I'm wondering a few things before I order my headers,
Do headers connect the cat or do they delete the cat?
If no cat do you throw cel with no cat? (is there a way to bypass)
Does gas mileage suffer with out a cat installed?

 

they connect to the cat ... a hi flow or cat delete is available seperately ... gas not sure id think headers alone would imrove milage if you can keep your foot out of the gas ...if you know what i mean

 

yea headers are a + when it comes to gas mileage... exhaust mods all add to your mileage because it does not burn any more fuel it simply makes more power... intake mods (ones that add air) are actually making ur car add fuel because it can literally burn more fuel now that the air allows it..

So intake mods will likely decrease mileage while exhaust will add to mileage.. things that reduce engine work like pulleys, weight shaving will also increase mileage.

hope this helps.... HEADER fuckin rock ur sox off... u'll have a smile glued to ur face for a solid day. WITHOUT A DOUBT

 

Neither headers nor intake should do anything directly to your mileage. Despite them improving your engine's VE, at cruise the throttle plate is regulating the flow of air into the engine, not the various restrictions in the intake and exhaust like at WOT. Your mileage will go down for a while because the power gains are above 5k rpm and you'll be visiting that more, but after you get used to it, mileage will be the same.

 

Most here, myself included, have seen a 1mpg bump after the headers have been installed.

 

theoretically any mod that frees up power lost from ur engine to the wheels should improve gas milage ... i.e. it takes a certain amount of hp to make a 3500lb car cruise at 60 mph ... headers make it easier on ur car to maintain that same speed and should therfore return better milage

 

No, that's not the case. Mods that add power do so by improving the maximum volumetric efficiency of the engine. This occurs at wide open throttle, where the inefficiencies of the intake and exhaust tracts are responsible for loss/gain when modding. During cruise, the car only requires about 10 hp to roll down the road at around 70 mph. The ECU is monitoring the air/fuel ratios and constantly keeping those in check using a real time closed loop system as well as fuel trims which are short and long term averages of the amount of correction the system is having to use in order to keep things in the "green box." 10 hp is 10hp....mods that add power just do so at wide open throttle. At very low throttle angles like during cruise, it's all the throttle plate and the ECU doing every thing.

There could be a slight mileage improvement due to the MAP based load system the J32 uses. There's no maf sensor, so the system guestimates how much is flowing through the engine using a variety of sensors as well as an ideal gas law model. This takes into account the VE of the engine which is very carefully mapped out at the factory on a STOCK engine. Adding things like intakes and headers improve the VE which actually makes the engine run slightly leaner than it otherwise would have. The ECU adjusts to this though through the 02 sensor and short/long term fuel trims. With that said, if it dance on the "lean" side of stoich instead of the rich side, say like from the factory, there could be a very small fuel econ improvement at cruise.

I used to help calibrate ECUs at Audi Performance and Racing (APR) when I was in college, and I also remote tuned Evos, AEMs, and all other kind of crap while I was in med school

 

Nicely said. Not sure if it's true but if you told me that I would believe it! Meaning it sounds totally legit. Since you seem to know what you're talking about and obviously got a good head on your shoulders (MD) how about helping us FI guys tune for high boost. I don't mean to beat a dead horse but it seems you're fairly new to the j series motors and I for one would like to go high boost eventually. What are your thoughts on the ecu issue in order to tune?

 

I'm not really sure. I'm kind of surprised that people can't get the piggybacks to work with the stock ECU regardless of it's abilities to apparently affect fueling dramatically. There are 02 sensor attenuation/skewing functions built into the piggies which will actually use the stock ecu's logic to keep things in check. To be honest, it seems like it's all really hear/say. If I were going to go that route, I'd have my own piggy and see for myself. Between me and a good friend of mine, I'm pretty certain we could get it work right after a bit of fiddling. The stock ECU is only as good as the inputs presented to it. Lean how it reacts, and you should be able to guide it's behavior regardless.

 

so youre saying that if my intake at WOT add 10 hp due to various factors that at cruise it cant be contributuing to slighly more ingested air , a more complete burn of fuel , a slightly leaner mix ... on the header exhaust side at the same 60 mph cruise ... these headers cant be contributing to less back pressure , better exhaust scavaging etc ... things like thermo blok spacers lowered t stats and fan switchs that have the ECU run more adavnced timng and again perhaps a slightly leaner mix wont contribute to a more efficient use ignition and that its not possible that the work nessecary to acieve 10 hp power has been lessened resulting in the same cruise speed requiring less energy from the motor to maintain speed ? is that what you're saying

in general if a mod has even a .5 hp increase in motor output at 2000 rpms then the end result should be improved milage

 

During cruise there is no extra "ingestion" of air with an intake, period. The throttle plate is by far and away the moderator of intake air mass in that situation. There theoretically is a very small increase in the VE of the engine at cruise with headers, but this is a very small fraction of a percentage because once again only a very small amount of air is moving through the engine as compared to the maximum it can move. This does not stress the intake and exhaust system at all. There is also no more of a complete burn with an intake. Headers typically do help with scavenging, though the shorties on our cars don't do a whole lot in that regard. But again, the effect is most pronounced at WOT where the largest volume of air is moving through the engine.

As for thermoblock spacers, anything that lowers IATs can theoretically allow the ECU to run a touch more ignition timing. That said, ignition timing on an NA engine is not always directly proportional to fuel econ, or power. A degree or two more at cruise may be good for 0.5 mpg in a perfect world...maybe. Lower temp thermostats many times actually decrease thermal efficiency as well as lessen a complete burn due to the lower head temps and less complete vaporization of the injected fuel as it enters the combustion chamber.

Back to the enleaning that can happen with mods. As I said before, with the headers the engine WOULD run a touch lean as compared to the factory setup due to increasing the VE without recalibrating the ECU. Fortunately, this is well within the ECUs adjustment ability and it simply adds the couple percentage points of fuel back in during cruise as it's constantly watching the AFRs in the region it cares about using the narrow band sensor. Like I said before, there could be a very small change where the ECU dances slightly leaner as before...but this difference is very slight.

As for what I am saying and what you are questioning: 10 hp is 10 hp. This is an NA motor which is running on a tightly controlled air fuel ratio during cruise as well as a very well calibrated timing map in this range from the factory. You could bump the ignition timing a touch, but at such low engine loads, the 1 or two degrees isn't going to do much as the engine is already well into the mid 30 deg range anyway. There are very slight efficiency improvements to be had, but nothing big. Mods that add power up top do so by improving the VE of the system up where the amount of flow is restricted by the factory intake and exhaust system. This is not an issue at cruising loads.