Hi
I got most of the bugs out of my 87 after installing a rebuilt engine.The timing is bugging me I timed it at 6 degree which is the timing for a stock and would like some advise on the new engine which is not stock,runs good but how do you know what timing to use.Some tell me I need to advance it a little.Not doing anything until I get some expert advise.

Thanks Jim

 

You're timing is computer controlled. Advancing the distributer will not give you the improvement as if it were mechanical.

 

Thanks for the info.
I live in Va. now,from the Uniontown,Pa. area where I lived for 65 years
Jim

 

Just set the timing at 6 degrees BTDC and your good.. Im running a 383 stroker and it's set the same as stock..Some do advance the timing a few degrees and say they get better performance, but the stock setting is safe.....WW

 

Hi
Thanks WW7 I might try 2 degrees to see if it makes any difference.

 

The ECM assumes the distributor is set to 6°. If you change it you should also change the corresponding parameter in the EPROM to match.

If you want to adjust the timing, it should be done in the EPROM.

The ECM limits the maximum advance to 42° (it adds its advance to the distributor advance). If you mess with the distributor timing you can get into knock from too much advance if you don't make the corresponding adjustments in the tune.

 

Jim,

I just completed a 383 rebuild and I feel your concern. I believe one of the most important things regarding your timing is the total advance under a load. To little and you lose performance... to much and you get pre-ignition (knock) which in time could take the tops off your pistons.

I have my base timing set at 8 degrees BTDC with a total advance of 40 +- 2 degrees and I have great performance and no pre-ignition with 92 octane premium gas.

IMO, I would say 6 degrees BTDC may be to close to TDC. Start the burn at 8 degrees BTDC to get a smoother and more efficient combustion and burn as the premium fuel tends to burn at a slower rate.

 

No. The two pasts above are incomplete, very misleading and not helpful to the OP.

First correction, the distributor has no advance, for the ECM to "add to", and you absolutely CAN gain power/performance, by "turning the distributor". While it is true that the ECM controls the timing curve, the initial/base timing, total and everything in between are EASILY manipulated (universally) by "turning the distributor"...just like in the old days. To say otherwise is passing bad info.

The stock timing setting is a conservative one, and for use in bad conditions and with 87 octane. So is there opportunity left on the table with the stock setting? YOU BETCHA!

So where should you set the timing? Since you have a customized engine combo, there is not "spec" at this point. You need to "feed it what it wants". Use some method to measure power in an objective, repeatable way -either a drag strip, Dynomometer, or even just a long, consistent grade and a stop watch or speedometer. Make adjustments to the timing to get the best power/performance while avoiding detonation.

Folks, this is some of the lowest hanging fruit there is in "hot rodding". Optimized timing provides the best power AND the best fuel economy.

 

Jim, I would I would not be afraid to set the base timing at 8* or even 10* to see how it runs. Mine is set at 6* but I have more timing programmed into the chip. I was surprised at how much advance I could run even with 10.76 CR. If you are worried, Buy an ALDL cable and datalog, inexpensive and easy, it will tell you if the knock sensor is picking up anything. Also makes it easy to set the tps voltage and minimum idle, and troubleshooting.

 

I beg to differ. The advance ADDED by the ECM is on top of the MECHANICAL advance set by the POSITION of the distributor. The ECM has no way to know how much "turning" you have done unless you adjust the parameter in EPROM.

The advance curve in the '89 ARAP EPROMs is much more aggressive. Use that one if you want more timing.

See the above comment on the ARAP advance curve. It's already been done for you. If you want to get really tweaky, start there and adjust to your heart's content (or your engine's content, as the case may be).

 

O.K. That is not "mechanical ADVANCE". That is what we call "base timing". The term "Mechanical advance" refers to flyweights inside the distributor that move outward as RPM increases, moving the rotor and pick-up hub forward in relation to the distributor shaft...thereby physically (mechanically) advancing the timing. It looks like this:


If you look inside the distributor of any '84-'91 Corvette Distributor, I can guarantee you that you won't see any flyweights in there (or a vacuum advance canister on the side). ECM controlled cars do no have "mechanical advance". To say that they do is wrong.



OR advance the distributor. More than one way to skin a cat.



