Test Pipe = Worse Gas Mileage?

[speed3 britz]

Ok so if had my SU test pipe installed for about a month now and I've been experiencing worse mileage since before installed. Before installed, all stock exhaust I was averaging 28 mpg (actually calculated, not dash computer)... now since then with installed I am barely getting 24, sometimes less. That is 40 mins driving with 90% expressway. I thought with free'd up restriction, more free flow, would mean maybe better if not same MPG... Not worse. Any Ideas??

 

[Jdm_Mica]

Has your driving habits changed any? Usually with power mods, people tend to go faster

 

[MSMS3]

^ Agree. Getting into the throttle more, enjoying the different sound. Or, you have something else going on that happens to be coincidence. Test pipe will not reduce fuel consumption at the same engine load and throttle settings.

 

[speed3 britz]

No actually not, I honestly thought about that... but I cruise the same stretch between 75 and 80, usually no higher

 

[b e z]

I have an ebay test pipe and I was thinking the same thing in the beginning but after this last tank of gas I think its ok. Ive had mine since Xmas.

 

[MSMS3]

There is no scientific, evidence-based, rational reason that a test pipe would increase fuel consumption when operated under conditions the same as with the stock mid-pipe and secondary cat.

I don't doubt that OP has a drop in fuel mileage. But it cannot be due to the test pipe. Something else is going on. Maybe he needs to clean/replace air filter, replace or regapped plugs. Maybe he has some other maintenance issue, has different fuel octane, maybe his MAF sensor needs cleaning, tire pressure checked, etc. Given the age of the car and probable high mileage it could also be build-up of carbon and crud on the intake valves. There is an almost endless list of reasons fuel mileage would drop. With respect, swapping in a test pipe for the stock midpipe and secondary cat is NOT one of them.

One way to be certain is to put the stock midpipe back on and drive. I'd bet hard money that does NOT solve his fuel consumption issue.

 

[chief_wiggum]

No actually not, I honestly thought about that... but I cruise the same stretch between 75 and 80, usually no higher

it's not the 'cruising' phase of your commute, but more likely the manner in which you get to your cruising speed which goes back to what Jdm_Mica and MSMS3 were saying. Agreed that some other factor has caused the change; weather, tire pressure, etc...

 

[speed3 britz]

I understand both comments but like I said, no issue until test pipe was installed. And a loss of 4 miles is quite substantial in my opinion specially with 94 octane at $4. The car is a 07 with 90k on it with all regular maintenance done on schedule. I agree that it is probably not the test pipe that is the result but maybe a cause of something else to happen to drop the MPGs. The MAF has just been cleaned, a new HKS element was just installed on my intake, and a new O2 sensor just replaced the old faulty one in the downpipe, along with tire pressure checked recently. Besides this, the car is all stock. I appreciate the "checklist" MSMS3, however I just find it too coincidental to have this problem all of a sudden... Just my .02

 

[chief_wiggum]

did you have normal mpgs after your new O2 sensor or was it done during the same tank as the test pipe?

 

[speed3 britz]

Yes, I had the 28mpg average after the O2. The pipe was installed last month and thats when I noticed the drop. The o2 was installed 3-4 months ago, maybe longer? It just strikes me as odd (uhm)

 

[MSMS3]

Well, since OP is determined to disregard the laws of physics and blame the mileage drop on an open pipe, the only suggestion I can offer is that he put the stock midpipe back on and report to us how making an exhaust section more restrictive improves gas mileage.

My apologies for sounding doubtful and a bit stern. But the test pipe causing a reduction in gas mileage would be what is called post hoc ergo propter hoc reasoning. "Because event A proceeds event B, therefore it causes event B." So, if I changed my socks this morning and decided to wear black socks, although I always wore blue socks before, and then I get in a car wreck, that must mean that black socks cause care wrecks. Bad logic.

There are known causes of drops in gas mileage. A more open exhaust on a turbocharged engine is not one of them. Maybe consider another known cause. EGR valve malfunction? Lots of others.