427W TWM starting - backfiring

[StreetSnake]

I have a 427 W with a TWM stack EFI. When the car has been sitting, overnight lets say, I try to start it and it backfires through the throttle bodies. It does this once or twice and then fires up. When the engine is warm it doesn't do it.

I turn power on, wait for fuel pump to cycle, crack the throttle and fire. I don't have an IAC plate on the stacks and that is why I crack the throttle. Is it that or is it timing, I'm at a loss and so is the guy that tunes my car. Any thoughts?

[kobrabytes]

How do you have your timing set?

[StreetSnake]

I'm not positive what "initial" timing is at but the other is at 28 or 29. Hopefully that makes sense.

[priobe]

Try pulling the plugs the day of starting it when cold. Check to see if the plugs are wet with fuel. Maybe injectors are leaking in that cylinder.

or shutdown down pump to release the pressure

[undy]

It sounds like not enough timing on tip-in. Your max timing looks a litlle shy too (34 to 38 typical). I'd bump overall timing about 3 to 4 degrees and see what affect it has on the backfire. You can always move the timing back. Not being familiar with the TWM that much, it could be some mis-set "open loop" programming. That could also explain the cold engine only symptom. Keep us updated on what you find.

Dave

[fkemmerer]

Originally Posted by StreetSnake

I have a 427 W with a TWM stack EFI. When the car has been sitting, overnight lets say, I try to start it and it backfires through the throttle bodies. It does this once or twice and then fires up. When the engine is warm it doesn't do it.

I turn power on, wait for fuel pump to cycle, crack the throttle and fire. I don't have an IAC plate on the stacks and that is why I crack the throttle. Is it that or is it timing, I'm at a loss and so is the guy that tunes my car. Any thoughts?

Take a look at your timing table (you'll need a laptop for this). Assuming you are using speed density mode, the starting cells would be the lowest RPM columns and the top 5 or 6 load cells. These cells should be set at around 15 degrees. This will allow the engine to crank easily and start fine.

This said, I doubt your issue is timming. There is usually a pre-start (cranking) fuel table that controls how much fuel the engine gets before it starts. This table is usually set up to control the cranking injector pulse width as a function of the engine's coolant temperature. My guess is that your calibration in this table is too lean when when your engine is cold. This causes a lean backfire the first few times you crank and try to start the motor. There is some called Tau that relates to how much fuel dropplets are built up on the walls of your intake manifold. The fact that there is no Tau when you first try to start yoru engine cold is probably contributing to the the likely lean backfire problem that you are having. After you crank a few times, you build up some fuel on the intake runners (Tau) which causes the engine to be a little richer and it will then start. The solution is to adjust your cranking fuel table in the cold start temperature range to increase the cranking pulse witdh and richen things up a little bit.

What kind of computer do you have? If you have a FAST system, send me your calbration file via a private message and I will take a look. I would suggest increasing the cranking pulse width about 5% from 85 degrees on down. Save your initial tables first make the change and smooth out the curve in that table and see if you car starts better. If this helps but you still have some problems or you need to crank a long time when the motor is cold, increase the pulse width a little more and try again. If you see black smoke when you start cold or the motor seems to "load up", you've gone too far with the pulse width increases. Remember that you have to wait for the motor to completely cool to test this after you make a change. Just do a moderate amount of increase at a time until you problem is solved. Here's what my cranking fuel table looks like (I have a 482 ci FE with a TWM setup and a FAST XFI computer). You should use this for reference only - my injectors are probably a different size and flow rate (and my displacement is different) than yours so my pulse widths would not be right for you engine. Note how the curve rises smoothly with lower temperature. You want to increase the pulse width by a constant percent from the temperature where you cold start problems are hapenning on down and then smooth the curve from that point up to the hot start termperature such that you don't add pulse width in the hot start zone.

