We have all seen it, heard about it, or done it ourselves: Filled up our street cars with race gas in the hope of seeing a performance gain. But what does it really do? If a motor is happy with 91 octane, what will race gas do to it? What possible changes would you need to perform to see a gain? We took a trip to Westech Performance Group to thoroughly test these issues on the dyno, and come to some sort of concrete conclusion.
The first issue at hand was choosing an engine to perform the tests. We wanted a common combination that most people could relate to. The Westech-owned "Gladiator" motor fit the description quite well. It's a 355-inch small-block Chevy with a COMP Cams hydraulic flat-tappet cam, ZZ4 crate motor internals, Vortec iron heads, Weiand intake, and Holley double-pumper carburetor. This combination ran without detonation on California's max-mandated 91-octane fuel at 10:1 compression. This choice of engine suited our test well: Its compression ratio is easily into pump-gas territory, yet high enough that it just might be able to benefit from race gas. Simply put, it's a lot like the engine in a lot of our cars.
Step one was to get some baseline numbers. With 91-octane pump gas being fed to the 355, we played with timing and jetting until the best curve and peak power were achieved. There are some misconceptions about how that tune is found, which we will explain in just a bit. Once the baseline was set, we switched fuels to Rocket Brand's 100-octane without making any changes. With those numbers recorded, we tweaked the timing and jetting and looked for the change. From there, we would return the engine to the 91-octane baseline settings, and switch fuel to Rocket Brand's 118-octane, followed by a repeat of the tuning practice to squeeze out max power.
Here is our mule: a 355ci...
Here is our mule: a 355ci small-block Chevy. It's basically a GM ZZ4 crate motor with Vortec iron heads, COMP Cams XE-274 hydraulic flat-tappet cam, Weiand Air Strike air-gap-style intake manifold, and Holley 750-cfm Street HP carburetor. It has a pump-gas-friendly, ping-free, 10:1 compression ratio.
The objective was to find the truth, and either discourage or encourage the practice of putting race gas in a pump-gas motor at the track to gain optimum performance. Our findings were hard to argue, and were not a surprise to any of the staff at Westech.
Tuning For Power
Tune for power-makes sense, right? Many engine tuners don't look at it that way. They have a misconception of using benchmark numbers, such as brake-specific fuel consumption and air/fuel ratio, to guide their adjustments. For example, if you were to decide that a 13:1 air/fuel (A/F) ratio was the optimum number for your engine, you would change jetting to achieve it, and then leave it at that. Engines don't create the most peak power or best curves at a predetermined A/F ratio. The A/F ratio is merely a number you should take into consideration when tuning to keep out of the danger zone. At the end of the day, you are looking for the best performance, not a specific reading on the A/F meter.
The first step is to add fuel until power starts to level out. Once it does, then perform what is called a timing sweep. To do this, pull two degrees of timing out to see if there is a change in power. If there is no change, add four degrees from there to see if more power is gained with the more advanced timing. If the motor likes the increase in timing, keep adding timing until there is no change, or until the motor approaches an unsafe level of ignition timing. Once the optimal total ignition timing setting is found, remove fuel by means of a jet change up to a point to a loss in power. On most engines, this is the path to finding the most power.
Here is where the dyno's subject...
Here is where the dyno's subject motor feeds from. We tested 91-octane fuel from a local Union 76 station in Southern California, and Rocket Brand's 100- and 118-octane unleaded race gas.
Pump Gas Versus Race Gas
Octane rating is defined as the resistance to detonation a fuel has in an internal-combustion engine. The higher the number, the more resistance it has. That is why engines with higher compression require higher-octane fuel. As a result of its resistance to detonation, it has a resistance to burn as well. This resistance to burn is a non-issue in motors tuned to the edge; the edge being just before detonation occurs. When a pump-gas engine is subjected to a higher-octane race fuel, it may result in a decrease in power from an incomplete burn.
The theory is the more fuel you can pass through an engine and completely burn, the more power you can make. The fuel has to be matched with air to create a burnable mixture. Without a power adder such as a blower or a turbo, air is hard to add. That is where oxygenated fuel is helpful. It contains a higher percent of oxygen, creating a quicker, more efficient burn. In the state of California, 91-coctane pump fuel is oxygenated, so the gains between the two fuels due to oxygenation wouldn't be notable.
91 Octane Versus 100 Octane
The first test was to compare the difference between 91 and 100 octane without making any changes from the 91-octane baseline. The engine, with pump gas, responded the best to 37 degrees total advance, and 71/77 jetting. The same tune, with 100-octane fuel, showed no change. Through the same tuning processes used to find the most power from 91 octane, we were able to get up to 7 lb-ft of torque between 3,800 and 5,000 rpm. Horsepower gains were similar from 4,600 and 5,500 rpm. The only change made was going from 77 to 79 jets for the secondaries. At $60 for a 5-gallon can, I personally think that 7 horsepower worth of extra acceleration can more cost effectively come from lightweight parts.
100 Octane Versus 118 Octane
The first test we did with the 118-octane fuel was with the settings that worked best for 100 octane. We saw an across-the-board loss in horsepower and torque. Again, using the same procedure, we found that the 91 octane's best settings were the best for 118 as well. That meant returning the secondary jet size to 77.
Filling your tank with high-octane fuel, when your engine runs fine with 91 octane, is a waste of money. In this engine's case, minimal gains were achieved after a dozen dyno tests were made, and would be too small to notice at the track. The higher 118-octane fuel made less power than both the 110 and 91 octane. It would be interesting next time to see what effects the fuel has on a boosted engine. We would expect to see more impressive changes there.
We limited our changes to...
We limited our changes to those that could be performed at the track between runs in between quarter-mile blasts. When performing a timing loop, you always want to begin by moving in a conservative direction, meaning less timing. You're looking for any loss or gain in power: A loss tells you that it's OK to increase the timing, where a gain indicates a motor on the edge.
We predicted the most gains...
We predicted the most gains would come from advancing the timing, but in this case, jetting yielded better results. An increase in secondary jetting from 77 to 79 jets resulted in 7 lb-ft more torque when fuel was changed from 91 octane to 100 octane-and both had been optimized for ignition and jetting.