Ask ExhaustVideos – Diesel Exhaust System Performance Questions

Erik asked us a lot of great questions on one of the FAQ pages, and I moved his question (and my response) over to the blog because there are a lot of people wondering about these same questions.

Here’s what Erik is wondering about:

I’ve got some thoughts about the exhaust system for a diesel – it’s regarding a 4.0 V8 diesel twin turbo. My goal is to optimize the exhaust system for performance.

Stock, it came with a true dual exhaust system with dual catalytic converters (one on each downpipe), and after that both pipes join into one large particulate filter, and then the pipes split up again into dual mufflers.

I have removed the particulate filter and replaced it with straight pipes, dual pipes from one end to the other. Thus the exhaust system does no longer have a crossover section there, as it had before. The mufflers after the particulate filter where replaced with straight pipes aswell. The replacing pipes has the same diameter as the stock pipes has otherwise.

1) How important is “back-pressure” for a twin turbo diesel engine?

2) Does removing the crossover section effect performance? Would I gain more power from having a crossover X- or H-pipe?

3) Does removing the mufflers effect performance?

4) How much can I increase the size of the pipes in order to gain performance? They are quite large even stock, I think the downpipes are dual 3″(or 2.5″) and then the pipes narrows to dual 2.5″(or 2″).

5) Will I be able to gain any performance by removing the stock cats? Replacing them with either sports/race cats or straight pipes.

In terms of boosting performance on a modern diesel, removing the particulate filter is a great way to add power as they are very restrictive…of course, it’s also bad for the environment and potentially illegal (every state has their own laws on this, but federal law prohibits removing this filter for sure) to remove this filter, so I would advise anyone removing their filter to be cautious and consider the ramifications.

OK – now that I’ve gotten that disclaimer out of the way – on to your questions!

1) How important is back-pressure on a twin-turbo diesel?

One way to think about backpressure is that it’s a “pipe diameter shrinker,” meaning that a little back pressure helps compensate for the fact that most exhaust systems are over-sized for an engine in the lower RPM range. A small amount of backpressure improves performance at lower RPMs mostly because the factory designed a little backpressure into the engine.

When it comes to a twin-turbo diesel, backpressure caused by mufflers is less important than it is on a normally aspirated engine. This is because the turbochargers themselves create enough backpressure in the system to facilitate scavenging. Therefore, you really can’t hurt a diesel by removing mufflers or reducing restrictions in the exhaust system.

2) Does removing an exhaust system cross pipe effect performance?

To my knowledge, no. X-pipes or h-pipes are used to balance the exhaust note on most vehicles, and they can help improve tone as well by mellowing out some of the higher frequencies. However, purely from a performance standpoint, they have no impact (only to be clear, I’ve never run a computer model to prove that conclusively – I’ve just never seen that setup add power on the dyno or at the track).

3) Does removing the mufflers effect performance?

Yes it has a positive impact, but again it’s not much on a twin-turbo diesel. The turbos provide enough backpressure for the system – the mufflers are mostly there to mute the exhaust note.

Speaking of which, diesels with straight pipes sound quite a bit better than gas engines with straight pipes. While a gas engine with straight pipes can sound like a top-fuel dragster at idle (and that’s too loud for my tastes), a diesel with straight pipes sounds a bit like a jet engine.

4) How much can I increase the size of the pipes in order to gain performance?

When it comes to diesel exhaust systems, I’m not sure that there’s a limit. I’ve seen numerous big trucks with 5″ exhaust systems, and they definitely have plenty of power. Having said that, 3″ seems plenty big for a 4.0L engine, and the factory size of 2.5″ is probably appropriate.

5) Will I be able to gain any performance by removing the stock cats?

Perhaps – replacing stock catalytic converters with racing catalytic converters could boost flow rates and give you a slight boost in power, especially on a turbo-charged or supercharged engine.

However, modern catalytic converters are not very restrictive – I’ve seen numerous situations where removing these on a stock engine had no appreciable effect on performance.

The fact is, if the catalytic converters aren’t clogged, there’s nothing in them that will restrict exhaust gas flow. Considering their environmental benefits, I would discourage you from removing them.

Hope that helps – good luck!