Test for octane requirement. |
By Bruce M. Fulper
(The Tim Conway of Pontiac Performance)
Inaudible detonation. A term I've introduced to our community in 1996 in an attempt to warn my Pontiac friends of a very serious condition. Unfortunately there isn't a "litmus paper" test, yet. That would be a terrific item if invented.
Octane by definition is a detonation resisting chemical. I'll get into it further in my book, but for now let's do a test to see if you're cheating yourself out of power, and possibly hurting parts. There are two ways you can approach this. Is there a chassis dyno in your area? or can you go to a race track to test? You'll need to do one or the other. You may have heard that you can twist the distributor advanced until the engine audibly "pings," but this is absolute foolishness. This is not an opinion, it's a fact. All you learn is; at the point it pings, it's too far advanced. So? It does not tell you if that you move the timing back 2 or 3 degrees, as the proponents of this test would have you believe, that you've achieved "optimum" timing. That this is the spot that's the engine will make the most power. We need to find the engines sweet spot, while under a load.
OK. We're going to capsulize this guys, so don't be pickin' me apart. Get to the track or a chassis dyno. Bring along five gallons of 108 or 110 race gas. Make sure you have less than five gallons of your regular fuel in the tank at the start of the test. Check your timing at 2500 to 3000 rpm without Vac. Adv. hooked up. (Capsule#1; dist. is assumed to be working right.) Make a dyno pull or a run as-is. Record the mph. We're not concerned with e.t. as the mph reflects your h.p. Advance the timing three degrees. Make another run or pull. Did it slow down, run the same, go faster? If it slowed down, pour in five gallons of race gas. (Do not use 112 or 115 octane.)
If it ran the same, or went quicker, advance the timing another three degrees and make another pass. Keep advancing the timing three degrees until you find the spot that the car slows down. At that point retard the timing three degrees, then add the race gas. Make two more passes. Did the mph come up? Yes? Then advance the timing three more degrees. Make another pass and? what happened? More mph? This tells you that you are not meeting the octane requirement on 93. Did the car slow down? Make another pass just to make sure the mix of fuel has gotten completely into the carb. If it slows down it would tell you that it doesn't need the extra octane. Also, if the car went quicker on the last pass, add two more degrees of timing and make another pass. Quicker or slower? Quicker tells you it really needs the mix of fuel.
Also, understand this. Too much octane will also slow the car down. That's why there's three basic levels at most gas pumps. You already know that if you put too little octane you will lose power, but did you know that you will also lose power if you put in too high an octane. The burn rate is incorrect, and while you will not detonate, you will not burn the fuel as efficiently. You'll get less mileage and have less power. So that's why I said not to use 112 or 115 unless you mix it two gallons to four of 92/93 octane. 108 mix 4 gallons to 5 gallons of 92/93.