Team Chevelle banner

Timing Question

2093 Views 15 Replies 5 Participants Last post by  vrooom3440
This may be a dumb question, but, here goes. I'm trying to get 36* total dialed in on my engine. I punched in 36* on my timimg light (vac advance plugged) and reved the engine. Should I see 36* at 0 on the tab?
1 - 4 of 16 Posts
This may be a dumb question, but, here goes. I'm trying to get 36* total dialed in on my engine. I punched in 36* on my timimg light (vac advance plugged) and reved the engine. Should I see 36* at 0 on the tab?
I don't use a dial back timing light myself, but my understanding of their purpose and operation says that your balancer TDC mark will align with zero on the engine pointer when your advance hits the value set on the timing light. These types of timing lights are nice when you do not have a degreed balancer as they "degree" it for you.
Thanks for the reply webfoot. What determines the RPM to hit the 36*? I'm still a little confused on this total timing issue.
This is all dependent upon the combination of springs/weights/center-plate in your mechanical advance. In general, and to a point, you gradually increase RPM until the timing stops changing. At that point you have found the total timing (with the vacuum advance disconnected and the carb/manifold vacuum port plugged). For a performance setup it is usually desired to have the total timing all in by 3000 RPM.
My engine builder scribed two lines on the harmonic balancer, one at 36* and one at 38*. My problem is that when I try to time the engine, I feel I should see the 36* mark at zero when I dial it in on my timing light. When reving the engine it seems to go beyond the 36* mark at zero on the balancer. Would that mean I'm getting too much mechanical advance?
To use those marks you will need the timing light set to zero. Then if it advances past the marks, without vacuum advance hooked up, then yes you have too much total timing.

We end up at the same point, but I differ a bit with Scott on approaches to timing setup. Too much total timing can damage your motor, which is why I always start my setup there. Too little initial timing just makes it run crappy (technical term ;) ). I prefer running poorly rather than damaging. I also start from total timing because the value is consistent across the entire range of Chevy engines no matter what their setup. Initial timing by comparison varies all over the map, largely dependent upon cam timing. Especially as cam durations go above about 270 the timing advance required at idle goes up. But there are multiple ways to get that idle timing. Again more complicated at initial than total.

I don't think you have shared your cam parameters? Without them it is rather random to suggest too much about what your initial timing should be.

Let me use my setup as an example. I run a 402 BB with a HR296 roller cam and a Muncie 4 speed. I have my total setup to around 36-38* which put initial around 16-18* on my no-name performance HEI. I have limited the movement on my vacuum advance from 20* down to about 12* to give me a cruise timing of 50-52* (which is another consistent Chevy timing parameter). And finally I run a vacuum advance can that starts at 3" and is all in by 9", but with my range limiter the start is moved up to 6". My vacuum advance is hooked up to manifold vacuum. This combination gives me about 30* advance at idle and 11" of vacuum.

Note that prior to adding the vacuum advance at idle I had to open up the throttle blades too far and had 3-4" lower vacuum. In a prior discussion here on TC it was found that a particular engine kept idling better and better all the way up to 40* of advance. Bigger cams really want more advance at idle :beers:
See less See more
Scott, I don't disagree with your end point at all. We agree on most of the parameters and on the desired total we are in violent agreement :thumbsup:

From your description it sounded as though you were suggesting setting initial and then tweaking the advance mechanism to get total. I am merely suggesting setting total and tweaking to get initial. We are both trying to arrive at the same destination just from different directions.

I also agree that 16-18* initial, 20* advance, and 36-38* total is a very common setup. After all it is what I put in my example and what I run :beers:
Sounds like a good starting point. The motor would probably like more advance at idle. Do you have any idea how your vacuum advance is setup?

I personally like the idea of adding more advance at idle via the vacuum advance (if you could not tell ;) ). The cool part of doing it this way is that your starter only has to crank with 14* and not 20-something. Makes the starter much happier and actually meets true engine requirements while turning over at the slow cranking speed much better. Then once it is up to speed/idle the vacuum comes up and adds the extra advance needed.
Yes given no other changes you will change the total when you change the initial. To increase the initial without changing the total you must reduce the amount of advance provided by the mechanical advance.

I spent an evening with a hand vacuum pump on my vacuum advance and it was one of the best things I ever did. You can slowly squeeze the pump and watch to see when the vacuum advance starts to move. Note the vacuum level on the gauge and this is where vacuum advance starts. Keep slowly squeezing the pump until the vacuum advance has moved to it's limit. Note the vacuum level on the gauge and this is where vacuum advance is all in.

Now compare the latter value with your vacuum at idle and in gear if you have an automatic transmission. That all in advance vacuum should be 1-2" less than you have at idle for a matched vacuum advance setup. Then hook it up to manifold vacuum and instant bump of "initial" (really just idle timing).

The hand vacuum pump tools run about $40 at your local car parts store.
See less See more
1 - 4 of 16 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.