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timing advancement questions

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Oct 30, 2001
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Glen Burnie, MD, USA
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1986 Bright Red Coupe
I was sitting there with a timing gun today watching my timing change as I played with the throttle cable (yes, I'm easily amused :)). Essentially, the guy I borrowed the timing gun from told me to make sure the timing is advancing as the RPM's climb. So... my questions are:

a. Where does the timing belong as you're accelerating, and

b. Which way is advance and which way is retard on the scale?

-|
| before
|
-| TDC
|
| after
-|


Pardon the crudity of the drawing, but my scale looks something like this. When idling, the line is right smack on-center. As I increase the RPM's, the line moves UP (towards the passenger side) into the range marked as 'before' on the scale. I thought 'before TDC' meant I was decreasing or retarding the timing.

Pointers or tips?

Thanks...
[RICHR]

<on a side note, finally got the fourth TPiS shock installed. Nice handling...>
 

Ken

Gone but not forgotten
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Hermosa Beach, CA
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1987 Z51 Silver Coupe
Before Top Dead Center, the timing is advanced.

Advanced timing: If the ignition timing on a car is advanced (to a certain degree), slightly more power is made because the air/fuel mixture is given time to fully combust and to allow the plug spark to travel. When the air/fuel mixture completes combustion, there is enough power created to push the piston downward.

Retarded timing: If the ignition timing on a car is retarded, then less power is made because the air/fuel mixture doesn't completely combust during the compression/combustion cycle. The flame front completely combusts after the piston has started its power stroke. This results in a loss of power.

_ken :w
 
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OK, that makes lots o' sense. Thanks, Ken.

And everything seems to be working like it should be.

[RICHR]
 
B

Bill C

Guest
rrubel

Did you disconnect the connector that puts your ignition in the base timing mode?? You have to disconnect a wire on the fire wall that puts the ECM in a mode that will allow you to set base timing or your car will run like chit. I do not know where it is on an 86.

Bill C
 
Joined
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1986 Bright Red Coupe
Did you disconnect the connector that puts your ignition in the base timing mode??

Sure did. You missed an earlier thread where I was trying to find that wire. I eventually did (thanks to some forum help) and was able to properly set my (way off) timing. Just FYI, on an 86 it is just to the passenger side of the brake booster (and in my case under some vacuum hoses).
[RICHR]
 

vigman

Motor head!!!!
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Valencia, CA,USA
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88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
So where are you set at

Stock timing is 6 deg BTDC ( with connector pulled)



Mike
 
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Re: So where are you set at

vigman said:
Stock timing is 6 deg BTDC ( with connector pulled)

Wait... now I'm really confuzled.

I could swear that my book says to set the timing line to the biggest valley on the scale (which I think is TDC). The car actually runs decently like this, as opposed to where it was before (which was probably around 4-6 degrees BTDC - I called it 'retarded' in the other thread, but it really was advanced).

This must mean that it was set RIGHT before, and I've gone and retarded it... which explains why the pinging went away, but also means that there's still some other problem I have to work out.

Oh, *(#$&*(#$&*().

<sigh> I miss fully-computer-controlled ignition and timing.

Back to the timing gun, I guess...

[RICHR]
 

vigman

Motor head!!!!
Joined
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Messages
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88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Does the timing light have a delay knob?

Some of the later ( read trick) timing lights have a knob so you can delay the timing strobe.

The reason it doesn't ping is the timing has moved so the spark happens LATER.

The PING is LIKE a pre fire where all the slop in the wrist pin & the con rod bearing get slammed back down but the crank continues to force it up.

The timing is critical in older cars for these reasons
1) Excessive carbon builds at the top of the piston, the top of the bore & the top of the head volume goes down= higher compression
by a small amount.

Stock lets say 9.5/1 really carboned 9.7 to 1

2) Cam chain strech ( roughly 4 deg in really loose chains) and the cam timing is a retard so the power point moves up in the rpm scale.

Shade tree timing 101
Set for stock
run car @ temp
each time it pings
RETARD timing 2 deg till it stops pinging.

BUT when you have to SMOG put the timimg back or she won't burn properly.


Once you have that sussed
Start running 92 Techron
& do the top end cleaner.

Remove the water pre heat in the throttle body.

Then you can start advancing again... now mind you were chasing SMALL hp gains.

So ADVANCE means move twds pass side means the spark fires(BTDC) Before top dead center and it's measured in degrees of crank rotation.


TDC top dead center is just that the piston is at the EXACT top of its travel along with the crank.( assuming the harmonic balancer has not moved throwing your TDC out the window)

RETARD is to make the spark happen AFTER TDC.


The confusion exists due to the reference of STOCK timing (6 deg BTDC) vs 0 TDC


Mike
 
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1986 Bright Red Coupe
Re: Does the timing light have a delay knob?

vigman said:
Some of the later ( read trick) timing lights have a knob so you can delay the timing strobe.

Nope...



2) Cam chain strech ( roughly 4 deg in really loose chains) and the cam timing is a retard so the power point moves up in the rpm scale.

New chain and gears. Pinged before and after the change.



So RETARD means move twds pass side means the spark fires(BTDC) BEFORE top dead center and it's measured in degrees of crank rotation.

This contradicts what Ken says earlier in the thread, where BTDC is ADVANCED.

[RICHR]
 

vigman

Motor head!!!!
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88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Oops ( you are correct!)

I'll edit

Didn't have all my morning coffee Brain retarded 12 degrees

Mike
 

Bluevette85

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Latham, NY
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1985 Blue Coupe
Could your harmonic balancer be slipping? It is two pieces of metal seperated by rubber and can move with age thereby moving the timing mark.
 
