Hesitation on acceleration

[TMP39]

hey nightshade, talked to guy at work today and you and i are both right, the different octane levels do prevent knock and prignition and detonation. but they do also burn differently. but anyways..... have you even checked your vacuum yet? 17 - 21 inch pounds is prety much average at an idle.

[_-Night-Shade-_]

hey nightshade, talked to guy at work today and you and i are both right, the different octane levels do prevent knock and prignition and detonation. but they do also burn differently. but anyways..... have you even checked your vacuum yet? 17 - 21 inch pounds is prety much average at an idle.

We are not both right, it doesn't burn differently. I've had every octane in my car and it doesn't give me more power or better fuel economy, it's the same sh*t. If an engine can run on 87 then using higher octanes will do squat. This makes sense as it's what every piece of literature is saying too. Maybe there are some magic gas stations in your area or something. The only thing that could be remotely different is that here 87 has 10% ethanol, 89 has 5% and 91 and up has none, but even still it's too small of a concentration to make a noticeable difference.

[Ryan]

Night_Shade, I think you're a little on crack about this.

If octane made no difference, we wouldn't ever need race gas, etc... octane ratings do correlate to tendencies of predetonation. Thats basically the whole reason we have the choices. Our cars aren't high enough compression to NEED more than regular gas though. Boosted, high C/R, and high power in general are a different story...

I notice a significant difference in how my cars run on 91 to 85. More of a difference on the car that runs the worst normally, though.

I notice a bit better mileage(50 more to a tank) but that doesn't pay off.

Not magic gas, just science

[Inodoro Pereyra]

Not quite.
Night shade is right that octane rating refers EXCLUSIVELY to the fuel's resistance to detonation. There is no correlation whatsoever between octane rating and either power delivery or fuel economy.

That said, gas is not a single substance, but a recipe. Gas is normally a carefully combined mixture of hydrocarbons, oxygenates, detergents, and a whole lot of other stuff.
Now, be it for economic reasons, performance reasons, whatever, companies tend to have different formulations for different octane gas. That means that, even when octane rating alone has nothing to do with it, different octane gas CAN, and DOES produce, in many cases, different results. I have personally tested Citgo 87 and 92 octane gas in 2 cars (a '91 protege B8 and a '92 MX-3 B6), and 92 octane gave me consistently a 10% increase in gas mileage over 87 octane.

Ryan, racing gas exists to allow racers to use higher compression ratios in their engines, therefore getting more power.

[Ryan]

You know what, Inodoro, you're right again.

The only time I buy premium is from Shell. That would explain the massive difference from CO-OP gas

[_-Night-Shade-_]

Thanks Inodoro.

[Inodoro Pereyra]

Haha...don't sweat it. I'm just a very lucky guy.

In the old times, before BP came and screwed it all up, I once put Amoco 92 octane in my car. I don't know if it affected the gas mileage, as I only filled one tank (it was way too expensive for me), but the difference in power was noticeable. Later on I tried BP, but it was the same sock juice as the cheaper brands.

[h3xt0r]

Inodoro, you're right on the dot about the Octane rating of fuel however I'd like add to it, to clear up some ideas.

The higher octane does effect performance (LET ME FINISH!) in COMPARISON to lower octane fuels. The reason for this being that the lower octane fuel, when under stress, will predetonate. This in turn will cause the engine's performance to deteriorate. The relationship between octane rating and performance isn't that the fuel itself has more energy, it is that it doesn't predetonate. That is what alot of people don't take into account, the fact that higher octane is harder to predetonate.

[mazdags94]

The higher octane does effect performance (LET ME FINISH!) in COMPARISON to lower octane fuels. The reason for this being that the lower octane fuel, when under stress, will predetonate. This in turn will cause the engine's performance to deteriorate.

I'm not sure how much I believe this.... you are basically saying that no matter what, if you use a lower octane fuel, and put the engine under a load, you will always predetonate.

[Inodoro Pereyra]

h3xt0r: it's not like that. Detonation has nothing to do with stress. Detonation is caused by temperature. And temperature is caused (among other things) by pressure.
An engine like the K8, for example, with a compression ratio of 9.2:1, and a typical ignition timing of about 10 deg BTDC, will work perfectly with 87 octane fuel, always. You could possibly go up to 10.5:1, and 15 or more degrees BTDC, before that engine starts detonating.
Detonation happens when the temperature in the combustion chamber (which is, everything else being equal, a direct product of the compression ratio) is so high that the air/fuel mixture reaches its autoignition point BEFORE the spark plug can do its job.
That said, detonation happens ALWAYS at low RPM, regardless of engine load. That's because at higher RPM the F/A mixture just doesn't have the time to get hot enough to detonate.

Higher octane fuel DOES produce more power, if you take advantage of that higher octane rating and increase the compression ratio of the engine, and/or the ignition timing. Otherwise, octane rating, per se, means nothing.

[mazdags94]

+1...

All cars are pre-programmed to function efficiently off of a certain type of octane rating; also to maintain the vehicle's intended performance and keep temperatures lower. Since are cars were programmed to perform with 87 octane, then simply dumping in 89, 91, or higher isn't going to do anything.

Modding your engine or adjusting beyond the OEM specs may require the use of a higher octane (as Inodoro suggested above) for temp reasons (to avoid detonation).

Heres something to try as an experiment:
Next time you are at the track, fill your tank with 87 and do a run.
Then, add some "octane booster" and do a run to see if there was any difference performance-wise.

You could also try to do this if you have access to a dyno.

Car and Driver did a test on this idea before only to find that octane booster did just what it says, Boost Octane but no gains were found in HP or torque.

Now, back to the hesitation problem

[stumsport]

Hi guy's

Thanks for all the info you have given me. I am busy at the moment working through all the possible solutions that you guy's have mentioned so will let you know if and when I find the problem. Once again thanks for all your help guy's.

stumsport