Carbs vs FI Options

[Type 4]

Here is the dyno sheet for the 2054cc engine with carbs.

[lapuwali]

Which ECU is, IMHO, not all that relevant, assuming any ECU can be tuned appropriately. What IS relevant to this particular comparison is the fact that carbs were swapped for throttle bodies. If you plot the two power curves on the same scale, you'll see that the difference in power rises as the revs rise. Under about 4000rpm, they make the same power. Above that, the EFI engine's curve rises away from the carb'd engines curve. This strongly suggests that the carbs used in the comparison were too small, and were flowing too little air at upper revs. The freer flowing throttle bodies kept flowing, so they produced more power at higher revs.

Jake's results show similar curves when layed atop each other. The divergence happens a tad earlier on those graphs (3500 rather than 4000), but it's still close.

This doesn't mean this is an unfair comparison. One of the advantages of EFI is that the effectiveness of the fuelling is decoupled from the air velocity through the throttle body. In a carb, if air velocity falls too much (carb too big), fuel control gets very erratic. So, an engine with big carbs tends to suffer from poor drivability, so for a daily driver, you tend to deliberately run carbs that are too small to keep good fuel control. In an EFI setup, flow velocity is irrelevant to fuel control (but NOT completely irrelevant to power), so you can run bigger throttle bodies with EFI and retain good drivability, thus getting the high rev benefits of big throats with none (or few) of the downsides).

If you used, say, 44IDFs with 36mm venturis, and compared them against 36mm throttle bodies, you'd see very similar power figures. Comparing 44IDFs with 50mm Jenvey throttle bodies is not an apples to apples comparison, even though a well-tuned EFI setup with those TBs would probably drive just as nicely as the same engine with well-tuned 44IDFs.

Sanity is restored. Now I understand how this 30% number is possible.

So, bolting aftermarket EFI onto a 914 engine using the stock intake plumbing will NOT give you a 20-30% power increase. It MAY help if you use a larger throttle body. Using a throttle body that flows more than the engine will take offers no help at all to power, and only makes the throttle action "jumpy", as you're changing the ratio of throttle pedal movement to airflow into the engine. Since it's very likely (but unconfirmed) that the stock throttle body is adequate for an otherwise stock 2.0, it's very unlikely adding a bigger one will help. So, you're not going to see much, if any, power increase by using any aftermarket ECU with an otherwise stock engine in place of an otherwise properly functioning D-Jet or L-Jet system. 5% is pretty much the outside, by simply cleaning up the fuel delivery a tad.