MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - What type of Gas should I use?

I have a 1968 mgb roadster..What type of gas would be recommended....I have been using High Octane unleaded...Thanks
Mike

Sounds good to me.
Mike MaGee

I have a 69 MGB and my personal favorite is Sunoco 94. Super Shell (93) is the other I'd use if I can't find Sunoco.
Paul Slice

If you want a real treat, find a local airport using airport transfers tonbridge and put a tank of 100LL av gas through....she'll love you for it!
Robert Dougherty

Robert,

I have access to 100ll but have been a little hesitant to try. What kind of mods have you done on your engine? How often have you used it? Do you have any evap issues since av gas likes to dissapear as you look at it! Any down sides to av gas that anyone knows?

Paul
Paul Hanley

You won't get any benefit from higher octane gas, except its resistance to knock. Tune the car properly, put regular gas in the tank. Take it to a highway near a construction barrier or wall. Roll down the window, get it in fourth gear and floor it. If you hear any ping, then use the next grade higher gas and try again.

Running on can be overcome with higher octane gas as well.

I don't have experience with aviation fuel, but if it's just higher octane gasoline, there would be no benefit. Octane doesn't make power, it only decreases pinging. Burning gas in a compressed state makes power. You can get more power from more compression, but that increases the tendency to ping. Thus, high compression motors make more power but require higher octane gas.

Sorry to sound like a high school teacher, but this is one of those myths that I hate to see continued. It causes people to waste money.
Matt Kulka

At 5,000 feet altitude in Denver, CO USA it doesn't make any difference, I understand.
J.T. Bamford

Mike,

If your gas station is like mine, the higher octane gas is stored in a separate UST and blended at the pump for the intermediate grades. How often does it get changed? Does water leak into the tank? I'd be more concerned about the quality of the gas.

Doug
D. Cook

I believe the Owner's Manual specified 98 octane
on the HC engines.

That was back in the day when "Premium" fuel
could be commonly had as 98-to-104 octane.
Daniel Wong

I'd be hesitant to put avgas in my car--over the years I've heard (and witnessed) some real horror stories concerning ruined engines. Avgas is for airplanes--blended for use at altitude, with additives designed for use in aircraft engines. Cars like ours will run just fine on what the pump provides. Quality of the fuel, storage conditions etc are far more important. Find a brand and octane rating that's just high enough to prevent pinging and run-on. Anything higher is a waste of money and performance/economy will atually decrease. Find a dealer that sells fresh water-free gas...in other words: high sales volume and is conscientious in changing his pump filters.
R. L Carleen

Keep in mind that there are different ways to measure octane index. The numbers currently used in the US are the average of "motor" octane (MON) and "research" octane (RON) -- which is why you'll see octane ratings specified as (R+M)/2. In the UK, I believe reseach octane is the unit of measure, so the same fuel would tend to be labelled with a higher number in the UK than in the US.

I also seem to recall that the US changed from RON to (R+M)/2 at some point in the recent past, but on a cursory search I couldn't confirm that. I did find some interesting discussion on octane however:

www.ozebook.com/suzi/octane.pdf
www.vmclinks.com/x/modules/tutorials/viewtutorial.php?tid=10

My personal philosophy is to run the lowest octane fuel that runs properly -- i.e. no preignition. That will be determined by many factors, including engine condition, state of tune, modifications, advance curve, temperature, air pressure, humidity, etc, etc, etc. Do as Matt says and listen to your car. If it wants higher octane, it will tell you! :-)

HTH!

Rob Edwards

I use Amoco 93 in my 66 HC engine. I've stopped losing sleep over wondering why a 8.8 CR engine needs 98+ octane - might have something to do with cam timing.
Glenn

Ok; once more, here is the difference between how the British measured octane and how we Yanks measure it.

England used the RON octane number as the measurement when MGB's were built (and may still do so).

The U.S. uses the average of (RON + MON)/2 and this has been the measurement standard for as long as I can remember (almost 50 years).

RON is measured using laboratory equipment and is based on volatility of fuel, % of octane molecules present, yada yada yada.

