Fast XFI afterstart tables re-visited

[norbs]

Taking a closer look at the Afterstart tables and how things work, my interpretation

The 1st thing we need to know is that we have 3 tables in the fast for afterstart

1. afterstart vs cts
2. revs vs cts
3. decay vs cts


Afterstart vs CTS is the % of fuel added to the current injector PW, after the engine exceeds the start to run RPM, below this value no afterstart fuel is active.

Rev VS CTS is the amount of crank signal pulses that must be seen BEFORE any after start fueling begins once the start to run RPM has been exceeded this count begins.

Decay vs CTS is the rate the afterstart enrichment is REDUCED in .8% per crank signal pulse. Example if we had this set to 100 revs/step, every 100 crank pulses the ecu sees the after start programmed value from table 1 . will be reduced by .8%. So if you had a value in table 1 as 20% it would be 19.2% after 100 pulses. and so on, next 100 pulses 18.4% etc.

The Buick crank trigger has 3 crank pulses per RPM. If you have a programmed cold start of 1000 RPM you will see 3000 crank pulses per minute or 50 pulses per second. So based on the numbers above( 100 revs/step) will mean every 2 seconds the afterstart will be reduced by .8% so after 50 seconds it will decay out to 0% if we started at 20% enrichment.

Or if you look at it this way .4%per step per second= 20%/.4%=50 seconds

A stock buick chip uses 140 seconds cold afterstart and about 90 seconds warm afterstart as an example, but I like to see about 60 seconds cold and about 30-40 seconds warm.


If I have anything in-accurate someone please feel free to step in and correct things.

 

[larry33kc]

Are these #'s comparable with BS3? I am having fast idle prob.

 

[norbs]

The idea is the same its the amount of fuel added to the base PW in %, and it works on crank events the same way, but not sure if the ratio is .8% reduction per step as the XFi states it is, but I'm sure its not much different.

 

[TurboBuRick]

Subscribed!

Allot of different adjustments for starting. It can be a challenge to make the right change to the right thing. It's a good practice to keep a stable file seperate just in case.

Rick

 

[norbs]

I agree its tough, but datalog it and you can see what is going on, and when you change injector size, you have to pull your % down in the same proportions.

 

[kidglok]

One thing I've found that works well is:

Have the after start come in after only a few revs
Decay out the after start quick (5 revs)
and use the warmup enrich vs coolant temp exclusively for cold start routine

 

[norbs]

One thing I've found that works well is:

Have the after start come in after only a few revs
Decay out the after start quick (5 revs)
and use the warmup enrich vs coolant temp exclusively for cold start routine

I agree up to a point a low rev afterstart is good so it comes in early, the decay revs will vary according to the engine rpm and the programmed afterstart %, but if gm is using a long afterstart in the chip there must be a reason for it, that is my theory, but use what method you like and the engine runs best with.

 

[kidglok]

I agree up to a point a low rev afterstart is good so it comes in early, the decay revs will vary according to the engine rpm and the programmed afterstart %, but if gm is using a long afterstart in the chip there must be a reason for it, that is my theory, but use what method you like and the engine runs best with.

It's funny you mention that, because I asked Eric the same thing about his SD chip (really long afterstart fuel). He said that he found it to work best that way.

 

[dennisL]

I don't use XFI but would like to clarify what "after start time" is in the stock 86/87 ecm.

- During run mode (after cranking) the fuel is a coolant temp determined value plus a time-out (choke) value. The cool temp when the engine started determines how long to wait before the 'choke' fuel is reduced. At 68 degF for example, the wait is 15 sec (Eric uses 20 sec). The base afr gets more lean as cool temp rises and the choke fuel is reduced.
- The times Norb referred to is the minimum time before the switch to closed loop (or O/L idle if the chip supports it). The cool temp when the engine is started determines if the time is 140 sec (start below 103 degF) or 5 sec (above 103 degF). Eric's numbers are similar, just a little longer wait above 103 degF (20 sec). After this run-time is up, the ecm will enter C/L provided the cool temp is above the minimum for C/L. Stock is 76 degF but most custom chips use a higher temperature.
- Summary: There are two delay times used after the engine starts. The first is the time before the 'choke' fuel begins a reduction. The second is the time before going into closed loop.

Does the XFI after start time have the same purpose as the stock minimum run-time before C/L?

 

[kidglok]

Does the XFI after start time have the same purpose as the stock minimum run-time before C/L?

that has its own parameter, you can set the cold c/l delay time, and hot c/l delay (as well as temperature)...but it wont enter closed loop untill all the afterstart is decayed out.

 

[Sydwyndr]

This is the way I have my street car, It will crank and stay running in 0 degree weather and never have to touch the throttle for anything, drop it right in gear and go... warms up around 12.5 to 12.8

 

[Sydwyndr]

One thing I've found that works well is:

Have the after start come in after only a few revs
Decay out the after start quick (5 revs)
and use the warmup enrich vs coolant temp exclusively for cold start routine

Agreed... Essentially once the car starts it jumps immediately to coolant temp

 

[larry33kc]

Here is my MAP vs Coolant table. Is it ok.

 

[norbs]

Well Larry, I;m not an expert, but the way that table works looks like it adds a % to the base PW based on coolant temperature and load, depending where it idles at (kpa) will determine which cells it uses, and when you hit the gas its going to go to a higher kpa value of course for a split second, and use those cells. On a XFI the correction goes by coolant temp only temp not KPA, however it does have a table for AE fuel vs coolant temp. I can;t see you needing a lot fuel in low kpa high vacuum condition, but for now I would make all the rows 0 and make sure the ve table is set correct for hot idle in gear to an open loop 13.7 af. Then start the car the next day and see how lean it is at the specific temperature and then start adding fuel, it it wont run keep adding numbers in the table until it runs, dont touch the ve table. Since its all speed density you can do the math, if you want a 12.5 af ratio cold and your getting 15, you know you will need to add 20% at that cell to brinig the AF ratio in line.

 

[norbs]

Larry is any of this sinking in????

 

[larry33kc]

Yes a little. I have been sick so I haven't had a chance to play with it. It sucks because I have no fast idle and it wont idle on it's own till it's warm so its hard to play with the tables.

 

[norbs]

Why do you have no fast idle If i may ask?

 

[kidglok]

is the iac functional?

 

[larry33kc]

Norbs I don't have any idea. I have asked and asked trying to figure it out but no help.

Kidglok I think that this is my problem. If you do a search under my name you will see all the questions and issues I have had.

 

[larry33kc]

Any help would be great. Thanks