Initial ignition timing

[77 K20]

Just wondering what you guys thought would be best to do in my situation.

I have the GM HT383. It says use a particular GM distributor (HEI distributor P/N 93440806), no vacuum advance and set it to 10* at an idle of 650 RPM. This setting will produce 32º of total advance at wide-open throttle by 4,000 RPM. I've had it like that, and it ran great. I then put on the Edelbrock TBI fuel injection a while back and in their owners manual and talking to one of their tech support guys they keep saying their system runs best with 18-20* initial timing. To me initial timing should be based on what camshaft you have, not what fuel delivery system is has... or am I missing something?

I tried bumping up the timing just to see what it can do- GM says the engine will run on 87 octane. I use 91 octane as it has no ethanol in it up here. The timing tab is almost impossible to see- the power steering bracket is in the way, but I advanced it to around 16-17 degrees and was rewarded with part throttle engine knock. I've turned it down to about 13*. And by advancing the timing now I can not get it to idle less than about 800 RPM (idle screw backed all the way out, and have 650RPM entered into the software). I liked having a nice low idle RPM for when I'm crawling down something steep.

So- leave it there? Or turn it down to 10* and then install one of those adjustable vacuum cans on the distributor so I can run a a little vacuum advance when cruising at least?

I've been reading on this for the last few weeks and haven't come up with much yet. Probably won't mess with it until spring. It is too cold out there in my garage to work on it for long.

 

[DoubleDingo]

May be worth a look at the specs recommended by gm performance for their fuel injected 383. That should give a good starting point, and also gives the opportunity to compare the inner parts of the engines to see if they're the same or similar.

 

[77 K20]

I looked at the HT383E documents/owners manual. The engine specs are the same- same heads, same compression, same camshaft. You are supposed to move over the intake manifold from the old engine along with the throttle body and injectors.
Ignition timing is not adjustable. The distributor figures out everything from the crankshaft position sensor.
I'm not sure what the stock ECM on a '96-'99 Chevy truck does for timing.

 

[rich weyand]

Read this.
http://www.gmsquarebody.com/forum/showthread.php?t=14508

 

[77 K20]

So I read thru that, but still think I'm missing something. Why is it when I set my base timing to 13*-14* does my idle speed go up to 800 RPM?
Is there no way to advance the timing and keep a 650 RPM idle speed?

I did find some specs on that distributor:
Mechanical advance:
1,100RPM- 0*
1,600RPM- 12*
2,400RPM- 16*
4,600RPM- 22*

Vacuum advance:
0* @ 3"
20* @ 7.5"

Is this a typical mechanical curve?

 

[Georgeb]

Back the idle Speed screw out a little. I tend to like 7 or 800 for idle even with a stick. Makes it easier to inch around with just slipping the clutch a bit but thats just me. But really I set my idle by ear anymore so I have no real idea exactly where its at. I set it where I like it and leave it there.

 

[Georgeb]

If you are trying to back the idle screw out and it won't go any lower check the high idle screw. It may be hitting the idle cam and holding the throttle open.

 

[77 K20]

Since I have a throttlebody there is only the one idle screw and it is backed all the way out.

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[Georgeb]

Since I have a throttlebody there is only the one idle screw and it is backed all the way out.

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Oh, my bad. I assumed carb. Must be a setting in the code for the idle air control. Will it self learn over time?

 

[77 K20]

There is a setup procedure for the idle. I can close the IAC valve then adjust the idle set screw to set the idle speed. Then re-calibrate the throttle position sensor. After that I then enter in desired idle RPM speed.
But it will idle around 800 RPM and bounce around by 50 RPM right now.

Before when I was at 10* of timing I did the above procedure and it locked right in at 650 RPM.

I think I'll try changing my timing back to 10* and then run a manifold vacuum and see what happens. I'm thinking I might get too much advance as this engine has very high vacuum. I get 20" at idle and often see 17" when cruising at 45 mph. I know they make adjustable vacuum advance cans to limit this.

The system does a pretty good job of self learning, but it takes a while. They released new software for it a little while ago and when I upgraded it dumped the learned values. Runs kinda crappy the first hour or so after doing this. Took 4-5 days of driving last time before it ran well.

 

[rich weyand]

Your vacuum can is wrong. With that much engine vacuum, 20* at 7.5" means you will never get out of vacuum advance, and it's pulling in way too hard at 20*. That's why you're knocking. You want more like 15* (crankshaft, which is 7.5* camshaft) at something like 12".

Switch to an AR23/VC1853 can.

 

[77 K20]

Thanks so much for all your info and help. I thought I had a good understanding of ignition, but then when I started mixing and matching parts and going away from stock questions and problems start coming up.

 

[77 K20]

Got it installed- I still hate working on the distributor. Back was sore, so I put a 2x6 across the fenders so I can lay down over the engine to do it. Now my rib cage is sore...
Got it put back together and then set the timing with the vacuum unplugged. Figured I'd go back to the GM recommended 10 degrees and drive it like that for a while. I *really* hate where the timing indicator is on this engine. It has a plastic timing chain cover and the indicator is part of it. It doesn't extend very far out so can't see it by looking past the fan belts. (power steering bolt/bracket in the way). And looking straight down from up on top is no bargain either. Since it is off to the side I'm not sure how much parallax error I'm getting.

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Is the worthless cover for those that haven't seen a plastic one. I really wish I would have known how bad visibility is on it and I would have swapped it out back then. Or maybe I can bolt on a metal one over it? Might have to grind the plastic down... wonder if anyone has ever done this. I'm talking about adding something like this to it: But the holes in the plastic cover are partially recessed and this won't fit (I don't think)

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These seem to stick out away from the block quite a bit so perhaps I could see it looking from near the side of the radiator. The plastic one doesn't stick out very far at all. Basically is is even with the back edge of the harmonic balancer.

So drove it around for about 40 minutes tonight. No knocking- which was good. And one added benefit was before anytime I would coast down a hill the exhaust would pop and backfire all the way down. No it doesn't do that anymore.

 

[rich weyand]

By "got it installed" I take it you mean the AR23 can.

"So drove it around for about 40 minutes tonight. No knocking- which was good. And one added benefit was before anytime I would coast down a hill the exhaust would pop and backfire all the way down. Now it doesn't do that anymore."

Yep, that all makes sense. Way too much vacuum advance, at too little vacuum, on an engine that sucks like crazy. So, sounds like the vacuum advance was the big problem.

As for the timing marker, realize that you can sight the timing anywhere you want. So all you need is someplace where you can see the balancer good and have some way of mounting a marker tag. The tag doesn't need to be marked off in degrees, the wheel can be marked with a timing tape. All you need is a pointer, which can be screwed down with anything, like a timing cover bolt.

So, you mount a marker somewhere where you can see it and the balancer wheel underneath it. Now turn the crank by hand until you line up the existing mark on the balancer with the zero mark on the timing cover, using an inspection mirror or whatever so you can see it. Now put the timing tape on the balancer wheel so the zero on the tape is below your new pointer.

 

[rich weyand]

BTW, your optimum base timing is probably around 14* BTDC. With 22* of mechanical advance, that gives you 36* of total timing at rpm, which is the sweet spot for all GM V8s.