post your MPG

[Roadkill]

Quoting: Titsy

Nope, sorry chump but your wrong... What he's seeing is as a result of the engine... In a higher compression engine you 'should' run a higher octane rated fuel to prevent predetonation and knock... If you run a lower octane rated fuel you will get knock and the cars engine management system will retard the ignition to compensate and reduce knock... With a higher octane rating it will advance the ignition and you will get a better burn... you get no bigger bang from a higher octane rating, but you can make more useful energy out of it in a higher compression engine... they have the same calerific content, it's just that higher octane fuels are harder to ignite....

So what you're saying is if higher compression cars ran UL 95 they wouldn't get as high MPG ?

[Titsy]

Yup, because it will constantly be running a retarded ignition angle... By having a higher compression engine you are able to extract more usefull energy from the fuel, but only if it ignites at the correct time...

[55starchief]

Quoting: Roadkill

So what you're saying is if higher compression cars ran UL 95 they wouldn't get as high MPG ?

yup exactly, my old SAAB turbo was 20-30 down on a tank of regular over super

[Titsy]

This should clear things up:

Octane and power
It's a common misconception amongst car enthusiasts that higher octane = more power. This is simply not true. The myth arose because of sportier vehicles requiring higher octane fuels. Without understanding why, a certain section of the car subculture decided that this was because higher octane petrol meant higher power.
The reality of the situation is a little different. Power is limited by the maximum amount of fuel-air mixture that can be jammed into the combustion chamber. Because high performance engines operate with high compression ratios they are more likely to suffer from detonation and so to compensate, they need a higher octane fuel to control the burn. So yes, sports cars do need high octane fuel, but it's not because the octane rating is somehow giving more power. It's because it's required because the engine develops more power because of its design.
There is a direct correlation between the compression ratio of an engine and its fuel octane requirements. The following table is a rough guide to octane values per engine compression ratio for a carburettor engine without engine management. For modern fuel-injected cars with advanced engine management systems, these values are lowered by about 5 to 7 points.

Compression ratio Octane
5:1 72
6:1 81
7:1 87
8:1 92
9:1 96
10:1 100
11:1 104
12:1 108

Octane and gas mileage
Here's a good question : can octane affect gas mileage. The short answer is absolutely, yes it can, but not for the reasons you might think. The octane value of a fuel itself has nothing to do with how much potential energy the fuel has, or how cleanly or efficiently it burns. All it does is control the burn. However, if you're running with a petrol that isn't the octane rating recommended for your car, you could lose gas mileage. Why? Lets say your manufacturers handbook recommends that you run 87 octane fuel in your car but you fill it with 85 instead, trying to save some money on filling up. Your car will still work just fine because the engine management system will be detecting knock and retarding the ignition timing to compensate. And that's the key. By changing the ignition timing, you could be losing efficiency in the engine, which could translate into worse gas mileage. Again as a practical example, my little tale above about our trip to Vegas on low octane gas. (Whether you want to believe some bloke on the internet or not is up to you). On the low octane gas on the trip down, we could barely get 23.5mpg out of the Subaru. Once I was able to fill it up again with premium at the recommended octane rating, we got 27.9mpg on the way back. A difference of 4.4mpg over 450 miles of driving.
Doing the maths, you can figure out that by skimping on the price during fill-up, you may save a little money right there and then, but it costs in the long term because you're going to be filling up more often to do the same mileage. My advice? Do what the handbook tells you. After all it's in the manufacturers better interests that you get the most performance out of your car as you can - they don't want you badmouthing them, and in this day and age of instant internet gratification, you can bad-mouth a large company very quickly and get a lot of publicity.

[Roadkill]

Quoting: Roadkill

So what you're saying is if higher compression cars ran UL 95 they wouldn't get as high MPG ?

Quoting: Titsy

Yup

Quoting: Roadkill

so that's not completely fair when compared to standard unleaded . . .

Looks like you've just completely confirmed my original post.

And that was that we're comparing MPG's on different fuels . . . hence, again for clarity . . .

Quoting: Roadkill

so that's not completely fair when compared to standard unleaded

MPG is irrelevant . . . It should be Miles Per Pound (£).

To get a FAIR result we should all be using the same (control) fuel.

I don't see anything I posted as incorrect.

Quoting: Roadkill

Martyn uses Shell optimax which offers more MPG

Is correct as if martyn used standard UL he would not obtain the same MPG !

Again.

Muppet.

[Titsy]

Quoting: Roadkill

Is correct as if martyn used standard UL he would not obtain the same MPG

Yes but it's more a case that the engine is not being used as designed if he uses UL 95, so there is no fair comparison that can be drawn... Yes he will get better miles out of UL 98 or UL 99, but that's because that's how the engine was designed to run... If he used UL 95 he would be at a disadvatage because the ingition would be permenently in retard...

On the other had, if a low compression engine used UL98 or UL99 then you would see very little to no advantage in comparison... So you cannot say that it is the fuel that is the reason for better mileage...

[Titsy]

Remember you're preaching to a guy who used to run a 5964MPG car....
Muppet...

[Roadkill]

Quoting: Titsy

Yes but . . . . . . . .

No buts.

I didn't go into detail as for why.  I merely said "Offers" more MPG.

Which, again, is totally correct.

[FUBAR]

[Titsy]

Quoting: Roadkill

I didn't go into detail as for why. I merely said "Offers" more MPG.

