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Old 07-10-2002, 12:54 AM   #1
DDMan
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Black as Midnight!!!

Default Power gap

I know the 2.5 Motors have a torque dip at around 3500-4500 RPM, but I never experienced it until today. I had to remove my muffler cuz it was being replaced, so I drove to the shop without the muffler on, and while in 3rd gear I punched it aroudn a truck and right at 3700 RPM the car lost its power, and then gained it back over 4500RPM.

Then i tired it again AFTER i got the muffler back on, and it didnt do it at all.

Anyone wanna try and explain this, the pipe is 2.25" and is cat back.
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Old 07-10-2002, 03:30 PM   #2
goobie
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Could either have been a "freak" thing like a hiccup or something or it could be due to a loss of back pressure with the muffler removed. NA engines do require a certain amount of back pressure to perform properly. Changing the amount of back pressure (i.e. adding a larger diameter mid pipe/catback or as in this case removing the muffler) will affect the engine's performance depending on the RPM. Changing the amount of back pressure will affect the engine differently at different RPM's. An exaust can be "tuned" (vary the lenght, diameter, etc) to give an engine maximum performance in a certain RPM range (this will usually cause a loss of performance in other rpm ranges).
It's quite possible that removing your muffler gave you better top end but hurt your mid-range performance

Chad
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Old 07-10-2002, 04:13 PM   #3
2.5GT
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I believe the hiccup is due to that rpm (~4000rpm) being the resonance frequency of the intake manifold. I'm sure a search (once you get through the zillions of posts on the subject) would give you the answer . . . BTW, I'm being VERY sacastic about performing that search . . . cause I'm guessing someone on this board will make some comment about doing a search

Jason
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Old 07-10-2002, 05:25 PM   #4
mrbell
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*sigh* One more time... backpressure in an exhaust system is NOT a good thing... IT"S NOT. Think of any reason you can that this would be good, and then realize how silly it really is.
So why do you lose power w/ a 4" exhaust on a 2L motor? Well because a 4" exhaust has more backpressure than a 2" exhaust! What?! That makes no sense!! I must be smoking crack... but I'm not... here's why... too large of a pipe will allow the exhaust gasses to cool and collect before they exit and this blocks the following gases and causes them to do the same... so now you have backpressure, not from a restrictive pipe, but from restrictive gases... this causes all sorts of issues w/ O2 sensors, and cylinder scavenging... so... BACKPRESSURE IS NOT A GOOD THING! GET OVER IT!
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Old 07-10-2002, 05:41 PM   #5
Section 8
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Backpresssure is not even a "thing". It is a RESULT. WHat people usually refer to "backpressure" is the ability to scavage exhaust from the cylinder. Like was said, if you have to large of a exhaust you lose exhaust velocity and the scavaging effect that higher velocity gives you. You cannot have an exhaust pipe for every variation of throttle and RPM, you choose the best comprimise. By taking an exhaust pipe that works "all around" (including noise) you loose out somewhat on the high end at WOT as you will have some restriction as that is when there is the most exhaust produced. Restriction = Back pressure. Resistance to flow = back pressure.

Backpressure is bad. You want as little as possible. What you want is a pipe that has a good comprimise to keep a decent torque curve. You will end up with a bit of back pressure because of it, but you are NOT doing it FOR backpressure. You can use a larger pipe that will have less resistance to flow like a 2.5" exhaust, but it will start to take away from low end in exchange for highend. You start to loose comprimise and start to lean one way or another at some point.

I drove my car with an aftermarket header/high flow cat to the muffler shop and it absolutly sucked anywhere below 4300 RPM. I only went that high once just to "see" sort of thing. It was really "soggy" driving around like that at lower RPM.

Cobb has chalked the dip up to the intake manifold. Your lack of exhaust likely amplified it.

cheeRS,

Greg
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Old 07-10-2002, 05:48 PM   #6
ObsidianRS
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While true, I think you're a little hard on your rant there.

Backpressure itself isn't good, as you pointed out, but what you did point out you want, does create backpressure as a side effect. It's a symptom not a cause, of the desired state.
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Old 07-10-2002, 05:56 PM   #7
Section 8
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Quote:
It's a symptom not a cause, of the desired state.
I didn't say that is was a cause.....

Quote:
It is a RESULT.
There is a difference between comprimise and completely eliminating.

cheeRS,

Greg
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Old 07-11-2002, 10:07 AM   #8
goobie
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Default I stand corrected!

Hmm I guess I had the wrong idea of backpressure.
You learn something new every day...

Thanks!
Chad
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Old 07-11-2002, 10:53 AM   #9
Section 8
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Quote:
While true, I think you're a little hard on your rant there.
OOps. For got about that part....

It wasn't supose to be hard. I just wanted strong emphasis.

Guess it looks the same on "paper"

Greg
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Old 07-11-2002, 11:00 AM   #10
Section 8
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Quote:
Hmm I guess I had the wrong idea of backpressure.
You have the right idea, just use a word or phrase that is reflecting what you are saying instead of the word "back pressure". It has turned into a generic term for muffler shops to use on people who have no clue (and they don't want to explain the whole thing to), and a more descriptive term gets a point across better.

cheeRS,

Greg
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