so a local 76 station has a pump with 100 octane racing fuel (it's a hefty $7/gal) but can we safely run this occasionally in the MCS then go back to plain old premium 91 octane?

and, will it really be noticeable and worth the extra $$$$ (obviously if I were gonna track the car, but I'm talking just for a weekend drive and a little more ummmph!)

 

You can run the 100 Octane and switch back and forth with no problems. Whether you'll see any change or not is really variable, it depends on if you're getting any detonation or pulling ignition timing on regular premium.

 

If the car is not tuned to run on 100 octane, you'll just be wasting money.

 

I thought we were supposed to run plain old premium 93 octane ?

 

93 not available in all states. 91 is tops here in good ol' CA.
I've used the 100 octane on occasion, spirited runs on hot days and track days, and my car does run better. But it is $$$$$$$!!

Jim

 

research what OCTANE means.....

to run higher OCTANE than an engine needs is wasting $$$$

It can not increase performance unless your car is improperly tuned....

 

True, but if your car is pulling timing due to heat-soak or whatever reason, the higher octane will prevent the onset of detonation. It's not that our cars aren't tuned for 100 Octane, but they do need to run Octane levels higher than the 91-93 we have in other parts of the world and so they are likely "tuned" from the factory to be able to allow as high as what would be considered a 95 Octane in the US (RON+MON/2).

 

know what OCTANE means??????

 

Make sure it is unleaded? A lot of racing fuel is leaded?

 

Thanks, Jim, I didn't know that.

Can I drop down to 91 ...or is that a not a good idea after 25K miles with 93?

Dean.

 

The car's computer doesn't do any long-term "learning", so you can switch to 91 with no ill effects. I've been stationed in California for the past 18 months, and frankly, the car doesn't feel any different compared to when I was using 93 in Virginia.

 

Yes I know what it means, do you know what the variances in the ratings mean? Read Dr. Obnxs (sp?) writeup on comparing intercoolers, he was showing ignition timing being pulled with normal pump gas and was able to evaluate some of the changes that occurred when he switched to 100 octane fuel.
http://www.fes-auto.com/upload/artic...valuations.pdf

 

I'm pretty sure that some people here on NAM, armed with dynos and dataloggers, have found that the stock 1st-gen MCS engine will ping and pull timing even with 91-93 AKI gas at extremely high RPMs with full-throttle.

Personally, I'm not too concerned about it, because the computer pulling timing will prevent damage, and I'm not going to tailor my gas purchases based on engine conditions that probably make up less than a tenth of a percent of my driving time.

 

There will be a difference with 100 octane. On the R53 you can never run a full timing curve on 91 or 93. You are leaving a lot of timing and power on the table on just premium gas.

 

This is probably the best thread dealing with running 100+ octane:
https://www.northamericanmotoring.co...cing-fuel.html

Basically says that if you are running a stock MCS then running 100+ will just cause you to spend more money on gas; if your car is modified then all bets are off depending on modifications etc

So to answer your question, for non-track weekend drives, save your money. And as long as it is unleaded then it will not damage the car; leaded gas will damage the car.

 

It's very easy to tell if 100 octane is helping or not rather than speculation or butt dynoing. Get a Scangauge or better yet Dashdyno or something that can log the timing out of the OBD port and run through a tank of gas with each fuel and look at the curves. The higher the ambient the bigger the difference. There will be a difference, the question will be whether you are willing to pay for it.

As a side note, this is why some of us have gone to water/methanol systems for the increased octane and knock resistance without paying for tanks of 100 octane.

 

The problem is, a Scanguage or Dashdyno doesn't monitor the output from the knock sensor - it just gives you the instantaneous timing advance.

I can see that being helpful for full-throttle dyno pulls, because any retardation in the timing at 100% throttle & high RPM is going to be from the computer pulling timing in response to the signal from the knock sensor. But at lower engine speeds and smaller throttle displacements, the optimum amount of timing advance that the computer will aim for depends on engine load and RPM.

For around-town or even spirited weekend driving, do you know what the timing advance is *supposed* to be at say, 35% throttle and 3200 RPM? How about 65% throttle at 4000 RPM? On a flat road or a 2% grade? In short, I think that it's going to be hard to identify knock-related timing retardation in normal driving conditions.

And let's say that you find out that 100 AKI does give you more timing advance under certain high-load conditions - if you don't dyno your car as a hobby or race it, do you really want to burn an entire tank of $7/gallon gas just to gain an improvement during 1% of your driving regimen?

You mentioned water/methanol injection as an alternative. I think that's really the better way to go. Set up a boost-activated switch so the injection occurs only when you need it, and then you don't have to worry about special gas. You could even go one step further and install a separate fuel cell/fuel pump to hold 100 AKI (or higher), with a boost-operated switch near the fuel rail to feed the "good stuff" once you're over a certain amount of boost.

 

Your question is a bit loaded as say 35% throttle and 3200 RPM can mean many things. Is it level road, 1st gear or 6th, uphill? I agree with the theory the 100 is only going to make a difference under higher load conditions and the higher the ambient temps. I think you have to define it more as load based than anything else. I can be rolling at 2000rpm in second and snap the throttle half open and at that point the 100 will help just as much as at the dragstrip or autobahn. The computer will run various degrees of timing based on load, since load is nearly infinitely variable, theres no correct answer. But, by logging the timing curves and driving in the fashion in which you normally would, you can see the dips in the timing curve where the ECU has pulled timing back(due to knock) and then started to advance it again. It actually shows up quite clearly on the graphs(and the Dashdyno will record the values and graph them). You will see a big dip in the curve then usually it will jump back up. The best way to see the effects would be to log a few runs of 100, then do same runs with 91, look at them side by side and look for the differences. You will then be able to identify this same knock occurrence in the curves at any time, not just at WOT. Tip in and heavy throttle applications are usually the worst occurrences, but..... this may not affect many drivers in a daily routine. Only testing tells.

And for the love of Pete, if you are going to get a W/M kit, get an IDC actuated one and not boost. The boost ones are all over the board and very inaccurate based on load. It can hurt as much as help the timing if you do not spray it right. The IAT's may look spectacular, but the timing curve probably will not.

 

Just curious, what happens if you go as low as 87 - 89?

 

A hand comes out of the dash and slaps you in the face. Then the car goes to limp mode briefly just to mess with you.

 

I didn't pay extra for the face-slapping feature. Does that come standard?

 

More timing retard in response to more knock, decreased power, more risk.

When I have to run 91 Octane I can see the timing pulled more in day to day driving than when I have 93. I don't feel any difference really but with a gauge I'll see varying response with different conditions but it's always more (never the same) at anything but idle, near steady state cruising and deceleration. For your commute to work 100 Octane is overkill, for a weeked drive where you want a little more spirited driving, why not? It won't be as useful as someone doing a track day would find it but it will give a tad more performance and it's a personal luxury rather than a necessity. The OPs three questions were can he switch between the two fuels (Yes), would he notice a difference (possibly) and would it be worth (not likely but that's up to him).

 

On several long (3000+ mile) cross-country trips, I purposely alternated between the highest octane I could get and the lowest octane I could get at each fill-up, and I drove until I the OBC was reporting less than 50 miles left before filling up, so that I was replacing as much of the gas as I could each time. The high-octane gas was often as high as 93 or 94, and the low octane was sometimes as low as 87 or even 86.

I never noticed a difference in highway drivability between tanks, and the average MPG didn't correlate with the octane rating, either. All of the averages were between about 28 and 31 MPG, and sometimes the highest averages were with the lowest octane, and sometimes they were with the highest octane. The overall low MPG was because a) I have a convertible, so there's extra drag, and b) I was often cruising at 80+ MPH, and that really kills the fuel economy.

So, if you're going to spend an entire tank cruising on the highway, I wouldn't have a problem with using lower-than-premium gas. For around-town driving, I always use the highest I can get, though.

 

Right... mach doesn't indicate if the pulley is stock. If not, the boost is increased and the effective compression ratio (the biggest factor here) is increased accordingly. In that case, 100-octane should make more power.

However, I don't think you could practically use it on the street and keep your license for very long. The track is probably the only place where, I think, it would be worthwhile.

 

+1
You will get no benifit other than a lighter wallet..