Transient fuel tables: Impact and evap factor

[redgto4u]

Hey all,

I've been trying to figure out why my car stumbles ever so slightly when accelerating from a stop with light throttle. My scans show I am getting a lean condition on tip-in. This got me looking into the transient fuel tables after reading posts on here. Can someone tell me in laymans terms what I need to do with the impact and/or evap gas table to fix this slight lean tip in?

I tried playing with only the impact today. I first tried adding 50% to the last half (hot ivt's) of the table. Seemed to idle rougher, but I think the stumble was gone. My idle was more jumpy though. Then I tried using the stock table again with 50% taken out. Idle was much better, the stumble was still there.

Using a clsd tune. Which way does everyone normally go when messing with these tables? From what I understand, if you add to the impact it sprays more fuel on tip in. What about throttle release? For the evap table if you decrease the values it adds more fuel? How do the 2 correlate. Which one will work for me?

Also I noticed that if I go wot off idle from 0 mph I will also get a bog/stumble slight lean condition. Probably not a good thing.

[mightyquickz28]

Don't you love it when you pose questions and nobody answers. Seems typical here. I suggest just doing what I've resorted to doing on my own. Just modify the tables, get your logs and portray what you find. Because it appears 90% of the questions you ask here won't get a response from anyone.

[GHuggins]

I don't understand it fully myself, but hope this more in detail description will...

http://www.hptuners.com/forum/showth...0Gain%20Factor"

Don't know if you've seen this or not. Lots of good info. It's in the little sticky "tuning how too's"...

Ohh, also thought I would ask if you have tried messing with injection timing any at all since you have a base fueling issue. I have found this helps about as much as anything since you've got different scavenging with headers and intake. Just some thoughts? You may even want to try changing them both slightly instead of one set of numbers a lot. Your looking for richer (fuel being taken away) numbers while manipulating the injection timing. Just don't change it much if it's still NA and nothing much else has been done to it. This should help with your lean tip in and increase power and fuel economy at the same time.

[mowton]

Are you sure your VE is tuned properly, it comes into play during transients assuming your are in the Blended MAF/VE mode of operation.

To test and validate transient performance, bring vehicle to a steady speed/TPS, depress accelerator about 20% more and hold for 5-10 seconds. Concurrently to check decal, hold Speed/TPS for a bit, then reduce the pedal 20% and hold there for 5-10 seconds.

Ed M

[mowton]

Don't you love it when you pose questions and nobody answers. Seems typical here...... Because it appears 90% of the questions you ask here won't get a response from anyone.

I think that's a little unfair.......

Ed M

[GHuggins]

Are you sure your VE is tuned properly, it comes into play during transients assuming your are in the Blended MAF/VE mode of operation.

To test and validate transient performance, bring vehicle to a steady speed/TPS, depress accelerator about 20% more and hold for 5-10 seconds. Concurrently to check decal, hold Speed/TPS for a bit, then reduce the pedal 20% and hold there for 5-10 seconds.

Ed M

Forgot to mention that one. The VE on gm's is used heavily for transients.

[redgto4u]

Ve is spot on. +/- 3 on my stfts. Ltfts disabled for a solid clsd tune. I am actually switching out my fast 46 injectors to some ls3/7 injectors with top mounted spacers. Maybe i can get better consistency with an oem injector thats got accurate data.

I did mess with injection timing at one point, but never understood how to actually do it. Since my cam has over 20 degrees of overlap I suppose this would be beneficial.

Im going to put the ls3 injectors in with the corresponding tables, stock injection timing and transient tables and start fresh from there.

[GHuggins]

Ve is spot on. +/- 3 on my stfts. Ltfts disabled for a solid clsd tune. I am actually switching out my fast 46 injectors to some ls3/7 injectors with top mounted spacers. Maybe i can get better consistency with an oem injector thats got accurate data.

I did mess with injection timing at one point, but never understood how to actually do it. Since my cam has over 20 degrees of overlap I suppose this would be beneficial.

Im going to put the ls3 injectors in with the corresponding tables, stock injection timing and transient tables and start fresh from there.

Yes, cam needs different injection timing. You can go to your ect timing scaler then depending on which way your slope goes as it gets hotter, go that way even further by typically somewhere around the added duration of your new cam. You may want to do this in smaller increments until you stop seeing richer A/F's then back up a couple of degrees Bet this fixes most of your fueling problems and even gives you better fuel economy along with more power and torque output on the dyno.