Which, again, is totally correct.

Sorry not at all... If you stuck super in a low compression engine you would see no better MPG... If you were being totally correct you would have said that using super unleaded allows a hig compression engine to be more efficient....

[Giblets]

Sorry to interupt the handbag fight ladies....

I haven't bothered to calculate my mpg although I would expect it to sit between Hard Rock and Fubars cars on account that it's got the performance gearing but is only the 305 TPi lump.

So I'll make a wild assumption on about 22mpg.

[Roadkill]

Quoting: Titsy

Sorry not at all... If you stuck super in a low compression engine you would see no better MPG... If you were being totally correct you would have said that using super unleaded allows a hig compression engine to be more efficient....

As I was referring to Martyns Camaro at the time I was totally correct.

It wasn't a general statement.  It was clearly, 100%, referring to Martyns (relatively high CR) Z28.

If you can't read, that's a different matter.

[300bhp per ton]

Quoting: Titsy

If you're flat out then the A/C turns off to give you more power... Does on the LT1's anyway...

yep will on the LS1 also, but most of the time don't need to worry. Only really lost to 1 car and the speed they left me it wouldn't have mattered if the a/c was on or not.

[300bhp per ton]

Quoting: Titsy

Yes but it's more a case that the engine is not being used as designed if he uses UL 95

If you mean the LS1 then yes 95RON is what it is designed to run on, in the US this would be equiverlent to 91PON.

[Titsy]

Quoting: Roadkill

As I was referring to Martyns Camaro at the time I was totally correct.

The implication you make is this that the fuel offers better mpg when in fact it is the engine that make better use of it... There is no bigger bang from super...

I guess you'll just have to keep beleiving what you have said is correct, and I'll carry on knowing that it isn't...

[55starchief]

Quoting: Titsy

I guess you'll just have to keep beleiving what you have said is correct, and I'll carry on knowing that it isn't...

Now now children play nicely

[Titsy]

Quoting: 55starchief

Now now children play nicely

I have, I told him the facts and he chose to ignore them and instead fuel his ego.... The facts are the facts, simple as...

[Roadkill]

Quoting: Titsy

I have, I told him the facts and he chose to ignore them and instead fuel his ego.... The facts are the facts, simple as...

"Mummy, mummy, that Roadkill boy is being nasty . . . tell him, tell him !"



You don't need to preach to me, Titsy, I can read books and, yes, I understand the concepts of higher CR's requiring different fuels.

You either mis-read or mis-understood my original comment indicating that Martyns' Z28 makes better MPG on Super UL then it would Regular UL (which I think we all agree is true).
My point is that when discussing MPG's we are normally, actually, discussing economy.

(Hence my comment on Miles per pound . . . )

This said, yes, Super UL works out more efficient but what has been overlooked until my post was that Super UL doesn't always work out more economical on the wallet.

For example :

My Cadillac runs 10.25:1 CR.  This requires Super UL to achieve maximum benefits and efficiency from the fuel.

Do I put in Super UL ?

No, because I don't believe the extra cost will be justified in the extra MPG.

Again.

MPG are completely irrelavent.

It's Miles Per Pound that we should be comparing . . . . Or not caring at all.

Side note :

My ego doesn't need fueling.  If it did I wouldn't be looking to you to score points off of.

[Titsy]

Quoting: Roadkill

For example :

My Cadillac runs 10.25:1 CR. This requires Super UL to achieve maximum benefits and efficiency from the fuel.

Do I put in Super UL ?

No, because I don't believe the extra cost will be justified in the extra MPG.

Again.

MPG are completely irrelavent.

It's Miles Per Pound that we should be comparing . . . . Or not caring at all.

Go back and read http://www.mkb.cc/forum/index.php?action=vthread&forum=1&topic=4204&page=0#29\" target=\"_blank\" rel=\"nofollow\">this post mate... specificly the 'Octane and gas mileage' section...

[Shifty]

Roadkill actually has an interesting point, although I only fill up with super (optischmuck) and usually get around 32MPG, if I were to fill up with premium that is around 4-5p cheaper and get say 1mpg less would this justify the extra 4p per liter which would save me approximately £2 per tank, although I would then be getting 340 miles/tank between fill ups instead of 350.

Bit of a mouth full

Therefore does the extra 4-5p a liter justify the extra 0.2ish miles extra I would be getting.

In my opinion no, but as it is better for the environment and has a noticeable effect on performance (however small that actually is) and is also supposed to reduce deposits in the engine I think I will be sticking with the super.

PS. My engine does not have a nock sensor.

Argue Discuss

[55starchief]

Quoting: Shifty

Roadkill actually has an interesting point, although I only fill up with super (optischmuck) and usually get around 32MPG, if I were to fill up with premium that is around 4-5p cheaper and get say 1mpg less would this justify the extra 4p per liter which would save me approximately £2 per tank, although I would then be getting 340 miles/tank between fill ups instead of 350.

Might be worth testing mate, i know with my SAAB's the difference outways the cost hence the reason i always filled up with super even when it was £1.08 a liter

[Titsy]

Quoting: Shifty

PS. My engine does not have a Knock sensor.

So to use primium instead of super would most likely damage your engine over time, as i'm guessing the gueda Gauda [sp] is quite highly strung...

[Fieldy]

Look what you started Phil!

[Giblets]

This could end up being a monster thread unless someone backs down!

[Fieldy]

I think they are both relevant at this point!