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A guy who reads my blog (his name is Bob and he's not a member here) recently posed the following puzzler in an e-mail he sent me:
Bob wrote:
Once again I seek your help.

Perhaps you or one of your readers can answer a question that has long perplexed.

My new scooter requires high octane fuel - at least 91.

Back in the day it was simple - there was a dedicated premium pump somewhere at the station that pushed only the good stuff.

Now all the pumps are dial-a-matic with one hose and three or four grade selections.

Since the single big fat hose and filter hold a considerable volume of gasoline, and I assume that the gasoline in the hose is whatever the last customer selected (likely lower octane), how can I possibly get any of premium fuel into my tiny 6.0 L tank?

I strongly suspect that when I select and pay for premium, in most cases all I'm getting is regular, and treating the next cheapskate to a couple of gallons of my premium fuel for free.

A friend has a clever solution: He bought a 2 gallon can which he fills firs,t then takes home and empties into his car.

Alas I lack a second vehicle.

The best I've been able to come up with is that I lurk at the entrance of the station and follow immediately behind the person in the Mercedes, BMW or Ferrari.

Surely someone must have a better idea.
To be honest, this never crossed my mind. I guess I always assumed that you got what you paid for.

Is this a misplaced concern?

Is there some kind of check valve re-sucking system that purges the 'wrong' fuel from the hose?

Enquiring minds want to know.

I Googled to see if this topic had been covered on MV before and came up dry, though there have been high octane discussions here and there is some fuel info in the wiki.
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The amount of fuel in the hose is negligible.
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This has been rehashed, disputed and debated before. My personal thought on it is that it's not really worth worrying about. I don't believe the amount of fuel in the hose is all that great (I've looked it up and done the math but don't recall off the top of my head how much it is). I have no doubt that I'm mixing some lower octane fuel into the tank every time I fill up. I can't imagine it's having any ill effect on my scooter or my fuel mileage.
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I wouldn't worry about it. Do you have any stations that have designated hoses for each grade? If so use the,. I try to put ethanol free in mine if i can make it to the stations that have it.
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Thanks for the link to that thread.

There is another thread linked to that one as well: Not getting the gas you pay for.

So far, reading over those threads, it seems like the consensus is that there may be 500ml at most in the hose, which is not a concern at all if you generally refuel when the tank is really low.

If I come up with what seems to be a really authoritative answer, I'll post it here.

I wish there were a better way to find older threads other than remembering that you saw it
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davidmasse wrote:
I wish there were a better way to find older threads other than remembering that you saw it
Mark a thread you want to remember as a favorite or do as Danny does and use the notes function of the form to keep track of threads and a summary of why you kept the tread link.
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Tell Bob if it bothers him he can always wait until a high performance car fills up and then sneak in behind. That way he will get 500ml of hi-test.
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cdwise wrote:
davidmasse wrote:
I wish there were a better way to find older threads other than remembering that you saw it
Mark a thread you want to remember as a favorite or do as Danny does and use the notes function of the form to keep track of threads and a summary of why you kept the tread link.
Sorry Cheryl, what I meant without being clear, was that I was having trouble finding whether a topic I hadn't come across had been covered.

I had searched and failed to find the other two threads before posting this one.

I can usually remember a past thread and will then scour until I find it.
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UTC quote
Raputtak wrote:
Tell Bob if it bothers him he can always wait until a high performance car fills up and then sneak in behind. That way he will get 500ml of hi-test.
And remind him to pull up his diaper.
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Well it turns out that this one has the MV crowd pretty much stumped, making wild guesses, and stabbing in the dark.

The consensus from this thread and the other two threads unearthed earlier is "fugetaboutit" and "don't worry, be happy" and I find myself agreeing, yet being totally unsatisfied with the outcome.

Otherwise very well-informed and totally trustworthy people are saying "there's hardly any gas in the hose, half a cup, max!".

Others say essentially the same thing, but estimates vary from half a cup (125 ml), to a cup (250 ml), to two cups (500 ml).