BTW, when all of this is set right, you probably should not need to crack the throttle to start the motor. Your closed throttle idle setting should probably give you enough air to start unless its really cold. If you have trouble getting the motor to idle fast enough to warm up once it starts, there are other tables to look at and you might also consider some idle timing trim if your computer can do this but that is probably another post...




I hope that this helps you. Let me know how this works out.

- Fred

[StreetSnake]

Fred,
Thanks for the info. I am using FAST. I will show your suggestion to my tuner and see what he can do. I'll let you know how everything works out. Talk soon.

[Rwillia4]

Fred- great work spelling it out!

Streetsnake, I also have a 427W with TWM and FAST. I just had mine tuned in the car by KC. If you want to try my table out to see if it fixes yours let me know. They are pretty simple to upload and download. It will need some fine tweaking but it should give you a good base to work with. Other things will makes changes cam, heads, air cleaners, ...

[Rwillia4]

I don't have an IAC also and I don't touch the gas when starting. You may need to adjust your plates open a little, realize you need to open both sides equal or you could get some backfire... which might be something you want to check to start off with. Does it backfire warm, it is the same cylinders? What is your cold, idle, open-circuit fuel ratio? Does it backfire when cranking to start?

[StreetSnake]

Rwillia,

I am going to check to make sure the stacks are synchronized tomorrow. They were, but maybe something has changed. It doesn't do it when its warm only cold and when I'm cranking it over to start it.

What does your car idle at? I'm interested to know, maybe thats an issue too.

[Rwillia4]

because I don't have a IAC my cold idle is low @ 750-800. After it warms up it is 1000k. It idles smooth at a lower RPM but my alt needs at least 750 to charge.

My two sides we not sync'd up and it caused me backfire problems when trying to cruise in town at low speeds and slowacceleration.

Rob

[StreetSnake]

Rob,
After mine warms up it runs great, no hesitation or anything. Just an issue with cold start-ups. I have the roush individual stack k&n filters, do you think they may be causing a airflow restriction in one of them?

[Rwillia4]

nope I've got the same setup.

I was also running way rich. Now no problems. Let me know if you want to try my file.

[StreetSnake]

Will do. Thanks for info. I'm going to try tinkering with Fred suggestion and see how that plays out. Hopefully have an update tomorrow.

[StreetSnake]

Rob,

A couple other quick questions. What is your total timing set at? What size heads are you running? What did you engine put down at the rear wheels? Thanks, Ben

[Rwillia4]

Crank timing is 35. I have AFR225 heads. I haven't dyno'd at the wheels, crank HP is 600-610 range.

[StreetSnake]

I have the AFR 205s. Crank HP was 550 (very rich at the time). I think the timing is set at 28 or 29. What is the downfall of increasing the timing, ie engine longevity, etc?

[Rwillia4]

I think it really depends on what cam you are running. I don't want to give you any bad info but I think you can run some more advance and still be safe.

[StreetSnake]

Sorry for the delay in getting back to everyone. I went to sync the throttle bodies and completed messed them up and had to start from stratch. One problem is solved. It starts up fine now, that is with the addition of a new battery. The optimu's look cool but don't seem to hold up. So one thing fixed. Now another issue. Because of the starting issue and the fact that when I ran it for long periods of time (interstate) it would start to "drop-out". I put a new USED MSD 6ALN on. Now it hesitates (small backfire through stacks) nearly and there all the time. I just expidited a NEW MSD to be delivered tomorrow. Do you think that the used box I put on could be "scattering" the spark. It doesn't seem to do it when you are excellerating decently, just when cruising or normaly increasing speed from shift to shift. I also haven't checked the timing yet, could that be an issue too?

[thorconstr]

Streetsnake, This is the correct way to synco the bodies. If you are popping back thru the stacks, that is most likely what it is. I can walk you thru it if you call me. 520-401-9142 8am to 10:30pm MST
http://www.twminduction.com/v8_kits/...tle_bodies.pdf