HEI Operation / Setting timing...'81
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
HEI Operation / Setting timing...'81
On an '81 with computer and 7 pin ignition module; if the four wire connector from the distributor is disconnected, how does the spark get advanced?
I was trying to check/set the timing and thought that the steps were to disconnect the four wire connector, set timing at idle for best idle, highest vacuum, then reconnect four wire connector, hold engine at 2000rpm and adjust timing to 36-38 degrees.
But then I left the connector disconnected, and increased engine to 2000rpm and saw that the spark advanced without the computer connected. I though the computer advanced the spark. Which is the correct method??
I was trying to check/set the timing and thought that the steps were to disconnect the four wire connector, set timing at idle for best idle, highest vacuum, then reconnect four wire connector, hold engine at 2000rpm and adjust timing to 36-38 degrees.
But then I left the connector disconnected, and increased engine to 2000rpm and saw that the spark advanced without the computer connected. I though the computer advanced the spark. Which is the correct method??
Abby Normal
Joined: Dec 2001
Posts: 53,432
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From: Press "2" for English
sounds way different then the way i used to do it. i seem to recall disconnecting the four pin connector and setting my idle (stick) to recommendation, i think auto cars are 700rpm in 'D'. set timing to 6* (i think)BTDC. lock down dizzy, reconnect plug and drive.
Race Director
Joined: Sep 2002
Posts: 10,362
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From: San Diego - Deep Within The State of CONFUSION!
On an '81 with computer and 7 pin ignition module; if the four wire connector from the distributor is disconnected, how does the spark get advanced?
I was trying to check/set the timing and thought that the steps were to disconnect the four wire connector, set timing at idle for best idle, highest vacuum, then reconnect four wire connector, hold engine at 2000rpm and adjust timing to 36-38 degrees.
But then I left the connector disconnected, and increased engine to 2000rpm and saw that the spark advanced without the computer connected. I though the computer advanced the spark. Which is the correct method??
I was trying to check/set the timing and thought that the steps were to disconnect the four wire connector, set timing at idle for best idle, highest vacuum, then reconnect four wire connector, hold engine at 2000rpm and adjust timing to 36-38 degrees.
But then I left the connector disconnected, and increased engine to 2000rpm and saw that the spark advanced without the computer connected. I though the computer advanced the spark. Which is the correct method??
Burning Brakes
Joined: Dec 2007
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Correct.
Melting Slicks
Joined: Mar 2006
Posts: 2,465
Likes: 0
From: Orange Park Fl
Lanny, You set the idle MIXTURE for max vacuum at idle. I think that's what you are thinking about. This is set independently of the timing (you don't disconnect the 4-pin at the distributor). The service manual gives a set number of turns out for the idle adjustment screws, but if you have any engine mods (andd I think you do), it may be different.
As far as timing, get the engine to normal operating temperature, then turn it off and disconnect the 4-pin at the distributor. Hook up the timing light and re-start the engine. Set the timing at 14 BTDC (the manual says 6 BTDC for emmissions reasons. 14 will give better performance and fuel economy). Re-connect the 4-pin and test drive. Accelerate hard up an incline and listen for pinging (detonation). If there is any, back the timing off 2 degrees or go to a higher grade of fuel.
God bless, Sensei
As far as timing, get the engine to normal operating temperature, then turn it off and disconnect the 4-pin at the distributor. Hook up the timing light and re-start the engine. Set the timing at 14 BTDC (the manual says 6 BTDC for emmissions reasons. 14 will give better performance and fuel economy). Re-connect the 4-pin and test drive. Accelerate hard up an incline and listen for pinging (detonation). If there is any, back the timing off 2 degrees or go to a higher grade of fuel.
God bless, Sensei
Pro
Joined: Apr 2007
Posts: 585
Likes: 2
Lanny, You set the idle MIXTURE for max vacuum at idle. I think that's what you are thinking about. This is set independently of the timing (you don't disconnect the 4-pin at the distributor). The service manual gives a set number of turns out for the idle adjustment screws, but if you have any engine mods (andd I think you do), it may be different.
As far as timing, get the engine to normal operating temperature, then turn it off and disconnect the 4-pin at the distributor. Hook up the timing light and re-start the engine. Set the timing at 14 BTDC (the manual says 6 BTDC for emmissions reasons. 14 will give better performance and fuel economy). Re-connect the 4-pin and test drive. Accelerate hard up an incline and listen for pinging (detonation). If there is any, back the timing off 2 degrees or go to a higher grade of fuel.
God bless, Sensei
As far as timing, get the engine to normal operating temperature, then turn it off and disconnect the 4-pin at the distributor. Hook up the timing light and re-start the engine. Set the timing at 14 BTDC (the manual says 6 BTDC for emmissions reasons. 14 will give better performance and fuel economy). Re-connect the 4-pin and test drive. Accelerate hard up an incline and listen for pinging (detonation). If there is any, back the timing off 2 degrees or go to a higher grade of fuel.
