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95 versus 99 octane petrol

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Thunderchild

New Member
ok so I'm probably going to open a BIG can of worms now ! does 99 octane really get you better fuel ecomomy ? now i know that it's supposed to give you more power and I'm figuring that if your charging down the motorway you won't see much difference as your already running the engine at peak performance, but what about trotting round town on a cold morning ?

not sure if its my imagination but the engine seemed to cope better at low revs (more power ?) meaning that I could do more in 4th gear than 3rd

any thoughts anyone ?
 

Ross Craney

New Member
unless your car is designed to run on 99 (not many are) then you are simply wasting money
 

gabeNC

Member
unless your car is designed to run on 99 (not many are) then you are simply wasting money

True. Some engines will "knock" if you use lower octane fuel than what they are designed for. I had a motorcycle that really liked 93 over 87. I used to use 93 in my van during cross country trips and logged the mpg and it never made a difference.
 

dknguyen

Well-Known Member
Most Helpful Member
If your engine does not use a higher compression ratio, then it makes no use of the extra octane.
 
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Boncuk

New Member
If your engine does not use a higher compression ratio, then it makes no use of the extra octane.

99 octane is no diesel fuel. :)

It's just a matter of arithmetic. Making 1,000km at an average speed of 160kmh burning off 120l of 95 octance fuel priced 1.05€ (total fuel cost 126€) against the same distance and average speed and a fuel burn-off of 110l of 99 octance priced 1.25€ (total fuel cost 137.5€) doesn't make much sense.

Using high octane fuel from time to time (one tank filling) over long distance has a better self cleaning effect on the catalysator (because of higher exhaust gas temperature).

I know what I'm talking about, having made more than 80,000km per year in active business.

Boncuk
 

Thunderchild

New Member
I have noticed that I can now run smoother at low speeds, for example I could just run at 33 mph in 5th gear, now this is quite feasable even on a slightly up slope, the bend and then light hill I just manage to cluimb while keeping the car in 4th is now easier to get round that bend (actually the 1st exit of the new roundabout) and up the hill and pick up speed fairly quikly and go into 5th gear again moving the car much more smoothly at just 40 mph whilst climbing, with 95 octane petrol it takes longer to pick up speed and change into 5th so I've use less fuel with 99 octane ?

I think that on a long motorway trip 99 octane is an utter waste maybe, the advantage I'm seeing is around town where the car would normally have trouble negotiating conditions that vary continuosly, I can stay in 4th and 5th gear longer so save fuel overall. I've only put a 1/2 tank in this time and done a run of 54 miles but I'll fill up and see what happens just going to work and back.

btw its a ford mondeo LX 1.8 L (Twin raised cams)
 

Speakerguy

Active Member
Sometimes older engines that were designed for regular unleaded will begin to knock with age. Switching to higher octane fuels helps fix the problem.

Such was the case for my 1997 Lincoln, which would knock under heavy acceleration on 87 octane. Bumping up to premium (93) eliminated it completely and the engine sounded just fine whenever I floored it.

This was with 140k+ miles on the car, and I certainly wasn't into spending more on it than absolutely necessary (piece of junk).
 
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Sceadwian

Banned
Thunderchild, you're judging from a very tainted perspective, you've done no qualitive testing before the switch and after to determine what is occuring so be VERY aware that you own perception is more of an influence here than the gasoline.

Although not the same you can kind of look at it like speed grades on micro processors, although not true for entire product lines there is always variability when they're making CPU chips, some batches just come out better than others, so they'll go through testing procedures to grade the quality of the chip. You know if you buy the higher end chip you're going to be getting the cream of the crop because it has to be to run stable at it's advertised specs. Thing is if they manage to get a really good batch or the moon is full at just the right point in time with supply and demand they may remark higher speed chips to fill orders for lower speed chips, it just makes sense if the supply/demand and inventory is just right. Same thing with gas. Higher octane fuels need to be 'the good stuff' but you'll often get good batches of lower octane fuel as well. It's all up to the exact qualities of the oil they pumped out of the hole in the ground at that particular spot and that particular time and how and where it was processed.

You've already switched to the higher octane gas so you've passed up any opportunity to collect actual data other than feelings and opinions (which might be good enough if you trust yourself) But you also have to figure out if any mileage gains you may possible have gotten can offset the increased cost of the gasoline, or if it'll lengthen the life of your engine or decrease it for some reason you're overlooking. If you like the way it driver better on the high octane fuel I'd say stick with it and screw the science, as long as you're not scraping pennies to get buy the increased cost of the gas isn't excessive and if it makes you feel better the simple increased peace of mind is worth the increased cost.

