TIMING AGAIN
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Instructor
Joined: Feb 2002
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From: margate florida
TIMING AGAIN
pre lars, your total time should be 36 to 38%. what if you have a loppy cam does that make a differance? do some engines need more advance then others? vacuum advance should be 16% which gives you 52 total with vacuum i think. what can i expect if i don't use my vacuum advance? jim
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Re: TIMING AGAIN (1970LS6)
Regardless of cam, the total timing (less vac advance) should be in the 34-38 degree range for peak performance on most engines. However, bumpy cams tend to require more initial timing to produce acceptable idle quality. More initial, while not changing the total, requires a shorter mechanical advance curve - many bumpy cams want 18-25 degrees of initial advance. If shortening your mechanical curve is not an option, you can plug your vacuum advance into a direct manifold vacuum source and use the VC1810 (B28) vacuum can. This will pull all of the vac advance in at idle (even with a bumpy cam that pulls poor vacuum), and will give you a bunch of advance to keep your bumpy cam running without over-advancing the total.
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Joined: Feb 1999
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From: Lenoir City Tennessee
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St. Jude Donor '05-'06-'07-'08-'09-'10-'11-'12-'13-'14-'15
Re: TIMING AGAIN (lars)
Regardless of cam, the total timing (less vac advance) should be in the 34-38 degree range for peak performance on most engines.
Tech Contributor
Joined: Aug 1999
Posts: 14,356
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From: At my Bar drinking and wrenching in Lafayette Colorado
Re: TIMING AGAIN (1970LS6)
Jim -
Vacuum is not used by guys running pure racecars because a racecar (drag car) always operates at Wide Open Throttle (WOT). At WOT, there is no vacuum, so a vacuum advance control unit is useless. Intial timing is advanced significantly to get the car to idle, and the advance curve is kept short to limit the total to an optimum number (usually as indicated above). Fuel economy and emissions are not a concern.
On a street car, elimination of the vacuum advance will result in poor gas mileage, poor part-throttle throttle response, poor idle, and high emissions. As noted, you can use the vacuum advance to kick in a bit more timing at idle if your cam combo needs it. If it doesn't need it, you can run off the ported vacuum source to get the benefit of the vacuum advance at cruise only. This will improve fuel economy and throttle response. I cannot think of a single reason to eliminate the vac advance on a street-driven car... why would you want to disable it..?
Vacuum is not used by guys running pure racecars because a racecar (drag car) always operates at Wide Open Throttle (WOT). At WOT, there is no vacuum, so a vacuum advance control unit is useless. Intial timing is advanced significantly to get the car to idle, and the advance curve is kept short to limit the total to an optimum number (usually as indicated above). Fuel economy and emissions are not a concern.
On a street car, elimination of the vacuum advance will result in poor gas mileage, poor part-throttle throttle response, poor idle, and high emissions. As noted, you can use the vacuum advance to kick in a bit more timing at idle if your cam combo needs it. If it doesn't need it, you can run off the ported vacuum source to get the benefit of the vacuum advance at cruise only. This will improve fuel economy and throttle response. I cannot think of a single reason to eliminate the vac advance on a street-driven car... why would you want to disable it..?
