About 3-4 weeks ago I completely redid the exhaust on my 86 with a 3.1 The exhaust exits towards the trunk, and I used to have a 2 inch downpipe to a muffler, and single exit exhaust. It was surprisingly quiet, and sounded like a typical 3.1 GM car. With this setup, I was getting between 30-35 MPG consistently. I used this setup for about a year, and since I had a GT rear bumper with dual cutouts for the exhaust, I decided to make a completely new exhaust system. What I have now is a 2" downpipe straight into a high flow cat, which splits at the bottom into a Y, with 2" straight pipes going to the exhaust tips. Obviously this is a lot louder, but burns cleaner. I thought changing the exhaust might change the gas mileage a little bit, but the 3 times I've filled it at the gas station, I've gotten at most 24 MPG. I dont drive it any harder than I did with the old setup, as Im usually just driving it on the highway to school 160 miles a week. I am guessing this new exhaust system has almost no back pressure, as it cackles a lot on downshifts, but I wouldnt think it would cut down on my gas mileage so much. Any ideas what is causing this? I might have to make the exhaust more restrictive and see if that helps some, but I miss getting 30+ mpg!

[This message has been edited by toadson (edited 12-02-2007).]

Did the O2 sensor become disconnected when you swapped out the exhaust? Thats the only thing I can think of that would change F/A ratio. Increased flow resistance should NOT increase MPG.

maybe it was the cat because when I pulled the car off my monza I gained about 15mpg.

quote Originally posted by toadson: as it cackles a lot on downshifts,

Sounds like you might have a vacuum leak somewhere also.

The 02 sensor is in the exhaust manifolds, so I shouldnt have even touched the thing. As for the vacuum leak, I highly doubt it. I went crazy looking for vacuum leaks a while back when I was having some idling issues, and never found a thing. Im not sure if I should even describe the exhaust as cackling. It more or less just pops a lot, kinda like a Harley without baffles. I think I will go ahead and make sure that the O2 sensor is plugged in though, just for good measure.

yes, the most likely part of the exhaust to affect mileage is the O2 sensor.
when the ECM gets no good O2 reading, it goes to "open loop mode" - and with a Fiero ECM - that usually gets you 17 MPG.
this also happens if the thermostat is stuck open, and the engine cant reach 180*

I could simply be a loss of back pressure. Most everyone thinks that the more open an exhaust is the better, but engines do need some back pressure. I believe it also helps for the scavenging affect on the exhaust gases as well as keeping velocity up.

you could have reduced back pressure enough that the O2 sensor is not getting up to operating temp and is giving false readings to the ECU

I agree that the change in backpressure mgiht be the cause.
Get a cable and Winaldl if you don't have it and see what is happening with the O2.
You could try the AB jump and watch the SES light and see what the flash rate is. That is suppose to be able to tell you if your running rich or lean by the flash rate. It's in the service manual if you need to read up on it...

Page 582 http://spad.sytes.net/fiero/manuals

[This message has been edited by Dodgerunner (edited 12-03-2007).]

It might be time for a heated O2 sensor.

quote Originally posted by The_Stickman2: I could simply be a loss of back pressure. Most everyone thinks that the more open an exhaust is the better, but engines do need some back pressure. I believe it also helps for the scavenging affect on the exhaust gases as well as keeping velocity up.

You do not want "backpressure". If there is pressure in the exhaust pipe pushing back towards the engine as you imply with backpressure, it will be harder for the exhaust to move its way out.
What you want is to reach a resonance frequency with the exhaust. It has to do with the length and volume of the exhaust system, not really how "open" it is. When the wave caused by the exhaust hits the open air at the end of the pipe, it suffers a relfection back (just like waves in an ocean bounce back if they hit a wall). You want that reflected wave to have a negative pressure when it reaches the heads or valves, thus "sucking" the exhaust out. This isn't constant either, as RPMs change, the frequency of the pressure waves change, and the wavelength changes. Thus you can tune the exhaust for a certain RPM, but as you deviate away from that RPM, the efficiency drops.
A drop in backpressure would be good, it won't cut the gas mileage. a straight pipe is going to give the best power, as long as it is the right length. So a decrease of power for him may have been caused by the tune being shifted to a different RPM, but it is not going to change it by 10mpg.

