Quote:
Originally Posted by ERTW
Diesels have "low vacuum" because they use turbos to fill the vacuum. By their nature they must open the valves late so that they don't hit the pistons. If they were naturally aspirated they would create lots of vacuum. The turbo is there to help fill the cylinders because of their limited lift duration.
if what you're saying were correct, then I should use a lopey cam and expect better mpg. the truth of the matter is engines need vacuum to run properly. If the intake valve opens too early you get lots of misfire. Vacuum creates the air speed that entrains, atomizes and emulsifies the air/fuel mixture, which creates a faster burn. A Toyota engineer gave me the Prius cam specs (which was at 40% BTE at the time). They phase the cam so that it closes at 100* ABDC, which means it opens the valve much later than typical...which means lots of vacuum. They give up a little pumping loss for better combustion and a much lower compression ratio (negative work).
I'm replacing broken sensors and fixing air leaks atm. My lean burn baseline should be around 40mpg...it's after that that I may need a little luck  |
I had a natural aspirated diesel and it ran no vacuum. Then when I turbocharged it, I ran the same cam and same timing because that's how the factory made the turbo version.
I have a 5.7L 1989 firebird with a t-56. One time I actually drove the speed limit it got right about 30mpg, but ain't nobody got time for that. If I could run lean burn I could easily eeek out another 10%. I installed an eprom with the early 1990s tune on it that caused the engine to favor a lean burn "under highway driving conditions", said to be good for about 1mpg, so a pretty mild lean burn.
My next mod is I found another t-56 locally and bought it. But its not just any t-56, it's the t-56 with the 0.5:1 topgear. My current t-56 has the more common 0.62:1 topgear.
It has plenty of power has no problem going over east coast mountains on I95, I81, I40, I84 in 6th gear so I figured might as well over drive those rear wheels a bit more.
The only time it lacks power in 6th is towing and the solution to that is called 5th gear.