Quote:
Originally Posted by Formula413
I will never give up my LT1! 
I never thought of that, very interesting. I'll have to give this more consideration and research. I'm leaning towards a Mazda3 with the Skyactive direct injection engine for my next car. |
Hey Formula, are you referring to the 1st gen LT1 of 1970-72, the opt spark LT1, or the new released for 2014 6.2L GDI LT1. The new GDI engines are more efficient than any previous LS or LT (at least until the intake valves begin to coke to the point of disrupting the airflow, after that they degrade quite quickly in efficiency). The LS was more efficient than the 90's LT1 due to a much more robust PCM and improved cylinder head design as well. After a GDI engine's intake valves begin to build more than a light deposit (generally 5-10k miles depending on engine and make) the airflow into each cylinder is affected so the cylinders with the more severe build-up (those closest to the point of entry, the PCV connection to the intake manifold) will have less air entering so they run richer, the ones with the least amount of coking, those furthest from the point of ingestion, will run leaner as the upstream O2 sensor for that bank is reading the bank as a cumulative whole, so the PCM commands the same fuel amount for all cylinders in that bank, so they soon become less efficient in not only power and fuel economy, but also hydrocarbons released from a less complete burn.
As designed when new, they are a good deal more efficient for the following reasons:
Fuel is atomized far better exiting the injector at 2000-3000 PSI than 45-55.
Compression ratio is much higher (11.5:1 common vs old 9.5:1 or so) due to
the elimination of fuel present during the compression stroke. The fuel is not introduced until mere milliseconds before ignition, and this requires a different complex topography of the piston top resulting in most of the fuel being burned before it can make contact with most of the piston top.
The greater amount of ignition timing advance also produces a more efficient burn and thus more power/MPG.
So tons of advantages to GDI, just the unexpected negative of fuel no longer touching the backsides of the intake valves allows deposits to begin forming as soon as the engine is run, and as there is no cooling spray of fuel either, the valves operate at a far higher temperature than port injection, so these are not soft deposits like the carbureted or port injection engines of past, but very hard crystalline bakes on deposits, thus the wear to the guides as every cycle that valve makes is pulling this abrasive coating into the softer guide (most are bronze alloy) as well.
Anyone wanting more in depth discussion or questions answered, just ask. I am not able to spend much time due to work load, but always willing to share as this is the field I work in daily.