Look at it this way... Right now with stock injection timing your just spitting raw fuel straight into the exhaust making it to where you have to deliver much more fuel than what's wanted to achieve your a/f ratio resulting in your lean tip ins and so on.

Give the timing a try and see what happens. Just make sure to use the ect end of injection timing to do the adjusting. From what I've found so far, the boundary when knocked down only 1% can be the same as adjusting the ect table as much as 20 to 30 degrees. Then if you have certain rpm areas where fuel may be heard igniting or popping in the exhaust you can use the rpm table to better dial in these zones if necessary-this is only if something odd is going on-normally you shouldn't have to touch this one either.

[mowton]

On some setups you may have to increase the base timing 2-3 degrees in those tip-in area's to help the stumble. Have see that a lot on CTS-V's.

Ed M

[redgto4u]

Thanks for the help. Here's my plan.
1. Swap out injectors to oem ls7 injectors and input all the necessary ls7 injector data into my tune file.
2. Make sure transient tables are stock. For now..
3. Use the eoit spreadsheet to determine where to set my eoit based on my exhaust valve closing point and injector size...duty cycle at idle? I don't think this will really have any effect on transients, just better smelling exhaust and fuel economy. Copy and paste that into my tune file.
4. Log and edit my ve table back inline. Shouldn't need too much.
5. see where I'm at from there..

[schpenxel]

If anyone has figured out transient tuning, I'm all ears. I'd about pay for a solid way to get that area adjusted. So far all I've found is "if it looks kinda like this.. try changing that table up, if it looks like this, try changing it down". For better or worse mine goes rich on both throttle increase and decrease right now, so at least it isn't going lean

One question though.. someone mentioned the VE table being used heavily during transient operation. Is that still the case even when running in MAF only mode?

[BigDaddyCool]

Transients hard.

A common thing to do as per my advice......... log your IVT..... sit steady state on and off across the diff IVT temps and jab the throttle around 20-30% and notice the AFR jump........ then multiply the entire IVT cell for that temperature by either under that or above that percentage.

Too much fuel transient wise not only kills power, but it will hurt idle..... most of the cars tho do adapt over a few days anyway.

The problem is......... timing itself can interfere with transient predictions via logging. ie. if you go from one cell to another and the timing is higher etc it in itself can cause a leaner condition.

.........

Try a starting point.......... start the car, drive until the cars hot, do a couple of wot runs........ Find what the peak % is overall........ that the afr spikes..... ie. it may be like 20% or something for an average, multiply the whole impact table by that......... drive for a few days and see how it goes.

PS. Your fueling has to be near spot on to get a fairly accurate idea.

[mowton]

Transients hard.

A common thing to do as per my advice......... log your IVT..... sit steady state on and off across the diff IVT temps and jab the throttle around 20-30% and notice the AFR jump........ then multiply the entire IVT cell for that temperature by either under that or above that percentage.

Too much fuel transient wise not only kills power, but it will hurt idle..... most of the cars tho do adapt over a few days anyway.

The problem is......... timing itself can interfere with transient predictions via logging. ie. if you go from one cell to another and the timing is higher etc it in itself can cause a leaner condition.

.........

Try a starting point.......... start the car, drive until the cars hot, do a couple of wot runs........ Find what the peak % is overall........ that the afr spikes..... ie. it may be like 20% or something for an average, multiply the whole impact table by that......... drive for a few days and see how it goes.

PS. Your fueling has to be near spot on to get a fairly accurate idea.

Make sure when you press the accelerator (or release it for decel) to get the 20% increase (or decrease) you hold the pedal there for about 5 sec's or so to allow everything to settle down. perform this test at several different rpm values working up from lower rpms/load to higher in order to characterize the table. Make small changes and watch your results.

Ed M

[mowton]

Transients hard.

A common thing to do as per my advice......... log your IVT..... sit steady state on and off across the diff IVT temps and jab the throttle around 20-30% and notice the AFR jump........ then multiply the entire IVT cell for that temperature by either under that or above that percentage.

Too much fuel transient wise not only kills power, but it will hurt idle..... most of the cars tho do adapt over a few days anyway.

The problem is......... timing itself can interfere with transient predictions via logging. ie. if you go from one cell to another and the timing is higher etc it in itself can cause a leaner condition.

.........

Try a starting point.......... start the car, drive until the cars hot, do a couple of wot runs........ Find what the peak % is overall........ that the afr spikes..... ie. it may be like 20% or something for an average, multiply the whole impact table by that......... drive for a few days and see how it goes.

