Timing Question
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Troy,remember,if you just set your TOTAL TIMING to 36 deg without 1st setting your initial timing the initial will very likely be retarded,that looses power,wastes fuel,and causes motor to run a little hotter too.
What happens is if your dist has approx 25 deg mechanical then at 36 total your only left with approx 11 deg initial which is approx 7 deg retarded as most perf cams require approx 16-18 deg initial.
If your running any kind of aftermarket perf cam you need to 1st dial in approx 16-18 deg initial which is done with idle low like 550-600 rpms so mech adv does not start to avtivate and upset the initial setting,vac adv unhooked & plugged too.
After you do that then tighten the dist & recheck timing to ensure it did not move after tightening,if it changed reset & check again.
Then leave the vac adv plugged and now check the total by reving the motor to see what the highest timing reading you get is & at what rpm too.
You may need to get a timing tape to hook on your harm ballencer as your stock timing tap only goes to approx 16 deg for reading initial timing.
With 16-18 initial your total could easily be over 36-38 if your running a stock gm dist and in that case you need to pull the dist and have the mech adv recurved/limited to approx 20 deg to run with 16-18 initial for a 36-38 total.
If you dont do it this way your leaving a lot of perf on table by setting the timing only for a total without 1st getting the initial set correctly and then getting the mech timing curve set correctly to achieve the 36-38 deg total.
Scott
What happens is if your dist has approx 25 deg mechanical then at 36 total your only left with approx 11 deg initial which is approx 7 deg retarded as most perf cams require approx 16-18 deg initial.
If your running any kind of aftermarket perf cam you need to 1st dial in approx 16-18 deg initial which is done with idle low like 550-600 rpms so mech adv does not start to avtivate and upset the initial setting,vac adv unhooked & plugged too.
After you do that then tighten the dist & recheck timing to ensure it did not move after tightening,if it changed reset & check again.
Then leave the vac adv plugged and now check the total by reving the motor to see what the highest timing reading you get is & at what rpm too.
You may need to get a timing tape to hook on your harm ballencer as your stock timing tap only goes to approx 16 deg for reading initial timing.
With 16-18 initial your total could easily be over 36-38 if your running a stock gm dist and in that case you need to pull the dist and have the mech adv recurved/limited to approx 20 deg to run with 16-18 initial for a 36-38 total.
If you dont do it this way your leaving a lot of perf on table by setting the timing only for a total without 1st getting the initial set correctly and then getting the mech timing curve set correctly to achieve the 36-38 deg total.
Scott
Scott,
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?
By the way, great info in your post, thanks.
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?
By the way, great info in your post, thanks.
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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.
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.
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:
Steve,you state "but I differ a bit with Scott on approaches to timing setup. Too much total timing can damage your motor"
I am wondering how you get that i am suggesting too much total when i suggested he run 16-18 initial(with aftermarket perf cam),then 20 mech,for a total of 36-38 deg ?
36-38 deg total in not too much for a good starting point for dialing in timing for many sbc/bbc perf motors with aftermarket perf cams IMHO.
I agree with you that too much total timing can cause detonation that can break pistons etc but again 36-38 deg total is not out of line at all,it;s a very common setup.
You can run the vac adv off full manifold vacuum at idle if the cam needs more timing at idle or run it off the ported if you dont need any motor timing at diel with a very mild perf cam. Yes somesites the vac adv may need to be limited or changed to get approx 50--52 deg for light load/part throttle cruise.
Scott
I am wondering how you get that i am suggesting too much total when i suggested he run 16-18 initial(with aftermarket perf cam),then 20 mech,for a total of 36-38 deg ?
36-38 deg total in not too much for a good starting point for dialing in timing for many sbc/bbc perf motors with aftermarket perf cams IMHO.
I agree with you that too much total timing can cause detonation that can break pistons etc but again 36-38 deg total is not out of line at all,it;s a very common setup.
