carb vs. efi

[Gregory R. Travis]

On Fri, 12 Sep 1997, Jim Davies wrote:

> > He argues that no top racer (NHRA is his game) runs efi.  I admit
> > that's probably right, but the reason isn't because it's not better,
> > but rather that it's too danged difficult to change at the
> > track ....
> > 
> > what do the Formula 1 guys (and others) do ??
> > 
> > Like I say, he'p me here .... 
> > 
> He's got a point. In any more-or-less steady state situation I think 
> carbs can compete, and may be better in some ways. When speed/load vary 
> as much as they do in road vehicles (or F1), well...
> 
> For an exhaustive discussion on this subject, look at the 1935-45 era and 
> the development of aircraft piston engines. One of Ricardo's editions had 
> a good discussion on this subject, IIRC. Basically, though, it was the 
> Brits vs the rest of the world, with the Brits defending carbs and 
> everyone else going to injection (or injection carbs)

Jim, and others, have hit the nail on the head.  The performance
differences between carbs and EFI vary as a function of two variables:
Frequency of state change and percentage of total output.

If the engine application is one that finds the engine in a roughly
steady-state, high power, situation then EFI gives you nothing over
continuous injection (CIS).  CIS, in term gives very little over
carburetors.  What it does give there is much finer control of individual
cylinder air mass/fuel control.

On the other hand, if drivability (meaning response to widely and frequently
varying throttle settings) is important then EFI wins hands down - especially
closed-loop EFI.

As for power output, virtually everything I've read on the subject says that
there is virtually no difference on a four-stroke engine between sequential
port injection EFT and continuous injection above about 2000 RPM.  Sequential
helps emissions and idle smoothness when the engine is loafing but as
the pulse width or pulse frequency increase with increasing RPM the
advantage over CIS starts to close until, at around 2000RPM, it's gone.

Aircraft engines fitted with CIS injection typically outperform the
same engine, fitted with a carb, by a BSFC of around 0.02-0.04 or so.  The
difference is entirely because the CIS injection allows the engine to
be leaned more aggressively before roughness starts.  There is no difference
in power.

gregory travis		greg at littlebear.com	http://www.prime-mover.org