Ever since high-octane leaded gasoline vanished from the scene, there’s been an undercurrent of concern among hot rodders every time they fill up at the pump. For the most part, daily driven hot rods have adapted to greatly reduced fuel quality by embracing stroker kits, nitrous oxide, specialized camshaft grinds, aluminum heads, and lower static compression ratios. It must be working; cars just keep getting faster.
To explore the impact of fuel quality on engine performance, we stuck a 10.4:1-compression-ratio 360 Mopar on the DTS dyno at Joe Jill’s Superior Automotive. Then we beat it up, making 40 power pulls to see if octane rating has a significant impact on power and if ignition timing can be effectively manipulated to ward off detonation without heavily penalizing output. Does fuel additive really increase the octane of pump gas? And does boosting the fuel’s octane really make more power on a typical street engine? The results are surprising.
87-Octane Unleaded: 396.0 hp/401.3 lb-ft
You pull up to the pump in your hot street whip; the needle’s on E. You’ve only got 20 bucks, and you have to make it count. Premium is 30 cents a gallon more expensive than the cheap stuff. You roll the dice, grab the nozzle marked “Regular,” and start filling.
Approximating this scenario, we filled our 2-gallon fuel cell with a dose of 87 octane and set the total ignition timing at 31 degrees BTDC. Despite the sleazy gas and heavy dyno loading, the smooth power curves indicated no sign of detonation. Then we tried 34 degrees and still no sign of detonation. Yet another counter-clockwise twist of the Accel Billetproof electronic distributor gave the Mopar 36 degrees total; despite the lousy gas, the motor liked the additional timing.
Could it take more? Pushing the envelope further, we dialed it up to 38. Detonation had found us. The most telling evidence of detonation was seen in the 300-rpm drop where peak horsepower was made and in the way the smooth power curves of the previous tests had become a jagged mess at 4,200 rpm all the way up to our self-imposed 6,000-rpm limit. The peaks and valleys on the dyno chart reveal uncontrolled combustion causing fluctuations in peak cylinder pressure and, as a result, hiccups in the force delivered to the business end of the crankshaft. See Test 1 in the sidebar below.
87-Octane Unleaded With 104+ Octane Booster: 397.9 hp/403.1 lb-ft
The rent is due and the kid needs new sneakers, so you’re running the cheap stuff… again. You add one 16-ounce bottle of octane boost to your 20-gallon tank, cross your fingers and hit the road.
To see if we could turn sow’s-ear 87- octane into silk-purse go-juice, we shut off the dyno’s fuel pump and let the test motor drain the Edelbrock 750’s bowl dry at idle. Then we added 2 ounces of Super 104+ additive to the 2 gallons of 87 in the fuel cell. With timing set at a conservative 31 degrees, we saw no appreciable difference, but we’d only just begun. Twisting the sparker up to 34 degrees BTDC delivered 6 hp, and we were still far from the motor’s likely detonation zone. At 36 degrees we noted that more peak horsepower was made at a higher engine speed, a clear sign that the chemical enhancement was keeping detonation at bay. Further proof of the benefit came when we cranked it well into what should have been rattle city with 38 degrees of timing. Power readings were on their way down due to mechanical factors related to the efficiency limits of the heads, cam, and induction rather than fuel quality. Despite this, the motor still made peak power at 5,800 rpm, a full 400 rpm more than without booster. Convincing proof that the Super 104+ was thwarting detonation.
If some is good, then more must be better, right? Doubling the dose of octane booster to 4 ounces in the 2-gallon fuel cell (like putting two 16-ounce bottles in a 20-gallon tank), and leaving the timing set at 38, we gained 1.5 hp. While power wasn’t improved significantly, the 5,700-rpm horsepower peak and smooth torque and horsepower curves indicated continued protection against abnormal combustion. Octane booster works, but double-dosing an engine like ours wasn’t worth the added expense. See Test 2 in the sidebar below..
91-Octane Unleaded: 402.1 hp/409.4 lb-ft
The bills are paid and you’ve got a few extra coins rolling around in your pockets, so you give your motor what it should have had in the first place—the good stuff: 91 octane.
While some parts of the country can brag about as much as 94 octane, left-coasters must make do with 91. With the timing set conservatively at 31 degrees BTDC, the sturdy 360 surpassed the best 87-octane numbers by 2 hp and 5 lb-ft. At 34 degrees, the numbers dipped, but recovered when we bumped timing to 36 degrees, delivering our highest numbers yet and breaking the 400hp mark.
There was no doubt that the higher octane fuel was good for a few extra ticks on both the torque and horsepower charts, but would it finally allow us to advance timing to 38 degrees BTDC without losing power? No dice: At 38 degrees, the numbers fell by 8.2 hp and 11.9 lb-ft, illuminating the reality that there is a difference between chemical potential and mechanical potential. If testing reveals that an engine is most efficient with timing set at 36 degrees BTDC, it will not necessarily produce more power even if high-octane fuel allows the use of more ignition advance. Still, our testing was far from over. See Test 3 in the sidebar below.
91-Octane Unleaded With 104+ Octane Booster: 399.8 hp/403.6 lb-ft
You’re off to the local bracket races. You know your pump-gas motor will be flogged pretty hard, so for insurance, you pour a bottle of octane booster in the tank and roll into the staging lanes.
