Dave, Samy, Mark, et al... If you gentlemen are correct then I'll be buying you some drinks and eating some crow. But...as reasonable men perhaps you'd want to know *what* particular thing is causing me to think that we'll have plenty of fuel from the stock 2.5 injectors. Here's a pic of the Motronic 2.5 table t3. The top row, going horizontally from left to right, are battery/system voltages for the 348. The bottom row holds the fuel injector time t3 for each voltage point in the top row. Now, what Motronic 2.5 and 2.7 does is to build a total time (Ferrari calls it Tj) that the injectors are held open/pulsed. This total time Tj is calculated (primarily) by adding the appropriate value from a t1 map to the appropriate value from a t2 map to the appropriate value from the (just one) t3 map. The sum of those times is then executed. OK, now look at that t3 table in the picture below. Notice how long the injectors can be open (larger hex values equal more time) when the system voltage is low. High rpm, low rpm...doesn't matter...if the system voltage is low, then Motronic is holding each injector open longer per that t3 table. Well, that tells me that I can open the injectors longer at normal system voltages (e.g. 12 volts or 14 volts). It tells me that I'm not maxing out the capacity of the injectors so long as I stay below the hex value for 8 volts. Anyway, that's just why I'm thinking that the OEM injectors can handle what we're doing. You fine gentlemen could be right, and I could be all wet. I've been wrong before and I'll be wrong again...but now you at least know *why* I'm thinking the way that I am...that it's not just a wild-eyed guess. That t3 table is telling me (if I'm reading/interpreting correctly) that the injectors can handle being open for longer amounts of time, even at high RPMs. Image Unavailable, Please Login
Heres the flaw in your logic. A fuel injector is a spring-loaded valve with a solenoid to open it. The spring force is constant. The fuel pressure is also acting to close the valve and is regulated to be a constant pressure. The components of the valve have mass (a constant) and therefore inertia (which causes a force acting against valve motion). Basically, it requires a constant force to open the valve at some desired rate. That force comes from the solenoid. The solenoid force is proportional to the magnetic field strength, which is of course a function of voltage. The lower the voltage, the longer it takes to get the valve to move to the desired position. The effect is that the valve opens slower at lower voltage, and a slower moving valve delivers less fuel in a given time. What ferrari is attempting to do in the voltage correction table is correct for the field strength and keep the fuel delivery constant and messing with it will make the mixture change with voltage turn on the headlights or A/C, the engine goes lean. Not good. The other important part is that reduced voltage also reduces current (v=IR), so less heat is generated in the injector coil. Heat is the reason injectors are normally limited to an 80% +/- duty cycle, but at a low enough voltage (about 20% below normal), there would be no issue with running a 100%. They might have written it by measuring the injector performance, but its probably just a calculated result. Without knowing how dutycycle at what voltage the injector is running at is a problem. If the engine really make the numbers you hope, you will most likely fry the injectors. All ECUs have a voltage correction built into them and that table is on the list on thing labeled the values are pre-set and should not require alteration This is exactly what makes working with factory ECUs so freakin hard .There just is no way to know for sure what the heck the unit is doing. Im sure you guys will get it, but the voltage correction table is probably not a good place to try to add fuel. Calculating the required injector size is pretty simple and is where the tuning usually starts .why put all the effort into injectors that are not safe to run? Injector size (lb/hr)= engine hp*.5 (lb/hp)/numbercylinders/.8 duty cycle cc/min=lb/hr*10.515 On a street car, you can probably get away with a 90% duty cycle, but youll probably have trouble if you decide to run it at the track. Always do the math with 80% For 400 hp you want about a 31 lb/hr (328cc/min) injector by that math. If you truly believe you are going to hit 400 hp, you want to have at least 30lb/hr injectors in that thing (they are very common and pretty cheap), or get the fuel pressure bumped up to where the stock injectors will flow 30 lb/hr.. Youd hate to spend a couple weeks tuning just to find out you need to swap injectors or have to bump the fuel pressure and need to start over.
