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Ram air intake, bigger exhaust and propane.
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Ram air's not going to do anything for off-the-line, right? No high-pressure area to "ram". And the pump'll have to be turned up to send more fuel to take advantage of the extra air, but since the extra air will only be available when you're moving at a pretty good clip, you'll get more smoke for slow-speed-heavy-foot operation.
Is the OEM exhaust really that bad, that increasing it makes a measureable difference. Sure, it's a big engine, and yes, diesels move a lot of air due to the intake always running at "full throttle" so to speak, but recall that this engine tops out at around 3300 RPM, it's not going to move the volume of air that, say, a 460ci engine will at 5500 at full burner.
I know that aftermarket exhaust is marketted as yielding more HP, but other than mfgr claims, does anyone have personal experience (with numbers) that replacing only the exhaust has improved performance (other than sound "quality")?