40 degrees initial timing

[JG1966]

My 340 is set at 18 initial, 34 total, all in at 2500. Today, I used a vacuum gauge to see what timing my engine "wanted." I rotated distributor to get max vacuum (about 15 lbs), and when I checked, damned if my initial wasn't at 40. It idled well there (idling at about 850) and didn't appear to be detonating. Obviously, I didn't leave it there. I moved it back to 18 initial. But does that sound alarms? I know timing marks/balancer are right.

 

[Hemirunner]

That's pretty typical. Most engines with larger than stock cams will want some timing at idle. I run my street hemi locked out at 32 degrees and use a start retard to fire it.

 

[67 GTX]

Did you get that 40 with the vacuum advance hooked up? Initial timing is without vacuum advance.
Normally the vacuum advances are between 14-16 degrees, so that would put your total initial around 35, which is fine.

 

[747mopar]

Mine likes the initial at 30, I have mine setup so the vacuum advance pulls it from 20 to 30 at idle for easier starting. If mine were set at 18 degrees it would be a complete dog off the line. I'm thinking you're in for a happy surprise if you get that car tuned right.

 

[JG1966]

Did you get that 40 with the vacuum advance hooked up? Initial timing is without vacuum advance.
Normally the vacuum advances are between 14-16 degrees, so that would put your total initial around 35, which is fine.

Vacuum advance was plugged.

 

[JG1966]

Mine likes the initial at 30, I have mine setup so the vacuum advance pulls it from 20 to 30 at idle for easier starting. If mine were set at 18 degrees it would be a complete dog off the line. I'm thinking you're in for a happy surprise if you get that car tuned right.

Thanks 747. I like that idea. Right now I have vacuum advance plugged. I may have to try something like that. My question is - and sorry if I'm a timing noobie - but how much mechanical advance should I look for with a setup like that? Right now, I have a limiter plate that gives 18-16-14-12-10 or 0, depending on where I have it slotted. When you figure total timing, do you add initial, vacuum and mechanical? Or because vacuum advance lowers as mechanical increases (if I use manifold port with vacuum advance), is the math different?

 

[beanhead]

That's pretty typical. Most engines with larger than stock cams will want some timing at idle. I run my street hemi locked out at 32 degrees and use a start retard to fire it.

Can you elaborate on the start-retard setup? My 440 may love 28 to 30 at idle but any more than 20 and the starter kicks back when warmed up

 

[747mopar]

I'm by no means an expert and required some help to figure mine out but I'll try to help.

Your total will be a combination of your initial plus whatever you set the limit at (20 degrees initial plus 10 degrees mechanical advance = 30 total). Your vacuum advance will pull more timing in at idle and at a maintained cruising speed but does nothing under acceleration since vacuum drops off anytime the throttle is opened.

As for your setup? Everybody's is different so you'll have to tinker with it. Mine likes 30 at idle and 40 total, I've seen small blocks that liked upwards of 50 so you'll just have to tinker.

 

[Hemirunner]

Can you elaborate on the start-retard setup? My 440 may love 28 to 30 at idle but any more than 20 and the starter kicks back when warmed up

I run an MSD brand digital programmable ignition. It uses a programmable start retard that pulls timing at below idle cranking rpms to eliminate the kickback. They make add on units for their standard ignition as well.

 

[Yatzee]

Mine likes the initial at 30, I have mine setup so the vacuum advance pulls it from 20 to 30 at idle for easier starting. If mine were set at 18 degrees it would be a complete dog off the line. I'm thinking you're in for a happy surprise if you get that car tuned right.

The vacuum advance can't add any more timing until the engine starts and makes vacuum.

 

[747mopar]

The vacuum advance can't add any more timing until the engine starts and makes vacuum.

Duh, I would have never guessed that the motor had to be running to make vacuum... thought it was magic or something?

O.K. I see where your getting that though, should have worded it better. My initial is 20 with the vacuum bringing in an additional 10 once started for 30. Better?

 

[beanhead]

I run an MSD brand digital programmable ignition. It uses a programmable start retard that pulls timing at below idle cranking rpms to eliminate the kickback. They make add on units for their standard ignition as well.

Ahhh...thanks

 

[moper]

All respectfully IMO...
I'm not a fan of checking timing by vacuum. That's a great way to set the perfect timing for an engine that remains at idle for it's life. My lawn mowers have locked out timing. One of my track only race cars used it too. Once one changes the throttle and rpm, that number is no longer "perfect". Most V8s with open chambers and/or low static compression ratios will want more initial timing. IMO if you need more than 20* initial there's something else off in the combo. Centrifugal is next - when it comes in and how much it adds is very subjective.
Vacuum advance can be added on top of those two (how I prefer to set things up), left off totally (sometimes necessary if the cam/engine combo puts the running vacuum levels outside the adjustability of the vacuum can), or plugged into manifold vacuum to provide more timing at idle (see above comment on wrong combo).

Some very base curves:
1. For static compression under 9.5:1, open chamber heads (typical factory displacement performance rebuild) - properly matched camshaft: 18* initial; 20* centrifugal set to start coming in at 2K rpm, all in by 3K; vacuum connected to ported vacuum and adding whatever it's got (10-18).
2. For static ratios above 9.5:1, closed chamber heads, factory displacement, properly matched camshaft - 18* initial; 12* centrifugal set to come in at 1800 and all in by 2400; vacuum in ported on top of that.
3. For static ratios above 10:1 in factory displacement, or above 9.5:1 in longer stroke combos, aluminum heads w/modern chambers, and built with effective quench; 18* initial; 10* centrifugal starting at 1500 and all in by 2200; vacuum in ported with whatever it has in it.

 

[TxDon]

I'm in the process of adjusting the centrifugal vs. initial timing on my car so am watching this thread with interest.

 

[Hemirunner]

I agree with Moper right up to the point where you take a BBM, stroke it, add a big cam and try to drive it on the street. It can be a real miserable experience. If the engine makes max HP at 34 degrees, it's never going to need less except to get it to start. (I'm excluding high rpm, high load gear retards like we use for the sake of simplicity) Low initial timing in these combos means a poor, choppy idle with low vacuum and excessive heating at low rpm. For most of you out there with mild combos , 18 degrees initial will suffice. For those that don't want to fight those issues, locked timing is an option. We all have different backgrounds and experiences, and opinions. Find what works for you and your combo.

 

[MarPar]