You need to be aware the is much speculation to these swirler's, as to the benefit derived from fitting them.
Increasing turbulence in the intake manifold changes air-fuel ratios of lean burn and can alter the emission standards on a modern engine
savings are speculative but an average of 5% can be expected , with the negative of added carbon emissions.
Adding inline fuel savers such as Fitch work as they agitate the fuel and clean impurities as well.
In the USA there is a standard where they are supposedly to qualify for emission standards, but it takes time to find them.As It's a general question
Have a look at these links and make up your own mind.
I use the Fitch fuel in line savers however it's in conjunction with fuel additives and twin oil in line filtration systems for maximum emissions standards, the cost you would not take it on it's too expensive as to the fuel savings in the long term , I do it for environmental testing For the European Justice Commission for transparency to comply for Euro 4 and 5 Testings on Nox and Co2 emissions as a guide line for Mercedes Sprinters engines down under.
I know they work on the Vito as these engines run hot But the sprinters, I don't know.
Links have good information
worth the read.
Richard.
Directory:Fuel Efficiency Retrofits - PESWiki
Fuel saving gadgets - a professional engineer's view
http://www.thegreendirectory.com.au...-Australia/Fitch-Fuel-Catalyst-Australia.html