Back-to-back testing of four different-octane fuels on a dyno takes the guesswork out of choosing the right one for your Spec E30.

So many times I’ve heard different opinions from various experts on which octane fuel makes the most power. According to Spec E30 rules, “Permitted fuel is unleaded pump gasoline, with a maximum octane of 93. Fuel must be from a mass-marketed supplier, e.g. BP, Sunoco, Exxon, or other independent mass marketer, e.g. track supplier or local independent gas station.”

It is true that some engines can gain performance by using a specific fuel, but there are some variables as to how a given fuel is able to make more power. Should I spend more money for 91-octane fuel, or should I spend a lot less for 87 octane and save some money over the course of a race weekend? Or is using cheaper fuel hurting my engine and the performance of the car? There are so many questions, but first, let’s understand what an octane rating means.

Octane is the measure of a gasoline’s ability to resist knocking or pinging during combustion. The higher the octane rating, the greater the fuel’s resistance to knocking or pinging. The knocking or pinging in your engine occurs when the air/fuel mixture detonates prematurely. In broad terms, fuels with a higher octane rating are used in high-performance gas engines that have higher compression ratios.

Another way to look at octane is that a lower octane number has a faster burn rate. It has a better ability to expel energy faster. A higher octane fuel slows the wave front of the combustion flame and raises the temperature at which combustion occurs. For example, you may have driven an older car that pings. Even though it has a low-compression engine, carbon buildup on the piston or on the cylinder head can cause a hot spot, which can pre-ignite the fuel, causing a ping. Using a higher-octane fuel can alleviate these symptoms.

- Advertisement -

So again, which one is right for my car? For that answer, you must find out what the compression ratio of your engine is. In this story, my freshly hand-built Spec E30 M20 engine is rated at 8.8:1 compression ratio. That’s a common compression ratio for an engine that was purpose built to be a commuter car. The minimum octane rating for the engine from the factory is 87. That might be fine for commuting, but what about for racing? One way to find out is to do a side-by-side test of available pump gas, and that’s what we did!

The Setup

After much thought on how to get four different fuels into the car for its testing, the simple solution was to create a fuel pump system that would simply sit on top of racing fuel cans. I wired in a simple on/off switch for the external pump. All that was left to do was to swap out the different fuel cans with their different octane fuels.

The other thing that needed to happen was to disable the stock fuel pump, otherwise we would have had a mess. That was simple to do and all it required was pulling the fuel-pump relay and capping the stock fuel line once it was removed from the fuel rail.

Since this was a fairly fresh build and the engine only had about five hours of run time on the track, I needed to do a baseline pull on the dyno to see what kind of figures we had. The first pull showed a healthy 155 horsepower and 156 pound-feet of torque, but the air-fuel ratio showed it was very lean. A quick adjustment and a few pulls later, the engine was running right where the fuel mixture needed to be and the power numbers increased to 158 horsepower and 158 pound-feet of torque. Just as when you fill up your car with different grade fuels, I was going to leave the mixture alone to better simulate what would be available at the track, where there is no safe way to adjust the air-fuel mixture accurately, for the most part.

Now that the engine was dialed in with the air-fuel mixture, I disconnected the main fuel supply line to the fuel rail and pulled the fuel pump relay. After I plugged in the electrical plug for the external fuel pump and fastened the fuel hose to the fuel rail, I turned on the pump to pressurize the fuel rail with the bottle of 87 octane fuel and did another dyno pull to make sure the pump being used was working properly. The same numbers came up (158 horsepower/158 torque), which made me confident the test would be accurate for the remaining fuel grades.

Using the 89-octane fuel showed a similar number as the 87-octane fuel. The more pulls we did on the dyno, however, the more the power dropped off as the engine ran hot. The 87-octane fuel maintained more power than the 89-octane fuel.

The dyno sheet for the 91-octane fuel showed horsepower and torque figures within one point of those measured on the 89-octane fuel. However, it showed a much higher decrease when the engine returned to its normal operating temperature, and as it got hotter on the dyno, showed an even greater a loss of power.

With all the discussion of E85, I decided to test it on the car last. As I mentioned before, I didn’t make any fuel-setting adjustments with the different fuel grades. Without changing anything, it was very clear that just running the car mildly in third gear to get some heat back into the motor showed that under light loads the fuel mixture would go into an extremely lean condition. So much so that we decided to abandon a test with the 100 percent E85. Instead, we opted for a test with four gallons of 87 octane fuel blended with 1 gallon of E85. That made the lowest power of the day and proved to be very lean.

We saw no reason to try a higher E85 mix ratio because it would only get leaner. To be able to use E85, larger injectors would be necessary and probably a fuel pump with a higher flow rate to keep up with the demand. It has been established that E85 can provide gains to other racecars, but we found that was not the case for a Spec E30.

What did we learn? Surprisingly enough, the lower fuel grade 87 octane made the most consistent power for our car and E85 is a definite no-no with the stock fuel system, without potentially causing major engine failure. We found that the more expensive, higher fuel grades did not add more power. On the contrary, the engine lost 2 to 3 horsepower versus the more affordable 87 octane.

It’s important to note that our findings apply fairly strictly to a Spec E30. Other classes of racecars that allow for adjustments to ignition timing might benefit from a higher octane fuel, but that was not so in the case of a Spec E30.

Now when I go to the track, I know that being thrifty on fuel also gives me the most bang for the buck.

Running 87-octane fuel, we registered 158.15 horsepower and 158.11 pound-feet of torque.

Using the 89-octane fuel, the more pulls we did on the dyno, the more the power dropped off as the engine ran hot. The 87-octane fuel maintained more power than the 89-octane fuel.

The 91-octane fuel showed a decrease in power after a 1-horsepower increase from the baseline pull. However, as it got hotter on the dyno, it showed an even greater a loss of power.

Here’s a graph showing all the dyno pulls with the different fuels. There were some temperature variances between pulls, as noted in the color key chart, but 87 octane still made the most power.


  1. The accuracy and resolution of a chassis dyno are way below that required to make this in any way a meaningful test.

Join the Discussion