View Full Version : Relationship of injector size and mileage
As the title says, can the injector size affect the mileage, and how can you figure what sizes you can use without causing other issues? Kind of a stupid question, but I really have no experience with any of this, and in my early driving since I started the registration process, I seem to be going through an awful lot of fuel. Don't know what the mileage is since I can't calibrate my speedo. Apparently an electrical glitch that Speedhut has offered to fix for free. I just can't remove the speedo because I can't reach the wiring plugs for the unit. This is what happens when building for the first time, and you don't think about access. But that's another topic.
They would only be a problem if way too big for the engine in which case there could be idle issues. Here's the formula to calculate injector size. "injector size in lbs./hr. = (Max. HP x BSFC) / (number of injectors x duty cycle)". Use .5 as the BSFC, and 80% as duty cycle. So at 400 HP you would need 30 lb. injectors minimum. You probably need to work on the A/F ratio and your timing table to get plenty of advance in the cruise range. I was just tweaking my timing curve more yesterday, and that's after 5 years and 12k miles. What I do is get data logs at cruise speeds and see where you are at various map/rpm points for timing and A/F. I'm running around 14.5 A/F at cruise and 42 degrees of timing (light cruise only). I'll be glad to take a look at your tune if you want to email it to me.
Bob
SJDave
06-25-2021, 07:33 PM
Rick, the Easy EFI system should be trying to adjust the Injector pulse width to achieve the Air/Fuel ratio numbers in the "target" Volumetric Efficiency table that FAST has created as a default. Unless you upgrade to the FAST Sportsman or XFI 2.0 you cannot adjust the VE tables. So let's assume the table they have defaulted to reflects your engine, and the A/F ratio's in the table as a function of Intake manifold Vacuum and RPM are reasonable, which they may not be a great match since you have the stack system and not a throttle body injection system. I don't think you can even datalog with the EZ efi system, so it's hard to figure out what is going on.
Injectors size is usually chosen so that they do not exceed 80% duty cycle to inject the required amount of fuel to achieve the A/F ratio under WOT. I haven't read anything that have way too big injectors and running 20% duty cycle creates a problem. My 427 injectors were 45 lb/hr and ran at 70% duty cycle worst case at 7000 rpm with 55 psi.
But, the A/F sensors in your exhaust should be running the show and telling the FAST unit to either reduce pulse width to the injectors or increase it. I assume they provided the Wide Band O2 sensors?? So they are probably sized properly and capable of over richening, but equally they should be capable of leaning it down to match the desired A/F ratio from the table.
If you can't datalog, you might have to rent some Dyno Time down the road to use their exhaust tip wide band sensor to check the mixture throughout the load and rpm range. That's why several other guys exchanged their EZ EFI for the "Sportsman" model, since you can adjust the VE tables and datalog.
No simple answer unfortunately.
Oh I didn't realize you had the EZ. It's really limited on tuning you can do. You could always get someone to video the handheld to see what it's doing.
Bob, thanks for your input. Sorry I forgot to mention the EZ.
Dave, I just started thinking about other possibilities besides the e brake dragging causing the drop in mileage. I've got another idea that involves the rear brakes, but I need to investigate it further. It could be another of my wild goose chases! I'm going to try to get some things done tomorrow. I'm finally feeling up to it. We'll see how long I can avoid the pain coming back.
Thanks guys.
Does your system provide a provision for free-air calibration of the O2 sensor? Sensor calibration can be affected by use and even big altitude changes so that the info sent to the controller may not be quite accurate causing the controller to over or under fuel. I've never seen a significant calibration error that would cause performance issues at the track but if you're checking fuel mileage it doesn't take as much error to notice a difference.
ej95Cobra
06-26-2021, 05:17 AM
As one of those SJDave mentions that ditched the EZ EFI 2.0 for the Sportsman, that is the first thing you should do. We could not get a stable baseline with the EZ EFI and switched to the Sportsman since it uses the same wiring. We got a dyno tune and that fixed the issues we were having with the EZ system. FAST also notes that the positive and negative wire should go directly to the battery terminals to eliminate any interference in communications to the ecu. Had I known before ordering the engine the issues we would have, I would have requested a different ecu set up.
rich grsc
06-26-2021, 07:50 AM
As one of those SJDave mentions that ditched the EZ EFI 2.0 for the Sportsman, that is the first thing you should do. We could not get a stable baseline with the EZ EFI and switched to the Sportsman since it uses the same wiring. We got a dyno tune and that fixed the issues we were having with the EZ system. FAST also notes that the positive and negative wire should go directly to the battery terminals to eliminate any interference in communications to the ecu. Had I known before ordering the engine the issues we would have, I would have requested a different ecu set up.
I agree, in fact I've replied to your issues in the past with the exact same recommendations, get rid of the FAST EZ system. Just unplug the EZ, and plug in the Sportsman. You won't believe how much better the car will run.
Just as a note, I really can't complain about how well the car runs. No hesitations, doesn't lug from low rpms on acceleration, idles well, and no fuel smell while running, or when sitting in the garage after a drive. Just trying to determine what to expect mileage wise with the stacks. Because of my extreme lack of knowledge on EFI you may be correct about the car running even better with the Sportsman. The precipitous drop of the fuel gauge during short drives makes me question if I'm going to be able to afford to take this on any longer trips! It may just be the e brake dragging, as the rear wheels area bit tough to rotate up on jack stands, and there was a lot of brake dust on the PS rear wheel. As a note, T-Bird IRS and brakes from the same.
Mine got about 17 hwy and 15 overall before I switched out the 3.55:1 gears for 4.10:1. Lost a couple of mpg but it sure is fun to drive.