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Exhaust Cutout Problems?

m4ld0.^

m4ld0.^

Loddi Doddi
Joined
Aug 18, 2005
Location
Raleigh
So, I've been throwing together a list of things to continue the build-up of my 244+T. I searched the forums, but didn't really find any answers to the questions I have in mind.

One of the things I have on my list is an exhaust cutout. Nothing fancy, no electronics or even manual cables, just a cutout with a block-off plate. The block-off plate has 3 wingnuts that unscrew to take the plate off and allow the exhaust to dump. My question regards loss of backpressure and its effect on a turbo. I know plenty of people have side exits and I've seen cutouts on other volvo's, but would there be any problems with not enough backpressure for the turbo?

I'm not really going for any super performance gain with the cutout, just a fun thing to have. I obviously wouldn't be running around with it open all the time, but when I do want to be loud I could be loud by unscrewing those wingnuts. So is there anything I need to be weary of before this makes my official list?

I have a 2.5" exhaust with a 13c +T if that plays into it all. Thanks
 
Backpressure is a bad word around here. Exhausts don't need back pressure they need gas velocity. Turbo cars need as little as possible. The pressure difference across the turbo is what makes it spool. So with your cutout the turbo should spool alot faster
 
With a turbo system, look at the exhaust as two parts:

1) Before the Turbo... you want this to be high speed and in a sense high pressure. This is the most important part of a turbo exhaust system, and the reason a good manifold is such a huge deal with turbo tuning. This side has the biggest influence on potential power of the system because it controls how much force will go into spoiling the turbo. It has a lot of control over over how responsive the system will be. As stated, this is the most important side of the turbo exhaust system, and has the biggest influence on all aspects, period.

2) After the Turbo... you want this to have as little resistance, ie back pressure, as possible. This side has some power control(less back pressure=less power needed to move the turbine), but response is the biggest result of freeing up the back side since the turbo can spoil more freely.

The only real concern in opening up the back side is that you don't do it so much you get exhaust gas stalling, leading to hot spots, and even increased back pressure if done bad enough.

The system you speak of sounds ok, but you might want to make it so that all the exhaust will leave the cut out so that you don't have any exhaust gas stall after the the cut out in your regular system, causing a hot spot. And if your cut off pipe is the same 2.5", or even larger like 3", you should see a power gain, but more importantly, a response gain.

A good read is Maximum Boost by Corky Bell. It does the best job, IMO, of going over the in's and out's of a turbo system.
 
Karl Buchka said:
Wow buddy, you probably shouldn't post replies like this if you're not sure what you're talking about.
Click to expand...

Sorry, did I miss understand what he was looking to do? When I hear exhaust cut out and dumping, it makes me think of fabricating a plate system that allows him to have a second exit for the exhaust before the muffler, thus a louder sound as he wants, and in the more common desire for it, a single exhaust system for a car that has dual duty as street and drag car. Correct me if I am wrong, but this was a common practice among hot rodders and draggers back in the day, and still is fairly common among this crowd.

And I didn't mean for it to come off like I was assuming he didn't understand how a turbo works(I am sure anyone that adds a turbo to their car has a plenty good understanding of the entire turbo system). His main concern was back pressure issues, so I explained how I look at a turbo exhaust system so that he would understand my answer better.

And as for exhaust gas stalling, that probably isn't the common term for it if there even is a common term, and it is very rare and not much of an issue to worry about unless you are doing a cut out, or use way too large of pipe in the system. I have only heard of this phenomenon a couple of times, once in a book on turbo charging (It had pictures of what it looks like too) and once from a friend that did a dump pipe right after the last collector... the regular exhaust tubing would get really, really hot a couple feet after the cut out when he opened up the cut out. He put in a redirect plate, and it quit doing it.

As for the book, it is good, check it out. Corky Bell has designed and built hundreds of racing turbo systems. No matter your experience level, the book is a good read, and has some very useful and helpful tips and tricks from a man considered to be one of the foremost experts on the subject of turbo charging.
 
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