Hello everyone!

To keep this as short as possible, would adding a smaller turbo to m52 bmw engine increase mpg? As it would spool qucik and fill the gap in torque in lower rpm? Not looking for more power,just better city mpg

 

Power with boost typically requires more fuel per hp so no real mpg gains to be had.

 

Thanks for the reply, makes sense!
My logic was, if dieseles have peak torque at lower rpm, thus lower consumption (also other factors play role in lower consumption part ofc), it might be a good idea to have smaller turbo spinning the engine faster to its peak torque rpm range.
The idea was to spend as much time as possible in that range while driving to reduce consumption, as any kind of accelerating uses a lot more fuel than just driving.
Also a remap etc..

 

Diesels are totally different animals, they're built for low speed torque production. Diesel can be run very lean, and efficient thus the better mpg.

 

Yeah, thank you!

 

Diesel also contains about 15% more energy by volume.

Using boost to keep the rpm down is sort of a mixed bag. Depending on your gear ratios, a little math will tell you if its better or not. Most gas engines run about 14.2:1 air fuel ratio, and go up to 13 or 13.5:1 under load, if you are in boost, you are under load expect to run around 11.5:1 afr, so is it better to change gears, or run richer with the turbo spooled?

Ultimately I think if you are doing this with the expectation of better performance, and willing to loose fuel mileage (simply because you are going to be able to go faster) then your on the right track. If you are expecting a turbo to increase mileage I think you will be disappointed. Unless you are going down in displacement and making up for it with boost you will very likely use more fuel.

There are also other questions to ask yourself. If you have increased power by 50% have you increased braking to match it? What about the rest of the towing capacity factors, how is your suspension going to behave at these new found speeds?

 

Turbos do not add fuel economy. The turbine is a restrictive component of exhaust and will reduce economy through pumping losses of a cyclic combustion engine.
The compressor side spinning will further induce velocity vector component of air intake charge which will reduce pumping loss associated with bringing the air to the throttle body, but this will create additional intake manifold vacuum since energy requirement to pull velocity fluid component of air towards the engine is reduced (the piston is no longer providing the energy- the compressor is) but the increased vacuum in the intake manifold will induce increased pumping loss of the descending pistons (working against a stronger vacuum costs more energy) And unfortunately when you add it all up there is a net loss of energy and economy.

On the bright side turbo make car go vrooooom

 

Are you adding a turbo or switching to a smaller turbo?

Switching to a smaller turbo will improve mpg by encouraging use of lower rpm. Assuming CR and tune is optimised for economy.