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MG MGB Technical - Miles per gallon 2

Continuing with the miles per gallon discussion, how much effect does one octane fuel as versus another have on the milage.

I've been running regular (that's typcially 87 octane) and adding a half bottle of lead additive. On the occasional fill up with 93 octane I haven't really noticed a difference in milage. I seem to pretty consistently get 18 around town, and can't say for highway driving as I haven't taken a trip in the car yet.

BH
BH Davis

Octane does not affect fuel mileage. Higher octane fuel burns slightly slower (more controlled) so as to prevent detonation/pinging/pinking. You can keep using the lower octane gas, so long as that doesn't happen. If you advance the timing slightly, you may find a bit more power and efficiency, right up until you start to get pinging/pinking under heavy load, at which point you return the timing to the previous setting.

The 2 biggest factors affecting fuel mileage are state of tune and driving style. My GT should be getting better mileage than it does, as the Peter B head makes it more efficient (along with some other tricks). However, the increased fun factor makes me put my foot down harder, but smiles per gallon is unbeatable!!

Paul K

Can't agree with that. In a high-compression MGB engine at least pinking happens much more with low octane, and the only way to control it is to back off the timing. This reduces performance, cruising economy (i.e. the reverse of advancing the timing until you *get* pinking as you say) and increases running temps as more of the energy released from burning the fuel occurs further down the expansion stroke, hence imparting less forward motion and more waste heat. So for a *given level of pinking* i.e. just short of it lower octane *does* result in worse mileage as well as performance.
Paul Hunt 2

As usual, both sides are right and in perfect agreement, and only pretending to disagree. It's true that simply changing gasolines without advancing the timing is unlikely to improve your mileage. If you've retarded the timing to prevent knocking, then run premium and tune it to spec and you'll see better mileage.

Late engines with dished pistons and an 8:1 compression ratio are almost always okay on regular, and there's no point to running premium on such an engine. Modified or higher compression engines usually get at least some benefit from premium fuel and proper tuning.

Sam Good

BH,

lower compression ratios lent to lower effectivity of an engine. If there are a lot of auxillariers bolted on, (air injection pump, fan, large generator, ac compressor etc.) all this will even lower the output at the flange of the gearbox at a given amount of fuel to be burned and asks for more gas to reach comparabble results, compared with an engine without all this 'helpers'. Ineffective carburation and exhaust layout are further points that effect the mpg readings on 1970 on MGB's exported to the USA.
When i imported my 1975 California B from Alabama, i first took off all this stuff from the engine and fitted another dizzy and SU HS4 carbs as used on the 1965 model year of the car. The result was +/- 9 litres at 100 Kilometers. Converted to mpg it would read +/- 30. Before i changed to this setup the same engine was undrivable at less than 15 Liters a 100 Kilometers, a mpg ratio of less than 18.

If it is legal in your state, do not allow any compromises to the setup of your engine that had been made after 1967 if fuel economy and driving plesure is on your list.

Ralph
Ralph

I have a 1974B with about 121K miles and an O/D. I get about 20MPG around town (I coast a lot) and recently got 29.5 mpg on the freeway for 500 miles. My in town driving is pretty low milage each time so the engine is really hardly heated up.
Bob Ekstrand

I don't know if ANYBODY has done the maths properly but I think that any reduction of efficiency must lead to an increase in overall pollution. If you reduce efficiency then overall pollution per mile MUST increase because you are less miles per gallon and hence polluting more per mile travelled. just my 2 cents worth ( after several beers at 1 am). I wonder if any of the pollution requirements are actually doing any good for the planet at all or are they just a lot of hot air. I had a conversation with some industry engineers from ford at a dinner a few months ago and the upshot of the (off the record) conversation was that overall and taking into account the pollution of actually manufacturing and disposing of the pollution equipment ( catalytic converters etc ) it led to a nett increase in pollution over a straight increase in engine efficiency. Several engineers said it would be a lot more beneficial overall to ditch all pollution specs and go for a mandatory miles per gallon system. Probably not a very politically correct statement, which is why it was off the record.

please excuse the above I have had FAR to much to drink lol
S S Tew

"or are they just a lot of hot air"

I had to smile at that. I know exactly what you mean but the choice of words is ironic. If all our cars produced *was* hot air there couldn't be any claims that they are polluting the atmosphere. Modern cars, whilst running, produce no actual pollution at all, indeed if you run them in polluted atmosphere the exhaust is actually cleaner than the intake. The 'pollution' these days is taken to be the amount of CO2 produced for each kilometer travelled. CO2 is a natural part of the earth's atmosphere and can hardly be classed as a pollutant, and whether or not it is contributing to climate change is simply unproven.

It has been said that the dust-to-dust cost of producing cars like the Toyota and Lexus hybrids far outweighs those of 'conventional' cars even including the Range Rover Sport. Some months back a UK TV program invited members of the public who thought they were 'ecologically sound' to appear on a program. I'm sure the people selected were chosen to show how ridiculous and misguided people are, as one had a power boat he towed with a huge 4X4, and another couple had swapped conventional cars for a Prius for her and a Lexus hybrid for him (which won't move at all under purely electrical power) and were told in no uncertain terms they should have bought 2-tear-old Astra Diesels. But the greatest scorn was reserved for a stupid woman who went on so-called 'ecological' holidays, by long-haul jet!

Paul Hunt 2

Regretably the rigours of engineering analysis evade both the politicians and the electorate. If you want to save the planet the easiest action is run your everyday car for one year longer than you intended. Reading a book on economy driving and applying the lessons would be no 2. If we have time we take the bus, luckily we both enjoy urban anthropology but I can see how viewing the results of our educators labours close up might put some people off.
Stan Best

I don't know how your gasoline is configured in the UK, but in the USA it is no longer quite the "gasoline" it was chemically when these cars were built. Today, it is a concoction of various chemicals to reduce harmful emissions including deletion of things like tetraethylead to accomplish that objective. To what extent these changes affect the combustion which takes place, frankly, I don't know, not being a petrochemical or automotive engineer. I do know that in general, my car, low compression, seems to run just fine, gets around 20-21 mpg around town on 87 octane "regular" gas and it seems to obey the rules of timing as mentioned above.

However, to what extent deposits, heat, and power are affected by these changes differ from when the cars/engines were designed for use with the fuels extant at that time, I would like someone who DOES know to explain. Having a good idea if it makes any difference to the functioning of the engine and its longevity is probably of the most interest and importance to anyone who drives one of these cars. My guess is that it may differ but not in any way that requires me to be concerned about it. But if what we burn confidently as gasoline today somehow varies significantly from the established theory we embrace as to what should and does happen, then perhaps that could shed some light on other issues we encounter today as we experience the operation and performance of these engines. I am probably splitting hairs, and raise a spurious question, but perhaps not. At least knowing that there is no significant change can free us from considering the possibilities or help folks in their diagnostics of how their cars/engines are working. Thanks!
Bob Muenchausen

This thread was discussed between 14/08/2007 and 17/08/2007

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