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Old 08-13-2007, 08:10 AM
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Originally Posted by Not A T 25
I am fine tuning my SBC (383) for fuel economy here are the stats:

Base timing 10 deg with no Vac
All in timing 34 deg @ 3200 with no Vac
Base timing 20 deg with Vac
All in timing 44 deg 3200 with vac
15" vac at idol
13-14" vac at 2200 RPM (65 MPH)

Plugs are dark brown a Little darker than should be but Car runs very nice response is good

I am confused about the vacuum not dropping out guess I don't know all I thought I did! It seems like I need more open butterfly on the carbs but still a little confused on how to get that with out a lean condition also not sure if I need to re curve the dist to bring the all in RPM down from 3200. The car is a rod not a racer do more cruising and the occasional burn out but very occasionaly just to prove I can still do it Any positive thoughts would be appreciated I am out in the hot sun trying to figure it out?
Change one or both advance springs to slightly lighter (less tension) to bring the "All In" or Total Mechanical timing down to the 2500-3000 RPM range.

Set your Total Timing to 36-38 degrees with the vacuum advance dis-connected and plugged. If you have Vortec heads you will find that the Total Timing will work better around 32 degrees.

Your idle RPM should be below 800. If automatic this would be while in DRIVE. If manual this would be in Neutral.

Your vacuum at idle seems a little low; but this could be due to whatever cam you are running. The dark brown plugs would possibly indicate a overly rich gas/air mix. Hard to say because it all depends on how you tested to check the plugs. Most folks do not know how to test for that. You can not just run the engine and pull over and shut the engine off and then look at the plugs. The correct way is to run the vehicle up in speed, shut the engine off while putting the trans in neutral at the same time and coming to a complete stop. Then pull the plugs to check for "color". Also with todays unleaded gas it becomes increasingly difficult to "read" the plugs.

The vacuum will not "Drop out" except when the engine is under a load.

If the vehicle is stationary and the RPM's are increased the vacuum will remain.

If you are driving the vehicle and are at a "cruise" or fairly steady speed the vacuum will still be there. This is how you get the "economy" as the timing is increased at cruise. If you were to drive your vehicle with the vacuum gauge hooked up, you would find that the needle would fluctuate under different engine loads. You would also see that when cruising the needle would be in the green colored section (which is good) and that vacuum would be present.

Your vacuum at speed looks good.

Basically, timing and fuel delivery that yield the best performance will also yield the best economy as the engine will be running near it's optimum. How you drive (fast starts, etc.) will also change the economy.
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