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[!tom]

So, my wife gassed up her car ('78 NA) the other day, and as always she put in Chevron 94 octane (the Premium Plus stuff). Ever since we got it, this is the fuel it worked best on -- anything less and it would ping considerably.

Well, the gas station had just put in new pumps, and they must have screwed up, since although she paid for the expensive stuff, her car started pinging like gangbusters.

I borrowed a timing light and couldn't find the mark in the window. I cranked and cranked on the distributor 'till finally I could see the 0 mark, then the -3 mark, and stuck it there. Now it runs without pinging, but the timing must have been around 10 - 15 degrees advanced -- maybe even more. With no markings, and a basic timing light, it's just a guess.

Now, granted, I don't have an external tachometer, so I'm not absolutely positive what the idle speed is. It did read right around 1000 at idle before, and now it's more like 850 or so, but there's no reason to believe it's particularly accurate.

I left the vacuum advance connected as the book suggested.

So, I know the advice is always to set the timing to where the book says. However, it is a common tuning technique to run more advance if you're going to run more expensive gas. Is there any reason not to crank the timing back up to the positive territory if we're going to use the more expensive gas?

I know the beater that I commute in has its timing cranked way up, and I put expensive gas in it. I've worked out that it costs less money per unit distance this way, even though the gas costs more, and I would expect I'd get better performance to boot.

My understanding is the octane fuel, and hence ignition timing the engine is designed for is determined by the ratio of the stroke to the connecting rod length, right? However, given any design, I was under the impression efficiency and power could be increased with more timing and octane beyond the design baseline.

Comments?
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78 924 NA
5-lug

[joecitizennn]

uh, is that a question?
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87 924s
88 924SE gone
447 hp cgt clone
87 924S gone
84 944 sold
83 944 sold
89 turbo S LSD, ABS etc
86 951 white rocket
85 944 sport everything.
84 944 sold.
87 944 sold

[endwrench]

Timing advance is more a condition of compression ratio and combustion chamber design along with air\fuel\ratio. I would go ahead and bump it back to 10° BTDC. This seems to be the norm. I ran this on my 79 before switching to EDIS and I still run it at idle now.

Todd
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'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....