You can run the vac adv off full manifold vacuum at idle if the cam needs more timing at idle or run it off the ported if you dont need any motor timing at diel with a very mild perf cam. Yes somesites the vac adv may need to be limited or changed to get approx 50--52 deg for light load/part throttle cruise.
Scott
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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:
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:
What i said was after i suggested to set inital timing i said to have the mechanical advance recurved/adj in the dist for approx 20 deg to get a total of 36-38 with 16-18 initial which is very common to do.
You know,its in the mechanicl advance where they replace the coller (older stock points type gm dist) in the mech adv mechanism with a different one to limit/reduce the mech adv. Then they also install lighter springs on the mechanical adv weights so it comes in earlier like at 2,800-3,000 rpms insted of like at 4k rpms like many of the older stock gm dist had.
I think you confused when i said to recurve the mechanical advance with tweaking the the vacuum advance which you sometimes have to do too depending on the setup.
Then he can run a vac adv with approx 12-14 deg adv off intake vacuum if its a hot cam that needs more timing at idle or off the ported vacuum if its a mild cam that doesnt need any more timing at idle.
Scott
You know,its in the mechanicl advance where they replace the coller (older stock points type gm dist) in the mech adv mechanism with a different one to limit/reduce the mech adv. Then they also install lighter springs on the mechanical adv weights so it comes in earlier like at 2,800-3,000 rpms insted of like at 4k rpms like many of the older stock gm dist had.
I think you confused when i said to recurve the mechanical advance with tweaking the the vacuum advance which you sometimes have to do too depending on the setup.
Then he can run a vac adv with approx 12-14 deg adv off intake vacuum if its a hot cam that needs more timing at idle or off the ported vacuum if its a mild cam that doesnt need any more timing at idle.
Scott
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Not to jump on Scott, but I suggest setting the TOTAL first. Most of the dizzy's I've played with have a 20 to 22 mechanical curve.
If you set it to say 16 initial, and you have 20 mechanical, great!
But if you set it to 18-20 initial, and you have 22+ mechanical, you risk damage as your total will be pushed further than 40.
I suggest, set the total to 36-38*, and see where the initial falls. Most likely will be 14-16*, most likely 16 - which should be enough for good performance. Drive it around see if it feels snappy enough. If not, you will need to mod the dist. for less mechanical.
And may I make 1 more important suggestion: when setting your total, rev the motor hard to make sure there is no advance left. Some of these springs are so strong that they don' let the curve in till ~5000rpm. Just imagine setting to 36* total, just to have more advance left and then you have holes in the piston.
If you set it to say 16 initial, and you have 20 mechanical, great!
But if you set it to 18-20 initial, and you have 22+ mechanical, you risk damage as your total will be pushed further than 40.
I suggest, set the total to 36-38*, and see where the initial falls. Most likely will be 14-16*, most likely 16 - which should be enough for good performance. Drive it around see if it feels snappy enough. If not, you will need to mod the dist. for less mechanical.
And may I make 1 more important suggestion: when setting your total, rev the motor hard to make sure there is no advance left. Some of these springs are so strong that they don' let the curve in till ~5000rpm. Just imagine setting to 36* total, just to have more advance left and then you have holes in the piston.
I appreciate all the responses fellas.
I must admit that I am "ignut" a little about timing. I checked it again, the total seems to be 36-38* and the initial is 14*. Does this sound about right? Since this is a new engine, I haven't gotten on it much at all.
The cam is 224/234 530/550, 427 ci, original points distributor, Quadrajet. The engine was dynoed and the builder said he marked the balancer for future use of setting the timing at 36-38*.
I must admit that I am "ignut" a little about timing. I checked it again, the total seems to be 36-38* and the initial is 14*. Does this sound about right? Since this is a new engine, I haven't gotten on it much at all.
The cam is 224/234 530/550, 427 ci, original points distributor, Quadrajet. The engine was dynoed and the builder said he marked the balancer for future use of setting the timing at 36-38*.
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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.
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.
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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.
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.
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