c2 timing

[Duke W.]

Re: c2 timing

I think 700 is a little low. Try 850-900. That will increase manifold vacuum and should improve the idle quality. Idle speed choice is somewhat subjective. The GM recommended idle speeds are low IMO especially on SHP engines.

I recommend the lowest idle speed commensurate with acceptable idle quality.

If you do stay with the 700 idle speed you might need the VC-1810 or equivalent, (stamped B28) but they are now very scarce and tough to find.

Install a new B20 and go from there.

Duke

[Robert T.]

Re: c2 timing

Thanks again, Duke.

I think I'm catching on to all the timing stuff. I haven't cramed so much since "finals" back in college. What do you think about the 38 degree total number, or is that one of those things you set and try. It sounds like if I get a correct vacuum can then it will add the advance at idle that I have been doing instead with the initial timing (again because my current can is not correct). That way I can back off the initial setting, thanks to the vacuum advance, and thus reduce that 38 total a little.

I'm also trying to dig up more details on this engine. The GM guys have not been of any help. It is a new GM crate engine model 454HO. I've attached a couple of pictures of the install.

Thanks for the help.

[Duke W.]

Re: c2 timing

I believe Lar's paper thoroughly discusses the requirements for a spark advance map:

1. total idle timing
2. total cruise timing
3 total WOT timing

and provides guidelines for various configurations. Combustion chamber geometry plays a roll and big block chambers have changed a lot over the years.

As a general rule the total WOT timing can usually be advanced to the threshold of detonation for best top end power. Then you juggle between initial and total centrifugal and the rate of centrifugal to handle low rev detonation.

The required total idle timing is basically a function of valve overlap and should be in the range to low 20s to low 30s with high overlap cams needing to be in the upper part of this range. This is generally achieved with the sum of initial and full vacuum advance.

Total cruise timing depending on the centrifugal curve and cruise revs will usually fall in the range of 40-55 degrees.

It comes down to how much time you want to spend optimizing the spark advance map.

[Robert T.]

Re: c2 timing

Thanks again. Sounds like more studying, measuring, adjusting...and drinking, are in order.


Thanks.