Lead Additive? Octane?

[ItBites]

Ron,
1) Thats not valve clatter you hear, its piston skirt slapping the cylinder walls.
2) Using a fuel with more octane than you need will reduce power output (in addition to waste money), unless timing is advanced to accomodate the slower burn.

CriticalMass,
1) Do a compression test. Make sure to hold the throttle plates open. Divide the number in psi by 14.7. This won't give you the compression ratio, due to cam dynamic effects, but will get you in the ballpark if you're lucky. If the number is over 160 and you have iron heads, you will want some additional octane or back the timing to maybe 30 or 32 full in. If the number is way over 160, use high octane fuel.
2) I've never liked octane boosters. Most are just toluene, which has a high vapor pressure and will evaporate faster than the gas they are mixed with, resulting in a reduced concentration as time goes on. Some also color the plugs (Moroso) so that reading for mixture is impossible...
3) You'll hear a lot of naysayers chime in after I say this: I mix avgas in my gasoline to raise the octane. It cools the intake charge. It cools the intake valve and combustion chamber. It is infinitely more stable than pump gas. It will not outgas through a plastic jug in the way race gas will (if you store your race gas for more than a couple weeks in a plastic jug - guess what? - it aint race gas anymore...). Avgas is also cheap and you can use it without significant jetting changes.
4) Using VP110 or other race gas will require jet changes in order to avoid losing power. With 12.5:1 CR, I had to go up 10% in (effective) jet size in order to bring the power back in line with what I get from avgas/pumpgas mix. The reason for this primarily lies in the density (or specific gravity if you prefer) of the various fuels as well as the slight change in stoicheometry (even though we run far from stoichometric) and also the burn rate (which is related to but not totally controlled by octane).
5) Don't listen to the inevitable naysayers about avgas. When they post a link (and they will) to a site discussing pros and cons of avgas, read the site carefully. The sites I was referred to by the usual naysayers around here all came to the same conclusion that avgas is not as great as racegas (in terms of possible power production), but, given the cost provide a generally acceptable fuel for your hurting high compression motor.
6) Avgas smells good when burned
7) Mix 50/50 avgas/premium and drive the sucker. you'll be fine.



I will now await the potshots...