Smitty

iTrader: (1)

What should the air to fuel ratios be in a close to ideal set up be??

At idle ???



At 2500 rpm ???



At 3300 rpm where boost starts ???



At 4500 rpm???



At 5500 rpm ????



At 5800 rpm????

Thanks in advance for some suggestions.

The motors are 540 CI Big Block Chevy. 9-1 base comp 8 lbs max boost at 5800.

Whipple Charged

This really is a far more complexed problem, but here's my rule of thumb given the info you've provided.
Idle = 13:1 (any leaner and off idle throttle is tough)
2500 = 13:1 (+/- .25 af)
3300 = 13:1 (+/- .25 af)
4500 = 12.25:1 (+/- .25 af)
5500 = 12.0:1 (or .5 af)
5800 = 12.0:1 (or -.5 af)

Thanks,
Dustin

Smitty

iTrader: (1)

Dustin,

What other info do you need from me ???

Let me know.

Smitty

zt260

1)All the engine specs.
2)Dyno sheet
3)Boost levels at all rpms in the water.
4)Timing
5)Max Rpm you want to turn
6) EFI? then computer specs

And several other things I forgot.

CcanDo

Smitty,only my opinion,fuel injected vs. carbureted will require a different set of numbers. A sophisticated EFI should monitor and effect changes the carb is not capable of. Therefore,the carb is typically set up "fatter".

You mention boost coming in at 3,300,doesn't boost occur at any RPM? Idle surge seems related to pulley ratio and boost.

Dustin,please straighten me out.

zt260

I also assumed he was running EFI, but the numbers work either way. It's just harder to hit with a carb.

While cruising, my boost doesn’t build until 4000rpm. Now if I pegged the throttle from low rpm and the drive did not explode. I would be building boost all the way. In a computer controlled EFI engine you change the AFR numbers at several boost levels for each rpm zone. So if you get on it at 3000 rpm and start making boost you move into another box with different AFR numbers. If you’re just cruising at 3000rpm with no boost it’s in another box and the AFR is leaner for fuel economy and general engine happiness.

Whipple Charged

I'm very opionated that you should NOT let the computer control AF except for base mapping in the beginning. It may work good in one boat, but I'm telling you, sooner or later, it will be a problem.

Carb and EFI should be the same, as long as the motor is the same, and you can get drive-ability.

Boost is controled by blower speed and the amount of air given to the sc, therefore throttle position is a direct relation to boost on a marine engine. You then have to have enough load to hold that rpm to get into boost. Run small prop, and you'll never go into boost, run oversized prop and you'll always be in boost. Remember, boost is what is measured outside the valves. Therefore, when the motor has "vacuum", the motor is consuming more air than the sc or throttle is giving it. When it's in boost, the sc/throttle is giving the cylinders more air than it naturally would ingest.

Thanks,
Dustin

Smitty

iTrader: (1)

The reason I am asking is because some guys like to go rich at around 11-1 and some go leaner at 12.5-1 as an average. Just trying to get a feel for what's out there and what different guys are doing.