I believe most if not all petrol engines can be fitted with a vacuum gauge, because let's face it traditional engine design has a manifold where air and fuel are mixed before it is sucked into the combustion chamber. But the MAP sensor measures manifold air pressure to feed to the ECU to control fuel intake, so why not just see what it's controlling and make adjustments to the way you drive to save fuel when you need to purely by having a visual representation as to what your foot and choice of gears is doing.
Locations for a tapping into the MAP sensor feed vary, some go as close to the manifold as possible other read directly under the sensor, but the feed is just a rubber pipe often clogged that effects economy, and a rubber pipe that can be got easily so experimentation as to where the best place to read from is no big problem.
Traditionally users of older cars without ECU's have tapped directly into the manifold via a screw fitting which requires removal of the manifold to drill and tap, or in some cases manifold vacuum adaptor plates were available for sandwiching between the carburettor and the manifold. My old '77 1600 VW camper had one of these fitted under the carb with a long rigid pipe running to the dash where the gauge was a Gunson's Lo-Gauge, far more useful than the TIM gauge I have fitted on my Clio now and I even adapted the Lo-Gauge for lighting.
But just some of the things a vacuum gauge can diagnose ;
http://moodle.student.cnwl.ac.uk/moodle ... gauge.html
http://www.secondchancegarage.com/public/186.cfm
http://www.iwemalpg.com/Vacuum_gauge.htm
Hey save money on car repair bills too, as many a modern mechanic just guesses instead of tests.
But as of diesels I am not sure as it has not been my area of interest, but one thing I do know the turbo fitted to some to bring the performance up to petrol car performance standards and beyond is the cause of poor fuel economy and there the hole in the pocket, you want the economy of a traditional diesel, don't buy a turbo diesel unless you have the ability to disengage it when you don't want the performance as performance on a diesel equals fuel detonated at higher rates and pressures.
But of diesels, an engine originally designed as a cheap power plant for German agriculture to run on vegetable oil, how long will it be before the diesel engine in it's present form, the exhaust gases produced from burning hydrocarbons are linked to cancer in humans and with that the unrestricted jet engine efflux in the air we breathe ?
As it is even our own H.S.E describes at least four known carcinogens in diesel exhaust gases that those who work with diesel engines have to take precautions against, yet we breathe it every day and in towns it is the worst, some say is like smoking forty a day.
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