I've got a 2007 Subaru Forester school-bus, with a EJ253 N/A mated to a 4EAT A/T.

So I've done some mild Googling, and came across This Great Post.

Deep in there is this gem:

THROTTLE BODY SPACER While not a new concept in N/A performance enhancement, suppliers for this part for the EJ25 seem to be just one. Area1320 sells a EJ25 TB spacer and has dynos on their site showing around 4hp gain. However most users have reported a significant gain in fuel mileage. Somewhere in the neighborhood of 4 mpg. At $150 this could pay for itself quickly. One caveat is that supplies seem to be pretty limited as the shop makes these on a part time basis. It is not their primary business.

My goals are:

  • Better Throttle Response
  • Easier Breathing
  • Freer Reving
  • Low vs Top-End Gain
  • Affordable, Simple, Smoke-wrench not necessary. (Lots of corrosion in my region)

With that list of goals, does anyone feel that a Throttle Body Spacer will provide anything towards the list, even if we're only talking about "Butt Dyno" effect here? Anything more than Placebo would be fine. Anyone out there with experience of modding on the N/A EJ platform?

Thank you for your time.

  • 1
    Are you running a less restrictive exhaust? Have you done anything as far as tires, brakes, suspension?
    – JPhi1618
    Mar 27, 2018 at 15:32
  • 2
    A spacer is usually just a hunk of material (metal or otherwise ... heck, could even be 3D printed, I'd bet), which fits between the TB and intake. The picture I see of one seems to be about an 1" thick. My suggestion for you is to just make one and see the difference. You could actually make it out of wood, if you wanted to, smooth sanding and sealing the inside surfaces (where the air passes through) and see how it works for you. If you like what you find, get the real thing. AFA N/A tuning, getting air in/out is what's needed for the kick in the pants. At 4hp, the spacer won't do it (IMHO). Mar 27, 2018 at 15:40
  • 2
    I was going to suggest what Paulster2 said about airflow (in/out), but I don't have any "real-world" experience on this platform. Just about everything else but that. Good luck.
    – CharlieRB
    Mar 27, 2018 at 15:47
  • 1
    Well, fit one of those spacers, then go for one of those magnets that clamp round the fuel pipe (so all the molecules point the same way...), after that you could have one of those intake fans to increase turbulence.... For 4bhp, a good service, quality filters and parts is probably a better spend of the same money...But that’s only my opinion.
    – Solar Mike
    Mar 27, 2018 at 17:43
  • 1
    @NitrusInc, I have put a high-flow cat-back on a car, and I have no idea if it added any real power, but it did make the throttle response noticeably better and that was fun for me. That and I loved the sound (not too loud, just a nice rumble).
    – JPhi1618
    Mar 27, 2018 at 20:56

2 Answers 2


I have only ever had one normally aspirated Subaru, and didn't mod it much, but from the mods I have had on various turbocharged Subarus over the years on some Imprezas, WRXs and my sorely missed Litchfield Forester STi, I wouldn't suggest bothering with this.

My preferred route with mods (backed up by Subaru specialists and modders) is:

  • brakes/suspension/tracking/wheels/tyres: not really weight reducing, but essential for better handling, safety and overall fuel reduction as you lose less through corners etc)
  • Then air intake and exhaust path: a smooth inflow (manifold and airbox) and tuned/freeflow exhaust gives your best bang for buck
  • Then turbo upgrades etc: everything gets much more expensive from this point

ECU upgrades at every step, but you expect that anyway, and any weight reduction you can do is obviously a benefit .

  • Thank you for your reply. When you are talking about the Air Intake are you talking about the Air Intake Manifold, the Air-Box, or both? Is the factory air-box restrictive compared to a CAI on a factory tune? Do you agree with the folks that claim that some year Intake Manifolds are more free-flowing than others? Do you have any experience with Throttle Body Spacers on N/A? I know I can google this stuff, but I'm interested in your personal experience.
    – NitrusInc
    Mar 27, 2018 at 20:30
  • 1
    Yes, some are definitely more free flowing than others. I have not done much in the normally aspirated arena.
    – Rory Alsop
    Mar 27, 2018 at 21:32

I discovered this seemingly scientific experiment performed by user wiliaty on the DI Rally Forums on the effects of a Throttle Body Spacer on a 2005 Impreza 2.5RS. This post is mostly just a regurgitation of that experiment.

After some googling it does appear that the 2005 2.5RS does run an EJ253. It is not clear whether the 2005 and 2007 EJ253 are similar within tolerable error rates. It does appear that there might have been an increase in plenum volume after 2005 based on this post at NASIOC on Manifold Testing but that needs more sources. For the scope of this query, this is the closest experiment I could find, and thus it's findings can be loosely utilized as a baseline in deciding if any effect is had.

The experiment presents us with two charts of interest:

The first Chart of Interest is the Engine Load Comparison. This line-chart shows two runs with the "stock" or Control setup and two runs with the TBS installed. The chart is shown below: Engine Load Comparison Chart

The second Chart of Interest shows In Gear Acceleration. This bar-chart uses data from the same set of 4 runs:

In Gear Acceleration Chart

The experiment presents this conclusion/Summary which I've summarized below for easier digestibility:

Summary: There was no measurable difference in AFR. With instrumentation and computer data-logging, I was able to measure a difference between stock and TBS even though I was unable to feel one. Stock has a peak engine load of 1.28g/rev. TBS had a peak of 1.31g/rev. This represents an increase in airflow of 2.3% with the TBS installed. Across the entire rev range, the TBS flows more air than stock, though mostly at much less than the 2.3% peak gain. In-gear acceleration from 3,000 RPM to 5,000 RPM was additionally calculated. It took longer to accelerate stock than it did with the TBS by an average of 4 tenths of a second (5.6%).

While this was only one experiment, with a small number of runs made with questionably accurate data logging, it stands as a base to explain whether or not the platform can benefit from the larger plenum volume provided by a TBS.

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