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 Post subject: Timing at idle ?
 Post Posted: Mon Jul 31, 2017 10:19 am 
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Frequent Racer

Joined: Thu Mar 06, 2014 9:20 am
Posts: 371
Location: 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?


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 Post subject: Re: Timing at idle ?
 Post Posted: Mon Jul 31, 2017 11:41 am 
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Frequent Racer

Joined: Fri Jan 19, 2007 11:21 pm
Posts: 379
Location: 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.

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 Post subject: Re: Timing at idle ?
 Post Posted: Mon Jul 31, 2017 12:18 pm 
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Frequent Racer

Joined: Thu Mar 06, 2014 9:20 am
Posts: 371
Location: Mountain Home Id
Thanks for your help. This car cruises very well with no problems. Thanks again.


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 Post subject: Re: Timing at idle ?
 Post Posted: Mon Jul 31, 2017 1:32 pm 
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Frequent Racer

Joined: Thu Mar 06, 2014 9:20 am
Posts: 371
Location: Mountain Home Id
We might bracket race this one sometime after we change the rear gears, 2.41's now going to 3.73's.


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 Post subject: Re: Timing at idle ?
 Post Posted: Sun Aug 06, 2017 11:02 am 
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MalibuRacing Junkie
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Joined: Sat May 15, 2004 7:26 am
Posts: 15672
Location: 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

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20k mile '80 Malibu, currently undergoing long term (aka no money) factory style install LS6 454, Cowl Induction, Legend LGT 700 5 speed. Full Global West suspension, 17" N90 wheels, 12" front/11" rear disc brakes, 9" 3.70 Auburn Pro posi.

"The older I get, the faster I was"


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