Right...but not everyone has the tools or experience to manipulate the ECM. While that may be the BEST way to manage it, the fastest/cheapest/easiest way is to simply advance base timing and the resulting "curve" moves up universally, giving similar results. You and Pr0zac said or implied that you can't do it (advance distributor) which isn't true. You CAN do it...and it DOES produce favorable results. As I stated above...this is some of the lowest hanging fruit in "hot rodding". Why would you steer a poster away from trying it?

 

It looks like we have a semantic issue here. Maybe "mechanical" isn't the most accurately descriptive word. How about "physical"? "Base" doesn't really describe it, even though that word is commonly used.

As I said earlier, changing the base timing runs the risk of knocks. The maximum advance allowed in the ECM is 42°. That's sneaking up on the limits of the distributor cap -- the terminals are 45° apart.

 

Well man...that's all wrong too...and it's not "semantics".
First, you "run the risk of knocks" regardless of HOW you advance the timing in an internal combustion engine. The risk is there. That risk is managed by engine temps, fuel choices, etc., regardless of the METHOD of advancing timing. To say that increasing base timing creates a risk of detonation but increasing timing in the ECM doesn't...that is wrong. The combustion process don't care HOW the timing occurs. Risk is there either way. In fact there is a "risk of detonation" with the stock timing! Which is why there is a knock sensor.

Second, when you advance the BASE TIMING, you move the CAP with the rotor...so you're aren't changing the relationship between when the spark fires, and the orientation of the cap and the rotor...at all! Unlike adding advance via the ECM which does change that relationship. The dist. cap and the reluctor ring (trigger) must move together (when rotating the dist) so you can't create the issue that you are claiming will happen.

 

Alrighty.....riddle me this, as I may have it wrong. When you advance the distributor, the rotor does not move, but the posts do. The timing relationship is determined my the position of the number one piston (TDC) and rotor and cap (with the number one plug wire on it) , so the distance between posts does not limit the advance to 45 degrees. It does not start over when it hits the number 8 post. If this was true, every time I stab the dist in the engine, it wouldn't be 180 out. The ESC adds "X" amount of timing to whatever the base amount is, and the puter has no idea what the mechanical timing is set at. So it seems to me that there could be gains to be had by adjusting the base timing, this would be determined by too many factors to list, such as gas quality, air temp, compression, carbon build up, elevation and phases of the moon. You may be able to increase performance by adjusting base timing either way and adding or removing timing in the tables, based on these factors. I think that is what the GM engineers were trying to do? Then throw in the knock sensor, to idiot proof the whole thing. Please correct me if I have it wrong.....

 

That is correct.



This is correct.


Yes. That is correct.

Changing the timing tables allows you (and GM engineers) to make customized adjustments to the timing, for specific conditions in a specific point in the RPM range, throttle position, temp, etc. This is the most detailed way to taylor the ignition to your combo.
In comparison, turning the distributor makes global changes...if you advance base timing 2*, every timing point in the entire programmed ignition timing curve has to go up by 2* for any given condition. The down side (compared to changing timing tables) is that you may (for example) run into detonation while cruising -this is where you will see the most total advance. That cruise detonation may limit how far you can go with a global (base timing) adjustment, when there may still be some timing left on the table at WOT. Again, just as an example. The "up side" to making global adjustments is that it's free, it's easy, it takes seconds to do, and because GM is so conservative w/the OEM settings...there is almost always low hanging fruit to pick with a simple tweaking of the distributor. I'f had cars pick up over 20 hp with this alone. That's some good Bang-for-the-buck that is hard to beat.

 

So....what is the point of the base timing number setting of the ECM?

 

The ecm uses the base timing to calculate total advance. If its at 6* and the timing table in the bin calls for 36*, the ecm will advance the timing 30* for a total of 36. If you change the base to 10* without changing it in the bin, the ecm will still add 30* but you will actually have 40*..

 

So changing the base advance number just changes the total advance in degrees, if the total advance number stays the same (in the tune). If you just rotate the distributor, you are making a global change. So the base timing number in the tune is used for calculating total advance by the ESC. So it seems to me the proper way to do it is to set the base timing for proper idle, and adjust the tables (total ESC advance curve) for performance and/or mileage, without triggering the knock sensor. Not ruling out a global change as an improvement by any means.......

 

100% correct. Well explained.

 

YES!! That would be the BEST way to set up the ignition timing. The way that you just described.