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I was catching up on my threads and read of the rejetting of a carb. for a trip to Colorado, as I read I remembered an article by Warren Johnson in the National Dragster(a weekly NHRA news paper). He said that too many carb. jets were being changed between rounds of racing. His comment's were that the carb. adjust's it's fuel flow by the air flow over the ventures to some degree. He had a formula I dont recall. I will find the article and post it. What I do think is that the advancement of timing as alitude increases is more important. As I remember the factory called for 4 degree,s on a 390 hp 427 C 2 that I bought in Nov of 1966 and we ran 12 degree's at 5000 feet alltiude. I would be interested in hearing any information on this agreeing or dis-agreeing. The Warren J. article really was very much controversial to what I had learned in 40 years around drag racing. Dale
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Re: rejetting carbs for alttitude(previous post)
Carburetor main metering systems meter fuel on the basis of volume air flow, NOT mass air flow, so as ambient air density decreases fuel flow does not, so carburetors naturally get richer as altitude increases.
Carburetors need to run a slightly richer cruise mixture than fuel injected engines so the leanest cylinder does not misfire. If you lean out the jetting just to the point of cruise hesitation, it will likely go away at higher altitude due to the richening effect.
Same applies to WOT. Best power is usually in the range of 13-12.5:1 A/F ratio, and going richer will reduce power.
When you get into big shared plenum mulitple carb manifolds, fuel distribution is a soccerer's nightmare. About all you can do it trial and error it and gather as much test data as possible. For example, variation in EGT between cylinders usually indicates an A/F ratio disparity. That's why many big plenun manifolds require "stagger jetting" - different jets between the four or more carburetor venturis.
The less dense the fresh charge, the slower the burn rate, so more timing at altitute should be beneficial. Most Corvette engines had CRs that pushed the available octane ratings of the era, so spark advance under most operating conditons was limited by detonation.
As a general rule you want the engine to operate on the ragged edge of detonation, especially at relatively high load to get the most power and efficiency out of the available CR and fuel octane level. This will provide best power and efficiency, and modern engines with knock sensors and electronic spark advance controls operate this way. Note that the new Z06 has an 11:1 CR, and 93 octane fuel is recommended, but not required. If fact, it should run just fine on 87, but there will be a lot of knock retard and the engine won't make as much power anywhere in the rev range as with 93, and 93 might ever cause some KR if inlet air temperature is high.
Vintage engines with their relatively crude centrifugal and vacuum advance cannot keep the engine at the ragged edge of detonation, so they were generally set-up conservatively to keep them out of signficant detonation under worst case conditions. This provides opportunities for "tuners" to optimize the ignition map by tweaking, initial, centrifugal, and vacuum advance curves to the specfic operating conditions and environment. The same also applies to carburetor calibration.
That's why vintage cars are more fun than modern cars. You can tinker with them and make them better.
Duke -
duke some modern Qjet carbs
have a aneroid assy to richen or lean the carb jetting according to altitude. these are not the computer controled ones.Comment
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