Originally Posted by dave w
Using this spreadsheet, running 100% duty cycle on my injectors indicates a supported horsepower of 306. I feel I have been wronged. I purchased a rebuilt TBI unit from SPR performance, specs below:
46mm TBI Kit
New TPS, New IAC, TPS adapter, 65lb flow-matched injectors, 18psi spring, thinned shaft, bushed shaft, 1/4" pod spacer, shaved air horns, adj reg cover, custom cut 46mm base gasket
I specified I was using a kit from Summit Racing (SME-K-355-400), link to cam specifics here. I'm considering selling all the GM TBI parts and going for an MPFI self learning kit from Fast.
I am by no means an expert but running your injectors at 100% is not good. You need to increase your BPW
IIRC, this is a zero sum game; increasing the BPW or increasing the duty cycle from the injectors. Either way, the amount of time the injectors are firing is the same.
One of my 5.7 liter TBI tuned engines ('7427 PCM using a Wide Band O2 Sensor to tune with) went on a Dyno ... the results were 330 RWHP using 65 cop car injectors with a stock fuel pressure regulator spring and 670 CFM (50mm Throttle Body).
The FAST system might be an option. I would use a '7427 PCM in MPFI mode. Personally, I fell I can tune better than the self learning FAST system.
Thanks for your reply Dave. I feel like I *could* tune better than a FAST or Holley MPFI system with enough time and trial and error. I have had this setup for over a year and not had the time to learn enough to make a perfectly drivable bin IMO. However, I feel as though there are some limitations to the 1227747 $42 ECM that can't be solved such as:
Inability to correct using a wide band
Low-resolution fuel/spark tables
Slow sensor polling intervals
The FAST or Holley MPFI kits are not cheap and do require standalone Ignition controls. Probably total parts cost of 4 grand. On the plus side, there is now a one to one relationship with the injector and cylinder. I would really like the current TBI system to run as smoothly as my daily driver but feel as though I am currently overwhelmed with tuning and have a need of my Jeep being ready for the winter.
Lastly, I feel as though the Holley or FAST kit would net me better fuel economy. Currently averaging 10 MPG with this setup in a 1990 Jeep Wrangler YJ which weighs in around 3300 pounds. I have the motor bolted up to the stock AX15 5 speed with a custom bellhousing, 4.10 gears and 35 inch tires. It will run around 2k RPM doing 70. The MPG isn't the problem so much so is the 15 gallon fuel tank. My commute into work is 80 miles round trip and having to fill up every day is annoying.
Last edited by 350yj; 04-10-2016 at 06:57 PM.
The 7747 seems to have less functionality than later computers. It is somewhat harder to tune imo and I have not opted to use one in years. I plan to use a 7427 or 7730 when I put my 4.0 / AX15 combination together. There are ways to do all you want with a stock ecm but it takes some work and time to set up hardware and software. Just like trying to use a stock engine for a specific, high-performance purpose, the stock computer was built for stock applications and will require "help" if you want to extend its abilities.
It is more challenging to tune for maximum fuel economy than to "get it running." It takes time. You need to log a trip to work, pour over the trip, and look for areas where you can make improvements. Do the same on the trip home. Watch for conditions like high MAP causing the ecm to enter PE mode. Watch to see if you can lean out fuel and maintain power. Advance and retard timing in small increments at cruise to see if it makes a change in economy. It is the same approach as with a carburetor but with the advantage of being able to log conditions and view them later. Opting for another system means making another set of tradeoffs... I personally have not seen that the significant additional investment allows one to skip spending time with the system to get best results. Still, I also know that there is no rational approach which will trump what a person "feels" will work best. If your heart is set on spending money for a solution I'd ask, in all honesty, that you report on your experiences should you decide to switch systems. I would also advise looking at Dynamic EFI as an option. Bob Rauscher's system starts with a stock ecm like a 7747 but adds a considerable amount of functionality through custom code, additional hardware, and proprietary tuning.