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 Post subject: Timing at idle ?
 Post Posted: Mon Jul 31, 2017 10:19 am 
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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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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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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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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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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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 Post subject: Re: Timing at idle ?
 Post Posted: Tue Aug 22, 2017 8:47 am 
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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*


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 Post subject: Re: Timing at idle ?
 Post Posted: Wed Aug 23, 2017 8:14 am 
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There is a good read a couple posts above (Timing Mech vs Vac advance) worth taking a look at.


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 Post subject: Re: Timing at idle ?
 Post Posted: Sun Aug 27, 2017 2:51 pm 
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78elk wrote:
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

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"The older I get, the faster I was"


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 Post subject: Re: Timing at idle ?
 Post Posted: Sun Aug 27, 2017 7:17 pm 
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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.

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 Post subject: Re: Timing at idle ?
 Post Posted: Mon Aug 28, 2017 7:57 am 
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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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 Post subject: Re: Timing at idle ?
 Post Posted: Mon Aug 28, 2017 6:59 pm 
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Locked distributors make engines in street cars run a little hotter, too.

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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 Post subject: Re: Timing at idle ?
 Post Posted: Tue Aug 29, 2017 5:58 pm 
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We have the same exact cam in my dads regal with a tunnel ram. Sounds good but its all mid to topend focused

When he finally upped the gears from 2.41 to 3.23 it really woke the car up.

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 Post subject: Re: Timing at idle ?
 Post Posted: Tue Aug 29, 2017 7:50 pm 
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He said it had a "racing" cam in it. I agree, locked timing is the best way to go. I never use anything but locked out for hot rod or race car.

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80 Malibu. 496 pump gas.
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6.33@109 n/a 1.38 60ft. With South Side bars!
9.75@135 100 shot.



A stupid question is better than a stupid mistake!


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 Post subject: Re: Timing at idle ?
 Post Posted: Thu Aug 31, 2017 1:30 am 
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Forewarning, I don't have as much hands-on as some of the guys here, but I'll say what's worked for me.

I don't I looked up the cam, it's 230 @ .050, .480" lift, both lobes. The cam I have in mine is almost identical, it's just the HE series. IMHO I wouldn't say it's a racing cam, but it does look a little aggressive in terms of an older stock SBC. Mine is an XE268 (12-242-2), the paper specs are almost identical, but from what I understand the lobe design is supposed to be different.

I've driven mine daily with a basically stock 350 (210,000+ mile TBI-era 350, original rotating assembly balanced with new rings/bearings)... Vortec heads, carbed intake, and a 600cfm Edelbrock (1405), and a 4-speed (not a 700R4, a Saginaw 4-speed). Very low rpm it complains a little, but not much, but it also doesn't have much more grunt than a smog era 350 down there either.

I'm running a 100% stock HEI distributor out of the 305 that was in my 2nd Malibu, I believe initial timing is at 17º or so, and the vacuum canister is connected to manifold vacuum (not ported). I'm not sure what total advance is because I don't have timing tape on the balancer (come to think of it I believe I put a new balancer on when I rebuilt the rotating assembly, something similar to this) or a dial-back timing light, but the number on the vacuum advance canister says 20, and from what I've read that's 37 total timing from stock. It does knock under heavy load with 87 octane, but I don't know enough about advance weights/springs to properly dial in what it really needs, so that may be on me. I started running 91+ and it doesn't knock, runs like a raped ape, and doesn't get hot, even with a stock, Autozone replacement-style thermostat and radiator.

PS, I also live in Arizona, one of the hottest places in the US... please buy me some A/C :awesome:

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 Post subject: Re: Timing at idle ?
 Post Posted: Sat Sep 02, 2017 9:41 am 
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Ed Cannon wrote:
He said it had a "racing" cam in it. I agree, locked timing is the best way to go. I never use anything but locked out for hot rod or race car.


The 12-212-2 is a Magnum hydraulic flat tappet. 110° lobe separation, 230° duration at .050", .480" lift. Hardly a "race" cam. I still think he should use the vacuum advance on full manifold vacuum.

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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