Thread: Timing help, sbc

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I have searched and read but still this is the first time I am trying to fine tune the dist timing so just want to make sure I am doing it right. Ok I have the base timing set at 14 degrees BTDC at 800 rpm. I am using a digital timing light. Ok so I bring the rpms up to 3500 and I have to push the buttons on the light to bring that timing mark on the balancer. It comes out to 16 degrees on the light to get it back to 14 degrees on the balancer at 3500 rpm. So that would make my total timing 30 degrees? Do I need to bump the base timing up 2 degrees to get me to 32 degrees BTDC? The engine is stock , 84 chevy truck, no modifications besides deleting some emission stuff 350/350 stock truck set up. Thanks.

I will assume you have the vacuum advance plugged off. I am not sure of your timing light but it sounds correct. YES, you need to advance your base timing to get the additional 2 degrees of total timing. Some distributors have a limit on the amount of centrifugal advance. Different springs may allow more timing but also may come in sooner.
Let us know if you get it where you want.

Originally Posted by 18436572

I will assume you have the vacuum advance plugged off. I am not sure of your timing light but it sounds correct. YES, you need to advance your base timing to get the additional 2 degrees of total timing. Some distributors have a limit on the amount of centrifugal advance. Different springs may allow more timing but also may come in sooner.
Let us know if you get it where you want.

yes i do have the vacuum plugged off. The timing light is this one
http://www.northerntool.com/shop/too...ct_19845_19845

Any other ideas? I am also wanting to put some different weights and springs to help performance mainly at low rpm. I am reading that I should get the total timing set to 34-38 and forget about the base timing.

Still reading up on this and found a couple links that might help others looking for a better grasp on the timing of a sbc.

http://67-72chevytrucks.com/vboard/s...d.php?t=290498

http://www.chevytalk.org/fusionbb/pr...hp?tid/235206/

http://www.youtube.com/watch?v=UYGU7mTwsZc

First, you need to understand that lean mixtures, like at idle and light throttle pressure or low speeds, burn slower than rich mixtures. These lean low load mixtures are a result of lower volumetric efficiency and the scavenge ability of the exhaust port to clear the spent exhaust gases from the combustion chamber this dilutes the oxygen levels and slows down the burn rate. This condition requires the flame front to be ignited earlier in the compression cycle (more advance), allowing more burn time so that peak cylinder pressure is reached at about 12-15* after TDC.
If your engine doesn’t have enough initial timing in it your Exhaust Gas Temperatures (EGT) will rise dramatically due to the excessive fuel that remains unburned in the combustion chamber and tries to complete the process in the exhaust runner and into the headers.
So we’ve determined that a lean mixture burns slower so on the other side of the coin a rich mixture will burn faster. So if you have a rich mixture you need to fire the plug later or retard the timing slightly. Remember we’re trying to achieve maximum cylinder pressure at 12-15* after top dead center, before that point we’re into detonation after that point results in drastic power loss.
The second timing event is what we refer to as “Mechanical Advance”.
This part of the advance curve is controlled by Engine RPM only and has nothing to do with load on the engine or cylinder pressure, it’s simply RPM activated through the weights and their retention springs. Sometimes this is referred to as “Centrifugal Advance”. When we add the static or initial timing to the amount of Mechanical timing we get what we refer to as Total Timing.
So if we set the initial at 10* and allow the weights to pull in another 25* of timing we would have a total of 35* of timing at the RPM where the weight travel is maximized or they hit what we refer to as the Advance Limiters.
The third and final timing event and probably the most misunderstood is the “Vacuum Timing”. Vacuum timing has nothing to do with total under load timing or performance tuning. Vacuum timing is controlled by “Manifold Vacuum”, as soon as you accelerate the vacuum in the manifold drops and the Vacuum Timing is released and the timing events are then controlled by the Mechanical and Initial timing settings.
So previously we determined that a lean idle mixture requires more timing to get a complete and efficient burn in the combustion chamber than the under acceleration or full power richer mixtures as delivered by the main circuits.
So to achieve this, the vacuum unit is connected to a “MANIFOLD or CONSTANT” vacuum port and can be set to add anywhere from 5 to 20* more timing in the motor when high vacuum readings or low load conditions are present. So now to calculate the total timing at idle or low load we add together the static or initial, the Vacuum timing and whatever mechanical timing we have based on RPM. Going back to the previous example you would add the 10* initial setting and say 15* of Vacuum timing for a total of 25* at idle or low load, low RPM cruise. Because the vacuum timing is progressive to and directly related to manifold vacuum as the manifold vacuum increases (Light Load) or decreases (As you accelerate) the timing in the motor changes accordingly to keep the efficiency at optimum. The mechanical timing will come and go as engine RPM increases or decreases.
So let’s look at an actual car rolling down the road at 65MPH on a nice long flat stretch.
We need to realize that a car in cruise mode at 65 MPH only needs to make about 50-75 HP (Depending on weight and wind resistance) to maintain a steady cruise speed.
We start with the 10* initial add to that the mechanical of 25* at say 3000 RPM and Vacuum timing of another 15* totaled up we see that we have 35* of total timing in this engine example and 15 in the can. Pretty standard for a near stock application, but probably not the best for a Muscle Car trying to burn unleaded 93-octane fuel.

