Which Petrol

[Alpine]

Jazz, Civic, CRV etc are all tuned for 91 octane...yet alot of us insist on using 98 octane in our rides claiming better fuel consumption, more power, etc. In our gen 1 CR-V however, using 98 octane actually DECREASES responsiveness, torque and fuel consumption appears to be the same as on 91. When I use 91 in the CR-V, the engine feels noticably more responsive and torquey. Even my wife notices the difference when she drives it and she has no idea about cars or the types of fuel...so if she notices it, then the difference is real. So we stick to 91 in our CRV, the bonus also it being the cheapest fuel.

Moral of the story - stick to the fuel recommended by Honda for your car. By all means try the higher grade stuff and experiment. If it makes your car better, then great, but if you find it honestly runs better to the fuel it's tuned for, stick to it and don't waste your money.
(this is of course assuming your car is stock and you haven't mucked around with the engine so it DOES require higher octane than stock)

[2K9Jazz™]

www.drive.com.au explains everything...




91 octane for me!

[debowen]

I certainly agree with Alpine on the cost benefit of sticking to the 91RON fuel, but when you try 98 RON you have to use a second tank as the computer takes about 1/2 tank to re-tune itself to the change. The 98 burns hotter and will clean out the carbon in the head, values and exhaust system. It adds about $5.00 to the cost of a tank (40lts) and is a bit cheaper than an injector cleaner. On that subject, I have tried a few brands and settled on FLASH LUBE. 250ml bottle is about $7.00 and I use 1/2 a bottle to treat injectors, cyl head etc. I do this every 20,000km in line with my auto trans fluid change and now stick to 91RON petrol.

[rossw]

I certainly agree with Alpine on the cost benefit of sticking to the 91RON fuel, but when you try 98 RON you have to use a second tank as the computer takes about 1/2 tank to re-tune itself to the change. The 98 burns hotter and will clean out the carbon in the head, values and exhaust system. It adds about $5.00 to the cost of a tank (40lts) and is a bit cheaper than an injector cleaner. On that subject, I have tried a few brands and settled on FLASH LUBE. 250ml bottle is about $7.00 and I use 1/2 a bottle to treat injectors, cyl head etc. I do this every 20,000km in line with my auto trans fluid change and now stick to 91RON petrol.

98 doesn't burn hotter, it burns slower. That's why it can be used with more ignition advance. The additional advance compensates for the slower burning.

If you really want to clean deposits out, use E10. The ethanol keeps water from building up in your tank and dissolves deposits that will not dissolve in petrol.

It's exactly like the old bushman's trick of putting a cupful of metho in your tank to get rid of any water that might have got in there. Ethanol will mix with water and still burn. Petrol will not.

I have tried 91, 98, 95 and E10 in my Jazz with little discernable difference. My car has no taste buds so "nicer" isn't relevant. I use whatever get me the most km for the least $ and ignore the marketing BS put out by the oil companies.

"Put in 98 if you love your car" ?? FFS!

[debowen]

Thanks Claymore, I got that wrong, but why did the 98ron fuel change the feel of my '91 Diahatsu Applause 1600cc when I drove it to the eastern states of Australia? I used to run it dry and fill with 98ron on my first leg out of Perth and by the time I went 200km the car would be running 'sweeter' and the colour in the exhaust pipe went from black to dark brown.

[rossw]

I hate to admit it but I used to be one of the people that said higher octane was a waste. BUT THEN a member on the fit site where they are real freaks did a nice little study using his scan gauge which shows ignition advance.

He tested using 87 and drove a circuit in his area that has flat and hills. He did the 87 first at a bunch of speeds and conditions then went to 95 the highest in America and waited a week and another tank full just to be sure it was all 95 now and drove the course again.

The results were amazing using the 95 the timing WAS ADJUSTED ADVANCED by the ECU (Stock Fit 1.5 Vtec with stock ECU). It varied by engine speed and load but averaged 2 degrees advanced and as high as 4-5 degrees under load going up hills.

That is a big advance and is enough to feel in the seat of your pants. So it just goes to prove that the Fit/Jazz is very efficient at the ECU and it will take advantage of higher octane fuel and now we have actual PROOF not just somebodies word for it.

So I tried it in my car and even though I don't have a scan gauge to prove his evidence I could feel the difference and got a whopping 3-4 kilo per liter increase around town. I used to use 95 here on trips and thought the better mileage was just from highway driving but it was really from using the higher octane also.

