Test Pipe & Gas Mileage

[Jimmer307]

Alright so i have read numerous threads about gas mileage and a test pipe. I was wondering what is the truth? I have heard it makes your mileage shitty, i have also heard it improves your mileage, so what does it do?

Mileage shitty explaination: I have heard that it makes it shitty because since the exhaust is so fee-flowing, there is no back pressure.

Mileage better explaination: I have heard that it makes it better because it is free flowing with no restriction.

So what does it do? Please any FIRST HAND (NO "I HEARD BS") users of test pipes feel free to chime in. Or educated DSMers. Thanks in advance.

 

[fwdmaster]

I have a 3" test pipe on my laser,doesent seem to waste gas that bad, only thing I noticed is that my turbo spools up quicker and exhaust sounds louder

 

[Jimmer307]

I have a 3" test pipe on my laser,doesent seem to waste gas that bad

Well that is quite a broad statement. What does "doesn't seem to waste gas that bad" mean? Did your gas mileage increase, decrease, or stay the same?

 

[SuzukiRidr326]

i had a test pipe on my 420a and i usually got around 320-325 miles per tank then i put on the test pipe and got around 350-360 miles per tank and the test pipe was an ebay one and i was running an XTune cat-back

 

[1992awdlaser]

Short answer:

A test pipe should increase mpg since the engine doesn't have to work as hard to get the exhaust out.

Long answer:

I'm going to assume you know a little about how a four stroke engine works. If you don't I can explain that too. This is just an explanation and is not specific to dsm's.

In order for your exhaust to get out of the cylinder there has to be a force pushing against it. The piston is doing this while it is moving up during the exhaust stroke. It is able to do this because another piston is moving down during the power stroke which is forcing that other piston up. The more restriction there is in the path of the exhaust (back pressure) the more the engine has to work to get the exhaust out.

So if you decrease the back pressure the engine works less to get the exhaust out. This means the engine is more efficient with less back pressure. What actually gets the exhaust out is the energy from air/fuel mixture igniting in the other cylinders during the power stroke. If the back pressure is decreased less of this energy is used for pushing out the exhaust. That means more of the energy can be transferred to the wheels instead of being wasted. This is how the engine becomes more efficient.

The car can however stay at the same mpg if it's a turbo car. This is because the turbine housing of the turbo might be too big of a restriction in the exhaust system. If this is true changing the cat out for a test pipe won't make the turbine housing any less restrictive, therefor it won't make the back presure any less.

Understand?

 

[fwdmaster]

Well that is quite a broad statement. What does "doesn't seem to waste gas that bad" mean? Did your gas mileage increase, decrease, or stay the same?

meaning, it stayed the same

 

[Jimmer307]

1992AWDLaser... All i can say is wow, you know your shit. Thanks a ton

 

[gofer]

Short answer:

A test pipe should increase mpg since the engine doesn't have to work as hard to get the exhaust out.

Long answer:

I'm going to assume you know a little about how a four stroke engine works. If you don't I can explain that too. This is just an explanation and is not specific to dsm's.

In order for your exhaust to get out of the cylinder there has to be a force pushing against it. The piston is doing this while it is moving up during the exhaust stroke. It is able to do this because another piston is moving down during the power stroke which is forcing that other piston up. The more restriction there is in the path of the exhaust (back pressure) the more the engine has to work to get the exhaust out.

So if you decrease the back pressure the engine works less to get the exhaust out. This means the engine is more efficient with less back pressure. What actually gets the exhaust out is the energy from air/fuel mixture igniting in the other cylinders during the power stroke. If the back pressure is decreased less of this energy is used for pushing out the exhaust. That means more of the energy can be transferred to the wheels instead of being wasted. This is how the engine becomes more efficient.

The car can however stay at the same mpg if it's a turbo car. This is because the turbine housing of the turbo might be too big of a restriction in the exhaust system. If this is true changing the cat out for a test pipe won't make the turbine housing any less restrictive, therefor it won't make the back presure any less.

Understand?

i have no where near the knowledge about cars that 92awdlaser has but im going to have to mildly disagree or maybe you can clear this up for me...

my GS-T was getting about 300 miles to a tank on mixed city/hwy driving before i put on my test pipe. once i put on the test pipe and bigger down pipe my mpg dropped drastically and i can't control my boost (totally different story) but now im getting about 225 miles on a full tank and my car sounds like a dump truck.

on to my theory of why i think its good to at least have a high flow cat... it might be true that less restriction in the exhaust allows the gases to escape faster which equates to less work for the engine. isn't it also true though that each exhaust stroke coming from the engine helps pull the next exhaust stroke out? if thats true then i think that with absolutely no back pressure (restriction) in your exhaust its flowing to freely and the exhaust strokes are to far apart to help pull the next one out of the motor. this would cause the motor to work harder "pushing" it out rather than it being helped along by the previous exhaust stroke which would = bad gas mileage?

i might have just sounded like a retard. does that make sense though?

 

[Mr Peepers]

I agree with everything Pat said, except for the fact that the restriction of the cat will "stack up" with the restriction of the turbine/turbine housing. The turbine housing is a restriction, but that doesn't mean something behind the turbine can't make it better or worse.

Speaking of exhaust pulses:
A turbine will "mulch up" the exhaust pulses. This is a big reason why you can go so large on a turbo car's exhaust compared to an n/a, there isn't much benefit to be had from scavenging after the turbine anyway. Also, you can get to a point where scavenging is so prominent that it will suck fresh air/fuel mixture straight out the exhaust during the overlap period, hurting gas mileage. This will depend on cams, exhaust design and many other factors but I highly doubt this would happen just by doing this and there is a fairly restrictive turbine in the way anyway; just thought I'd mention it

Quick and dirty general answer is that it should increase mpg; albeit probably only slightly. We are talking about light throttle cruise here, with driving habits aside. YMMV. Any resulting change in A/F will quickly get compensated for by your fuel trims and the only thing left is less restriction/pumping losses.

You may however get a glowing CEL in your face from the secondary o2 sensor thinking the cat is faulty. It does nothing other than that so don't worry, and it can usually be fooled by using an o2 simulator. Whether it's mechanical(screw in anti-fouler) or electrical.

 

[1992awdlaser]

I agree with everything Pat said, except for the fact that the restriction of the cat will "stack up" with the restriction of the turbine/turbine housing. The turbine housing is a restriction, but that doesn't mean something behind the turbine can't make it better or worse.

I guess I should have explained that part better.

What I meant was if your exhaust after the turbo already has the ability to flow more than the turbine housing can flow. Then if you make the exhaut after the turbo less restrictive there shouldn't be an improvement in mileage since the exhaust can't flow better due to the turbine housing restriction.