Thread: timing at idle and wot

Tweet

im wondering what is the base timing on a small block chevy. when i had the orginal mechnanical advance it would be to advanced at start up and i went thruogh 2 starters.i put a vacum advance in it and i want to know what should the base timing be at idle and start up and how much should it be advancing thruogh idle -5000.and when should all the timing advance be pulled in

I think the Chevy manual says something like 4-6 degrees initial at the crank. Denny will have straight scoop on this, but most everybody uses more initial, like somewhere between 10 and 14. When we had good gas we could run 36 degrees initial and centrifugal combined, but I might pull that back a little these days to 33-34 with good pump gas. You'll get varied opinions on the rpm point where all the advance should be in, from 2,400 to 3,200.

When you say SBC, I hope you realize that there are numerous configurations under this heading, and they most certainly do not have a “one setting fits all” category, not all SBC engines fit under the same classification.

For instance, you can have a SBC as small as 4.4 liter or 267 cu inch all the way to a 7.4 liter or 454 cu.in.

Having said that, the smaller the displacement, the more timing you can get away with. I would think that a stock 305 with flat tops and with 60 cc chamber heads, you could set at 14 initial and 40 total with premium fuel (94 octane).

Bolt those same heads on a 5.7/350, and you would have to back off the timing by 3-4 degrees ie:10 initial and 36 total with (96 octane).

Bolt those head on a 40o/6.6, and you will have to retard the timing even further, say 6 initial and 33 total (with 100 octane).

Take those heads and bolt them on a 7.4/454, and you are asking for trouble. Probably 30 degrees total (with 114 octane)

You see, here-in lies the problem....... the larger the stroke, the more travel a piston has to circulate, given the same distance before TDC (top dead center).

What I’m trying to say is this, if you have two SBC engines and both distributor timings are set at say 40 degress BTDC, the engine with the bigger stroke will create the most compression because it has a greater distance to travel given the 40 degrees marker.

The higher the compression, the more power you produce, however one has to compensate for this either via backing off the timing, using higher octane, or going with dish pistons or perhaps even using thicker head gaskets, but that brings into this calculation yet another problem, which I will not introduce at this time.

On the matter as to at what point you should reach the total timing, I would give you only a general rule, which is the 3000 rpm marker, which the Inspector already informed you of. I personally have my initial at 24 degrees with a total at 3800 rpm. But then I also have a start retard of 20 degrees. I have a 406 SBC.

As to why you had a problem with the starters, my guess is that your mechanical weights were not collapsing and therefore you had two much timing at start up.

the motor in question is a stock 305 from 1982.the compression is about 8.6-1
the reason i was asking is cause i took out the mechanical advance distributor cause it would be too advanced for the starter and i broke 2 of them
but when i retarded it would run like crap.
so i put a vacum advance in
i need to know what should the base timing be and where should all the advance be pul;led in and how much advance
i got one of those spring advance kits
with gold black and brown springs