04 Cobra Exhaust Back-Pressure Issue/Question.

[leonardo]

Got my first cobra about a month ago. It has 72,000 Kilometers its a Canadian car. Car had a 2.93 top pulley, a 4lb metco crank pulley, a 3.2 alternator pulley and a aftermarket filter. It came with a SCT programmer with a 91 Octane tune on it. The guy I bought it from had the car for a month and wanted a 2008 Cobra and did not know what the tune was. I took it to a reputable tuner to see if the tune would hurt the car. They put it on the dyno and did a pull. I wish I knew how to add it. The results were:
CEngTq 444.8
CEngPw 398.3
WhlTqr 392.7
SAEPwr 346.2
They said tune was Ok and would not hurt the engine but they wanted to do a exhaust back pressure test to see how that was. They ran it again and told me there was 1.5 bar or 21 psi of back pressure on drivers side before the cat advised me to drive easy and they would put high flow cats on. They installed cat and measured a 1.4 bar of pressure, really did not change much so they unhooked the mufflers and still had a .4 bar exhaust pressure. I was upset that after 1500 dollars things had not changed much so told him to put mufflers on and I took the car home. I did a back pressure test at the EGR valve and at 4000 rpm did not get more than 2 psi with a manometer. Wanting to verify that I had been lied to another shop did a back pressure test that verified what the first guy told me. So now I am going to put the car back to stock and put the stock tune back on. Well I'm glad I got a Cobra there must be some reason why that Back Pressure is present. I am open to any ideas or reasons why.
Thanks to Bob from this forum for suggesting I post this

 

[94slowbra1]

Interesting. Back pressure issues are usually a concern of turbo cars. I have never seen a discussion on here about back pressure. I'm curious to see what others have to say

 

[CobraBob]

I talked to Leonardo (Bob) on the phone and suggested he post this thread. Hoping for some input and comments from folks. I'm as interested in this topic as he is.

Strange that his exhaust back pressure is so high, and yet the car runs perfectly normal. A/F is good. WOT is good. Idle is good.

 

[zredfire04]

Anybody ever measure "normal" backpressure as a reference?

Was the measured backpressure at idle or in boost?

 

[leonardo]

Anybody ever measure "normal" backpressure as a reference?

Was the measured backpressure at idle or in boost?

It was in boost I did not get back pressure during idle. This back pressure was on the dyno run

 

[c6zhombre]

I talked to Leonardo (Bob) on the phone and suggested he post this thread. Hoping for some input and comments from folks. I'm as interested in this topic as he is.

Strange that his exhaust back pressure is so high, and yet the car runs perfectly normal. A/F is good. WOT is good. Idle is good.

View attachment 1439595

Who said the a/f was "good"? Does Bob have his own wideband on the car or is this the opinion of the shop that just charged him 1500 for 2 high flow cats?

Personally, I'd take a step back and start over. I don't like the sounds of this shop. 1500 should have got him a brand new catted xpipe AND quality catback. That would eliminate any concerns of backpressure issues. The factory manifolds are already proven to flow plenty for double factory HP and TQ

Find another shop, get a real wideband on your own, and get a high quality retune from someone with a proven track record on these cars. The dyno numbers sound really weak

 

[whitedevil95]

Who said the a/f was "good"? Does Bob have his own wideband on the car or is this the opinion of the shop that just charged him 1500 for 2 high flow cats?

Personally, I'd take a step back and start over. I don't like the sounds of this shop. 1500 should have got him a brand new catted xpipe AND quality catback. That would eliminate any concerns of backpressure issues. The factory manifolds are already proven to flow plenty for double factory HP and TQ

Find another shop, get a real wideband on your own, and get a high quality retune from someone with a proven track record on these cars. The dyno numbers sound really weak

Great words

 

[leonardo]

Thanks for the info on the stock manifold. What I would like to find out is what is the exhaust back pressure on a stock 03-04 Cobra? Also is there a way boost can get into the exhaust manifold through the Egr valve seems like there is something between this back pressure and boost. I am no mechanic but something must be causing this

 

[speedoflife]

Why were they measuring backpressure in the first place?

I've never heard of anyone doing that test on a 2003-2004 Cobra. Especially one that put down decent power and drove fine....

I don't like the sound of that shop honestly. Maybe I'm judging too quickly.

 

[leonardo]

Why were they measuring backpressure in the first place?

I've never heard of anyone doing that test on a 2003-2004 Cobra. Especially one that put down decent power and drove fine....

I don't like the sound of that shop honestly. Maybe I'm judging too quickly.

Oh that is a given I will never go back there again but the second place did not take it to full run and stopped because back pressure was at 19 psi at about 5900 rpm. They would not do another run to check total boost because of it. I am in the process of getting the parts and going back to stock which is why I was looking for the billetflow 3.65 ring so I could go back to way it is now later without pulling the adapter. I appreciate all help from this forum and will not quit until I find out why the back pressure is there
Bob

 

[zredfire04]

yeah...this is the first i'm hearing of a backpressure test as well.

it's obvious that you're going to have more pressure in the exhaust system as you get more heat in it (i.e. making more power). whole thing just seems dubious on the surface of it to me.

BUT...there's a whole lot more that i don't know than there is that i do.

 

[whitedevil95]

I also have never in my life seen a shop do a back pressure test. Seems so strange. So now 2 different shops are testing this?


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[leonardo]

Of all the tunes done on 03-04 Cobra I cannot believe that no one has checked the exhaust back pressure except the first shop that I took mine too. Or is back pressure not a concern for these engines?
Bob

 

[01yellercobra]

I believe back pressure is only a real issue for turbo cars. For blower cars unless you have a bad cat or muffler it's basically a none issue. It sounds like the shop was feeding your BS and your car was fine. Just my opinion of course.

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[leonardo]

I believe back pressure is only a real issue for turbo cars. For blower cars unless you have a bad cat or muffler it's basically a none issue. It sounds like the shop was feeding your BS and your car was fine. Just my opinion of course.

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Thanks for your opinion I an eager to hear more answers like that one if it is true. My exhaust is 2 high flow cats and a stock cat back I am thinking if a new cat back in 2.5 inch is installed I should be OK?

 

[whitedevil95]

Thanks for your opinion I an eager to hear more answers like that one if it is true. My exhaust is 2 high flow cats and a stock cat back I am thinking if a new cat back in 2.5 inch is installed I should be OK?

I was going to ask you if your catback was still stock. The stock mufflers are pretty restrictive. Back when these cars were being built they were under some new federal noise restrictions and Ford put some really quiet mufflers on them. I remember when these cars first came out people were reporting like 20-30hp gains from just a free flowing catback.

 

[01yellercobra]

I was going to ask you if your catback was still stock. The stock mufflers are pretty restrictive. Back when these cars were being built they were under some new federal noise restrictions and Ford put some really quiet mufflers on them. I remember when these cars first came out people were reporting like 20-30hp gains from just a free flowing catback.

I'd be curious to know what else was done when those catbacks went on. Almost everyone I know that put exhaust on also put a pulley and tune in at the minimum.

FWIW, I ran around with a stock catback earlier this year for a ticket. While I didn't get it dynoed I can say it didn't lose 30hp. Nothing else was changed for that. I kept my Bassani mid pipe on.

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[leonardo]

Here is the dyno run that shows the 1.5 bar of exhaust pressure