Were you getting the better gas mileage during the Summer? It could be the Winter gas you are using is to blame for your gas mileage going down.

quote Originally posted by Draekon: You do not want "backpressure". If there is pressure in the exhaust pipe pushing back towards the engine as you imply with backpressure, it will be harder for the exhaust to move its way out. What you want is to reach a resonance frequency with the exhaust. It has to do with the length and volume of the exhaust system, not really how "open" it is. When the wave caused by the exhaust hits the open air at the end of the pipe, it suffers a relfection back (just like waves in an ocean bounce back if they hit a wall). You want that reflected wave to have a negative pressure when it reaches the heads or valves, thus "sucking" the exhaust out. This isn't constant either, as RPMs change, the frequency of the pressure waves change, and the wavelength changes. Thus you can tune the exhaust for a certain RPM, but as you deviate away from that RPM, the efficiency drops. A drop in backpressure would be good, it won't cut the gas mileage. a straight pipe is going to give the best power, as long as it is the right length. So a decrease of power for him may have been caused by the tune being shifted to a different RPM, but it is not going to change it by 10mpg.

Finally, an intelligent explanation of this "backpressure myth". The only people that claim backpressure is the cause of power or efficency loss is just confused with exhaust flow characteristics being taken out of an exhaust system.

Maybe we're throwing the term backpressure around meaning different things.
Backpressure can have a effect on fuel uses, as said maybe not 10mpg.
http://www.uucmotorwerks.co...essuretorquemyth.htm
However the pressure change, weather change, tire pressure etc. could all come together to be affecting the mileage drop.

Yes I was getting 30+ miles a gallon all summer until I changed the exhaust. I dont think the winter gas is to blame, because my truck has been getting 18 mpg consistently all year long (til I hit a deer last night in it). Since my truck will be out for a while, I would like to figure this gas issue out. I ran out of gas the other day with less than 230 miles on the trip... so thats pretty crappy mileage for 10 gallons of gas. I really dont want to run out of gas again, so I hope I find the root of the problem. I have an obd1 scanner, but dont have an ALDL cable too hook up to my laptop. I only have a cable for OBD2 vehicles as of right now. I think I will scan it tomorrow and see what the o2 sensor is doing.

Free or changed flow in exhaust could finish an otherwise working but taxed o2 sensor.

quote Originally posted by Draekon: You do not want "backpressure". If there is pressure in the exhaust pipe pushing back towards the engine as you imply with back pressure, it will be harder for the exhaust to move its way out.....

Actually for high fuel economy you do. The hyper milers (people that mod their cars for highest possible fuel economy) add in exhaust restrictions to gain an extra 5-10% in fuel economy. That is why 44mpg geo metros don't have a 3 inch exhaust and even have a quite restrictive cat to improve fuel economy. I lost fuel economy on my old metro when I replaced the CAT with a high flow type. I replaced the new one with a stock one and the economy came right back.

so stright piping a cat would cause a loss of mpg??

Did you change the cam? Sounds like scavenging out the fresh charge because of overlap to me.

Norm

quote Originally posted by gt88norm: Did you change the cam? Sounds like scavenging out the fresh charge because of overlap to me. Norm

Did you read the thread?

quote Originally posted by 86_IRM_TURBO: Did you read the thread?

YES!
It's not exactly like he went "in depth" about the engine swap!

my money is on the O2 sensor not heating up enough or being disconnected..

9 times out of 10, when you increase the flow of an exhaust, the o2 has rouble operating UNLESS it is a heated O2 sensor. the only way to find out for sure is to scan the computer to see if it is working

Just a suggestion. It sounds like everyone agrees that its an O2 issue. Either there is not enough back pressure for the sensor to pick up the exhaust pulses, or it just is not getting up to operating temp. If you don't have an ALDL cable for your PC then you could try another solution. Stuff some steel wool onto the exhaust pipes and run a tank of gas through it. It will give you some back pressure AND hold some heat in the exh system.

Just a thought.......

surely the exhaust gets hot enough for the o2 to work without a cat? someone needs to use a laser temp reader and point it at the o2 before and after cat removel.

Seems like theres a bit of confusion going on here. I swapped a stock 3.1 from a 90 Lumina in, with the ECM. Ive been getting 30 mpg with it all summer ever since I got all the quirks worked out, and had NO cat on it and just a muffler. I switched to a setup with a free flowing cat and no mufflers, and ever since then it has gotten much worse mileage. I checked the o2 sensor yesterday and it is still hooked up, so that is ruled out. Tomorrow I am planning on using our scanner on it. I can get the mv reading for the 02 sensor, which should tell me something. If the readings look way off, I have plenty of extra sensors lying around to swap in. I'll post my results tomorrow.

