All right, we've almost perfected it: running high compression on pump gas, that is. And while it's still not something we'd recommend for your everyday commute, it sure is an easy way to make stout power! Last month some dissenters deemed us certifiably insane when we increased Danger Mouse's compression ratio to 12.5:1 and then dyno'd it on 91-octane pump juice! The results we got, over 550 hp, proved only temporarily insanity and this month we wanted to push the envelope further to see if this type of combination can live and make a whole lotta' power doin' it!
Last month, peak power was high, but the low end suffered from detonation. That's because the distributor we'd been running had a locked-out advance curve. So we simply set it at max-38 degrees-and let 'er rip. Turned out that the one thing we were lacking in last month's test, but desperately needed, was a timing curve. So this month, the first and only bolt-on, or, more precisely, plug-in, we tested was a new MSD Multi-Function Ignition Controller, (read: computerized ignition box). With MSD's new box plugged-in and a trusty old Windows 98 laptop running at full speed, we tweaked DM's timing curve enough to rid us of that pesky detonation. But, as seems to lately be the case, the power we got perplexed us more. The end results were far from what we'd expected and will probably confuse the issue even further, but we've no choice but to share them to you.
Perplexing PowerWe ran this month's tests again with 91-octane pump gas to see if we could tune power into the combination by tuning out the detonation. The weirdest thing happened to us on the way there. We were surprised when, after all of the day's 25 tests were through, DM made the best low end power without much of a timing curve! In other words, it didn't detonate, even when we put 35 degrees of advance back into the engine at 2,500 rpm! In fact, DM made even more power from 2,800 to 4,800 rpm then it did last month, but then things went all weird on us again. Power began to fall off, so much so that by the top of the rpm band, DM was down by 29 hp! Why? We haven't figured that out yet.
One possible explanation is that this month's tests were done at a different dyno facility, Vrbancic Brother's Racing vs. Speed-O-Motive, and running a different set of headers, which could have had a pronounced effect on top-end power in an engine of this caliber. Another could be that we obviously were not running the exact same fuel as we ran last month, and the quality of gas that comes out of the pump has long been questionable.
It could be that last month's 91-octane really only rated about 89 on the R+M/2 scale and that could have caused the detonation. Maybe this month our octane was a little bit higher? Since we have no way of testing fuel for its octane rating, we'll never know. But higher octane or not, we still don't know why DM lost so much power at the top end of the scale? That could be more the result of the different dyno headers.
We're not saying that Vrbancic's dyno is a dud. Quite the opposite in fact. We've used their shop time and again for tests and will continue to use them long into the future. We trust their numbers and have seen nothing but good, repeatable results in the past. It's more probable that the air in the shop itself was partially to blame. Last month we tested on a cool, dry day, but we can't wait around forever for the weather to change and this month we tested in hotter, humid conditions. Now, we know that the dyno is supposed to correct for this, but correction factors are just mathematical calculations and the engine doesn't breath in and spit out numbers. It breathes air and spits out power. If the air's too hot and wet goin' in, the engine won't spit as much power goin' out-plain and simple.