MON is measured by running the fuel sample through a calibrated test engine and changing compression until pinging occurs. This is used along with corrections for air temp, pressure, etc. in a very complex formula to arrive at an octane number.

RON is always higher than MON and there is typically about a 10 point difference. Basically this is the difference between theoretical maximum and real world under specified conditions. Also, way back in the past, there was disagreement on which one should recalibrate to be equal to the other so it was agreed that both would go their merry way separately.

Therefore, the (RON + MON)/2 method is usually 5 points lower than RON only. The MGB manual is based on RON octane numbers and thus the 98 octane referenced in the manual is equal to 93 octane in the U.S.

One other thing to remember is that this was the recommendation back in the 1960's; and to be honest, gasoline wasn't quite as clean and consistent as it is nowadays. While the octane number may have been there (usually through adding a lot of tetraethyl lead), there were other contaminants present that inhibited good combustion and could cause pinging. MG played it safe and specified the best octane that most motorists could buy. Nowadays, gasoline is a much more consistent and cleaner fuel (at least in the U.S. to meet EPA requirements) and the 92 premium available at most pumps is probably better than the old 93.

For the amount of miles most of us drive our MG's, paying the extra 20 cents a gallon for premium is not going to break the bank and is cheap insurance against destroying a piston through detonation. If you do drive a lot of miles and want to economize, then following the previous suggestion to put in a tank of lower grade and seeing if it pings is on the money.

One last item is don't waste your money on the supposed octane boosters that are on sale. The amount of octane kick they profess to give would require purchasing significant amounts of $8 bottles to boost one tank of gas to the advertised octane (read the fine print guys).

If you truly need, or want, high octane gasoline; buy a couple of 5 gallon fuel jugs and go to your local off-road motorcycle/quad-sport/jet-ski store and tell them you want to purchase high octane off-road gasoline. Do not tell them you are going to use it in an on-road automobile!!! They don't want to hear that and may not sell you the gas if you do tell them (liability, EPA, and all that stuff). Once you have purchased the high octane gasoline and get home, which fuel tank you put it in is between you and your environmental conscience.

Also, don't faint when you see the $4 to $5 per gallon price. This is not a rip-off mark-up just because they are the only ones that sell it. Since we cannot use tetraethyl lead in the U.S., the only way to kick up octane is to use some really expensive blending components that have far greater value for the petrochemical industries than they do for fuel. Thus, you pay for the petchem value, not the fuel value (Econ 101 - the refiner is not going to sell at a discount just because it is for fuel).
Ron Kluwe

Wow. I am at the other end of the spectrum. I always get the lowest priced and octane available. My MGA doesn't seem to differ with gas grades and the daily driver B doesn't seem to mind either. I think I would rather buy a six pack of Speckled Hen in lieu of the higher octane stuff. IMHO
Bill Haglan

I used to put in 92 but then that broke 2 dollars a gallon. I switched down to 89 and really didnt have any problems with run on. At 87 it is terrible. I currently am running with 89 but when i went to fill up today it cost me 25 dollars for a full tank. $2.07 a gallon. Insane.

Brooks
Brooks Twist

Ah, Brooks, you poor lass. You live in an enchanted land where fuel must be piped in from afar. Here in San Francisco, the tankers anchor, the refineries are in the neighborhood, and our fuel is only $2.20 for the cheap stuff. A big boost is anticipated by May 30. Go West, young lass, go west.
Dan Robinson

I don't really feel sorry for you US guys. Yesterday Shell increased the price for regular unleaded here to Euro 1.28 per litre, that would be $4.84 per US gallon.
And my C doesn't even run on regular, but wants "super" (98RON) which is even more expensive....
I live in the wrong place I guess.
Rufus
R Pool


For those that need high octane, a small number of Sunoco stations across the US carry Sunoco GT unleaded which is rated at 100 octane.

I have a 10.5:1 compression 1950cc motor that is relatively highly modified. I had a severe run-on problem when running 93 or 94 octane and had to significantly retard my timing to prevent pinging.

