sluggish 03 Forester

215 60 16 is stock size so yes im sure that plays into my problems a little, it had stock wheels when i bought it though and it didnt seam much different. Im going to replace the fuel filter, plugs, air filter and check the throttle body this weekend and see if it helps.

 

I know my 99 forester had 2 air filters in it but idk about the 03. Plus I've got a 5 speed so that helps but I've got headers back to a flowmaster 40 and it helped over the tsudo muffler that was on it

 

Yes. Also the 17"w+t combo is going to be heavier I imagine, which won't help either.

 

I sort of remember figuring that out back when i had my comanche with 33" tires on it. I think it was technically linear from stop to accelerate to stopped again
edit: EricS already answered it.

 

In terms of power adders with the NA engine, you have two main components and then a third optional component.

One, ECU tune for higher octane. You can bump up to running 91 octane and basically double the stock timing. This nets around 9 ft-lbs across the board.

Two, exhaust, primarily good, equal length headers plus a little freer flowing exhaust (but headers are the main benefit). TWE has the best ones available at a bit of cost. More affordably are the Cobb/OBX/Rallitek/knockoff EL headers. TWE's header dyno plot shows a solid 15 ft-lbs gain across the board. I've seen some people state north of 20 hp gain with a TWE header. The Cobb style tends to show similar torque gains. Even a poorly designed header or UEL header will yield around 10 ft-lbs over the stock manifold. The stock manifold is just crappy for power.

Step three is cams. Once you do ECU tuning for high octane and fix the exhaust, there really isn't any other improvement. Modifying the intake yields little gains (it just breaths plenty well plus is sensitive to ECU tune (you can lose a lot of midrange torque removing the stock intake). All you can really do is start shifting that torque up or down the power band by modifying the cams. There are sport grinds that maintain good low end torque and a little bump in upper end breathing. There are also hot cams which can shift that torque up high and make good hp. However, once the cams start getting big with a lot of overlap, dynamic compression drops and you are forced to run higher compression pistons to compensate. A mild sport cam is the easy route for mild gains and normal day to day drivability. If you want it all to be about top end, there's a lot more work to do including pistons, head porting, oversized valves, etc.

With cams you do not make more torque. You simply move it up or down the power band. You have a 3k to 4k hump you can shift around a little. Shift it up, and peak hp goes up. Hp rises based on how far you shift that torque hump up. There may be a dyno plot here or there showing some different cam grinds, but not a lot of people go this far.

The downside is our engines are designed quite extensively for low/midrange efficiency. If you want it to rev and make power like a hot Honda engine, you have to modify a LOT of parts. Piston speeds are actually pretty slow for the engine design, so a 9k redline isn't really a problem but so much of the engine, intake, and exhaust are are so heavily geared for 3k to 4k rpm operation. It's all wrong for high rpm use and just chokes air flow up top. TWE does offer a lot of parts for a hot engine include big cams, high compression pistons, and large diameter headers. You can get the porting and up sized valves done on the headers. The one thing that doesn't exist is an intake manifold geared for high rpm operation. No one has ever made one. You would need a large diameter intake runner and may or may not run a short runner with some strange form of plenum design (subaru intake runners are pretty long).

The short answer is high hp is a lot of work simply because our engines are fundamentally designed incorrectly for peak hp. They are all about midrange torque, which is odd for the oversquare design which is particularly beneficial for high rpm use. It's kind of why the newer Subaru engines are now undersquare with better efficiency and torque.

 

I've seen some short-runner intakes, but they've been billet $$$

 

They could be built cheaper. You should be able to build a custom one of any design for $200 (mild steel pipe, two stamped halves, a few machined flanges, some welding).