Thread: 30 degrees of advance at idle?

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I was reading a very good article which made sense to me which I will use when tuning my SBC with Qjet (I think follows what Denny recommends, I'll let him comment on that):
http://www.73-87.com/7387garage/drivetrain/hei.htm

With the above setup, it will yield 30 degrees of advance at idle, that sounds very high. I have not tried it yet, just seems very high.

I have bought the Crane kit and installed it, but have not run the engine yet.(I just discovered my NEW China-Ebay HEI's vacuum advance canister diaphram was broken).

Originally Posted by pnut

I was reading a very good article which made sense to me which I will use when tuning my SBC with Qjet (I think follows what Denny recommends, I'll let him comment on that):
http://www.73-87.com/7387garage/drivetrain/hei.htm

With the above setup, it will yield 30 degrees of advance at idle, that sounds very high. I have not tried it yet, just seems very high.

I have bought the Crane kit and installed it, but have not run the engine yet.(I just discovered my NEW China-Ebay HEI's vacuum advance canister diaphram was broken).

Your link does NOT work.

Tried the link again and it worked this time. Good article! Read my first sentence below.

30 degrees BTDC at idle is very excessive unless you are checking the timing with the vacuum advance connected to full manifold source at the time.

Your timing (both initial and Total mechanical) is checked and set with the vacuum advance disconnected and plugged.

The Total Mechanical (initial plus the built in mechanical advance) should not exceed 40 degrees. You will see figures above 40 degrees if you have the vacuum advance hooked up when checking.

That confirms what I thought, thanks.

First set timing to about 12-14 with no vac advance, then add in the vac advance and adjust to about 14-16 additional degrees. Of course both of those numbers are dependant on each engine, but that is my starting point. And I DO plan to use manifold vacuum (i've read so much on the subject and finally decided that is the best way to do it for my engine).

I can't see hooking my vacuum advance to manifold vacuum instead of ported vacuum,you'd lose all vacuum advance when you open the throttle,and still waiting for the mechanical to come in. Hank

If you read the above linked article, I think it explains this better than I could defend.

I understand your point of view, I used to be there. But now I am (so far) convinced that Manifold vacuum is the right way to go when set up properly in combination with the other factors.

I conclude this from reading many articles here (mostly from C9x), and the linked article. Read these:

http://www.clubhotrod.com/forums/sho...vance+canister

http://www.clubhotrod.com/forums/sho...vance+canister

http://www.clubhotrod.com/forums/sho...=ported+vacuum

Originally Posted by DennyW

.....Besides, the vacuum wouldn't start coming in until you reached the mid-upper rpm's anyway. Same as the ported vacuum does. ...

Denny, I am very confused by this comment, do you mean the vacuum would not start coming in until mid to upper rpms with manifold vacuum? When my engine is at idle, it pulls vacuum of 20. That is the max vacuum ever from the manifold, and therefore maximum vacuum advance.

Also, the one part about this I still don't get (about the overheating issue) is that I was taught that the more advanced the timing is, the hotter the engine will run. Using manifold vacuum will have the most timing at idle when you are sitting in traffic. I'm sure there is something I'm missing if you can help clear it up for me.

So as to not throw this discussion off track, I'll post a manifold vacuum vs ported vacuum article I did a while back and - far as I know - never posted here.

Titled: More about vacuum sources and timing.

Originally Posted by pnut

I was reading a very good article which made sense to me which I will use when tuning my SBC with Qjet (I think follows what Denny recommends, I'll let him comment on that):
http://www.73-87.com/7387garage/drivetrain/hei.htm

With the above setup, it will yield 30 degrees of advance at idle, that sounds very high. I have not tried it yet, just seems very high.

I have bought the Crane kit and installed it, but have not run the engine yet.(I just discovered my NEW China-Ebay HEI's vacuum advance canister diaphram was broken).

At idle my trucks motor runs 34 degrees! Runs like a top, and trust me, I"ve had many people saying, I cannot beleive that things running with that much degrees at idle lol. Who knows, but mine runs fine.

I'm hoping there are enough people reading this thread for me to not start a new one....

I started the engine with the vac advance hooked up, using the new adjustable canister. After I let it warm up, at idle (800) I read 30degrees timing (static plus vacuum).



I tried to disconnect the vac advance, and the engine died. Even when I turned up the idle this happened. I wanted to check/set the static timing. Why can't I run it on static only without the engine shutting down?



Also, I did take it for a quick spin. It seemed to run fine until I stepped on it and it blew black smoke (super rich) and bogged bad. No pinging at all. Felt like 30HP. I am using middle springs on stock weights intended to come in full at 2600rpm. I'm sure I have more timing work to do but thought I would ask opinions. The weather was too bad to continue trials, but I suspect the static timing is too far retarted and that is why it was horrible under load (only), opinions?



Finally, I have never checked that my timing marks are correct by using a piston stop. Anyone have good instructions for using a piston stop to make sure TDC is the zero mark on balancer? Hard part is manually turning engine over.

screw piston stop into your number 1 spark plug hole, turn engine by hand until the piston hits it and you cant turn it.... make a mark on the balancer where the pointer is pointing, turn it the opposite direction until the piston gets stopped again, and make a mark on your balancer where the pointer is pointing, then measure with a precision measuring device such as a quality seamstress measuring tape and mark exactly dead center of those two marks, and thats your true top dead center mark

take note that it doesn't matter weather you are on compression or exhaust stroke seeing as the mark will always be in the same spot in relation to piston position regardless of valve train..... if your timing mark is off, buy timing tape for your size balancer and you'll have nice quality timing marks that are easy to see.....

and yes it sounds like your static timing may be a little retarded, which will cause engine to not run well or at all with out some advance added to it.... check with the piston stop and add the timing tape to be able to get here better in sync

OK, one more generic question. Let's say the timing needs to be advanced (in any engine tuning scenario, not just this one).....

How do you decided WHICH aspect of timing to change? How do you know when to change the centrifugal weights, vs vacuum advance (adjustable) vs. static? I understand what each does, I am having a hard time determining which would need changing under different scenarios.

Are the dist weights in the correct orientation in the pic below (not upside down center part)? Not my pic, borrowed.

well to determine which timing needs to be changed would depend on what rpm the engine is at when you're seeing a problem.... if its at idle and you have vac disconnected then its obviously static, if it doesn't seem to be right at say 1500-2600 then its probably vac, and above that its probably mechanical.... pretty simple eh?

Edit:

SUCCESS!! (mostly):

I followed Denny's instructions, and using the article I first linked to, I finally set it up like this:
14 degrees static +
22 degrees centrifugal (starting 800, coming all in by 2800) +
16 degrees vacuum

I still have work to do, but I am 90% there. I am pretty sure my timing marks are off though. With the above setup, it still bogs REAL bad at full throttle acceleration. I advanced static more and more until my marks show 48 degrees at idle (600-700). Only at that setting I am really starting to get some power out of it, and never had any pinging anywhere. Full throttle power still seems off, so now I have to figure where REAL TDC is, and get some timing tape.

I still want to be sure, are the timing weights and pivot shown in the pic correct?