Gas Question

One of the guys at the shop I bought my moped, told me to only buy regular gas all the time (This was back in july). He said anything else would burn at to high of a tempurture and burn out the engine. I took his word for it, and bought regular for the first 550 miles. Then after going over the service manual again and again, I noticed this:

Fuel tank and fuel: Fuel: 4l (1 gallon), reserve 0,5l (1 pint) - 98 oct. gas

Hmmm...So who do I listen to? The manual, or the guy? I wasn't sure what to get on my next fill up, so I bought the middle one (not premium, or regular).....The highest octane gas (premium) is like 93...So what should I do? Obviously, if it's not going to hurt the engine, I wanna buy the best gas possible.

Re: Gas Question

Kold,you've gotta tell us the make,and year of the bike and what type engine.Usually 87 octane reg. is o.k.,but there are some Puchs with 10:1 compression ratios which would be better on 89 octane I'd think.

Sorry, forgot...

Its a 2001 tomos targa lx.

Re: Sorry, forgot...

XBrandon EdgeX /

Maybe your manual had a typo. They may have meant to type 89 instead of 98. Regular gas should be fine (87 - 89). High octane gas will have a harder time burning.

Re: Sorry, forgot...

can you even buy 98 octane anywhere? ;-) it has to be a typo.

Re: Sorry, forgot...

I have run Racing Gas which is still leaded and it runs Super! with no bad effects in my 94 Tomos TT Doug D.

Euro octane specs

I am just guessing... but I'd say they are using Euro octane ratings... which are different than US octane ratings.

That or its a typo.

I bet you that you can use 87 octane or 92 octane... and they don't run any different... you just have less money in your pocket when you buy the 92.

Re: Gas Question

I believe the fellow who told you that was passing along an old wive's tale. There is a lot of nonsense floating around about gasoline (high octane burns hotter, it has more "power", etc).

The only real difference between lower and higher octane is its tendency to predetonation. That is, at high compression, higher octane is more likely to wait for the plug to spark before it starts burning, whereas lower octane may ignite under compression all by itself. (This is bad... it's called "knock" and puts a lot of stress on your engine, not to mention not pushing you forward the way it's supposed to.) That's why high octane is important for engines with very high compression ratios, and the reason the "more power" myth is so enduring. (High compression engines being higher performance, so obviously they need "more powerful" gasoline, right?)

If your ped generally runs okay on regular gasoline, then the only time you may notice a difference is if under extreme load (struggling up a steep hill, perhaps) your engine knocks. If so, you should definitely go with higher octane, since even an occasional knock is very hard on your engine.

I personally just ignore all that stuff and put in the highest octane available, with attention to make sure it's not a gasohol blend. (You definitely want all-petroleum gas). Since I have a 1 gallon tank, I generally add fuel 1/2 gallon at a time, so I can easily mix the oil correctly. The difference between spending 49 cents and maybe 53 cents on a visit to the gas station is, IMO, not worth worrying about.

Just my two (or four? <g>) cents...

Re: Gas Question

Ron Brown /


There is good and bad advice in this thread.

There are at least two ways to measure octane and the numbers are radically different for the two methods. If you search the web, you can get details, but in a nutshell, here is what I remember and what I think is important.

The higher the octane of gasoline, the slower it burns. By burning more slowly, the pressure in the cylinder increeses more slowly, allowing the piston to "get out of the way" (technical term : )) of the expanding gasses. In a high compression engines, fast burning, low octane gas will acheive a high enough temperature to spontaneously ignite, or knock. This is when all the remaining fuel ignites at the same time instead of burning evenly through the combustion cycle. This usually occurs when the engine is running slowly under increased load as the gas always burns at the same rate, but the piston is moving slowly causing excessive combustion chamber pressure.

Pre-ignition is a different problem and is caused by "hot spots" in the cylinder igniting the fuel, typically as the piston is rising. This creates very excessive pressures and knocking.

I allways use low octane fuel unless I have a knocking or overheating problem.

Hope this helps.


Re: Octane 101

Octane is gasoline's resistance to preignition as rated on a scale. The higher the octane number the less likelyhood of preignition (spark knock).

Two methods of measuring octane are available: Research and Motor. The numbers do not correspond between them, but are close. The number on the fuel pump, 82/87/92/101 is an average of the two.

Generally, you should use the lowest octane number that allows you to operate your engine without knocking in the way you use it.


Usable BTUs

In a gallon of premium gas,there are supposedly more BTUs(potentially)than in a gallon of regular unleaded 87 octane.The trick is to see if your car or `ped LIKES the higher octane.I've always done it by checking my gas mileage under identical conditions and trips.I KNOW,some won't believe this,but little brother does.Back in 1979 he had a new Ford Fairmont,4-cyl..He and I were working together in the evenings loading pop trucks for extra income.He was complaining about the pitiful 23 mpg or so he was getting.I told him to run the premium gas and it would pay him back in gas mileage.He didn't believe it.We did the test on a couple of long trips and he gained 4 mpg! He was amazed `cause he did the driving and comparing and knew there was a considerable difference.Now I've tried 92 octane in my Franco-Morini and my Aerostar van and it just doesn't seem to help at all.So to really find out just run your car to empty and fill her with premium and do the same driving you did with the reg. gas and compare mileage.But if your car is pinging and it's tuned properly,you shouldn't run 87 octane anyways.

Re: Usable BTUs

Ron Brown /


If you consider the relationship between piston/crank position and speed of burning of different grades of gasoline, then ignition timing and engine rpm can have a sigificant effect on gas mileage.

As an example, if you have "knocking", you can usually cure it two ways.

You can retard the timing, this allows a greater volume of combustion chamber for the burning fuel to expand into, as the piston is a little further down the bore as combustion is complete.

You could, instead, switch to a higher octane gas. The slower burn rate would have a similar effect to retarding the ignition.

This is a very simplistic view as compression ratio, bore and stroke, engine rpm and temperature all have a significant effect.

Sometimes, running premium gas will allow the engine to run cooler by reducing pressures during the early part of the combustion cycle. Hopefully, this translates into more pressure applied to the piston near the center of its stroke where it can do the most good, possibly improving gas mileage.


Re: Usable BTUs

Right,Ron! There are a load of variables.I was only saying that if you want optimum gas mileage,do the mileage check with ALL grades ,`cause it's a great indicator of the torque received per BTU input.The more torque,the less gas you need to give it,thereby,putting it in simple terms,the less gas goes thru your injectors or carb into your engine for that mile of travel per that speed.I have a 460 c.i.d. engine in my F-250 that just wastes gas constantly because of low rear-end gearing.I'm thinking I'm gonna sacrifice a little of the heavy-dutiness of the C-6 tranny and opt for an AOD out of the lighter built F-150.If I'm careful I think it will last and give me a lot better gas mileage.The point of saying all this? The 460 HAS the torque,(BOY,does it have torque)so I'll be letting off the gas quite a bit.

Re: Sorry, forgot...

SteelToad /

Go to a small local airport w/ private planes. You can get 104 or 108 octane. DO NOT run that in your ped, but if you can get them to let you put it in your car, it does wonders for cleaning out the carb and engine.

Re: Sorry, forgot...

SteelToad! Does it have road tax on it or HIGH way tax?Seriously,I'll bet that IS some really clean gas.I've hauled it before,but never made out an invoice on it.

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