Petrol Grades

[Gazbon008]

Question regarding Mega Squirts behavior when using different petrol grades.

My motor originally used 91 octane - so the factory compression etc was set to be safe using 91 - no internal modifications were done before installation

If I fill with 98 octane now - does the Mega Squirt re-tune - so to speak - to get the most out of the better fuel. or am I just wasting money.

or does one have to re-tune ones MegaSquirt when running 98 and does that mean that 98 is required from then on?

[elutionsdesign]

Wasting money.

[jsiedlicki]

If the motor's compression ratio and cam combo were designed to run safely on 91 octane, there is no point to running any higher octane. It only costs more money

Assuming you are controlling the timing in addition to the fuel with Megasquirt, using higher octane may allow you to increase the timing to get more power if it was not tuned for the max break torque previously. Odds are you would not gain much however.

[Suprazz]

If your tune is made on 91 octane, you wont have benefits, but if you tune your car with 98 octane, then it will worth it. But you'll have to always use 98 or use table switching to two timing maps.

[turbo conversion]

If you use 98 octane you will have to tune fuel along with timing verse 91 octane, at least that has been my experience.

Higher octane fuel burns slower than lower octane so you can add timing so more torque.

What I have seen is the mixture leans slightly the higher the octane rating so a little more fuel is needed to maintain afr target.

If you want to use 91 and 98 octane fuel then ll you have to do is make a 91 octane tune and 98 octane tune and save as such.

When you are using 91 octane then load that tune or when using 98 octane load that tune, it's that simple.

David

[racingmini_mtl]

Higher octane fuel burns slower than lower octane so you can add timing so more torque.

I'm sorry but the octane value is no indicator of the combustion speed. The way the higher octane is obtained might create a slower burning fuel but that depends entirely on the method used (type of additives used). So you could have a faster burning high octane fuel.

And adding timing over the amount needed for maximum torque will decrease torque. So if your engine doesn't knock on low octane at the optimum timing, adding more timing will do nothing but reduce torque and potentially cause issues.

There is a lot of misunderstanding about octane, combustion speed and ignition timing. A lot of what is stated as common knowledge over the internet and amongst car enthusiasts is simply not factual.

Jean

[turbo conversion]

Higher octane fuel burns slower than lower octane so you can add timing so more torque.

I'm sorry but the octane value is no indicator of the combustion speed. The way the higher octane is obtained might create a slower burning fuel but that depends entirely on the method used (type of additives used). So you could have a faster burning high octane fuel.

And adding timing over the amount needed for maximum torque will decrease torque. So if your engine doesn't knock on low octane at the optimum timing, adding more timing will do nothing but reduce torque and potentially cause issues.

There is a lot of misunderstanding about octane, combustion speed and ignition timing. A lot of what is stated as common knowledge over the internet and amongst car enthusiasts is simply not factual.

Jean

As I said this has my experience on the dyno with carberated and fuel injected engines.

So if this is the case can you please tell me the reason for all the different octane ratings then.

David

[billr]

Put simply, higher octane rating allows for higher compression and/or manifold boost. The OP here has neither, hence several of us asserting that the OP will gain little-to-nothing by using a higher octane-rating fuel. If the OP starts fussing with spark timing trying to take any possible advantage of the higher octane, there may very well be a decrease in performance unless happy hours are spent on a dyno.

[racingmini_mtl]

So if this is the case can you please tell me the reason for all the different octane ratings then.

The octane rating is only related to how much a fuel can prevent knock. And different engine setups need different ratings to reach the optimum ignition timing. Any octane rating higher than that is wasted money (everything else being the same).

Jean

[benckj]

Octane is s measure of the carbon bonds within petrol. The longer the carbon chain then the higher the octane and thus the slower the burn. Higher octane fuel is less volatile and more controllable which is why it is required for high compression engine including forced induction applications. It is less likely to explode during high pressure stroke or commonly known as detonation. As a matter of fact increased ignition timing can be used when the fuel is controlled.

As pointed out fuel octane has no relationship with economy or performance. The gains are by adjusting the quantity and ignition to a specific tune. If one of these factors change then the others must be adjusted to suit. Using VEAL makes this a simple process but must either run duel maps or have some way to select tune such as a Flexi-fuel system.

As I run a petrol- methanol mix I tend to use VEAL based on AFR data. Having WI also allows an additional fuel supply during high boost applications to keep engine in a safe operating zone.

[racingmini_mtl]

Octane rating is a measure of knock resistance. It is not a measure of combustion speed but rather of controlled vs uncontrolled ignition (auto-ignition) under very specific testing condition (MON, RON, and others).

[racingmini_mtl]

By the way, if you don't believe me you can always argue with the ones who are actually making the fuel: http://www.whitfieldoil.com/www/docs/17 ... cing-fuel-

Jean

[turbo conversion]

By the way, if you don't believe me you can always argue with the ones who are actually making the fuel: http://www.whitfieldoil.com/www/docs/17 ... cing-fuel-

Jean

If you think I am arguing with any one then you need to reread my post.

AS I SAID, this has been my experience on a dyno.

David

[Yves]

Hm, knock is a product of pressure and heat in the combustion chamber.
That being said, a higher octane fuel has properties that allow more pressure and heat before it self ignites. Has nothing to do with a faster or slower burn speed.

If the engine in question was tuned on 91 octane fuel, you could employ more ignition timing (assuming your are doing this on a dyno) and get more torque. Assuming the engine runs a constant cruise rpm, in essence the total cost of fuel might not be that much different since you will be burning less fuel on 98 if ignition timing is optimized.

[subwoofer]

Incorrect about the timing increase! It is only when the advance is limited by knock there is anything to gain by increasing the octane rating, and cruise is not where that situation occurs. More advance DOES NOT mean more power!

[turbo conversion]

Incorrect about the timing increase! It is only when the advance is limited by knock there is anything to gain by increasing the octane rating, and cruise is not where that situation occurs. More advance DOES NOT mean more power!

My takeaway on what you are saying is your first and second statement contradict each other.

From my understanding this is why OE uses knock sensors for optimum timing for all conditions (to many to try and list) but octane is included.

I have tested this on the dyno and the results were more timing(optimum) with 93 octane produced more HP and TQ than less timing(optimum) with 91 octane.

David

[Paul_VR6]

Best torque may or may not be near knock. A lot of variables are at play. The more compression, boost, earlier exh valve closing, heat, etc the closer they will probably be. I have tuned engines where taking timing out increased power, or kept it flat. Always run the least you need for best torque.

[turbo conversion]

Best torque may or may not be near knock. A lot of variables are at play. The more compression, boost, earlier exh valve closing, heat, etc the closer they will probably be. I have tuned engines where taking timing out increased power, or kept it flat. Always run the least you need for best torque.

I agree.

David

[billr]

Those "happy hours on the dyno" that I mentioned earlier, to be sure you aren't hurting performance with too much spark timing!

[turbo conversion]

Those "happy hours on the dyno" that I mentioned earlier, to be sure you aren't hurting performance with too much spark timing!

The one thing I know for sure is, you don't argue with dyno results.

David