So I started Googling.

The smartest answer from the best source I could find was this, from the Wall Street Journal:
Wall Street Journal wrote:
Q: I ride a motorcycle with a typical three to four gallon gas tank. I ride where fuel stations are farther apart, so I fill up when the tank is still half full. The bike requires premium fuel, and doesn't run well on lower octanes. If the previous customer was pumping regular fuel, I assume the refueling hose is still full of regular fuel, perhaps a couple of gallons. This would mean I'm initially getting a mix instead of pure premium fuel. Is this a genuine concern, or does the system have a mechanism for evacuating the gas pump hose between uses?

-- Paul Kowacki,
Orange Mass.


A: It is a genuine concern, but one that motorcyclists tend to appreciate more than car drivers. According to the American Petroleum Institute the gas-pump hose typically retains about one third of a gallon of fuel. So when you pump a couple gallons of 93-octane premium after the previous customer pumped 87-octane regular, your fuel load would be diluted (not to mention overpriced).

This is more important to motorcyclists because bikes have smaller fuel tanks and a lower tolerance for low-octane gas compared with most cars. I have found that high-performance motorcycles designed to burn premium fuel run poorly on regular. They generally do not have the complex engine-control systems that allow cars to run on fuels of varying octane ratings.

I don't think diluting your premium fuel with a little regular will harm your motorcycle, especially if you always select the highest octane rating available. However, next time you're filling up you may want to get in line behind the driver with the highest-performance car in the station.
So if the WSJ is to be believed, and they are so far the most credible source, there is not half a cup, not a cup, or even two cups, but five cups (give or take - 1/3 of a gallon is 1.24919 liters, which we can round to 1.25 liters, which is five cups on the nose) of fuel left in the hose.

Just in the interest of trying independently to get to the nub of truth, let's say that the average gas station hose is 12 feet long. According to Goodyear's web site, gasoline pump hoses are either 3/8" or 3/4" outside diameter hoses. Let's take the worst case scenario, that's a 3/4" hose. All 3/4" gasoline hoses have an inside diameter of 19.1mm (don't ask me why Goodyear's specs give the outside diameter in SAE and the inside diameter in metric). The volume of gasoline in twelve feet of that type of hose is Pi (3.14159) times the square of the radius of the inside diameter of the hose (19.1mm divided by 2 = 9.55mm, squared = 91.2025mm) times the length of the hose (12ft, which in mm is 3657.6mm) equals 1,049,416.44 cubic millimeters, or 1,049.41644 ml, or, rounded down is 1.05 liters, which is just a tad over four cups.

Based on the credible WSJ source, as somewhat corroborated by my mathematical, semi-scientific, guesstimated calculations, if we average the anecdotal journalistic number (5 cups) with the semi-scientific guesstimate (4 cups), we get 4 1/2 cups of questionable fuel in the hose.

In the case of the Vespa GTS 300 i.e., the tank capacity is 9.2 liters. But even if you fill up only once the low fuel light comes on, the tank is not empty. There are still about two liters of fuel in the tank, as near as I can tell.

Basically I'm buying 7 liters of fuel when I fill up. That means that I'm getting 1.1 liters of questionable gas, and 5.9 liters of the good 91 octane gas that I need and want. Assuming that the octane rating just dilutes like any other liquid, I'm not really getting a full tank of 91 octane, I'm really getting 90.37143 octane.

I think that's pretty well within spitting distance of the truth.

Based on everything I've heard and read about octane ratings and internal combustion engines, that's close enough to 91 octane that the Vespa engine won't suffer for the difference.

So I am declaring this one done, solved and one for the history books. And I am going to join the ranks of the MVers who don't worry about the fudge in the gas that the local gas station is selling me.

Thanks to Bob for asking the question. I thoroughly enjoyed digging to the bottom of this week's puzzler.
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Busted
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davidmasse wrote:
If I come up with what seems to be a really authoritative answer, I'll post it here.
David-

How's this:

Typical dispensing hose is 5/8 or 3/4 inch inside diameter. Thus, a typical hose would contain 70.45 to 85.25 cc of fuel per foot.