All that being said, it could just be because the gas is cleaner..
 
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Ross Craney

New Member
Sometimes older engines that were designed for regular unleaded will begin to knock with age. Switching to higher octane fuels helps fix the problem.

Such was the case for my 1997 Lincoln, which would knock under heavy acceleration on 87 octane. Bumping up to premium (93) eliminated it completely and the engine sounded just fine whenever I floored it.

This was with 140k+ miles on the car, and I certainly wasn't into spending more on it than absolutely necessary (piece of junk).

Maybe a decent service would be money better spent
 

BrAdAsS

New Member
98 octane fuel has been refined more than lower octane fuels. Per unit, 98 octane has a higher efficiency, and this is what equates to the power difference and extra mileage. If your vehicle is tuned for 98 octane you will see the full benefit, but, on cars like mine where the ECU uses a 'range', the range will need to be remapped to get the full benefit. Old cars with a distributor simply have to advance their timing a few degrees to get the full benefit.

I don't know what you guys have over there, but, we have another type... Ethanol... E10, E15, E85 whatever you want to call it.... This has a higher octane (100-104) then 98 petrol, and is cheaper, but, it's not as efficient per unit as 98 octane petrol. The effect this has is you use slightly more Ethanol in comparrison to 98 octane, so you get to watch your fuel gauge go down and the difference in pricing get eaten up due to using more.
 

Thunderchild

New Member
well there is another possibility, and yes there has been a difference I can trust myself on that as I am being objective (make me a good QC inspector!). I just bought thre car from a work mate that had not hadit serviced in a long time, air filter looked like it had done 50K miles and the plugs like they were the original ones 80K miles ago. I am on first half of the second tank since I serviced it myself, perhaps the engine has "cleaned" itself out ? I guess that due to lack of air and much larger spark plug gap than recomended the mix was richer and maybe got a lot of deposits in the engine ? or am i talking out of my hat. I could go back to 95 next time and see if it makes a difference. I'm sure the car should perform at least as good as it does now as its a big engine and was driving pretty bad when i bought it.
 

grim

New Member
if your have a learning ecu, disconnect the battery leave it for 5 mins and then reconect, and the ecu MAY POSSIBLY play around with timing to see how good the fuel is.

i just had my car remapped for 99 fuel, and consumption is much worse, but as the other part of the work was fitting a boost controller and pushing the output up by 80bhp, I can't complain :)
 

Thunderchild

New Member
Well what I'm thinkig is maybe the engine was messed up through the lack of service and maybe now is starting to run better, I'm no expert so am at a bit of a loss: performanced improved by fuel or car recovering from lack of service
 

Thunderchild

New Member
Engine designers call it DOHC (dual overhead camshaft). :)

yes thats the one, apparantly a powerful engine although I'm not into speed but I figure that a powerful engine will have more power at low revs and so I should recover fuel by being able to loose gears at lower speeds instead of having to change down more often, for example I have no trouble approaching the roundabout in 4th gear, dropping to 25 mph and then going round the corner and up the hill picking up speed quikly and getting to 5th gear more quickly
 

Speakerguy

Active Member
Wow, you guys in the UK call a 1.8L engine big? That's a large motorcycle engine over here in the states :) I've got a 5.3L V8 in the car I'm driving now, and had a 4.6L in the car that's in the shop. Both SOHC I think (maybe DOHC), but not particularly high-revving.

The gas mileage isn't great, but it's not as terrible as you'd think it would be. The 4.6L will get 22mpg on the highway. Fortunately I live but 1.7 miles from my work.
 

Boncuk

New Member
DOHC's usually have more power at high revs, as it enables more air to enter the engine.

Not quite.

DOCHs make the pushrods and leverage for valve operation unnecessary saving power. Valve opening and closing time generally depends on the steepness of the cam, hence controlling fuel/air amount. Omitting several mechanical elements increases the overall engine performance. Further the cam is adjusted to have direct contact with the valve pushrod via a small piston, thus eleminating the need for length compensation like pushrods require. Ford uses hydraulically driven self-adjusting cam drives.

Increasing just air results in a lean mixture, increasing cylinder head temperature - in extreme cases leading to engine fire, to be recognized by white smoke. :)

(Cylinder heads are usually made of aluminum/magnesium alloy.) :D

Boncuk
 
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