I've got my 308QV heads (which are very similar)on my bubby's flow bench now. On the QV, all the part match almost perfectly, so I've got to fix everything to get more flow. Shoot me a PM if you want to compare notes or I've got a thread running: http://www.ferrarichat.com/forum/showthread.php?t=129621
You let the cat out of the bag! I was going to give them that formula, but I was shot down earlier by skeptics. Yes, its that easy. Here goes, look at this way. First, get an Air/Fuel monitor for both banks. Put the larger 12-16 Ohm 30lb injectors in. Start the car, turn the AMM screw on the air meter to incrase Ohm resistance until you get a Stoich reading on the A/F monitor, DONE! If you feel you have to tweak the chip for performance, now you can take out fuel instead of adding, yes, take out fuel. but most importantly, stock 348 injectors are rated at only 194 cc per min., 18.39 lbs per hour, thats only 39 hp per cylinder or 312hp, period. Now your only weak "air" link is the AMM itself. It might only flow enough air to supprt 24lb injectors before it(AMM) maxes out its voltage. I have the solution to that also.
Plugzit, why don't you test out Dave's idea above by dialing up the resistance via the MAF screws while I'm out skiing.
Turn it up to 415 ohms to start. Then try 450 if you're still lean. Hey FBB, where's that A/F meter?!
Whoa. Don't look at me! Maybe the other way is right. What do I know. I'm a stooge. The one that no one remembers...
You only need to adjust the screws after you put in 24lb injectors for 360hp or 30lb injectors for 400hp on a stock engine. 450hp with head work and did somebody say custom cams, nyuk, nyuk, nyuk. Man, I gave my combo out for the brothers stooges. People pay me big bux to get their cars to haul some A**. For your car, put your stock chips back in, then I would put in 24lb for now, because you will need larger adjustable Mass Air Meters for 30lb set up. The stock meters will only support about 360hp of air flow before being maxed out on voltage. The lower the resistance the richer the mixture. 383 ohm is stock. There is no need to adjust the screw if you have the stock 348 injectors. Someone you know must have a set of 90's mustang or corvette 24lb injectors around to test. I have a calibrated 24lb set fresh from the flow bench if you need them. BUY your A/F meter of choice ASAP! I use the FAST dual setup, works for both cylinder banks in one screen module with 50 minutes of data logging.
Man after reading this stuff I have ended up with a question for the tuning gurus: Is it really necessary to swap out the intake for 8 tbs to get the desired Hp as opposed to just replacing the injectors, fuel pump, chip and MAS? Although Bruce's project is very cool and much prettier, was it necessary to get the ponies, or can we get them with the stock intake?
My guess is yes, it is necessary. Ferrari is pretty good at matching components, but there is always one the flows less than the others. On the 348, it appears to be the intake, although I haven't had one on the flow bench. If the mixture is correct, just adding fuel will have no effect on hp. Sometimes the factory sends them out a touch out if idle for one reason or another, but in general making hp is all about getting air into the engine
Yes, it is necessary to change the stock intake system, among other components that move air. The factory "air" package is pretty well matched. As I stated earlier, bolting on 8 tb's is cool but the rest of the air flow components need to be upgraded to match as well. The fuel required to obtain higher hp levels has to be matched to the volumes of air the engine is able to flow. Air, Fuel, Spark, thats it guys. You can't upgrade one without matching the others, for serious power gains. Sure, you can do exhaust swaps, air filter changes and test pipes but your only getting minimal gains.
Thats correct, that is why I put larger injectors in my customers modifed cars and tune the AFM at idle to correct a/f ratio, then when you need the power up top the actual larger injector flow will supply the necessary amounts of fuel for the increase in airflow that was bolted on. Alpha n in high loads with stock injectors in Plugzit's 8tb project will always be lean up top where the engine is capable of most of its power. Must change to larger injectors for best results.
