New injection installed Looks just like the Webers!

[dlampe]

Thanks for the clarification on the open loop terminology. What determines how much HP a system can handle? Is it the injector size? Fuel pressure? Throttle body diameter?

[Aussie Mike]

Originally Posted by dlampe

Thanks for the clarification on the open loop terminology. What determines how much HP a system can handle? Is it the injector size? Fuel pressure? Throttle body diameter?

Injectors are matched to specific fuel pressure. It can be varied but they have a specific pressure range they work at their best. Injectors need to be sized to support the amout of power the rest of your motor will make they will flow X ammount of fuel at a particular duty cycle. If the ECU is having to hold the injectors open 100% at full power then you probably need bigger injectors.

FI systems seem to be able to handle a lot more airflow than an equivalent carb setup. I'm running a 1000CFM throttle body and it runs great but a 1000CFM carb would be a pig. FI will probably make similar top end power to a properly setup carb motor. Where the gain is is in drivability and low end mid range power. With the injector placed right at the port in the head the fuel doesn't have a chance to fall out of suspension at low gas velocities i.e when you open the buttterflies on a big carb at low RPM. The carb uses a big shot of juice from the accelerator pump squirters to fix a mixture that's suddenly become lean because all the fuel is now sitting in the bottom of the manifold.

Cheers

[Anthony]

Originally Posted by Aussie Mike

.FI systems seem to be able to handle a lot more airflow than an equivalent carb setup. I'm running a 1000CFM throttle body and it runs great but a 1000CFM carb would be a pig. FI will probably make similar top end power to a properly setup carb motor. Where the gain is is in drivability and low end mid range power. With the injector placed right at the port in the head the fuel doesn't have a chance to fall out of suspension at low gas velocities i.e when you open the buttterflies on a big carb at low RPM. The carb uses a big shot of juice from the accelerator pump squirters to fix a mixture that's suddenly become lean because all the fuel is now sitting in the bottom of the manifold.

given the same venturi diameter, yes the fi system will flow more air (maybe 30% more) because it doesn't have a booster stuck in the middle of the venturi to restrict airflow. All you need to do is get a bigger diameter venturi in the carb to "match" cross sectional area, then I would think flow would be the same.

I have read several comparisons of Fi versus carb, and the one that sticks out in my mind is a boat comparison (same boat), mercruiser 502 HO and 502 magnum, I think, anyways, one was a carbed engine, the other FI, otherwise the heads, cam, compression, were the same. Just different intakes. The carbed version was slightly quicker/faster. The explanation I read later was that there is an optimal size of fuel droplets, to make the most power. You don't want the fuel droplets too small, or the fuel to vaporize, as the vaporized fuel then takes the place of air (oxygen) thus diminishing power potential. An FI system can't supply the fuel in large enough droplets, and therefore, a properly carbed engine will make more top end power.

The reason you need an accelerator pump on a carbed engine is not because the fuel sits on the bottom of the intake, but because gasoline has a larger inertia than air, and thus takes slightly longer to increase flow when you open up the butterflies, and therefore, you need to supplement the fuel supply with a squirt to prevent a momentary lean condition/hesitation.