Hey all, can someone please explain how base timing affects start up/craning, vacuum and idle quality?
I am sure to have more questions.
Background info: Starting to tune my 340 with an MSD with an unknown/aggressive solid lifter cam. Currently base timing is at 20° and curves to all in at 3,000 @34°
Many Thanks,
Ron
I didn’t read every post so I may be repeating things.
Think of the curve in segments because that’s what it is. So where you start and where you end are not the only two points to consider.
Starting point is initial. The end point is total. The combination of initial timing, plus mechanical advance and vacuum advance is your total. And you should be using VA if the engine sees the street. You can’t always but you should give it a try.
The issue becomes getting the initial right, then starting the curve at the right time and shaping the timing curve to follow pretty much your volumetric efficiency curve.
That means that at and around peak torque the engine will want LESS timing that it does at peak power. Sounds simple but you can get boxed into some hard places if your not careful.
Low compression, big cam engines will take a different curve than the same engine with the correct cam or more compression.
Or, when you are getting near the limits of the fuel you are using. As in let’s say 12:1 pump gas. That stuff gets tricky.
If you have to run vacuum advance to get the engine to idle you’ve probably over cammed it. This is easy to grasp if you think it through.
Let’s say we have a 10:1 engine with the best cam timing we can get (it’s always a compromise) and then we get the compression to 12:1 and we change the cam to match the need compression.
The timing curves will be totally different everywhere. One will want more initial. The high compression engine will most likely want less initial and a slower curve.
At idle the chambers aren’t packed as tightly with less compression so it needs more time (timing) to burn that less dense mixture. Higher compression is packed tighter and need less timing
And then there is rpm. Both combinations will have different power curves. Peak torque will be lower with the smaller cam, low compression engine.
So you have to get the weights and springs to get the curve as close as you can to match it.
This is hard to explain while typing.
At any rate, let’s say the first combo is all done by 6k and the other combo makes peak power at 6800.
Using rough numbers we can say the first combo will have a torque peak about 4000-4200. And we’ve tested and learned at 6k this engine wants 34 total and you need 24 initial to make it idle.
You only get 10 degrees of curve to play with. That tells me it’s going to be difficult to slow the curve down through the middle so you don’t kill power at peak torque. Too much timing at peak torque will kill power.
If you get the peak torque timing number wrong it will be wrong for the rest of the curve.
Now let’s see what I expect my timing to be on my garbage.
12:1 on pump gas. Peak power at 6200 and shifting at 6700-6800.
That means peak torque will be near 4500 and at 6800 it will probably only need 32 total. I’m expecting 18-20 initial. I’m hoping it’s that high. If it’s not and I need less initial it makes it harder to not kill power at peak torque with too much timing.
Just some examples of what timing curves can look like. The upshot is if you have peak timing at or worse before peak torque you are probably trimming 20-30 horsepower or more right off the top.
That’s why getting the engine on a crank dyno is so important. You have to be able to load the engine down to figure out what it wants at those rpm and loads.
It’s like making hot dogs. You don’t want to see it done but you love the end result.
And, you can’t do that type of testing on a chassis dyno because you can’t just add load IF it’s an inertia dyno. You can do the testing on a chassis dyno IF can be loaded down like an eddy current dyno.
