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  #16 (permalink)  
Old 02-09-2011, 10:31 AM
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If all this in an attempt to achieve the same high vacuum reading at cruise as at idle, I seriously doubt that will ever happen as long as your fighting wind Resistance. They used to have a vacuum gauge to read while driving for best fuel mileage and even that had a green ZONE that you would try to keep the needle in. If you want more performance, swap the cam. If you want better mileage, loose the 4:10's. I have the same combo in my truck and the best I've ever gotten is 10 mpg in my 4x4 and that's in the summer time when I don't have to use the choke or warm up time. It seems to me that something is goofy with the TDC because I tried as much initial as you are now and my engine rattled like a baby toy at 14* initial. Maybe it's the curve kit or what, I don't know, but I can't wait to see what you finally come up with. Right now I have a performance HEI, adj. vacuum advance, weight and spring kit, 65,000V coil, 10.5 mm procomp wires to R44T AC's. I've got the vacuum can adjusted all the way in for the closest to stock HEI advance rate right now.

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Old 02-09-2011, 01:41 PM
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I have had the vacuum reading at 20 idle with 14 to 16 at cruise on level ground. I know I will never reach 20 at cruise. The idea hear is just as you suggest. I figure the higher the vacuum the more efficient the motor is running. I am using my vacuum gauge like the instant mpg gauge in my Buick. The current set up seems to be a little low at cruise (12) and that was my initial question. I was also getting between 14 and 16 mpg with the quadrajet. As far as the 4.10 gears, I have 31" tires on the 3/4ton truck and I have no problem doing 60 mph and staying at around 3000 rpm. I have hit 85 but did not have an interior tach to check rpms. I don't believe I have all the power out of the motor that it has in it. I can be just off a little with the timing and be missing out on 40 hp so I am going to take it to the limit and see what it will take.
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Old 02-09-2011, 10:18 PM
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Let me go check my driveway.................yep, it's still there! I too have 31" tires and turn 2,800 rpm at 60 (timed between the mile posts). My 3/4 ton 4x4 weighs 4,848 with half a tank of gas and my butt not in it. Add me and it's 5,078 and with that amount of weight it stalls at 2,000-2,200 rpm which is good for what I do which is off roading on the week ends and a 10 mi round trip to work every day.
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Old 02-10-2011, 09:22 AM
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My truck is only a 2 wheel drive and weighs 3,865lbs dry. Full tank and me in it it weighs 4200lbs. That probably contributes a lot to the difference in gas mileage. Stall is at 1800 rpms. I have been using a advancing timing light with my MSD distibutor. I have read that that type of timing light is not always accurate with electronic ignition like MSD especially if you have a box which I don't. I will double check the timing with an old cheapo sears light I have and confirm TDC.
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Old 02-10-2011, 09:54 AM
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As long as you don't have a box and it's just a MSD distributor, you'll be fine. But some dial back lights (probably most if not all- if being used in the dial back mode) are still not as accurate as a standard light, so it wouldn't hurt to check, IMHO.
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Old 02-10-2011, 12:20 PM
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I am using a MSD Ready to Run which is supposed to have multiple spark without the box so it might have an affect on the light.
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Old 02-16-2011, 03:07 AM
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I like to use a vacuum guage too, I installed one permanently just last year in my Bel Air. I did it to help get maximum fuel economy.

I see a problem with your 18+16 though. That will give you 34 deg. at idle, which will probably be too much. Idle will tend to be rougher and vacuum will be lower because the engine is fighting against itself. My suggestion is first find the perfect igntion point at idle, tune the intial and vacuum advance accordingly to achieve this at idle. Then go on to try to achieve the best vacuum at cruise.

To determine best idle advance simply turn the distributor back and forth to find the point where the engine idles at the highest RPM (you will hear this easily - no need for an RPM guage if your timing light does not have one). In my experience this will be between 27-29 degrees. Using as much intial advance as your starter can easily handle is best for off-the-line performance, so if it handles 18 deg. stick with that. But then reduce the vacuum advance travel with one of those limiting plates to 10 deg. Then you will have 28 at idle, where you have max. vacuum. 18 degrees mech. adv. will give you 36 after its all in at WOT. You will cruise then at 46 degrees, which is a little low but won't affect mileage much because there is a wide band between 46 and 56 deg. where economy will remain relatively flat at a high level. If you want max. economy at cruise you can sacrifice 3-4 deg. intial timing and add 3-4 more vacuum and mech. adv.

I've gone through all this and its worth it. My car has great throttle response for a little 283 in a 3800 lbs. car with very good fuel economy to boot (25+ mpg). I also use an AFR from Innovate. Highly recommend it, you can tune the carb absolutely perfectly with one of those.
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