4 replies to this topic

[Dr Terry]

Hi Guys.

This is an excellent example of the need to regraph the dissy to suit a new camshaft.

Here's the problem. Standard Chev 350 (we will assume a pre-pollution version '68-'71) is around 4 degrees, the mechanical advance in the dissy is around 30-32 degrees. That means the max mechanical advance will be around 34 to 36 degrees, which is about right for a motor like a SB Chev V8.

The new cam will need around 10 degrees initial ignition timing, because it will have a longer duration than the stock cam & will start & idle better at 10 degrees, otherwise your manifold vacuum & low down torque will be awful. If you advance the stock dissy to 10 degrees, you will then have 40 to 42 degrees of advance at high speed, it will detonate & ping like mad.

You need about 6 degrees LESS mechanical advance in the dissy, so it will still idle & drive low down well & still have 34 to 36 degrees up top.

The basic rule is, the bigger the cam, then the more initial timing is needed, therefore LESS advance is required inside the dissy itself, to maintain the same total.

When I ran my 350 Monaro drag car, we had a very unstreetable roller cam with around 600 thou lift. We used 22 degrees initial timing & only 16 inside the dissy. 38 degrees total is about all an NA Chev will with stand on pump gas.

Dr Terry

[Dr Terry]

Hi again Chris.

I did forget that you asked about point dwell. If it's an original Delco dissy the factory specis 27 to 31 degrees.

Dr Terry.