Well, found out that GM sets the stock tbi cars to have pretty high injector duty cycles adn even go static at only moderate rpms. Looks like GM did this as a tradeoff so they could obtain reasonable duty cycles at idle but damn, I really wish I would have known about this earlier. I didnt know that I was completly out of time by 3000 rpm and I just kept adding fuel in on top of the stock stuff wondering why it didnt do anything. As of now, I set it up to run rich for the inevitable day I have to put a cat on and above 3000 rpm, I get the vtec effect from the injectors going static and the mixture leaning out. At 4500, it just plain breaks up from being lean. On top of that, the ecm goes in and out of async at idle because the pulsewidths are too short. Guess Ill have to get an aftermarket pump and rig up a VacAFPR.

Figures... Even the stock injectors that come with stock tbi motors are too small.

Heres a nice chart to illustrate it. I used my stock bin and my test bench to get the data. The pulsewidths are plotted with the maximum time available for the injector to fire. Above 4000 rpm, the commanded pulsewidths start tracking along the max time thats available for the injector to fire. Coincidence?

This is all at WOT, of coarse.

 

Have you brought this up on thr diy-ecm board yet? I bet Rbob would have a comment or two on this subject. Grumpy too for that matter. I guess it's time to start researching the 749ecm to run our tbi's.

Steve

 

Well, its not the ecm, its really the fact that the relatively large pph tbi injectors have to have short pulsewidths at idle since only a small ammount of fuel is needed. From what Im getting with mine, it looks like with a non-variable fuel pressure its a balancing act between the needs at idle and the needs at WOT. Way I plan to get around it is to run a VacAFPR and throw some code on the ecm to adjust the BPW as the vacuum (and pressure) changes.

Lol, im sure theyll be along soon enough to comment on this. I dont think its really too much of a secret. I suspected that theyd have high duty cycles at WOT but not quite this bad. This is really more of a tbi issue as its the injectors and the ammount of fuel thats capible of being delivered while still staying out of async. at idle. Pretty important to know, that with the stock injectors, there isnt much room to grow.

 

Why do you think GM used larger injectors on the LO5 cop car motor. I believe the L03 cop car motors came with L05 injectors as well.

 

Probably because they need more fuel throughout their operating range. I remember hearing that some truck calibrations ran in async fueling all the time rather then sync when the PW where long enough. This may have been for the same reason, that they couldnt supply enough fuel as well as have sufficient time for AE during transition to high throttle loads and still fuel down low with syncronous fueling.

 

it is very interesting you have those results,

with my UD cam in the car, i add quite a bit of fuel, and i don't go static until above 6k.

 

What pph injectors do you use? With mine im having trouble with the PWs going too short but that may be due to low cylinder pressures driving down the VE at low speeds.

 

they are the stock injectors. so i think they are 43# or 45# or somewhere in that range.

 

instead of adding more ontop why not increase pressure and take away on bottom?

 

I'm not sold on the argument that its completely the injector sizing that causes idle issues.

Some SEFI guys report running Peak-Hold injectors in the 65-70lbs/hr range without idle trouble.

I think the trouble is from the TBI injector firing strategy. Each injecotr is fired twice per crank revolution whereas with MPEFI they fire once per revolution and SEFI they fire once every two revolutions.

This is why TBI runs out of Duty Cycle so quickly. The injector has to turn on and off twice as often as batch fire.

 

because typically there are less injectors so it does make sense gm did design it all this way... they dont think a lot about what people are going to be doing modification wise 20 years down the road when they're designing this stuff

can we hack the chip to completely reprogram this for idle?

 

To me, the problem is really twofold. The firing stratagy is part of it. The pulsewidths are shorter since there are 2x the number of firings per revolution, but the second problem is the fact that there are only two injectors to supply the engines entire fueling demands. That necessitates that large injectors are used to adiquatly fuel the motor up top but then those same injectors must have very short PWs down at idle in order not to overfuel the motor. While those high pph injectors on the mpfi/sefi are only really found on high output engines, around here, everyone has to have at least 55 or 65pph injectors if theyve done some mods and they want reasonable ammounts of saftey margin. Not also to mention that, as you already know, people even run the high pph injectors at high pressures to get over 100+ pph of fuel out of each one, so naturally problems are to be expected, regardless of the firing stratagy.

More my opionon then anything else but the fact that tbi only has two injectors is really its biggest downfall. Airflow is ok but the fueling will always be somewhat of an issue that places upper limits on what one can run. Not only does there have to be enough time to meter out the fuel, but you also need additional time available for AE on sudden transitions to high loads. Big problem if you have a larger manifold. Probably the same reason why some people keep adding and adding pulsewidth for AE because it goes lean when slamming the gas down but still no matter how much they add it doesnt help, simply because there isnt any off time available for anything else.

 

Not really, the firings are controlled by the hardware. Either sycronous with the engine, or at 80 Hz. The only thing you can select is which mode and how much pulsewidth.

 

dual 2bbl, 4bbl holley w/ commander eh?

what was the result of the dual tbi dual ecm topic, i dont remember..... was it that you just would never be able to tune it?

