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This is from another forum, going from a 2.5" exhaust to a 3" exhaust. There was a little more boost added and the downpipe type was changed from a 2.5 split straight tube downpipe (this is for a gasser) to a 3" split bellmouth downpipe.

Quite an improvement! We cannot see how much was due to the exhaust diameter alone and not to the amount of gain is definitely more than just due to higher boost.

 

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I saw that too. I think that they said it was also tuned. So for 70-100 ft lbs and hp gained, lets say, 50-70% of that was due to tuning and the ability to raise boost due to better exhaust. So about 30-50 hp and torque was due to the exhaust. This is all a guess, of course, since there is no apples-apples comparison.
 

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for a 300zx twin turbo...upgraded fuel system, and upgraded turbos and supporting mods. It's a 3.0L gasser. This person had the car fine tuned and switched from a 2.5to a 3" exhuast so it's not an apples-apples comparo.

Yeah, just looking at it you can tell it's not a diesel. The powerband and the rpm range....I don't think there are any passenger car diesels that spin to 6600rpm :) His point was the exhaust upgrade.
 

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yep, the point is that going to a larger exhaust will always benefit you...if you're a turbo engine.

Keep in mind that the 300zx is a dual layout, each side is 1.5 L and cam go to almost 7000 rpm
A TDI is 1.9L but it maxs out at about 5000 rpm.
 
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yep, the point is that going to a larger exhaust will always benefit you...if you're a turbo engine.

Keep in mind that the 300zx is a dual layout, each side is 1.5 L and cam go to almost 7000 rpm
A TDI is 1.9L but it maxs out at about 5000 rpm.
I totally agree with you about the larger exhaust almost always being better on turbo cars, but again, this example is somewhat lessened due to the lack of an apples-apples.

PS, DAANG, that's a lot of power!
 
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