Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 95 or 98 octane gasoline?

The best for engine is 98
If you cant put It use 95 with additif

Excuse me for being a little blond, but isn't the needed octane number related to the compression rate? That is, if an engine has a very high compression rate, it might need 98 or even 100 octane, if not 95 is enough. No modern engine I know of in road going cars has anything that really needs 98. Of course you can put it in, but you will achieve no benefit. I had an old Sierra that was specified for 98, if you put anything lower in it you could risk premature firing just by the compression (like a diesel), not the spark from the spark plug. But not so with a modern car. I also had an Elise - tried with both 98 and 95, noticed absolutely no difference. I hardly think that the MW3 S&S engine is high compression and really needs 98. So, if the engine can tolerate 95 anything above is a waste money, if 98 really is needed anything below can/will cause problems. Or am I totally wrong?

Bear in mind that the numbers are quoted differently on the other side of the "Pond" so adjustment is required to be comparable

I was advised to use 98 by Aero Racing and have done so out of habit. I also use Shell Optimax in my day car and have frequently recorded better mileage...

I think the best advice is use what the engine manufacturer says, most recommend 95 on modern cars.

As you say, most recommend 95. But many still recommend 95 or 98 (both).

If I remember correctly (my classes of combustibles at the engineering school are so distant now...), the 98 fuel will "burn" smoother, and more complete, than the 95 fuel, as its hydrocarbons molecules are shorter and easier / faster to "burn". But being shorter, the molecules have less energy than the long ones in a 95 octane fuel. So benefits are diluted by the "lower energy" of its molecules in most of the engines.

But as our S&S V-Twin is American, and quite "peculiar" compared to a regular car engine... maybe we can feel a real difference between using both fuels… That’s why I asked.

For example, in our 1988 Range Rover, we Only use 98 fuel. If we use 95, the engine is noisier and the consumption a little higher...

octane rating are a way of rating the speed in which a fuel will burn. It initially was based upon the amount of "octane" molecules in the fuel. There are a lot of variables in the octane rating for any engine. The S&S (and HD's) are high compression engines. Higher compression engines need higher octane (the higher the octane the slower the speed of the burn). This is because higher compression in and of it self makes fuel more likely to burn. Put too low of an octane in these engines risks pre-ignition. Recall that diesel engines only use a glow plug to start, but burn due to compression once the engine warmed up.

Often, higher compression engines are also higher performing engines. Therefore people think that using a higher octane fuel is what causes the higher performance However, taking a standard engine that needs a lower octane fuel, you will have no higher performance, instead, you may actually have poorer performance as the slower burning fuel will not have enough time in a cycle to do a complete burn. Or worse yet, you will have wasted money.

Not wrong Robbie, but maybe a little behind the times. Modern high output engines , particularly those using forced induction or leaner air/fuel ratios, have particular need to avoid pre-ignition or detonation and manufacturers will specify (Europe) 97/98 Ron fuel for certain models. My AMG has a little notice by the fuel filler cap as a reminder!

To protect these engines (for example where lower grade fuel is used, or conditions cause cylinder head or inlet charge temperatures to exceed the normal range) a piezo-electric sensor is built into each cylinder head to detect incipient pre-detonation and change the spark advance (via the ECU) to protect the engine. Sophisticated engines can do this individually by cylinder and highly-boosted engines will usually bleed a bit of inlet boost pressure at the same time.

Not relevant to three-wheelers I know! But there are very good reasons why some engines specify 98 Ron.

Probably irrelevant but in my little Abarth I definitely get more mpg if I use 98. No idea why and I've always meant to do the maths and see if it's actually cheaper to buy the dearer petrol. Never got round to it though.
Nick

Actually very relevant to Three Wheelers Two Tribes, the S&S X-Wedge does have a knock sensor with it's ECU managed engine (page 8-4 item 20 of the S&S service manual). It works pretty well unless it is paired with a standard Morgan exhaust system when it cannot adjust the ignition enough to compensate for its very restrictive but compliant design.