Will the engine management system on a stock S2 take advantage of higher octane? If so, how high can I go and still get some benefit? I'm in CA, and we've got 91 octane, R+M/2, at the pump.

 

If the DME is retarding your timing allot then yes Higher octane will help....but I think you will find it will get expensive quick.... what mods do you have ?

 

I've always used 93 octane in mine. With about 11:1 compression I wouldn't think that it'd be healthy to try and run on regular...

 

Hey Deepice,
Engine is completely stock, save for a set of underdrive pulleys. I never detect any sort of pinging/detonation, even under the hardest acceleration...I just wonder if the knock sensor/engine management works so well that it pulls back timing without any noticeable effects. I'll have to check the owner's manual to be sure, but I'm thinking the engine was designed for higher octane than Calif.'s "premium" 91.

 

FWIW - higher octane just means the fuel burns slower... less explosive (less power if you use higher octane than the car needs). On motorcycles, it's easy to tune - you just run the lowest octane you can, and if it pings you go up in levels till it stops. a Little more difficult with a car that has a brain w/knock sensor.

I'd think your best bet would be to pull a few dyno runs. A set with normal pump gas, and a set with highter octane fuel. I doubt the with a stock or even moderatly upgraded engine you'd notice any power increase. The fools spending the money on high octane are either a.) truely fools or b.) have engines modified to the point that it's necessary.

My 87 Honda VFR puts down 90.3HP with regular (this was Arizona in the summer, not sure if they run METB/additives). My numbers dropped to 86.7HP with 'premium' on the same day (after an extended lunch-break - heh), same dyno. Even armed with this info, there are lots of listers on my VFR list who still insit upon premium/higher octane and 'swear' that 110 race fuel boosts the power even more. A testiment for the imagination and 'seeing/feeling' things just because you believe in it enough.

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

I just like the smell of racing fuel in the morning.

Stef

 

[quote]FWIW - higher octane just means the fuel burns slower... less explosive (less power if you use higher octane than the car needs).<hr></blockquote>

I'm gonna have to disagree here. Higher octane makes fuel less prone to early detonation, not necessarily the speed at which the fuel actually burns. This is why you need it in turbos and high compression motors and cars with advanced timing. I guess you could say the higher octane fuel is "less explosive" but the engine will still explode the fuel without any problem when the spark arrives. As for the power of the car, higher octane than needed will not lower the power. You may be paying for premium gas with more octane than you engine has use for (and a few extra detergents that are good for your engine) but you aren't going to lose any power. Running on low octane will make you lose power - especially when the knock sensor begins to retard the timing.

Now, there may be a point of diminishing returns where the extra concentration of octane may be so high that it is taking up the space of fuel. Then you could have a drop in power. But the octane difference would have to be huge (not 93-87, maybe 87-110). Interesting dyno results though, I would love to have a test of all of the octanes compared on an engine - that would be very useful. Remember that fuels themselves vary also based on factors other than octane.

 

"I'm gonna have to disagree here. Higher octane makes fuel less prone to early detonation, not necessarily the speed at which the fuel actually burns. "

Ok, I'm going to have to agree with that disagreement. Octane is just a measurement of knock-resistance, like the Richter scale for earthquakes. It's simply one property of gasoline. Frame-front propagation speed is another independent factor that can be custom-brewed separately from octane. Although ingredients used in the blend may affect one, the other, or both factors. So it's actually possible to mix a gas that has high-octane with fast-burn rate or low-octane with slow-burn or any combination you want (4 possible with these two properties).

One of the blending ingredients, isoprene, has a very fast and hot burn, leading to more power, yet it has a very low octane rating. Increasing the concentration of this component would have a resultant octane rating lower than pump gas, so you'll have to reduce your ignition advance to use it, but the blend will be more powerful.

Some other properties in gasoline that's intentionally engineered may be specific gravity, the D86 distillation curve, Reid Vapor Pressure (RVP) and others. The gasoline blend changes seasonally to meet environmental factors. RVP could be as high as 11-13 psi in summer, and as low as 5-6 in winter, while most racing fuels are around 4-5.

"Now, there may be a point of diminishing returns where the extra concentration of octane may be so high that it is taking up the space of fuel. Then you could have a drop in power."

