So Don from FBO Systems was spot on.
I had messaged him and explained the rough spot I was feeling around 2500 rpm in every gear with the 18 degrees of initial timing and 34 degrees total timing with the 2 blue springs in the firecore distributor that had the timing all in around 2400-2500 rpm.
He asked me all the details, what engine, what vehicle, what tire size and what gearing.
I answered all of his questions and he said I think your initial is good and you could back off the total timing a couple degrees, the magnum heads are more efficient and don’t need as much total timing, he said try 18 initial and 32 total.
Then he said your biggest problem that I can see isn’t your advance rate is too fast, all in by 2400-2500 is to quick in that heavier truck with bigger tires.
He said slow that down to all in by 3000-3500.
So yesterday I pulled the distributor back out and took it apart and swapped the 2 blue springs for one silver spring and one black spring. Set the mechanical advance to 14 degrees.
This has the advance rate start at about 1500 rpm and has it all in around 3400-3500 rpm.
Set initial timing at 18 which lands the total at 32 degrees.
Hooked the vacuum advance up and turned it up to add 12 degrees at idle for a total of 30 at idle with vacuum advance.
Took it out for a drive and the rough spot at 2500 rpm is completely gone.
Smooth as butter all through the throttle now.
What confuses me is this engine is advertised at 9:0.1 compression, I’m running 4.56 gears with 33/12.50/15 tires.
So my gearing is more then adequate for the tire size and the truck.
Why was having the timing all in by 2500 an issue?
I’m running premium fuel as well.
I was just shocked that all in at 2500 was too fast, specially with the lower gear ratio.
Is it because the engine had more of a load on it being in a truck and the smaller camshaft is building a lot of cylinder pressure lower in the rpm range?
Those are the only conclusions I can come too.
The throttle response is lacking a little bit compared to when it had the two blue springs in it, but it feels so much smoother going down the road now so I’ll deal with that trade.
I’m sure it could tolerate having the timing all in by 2800-3000 but there isn’t a spring combo available that has the timing all in by 2800-3000 with 14 degrees of mechanical advance.
The closest I could get is the one silver and one black spring which starts the curve around 1500 and has it all in by 3400-3500