect match for them. I use stock size tires and wheels at 19/28 pressures, and 5W-30 oil. I have original breaker point ignition systems in all my cars, nothing in my vehicles represents any trick technology. I use 91 Pump Octane fuel in all high compression Corvairs. They will all operate without ping on 87 if initial advance is significantly reduced or the vacuum advance unit hose is disconnected and capped until proper fuel is availible. In rural Canada you can't always count on a premium pump being availible.

Back when I was doing Corvair repairs full time there were certainly lots of cars getting miserable fuel economy, but for very obvious reasons and all could be restored to efficient operation when the defects were corrected. Typical issues are too high a fuel pressure, late ignition timing, leaking needle/seats, wrong distributor, flooding secondaries on 140, corrosion/deposits at the siphon breaker port, oversize tires, bad alignment, dead ignition cables, sticky distributor weights, out of adjustment transmission kickdowns etc. Having an effective vacuum adance seems very crucial to decent highway economy, especially.

In summation, 110 Powerglide seems to be a very efficient combination that is not very sensitive to which axle ratio is used. It appears to enjoy around 15% advantage over 140 HP in round terms for fuel economy. 140 Powerglide seems to get the worst mileage but the late '65-onwards style RN engine with 3839889 and special crank may pick that up somewhat. Powerglides seem to get slightly better mileage on the highway in pretty much every case and a little better in traffic for most people in non-140 HP cars. I would say if a 110/Powerglide gets less than lets say 24-25 mpg in conservative highway driving (~60 mph) once broken in, theres some scope for improvement, and reason to keep looking for some obvious issue. There is every reason to expect mileage to occasionally hit the high twenties zone if everythings in good shape with a 110HP engine and favourable conditions.

I remain mystified about the cause of Mikes very poor economy and am meditating on what might be behind it, I am very certain he can do vastly better, especially considering the care and quality of his project. I'm sure he will improve his economy by about 20-30% when he has tracked down the issue(s). 

I certainly would be interested in hearing what kind of mileage people get with various combinations and it would be worthwhile knowing the normal highway driving speeds and location for some idea of the circumstances that produced these figures.

I always enjoy your comments Bob,
Brent Covey
Vancouver BC

> bias ply tire new Corvair was and a regular 110/Powerglide/3.27 gets around
> 18-20 in town and 26-28 on the highway. 


  All,
  While I highly respect Brent and appreciate his sharing Corvair knowledge with us all, I have to be critical of these gas mileage figures. They are just unrealistically too high!. Chevrolet's own data place mileage figures closer to 20 MPG at 60 mph for a 110 hp MT w/3.27.  A PG would be worse. And none of the car test mags ever got mileage this high (as Brent's). The Corvair was always known for its sweet handling and sporty performance, but as for gas mileage......it was always ranked as poor to moderate by them.

  Regards,
  Bob Helt