Apples to apples, how does i mileage compare to eta?

[dlhoovler]

Most of the threads I've seen discussing fuel economy don't really address one of the biggest differences between the i and eta setups. I suspect that the engine differences are possibly far less significant than the extra point in the differential ratios between the two cars (early eta's 2.79 to i's 3.73).

Has anyone ever been able to do a gas mileage comparison between a 2.5 "i" engine and 2.7 "eta" engine with the same gearing?

-Dave

[jlevie]

If just a change in differential ratio would have "done the job" BMW would not have gone to the trouble (and expense) to produce the ETA engine in the quest for a more fuel efficient car. The differential is just a consequence of the engine's realm of operation.

[VinniE30]

The eta is a 2.7 L with a longer stroke. Everything was optimized for lower rpm torque. The eta has even more peak torque than the i, and it has way more torque at low rpms. There are many differences in the engine that contribute to the better fuel economy.

The longer diff keeps rpms lower on the highway but there are other reasons for it getting that lower diff ratio. Lower ratios are better suited for engines that make more torque at low rpms. Also since the rev limit is way lower the gears would be very short with a higher diff ratio like the one the 325i got.

[F34R]

I have never had the same gear ratios in a b25 and b27 to test, but I got better MPG with my b27/2.93LSD. The low end power with the top end ratios make it a perfect daily driver IMO.

[dlhoovler]

Yes, I know that there are significant differences between the m20b27 and -b25. But my main point was a question of how much BMW actually gained with the engine tuning alone. To eliminate one big variable, has anyone ever put a 3.73 under an eta and kept track of fuel economy on it?

Alternately, has anyone kept the 2.93 with a 2.7i/885 head swapped eta conversion and noted the resulting mileage?

FWIW, I've seen about a 2 MPG reduction going from the stock chip in my eta to a MarkD chip, so the economy-minded fuel and timing maps definitely make a big difference. I went back to stock once for a few weeks just to see if the performance increase was really worth it. Yes, yes it was... :)

-Dave

[mrgraphics]

These aren't exact, more theoretical to make the point:

ETA with 2.93 - 2000 RPM - 70 MPH

ETA with 3.73 - 4000 RPM - 70 MPH

So which one will be getting better gas mileage?

The above explains why the optimal diff swap for an ETA is the 3.25.

ETA with 3.25 - 2500 RPM - 70 MPH . . . not that bad of a hit fuel economy wise.

[dlhoovler]

OK, for some reason my point is not getting across at all. Maybe it's a stupid one. I know that the eta gets better gas mileage than the i, and it's due to a combination of the engine mechanicals, management software and gearing. I know that an eta with shorter gearing will get worse fuel economy. I think both of those go without saying.

What I'm wondering is what the fuel economy difference would be between an eta and an i engine with similar gearing. Like I said, maybe it's a stupid question, since the engines are tuned for such different RPM ranges, but I was just curious. The most practical application would be a 2.7i conversion: What happens to economy when switching to the i head, intake/exhaust and ECU but retaining the eta gearing?

-Dave

[SmokeE30]

Most of it will come down to driving style and vehicle maintenance, theres no way to say difinitively that a e engine will get Xmpg vs and I engines Xmpg with the same gearing. You can pull 30 mpg highway out of either one stock if you drive it conservitively and they are running properly