Dan,
You have invested a lot more time in figuring this stuff out than I have. Great work. I was thinking about the dead times. Those are at 3 bar. If you're running at a different pressure, you'll need to determine what the dead times are. Or doesn't Megasquirt allow for dead times?
As far as running at 60PSI, per Scott's suggestion, I did find one chart that lists flow at 55PSI, 334 cc/min versus 297 cc/min at 43.5PSI.
Stan Weiss' - Electronic Fuel Injector (EFI) Flow Data Table
Dave
Quote:
Originally Posted by dannichols1474
Hi montauk,
I set the fuel pressure based on my observation of the fuel pressure running on other cars I have owned or worked on and not on any engineering calculations. When I initially installed the ITB EFI system on my stock CIS motor and checked the fuel pressure, the fuel pressure regulator adjustment was screwed all the way loose so I saw the max pressure of the fuel pump at 90 psi and turned it down to 40 psi and left it there.
Since the stock CIS motor with ITB EFI was making 205 hp (crank) on the dyno before I rebuilt the motor, I was way below the estimated max hp for the given injector size (I remember reading the 30 lb/hr injectors would be good for 288 hp), so if I was running the injectors with 38 psi versus 43.5 psi I wasn't in danger of running out of injector capacity. But now with rebuilt motor making 274 hp (crank) I am much closer to the edge.
I did a quick AI internet search:
Calculating Maximum Power for a Given Fuel Injector Size
To determine the maximum power a given fuel injector can support, you'll need to use a variation of the fuel injector sizing formula. The key components are:
• Injector Size (lbs/hr or cc/min): The flow rate of your specific injectors.
• Number of Injectors: The number of injectors in your engine (e.g., 8 for a V8).
• Brake Specific Fuel Consumption (BSFC):This represents the amount of fuel an engine uses to produce one horsepower per hour. BSFC values vary depending on the engine type (naturally aspirated vs. boosted) and fuel type (gasoline, E85, methanol). (0.5 for naturally aspirated gasoline engines)
• Maximum Injector Duty Cycle: This is the percentage of time the injector is open and flowing fuel. It's generally recommended to use a maximum duty cycle of 80-85% for safety and consistent fuel delivery.
Formula: Estimated Crank HP = [(Injector Size in #/hr x Number of Injectors) ÷ BSFC] x Maximum Injector Duty Cycle
For 30 lbs/hr: Est HP = [((30 x 6) / 0.5) x 0.80] = 288 hp
For 31.4 lbs/hr (43.5 psi): Est HP = [((31.4 x 6) / 0.5) x 0.80] = 301 hp
Now assume those injectors at 38 psi flow only 27 lbs/hr (down 10%),
For 27 lbs/hr: Est HP = [((27 x 6) / 0.5) x 0.80] = 259 hp (80% injector duty cycle)
For 27 lbs/hr: Est HP = [((27 x 6) / 0.5) x 0.85] = 275 hp (85% injector duty cycle)
Now, with more experience and considerably more thought, I see that I should turn the fuel pressure up so that when the engine is running the fuel pressure will be at 43.5 psi to take full advantage of the Bosch 62672 (0280155831) injectors that are rated at 31.4 lbs/hour at 3 bar (43.5 psi).
But unlike your Haltech ECU, the MegaSquirt2 ECU doesn't monitor fuel pressure nor fuel temperature directly, so if I adjust fuel pressure, then I will need to re-run auto tune to adjust the values in the VE table since for a given injector pulse width the injectors will deliver more fuel per shot at the higher fuel pressure.
Another thought: My motor makes max hp at 6600 rpm and that carries flat out to 7200 rpm and I have the rev limiter set to 7500 rpm. The ECU fires each bank of injectors once every 2 revolutions, so a given injector fires 3750 times per minute which works out to firing every 16 ms. Per your dead time data and given my Anti-Gravity battery is at 13.2 V (13.8 V while running), my injector dead time is about 1 ms which leaves 15 ms for the injector to deliver fuel at 7500 rpm. At 85% duty cycle, that translates to 12.75 ms maximum pulse width available at 7500 rpm. I need to go look at a recent data log with the a 100% TPS pull to 6000 rpm and see what injector pulse width the ECU was calling for - I wonder if I'm already at the wall?
Thank you for asking the question.
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