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MG MGB Technical - Avgas mixed with 98 Octane

Has anyone had any experience with mixing avgas with
BP 98. Either here in NZ or elsewhere. If so what percentage would you reccomend.

Tom Rostron

Why would you want to? What is your compression ratio?

Colin Parkinson

Many years ago I mixed Avgas with 97 (super unleaded) octane 50/50

If I remember correctly the Avgas was about 113 octane and the resultant mix was tested (I had friends in the labs at a major oil refinery in the UK who used a knock engine) at 103

When used in an engine (Raced) after a few laps a valve picked up in the guide and caused a retirement. I originally blamed this on excessive heat from the fuel, but subsequently realised that the new valve guides we were all begining to use at the time for unleaded fuel required a little more clearence than the originals so it was a steep learning curve at the time.
Bob England

MGBs originally ran on 4-star leaded which was about 99 RON, so it hardly seems worth it if you can get 98 RON. I assume you use RON, as 98 PON is about 103 RON! In my experience the loss of lead seems to have had the greater effect on pinking and running-on, and that's compared to 98 and 99 which are quite commonly available across large parts of the UK BP in the UK were at one time selling 102 at a few outlets, I've never tried it and at 2.50 per litre it's hardly surprising. The more resistance to pinking the more advance you can run, which converts more of the energy into downward motion near the top of the stroke and less to waste heat further down the stroke. For that reason 98/99 gives significantly better performance and mileage than 95.
P Hunt

Thanks for your comments. I am experiencing pinking and occaisionally run on. I just thought a bit of lead and an increase in octane rating may help resolve this.
Tom Rostron

You can still get Castrol Valvmaster lead additive here in NZ which boosts the octane a little I believe. In NZ our fuel is rated using RON.

How much advance are you running? What year car have you got and does it have any of the US emissions gear on it?
Simon Jansen

Hi Simon it is a 1979MGB. The previous owner raced it for 17 years and only ever used avgas. Thanks for the Castrol Valvemaster tip I will try some thing Before I do anything else.
Tom Rostron

Generally the only aviation fuel I've encountered is 100 octane low lead. Good enough for our Harvards etc.
Clive Reddin

Pinking and run-on - sounds like it's a little far advanced. If the valvemaster / AV-gas doesn't work, you might need to retard the timing by a few degrees. Sure, less power, but also cheap & proven :)

FWIW, I run 98 RON BP Ultimate in my B - anything less and she does pink and run on a bit (high compression 18V).
Curtis Walker

I tried Valvemaster Plus i.e. the octane boosted version of the valve protection product but found it made no discernable difference to pinking in my 73, so I had to retard the timing a little. Still pinks a bit when visiting hills (relatively flat here) and as well as backing off a bit or changing down ironically I've found *increasing* the throttle opening stops it. Maybe the bigger charge cooling more, this era of MGB engine seems particularly prone to it. I use 'ordinary' Valvemaster now purely for its valve protection, although on an engine that has done many tens of thousands on leaded and is rarely used for long distance high-speed motorway journeys it probably isn't really needed.

Running-on went from slight on leaded to dire on unleaded, I hated stalling it in 4th, and opening the throttle just as I switched off did nothing. Neither did an aftermarket anti-runon valve plumbed in as recommended, but it now works fine plumbed as the American anti-runon valve is, see and click on 'Running On'.
P Hunt

Thats a pity Tom thought you were building a fire breathing monster!! :-)

Paul I have never rated the off the shelf types of octane booster, same results as you. However I think one reason is that to increase octane rating needs some really nasty stuff to be added to fuel. The type of stuff required would never be allowed to be sold over the counter of a motor store or such like! So I wonder what active ingrediant is in these boosters that is safe for sale in motor stores?
Robert (Bob) Midget Turbo

Thanks Paul for your info. Good oil for sure. A fire breathing monster it may not be. But with the delorto carb it sucks enough fuel to create a firestorm.
Tom Rostron

I've a reasonably tuned engine (big valves, 285 cam) and it pinks all the time on ordinary unleaded as it's set up on Optimax/V-Power. To get over the pinking when V-Power is not available, I've tried several octane boosters. The Aldon stuff seems to work (stops pinking), but I've used some stuff in a recatngular metal can that had no effect and was so nasty I wore industrial rubber golves to put it in! Valvemaster Octane Boost seems to work, but turns the plugs red...

Supermarket fuel is cheap, and not necessarily nasty - the fuel at the local Waitrose comes in a BP lorry, so their 'super' might actually be BP Ultimate? And I've seen Shell lorries delivering to Sainsburys.


In the US, it is illegal to use Avgas except for aircraft and off-road racing cars/boats. The reason is less than scientific: the US Government collects all the tax on Avgas, while the state of California gets the biggest piece of the tax dollar for Regular at the pump.
Dan Robinson

This thread was discussed between 27/12/2009 and 29/12/2009

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