PS. Your fueling has to be near spot on to get a fairly accurate idea.

Again the key is to determine where your fueling issue exists....VE, Transient tables etc.

Are you saying IVT open loop table? Can you explain?

When I say the tip in area's, you watch the path of the timing as you accelerate from light load to heavier. You will see, because you have increase the cylinder charge capabilities, you g/cyl will increase and point to area's in the timing table you never would hit as stock. Reducing TM/TC further adds to this condition. The oem timing is traditionally very low in these transitional area's, sometimes only 8-10 degrees. Increasing 2-3 degrees goes along way in reducing the "flat spot" you sometimes get after cam/power adder mods. You "blend in the increase so there is no sudden changes.....

Ed M

[schpenxel]

Again the key is to determine where your fueling issue exists....VE, Transient tables etc.

Are you saying IVT open loop table? Can you explain?

I assumed (right or wrong) he was talking about the intake valve temp column in either the evap rate or impact factor transient tables, but who knows

That is one of the biggest problems with transients.. the few people that have tried to explain it don't do that great of a job. Or maybe I just want to be spoon fed, who knows.

[mowton]

Just as a note, en lieu of calculating EOIT, you can try increasing/decreasing the value a small percentage (5-10%) towards the value that the OEM has picked when completely warmed up. Some PCM's use milliseconds, some crank references etc, your may vary. With a wideband, closely monitor your LAMBDA/AFR during idle, note it. Make the above change, flash the car, run it for a bit and then monitor the idle LAMBDA/AFR. If you are truly in need of adjusting the EOIT, you will see the LAMBDA/AFR will have actually gone richer. This is because if you are "short circuiting" fuel to the exhaust valve because of cam overlap, the WB will actually read this unburned fuel as lean. Remember the WB reads O2 and the only way you can get that is to burn the fuel. This actually makes the tuner add more fuel because he thinks its lean and adds to the "stink". By delaying the injection until the exhaust valve is nearer to closed, more of the fuel gets burned and less is read by the WB and the readings will go richer with no other changes. Repeat the process until you see no change in the LAMBDA/AFR. Then reduce the fueling to bring the LAMBDA/AFR back to your desired value.

Hope this helps

Ed M

[mowton]

I assumed (right or wrong) he was talking about the intake valve temp column in either the evap rate or impact factor transient tables, but who knows

That is one of the biggest problems with transients.. the few people that have tried to explain it don't do that great of a job. Or maybe I just want to be spoon fed, who knows.

I agree, Gregs book (think it was his first) does a pretty good job in explain the phenomenon, but as you say the process for tuning is still very much undefined and varies from Operating System to operating System. It is all trial and error. Larger injectors exacerbate the issue. :-)

Ed M

[BigDaddyCool]

For those wondering I was referring to the impact tables........ not the evap.

Essentially monitoring the intake valve temp which is the axis basis for it.......

I have not had any success in adjusting evap ...

-----

I am rather curious tho about transients........... eg. corvettes use the exact same transient and charge temps in both the ls2 and the ls3 engines.... I honestly think with transients it comes down to the intake you use.....

I have a commodore, it has a crappy stock intake...... I now have a ramjet which is much more like a corvette's...... so things like charge temps, transients ....... I think would be good to workout. But really its too hard, you need computers to work all this sort of stuff out, the best we can do is compensate from what we see to make it run good maybe not the best it could be but good.

[schpenxel]

For those wondering I was referring to the impact tables........ not the evap.

Essentially monitoring the intake valve temp which is the axis basis for it.......

I have not had any success in adjusting evap ...

-----

I am rather curious tho about transients........... eg. corvettes use the exact same transient and charge temps in both the ls2 and the ls3 engines.... I honestly think with transients it comes down to the intake you use.....

I have a commodore, it has a crappy stock intake...... I now have a ramjet which is much more like a corvette's...... so things like charge temps, transients ....... I think would be good to workout. But really its too hard, you need computers to work all this sort of stuff out, the best we can do is compensate from what we see to make it run good maybe not the best it could be but good.

Have you ever looked at the 2005 GTO's (LS2) vs. 2005 Corvette (also LS2)?

How transient section is totally different. Maybe there's something different about each of the engines, I'm not 100% sure.

The stock 2005 corvette transient table is weird though for sure.

[schpenxel]

http://www.megamanual.com/ms2/xtau.htm

Came across this on transient fueling and thought it may interest some here..