Without doing a full scan of the ECM/PCM, it is impossible to tell exactly what is going on. You will need to buy or borrow a scanner or use your laptop with a scanner program and an ALDL cable. If its an ECM that you are using first make sure that the computer is going into open loop then look at the BLM , O2 sensor and CTS numbers and look to see if they are within normal range. That should lead you in the right direction

------------------
87GT - with 3800SC Series III engine, 4T65eHD
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by Draekon: You do not want "backpressure". If there is pressure in the exhaust pipe pushing back towards the engine as you imply with backpressure, it will be harder for the exhaust to move its way out. What you want is to reach a resonance frequency with the exhaust. It has to do with the length and volume of the exhaust system, not really how "open" it is. When the wave caused by the exhaust hits the open air at the end of the pipe, it suffers a relfection back (just like waves in an ocean bounce back if they hit a wall). You want that reflected wave to have a negative pressure when it reaches the heads or valves, thus "sucking" the exhaust out. This isn't constant either, as RPMs change, the frequency of the pressure waves change, and the wavelength changes. Thus you can tune the exhaust for a certain RPM, but as you deviate away from that RPM, the efficiency drops. A drop in backpressure would be good, it won't cut the gas mileage. a straight pipe is going to give the best power, as long as it is the right length. So a decrease of power for him may have been caused by the tune being shifted to a different RPM, but it is not going to change it by 10mpg.

One word...or name rather. Helmholtz

I am willing to bet its the O2 sensor went bad and it just happen to go bad around the time the exhaust was changed out. This happen to me when I was in the process of changing out the clutch in my 88GT many years back. Car came up from swap and my mileage had dropped by 10 MPG or so. After 3 times of running out of gas, I swapped in a new O2 sensor and gas mileage went back to normal.

Try swapping the old exhaust back in and see if that brings the mileage back up.

I don't think the 02 sensor is bad. With a standard or "turbo" chambered muffler in the CAT position, you are getting lower flow than a high flow CAT straight out.

When I used the Dynomax turbo muffler in the cat position, I lost noticable torque. I put a Moroso spiral muffler in its place with a single out, and the car got loud, but really picked up in the seat of the pants test.

The question is whether, with 25 mpg, you're feeling more oomph in the seat of your pants. If so, the engine will be using more gas to give you that oomph. 25mpg is not unusual for a mildly stepped-on 60*v6. 35 mpg is excellent gas mileage on the 60*v6, and likely what EPA wanted from GM.

The other issue is the overall length of your new pipes. Once the length gets to 72" (or 6') you get higher backpressure caused by conflicting pulses in the pipe. This causes the engine to work harder at any given rpm. I'd measure the pipe length. Your new duals might be the cause.

One of these two issues is your problem. IMHO

Arn

I just did the simplest thing and changed out the O2 sensor. Now i just gotta drive and see what happens.

so 6 feet of pipe is the hp limit? is this fiero only?

From my reading, (I've done alot to design my headers) all engines suffer the same thing. The way they overcome this on big cars is to put in a resonator or crossover X about 4' after the collector. The engine pulse reads the resonator as an open end pipe and the pulse is interrupted. this is a complicated science and not easily explained without a huge amount of reading.

On any v6 engine you don't want the exhaust pulse to work anything past 36" for performance purposes.

Hope this helps.

Arn

So my 86 gt auto without its cat should get worse gas mileage than stock but yet I am getting 20mpg city and my 78 monza with a duke and no cat and custom 2.5 pipe and glass pack(no back pressure) wasn't getting 40+ mpg and tops 52mpg in the winter. I am sorry but every car I have pulled the emissions off get better mpg over stock numbers and I always drive with a lead foot. If you do not need emission equiptment in your state or county I say take them off and get a chip made / tune and you will see better gas mileage or just go carb and learn how to adjust it and if you don't want to mess with a choke get one with an electric choke and gett better mpg and better performance.

Something isnt making any sense. Tonight I filled up and somehow got 36 mpg..... I drove another 50 miles and filled up, and it took 2 gallons, thus getting 25 mpg. I think it was a fluke though, because when I filled up the first time, my gas gauge read between 3/4 and full. The second time, it went all the way to full, and now it is between 3/4 and full after 25 miles of driving. I'm guessing it didn't fill up completely the first time, making my mileage higher, and when I filled up the second time, it took more gas to get full, making it look like I got worse mileage. I think this car just likes to screw with my mind or something.

Just a note on the gas gauge.... I have a 97 Camaro dash and cluster in the car, and the gas gauge never would read full. I messed with the float on the sending unit numerous times when I would drop the tank and clean it out, so I dont really know how accurate the thing is. When I ran out of gas, it was still reading 1/4 of a tank, so obviously it is still off.

Do you have any RTV upstream from the O2 sensor? It may be possible that the silicon has messed up your sensor. Maybe it doesn't have to upstream; since i've read the explanation on backpressure, perhaps it could even be downstream.

I have a 2.8 and getting 23 - 24 mph. After reading this, I checked my O2 senser, saw a little crack, gave it a little wiggle and it broke in two. The wife gave me one of those looks. Put the new one in today. Hope it works!

Rev.