I've been putting 92 in it because it's got forced induction. Should I be running 87 or 89? What does everyone else use?
I run 92 in my Hijet, because the compression ratio is up around 10:1. Higher elevation, and the corresponding less dense air decreases teh amount of octane, you need. I think you are pretty close to sea level, so the chart will be very close for you. To get your effective compression ratio with the boost, add the boost in psi, to fifteen, and then divide the result by fifteen, and multiply your static compression ration by that number. For example: if you are running 5-psi boost, and the compression ratio is 8:1. (5+15)/15= 1.3333. Multiply the 8 x the 1.3333, and effective compression is 10.7:1. From the graph, you need to be running around 97-octane to prevent detonation. The alternative if you have an engine management system which includes a knock sensor, you can figure out what your engine needs by varying the octane and tracking the fuel economy. Run, two tanks of the highest octane available, and track the mileage. Then drop the octane one available grade, and run two tanks. If the fuel economy dropped go back to the higher octane. If it stayed the same, drop the octane for two tanks, and track the fuel economy. When you get too low, the engine management unit will retard the timing, and enrich the mixture to prevent knocking. This results in a drop in fuel economy.
So in a non-supercharged, carburated truck, would you recommend just going with the highest one available? I always use the "premium" option at my gas station because with such a small fuel tank, i'm saving anyway.
if you run a compression test, and divide that by 15-psi, you’ll be close to what your compression ratio is. For example, you run the compression test, and you end up with an average of 148-psi. Divide the 148-psi, by the 15-psi, (one atmosphere at sea level), and you’ll be just under 10:1. At an average compression test reading of 135-psi, you’re at a 9:1 compression ratio.
I use shell V-power nitro 91 octane fuel in my truck simply for the fact it doesn't have any ethanol (the 93 octane version does). I keep some fuel additives in the toolbox in case I'm not close to a shell station and have to use regular fuel or a premium grade fuel that does have ethanol in it.
Ethanol, and MTBE got a bad rap, when they first started putting them in the fuel in the late eighties, because it does breakdown some of he rubbers which had been used in fuel systems up through the early eighties. So, it would eat o-rings in your carb, the diaphragm in the fuel pump, and fuel lines. I had a 73-Dart Sport 340, that it dissolved the fuel tank lining out of. I had to put a bank of four fuel filters in parallel, in teh discharge line before my electric fuel pump, and change them all out every weekend, in order to get gas to the engine. That lasted about a month, then it was every couple of weeks, and it took almost a year for all of it to dissolve out. But, MTBE and Ethanol, are both very good at reducing knock, and fuel systems from the mid eighties have been designed to not be harmed by it. I’m not sure you can even buy componenets at the parts store any more which aren’t compatible.
You're pretty much always on boost with these trucks and there isn't an intercooler. Intake temps get very high and you need the highest octane you can reasonably get to keep it from knocking and pulling timing(and reducing power). I'm gathering parts right now to add an intercooler to my truck. I'll be posting a thread on it that when I have all the parts in a few weeks with dyno results too. Totally agree on the ethanol. If I had an E85 pump nearby I would be running that. E85 loves boost. The alcohol creates a cooling effect when it is injected and evaporates. It's also around 110 octane.
