Timing at idle ?

78elk

Dragway Regular
Mar 6, 2014
1,294
81
48
Mountain Home Id
I have a question about timing. The Cam my son put in this engine back in 1995 is a 12-212-2 Comp Cam. It design is for racing. This engine has about 30,000 + miles on it . I know that GM wanted the initial timing at 10* , this engine wants it about 17* initial timing. When I time it I pull and plug the vacuum line. I check my 17* initial the set the timing light to 36. I rev it up to 3500 rpm and it is 35* there. I connect the vacuum line back to the canister it shows 35* at idle. I have since adjusted the vacuum canister down to 24 *. It runs well and doesn't heat up in 95* weather. What harm will this do to this 350 ci engine?
 

1982 SS

Frequent Racer
Jan 19, 2007
382
0
0
Clearfield, Utah
What you are experiencing is the vaccum advance and the difference between ported and manifold vacuum at the vacuum advance. Some cars will drive better with the vacuum advance hooked to one or the other, you'll have to drive it both ways to find out.

Now for some theory, at cruise where the mixture tends to be lean, and essentially has a slow burn to it, it needs more advance to burn the mixture efficiently for good road manners and drivability. The total amount of vacuum advance also depends on the combination, some like more vacuum advance, others like less. Make adjustments and drive it to see how it reacts. This is tuning for drivability.

High amounts of advance with the vacuum advance plugged in is normal and will not hurt your engine, as cruise is a low load situation. At and near WOT, the vacuum drops off to 0 and there is no advance brought in from the vacuum advance canister. That is where your mechanical advance has total control over the timing. If you had that level of advance at WOT your would likely be having detonation issues and you would be able to see it on the spark plugs and hear the pinging (provided the car is quiet enough). I've driven my El Camino with its locked out timing on the street and its a little lazy at about a 55 mph cruise. It did drive better with the vacuum advance and the mechanical advance, but it sees so little street time that it hardly matters. Its a bracket car first and it will be what it is on the street.
 

LS6 Tommy

MalibuRacing Junkie
May 15, 2004
15,847
1
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North Jersey
Stock distributors on emissions engines used ported vacuum. All others should be on manifold vacuum. It sounds like you have a decent performance curved distributor. A decent starting point for Chevies is 12° -15° at idle with the vacuum disconnected and plugged, roughly 36° "all in" by about 3000 RPM. The vacuum advance should be set to about 15° max.

Tommy
 

78elk

Dragway Regular
Thread starter
Mar 6, 2014
1,294
81
48
Mountain Home Id
Thanks for your help. I lowered the vacuum at idle to 27*at idle with vacuum hose connected. This is pretty close to what you asked me to do. I'm new to this, what's the reason that vacuum should be limited to 15*
 

LS6 Tommy

MalibuRacing Junkie
May 15, 2004
15,847
1
38
North Jersey
78elk said:
Thanks for your help. I lowered the vacuum at idle to 27*at idle with vacuum hose connected. This is pretty close to what you asked me to do. I'm new to this, what's the reason that vacuum should be limited to 15*

If the mechanical (centrifugal) advance is 35° and the vacuum advance is 15°, you have a total of 50°. Adding much more than that will cause preignition.

Tommy
 

Ed Cannon

Pro Stocker
Jul 17, 2009
1,682
0
0
Johnson City Tn
I would disconnect and plug the vacuum advance, Install a recurve kit in the distributor with the lightest springs. Set my total timing at 36deg, all in by 3k rpm let the idle timing fall where it may as long as it starts ok.
 

lilbowtie

Pro Stocker
Oct 5, 2004
1,736
0
0
Canton Mi.
Ed I would agree with you if it was a race car but on a street car your taking away drivability-economy. You need that vacuum advance to maximize part load performance. In our case it's WOT - I actually run with a locked out distributor and my car loves 40 deg.
 

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