B

Bill C

Guest
When I thought that I had a balancer problem, I inserted a small brass rod (about the size of a pencil and 8" long ) into #1 cilinder as the piston was approaching TDC. I guided the rod up and held it on the same place on the piston as some one turned the flywheel. The exact monent the rod stopped raising (engine TDC) I looked to see where the balancer and timing pointer lined up! To my amazement I still had about 10 degrees to go to get to the TDC line! It wasnt the balancer that was at fault but a timing chain cover from an engine of a different year! We scribed a temp TDC line, timed it to spec and we had the performance engine that we thought we had built.

This process will tell you if you have a big error! Give it a try.


Bill C
 
Joined
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Bluevette85 said:
Could your harmonic balancer be slipping? It is two pieces of metal seperated by rubber and can move with age thereby moving the timing mark.

How can I tell, other than the obvious "it seems to be in time but runs crappy?"
[RICHR]
 
B

Bill C

Guest
Are you REALLY sure its a timing problem??? Could it be somehing else like a restricted Cat or exhaust system? That problem will let it idle ok but as soon as you put it under power and a load she will loose power and run like CR@&.. Been there done that! Took the cat loose and gained all the power back plus some!!!
Open exhaust sounded like a Winston Car!!! Wheeee Haaaa!

Mine went at 150,000 miles. Guess I got my moneys worth out of that componet!

Check it out. I think you have three cats. Two pre cats and a main cat. If you find some way to measure your exhaust back pressure you could tell real easy if it was clogged.
If you could unscrew one of your AIR injection check valves and insert a pressure gauge (0-10 psi would do) in it's place, rev the eng and read the guage. It should be no more than 3-4 psi with a blast up to about 4000-5000 rpm in nuteral.

Bill C
 
Joined
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Glen Burnie, MD, USA
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1986 Bright Red Coupe
It's not so much a loss of power as the engine pinging under WOT. I checked the timing and thought it was out but apparently just retarded it. Now no pinging, but a bit less power (for obvious reasons). Nothing majorly noticeable in the butt-dyno dept, but it's there. So I have to set the timing back to factory and look for another cause to the pinging.

One of the cats IS going, according to my mechanic, but it's not at the "replace me!" stage yet - just a small bit of rattle when cold.

[RICHR]
 
B

Bill C

Guest
RICHR

You live close to my home town, Dundalk!! I live in CT now. If that cat is ratteling It meeds replaced. The little CAT CRUMS from the loose pieces break off and clog up the main cat. It may not be the main problem but it's something to think about.

Is your SPARK KNOCK/ timing retard system working?? Tap the block around the sensor with a hammer (while you have a timing light connected) and see if you get retard. You should get retard after each tap. I cant remember the amount of MAX retard the system provides but it is quite a bit. The factory manual explains the entire process. I just don't have it right now.

Good luck
Bill
 
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Bill C said:
RICHR

You live close to my home town, Dundalk!! I
...
Is your SPARK KNOCK/ timing retard system working?? Tap the block around the sensor with a hammer (while you have a timing light connected) and see if you get retard. You should get retard after each tap. I cant remember the amount of MAX retard the system provides but it is quite a bit. The factory manual explains the entire process. I just don't have it right now.

Dundalk is pretty close; I've friends who live there.

I'll try your experiment this weekend. Others have suggested the ESC/knock sensor, but not a simple way to test it. Need a timing gun again :(
[RICHR]
 
B

Bill C

Guest
The ECS Knock sensor is in the circuit that I'm talking about. It's just forward and above the starter. It's a piezo electric crystal that detects noise and is tuned to pick up the same frequencies as KNOCK. They can go bad, be installed too loose or too tight or the wire can be loose or corroded and the circuit is easy to test. YES you need a timing light and the car should be in the normal mode (timing connector connected) to test the sensor but I've found a way that you can test with out one.

If you tap around the sensor and there is no retard, I would check the module and the sensor.

If you get the Corvette factory shop manual and look on pages 6E3-104 - 6E3-109 it explains everything down to the last electron. The circuit can retard the spark up to 20 degrees. The ECS module is mounted on your AC/Heater blower motor.

Check the ECM for stored codes (ground pins "A" & "B") in the ALDL connector. If you see any of the following codes you have an EST timing circuit or a ESC Electronic Spark Control circuit failure; 42 = EST circuit, 43 = ESC circuit.

The book says that you CAN test the ESC circuit without a timing light. If you tap next to the sensor and if the retard happen properly, the IDLE should decrease.

If you disconnect the ESC module the timing should also retard and 4 seconds later you WILL get a code 43.

It also states that you can test the knock sensor it self by dicconnecting the plug from the ESC module and measuring the DARK BLUE wire (pin E) and the BROWN wire (pinD) on the ESC module connector wtih a digital volt meter set on the AC scale. With the engine at 2000 rpm you should get .080 volts Tap the area around the sensor and you should get a spike in the voltage. (Voltage spikes are hard to see on some digiral volt meters) That will prove that the sensor works and the wire to the module is good.

If all else fails Disconnect the knock sensor and take it out for a speed run. If it runs better its because of a too sensitive sensor.

I would guess that it isnt retarding at all and allowing the engine to knock resulting in a power loss.

I will be home and checking e-mail all weekend if you have ECS/ ESC questions.

GOOD LUCK
BILL C

bill327@msn.com
 
B

Bill C

Guest
rrubel

Did you get a chance to play with the engine this weekend???

Bill C
 
Joined
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Unfortunately, no. My wife decided we needed to go Outlet Mall shopping on Sunday, and Saturday was housework day. Maybe Friday...

I did get some neato parts bins from Black and Decker for cheap... :)
[RICHR]
 

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