Correct Grade of Petrol

[Graham Barker]

Apologies if the answer to this query has already been provided in an earlier forum post.  
I run my Ruby, which has the standard high compression head, on 95 octane unleaded and ethanol-free petrol. 91 Octane is also available, but I have been using the 95 octane in the belief that it should give me a little more power.
Recently another Austin 7 driver said that for the low compressions encountered in Austin 7's, it could be better to use 91 octane and that this might give better performance.
Does anyone have a technical explanation to confirm this or a well-documented trial?

[John Cornforth]

Hi Graham

Looking at my notes for UK petrol, Prewar grades were about 68,75 or 81 octane. Wartime and post-war 'Pool' petrol was about 72. By 1953 80 and 92 were available.

I think the term 'High Compression' is relative, it's still pretty low by modern standards.

91 octane should work fine and may be marginally cheaper, so worth a try but I doubt if performance is better. No downsides to higher octane as such, but it depends on the makeup of the fuel.

[Fencepost]

I think that might have been me! Some light reading.
https://www.austinsevenfriends.co.uk/hin...lmain.html

[Reckless Rat]

This is purely a personal view, but over the last 40 years I have run my RP (with SU carb and 37 head) on every type of commercial fuel and not noticed ANY difference. It currently runs on the cheapest available at the pumps, 95 E10 and I cannot understand why anyone would want to spend more money on petrol that makes no appreciable difference to performance or economy. During the post war years it wasn't unheard of to add paraffin to eke out the fuel ration, and you could probably get away with adding some diesel as well.

I wouldn't worry unduly. If your car is in good order it will run on whatever you give it.

[Ruairidh Dunford]

+1

[Steve kay]

Come now, mon ami, a teaspoon of paraffin or TVO is one thing, but topping up with red diesel is another matter!

[Reckless Rat]

I've even used some spare two-stroke mix for my trials bikes.

[Steve kay]

No doubt that will add clouds of white smoke, but does it make a Seven sound like a Deltic? The path to the workshop is blocked by snow, or maybe I should just stop looking at the computer and start work.

[Reckless Rat]

Nah, doesn't smoke much if you use a 50:1 mix and synthetic two stroke.... no worse than a shot of Red-X

Just as an aside to this current thread, I checked the oil on the car prior to a run out on Sunday morning. The engine has done 600 miles since the last oil-change and this is where the level is on the dipstick. I reckon it has used about half a pint, and the oil is still clean. Supermarket 20W-50.

[Ivor Hawkins]

I’ll second what Rekkers and Ruairidh have said, I run mine nearly every day on a variety of petrol and treat it to a tank full of premium under the illusion that it will stop the valves sticking, but Perfo4mance wise it doesn’t appear to make any difference.