"PSA: 1.5T / 2.0T Octane Ratings & Why You Probably Don't Want 87oct"
First off let me introduce myself. I'm 33 years old and have been working in automotive my entire adult life. I started out as a kid racing 100cc 2 stroke karts in Europe, moved to cars as I got older, and I worked in mod shops in my teens doing basic upgrades and repairs. I took a brief hiatus from technical work and became an instructor in the state I lived before moving back to building/modifying cars. In 2016 I discovered I was soon to be a father and so I explored employment opportunities that would be aided by my experience. Tuning was an obvious choice so I racked up online classes until 2019 when I flew out to Ben Strader's EFI-U in Lake Havasu, Arizona which I enjoyed thoroughly.
I think my experience and expertise in the field qualifies me to speak candidly on this topic, and I do so for your (the owners) benefit.
Let's talk about what octane is. Octane is simply a number given to quantify how stable the fuel is. The higher the number, the more stable/more resistant to knock or detonation. What is knock/detonation? Knock or detonation is simply when the fuel and air mixture auto-ignites before the spark plug is fired. This can happen from simple compression alone, but is often accompanied by poor intake air charge density (from high temperatures), high cylinder temperatures, or poor fuel quality.
So what happens? Let's imagine a piston coming up on the compression stroke. As it approaches the top of it's stroke BANG there's knock, the piston is traveling up the cylinder and collides with the massive concussive wave from the knock event. When this happens you can break the support sections that the piston rings sit on (ringlands) and you can even crack it all the way across. You also have a massive and sudden increase in cylinder pressure which can be in the neighborhood of +40%. Such an event can and will take a massive toll on your head gasket AND your fuel injectors. Since these engines are direct injected, the tip of the injector is also hanging out in the combustion chamber and stands to get damaged as well.
What's done about it? Modern knock sensors have come a LONG way in the last 10 years which gave way to new fueling strategies used by manufacturers. When it's not receiving knock feedback an engine can hit it's target ignition value (when the spark plug is told to fire), and similarly when there is knock detected the ECU will retard timing to initiate combustion before knock can happen again. This is as a safety net used by the manufacturer to prevent engine damage in case something isn't running right or if there's bad fuel. In my opinion this is where Honda made a mistake, they specified a minimum octane requirement of 87 for their line of modern turbocharged engines which results in them CONSTANTLY running in a retarded state thanks to the knock sensor being triggered. This constant triggering of the knock sensor results in high cylinder pressures during the regularly occurring knock events, potentially damaging the head gasket and fuel injectors over time. I'm betting Honda simply weighed out whether they would lose more money from people not wanting to buy an economy vehicle requiring premium, verses coughing up if any damage happened during the warranty period. I'm also betting they knew folks would be paying for repairs outside of warranty, further offsetting any costs in warranty. This can only have been a calculated move IMO.
Why wasn't this an issue before? Several things have changed over the years. One, we now have direct injection which is able to atomize the fuel significantly better which makes the fuel/air mixture more stable, and two we also have turbochargers that can build boost ridiculously low in the RPM range. The result is something called Low Speed Pre Ignition or LSPI for short (a kind of knock). Essentially since the motor is rotating so slowly in the lower RPM's it becomes very difficult to achieve combustion at the optimal moment because combustion is not instantaneous, it is still a chemical reaction and thus happens over a period of time. When you have a slowly rotating engine, a low octane air//fuel charge, AND now you're adding the stress of boost, you have a recipe for LSPI. The engine simply isn't moving fast enough for combustion to occur optimally, and with the lowered stability of the fuel/air charge you get knock. LSPI is the biggest killer of small, modern, turbocharged engines. I bet you didn't know turbo lag was actually useful for something lol.
I noticed there was a problem the first time a customer gave me their 1.5T with 87 in the tank. Before I start tuning I like to datalog the car with the factory flash on it just to see how everything is behaving. Yikes! The knock sensor is working hard with 87 in the tank, even on stock boost! It was to the point where if I saw that on a car that'd been tuned, I'd say the tuner left it in a dangerous condition. I called the customer and asked what fuel he pumps, and sure enough.... 87. I explained that needs to change, and he agreed to make it a dedicated 93oct car. After refueling with 93 the knock sensor calmed down and we were able to make more power taking advantage of the better fuel.
A long time ago my tuning partner who lives in Thailand was shocked to discover that we even sell 87 octane here. Their lowest equivalent octane is 91! Since they have exactly the same powertrain I got curious and asked, "how bad are the head gasket and fuel injector problems over there?" He responded with confusion, "we don't have any. Why? Do you guys?" The lightbulb went off at the same time for us both as we agreed on the cause and solution to the problem. Even with the intense heat and the resultant high density altitude, they still don't get knock. The only difference is the fuel.
What we have here is somewhat of a compounded issue. We have a small displacement turbocharged (small turbo too) engine fed poor quality fuel and a transmission that keeps the engine at low RPM's. This is a recipe for LSPI, which can and will blow a head gasket over time, and absolutely will damage fuel injectors for the same reason. Exactly the problems seen by consumers. The worst part about it is folks who think they're doing the engine a favor by babying it at low RPM's are actually causing regular LSPI events and making the problem worse. At least if you're floored the higher engine speeds mean combustion events can happen in a larger and more optimal window.
I leave you with a recommendation in conclusion to my explanation and warning. $6-$8 bucks per tank is a lot cheaper than a head gasket and injector replacement.
