Bentley 6.75 Turbo

most probably engine management with a differently sized turbo.

cummins and detroit diesel have been doing this for years on their 12.7L engines.

Turbo size is generally governed by other breathing factors and ignition timing map. That is you can't just turn up the boost. you have to balance charge temperature, exhaust backpressure etc. Essentially a bigger turbo requires a more efficient engine if its going to make any improvemnt.

those numbers of yours suggest they have changed the valve timing to miller cycle (Plus 28-30 degrees of inlet opening). This also requires a significant boost in either engine compression ratio or turbo boost pressure. It often means a bigger or more efficient intercooler (particularly if you go the boost pressure route).

they are getting about 75 HP per litre which is pretty hot for an off the shelf car!

I have to say that that sort of increase probably means lots of other 'minor' alterations as well such as different bearing shells, head gasket type etc.

Why do you ask?

they are getting about 75 HP per litre which is pretty hot for an off the shelf car!

In 1960, anything over 60 BHP per liter was considered pretty efficient. But now, 75 BHP per liter is very much run of the mill, even for normally aspirated sedan engines.

The Volvo s60 T5, a very nice soccer-mom sort of sedan, gets 257hp from its turbocharged 2.4 liters (over 100 BHP/liter). These days, 75 BHP per liter is considered very low performance for a motorcycle engine (with 1000cc crotch rockets routinely producing 150 hp, or more), moderately low performance for a normally-aspirated sports car (with the Honda S 2000 putting out over 100 BHP/liter) low for an econocar with sporting pretensions (the Honda Civic SI produces 98 BHP per liter), and even a bit low for a basic econocar (the Honda Civic sedan gets 140 BHP/1.8 liter = .77BHP/liter).

The standard Porsche Turbo puts out 480 HP from its 3.6 liters, and the S version will probably put out 510 (141 BHP/liter -- more than twice as efficient as the 6.75 liter 450BHP Bentley).

So, although 75 BHP/liter could hardly be considered "hot," one could at least expect the Bentley's engine to be long-lived, and the cars can be very pleasant to drive.

400HP isn't very impressive for a twin-turbo 412CI engine, you're right.

However, Bentley is much more interested in Torque, which is why this 5700lb car can do 0-60 in 6.0 seconds, and tops out at 155 (electronically limited). The new 500HP version I believe is doing 0-60 in <5.0 sec, and tops out somewhere above 170.

There's an old quote, I can't remember who said it "Horsepower sells a car, but torque moves them"

And there is a great deal more to a car than HP values. The Bentley shines in many areas.

It may very well be just the chip. I was really disappointed when I found out my Porsche Turbo didn't have the promised 450 HP because it didn't have the X50 option... The factory tweaks the computer to turn on the extra 50 HP. And I couldn't find anyone willing to touch the chip for me. It was like they were being watched or something. Go figure!

My family is in the process of buying an Arnage R with the "low performance" 400HP engine, and I have not been able to find out what the difference between engines is.

The turbo's stay the same accross all variations of the engine (Garrett T3's). As they are all hand assembled, it's entirely possible that each version may include different parts, since you wouldn't have to deal with changeover, or multiple lines.

The upcoming Bentley Brooklands coupe is supposed to have 550HP and 778 lbft out of the same engine with the same turbos.

I don't expect the difference to be noticable without a stopwatch!!! The 400 BHP version may even use less gas as a result......