God bless, Sensei
Please elaborate on the 6 degrees (recommended) versus the 14 degrees you are suggesting. What is this actually doing, and what are the pros and cons of each setting (or anything in between)?
LO PHAT
Melting Slicks
Joined: Mar 2006
Posts: 2,465
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From: Orange Park Fl
There are a lot of posts on timing on this forum. Reasearch them and you will find what an optimal performance timing curve is. With a dial back timing light, you can see what you have now, and you will see it is far from optimal. While you cannot adjust the curve itself without replacing the computer chip, you can get a good bit closer to optimal by advancing the initial timing to 14 BTDC. I run a Hypertech chip and 14 degrees base timing. I have to use premium fuel, but the car gets 18 mpg in town and spins the tires easily (I do have mods beyond the tuning).
Tech Contributor
Joined: Jun 2004
Posts: 20,905
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
The EST distributor module (STM, seven terminal module) has an internal electronic "centrifugal" advance curve built into it to allow timing advance with RPM in the event of an ECM failure (or loss of ECM connection, as you witnessed). This improves drivability until the ECM issue is repaired.
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Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
I also have the Hypertec PROM. The manual says to disconnect the four wire connector, set timing to 8 BTDC at 700rpm for a stock engine. Mine is not stock, mild rebuild.
Others have said not to worry about base timing, but to set the timing so that full advance is in (36 - 38) at about 2500rpm and then let the base timing be whatever it turns out to be.
Engine will barely idle at 8 degrees....it needs more advance due to the cam and head changes. I have it at about 15 right now. I was just tyring to determine if setting for full advance at 2500rpm if the computer should be connected or not as I was surprised to see the advance with the computer disconnected...but now understand that part at least.
As far as setting the idle mixture for max vacuum....that is still a problem as I must still have a vacuum leak somewhere or valves not closing. The idle mixture screws have no effect on engine rpm, except when either all the way in or all the way out. I have several problems to work on.
Thanks for all the help....keep them coming.
Others have said not to worry about base timing, but to set the timing so that full advance is in (36 - 38) at about 2500rpm and then let the base timing be whatever it turns out to be.
Engine will barely idle at 8 degrees....it needs more advance due to the cam and head changes. I have it at about 15 right now. I was just tyring to determine if setting for full advance at 2500rpm if the computer should be connected or not as I was surprised to see the advance with the computer disconnected...but now understand that part at least.
As far as setting the idle mixture for max vacuum....that is still a problem as I must still have a vacuum leak somewhere or valves not closing. The idle mixture screws have no effect on engine rpm, except when either all the way in or all the way out. I have several problems to work on.
Thanks for all the help....keep them coming.
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
I also have the Hypertec PROM. The manual says to disconnect the four wire connector, set timing to 8 BTDC at 700rpm for a stock engine. Mine is not stock, mild rebuild.
Others have said not to worry about base timing, but to set the timing so that full advance is in (36 - 38) at about 2500rpm and then let the base timing be whatever it turns out to be. That works on non-ECM engines. If you try that with your setup you will still have the MAP sensor reading a non-zero level of manifold vacuum (at 2500 RPM in neutral), and end up putting in some additional advance due to the light load. Once you actually drive it, at WOT the timing will back off from what you were reading, and you will be spark retarded. You need to contact the chip manufacturer and find out what curve they programmed into the chip.
Engine will barely idle at 8 degrees....it needs more advance due to the cam and head changes. Makes sense. Cams with overlap will require additional ignition advance to overcome the effects of residual EGR. I have it at about 15 right now. I was just tyring to determine if setting for full advance at 2500rpm if the computer should be connected or not as I was surprised to see the advance with the computer disconnected...but now understand that part at least.
.............................
Others have said not to worry about base timing, but to set the timing so that full advance is in (36 - 38) at about 2500rpm and then let the base timing be whatever it turns out to be. That works on non-ECM engines. If you try that with your setup you will still have the MAP sensor reading a non-zero level of manifold vacuum (at 2500 RPM in neutral), and end up putting in some additional advance due to the light load. Once you actually drive it, at WOT the timing will back off from what you were reading, and you will be spark retarded. You need to contact the chip manufacturer and find out what curve they programmed into the chip.
Engine will barely idle at 8 degrees....it needs more advance due to the cam and head changes. Makes sense. Cams with overlap will require additional ignition advance to overcome the effects of residual EGR. I have it at about 15 right now. I was just tyring to determine if setting for full advance at 2500rpm if the computer should be connected or not as I was surprised to see the advance with the computer disconnected...but now understand that part at least.
.............................
Melting Slicks
Joined: Mar 2006
Posts: 2,465
Likes: 0
From: Orange Park Fl
