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| Race carb/vacuum advance https://slantsix.org/forum/viewtopic.php?t=35346 |
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| Author: | Joshie225 [ Thu May 14, 2009 2:14 pm ] |
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Brennan, Were you able to achieve any kind of vacuum advance with the 390 CFM race carb on the slant? When I pulled your old carb out of the box I noticed that there is no vacuum advance provision on the carb. I know you had an electronic timing control on the car, but did it have a MAP sensor to give vacuum advance? I plan on using a digital programmable ignition system to provide vacuum advance so my fuel economy doesn't suffer too much. P.S. Now that I've thought about it a little more I think full manifold vacuum connected to the distributor through a valve which would vent the vacuum under closed throttle would do just fine. |
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| Author: | Bren67Cuda904 [ Thu May 14, 2009 2:33 pm ] |
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Quote: Brennan,
No vacuum advance, but It's my understanding that the advance would only have a 2-3 mpg advantage.Were you able to achieve any kind of vacuum advance with the 390 CFM race carb on the slant? When I pulled your old carb out of the box I noticed that there is no vacuum advance provision on the carb. I know you had an electronic timing control on the car, but did it have a MAP sensor to give vacuum advance? I plan on using a digital programmable ignition system to provide vacuum advance so my fuel economy doesn't suffer too much. BTW the vac can has not been hooked up yet and the 318 is getting 15mpg. Are you suggesting that if hooked up I could yield 22-25 mpg |
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| Author: | Joshie225 [ Thu May 14, 2009 3:17 pm ] |
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Vacuum advance will help fuel economy, but how much depends a lot on the engine's state of tune. High compression engines with long durations cams tend to tolerate less vacuum advance. I found a Mead valve that will allow me to switch the distributor between atmospheric pressure and manifold vacuum for $15 with shipping. This Holley 80507-1 has idle circuits that are meant to feed a 358ci NASCAR engine. I'm going to reduce the idle feed restriction and see if I can get the carb to behave itself on a slant and return better fuel economy. |
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| Author: | Bren67Cuda904 [ Thu May 14, 2009 3:27 pm ] |
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They have been all ready reduced. http://www.renaissanceracing.com/Albums ... Label.html |
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| Author: | Joshie225 [ Thu May 14, 2009 4:48 pm ] |
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Well, that's a step in the right direction, but that .029" jet is still 77.3% of the area of the original .033" jet. Actual jet area is .000661" square now Vs the original .000855". I would expect the idle jet area to be around 63% or less of original due to the engine size difference which puts us very close to an .026" jet which is .000531" square (62% of original). If the manifold vacuum on the new engine is greater than Holley expected in the original application then an even smaller jet might be needed. Was the idle feed restriction changed on the secondary side as well? |
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| Author: | Dart270 [ Thu May 14, 2009 6:00 pm ] |
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I have found vac advance to yield 15-20 % MPG improvement, even on high perf engines. Lou |
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