FollowupID: 611403 Submitted:
Saturday, Jan 10, 2009 at 11:44
Thermoguard Instruments posted:
Hi Lance,
I'd like one of these exhaust
places to come up with some proper
technical evidence that "The 3L turbos need a little back pressure". What a load of cobblers.
Turbo diesels do not 'need' any back pressure. All back pressure does is reduce the potential energy that is available to drive the exhaust gas turbine. This energy is related to the difference in both pressure and temperature of the exhaust gas in and out of the turbine. The cooler and lower the pressure at the outlet the better.
Now about exhaust sizes, let's suppose a 3" system is the optimum size for a typical 4.2L TD that may well have been fuelled-up or chipped. This seems to be the general consensus.
If this is agreed, then if it follows that a 3" system on a 3L is probably a bit of overkill. The capacity ratio is 3.0/4.2 = 0.71.
The cross-sectional area of a pipe (let's assume all sizes are inside diameter) is proportional to the square of the pipe radius. So, a 3" pipe is about 7.07 sq in., a 2.75" pipe is about 5.94 sq in and a 2.5" pipe is about 4.91 sq in.
The pipe area ratios are: 2.5": 4.91/7.07 = 0.69; 2.75": 5.94/7.07 = 0.84. Therefore a 2.5" system on a 3L is a little less than equivalent to a 3" on a 4.2 and a 2.75" system is a bit more than equivalent. Given that a 3L will typically have a higher specific output (kW/L & Nm/L), the 2.75" would seem the best choice.
All that said, I doubt many would detect any difference between equally well-made 2.5, 2.75 or 3" systems in 'blind' testing, so whatever you can get for the best price would probably do quite well. The quality and restriction of the chosen muffler will probably have a lot more influence on the overall result than the actual pipe size!
Also, people seem to get so obsessed with "mandrel bent". Unless the system is really contorted, I doubt the small extra restriction of a few conventional exhaust bends would have a lot of overall effect. If I could get a conventionally bent 3" system for significantly less than a 2.75" mandrel bent system that's what I'd go for.
Of course, Land Rover owners don't need to worry about any of this. Their 2.5L TDs come standard with properly-sized (65mm = 2.56") systems... And should you have trouble eating any STANDARD Japanese 4WD up to and including 4.2Ls, just cut-out the main muffler and replace it with a bit of straight pipe - they're still quiet enough. (And, Yes Virginia, this last paragraph IS bait...)
Ian
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