[66chevy2ss]

My car has a rough idle - - with a 0.52 inch lift cam, I understand that that's not unusual. Other engine specifics: '72 sbc 350. 10:1 compression. Headers. 650 Holley. Accel HEI. Est. 410 hp. Other details: M21 4-speed. 3.73 Richmond gears.

So the question is, can I set my timing by targeting the advance at a higher RPM? Say 1,500 or 2,000?

More to the point, does anyone know how far advanced I should be at about 1,500 RPM?

[Limey Simon]

I'm no expert but I've been advised on this site to set my 350 SBC to about 35 degrees of maximum advance at 3000 rpm, with vacuum advance disconnected & plugged. Don't know if this will apply to your spec engine, but it might help?
Limey

[66chevy2ss]

Thanks for the reply. I'll try to target 35 degrees total advance (sans the vacuum advance).

I guess I was exaggerating a bit when I said "1,500 rpm," but I don't think I can time it below 900-1000 rpms.

[trees]

I have a new toy and it is proving to be very interesting. My 383 has a new Edelbrock PRO FLOW siting on top and it comes with it's own diagnostic system. To set it up IAW the manual, the motor must be set at 10 degrees and then the computer takes over from there. Now the interesting part kicks in. My idle is at 600RPM (cam is mild) and the TOTAL timing is 28 degrees!! ie the computer advances the timing 18 degrees. I have not had time, nor am I at the point to complete all the initialization as per manual. I have taken it to 3000 RPM and the total timing is 40 degrees. These figures go along with the manual and some of discussions on previous threads. So, Limey, you are close to the same even tho your vacuum advance is plugged off, but you would not get much change if you remained steady at 3000RPM and reattached the vacuum. 66, looking in my charts, 20- 22 degrees would be close at 1500RPM. You could try that and see where it goes at idle. When you start mixing and matching on engine builds, it is easy to get the wrong timing indicator (timing gear cover) so unless you use a degree wheel to identify TDC, then you can not be sure where your timing really is.

Trees

[66chevy2ss]

Good info, Trees. Thanks!

What do you mean by "IAW the manual?"

I should be o.k. with top-dead-center. My vibration damper has a labeler on it that shows TDC, etc...

Plus I installed the distributor last summer (though, I had about 2 weeks of experience as a mechanic, then!) so I'm fairly certain the timing is accurately readable (Limey, please correct me if I butchered the English too much there, i.e. "accurately readable"!)

[Bartman39]

What "trees" said applys mainly to fuel injected engines... What I mean is they have the timing set to a certain point and will vary quite a bit and most notably at idle... The computer leans the engine out and advances the timing to compensate... So dont be fooled by what the timing is on a computer controlled engine...

66chevy2ss... In answer to your question yes you can set the timing by setting it by the total amount which means using both the vacuum advance and mechanical advance... Run the engine at 3500-3600 and check the total amount of advance 38-42 degree`s should be the max (either have a balencer or add on tape to the dampner to see this)... I run a SBC 355 w/Vortec heads and flat-tops (all forged & balenced w/fluid dampner) the cam I run is a performer RPM basicly (a Summit copy) the engine has to have 16 degrees of initial timing to run correctly on the topend 16+12+12=40 total (the 12`s are the vacuum and mechanical advance amounts in the distributor which is also a performance HEI...) Hope this answers your question...

[ April 25, 2003: Message edited by: Bartman39 ]</p>

[66chevy2ss]

O.k., so I WILL leave the vacuum advance attached?

[Frisco]

[quote]Originally posted by 66chevy2ss:
<strong>O.k., so I WILL leave the vacuum advance attached?</strong><hr></blockquote>

NO!!! Total timing is the initial advance plus the mechanical advance ONLY. The vacuum is not included to these figures.

Start your engine with your timing light hooked up. Disconnect your vacuum advance hose and plug it (a bolt will work for this). Using either a degreed harmonic balancer, timing tape attached to the harmonic balancer, or a dial type timing light; set your total timing to 34-38 degrees BTDC at 2500-3000 RPM. I use 36 degrees at 3000 RPM for my setup. You will have to experiment to match your particular setup. The idea is to get the highest advance without any ping (detonation). After you have set the total timing your initial advance should be in the 10-12 degree range (with the vacuum advance line still disconnected and plugged). You can now reconnect the vacuum advance hose. If you were to check the timing with the hose connected you will see a much higher reading. This is because there is no load on the engine and will go away when you place a load on it (driving it).

Another point that is controversial: I have found that using the 'FULL' vacuum port for the vacuum advance works best for me. Some will say you should use the 'Ported' vacuum source. Try both to see what works for your application.

[Frisco]

[quote]Originally posted by Bartman39:
<strong>What "trees" said applys mainly to fuel injected engines... What I mean is they have the timing set to a certain point and will vary quite a bit and most notably at idle... The computer leans the engine out and advances the timing to compensate... So dont be fooled by what the timing is on a computer controlled engine...

[ April 25, 2003: Message edited by: Bartman39 ]</strong><hr></blockquote>

This is correct.

Your statement about what 'Total Timing' is was not correct. The vacuum advance is not included in the Total Timing because it does not exist at WOT and is limited under varying engine load conditions. Vacuum advance is used to change the timing under varying load conditions to improve driveability. It is not used when setting either the initial timing or total timing.

[66chevy2ss]

Is it possible that I won't be able to detect (at least a certain amount of) ping given how loud my setup is?

[Bartman39]

Frisco

With a ported vacuum from the carb the vacuum will hold the vacuum advance to full movement to allow you to check for "total timing" But if you use manifold vacuum then no it will not work worth a crap... There is a difference and for the street and everyday usage using the vacuum advance is a must for driveability and fuel economy (a little anyhow...) When using just the mechanical advance and setting total timing this is a pure strip setup... And you might even need a retard system just to crank the engine...

Also ported vacuum does work at WOT... Thats what it was designed for...

[ April 28, 2003: Message edited by: Bartman39 ]</p>