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MG MGB Technical - Gas Mileage and advance keys

When I built the 77 mgb roadster 9 years ago, I used a 3* advance key, with 60 over forged at 9.3 CR, Peter Burgess FRBV head, Doug Jackson rollers, and a 285 Piper cam. The car sings at 70-80 mph ( 3700-4200 rpms) freeway and always brings a smile. Now with the broadband AFR showing cruising at 14-16 a/f, I am logging a bit over 28 MPG with OD and all synthetics including the trans: Royal Purple/Redline. I am aware of all the yada yada re OD and gas mileage, overtuned and gas mileage, but wonder now if I didn't stumble onto a recipe back when.Petal down at 80 and the power is impressive with the speedo up to 108 on a flat and straight before I quit.
Seems to me the BP head, 285 Piper ( idles around 900 without a perceptible lope), and larger volume have come together in very efficient play. Anybody else have a similar story, or better gas mileage with other recipes? Nowdays 28 mpg on 87 RO is ok w/me. vem
vem myers

This is all in the archives. If you get your car to flow more air you can either go faster or more economically. It's called volumetric efficiency. When you are off the cam it will be less efficient.
Stan Best

Stan, is "on the cam" defined as the being close to the torque peak?

Vem, sounds pretty sweet.

Charley
C R Huff

It does not need to be at mid revs you can feel when the car starts to get some scavenge effect and with a fast road cam as opposed to a race one it will be way below peak torque. If you drive a cammed car you will feel when it happens, the car just comes alive and takes off and the exhaust note crisps up, everything we like about these cars :-) . However you have instinctivley homed in on an interesting point. Everthing else fixed peak torque is where the engine is breathing best, and so you will get best specific fuel consumption. Not lowest ourright, they are different.
Stan Best

'69 B, stock cam advanced 3º, ported head, +0.040", lightened flywheel, stock exh manifold with Falcon big-bore exhaust, stock SUs, all emissions gear fitted & functioning, O/D. 32 mpg at highway speed. I dynoed the car shortly after the engine was broken in and got about 77 rear-wheel hp @ 5000 rpm and 95.8 lb-ft @ 3300 rpm. Car pulls strong, revs freely, and is a joy to drive!
Rob Edwards

Stan,

Thanks, I hadn't thought about the difference between specific fuel consumption and outright fuel consumption.

My GT and my Sprite have both been re-cammed, but they are mild being a Delta KB for the GT and an APT VP-7 for the Sprite. As such, they don't really seem to me to have the rush of coming onto the cam. I think the only thing I drove that had that feeling was a Honda S2000.

One of these days I need to get around to putting the new spindles and bushings in the carbs of the GT so that I can tune it properly and see what kind of fuel mileage I get.

Charley
C R Huff

Not the car to mention in the MG pages but, the Hyundai Tiburon has this lovely coming on cam feeling. At about 3500 to 4500 it just opens up and sings. For a 2.7 v 6 it is rather thirsty, but already this has been improved with a free flowing exhast system. I will get my GT to sing as well! Mike
J.M. Doust

Lowest total fuel consumption would be at idle! ;-)
Rob Edwards

"Lowest total fuel consumption would be at idle"

Yeah, but negative mpg ...
PaulH Solihull

Specific fuel consumption is a commercial vehicle thing, ton miles per gallon I think. If you drive slower you use less fuel absolutley which is the way most car users think of it which is reasonable. However as Paul notes being at a standstill with the car idling has a similar effect on MPG as getting a duck on your batting average.
Stan Best

Stan,

Gold, silver, or bronze;)

L.C.
Larry C '69 Midget

Hey Rob- A few months ago I had the pleasure of ferrying up to dinner in the 77 MGB, none other than the Guru Grandaddy, Mr. John Twist. During our drive and ramble I mentioned I'd gotten 31 mpg on the car all highway ( didn't mention it was coming done from Donner Summit California USA at 7227 feet). Mr Twist proclaimed Bull Sh*te Sir. John felt anything above 25 was miraculous and approaching 30 was a wet dream pun intended. So, Robert, can you share your apparent mileage record with us a bit more. I'm riveted as I've only seen 30 plus in a midget. Did you flow the head? Estimated ACR? Gas RON? Special tricks or notions? Cheers, Vic
vem myers

I can achieve between 25 and 28 mpg on an average trip. This is with a .040" overbore, 9 to 1 compression, a Kent 280 cam, a head that I ported and polished, a stock exhaust manifold, a Falcon Bog Bore exhaust, a custom DUI ignition system and a Moss supercharger system breathing through a SU HIF44 carb. It has great power and if I keep my foot out of the blower, I can get decent mileage. RAY
rjm RAY

Did not have the head flowed but it was given a fairly typical porting job by me under the tutelage of the crew chief of the local "B-Stingers" race team. I forgot to mention that I had the distributor recurved by Jeff Schlemmer and it has a pertronix installed. I'm running about 9:1 compression and with the timing set where it is, I must run 91 octane -- that's R+M/2, not RON. I have a 195º tstat installed which helps economy & power, and I've removed the oil cooler as well.

I also typically drive with an eye towards economy when just driving to get somewhere (as opposed to driving for the fun of it! ;-) No sudden stops or starts, trying to time traffic lights so as to maintain momentum, and I try to feather off the throttle as I drive to maintain minimum throttle opening for a given speed. Kind of hard to explain, but it involves gently lifting off until the speed starts falling of barely perceptively, then adding just the tiniest amount of throttle back in to maintain speed. I've been driving this way long enough that it's second nature and I no longer think about doing it.

I've read that driving technique can change your fuel economy by 30%!

It's sometimes a little difficult to get a consistent fill when fueling, but I've seen 30-32 mpg often enough to believe that that's really the figure and not an artifact of inconsistent fueling.
Rob Edwards

I do a few long trips with the supercharged B and regularly get 34MPG "imperial" this works out to about 28.7 US MPG. Which is the same as when the car was stock. I must admit to being a bit of a sleeper on the road. Denis
Denis4

I have one of Moss's vacuum/boost gauges. If I monitor the position of the vacuum side, I can easily increase my mileage. In the '60s, many cars came with "economy" gauges which were no more than vacuum gauges. RAY
rjm RAY

Well, I guess Papa John may have to rethink his think. I carefuly measured mileage on a recent trip and the math said 30.8 miles per gallon. Turns out it didn't make any difference if I ran 87 or 91 octane, which surprised me. Ray's vac gauge is a good idea for best of best mileage. So, on the MGB, the best measured gas mileag is Rob Edwards at 32 miles to the gallon. VEM
vem myers

Hi,

On tours I get around 36 mpg, stockmotor, with 60 thou oversize pistons.
Paul Hunt gets better than this with his V8.

Herb
Herb Adler

Increasing the octane rating, leaving everything else unchanged will not improve your mpg. Using higher octane means that you can have your ignition timing further advanced without pinking... this ought to give you improved mpg.
Geoff E

Using Imperial gallons rather than American gallons always helps! ;-)
Mike

Not quite, 34 on the V8, with 40+ on the roadster. Barely get 30 on the ZS180, until last months snow when I kept the speed down to around 60 for about 100 miles. Refilling the tank after about 240 miles with the nest at my usual speeds got 33, its best ever. Dread to think what I would get if I drove at 60 all the time.
PaulH Solihull

This thread was discussed between 10/12/2010 and 12/01/2011

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