Quote:
Originally Posted by Insider
Actually the above statement is totally wrong. Diesel's are the high-boost engines, petrol low-boost. The 335i has max boost of 8.5 psi, the 335D has max boost of 26psi. Diesel's work off heat, and need high compression and high turbo boost to generate lots of heat, this is how the fuel is ignited. The petrol relies on a spark plug so does not need the heat (would cause detonation, not a problem with diesel). Diesel fuel is very slow burning, so diesels by nature can't rev very high, thus their high torque, low horsepower character.
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This is right. Diesels have much higher compression ratios than petrol engines
http://auto.howstuffworks.com/diesel.htm
They need to run with higher compression ratios to generate sensible amounts of power. This means that they tend to be somewhat heavier and more robust than petrol engines.
One of the reasons why the 335i engine is all new is that the 330i engine was not really robust enough for turbo charging because of it's exceptionally lightweight alloy construction
http://en.wikipedia.org/wiki/List_of_BMW_engines
The 335i's engine is more similar to the e46 330i engine.
http://en.wikipedia.org/wiki/BMW_N54
Because of their inherent robustness diesel engines are generally easier to turbocharge than petrol ones.
The latest most competetive diesels are very highly turbo-charged, which is why turbo lag becomes a problem and why the sequential set up in the 335d and 535d is so damn good.
By contrast the turbo charging of the 335i is much more delicate. The turbos are relatively low power and by using 2 small ones turbo lag becomes a non issue.