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Old 08-11-2004, 11:10 PM   #1
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Default Poor peformance after mods, help

I had some cash to blow so I did a 2.5" single out exhaust from the cat back with a Magnaflow SS muffler and dual Borla tips. The car doesn't jump off the line anymore like it use to. Theres not even any torque steer during WOT take off'*. Also, it use to down shift and pull hard during 55 mph+ passing situations. It'* very sluggish now doing this as well and won't move to save it'* life. It'* as if I lost all the torque. I took off the cone filter intake and went back to my cut airbox and Fram filter but that didn't help get back any low end torque. Is there anyway to get it back? Thanks.............
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Old 08-11-2004, 11:29 PM   #2
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sounds like you might have gone with too much of a free flowing exhaust. You might've lost your backpressure which is extremely important for torque production. I hope that it'* not the case though, but I don't really have any other ideas.
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Old 08-11-2004, 11:37 PM   #3
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Have you scanned it for codes?
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Old 08-11-2004, 11:41 PM   #4
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You went too big with the exhaust. The N/A 3800 doesn't need more than a 2.25" exhaust. As you found out, the 2.5" lost you power. My dads GTP has a 2.25" exhaust on it and it runs 12.2'* in the 1/4...leading me to believe that the 2.5" you went on your N/A 3800 is too big.

The bottom line is that the exhaust flowed much better than the stock one, so you lost a lot of backpressure, thus reducing the amount of torque the engine produced.

My suggestion would be to put the stock exhaust back on there if you can and put some nice mufflers with the existing pipe.

Shawn
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Old 08-12-2004, 05:52 AM   #5
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Did you remove the resonator too?
I would think you could get back the lower torque with a 2.25 reduction from the cat to the mufflers?
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Old 08-14-2004, 02:56 AM   #6
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My car has a STOCK 2.5" exhaust. I think you may have gotten bigger then that, or something else is amiss. My car'* Torque curve was smoothed out, but nothing fell off, when i switched mufflers.
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Old 08-14-2004, 03:32 AM   #7
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Ya know every other random post I like to lay down a little of what I'm learning. This seems like a decent chance to lay the 'it needs backpressure' thing to rest. And as always, lemme know if I'm off on something

No engine works better with backpressure. Read that twice if you need to. Backpressure only makes the engine work harder to get the exhaust out. What you're experiencing when you make your exhaust too large is a combination of two things. One is that (as most of you may know) the exhaust from one cylinder actually pulls exhaust from the next. The problem with putting huge exhaust on is when the quick moving exhaust is dumped into a large area it slows down and hence doesn't 'suck' the exhaust as well. The second problem (and more noticeable at lower end) is that a large diameter exhaust doesn't hold heat as well. You want your exhaust to be as hot as possible because hot air is less dense and hence easier to push along. When your exhaust hits that cold dense air in your huge pipes it slows down and its more difficult for the engine to push it out. This isn't AS much of a problem at high rpm'* because it has more force behind it and its a bit hotter.

I'm tired but I hope this cleared a few things up.
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Old 08-15-2004, 06:23 AM   #8
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I'm with you on this, it might not feel like it has all the power, but i've read on 3800 f body site had the same stuff, more power, just smoothed.
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Old 08-18-2004, 03:11 AM   #9
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Any updates? Would like to know what you came up with?
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Old 08-18-2004, 01:22 PM   #10
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Quote:
Originally Posted by DrJay
No engine works better with backpressure. Read that twice if you need to. Backpressure only makes the engine work harder to get the exhaust out.
I think that statement is only true if you put it in the proper context - less backpressure works best at WOT runs such as 1/4 mile acceleration.

Quote:
Originally Posted by DrJay
What you're experiencing when you make your exhaust too large is a combination of two things. One is that (as most of you may know) the exhaust from one cylinder actually pulls exhaust from the next.
Cylinder scavenging is an important component in achieving volumetric efficiency. At low rpm, or part throttle, if there is little or low backpressure, you're not going to exhaust suffiently well enough because of low volicity. By using the pulse and pressure difference of other cylinders, the velocity can be maximised therefore allowing more complete exhaust of spent gas front the cylinder and a better intake charge - this accomplishes better volumetric efficiency and gives better throttle response.

Since most of us drive our cars under a variety of load conditions, it'* neither correct to say you need a lot of backpressure nor to say backpressure is your enemy. If you want only to maximise the performance of your car for one driving conditon, like WOT, then you could optimise the exhaust for that condition only whereby you'd want little backpressure. On the other hand, if you rarely went WOT and wanted better drivability and throttle response around town, then you'll need backpressure but you'll suffer at the track.

Like many things, backpressure is a comprise condition in response to a variety of demands we put on the engine to respond under loads.

Hope this makes sense...

Cheers,
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