So the bottom line is you can use what ever fuel you prefer but using the higher octane will give more performance and fuel mileage proven with FACTS not just the "Old stories"

I read that article too and you're right. Using higher octane fuel DOES increase ignition advance, based on this evidence.

Remember though, as you have said, the higher the octane, the slower the fuel burns. That's the only difference between "grades" of fuel.

So lets look at the example of an engine that is using higher octane fuel than it is designed for and that is therefore burning slower. All other things being equal, it will then make slightly LESS power, run hotter and use more fuel because it is effectively running retarded.

In our case (Honda Jazz) the ECU can compensate for this by increasing the ignition advance. This has been proven, so we are back close to where we were. The engine is giving the fuel longer to burn to compensate for its slower burning.

Maybe under such conditions it is possible to get 3-4MPG difference. That's a difference that can be obtained by a change in driving style, terrain or wet weather, or using the air-conditioning.

Even if the fuel consumption improvement is real, (see the Drive comparison that suggests that it is, at least for a Camry) the 13c/l difference in price in Australia means you are still on the losing end. Maybe in Thailand, the price difference is less. I don't know.

[rossw]

"So lets look at the example of an engine that is using higher octane fuel than it is designed for and that is therefore burning slower. All other things being equal, it will then make slightly LESS power, run hotter and use more fuel because it is effectively running retarded.

In our case (Honda Jazz) the ECU can compensate for this by increasing the ignition advance. This has been proven, so we are back close to where we were. The engine is giving the fuel longer to burn to compensate for its slower burning."



Your close but no cigar. The longer burn time is where the extra power comes from no matter which type of fuel is recommended for any engine. The longer it burns then the more cylinder pressure it creates which means there is more pressure on the piston top which is extra power. The extra heat is what makes the power it's does NOT cause the engine to run hotter it is only hotter in the combustion chamber and that is a good thing more heat = more power IN THE COMBUSTION CHAMBER. Now this extra heat is made in milliseconds and would not even be measurable in the engine coolant.

I didn't change fuels for increased mileage I changed to take advantage of increased power the extra fuel mileage was an unexpected bonus.

Here the difference between 91 which is the recommended fuel and 95 which is the highest octane available here is 3 baht per liter which is 0.08 US cents times 25 liter fillup the difference is I'm paying a grand total of 2 that's right 2 USD per fillup on top of the normal $28.00 to get the extra power and extra fuel mileage so that drops it even more so I'm paying less than $2.00 for the gains and it's very worth it to me. If you can't afford less than $2.00 USD per fillup for more mileage and power than stick with the lower grades.

Sorry, Claymore but I think you might have missed something there.

Regardless of octane, petrol has the same calorific value and has the same amount of energy in it, unless it's E10, in which case it has about 3% less.

Lets look at our scenario changing only one thing at a time. I'm sure your familiar with this as a principle. I'm going to go through some physics that are poorly understood (probably not by you, as an experienced mechanic, but by a lot of people)

If we start with our regular 91 octane fuel, and run our engine at constant speed, with an ignition advance setting that makes sure that the spark happens soon enough on the compression stroke so that all the fuel is burning by the time the engine reaches the optimum point, in other words, all of the fuel is being burned and contributing it's energy to driving the piston downward. Call this scenario A.

OK Lets change one thing. We'll advance the timing a bit.

Now the fuel is being burned too early and causing the flame front to meet the piston while it's still going up - bad! and this is the cause of detonation in cars that are being run on fuel with an octane rating too low for them. Call it Scenario B.

OK, let's now retard the timing again so that the spark happens later than in Scenario A. Now the fuel starts burning too late and the fuel is still burning and expanding after the piston has already gone. We didn't get detonation, but neither do we recover all the energy from our fuel. The fuel still burns, but the energy not transferred to the piston is lost as heat to the cooling system. This is scenario C.

OK returning to Scenario A, which is our optimum scenario, without changing the timing we alter the fuel Octane.

If we use a higher octane fuel, it burns more slowly. thus taking longer to burn completely. Remember we have injected the same amount of fuel and therefore introduced the same amount of energy into the system. Since it's taking longer to burn completely, and we haven't changed anything else, we have wound up with Scenario C, where the flame is chasing the piston back down the cylinder - at least partially, because it has taken longer for the flame front to reach it. Remember we started with optimal timing for our lower octane fuel.