While running this gas, I have absolutely no pinging, no run-on whatsoever, and I was able to add back an additional 5 degrees of initial advance.

Now the drawback: I've paid between $4.39 to $4.79 per gallon. Fortunately, I don't put high mileage on the MGB, but the fuel allows me to run the motor at a strong state of tune.



here is a dealer locator site
http://www.racegas.com/gt100locals/default.asp

Here are the specs for the fuel:
http://www.racegas.com/fuelspecs/default.asp
sliedavs

An advantage of the higher octane fuels is that they are usually have a higher density. For example here the 98 RON no. is usually around 0.75kg/l while standard 91 RON is around 0.72kg/l. So your car runs a little richer.
For all those thinking about converting to LGP you're only getting around 0.52kg for your litre which make the lower price a good deal less attractive.
The 98's also run a good deal of toluene (toluol) in the mix, which gives your fuel system a clean out, but is a bit unfortunate if you have old style rubber fuel lines as toluene is a very strong solvent. Toluene has a plus 100 RON, which is why you can't buy it over the counter in Australia. The goverment likes to exact a heafty tax on petrol. There was a big scandel here a while ago when some enterprising gentlemen were buying blendstock and tax free "cleaning" toluene and making a bundle blending up high octane fuel. I would give odds that the "off road" people mentioned are useing a high toluene blend. Be wary of using toluene though as it fairly unpleasant and you do develope an intollerance for it over time. It especially seem act as a Hapten and promote allergies. The odour is quite distictive.

Avgas consists of light alkylate, which is around 96/97 RON and they add lots of tetra ethyl lead (TEL, which in concentrated form is seriously toxic and will kill you if you spill concentrate on your skin). The only other additive is dye and absolutly nothing else, by strictly inforced law. You can run this in your car with no ill effects (it even smells nice) but there's really no point unless as others say, you have a seriously high compression motor.
Peter

ps Adding TEL to light alkylate pushes the octane up to around 112 MON which is about 107 aviation number. I've no idea what the RON is, way high but irrelevant.
Just to be clear-the RON is the reseach number and a bit of a theory. MON is more "real life" conditions, with the motor run harder and hotter.
Peter

FWIW, I run 93 octane in my '66 which has an MSX head and 10.56:1 CR. I sometimes get very light pinging when the engine is really hot and under heavy load. I do, however have my timing advanced to the ragged edge. The alloy head allows, from what I am told, one point higher compression than an iron head. I have a Nissan with a knock sensor. It needs 93 octane and gets sluggish with low grade gas.
David
David

In my 79 and daughter's 77, we run Unleaded Plus. Both have rebuilt engines with LC pistons because the quality of available gasoline has been declining over the years. My Scout and Bronco, both of which used to run fine on Unleaded Regular, now require Unleaded Plus. The octane ratings have dropped a couple of points over the last five years, which may be the cause of this requirement. As to the cleaning value of any additive's used, all of our available gasoline contains either ether or alcohol in all grades of gas. Hence, running a higher grade of gas because it may have some additional cleaning properties is probably a myth. Les
Les Bengtson

I'm not trying to start an argument, but it's my understanding that an aluminum head also requires about 1.0 more of CR to make up for lost power since they lose heat (power) quicker. Fortunately, their ability to dissipate heat allows them to run some extra CR.

Point being, of two identical engines other than one with an aluminum head and the other one with an iron head, the one with the iron head will make a little more power.

I know it's true - read it on the internet.
Glenn

<<<I'm not trying to start an argument, but it's my understanding that an aluminum head also requires about 1.0 more of CR to make up for lost power since they lose heat (power) quicker. Fortunately, their ability to dissipate heat allows them to run some extra CR.>>>


"The alloy head allows, from what I am told, one point higher compression than an iron head."

I think we are saying the same thing. If I had an iron head I would have to run one point less compression to keep detonation at bay with pump gas. I've always been led to believe that about 9.5:1 with a B engine and iron head is appoaching the limit.
Cheers,
David
David

Les,
Its the toluene that does the cleaning, not the higher octane no.
Peter

This thread was discussed between 04/05/2004 and 15/05/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.