Once again, Jill shut off the dyno fuel pump and let the 360 idle itself dry. Then the customary 2 ounces of Super 104+ were added to 2 gallons of 91-octane, and the torture test resumed. Starting again at 31 degrees of timing, the motor dropped a few points. It recovered some ground at 34 degrees, and just like the other tests, made best power at 36 degrees total. Going further, we advanced timing to 38 and lost a little more power; double-dosing the booster with the timing set at 38 brought a slight improvement. The power numbers generated with the boosted 91-octane are lower than those made with non-boosted 91, an indication that the fuel additive may have slowed the burn speed and reduced cylinder pressure. One thing is certain, there was no detonation present or we’d have seen it on the dyno charts and in reduced peak crank speed numbers. See Test 4 in the sidebar below.
100-Octane Unleaded: 403.5 hp/407.5 lb-ft
You’ve heard some of the local street rats whisper about 100-octane unleaded being sold straight from the pump. Its like some flashback to the ‘60s, but is it too good to be true? You just have to try some.
Even though we were pretty sure detonation wouldn’t be a problem with so much octane coursing through our 10.4:1 360’s veins, we began with the 31-degree setting to maintain consistency and to see if any noteworthy patterns emerged.
The dyno video monitor flashed just over 400 hp. Moving up to 34 degrees BTDC delivered virtually identical results, and 36 degrees bought almost 3 hp while torque remained nearly constant. At 38 degrees, the numbers were largely unaffected. The motor seemed indifferent to the increased timing, but in contrast to previous cycles run using the lower fuel grades, the amount of power sacrificed with timing set at 38 was negligible. To see if more timing would translate into more power, we did the unthinkable and moved up to 40 BTDC and let it rip. The results amazed and confounded us. Testing thus far confirmed that this particular motor combination really liked 36 degrees total, regardless of fuel quality. Any more or any less cost power—not much, but the numbers consistently fell. But now with 100-octane, the power seemed to remain stable despite the substantial 4-degree jump in timing. What’s more impressive is the fact that the 100-octane fuel was the only grade tested thus far producing maximum torque and horsepower numbers that never fell below the 400 mark. Our conclusion was that octane was not the sole factor at play, and that the 100-octane had superior burn characteristics to the MTBE-laden pump gas available here in California. See Test 5 in the sidebar below.
114-Octane Leaded: 408.3 hp/414.7 lb-ft
You love to watch professional drag racing on TV and jump with joy when the Pro Stockers run. So why shouldn’t you also jump at the chance to run the very same gasoline in your hot rod? It’s gotta run faster, right?
Taking our motor through its now well-established paces, we rang up our highest numbers yet at the 31-degree setting. We couldn’t wait to get to the 36-degree sweet spot, but exercised restraint and followed the plan, dialing in 34 degrees. What? Power was dropping? A backup run at 34 BTDC confirmed it. “Gotta be some kind of fluke. We’ll get it all back and then some at 36 degrees.” Or so we thought. We saw another drop at 36 degrees, and crumbs of no significance at 38 degrees. Through it all there were no signs of detonation. To rule out the possibility of error, we restored the timing to 31 degrees and watched the output jump back up to 406.6 hp at 5,700 and 413.7 lb-ft at 4,500. Further exploring the apparent benefit of retarded timing, Jill cranked in a mere 29-degree setting and output began sliding downward, losing 5.1 hp and 4.2 lb-ft. Why hadn’t more timing increased power? Probably because the 114 had even better burn characteristics than the 100. Its hydrocarbons vaporized and burned more readily, releasing energy sooner and accounting for why it required less spark lead to reach complete combustion. The octane level was not the operative here—rather it was the superior hydrocarbon content and vaporization characteristics of the racing fuel. See Test 6 in the sidebar below.
Conclusion
Frankly, the results of our test were a bit confounding. We consulted the chemists at Super 104+ and our pal Tim Wusz at 76 to help figure out what had happened. Here’s what we learned:
First, the octane booster did work. However, we saw that octane alone does not deliver horsepower; it only allows more complete utilization of the hard parts in the engine. Wusz said, “An engine does not know what the octane rating of the fuel is, unless it is too low”; note that we made less power by adding booster to 91-octane fuel. The lower the octane of the base fuel, the more benefit you’ll get from octane booster.
Also, the Edelbrock heads on our test motor have high-efficiency combustion chambers that are very tolerant of low octane levels, and their aluminum construction helps, too. Older chamber designs may not be as efficient and may succumb to abnormal combustion more easily.
But most of all, we discovered that our presumption that higher-octane fuels burn slower than lower-octane fuels (and therefore require more ignition lead) is largely incorrect. There are too many other fuel-formulation issues at work to assign a general rule about octane. Race fuel tends to have a more powerful formulation than pump gas, regardless of octane rating, because it is denser and can release more power and heat. (Note that we made the best power with 114 octane with the least ignition lead, indicating it had the fastest burn time.) California pump gas is blended with methyl tertiary-butyl ether (MTBE), alcohol, and other ingredients damaging to performance. Knowing what we know now, we’ll always experiment with ignition timing—both higher and lower settings—when we change fuels rather than presuming that more power can be made with more octane due to more timing.
