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Phrank said:

>but remember, the gas had lead in it back then. The lead helped cool the engine and prevent detination.

Oh please - lead had nothing to do with keeping the engine cool. It did three things - helped increase the octane rating, helped lubricate valve guides in cast iron heads, and helped prevent Valve Seat Recession in cast iron heads - it had nothing to do with "cool".

>As for plane engines, almost nothing crosses over from planes to cars. It's a totally different environment and duty cycle.

Very true - the average car engine is working at about a 25-30% duty cycle (it takes about 12hp to run a bug at 50mph) but plane engines run at 70-80% power in the cruise, and 90-95% in the climb, so have a much higher duty cycle.

>You also have exhaust gas temp gauges in airplanes (usually), allowing you to keep much closer track of combustion temps.

Also true.

>Personally, I tune my engines for power while keeping the temps in line so I always end up running premium. It makes the most power.

Absolutely untrue. The octane rating is just a number which tells you how good the fuel is at resisting detonation - it's nothing more than that.

Have a read of this article - you might find it interesting.
http://www.geocites.com/aussiebug1970/octane1.html



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Rob
Rob and Dave's aircooled VW pages
Repair and Maintenance for the home mechanic
http://www.geocities.com/aussiebug1970/

Dual port heads have harden valve seats so you don't need leaded fuel. Octane was not the question.

Aussiebug, I DID find that artical interesting. You obviously know much more about the combustion process than i do, but I think you misunderstood a couple of my points.

When I drive my wifes Ghia to the store, it may go from 1,000 to 4,000-5,000 RPM and back twenty or thirty times, for a few seconds at a time, at 1/8 to full throttle randomly. It's just a completely different environment and duty cycle than the typical airplane usage. Making decisions for one based on experiances with the other doesn't make sense (to me).

Higher octane fuel allows me to advance my ignition timing more than the lower octane fuels without detinating. This results in more power. And in the case of the aircooled VW engine, higher octane fuel allows me to build my top end with a higher c/r without detinating, making more power.

I have long understood that lead, in addition to keeping the old (soft cast iron)valve seats from receding, lubricated the valves, slowed down flame travel, and provided more cooling for the exhaust valves (something about the lead deposits on the valve). is this not true?
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"People who busy themselves telling what can't be done only annoy those of us out there doing it."
Tracy's Ghia Page (GhiaGirl.com) www.ghiagirl.com

[This message has been edited by phrankentruk (edited 02-25-2003).]

ok I asked this question way back when I started so I got that, but what happens when you put Leaded fule into a FI engine?