Hi Brian, I'm really most interested in the plug electrode not hitting the big intake valve. If I look into the spark plug hole, I can definitely see the intake valve, not so the exhaust valve, but that said, I'll look again. Presuming I can't see the exhaust valve, then I could theoretically place the gap toward the intake valve and the electrode would not contact the exhaust valve. I'm not expecting any measurable performance gain, but hypothetically, there is an optimum position for the gap postioning. My copy of Ricardo's "The High Speed Internal Combustion Engine" is in storage, so I can't check that to see if he has anything to say on the subject. The only way to test this for sure, would be to do a series of dyno tests and that's not in the budget.
If I use the plugs that require indexing, the hypothetical questions have only to do with optimization of what I have to do anyway. Presuming the prime directive is to not have the side electrode at 90 degrees facing toward the intake valve, it would seem the gap could be anywhere from 90 degrees to the valve (in line with the crank shaft) to the 180 positions (perpendicular to the crank shaft) or at 45 dgrees to the intake valve, with the electrode toward the exhaust valve. The almost purely intellectual question is: Is there a particular orientation that is better for combustion? I would think that an unmasked spark, in an Alfa combustion chamber with a flat top 10:1 piston should be pointing toward the center of the combustion chamber for best ignition. Even if this hypothesis is correct, I expect little practical value from it, that said, is ther a theoretical optimum? So! Now I've just called NGK technical. The guy said they recommend straight up and down with the open part toward the center of the combustion chamber, OR up to 30 dgrees toward the exhaust valve. He commented that this might get you 1% more power, which on a 500 hp motor is 5 hp. The other thing was the BP7ET 3 electrode plug. This has the same extension as the BP7ES, but are not available in the USA. Anybody know where I can buy some? Stevan Thomas 73 Berlina In a message dated 7/12/2010 11:21:13 AM Pacific Daylight Time, [email protected] writes: > >I have a big valve head, so I have to index if I use the extended nose > plugs. One of the things I've heard mentioned, is that indexing allows you > to >orient the electrode/gap so as to allow the best combustion. Agreeing > that the orientation should be 180 dgrees off the oversize intake valve, left > to >my own devices, I would tend to put the open space toward the center > of the combustion chamber. Does that make sense? > > > > If you buy into the notion that indexing is going to gain you some > performance (Ibm not sure I do, or that it would be noticeable), then I think itb > s contradictory to where youbd want to index things to avoid valve contact. > > > > Indexing for performance would seem to indicate that you want the gap > facing towards the intake valve. Or, as Ken suggests, facing away from the > intake valve. Flip a coin as to which is best for an Alfa motor. Then toss > both of them to the wind, because indexing them that way will maximize the > chances that youbll contact the valves. To avoid contact with the valves, > you want the plugs indexed for/aft. > > > > Personally, I think youbre worrying too much over something that wonbt > make any difference youbd ever notice. And as Les says, if you did index > them boptimallyb, but the spaces moved the plug that much further back from > the combustion chamber, would you still reap the benefits? > > > > On my big valve Spider, I mark the plugs, then shuffle them around so I > get the best combination of fore/aft orientation to void valve interference. > > > > On every other Alfa I own, I donbt bother with any of the above. > > > > bs -- to be removed from alfa, see http://www.digest.net/bin/digest-subs.cgi or email "unsubscribe alfa" to [email protected]
