Hello - I've been reading the forums for some time now, but have never posted. Just have a question I'm sure can be easily answered. What is the proper timing for a Chevy Small Block 400 with an unidentified cam?
I have no idea but this will get you back up. You could put a vac guage on it, get max vac and back it off a touch.
If it is not a "stock" engine then you will have to find it. Every engine has its own "sweet spot", my 408 sbc is 38-1/2 degrees. I would start at 32 degrees and go up 1 degree at a time till the performance drops off. Then back it off to before it dropped off. That will be your engine's timing sweet spot. By the way, I'm referring to total timing.
Awesome, thank you both for the tips. When you say "till the performance drop off" what am I looking for? The the vac to drop off like chopped says or should I actually be looking for the engine to bog?
Well, I drag race so I go by my time slip. When the car slows then you've gone 1/2 to 1 degree too far. But on the street I go by sound and the seat of my pants.
Set your idle speed, then with the vacuum gauge installed to the intake manifold, rotate the distributor to obtain the highest steady vacuum reading. This setting will usually spark knock under load, so leave the dist snug and then adjust on the road so there is no ping under load, climbing hills or WOT acc, then lock dow the dist.
That is the way I've always done it. When these engines were new we could get 101 octane gas at the pump. I remember being so proud when I bought my first timing light in the early sixties. I was a mechanic now. It's been so long that I have no idea where my timing light is today. I have not needed it in over 30 years. This way you tune your engine to the fuel available today and not to the dated information in the 40 YO manual when they were expecting you to run the high test fuel that we had back then but is unavailable today.
And if you don't have a vacuum gauge, start around 10-12° initial and then start bumping it up until it pings and back it of till it doesn't. This won't necessarily be the optimum timing, but unless you are going to have the distributor curved for your motor it is about the best you can do and it should run fine.
I do have a vacuum gauge. I'm going to give it a shot later this evening. The truck is running fine at the moment, I just think it could run better. I started at 12 and ended up around 14 somewhere. I backed off of it when the carburetor started popping.
Exactly. I think that the smog motor ran a little less lead than this but 12 is a good starting place.
If you have a de-smogged EGR engine with a stock distributor it will have WAY too much total advance. If you need it, there are kits available to re-curve, and limit mechanical advance.
I don't see how setting timing at idle with a vacuum gauge will be optimum timing without taking the distributor curve into the mix. Some dist are all over the place when it comes to amount of advance built into the dist, and the amount in the vacuum can Ago
Exactly..... static/initial advance has no effect on the advance curve and in no was insures that total advance is correct.
When I was a kid one of my mentors used to say "ya gotta let a Chevy run where she wants to run" meaning they were all a little different. I usually will bring her up to about 1500 rpm and start to advance the timing then back it off when it starts to run ragged. If you listen more than you look you will hear where that "sweet spot" is. Then tighten her down and when she hits the asphalt see how "clean" she launches from a dead standstill. It's still trial and error but controlled madness.
He said he was looking for a starting point, and without other information regarding the condition of his distributor, I guess the assumption is that it works like it is supposed to. So setting the timing at idle with a vacuum gauge gives you a starting point, driving it and and adjusting further under load lets you tweek it, So whats not to get??
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