Now THIS, I will have to disagree with. You are confusing a rating with a chemical compound. There's no such physical material called 'octane', it's simply a rating number. In the early days of octane-ratings, the number referred to a gasoline's knock-resistance in relation to two hydrocarbons. Normal n-heptane (octane rating=0) and 2,2,4-trimethylpentane (also called iso-octane, with a rating of octane 100). These two chemicals are similarly sized, and also have nearly identical flame front speeds, but one has an octane of 0 and the other an octane of 100. In fact, heptane was so sensitive to knock that if you just tapped the beaker, it would explode.

A gasoline's octane-rating was then based upon where it fell in betwen these two compounds knock-resistances. If it had fell right in the middle of the range between the two, it would have an octane-rating of 50. If it behaved closer to iso-octane, then it may have a rating of 70 or 80. Later on with advent of high-performance aircraft fuels, knock-resistance surpassed the 100-octane rating of 100% iso-octane, so a virtual or effective octane number was used above 100. Nowadays, octane is tested on a CFR knock engine.

So really, at the nitty-gritty level, 'octane' refers to a chemical property. Specificaly chemical bonds that are prone to generating radical alkyl groups under the heat of compression. These compounds are particularly sensitive to self-immolation and can ignite surrounding hydrocarbons as well.

 

I am not going to get into a leanthy talk about octane. But, in my 85.5na. After I installed a autothority chip the car responds very well to increased octane.

I use one bottle of super 104 octane boost in the black bottle per tank once in a while. It makes a noticible difference in the car.

 

Thanks for your input, guys. I plan on doing some dyno runs this Saturday, so I'll have a good baseline. Don't know whether it's worth another $65 for three pulls to do a back-to-back on 91 vs. 93 octane though! I checked the owner's manual, and the recommended S2 fuel was 95 RON, or 90 R+M/2, so my guess is there'd be no benefit. Oh well, so much for the quick & easy bhp!

 

I did screw up when talking about octane. It's only a rating and I totally agree with your explanation - I was talking out my ***. I do actually know what octane ratings are (my dad used to design fuels for christ sake) - I wasn't thinking. I'm disappointed in myself - especially after finishing studying my chemistry lecture notes only minutes before writing that post! Thanks for catching my dumbass mistake.

Too many parties...too much homework = College

 

*chuckles* Uh-oh, I think I see a gauntlet. Damn, gonna have to dig through my notes (damnit!! you don't know, but there are something like 16 3" binders FULL of my notes).

In the short (for the mean time, while I get my sources), while 'higher octane' may actually be fomulated to 'burn fast' - just as Danno said, it's primaraly a measure for antidetonation/preignition - the higher the octane #, the more time it takes to burn completely or the more energy used in the burn process to complete a cycle.

There ARE ways (lots of them) to take advantage of the increase in octane ratings (er, rather there are points in an engine's build/perfomance level with the additional octane is useful). On a STOCK, or mostly stock engine though - it is a waste. There's not a company in the world that would design an engine to perform better with fuel that isn't readily available.

My info may be a Little different because I deal in motorcycle/motorcycle perfoamcne (not many turbos or CDI's with knock sensors), but I have some solid chemical info that applies to all fuels aswell (somewhere).

<img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> woohoo.. a little friendly debate <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

 

Well, I WAS going to put in my .02 on octane being a rating but now that Danno's checked in....

What hasn't been brought up is the octane requirement for Doug's S2, 89 should cover it 91 gives a safety margin. Your DME (stock) is programed for a certain octane rating, higher octane will not be benificial but lower will be detrimental. Most of the german cars from the 80s that I've seen require 90-91 octane (RON method), most US fuels rated 89 (R+M/2) meet this requirement. Your S2 may be even higher. This is where chips come in, a chip gives a few HP by changing the ignition and fuel maps in the DME at the cost of a higher octane requirement. My own car runs like crap with anything less than 93 in it. I've been meaning to experiment with higher octane to make sure that 93 is high enough, but haven't figured out how I want to go about it.

Class J, search the archives for "rocket fuel", that little bottle isn't doing a damn thing for you.

EDIT; Damn I type slow, guess you have Doug's octane requirement covered!

 

octane boost : (
100 octane + one bottle of octane boost=100.1

 

I always run half gallon or so vp c-12 to 10 gallons of premium anytime I'm gonna turn the **** up a little (every weekend). Although that ****s high priced I don't have anyworry of detonation and the damage that comes with it. If you mix it too much race gas the car will run like **** till it gets diluted. But I love the smell of that stuff and the little ricers always wonder whats up with your car on cruise nights! The added effect is that when shifting my car often blows a nice flame out the exhaust and lights the interior of any persuing cars up.(or so my friends tell me when they follow me while in it) <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />