Alright, I normally run 91 octane in my supercharged MC with a CIA, but I recently did a pull all the way into third gear and got 5.7 degrees of KR at the bottom of 3rd.

So not liking that huge KR number I got desperate and got some octane booster, I won't name the brand because I'm too ashamed to admit what it was , but it claims up to a 3.1 octane number increase. After a run through 1st and part of 2nd up to 65 I got 0.4 of KR at the end of 1st.

So question time, does octane really play that huge of a role in KR reduction? I know that higher octane is for higher compressions and to reduce pre-ignition and detonation but from 5.7 to 0.4 on the exact same gas, from some octane booster? Any downsides to octane booster besides price and possible fuel line wear with the wrong type?

I'd really like to start adding parts to my car but is it possible/sensible to try and tune for a lower octane? The best I can get within 50 miles is 91 with >= 10% ethanol.

 

Yes, octane matters a LOT when it comes to KR. I had 10* of KR at one point and then filled up with 110 octane. No KR at all.

 

Curious why you're comparing two totally different operating conditions? If you're really trying to do an A-B comparison, you should be looking at the same RPM vs load (MAP) cell. This should be done in the same gear, with similar road conditions.

At WOT, I would expect less KR at the top of 1st since the engine is under less load. It's making more power in the upper RPM, and is physically under less load since the mechanical advantage of the steeper 1st gear lugs the engine less.

At WOT in the bottom of 3rd, you're now making less crankshaft HP than at the top of 1st, you've got less mechanical advantage, and now you also have aerodynamic drag adding to the load the engine is trying to overcome.


Yes, of course it does. Put lower octane into any of the newer boosted or high compression vehicles that recommend premium- and you'll get less performance. It's pretty simple.

Understand that this isn't just static compression ratio. Your engine may only be 8.5:1, but all of the added boost is not any different from high static compression in terms of octane requirements.

Sure, it's horrifically expensive. The little bottles of booster from the parts store have very little octane change when combined with a full tank of gas- you're typically measuring increases in TENTHS to your base octane number (ie 91 octane to 91.7 octane for a 7 point increase). It's a massive waste of money for the little octane increase. If you want to improve octane on the cheap- convert to E85.

It doesn't make any sense at all to tune for lower octane, your engine is supercharged. Running lower octane fuel reduces power output which shoots yourself in the foot for buying a higher performance engine. Buy the highest octane gas that is regularly available at the pumps, and base your build off of that.

 

I probably should've been a little more clear on some things.

The 0-65 pull with octane booster only had a 0.4 blip at the top of 1st, likely false knock, all other RPMS had 0 KR. On straight 91 I was getting a max of 5.7, there was KR through out the gears, I'd have to look at the logs to see how much and where, but it was significant throughout, but yes I'll admit, slanted comparison.


The stuff I bought was $10 for 31 points or 3.1 octane numbers and a minimum of a 16 gallon mix. I would love to convert to E85 but there's exactly 0 stations with it within 75 miles of here.

I actually meant lower octane than what most modders run, not lower than 91 . 91 would be the highest I can easily and regularly run so I was asking if 91 would be doable for a build as long as I mod for more KR reduction than normal for a given pulley size.

By the way, how do most of you guys scan KR? I went the cheap route and got the Torque app on my phone. I could make a guide on how to set that up, it's got a really nice logging feature and can give you a pretty good HP and Torque estimate if you know the weight of the car.