Exhaust Header
 

I am wondering about making an eco-mod purchase.

My 2001 Subaru its plagued by by the issue of power loss through the AWD system. Being mated with an automatic, the car is persistently wanted to downshift on any uphill. This only compounds the issue as the quick shifts leave the car even more loss going uphill. I would simply just drive faster, but i can't stay above 2K rpm unless I am speeding or rear ending the next person in front of me. Living in PA this occurs just about every mile, and every back road is marked 45mph (perfect shift point for the Subaru's 3-4 gears).

Now, in my mind's theory, if the subaru had just a little more power it would be able to maintain speed/gear longer and hopefully reduce downshifting on hills. And even once it does downshift, it will be able to run at a lower rpm thanks to the header. I am also hoping that I will be able to gain something on highway since going 55-65 falls between 2100-2300rpm. So better flow may reduce engine speed thus using less gas.

Now I understand that at idle I will probably lose some because of increase flow, but what I gain in actually driving will probably outweigh the idle loss. The Subaru has an amazing low idle at time (~500-600 rpm), so it maybe not be too bad.

Any one else have concerns or maybe feel I would loose more than gain? Its hard to find any comments about performance enhancements to improve cars mileage :D

PS

By quick math, if the header would bump the Subaru from 27 to 28 mpg @ current price of gas, the payback on purchase would be ~40,000 miles or 3 years.

@ ~80K on the clock now, the payback period would include the timing belt replacement, so spending the ~$150 dollars will be nothing compared to the ~$750 the will need to have the timing belt replaced in the next 3 years.

equal sign
 

Engines are like Algebra.Whatever yo do to one side of the equation,you've got to do the the other.
An exhaust mod won't see top value without mods to the induction system and vice-versa.And new camshafts are usually involved.
Relieving back- pressure for the rpm and flow range in which you'll typically be driving,might, and probably would exacerbate your situation.It's a challenge to beat powertrain engineers at their own game.
You may just want to respect the limitations of the car and enjoy it's good points when the driving environment allows.
A large-displacement engine with flat torque curve would allow you to walk up grades without a downshift,but then you'd enjoy all the dubious benefits of a large gas-guzzling engine.
You might just stay in the slow lane,let your speed fall,and let other traffic pass.

http://www.submariner.org/thepno95/P...0vs%20DOHC.jpg

I have the 2.5 SOHC, the torque is pretty flat for either engine. I'm trying to find charts for before and after header on the 2.5 sohc. I know i've seen them before. I believe it was in the range of a consistent 5-8 torque gain and ~10hp at the peak, and a few on the low end.

The intake tubing for my subaru has also been modified, result yielded worth wild difference in pull especially on hills.

Probably being subaru could fart up the hills in 4th @ 1700-2000 rpm, but instead it dumps to third and runs up to 2700-2800rpm, which the car begins to poop fuel economy at and above 3K.

IMO, the best solution would also be the most direct - recalibrate your shift points.

Installing a set of headers alone generally gives only minimal part-throttle power improvement unless you also recalibrate the tune in your PCM.

In most instances I've seen FE drop with headers. In some instances where the vehicle is very underpowered or undergeared, headers can improve FE during some driving conditions.

First I would determine conclusively if your trans is actually downshifting, or if your TCC is unlocking, or both.
In many vehicles you can use a switch to over-ride the TCC function and force it to stay locked while you are cruising. It must be unlocked for stops and accelerating from a stop.

If you have a shop install the headers it might not be worth the money for the gain. If you install them yourself it could be worth getting them. The best mods I have done to get the most out of vehicles is 1st all the maintenance is done and up to date. I swear buy switching all the fluids to royal purple synthetics. a good intake and exhaust and a nice chip from a a major company like superchips.

Many headers are optimized for power, I think that getting one made to low rpm torque could be worth to consider, of course rest of motor must then match to that, so new program to ECU might unleash the potential of new manifold, but intake manifold can't be ignored either, depending what kind manifold that is, bit longer runners should help to get again more potential out from motor, then one likes to replace camshaft to offer better low rpm torque, but depending from original exhaust manifold type, it may be very good place to start.

It varies case by case. Good thing is that modern ECU units can adapt quite well, with knock sensor they can advance timing in certain window, putting new program then enlarges that window, it is not very easy to say when to chip as there are quite many variables.

Its funny when old threads are dug up...no really it is :-p

But anyway, I actually do have a header currently installed. I've been working on the mileage difference, but I've also removed some of the forward grill/aero pieces.

It's hard to tell difference currently. I have a good set of pre-header data on 135miles where I had nice weather without terrible traffic incidents. I have yet to get a good spot of data because traffic/weather has been terrible in PA. As far as I can tell, the header is still leaving me within 10% of the original high-mark, and that 10% can quickly be made/lost due to rainy weather and poor traffic conditions.

I think the idea has merit, if it increases the VE of the Engine enough around the rpm band you are most likely to use it.

I will be doing this mod on mine, since the original header is much to be desired, but the length and diameters will be mathematically determined (via Pipe Max).

:)

Do you have a fuel consumption meter on the car? Didn't mention that you did, but it would be a good idea if you got one along with a A/F ratio monitor to see what the behavior of the EFI is like.

At 80k, for sure the transmission should be serviced and it would be a good idea to go synthetic front to back, as a previous poster has said.

I'd be curious about reprogramming first, before the header, since the exhaust manifold on the car stock will probably have higher velocity at that engine speed range ( 1700-2000 ) than a comparable header along with the heat retention.

Another option would be to take a stock exhaust manifold and get it Extrude Honed. Won't have to worry about fitment problems and a far sight cheaper than a custom header. Maybe get it barrier coated to reduce radiated temps so the computer does not back the timing off.