Load is generated by the maf normaly the m2.7 isn't reading in the 12x12 lookup table under full throttle. It is only reading in a 16x1 table wich revers to the engine rpm. You can adjust when it goes into alpha n mode thats the table nodoubt called full throttle lambda trim or something like that. When its reading there the other bigger 3d table isn't effecting anything. If plugzit raises the fuel pressure to 5-6bar he can get ~400hp out of the stock injectors with a high dutycycle. To get near 100% dutycycle you have to change more then the shown mappings that are no mappings with a direct injection time in it. There are additional mappings for maximum injection time. If that is 75% in stock f.e. you can go up to 100% in the full throttle 2d mapping and the injectors are at 75%. So if you don't change this till now and are far away from 100% in the 2d mapping you have planty of fuelflow left in the injectors. If its near 100% and he raises the fuel pressure he has to keep an eye of the fuel pressure under full throttle when the injectors going to 90% DC i don't know how much the stock fuel pumps flow at that pressure. And also it could be necassery ,or it will be , to change much mappings after that to keep the fuel amount correct in idle, low loads , cold start. In hot idle, and partial throttle the ecu lerns to lambda 1 but it takes some time and the cold start behaviour could be very bad because there he don't regulate to lambda 1 and uses fixed injection times. To setup that all a eprom emulation system is needed because you don't have much time to setup it till the engine is out of cold start mode.
It's a really bad idea to run injectors in a street car past 90%, and that is pushing it. I know people do but..... The best answer is just install bigger injectors, they are only a couple hundred dollars. The next best thing is to bump up the fuel pressure, but as mentioned, be sure and watch it. The fuel pump flow rate drops pretty fast as pressure goes up. I've got the fuel pressure in my car bumped to 60 psi and the pump started blowing 15 amp fuses. I'm told larger gauge wire helps on some cars.
I don't know if the tables on the Motronic 2.5/2.7 chips can be used to (reliably) gauge that fuel injector duty cycle, but if they can, then the Stooge V0.4 chip (est ~~ 310 rwhp) that I'll send to Plugzit toward the end of this month will only be to 80% (at peak).
How to bump up the fuel pressure? The FPRs on the 355 fuel rails are stainless ($250ea used at GT Car parts), bolt directly on nipples on the rails, and are o-ring sealed into the female aperture on the rail-way different from the 348. I'd really like to avoid the expense of adjustable regulators. Am I going to be using that much more fuel that the fuel pressure will drop? I don't think the pump for the 355 is different, the car will make less (or maybe marginally more in tuned form) horsepower than the 355. It is sounding like either I have more pressure to force more gas thru the current injectors or bigger injectors. Can't I just leave them open longer to deliver enough gas without exceeding the open time of the intake valve?
Now we're gettin' shadetree mechanic here with the regulators. OK, the stock regulator has a vacuum hose attached. correct? Well, that hose pulls vacuum on a disc diaphram at normal driving to lower pressure, at high engine load (low vacuum) the disc goes to its normal pressure state. To increase fuel pressure, remove and plug vacumm hose, find a suitable fine threaded bolt with jamb nut and screw the bolt into the vacuum port on the regulator until you feel it touch the disc. Now you need to get a FP guage and record the fp reading at you turn in the bolt. It was an old low buck trick we did on our mustangs and vettes. It would be nice to leave our injectors open longer but "time" is the major limiting factor. The higher the rpm the less time is available to open the injector. Do the math, so you really need larger flowing injectors to offset your increase in air flow and to keep the duty cycles near 80%. Start with 24lb/hr about a 240cc per/min flow. Remember stock is only 18.39lb/hr about 194cc per min. Using the fuel flow formula thats only 312hp at 95% duty cycle. The 24lbs should get you 330hp at 80% duty cycle. These numbers are based on 44psi fuel pressure.
