Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Fuel economy of 7.4L (454) powered 3/4 ton suburban

mwilliamshs · 04-29-2015, 06:26 AM

A carb on a 5.0 should need less main jetting than same carb on a 7.4 but the 7.4 would need more idle fuel and more accelerator pump fuel, as you've observed. The bigger engine will pull more fuel on its larger vacuum pulses on the main circuit (so a smaller jet will work) but will need more fuel to idle (bigger jet and/or "looser" mixture screw) due to breathing more deeply at high vacuum (idle) and will likewise need more fuel to prevent stalling at quick throttle angle changes due to that higher volume of air being sucked in by bigger cylinders (so bigger acc pump squirter/cam/etc).

Very interesting project you have here. Looking forward to the diesel stuff!

15-16:1 AFRs can work quite well depending on valve type/seat condition. Worn valves burn easier due to smaller margins.

14FF in vans had 3.21 R&P somewhat commonly in my experience. I'm a van nerd.

04-29-2015, 06:26 AM	#41 (permalink)
mwilliamshs EcoModding Apprentice Join Date: Apr 2015 Location: USA Posts: 148 VanDelay - '89 Ford Econoline E-150 XL 90 day: 15.93 mpg (US) Old White Civic - '98 Honda Civic LX, AT 90 day: 33.18 mpg (US) Thanks: 5 Thanked 50 Times in 43 Posts	A carb on a 5.0 should need less main jetting than same carb on a 7.4 but the 7.4 would need more idle fuel and more accelerator pump fuel, as you've observed. The bigger engine will pull more fuel on its larger vacuum pulses on the main circuit (so a smaller jet will work) but will need more fuel to idle (bigger jet and/or "looser" mixture screw) due to breathing more deeply at high vacuum (idle) and will likewise need more fuel to prevent stalling at quick throttle angle changes due to that higher volume of air being sucked in by bigger cylinders (so bigger acc pump squirter/cam/etc). Very interesting project you have here. Looking forward to the diesel stuff! 15-16:1 AFRs can work quite well depending on valve type/seat condition. Worn valves burn easier due to smaller margins. 14FF in vans had 3.21 R&P somewhat commonly in my experience. I'm a van nerd. Last edited by mwilliamshs; 04-29-2015 at 07:30 AM..