Math used:

5/8 inch:
Radius = .3375 in
R squared = .1139
X3.14 = 0.358
x 12 inches = 4.296 cu inches/.061 = 70.45cc


3/4 inch:
Radius = .375 in
R squared = .1406
X 3.14 = 0.4415
x 12 inches = 5.298 cu inches/.061 = 85.25cc
⚠️ Last edited by Aviator47 on UTC; edited 1 time
OP
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Aviator47 wrote:
davidmasse wrote:
If I come up with what seems to be a really authoritative answer, I'll post it here.
David-

How's this:

Typical dispensing hose is 5/8 or 3/4 inch inside diameter. Thus, a typical hose would contain 54.07 to 66.78 cc of fuel per foot.

Math used:

5/8 inch:
Radius = .3375 in
R squared = .1139
X 2.414 = 0.275
x 12 inches = 3.2995 cu inches
3.2995 cu in = 54.05 cc

3/4 inch:
Radius = .375 in
R squared = .1406
X 2.414 = 0.3395
x 12 inches = 4.07 cu inches
4.07 cu in = 66.78 cc
OK... math was never my forte, so Google has to be my friend.

Plus what with SAE and metric, and cubic centimeters vs milliliters... it's all enough to make me dizzy.

I redid the calculations using inches to get cubic inches (checked and 19.1mm given by Goodyear as the inside diameter is 3/4" - doh!, so we're on the same path there), but you're using 2.414 to get to the volume but pi is 3.1459. So if I make that change, I then get 5.30 cu inches for 12 inches which is 86.85 cc.

If you assume that the hose is 12 feet long, that gives you 1.04 liters, so basically the four cups of fuel that I guesstimated.

I think that 4 1/2 cups of gas (averaging the Wall Street Journal estimate of five cups and the guesstimate of 4 cups) is as close to the truth as I can get.

I'm not planning to go to my local gas station to measure the actual length of the hose.

One way or the other, with my poor math skills, and the extent of my curiosity, I am satisfied.

Thanks for helping out
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WOW - do math before my morning coffee and I come up with 2.414 for pi! I am getting too old to work from memory without coffee.

Calculations above corrected for the proper value of pi.

Thanks, David.
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Both the Jims (c & h) state that an entire tank of regular will not hurt the engine once in a while.

https://modernvespa.com/forum/topic111992?highlight=tank+regular+gas

So it MUST be true!
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Wouldn't the amount (%) of Ethanol in the gas have a greater effect? Asked in all innocence.
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Not much - the mix is done to maintain the octane status - which is probably within a range of +/- an octane point anyway. The whole issue is pretty much irrelevant to daily use - just fill up with roughly the right fuel, and all will be fine.
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See ^^^^^ he said it again!
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YaYaDave wrote:
Wouldn't the amount (%) of Ethanol in the gas have a greater effect? Asked in all innocence.
EtOH added is part of the Octane computation. A 91 Octane E-10 fuel is 91 Octane. The question David raised was the effect of a blending pump mixing the lower Octane fuel in the hose from the previous customer's purchase with a higher selected Octane for your fill up.
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I just put in 93 and don't worry about it
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WEB-Tech wrote:
I just put in 93 and don't worry about it
Like that!!
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Not to beat a dead horse, but my question is what's the volume of the mixing device itself and any internal plumbing between the mixer and the external hose. I'm not sure the precision of the hose volume calculation has led to an accurate conclusion on the total volume in the system from the previous sale.
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jwestcott wrote:
Not to beat a dead horse, but my question is what's the volume of the mixing device itself and any internal plumbing between the mixer and the external hose. I'm not sure the precision of the hose volume calculation has led to an accurate conclusion on the total volume in the system from the previous sale.
Good point, because the mixing valves are in the bottom of the pump housing not at the top where the hose comes out Laughing emoticon

Again, why I use 93
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