EFI Performance?

[easystep]

Can you give me a general idea of the performance difference seen between a carbed engine and a fuel-injected engine?

[Bart Carter]

If you set up both carb and efi right, there is no difference in power output. The difference would be in driveability, with the efi far ahead.

Most people don't set up efi for max performance. They usually use runners that offer good velocity (smaller cross section for low to mid-range performance) and neglect the need of a larger cross section for peak HP. There is a balance there that you have to decide on and then build accordingly.

A stand alone system such as Autronic, F.A.S.T., MoTec or Redline can be tuned to anything you want.

Of course, you also need a tuner that knows what he is doing.

[shelbygt500]

I thought carbs were a plus for improving torque and throttle response but I'm no expert...

[map]

Originally Posted by Bart Carter

If you set up both carb and efi right, there is no difference in power output.

Maybe with Webbers you mite get close the EFI if you know how to tune them! With a carb ( 4 barrel) you'll always have 4 lean corners because of the runner length, put a EGT in all 8 EX and data log a run you'll see higher EGT in the 4 corners lower ( fat) on the other 4. With EFI you can tune your A/F ratio get all 8 EGTs in sync and that adds up to power fuel economy. You can also run a more accurate timing and a improved throttle response..

[Cobrabill]

There is NO peak HP number improvement with EFI over a PROPERLY set-up carb.Midrange and driveability will see improvement but that's it.

[Slither]

In my case with my TWM's, I experienced a smoother idle and my exhaust is much quieter. That's because the EFI system is much more efficient. It is much more driveable, but it seems to pull much harder in 3rd gear. Hard enough to change the 3:54 rear end gear to a 3:31. It has gobs of low end torque. In my opinion, my car is much quicker with the EFI. Plus, it looks better, too!

[scott coyle]

Cobrabill,

There is a peak hp improvement with efi. Mostly because of what map said in an earlier post. With a carb, you have to have a rich enough mixture to cover the leanest cylinder and you always have lean cylinders because of the varying length of the intake runners and the distance from the carb to the intake valve.

With efi, the o2 sensor in the exhaust collector can read a 13 to 1 air fuel ratio and I know that represents all cylinders. With a carb, a 13 to 1 reading would mean 12.7 to 1, in the rich cylinders and 13.3 to 1 in the lean ones. You would want to richen the carb to cover the lean cylinders. You would loose power but save the motor.

The increase at max hp is in the 3 to 5% range on a comparison basis for well set up systems. The biggest benefits are low end and mid range power which have to do more with big plenums hurting low end power in carbs because the fuel is so far away from the combustion chamber on tip in acceleration.

Scott

[Toby]

Easystep,

I went from a carbed system to Massflo EFI in my 347 ford sb and the throttle response is insane! There are also cold starting and drivability improvements too, but the throttle response is what really gets noticed. The massflo system also doesn't need computer tuning. It really is plug and play.

Toby

[Cobrabill]

Scott-sorry,but i've seen dyno results exactly the opposite.Heads/cam/intake (flow) determine HP(peak #s) NOT what is dumping the fuel in.

If you change to a EFI intake from a 4bbl manifold and carb and gain peak HP,that means you carb/manifold was not optimized for your set-up.

This is the KEY POINT here,folks-PROPERLY OPTIMIZED CARB/manifold for your application!


Slither-if you installed a Webber set-up you would have notice the same.The rumpty-rump of the cam goes away.This also happens on a in-line six when you put Webbers on.The fact that is happen when you put EFI on your car is happenstance.
If you dynoed the car before and after,you'd be disappointed.But if it feels better-enjoy.

[scott coyle]

I got my motor from a cup builder, 358 ci, 660 hp. The dyno was with a cup carb on a short track intake. We took the intake drilled it for injectors and had a 4 hole throttle body. The cup buider is the one that told me he was getting 30 to 40 more hp with efi on the same intake and cfm set up as a carb. He could run leaner and not burn a piston. This was done with the same intake for both set ups and the same cfm out of the carb and the throttle body.

I have seen some great carbed motors, well set up, they are cheaper and close to as good as efi on the top end. But, the efi will make more power in an apples to apples comparison with a good tuner.

Scott

[ItBites]

EFI can produce more max power than a carb. Period. The reason is in the physics. All other things being equal, cylinder filling is a function of manifold pressure. As we all know with higher manifold pressure, more power is produced (assuming correct mixture - remember this)... Thus blowers and turbos, but let's stick to normally aspirated engines for the time being to keep life simple.

For a normally aspirated engine, tha max pressure available is atmospheric pressure; lets call that 14.7 psia for now. If you could get 14.7 psia in the manifold (after the carb or EFI throttle blades) with the intake valve open, that would result in the max power out of the engine (again -assuming a proper fuel mixture).

Since a carb requires a vacuum (lower than atm presuure) in the manifold in order to provide significant velocity at the carb venturi(s) in order to properly mix the fuel, the manifold pressure at WOT cannot be or approach atmospheric pressure. As the manifold pressure rises, the velocity through the carb is reduced and the fidelity of the "signal" to mix the fuel becomes unstable and ineffective. This is simply the nature of carburetion. It requires higher velocities which must be obtained by providing restriction (venturis) and resultingly lower manifold pressures in order to produce those velocities.

EFI on the other hand can allow the manifold pressure to approach (it will not equilize to atm pressure or no flow would occur at all) atm pressure without the associated reduction in fidelity of the "signal" to add fuel since the "signal" comes from a computer. The computer can be programmed to recognize the right mixture, even as manifold pressure just approaches atm pressure. Effectively think of the intake valve opening to a giant box at 14.7 psi with the fuel pre-mixed at the right ratio for max power. With this simple analogy, it is clear that the sheer fact that the carb MUST create a restriction in order to increase the flow velocity to get the venturi signal means it CANNOT produce that same power possible from EFI.

Now, does this usually happen in practice? No, but the answer to the question of which system is CAPABLE of producing more max HP is EFI. System design and tuning play an important factor in the result (DUH). But with the best possible design for each system, EFI can produce more power.

BTW, frankly, unless you can explain the physics behind why you've seen EFI make less power than carbs, do not shoot back your anecdotes. Unless you've figured out how to break the laws of physics...

[Slither]

Cobrabill,
I did dyno this motor before and after. With a carb, it dynoed 603 horsepower. With the TWM setup, it dynoed 613 horsepower and that was de-tuned. We also went to a crank trigger setup and deleted the distributor!

[Cobrabill]

Slither-that's not exactly a fair comparison.Dumping the dizzy and going to a crank trigger made the difference.

BTW- i just noticed you had Dralle do the motor.He did my heads.He's a little different-ain't he?

[Slither]

Cobrabill,
Dralle is a little different. A little cranky too! But, he does nothing but live, breath and die by Cobras and Mustangs. He initially set up my car as a race car. Probably because he feels that Cobras belong on the track and not the street. It was his idea to go to the crank trigger.Have you ever gone for a ride with him in a Cobra around Willow Springs? It is a ride that you will NEVER forget. God bless him!

[BlackBDR]

Toby,

What was your before and after MPG, carb to Mass-Flo EFI?

Thanks,
John