Originally Posted by J08M3
Did you give your ECU a chance to adjust to each octane change before the dyno? I think it takes about a full tank of gas of a certain octane to do that (so I've been told on here). In which case how could you do this in one day? If it's more than one day between runs you could easily see +/- 5 hp just by the weather that day.
SAE is trying to adjust for the weather. It was about 10-degrees hotter in the dyno bay today, which isn't a lot, but might have accounted for 1 or 2 hp.
I do at least 50-miles on the newly topped off tank before the dyno runs. I include plenty of full throttle, mixed with light throttle. I had more miles on this 50-50 blend than on the 30-70 blend, so the ECU should have been fully adjusted to it.
Also so of importance, we keep running with only short cool downs (allowing the fans to all get off high speed) until the hp and torque peaks (usually 4 or 5 runs). The engine likes to get a little heat into it before it gives its max.
About running in 5th gear, I've got a 4.10 final drive and the speed governor off and the redline raised to 8600 rpm. I noticed that fifth was right at 155 mph when we hit the 8600 rpm redline. A stock 3.85 FD if going to be dangerouly close to being speed limited right at 8400 rpm. You should hit peak hp before the governor kicks in, but you might not get a "full pull", in case you value that. I think it'd be worth getting a more accurate run if 100 or so rpm were not used. Just a thought. On a Dynojet, 5th will give you a higher result than 4th.