Does a hi perf cam require timing adjust?
Thread Starter
Heel & Toe
Joined: Feb 2010
Posts: 20
Likes: 1
From: fort worth tx
Does a hi perf cam require timing adjust?
Recently purchased a 1973 with 60K miles. Do not know history of car but sounds to have had a high performance cam installed at some point. If so, does it require the standard timing to be advanced or retarded? Thks
Race Director
Joined: Jun 2000
Posts: 10,046
Likes: 675
From: FL
Why set timing at idle? Do you drive with the engine at idle? Standard timing is for emissions, not performance.
Most small blocks like 36 degrees of centifugal (mechanical) advance all in by 2800 rpm or so. Add another 14 degrees of advance from the vacuum can. This is regardless of cam.
Read the timing sticky at the top of the page for more info.
Most small blocks like 36 degrees of centifugal (mechanical) advance all in by 2800 rpm or so. Add another 14 degrees of advance from the vacuum can. This is regardless of cam.
Read the timing sticky at the top of the page for more info.
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
Usually a larger cam works best with an increase in initial timing.
Don't change the initial timing too far though, without consideration for your total timing setting.
As SteveG mentioned, take a look at the timing sticky.
Don't change the initial timing too far though, without consideration for your total timing setting.
As SteveG mentioned, take a look at the timing sticky.
Racer
Joined: Nov 2009
Posts: 384
Likes: 2
From: Tulsa OK
Agree with what has been said about 36 degrees total timing, see Lars' paper and/or the timing sticky. Most mechanical advances are good for 24 degrees, so initial timing should be set at 12 degrees at idle.
A critical part of the issue with a replacement cam is the change in vacuum. A hotter/longer duration cam will pull significantly less vacuum and thus mess with your vacuum advance. I bought a car where the p/o installed a hotter cam. I read Lars' paper on GM vacuum advance specs and got a vacuum gauge. You want a vacuum advance can that pulls all the way in at about 2 inches Hg below prevailing idle vacuum. Mine pulled 14 inches Hg, and I ordered the B26 vac can from Napa (all in at 12 inches Hg). Idles much better and better off idle performance. This can gives 16 crankshaft degrees (8 distributor degrees) additional timing at high vacuum conditions (idle, light cruise).
Read the timing sticky. It and Lars' papers are a real education and not difficult to understand once you spend a little time reading that info.
A critical part of the issue with a replacement cam is the change in vacuum. A hotter/longer duration cam will pull significantly less vacuum and thus mess with your vacuum advance. I bought a car where the p/o installed a hotter cam. I read Lars' paper on GM vacuum advance specs and got a vacuum gauge. You want a vacuum advance can that pulls all the way in at about 2 inches Hg below prevailing idle vacuum. Mine pulled 14 inches Hg, and I ordered the B26 vac can from Napa (all in at 12 inches Hg). Idles much better and better off idle performance. This can gives 16 crankshaft degrees (8 distributor degrees) additional timing at high vacuum conditions (idle, light cruise).
Read the timing sticky. It and Lars' papers are a real education and not difficult to understand once you spend a little time reading that info.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
Lars
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
Answer to your question is "Yes". The cam sets up the [mechanical] 'timing' of the valves relative to the crankshaft/pistons. But the distributor sets up the electrical timing of the spark to the plugs. Your distributor has a mechanical advance system (flyweights and springs) which causes the electrical timing to advance with increasing engine rpm; it also has a vacuum cannister on it which is the 'vacuum' component of managing the electrical spark timing and helps to regulate timing based on engine load.
The distributor timing components can be [and need to be] adjusted for best performance. The cam timing is built into the cam lobes and via its indexing to the crankshaft with the timing chain and, once installed to the engine, is not adjustable.
Follow lars advice on how to set distributor timing.
The distributor timing components can be [and need to be] adjusted for best performance. The cam timing is built into the cam lobes and via its indexing to the crankshaft with the timing chain and, once installed to the engine, is not adjustable.
Follow lars advice on how to set distributor timing.
