Hey Gang
What should the total timing advance be on a '86 Tuned Port w/ the Doug Nash 4+3?


I currently have base timing set at 8BTDC and total advance @ 2500rpm is 39degrees.


35-40degrees total advance is typical but this is my first 'vette and I haven't been around a tuned port 5.7L since they were new.




Thanks
Jonathan.....

 

You need to look at the computer program to determine that and use a scanner to see what it it reading. The timing is built into a table and it will be all over the place based on RPM and load and temp.

 

Back in the day as a Chrysler tech, we had a diagnostic procedure on the early electronics where we had to set the engine RPM at 2000-2500rpm depending on package and allow a specific time period to elapse then chest total timing.


I presumed GM released a similar test for the tuned port systems.

 

Are you programming a tune? Anyways I'd assume the total timing could be as high as 50* btdc, you get that kind of advance at low load while driving, it helps increase mpgs. Think of all the advance in a regular distributor, at or near WOT the vacuum advance doesn't work because there is no vacuum so the timing would be around the sweet spot 35-40*w/out vacuum advance. I'm sure the EST system runs timing high like that under low loads.

 

The maximum timing is a parameter in the tune. My recollection is that it is 42°.

 

Usually max timing that can be added is 42 degrees plus the base setting of 6 degrees for a total of 48 degrees of timing available before TDC. The computer can only vary the timing by 42 degrees but the base setting adds to the total available range of (6 to 48) degrees, if the base is set at 6.

As noted at light load the timing may be 45 degrees at 3000 RPM and at heavy load the timing may only be 32 degrees at 3000 RPM. It all depends on what the chip has programmed into it.

 

Here's an 86 BUB spark table. There no power enrichment vs spark in 86. Add your base of 6 degrees to all numbers shown.

 

Set your timing to 6 degrees with the EST unplugged. The rest of the timing advance is controlled by the ECM depending upon RPM and Load.

 

I am pointing out an error in this reply. With tunercat the numbers displayed in the timing table already take the base timing into account. You do not add 6 degrees to the numbers shown in the table. The program does the calculations in the background and removes the initial timing shown in the constant table from timing shown in the table. That is why it is important to set the initial timing at the number shown in the constant table.

 

I disagree and we've had this discussion before. If you have 30 degrees at WOT in the table the car gets 36 degrees.

 

If this is true (which I don't believe it is), would you please explain how the system can know what the base timing is? There is no sensor to determine TDC in the L98 engine, so how can it know what the base timing has been set to? I believe the ignition curves programmed into the ECU can only add to the base timing! I am challenging you to explain your statement!

 

You are correct to a point, there is no sensor to determine base timing so the base timing must be set at what the computer has programmed in as base timing and then the total timing curve is a combination of base plus advance. My assertion is was that TunerCat has a setting for initial timing under constants and it is set at 6 degrees from the factory but it can be changed by the tuner to whatever he desires. Once the base timing is set the tuning program adds in the extra timing it needs to get total timing to reach the numbers shown in the main timing table. If your constant number for initial timing shows 6 degrees and your main timing table shows 36 degrees the computer will add in 30 degrees of timing to arrive at 36 total for that load position. If you do not set the base timing at the same number that is listed under constants the total timing will be incorrect because there is no way for the computer to verify the actual timing.

I was commenting that the Tunercat program does not add in the exact number shown into the main timing table it adds in the amount of timing needed to reach those numbers taking into account what the constant for initial timing shows. (36 degrees desired total - number for initial constant = total timing added at that point in the curve)

I realize that some people do not agree with me but this is my position.

I have verified this by setting the timing at 20 degrees (when the base was set to 6 degrees and the initial was set to 6 degrees) at all positions and then used my adjustable timing light to read the timing and it was at 20 degrees. I then changed my initial constant number to 0 degrees and I ended up with 26 degrees of timing everywhere verified with the gun. That is because the computer then added in 20 degrees of timing because the initial constant was set zero and it wanted to achieve 20 total, but I still had the 6 degrees of base actually in the car. The computer had no way to check the accuracy of what total timing is and assumes base and initial timing are the same.

This is in my opinion how TunerCat works, I cannot say for sure if this is how your tuner program for TunerPro works or any other software. You need to understand how the tuner program operates before you can tune the car.

 

I also tested this out. Set base timing to 6*, reconnected EST and timing was at 20*. My tune says 20* at idle in the Spark Advance table.

 

I haven't found anything that suggests a retuned ECM so I'm looking at stock timing advance.


I set the base timing at 8 BTDC and w/ the RPM at 2,500 in neutral, total timing was 39 BTDC. This is pretty much what we ran back in the day of mechanical timing advance.

 

Daniel, can you prove your point by setting the base timing at 3*, and then at 9*? Do you still get 20* at idle? How does the car run at 3* and at 9* base timing? Please convince me!

 

I use Tunerpro,I've never used TunerCat, but does it not have initial-SA setting where you tell it what the base timing is? In tunerpro I set my base timing at 6* and set initial-SA at 6*. With a warm engine my timing light agrees with the tables.

 

Look at the table that Mr *****'s posted, that is a stock table and you can see that the timing at 2500 RPM can be all over the place depending on the load on the engine when it is checked.

 

Actually it is not stock. Far from it and has the PE spark zero'd out.

edit: I'm wrong, it is a stock 86 BUB which uses no PE add.

 

For what its worth, last year I refurbed the entire fuel system end to end on my 150k mile '87. I also replaced all the ignition components (Taylor) right down to the capacitor and HEI chip.

It ran fine anyway, but I was advised that it would benefit from a bit more initial advance, so I moved it from 6deg to 10deg and it does run way better, pulling strongly and generally happier to idle smoothly and start easily. My ECU is stock, and it still runs out of breath around 4500rpm, but overall, it does feel all the better for the extra 4deg intitial and without pinking too.

 

You are correct. I advanced my distributer to 8 degrees and then changed it to 8 degrees also in my tune in the advance spark constant setting