I know using too low an octane in an engine is clearly asking for trouble. But I have been asked about using modern fuel in an older engine which only requires 70 octane as a minimum. In fact a 1943 petrol GMC straight 6. I presume very low compression and suitable for pool petrol used in the war. Owner is currently using unleaded and diesel mix. I don't think using 95 octane will cause any problems (valve seats aside) but does anyone have any observations on this? Thanks, David.

 

What is the purpose of the diesel content? Valve seat lubrication??
Other than talking to aviation fuel suppliers, I would have thought 95 + valve seat lube. would be ok. I assume he is using a "classic" oil grade?
Would love to see that big long six running!
JohnD

 

I'd be wary of mixing diesel in the fuel, whats the point of that?
if its for valve seat lubrication , as pointed out above, there is proper addatives for this, Diesel fuel is simply going to create sooting problems on the plugs and valve stems.

 

Thanks guys - I haven't a clue why he is putting in diesel - the first thing I wondered. Am hoping to get a photo of the vehicle. He is off to Normandy for the 60th Anniversary in a couple of weeks.

PS. John - you'd have liked my straight 8 5.4 litre Daimler then. Built as a hearse in 1948 which we converted to a 10 seater open tourer