Quote:
Originally Posted by Piotrsko
What about lean mixtures running hotter? Ever fry a piston top in a VW bug? In the diesel F250, you watch EGT on hard pulls because the aluminium stuff tends to melt.
|
That's somewhat of an old wive's tail.
In my 1972 1600cc VW Super Beetle with mostly stock air cooled engine (stockish cam grind, stock VW heads, stock 34-4 Solex carb, stock oil bath air cleaner, stock VW heat exchangers, stock VW muffler, etc. etc. etc. just electronic distributor and heads machined for higher CR, did adjust pistons for .030 quench),
I ran the compression ratio up to 10:1, ran lean, about 16:1 or leaner at cruising (with the aid of an O2 sensor and AFR gauge) and adusjusted the ignition as far advanced as possible (most power without pinging) with a digital timing map and drove it mostly over steep mountain passes at full throttle
and never had any problems at all. No pinging, no knocking, no fried pistons. The engine ran like a charm.
Why? Because leaner doesn't always mean hotter. Hot is around stoichimetric. A little leaner, like 15:1, is still hot. Go richer, 13:1, 12:1, etc. and it gets cooler. But going leaner, 16:1, 17:1, 18:1, (at 18:1 my Bug started misfiring) also runs cooler. In fact I actually had a hard time keeping my oil temps up as 180°F was the hottest I could get, and head temps never wanted to go over 325°F either.
I did make sure I got at least 12:1 at full throttle, but I drove all over, both going up steep mountain passes and along long stretches of road at highway speeds and never had any problems. I also averaged 30mpg.
I'm not the first VW aircooled owner to do this. Lots of others have found that they can do the same thing. Many airplane owners have found their planes run better leaner and cooler at loads below 75%.
The lean = hot and rich = cool depends on the context. Are we building a race engine and are getting AFR's of 13:1? Then yes, leaner is hotter and richer is cooler. And maybe you could do 16:1 in a race engine, but you wouldn't have the power.