Snorkel

The effect varies a lot between vehicles and can be negative on some vehicles.

95series TD Prados typically use 0.5-1 L/100k MORE diesel after you fit a snorkel.
Why? The standard air intake is shorter and better designed than the common snorkels. Air is directed from behind the LH headlight into the LH mudguard and this pressurises the space in the mudguard where the air is drawn. The snorkel has at least two 90 degree bends and is a metre longer than the standard intake.
There is no significant air ram effect because the top has 4 water bleed holes and the air cleaner box has two rubber valves in the base to let out water (and air!).
And any difference in temperature and pressure is made insignificant by the turbo.

I suspect thats why normally aspirated diesels may see some improvement and turbo diesels are unlikely to see improvement.

Reputable brands such as Safari and Airtec make no claims regarding improvements in fuel consumption or performance. That's because on most vehicles there are none.

Phil, I agree with your comments on the prado (hence my reply above) I dont think my prado went as good with the snorkel fitted, seemed to lack some puff at higher speeds then when I didnt have it fitted. My fuel usage seem to go from 10.2 L per 100 to over 11's freeway driving and around town 13's to mid 14's. It seemed like it was having to suck harder to pull the air through. Your comments about the bends etc make sense. Regards Steve

HDJ100 (Factory TD) comes with a better exhaust than the 1Hz, but unfortunately has a cat converter. If the cat converter is blocked by soot, then you'll see a difference with the Beaudesert exhaust, but if the cat converter is OK, then the difference is minor, and not noticeable when driving normally.

After writing this I noticed the above

I agree with reply.

A 3" exhaust mandrel exhaust will give you far more benefit than a snorkel, by a country mile.
Did on mine anyway.

I 2nd that. And if you haven't already got a chip in it, get one of those too. Much better response in lower rev range.