My son’s car had a hole in front of the muffler. The whole system was rusty and marginal, so we replaced
all the pipes and the muffler. The car now seems to have less power. I know that opening the exhaust can
increase power, but I wonder if the effect is large enough for a naive driver to notice. Before, all the gases were going out before they got to the muffler, but now they have to go thru a new one. So, does anyone think this is normal, or should we investigate further??
Is the Check Engine light on?
Tester
Both. I think this is normal, but you should investigate further.
Did you use a direct-fit OEM replacement or jury-rigged aftermarket parts?
What year is the car? If it’s 1996 or newer you should have had a Check Engine Light due to the hole upstream from the muffler. Did you? What were the codes?
Are there other problems that you’re aware of?
Has the maintenance been kept up with?
I suspect there’s more going on here than is obvious from your post. But it may need to be found hands-on. I would not expect the difference in flow resistance to be noticeable to the naïve driver, but if the exhaust was rotted through upstream from the upstream oxygen sensor it will affect the performance of the engine by affecting the sensor’s output to the ECU. The upstream sensor helps the ECU calculate how much fuel to feed the injectors.
No check engine light before or after the repair.
The hole was way downstream from the oxygen sensors.
The muffler shop bent their own pipes and used a muffler.
Oh-oh. I smell a “chain shop” whose name starts with an “M”. You know, the one that welds cheezy generic mufflers into exhaust systems.
I think I’ll bow out of the discussion now. But it could be the muffler.
Not a chain shop. A local with a good rep.
A thought occurred to me. They cut off pipes to replace them from near the engine on back.
So I presume they had to disconnect the cats and O2 sensor then put them back in.
So, maybe, the O2 sensor is having to “relearn” its situation. The car is located at about one mile
altitude so the default settings are probably not correct. Does this make any sense?
I wish I could say it does, but I can’t. The sensor doesn’t have the ability to learn. It has no program in it, it’s only a sensor.
It is, however, possible that they put in a generic (cheezy, cheap) oxygen sensor.
There are too many unknown variables here. I wish I could offer a suggestion, but I cannot. Perhaps others here can.
Have him do a 0 to 60 run on a deserted road. If it’s under 8 seconds…no problem.
I’d suggest that it is simply a case of noise. I think it is the opposite response to a kid installing a loud muffler on his car to make it faster when it only makes it louder. Same with Harley riders with open pipes.
The louder muffler gave the impression of more power without actually GIVING more power. You’ve fixed the muffler, it is quiet, and now it seems to a naive driver that the car is not as powerful.
My son might be fooled by the noise, but not me.
Basic question: could an open exhaust system (bypassing muffler) add 10-20 horsepower to such a car?
Single pipes and muffler and clamps
I think with the hole in the exhaust system, it reduced the back pressure on the engine. And this improved the scavenging of the cylinders improving performance.
http://www.veryuseful.com/mustang/tech/engine/exhaustScavenging.pdf
Now that the exhaust back pressure has been restored, the engine doesn’t perform as well when it had the exhaust leaks.
If the Check Engine light isn’t on, then it is what it is.
Tester
That is what I suspected and hoped was true. It was just that the effect
seemed a bit larger than I thought it might. The last time I had any experience
with something like this was with a 57 Olds 88!
Thank you.
If you’re curious about the magnitude of the effect, it’s probably possible for a shop to measure the back-pressure in the exhaust system. To get a controlled experiment though you’d have to measure it with the exhaust system open like it was before , and then closed like it is now. I wouldn’t have guessed a 10-20 HP difference myself, maybe 5 at most. After all, presumably the prior version of the exhaust system wasn’t entirely open, just had a big hole in it. And the cat was still there, right? But 5, 10, 20 HP, just speculation, and who’s counting anyway … 
I’m surprised it made such a difference. Makes me wonder if something’s blocking the flow somewhere. If this was that easy, I’d expect all kinds of articles on doing this to millions of Camrys and ESs out there with that V6.