I know that the minimum octane for the R-670 is 65 octanes (W670-K variant). I also noticed that other variants increase compression ratio and power using a minimum of 80 octane. Why there is an increment in octane? My guess here is to prevent detonation.

Could a Continental R-670 piston engine work with regular car gasoline (85-97 octane) or Avgas (100 octane)?

My guess is yes, but would it develop some other problems as time goes by?


2 Answers 2


Yes the higher octane is specified because of the higher compression ratio.

An R-670 would run perfectly fine on 94 octane mogas (almost all Regular car gas has alcohol - you need to run alcohol free Premium), and would run fine on Avgas except for lead fouling over time that plagues 80 octane engines run on 100LL.

The only benefit to the engine that avgas provides, other than the lower vapour pressure and therefore resistance to vapour lock, is that a combustion byproduct compound from the lead forms on exhaust valve seats that has the effect of lubricating the interface between the valve and seat (there is always a bit of sliding motion as the valve centers itself in the seat on closing), reducing wear somewhat.

Otherwise it will be happier on mogas than avgas. Supercharged P&W R-985s are approved to run on mogas with the Petersen STC and I imaging the Continental is as well.

  • 1
    $\begingroup$ It is possible to put hardened valve seats in an auto engine to "upgrade" it from leaded to unleaded gas. I'm sure that could be done to an aviation engine, as well. Whether or not the engine would then be legal to fly is a whole different topic... $\endgroup$
    – FreeMan
    May 4, 2022 at 16:58
  • $\begingroup$ Cars went to seats made from a harder alloy called Stellite back in the 70s as unleaded gas came on line. I believe aircraft cylinders also do these days, but not sure. $\endgroup$
    – John K
    May 4, 2022 at 17:01

In the design of a piston engine running on gasoline, the required octane rating for a particular engine will depend on the compression ratio and the cylinder head temperature (which varies according to how hard the engine is working). High compression ratios and high power settings require high octane fuel to prevent preignition, which damages the engine.

Taking the old-school air-cooled VW flat four as an example, the engine's compression ratio was set at 6:1 so it could run slightly hot on the low-octane (less than ~70) car fuel typical of the late 1930's. This worked out OK because the engine was not continuously run at full power in a passenger car.

Upping the compression ratio to 10:1 or even 11:1 in a car required 100 to 120 octane gas and water cooling, and this was OK again because the car engines did not run continuously at full power. This development occurred after WWII when high-octane gas (originally developed for aircraft engines) hit the car fuel market.

Since airplane engines run at high power setting for long periods of time and have air cooled heads, they will necessarily 1) have relatively low compression ratios (around 6:1), 2) have minimum octane specifications placed by the engine manufacturer on permissible fuels, 3) run rich at takeoff power to scavenge heat, and 4) still have 3-minute time limits on maximum power.

  • $\begingroup$ That last paragraph is just plain wrong. Many aircraft engines run on lower octane than modern automobile engines, comparable even to what a lawnmower requires. Yes, it's due to low compression that they can do so, but "highest-octane gasoline available" is just not the case. $\endgroup$
    – Zeiss Ikon
    May 4, 2022 at 17:28
  • $\begingroup$ @ZeissIkon, will edit. $\endgroup$ May 4, 2022 at 17:32

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