Thanks for the help. I am getting it but confused on the vacuum advance but for now running s stock setup so I deal with that later. Would it be a better idea to rev the rpms up to 3500 and set the total timing to 32-34 degrees and forget about initial? How would I keep the 32-34 total and lower initial if needed. And do this I set the timing light to 32-34 degrees and set the balancer degree tab to 0? I apologize for the basic questions but I have read so much I am kinda confusing myself,lol. I have read 10 different ways of reading the timing and curve with different lights and different ways of going about it.

Originally Posted by BigTruckDriver

Thanks for the help. I am getting it but confused on the vacuum advance but for now running s stock setup so I deal with that later. Would it be a better idea to rev the rpms up to 3500 and set the total timing to 32-34 degrees and forget about initial? How would I keep the 32-34 total and lower initial if needed. And do this I set the timing light to 32-34 degrees and set the balancer degree tab to 0? I apologize for the basic questions but I have read so much I am kinda confusing myself,lol. I have read 10 different ways of reading the timing and curve with different lights and different ways of going about it.

you can do that to a point were to much initial will make the engine buck you put to much load to fire the engine and on big CR engines you can end up with a busted starter and it on the floor so you want to try to put is much as you can in the dist really no need to make this to hard for you set it at 32 drive it see how it feels then try it at 34 if it feels the same you can set it back if it start to rattle pull it back . all the vacuum lets you do is pull more timing at no load so the engine runs cooler and you may get better mpg? so under a load the vacuum drops out so will the timming you do not metal the pistons in the oil pan.i use dammper that are degee or tape .so you know were you are and the pointer is @ O so it you rev it to 2500 rpm and its 32 thats what you have you can watch the curve as to when and what rpm it stop at .liter spring make the curve come in faster this is best tune in on the road .what ever you do with the spring s and timming you must make sure the engine stop pulling advance curve or you can blow head gaskets and metal stuff it better to have a bit less then to much so if you run it to lets say 2500 thinking every thing has stop it better to go past that so you know you total timming has stop or she still could be pulling timming and be at 40+at WOT .i would not worry to much about this engine to much if it a driver set it were it run the best .the damper ring can be off you could have the timming right on but not were it needs to be. i do not trust timming lites with curve worked in to them with a button or knob

Thanks Pat that clears it up a little more. I agree with the regular light and timing tape. Its just I have this digital light thats get me mixed up on things. Thanks though for you guys input I'll get it.

Originally Posted by BigTruckDriver

Thanks Pat that clears it up a little more. I agree with the regular light and timing tape. Its just I have this digital light thats get me mixed up on things. Thanks though for you guys input I'll get it.

yep i never was good with the tape getting it to stick BUT if you want lets say 32 then clean the damper lay the tape along the damper then use a paint marker to mark 32 then you have the tape use as long as you want .we could not use them at work they said it would screw up the thunder bolt merc curve boxs?

Ok I found it easier for me to set the degrees on the light to 34 degrees. Vacuum advance unplugged I revved the engine to 3500 rpm ,actually the balancer mark stopped moving at 2500 rpm but I still took it 3500 rpm. At 3500rpm the balancer mark lined up with the 0 degree mark. Dropped the rpm back to 740-750 rpm and the base was at 12BTDC.
So what I have is 12BTDC base timing, 34BTDC total timing, and 22 degrees mechanical all in by 2500 RPM. Does this sound right for a early 90's goodwrench crate engine (hecho en MX) low compression old style heads in a 84 chevy short bed truck? Thanks

sounds good to me. take it for a spin and see