If we now try a lower octane fuel than in our original Scenario A, then we get detonation again, because our now much faster expanding flame front reaches the piston while it's still on the way up, or at least before it's ready, so we have Scenario B again.

If we change both parameters, higher fuel octane and more advanced ignition timing, we start the spark sooner to compensate for our slower burn. Once again our piston is being pushed down at exactly the right moment, and our engine is happy.

The amount of energy being produced is the same in both Scenario As because the calorific value of the fuel is the same.

What I have described is simplistic - deliberately so. There may be other factors like valve timing that have an influence, but unless someone can show me dynamometer figures or consistent lap times, I remain unconvinced.

I am trying hard to be nice about this too, and making every effort not to be insulting about this, but there is a lot of misinformation on this topic and a lot of people spending money they don't need to.

I don't think it's a matter of what one can afford, it's more where one's priorities lie. I didn't buy this car for it's performance (entirely). If I wanted a track weapon I'd have bought something else.

To sum up though, my argument is that just because you are getting more timing advance, doesn't mean you are getting more power. It does mean your engine is adjusting to the different burn speed to avoid making less power. Maybe you do get more power out of this, but not as much as you'd expect I think.

I'm still waiting for someone with the resources to do a back to back dyno test that would resolve this issue once and for all.

[fundies]

Very interesting to read the "technical feud" above, but I'm with Claymore. Been using mobil 8000 exclusively for the last 3 months, and its great. No doubt smoother, quicker, more responsive. A small price to pay I say.

[debowen]

"So lets look at the example of an engine that is using higher octane fuel than it is designed for and that is therefore burning slower. All other things being equal, it will then make slightly LESS power, run hotter and use more fuel because it is effectively running retarded."
So that's the reason my Diahatsu ran hotter, cleaned the exhaust system and change the exhaust colour from black to
brown. It couldn't adjust the timing electronically! Led me to believe that higher octane ran hotter , but now I know it burns slower.

[rossw]

Not trying to be insulting or condescending but your posts reflect a nice young guy who has had some scientific training but no mechanical experiance.

On the contrary I'm flattered. the rest of you, Off my lawn!

You have all the data you need from his post all you need is some time actually working on and tuning a car to understand his data and what it means in the real world.

I defer here to your obviously much greater experience. And thank you for making it clear. I am merely trying to understand what I know is a poorly understood subject, and I have this habit of challenging what appear to be "sacred cows". I am a mechanical engineer, but Automotive technology is not really my field. (I do other more boring stuff.) You would know yourself that there are a lot of commonly held misconceptions out there, mainly due to marketing of products of dubious worth when it comes to producing more power.

I myself got sucked into it in my student days when I bought a "sports" air filter for my Mini 1100. It made stuff all difference to a car that had only about 50hp anyway. What did make a noticeable difference were things like keeping the original filter clean, and diverting the crankcase breather into a catch bottle (the rings weren't the best and removing the oil fumes from the intake made it go better).

I didn't explain what I meant when it came to the flame front being behind. What I meant was that if the spark is retarded, the cylinder pressure doesn't reach such a high peak because the piston is already on it's way back down, and the expansion happens at a lower pressure.

Now lets see if I've got it straight this time. Higher octane fuel allows a higher peak cylinder pressure without causing detonation. This in turn means you can start combustion sooner and get more cylinder pressure which means more driving force during the combustion stroke - ergo more power.

Thank you for helping me to improve my understanding in this area, and I apologise if I have caused offence. My goal is to establish the truth not to be right.

As you say, in the end, what fuel you use depends on your priorities. Myself, 88kW is more than I can use in everyday driving anyway, and if I tried, I'd probably get arrested. On this basis there is no value to me in spending money on higher octane fuel.

I think that after this discussion there will be people who go either way, and both will be happy.

[Alpine]

I don't think the 2.0 gen 1 CRV does...in fact it seems more responsive on 91 than on 98.

Not sure about the Civic though.

[gmonkey]

i use 95 octane, but the fuel octane amount will make no difference in performance and little amount in economy in a 1.3 or 1.5ltr car.. but i dont mind spending bit more for something a bit cleaner.. dont trust bp ultimate anymore after they were caught mixing the standard and 95 together and selling it off as 98 :P dodgy to me