 

I'm surprised more people haven't ran into the static injector problem. I ran into and banged my head on the wall for quite a while till Rbob showed me how to output my PW and I realized I was going static. I wouldn't be too worried about a-synch. I run it anywhere below 2000 rpm. I'm right on the boarder down low PW wise at 1 MS. Grumpy says he's gotten his P/H injectors to take .8 without too much problems. On the top end I'm right at 4 ms which is the safe 80% limit. I was running right up againtst the 5 ms max wall for a while as I was tuning and didn't have much problems other than every once in a while getting a lean pop when the pump shot pushed it over 5 ms. I'm runnin 75pph injectors at 22 psi which is right around 110 pph corrected, this is feeding a luke warm 350 that makes around 320 hp.

This is kinda off topic but not really. I just got a summer internship with Pratt & whittney in their engine overhaul operation here in WV. Their having real durability problems with their PW305 series engines. They are running within 50 deg of melting all the time. We're talking melting titanium and inconel so as you could imagine it's not cheap when you can melt a million dollar engine in a matter of seconds if you push it too far. So the moral of the story is learn how to run things on the verg of blowing sky high...... you might just have to do it for a living.

 

Man... Really gotta blow the cobwebs out to remember that one. It would be alot harder then just using one ecm that commands the second set of injectors to come on when needed. Im pretty sure I could at least somewhat get it to work with the ecm running the second set of injectors as an on demand sort of deal. But, with the injector driver boards defunct and the holley units going for something like $800 dollars a pop its pretty much all but dead.

 

Thats no good. The metal really weakens up when you get close to its melting point.

On the injector side of things. I finnaly expanded my VE tables so I could pull the VE adder table in with the main VEs and below 1200 rpm the VEs just drop like a rock. Explains the really short PWs at idle. BLMs all look good but it smells like it has a lean miss. Smells good when I first start it warm with the extra fuel from the choke but after that it smells like gas and gets that 'its got a cam' idle. Idles pretty steady but still shouldnt do that. Maybe ill just ignore the blms and tweak it to richen things up a bit at idle.

 

I added the two VE tables togather through 2800 rpm in the main table becouse it makes it simpler to adjust. ( I have all zeros in the #2 table through 2800. Do not combine 3200 because the #2 table adds each rpm correction value above 3200 to the 3200 block in table one. I found this out the hard way) . The BLM value is for both tables combined. Otherwise to correct VE in the main table, you have to add the value of the corresponding block in the #2 table to the block in the main table, multiply times the correction factor, then subtract the #2 table value from the sum and enter that number in the main table. When I started burning chips I couldn't figure out why correction only changed my BLM's about half each time.

 

Oh, guess I should add that my ve table goes up to 6400 rpm now. Basically havign the adder is jsut a cheap way to save space by eliminating the need for a larger VE table. I did know about the fact that my VEs where the two added togther so my blms applied to both but I completly forgot about that most of the time by about the second chip I burned

 

I wish I was alittle more versitile in the chip burnning arena. I'm just using tunner cat for now. Some day when I have more time (Maybe next winter) I can get into sorce code and patches and neat stuff like that. I may give you a shout when I do .
Good day, Dave

 

im glad i quit trying to pioneer any of the chip burning stuff when i did. This would have taken over my life. - I'll probably never mess with it now that $ isnt as tight as it was while i was in school.... ill probably end up going with an aftermarket fuel/spark management system... but it is still good to know some are out there trying to make this still work.

 

As for the metal weakening at that temperature...your right, but everything is engineered to work like that. The blades in the turbine are super loose in their holders but when they get hot all the tolerance is taken up and everything works well together untill the pilot decides to ignore his "over temp" light and burns the thing up. My point was that you really have to pay attention to the details if your going to push something to the edge. In that buisness several hundred thousand dollars can hinge on a couple of thousandth's of an inch.

I think the key to making big power with TBI is paying close attention every detail and making sure everything works together. The biggest problem with TBI is getting enough PW to feed your motor on the top end as well as getting one low enough to idle well. I think the real solution to the low idle PW problem is to get the injectors to fire less. Something less than a-sync would be nice, like every other ref pulse. The neat thing about TBI is that it is still alot like a carb in that you won't miss the extra time between pulse's. The fuel will have plenty of time to mix with the air as it travels through your intake. You don't have that with MPI. Of course doing something like that will require code change and unfortunatley thats over my head at the moment. Once you get that figure out though running the big 4 barrel units and the big injectors starts looking like a good idea.

 

Thats pretty wild. Never knew the tolerances where that close in ajet engine.

I do wish the 4 barrel units with the good injectors where cheaper and that the injector driver boards where still available. Would be neat to jsut have something that I could bolt on and button down and be done with it. I really do like tbi alot more then the stock tpi systems. Much simpler and easier to work on.

 

Thats one thing I like about efi tuning. Once you have the stuff its free power. Although ill probably loose interest somewhat when I can actually afford a decent car and drivetrain that doesnt have to be pushed hard to make power. crap 350 + 2.25 in exuast + crap rear gears = slow unless its really pushed. My brother had the same stuff with a holley 650 carb and non CC dist. and now this one with tbi the difference is night and day. Like and LG4 compared to an L98.