383 ignition timing

[Heavy Z]

Hello everyone, I'm trying to find out some info on timing the 383 in my blazer. It's got the stock distributor w/ vacuum advance, help me out and tell me what the numbers are for setting it at idle. What degree should I shoot for at what idle rpm?

 

Thanks!

[Guest greimann]

Assuming this is a fairly mild motor without a lumpy cam, I would think that 12 degrees at about 750-800 RPM would be about right. Make sure your vacuum line is unplugged and capped when setting this. The lumpier the cam, the more initial timing they like. If it is a high compression motor, a little less initial timing is usually needed for detonation free running.

 

Really though, need a little more info. Are you having problems with something now? Do you need stock specs for tuning for a smog check? If so, what year is the Blazer? Whatt are the cam specs, and the compression specs?

[Heavy Z]

Thanks for the help Dave - sorry I was short on engine specifications.

 

9.5-10 to 1 compression (not sure)

comp cams XE268H (224/230 duration@.50, 477/480 max lift)

 

Thanks,

[Heavy Z]

Oh yeah, this is a '73 blazer w/o smog, so I'm just trying to get a feel for dialing in a 383.

[Guest greimann]

The one thing about setting timing, every motor is different, and you basically have to fiddle with it until it feels right. There can be LOTS of horsepower locked up in timing and a few adjustment can be a real eye opener .

 

One thing you may consider is how your vacuum advance is plumbed, because you have 2 choices, ported and full. In general, emissions controlled low comperssion, stock cam motors want ported vacuum and high compression, hi-po street driven motors want full vacuum. The reason is this:

 

A high compression motor does not want much initial advance (4 to 6 degrees) in order to resist pinging under heavy load, however, that small initial timing makes the motor idle like crap-ola (to use a technical term). Full vacuum at idle brings the timing up and the idle quality as well. As soon as you put your foot in it, the vacuum advance goes away and you have purely initial + mechanical advance working for you. Part throttle cruise brings vacuum advance back into play for good economy.

 

An emission controlled, low compression motor uses the ported vacuum to bring in vacuum advance that is more dependent on the engine rpm, rather than load.

 

I think with your setup, that cam would like a lot of timing at idle that should be set up with full vacuum.

 

You might have to play with all of this to get the best combination.

[Heavy Z]

Thanks Dave, I appreciate the vacuum info you shared. I've got it set at 12 but thought checking with the hybrid crew was worth a post to get some other opinions.