Exhaust back pressure

Dessertrunner · #1 10-24-2007, 03:27 AM

Has any one mesured the exhaust back pressure before and after they changed it. I recently read a artical were they tested it before and after to see how good a job they did. I will try to find it again.
Tony

Dessertrunner · #2 10-24-2007, 03:33 AM

here it is

http://autospeed.com/cms/A_108185/article.html

Trevor · #3 10-24-2007, 04:27 AM

Quote:

Originally Posted by Dessertrunner

here it is

http://autospeed.com/cms/A_108185/article.html

All should thank you Tony, well worth posting.

In the past there has been some absolutely stupid stuff posted in regard to tuned length by one in particular, together with lots of carry on about cat back stuff. All should note where the important tuned length starts and finishes, i.e., at the first point of expansion.

The after market thieves, knowing their market is restricted to the exhaust system only convertor onwards, whistle some outlandish tunes.

BY the way, a couple of times in different threads I have suggested that a few words from you, regarding your off road experiences, would be well received. Did you miss my posts?

Thanks again, Trevor.

Dessertrunner · #4 10-24-2007, 04:30 AM

I will get the photos etc together for you over the next week.

bwb3 · #5 10-24-2007, 09:23 AM

Thank you for sharing. That is a must read for anyone contemplating exhaust modifications.

Gene

Myxalplyx · #6 10-24-2007, 10:37 AM

Autospeed has some nice articles. They influenced me into making a ram-air setup with one of my XT6s with the stock box (Negative Boost Article). I like their articles but some people do not.

Would be interesting to see what the stock SVX exhaust back pressure is measured at.

Quote:

Originally Posted by Trevor

The after market thieves, knowing their market is restricted to the exhaust system only convertor onwards, whistle some outlandish tunes.

Trevor.

immortal_suby · #7 10-24-2007, 07:53 PM

Quote:

So (and this is the classic case that I love repeating each time we do a story like this!), by looking at the measured pressure drop after the filter (7.2) and the pressure drop before the filter (6.2) we can see that the pressure drop across the (new) filter is just 1 inch of water, or 0.036 psi! In fact, as it is always the way when you measure pressure drops, the filter contributes stuff-all to the total intake flow restriction. In all the testing we have done, we have never seen a factory filter contributing more than a trivial amount of restriction to the total – never! So, upgrading the filter in the standard airbox is a complete waste of time.

Very interesting!
From this link from that article.
http://www.autospeed.com/cms/A_107827/article.html

Dessertrunner · #8 10-25-2007, 02:36 AM

I have an old O2 sensor that we will drill and hook up a hose to get a reading from on the weekend. WE have two SVX's wifes with 140,000 kilometers on and mine with 420,000 both have the standard orginal exhaust. Mine has bent and squashed pipes do I am interested to see the difference in the readings. Should be fun. Will let you know how I go.

Myxalplyx · #9 10-25-2007, 06:25 AM

Good luck! Make sure the wife is a co-pilot (or someone) so you can get an accurate reading. Do not want to read about an accident due to reading the gauge while driving.

Dessertrunner · #10 10-26-2007, 08:25 PM

Took out O2 sensor and measured the back pressure on my car 420,000, would have thought the exhaust would have been all blocked up but it really didn't give any reading till 4,000rpm and that was about 4# at around 6,000 it was 7-8# was a bit hard to read as the engine had to be under load and at that reves you had to change gear before it red lined. I have a few minor leaks in the exhaust but with all the bashing from rocks and age you would expect for it to be higher.
Tony

Myxalplyx · #11 10-26-2007, 08:35 PM

Quote:

Originally Posted by Dessertrunner

Took out O2 sensor and measured the back pressure on my car 420,000, would have thought the exhaust would have been all blocked up but it really didn't give any reading till 4,000rpm and that was about 4# at around 6,000 it was 7-8# was a bit hard to read as the engine had to be under load and at that reves you had to change gear before it red lined. I have a few minor leaks in the exhaust but with all the bashing from rocks and age you would expect for it to be higher.
Tony

According to that article link you posted, your stock exhaust is not bad but it is not good either. Also to note is that you have an exhaust leak. If you did not, your exhaust pressure would be higher than it is now. The leak is relieving some of the unwanted backpressure. How much higher the pressure would be would determine whether it is not a good exhaust or if it is a bad exhaust. Thank you for testing this out.

Dessertrunner · #12 10-26-2007, 11:03 PM

Also since I wrote the last post I have removed the hose and fitting I tend to agree withyou because the hose on the end of the fitting was breaking up from the heat so not sure if it was leaking as well. I may test the other car this afternoon if I get a chance.
What pressure do you think we should target for? It would be good to get some of the better built systems that guys have put on tested.

Myxalplyx · #13 10-27-2007, 07:18 AM

Quote:

Originally Posted by Dessertrunner

What pressure do you think we should target for? It would be good to get some of the better built systems that guys have put on tested.

If you must shoot for a target, shoot for 0 pressure. However, even a free flowing exhaustwith a muffler have at least 1-2psi of back pressure, according to the article. I would shoot for 1-3psi to be reasonable.

Dessertrunner · #14 10-27-2007, 04:24 PM

My problem is that the standard SVX exhaust is brillant I have bent & squashed pipes, mounts that are now 2in back from were they were suppose to be and its still origanal and still works. In 400,000 k the only thing I have replace is the centre resonator that I removed. I am just not sure that I can build one that will last the punshment like the orignal. We don't have to test car admisions at rego as you guys do so I figure I might smash out the centre cats to help the flow.
Tony

Trevor · #15 10-27-2007, 06:31 PM

Quote:

Originally Posted by Dessertrunner

My problem is that the standard SVX exhaust is brillant I have bent & squashed pipes, mounts that are now 2in back from were they were suppose to be and its still origanal and still works. In 400,000 k the only thing I have replace is the centre resonator that I removed. I am just not sure that I can build one that will last the punshment like the orignal. We don't have to test car admisions at rego as you guys do so I figure I might smash out the centre cats to help the flow.
Tony

Tony, I agree.

The OEM SVX exhaust system demonstrates brilliant scientific design and achieves ultimate performance, given the constraints involved relative to noise/silence and emission issues.

An important aspect is the location of a very large terminating silencer at the rearmost position possible. In this regard the sideways positioning of this final opportunity for gas expansion is noteworthy.

Eliminating the centre “cats” will improve flow as you suggest and should not affect the original design strategy.

Your ideas make sense, Trevor.

P.S. Those intent on more or different noise, or a minor improvement in efficiency, should simply modify and reduce restriction in the internals of the large final element in the system. There will be no after market "cat back system" which can better or equal the original. A home grown twin pipe system remains in another ball park.