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Discussion Starter #1
Hello,
Sorry if this has been discussed before, but after some internet searching, I can't find any information on boring out an outback throttle body. I have a 2001 limited, and I'm wanting to get a throttle body bored out and put in.

If anyone has the inside diameter for the TB on the 2.5 L, and if it is tapered, that would be greatly appreciated.

I read on a different forum that the Subaru TBs are touchy, and one person had trouble with the computer after a simple clean. Is this really the case? If so, is it feasible to bolt on a bored out TB to my outback?
 

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2016 Tungsten Outback 2.5l Premium w/ES, OP 14, PP #4
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Hello,
I read on a different forum that the Subaru TBs are touchy, and one person had trouble with the computer after a simple clean. Is this really the case? If so, is it feasible to bolt on a bored out TB to my outback?
I don't have a comment on enlarging the TB. However the "computer" uses an adaptive learn process. As the TB becomes "dirty", fuel adjustments are made to compensate for the dirty TB. If suddenly cleaned the computer is still supplying fuel based on a dirty TB. Over time it should relearn but rough idles and less than optimum drivability might be experienced. I haven't had a need to check but on several cars I've owned disconnecting the battery as part of the cleaning process causes the computer to "reset" it's learning curve.
 

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03 H6 OBW & 06 WRX Sportwagon
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seems like an extremely uncommon mod for our cars. I've read a lot across 3-4 forums for 15 years and this is the first time I've read about boring the TB out.

But, I'm willing to learn.
 

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2010 2.5 CVT Limited
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Admittedly, I'm not current on what tuners are doing these days, but I recall swapping throttle bodies to larger bore and installing bigger injectors is a common mod to increase power. Boring out the TB? New to me. Wouldn't that also require a new throttle plate to seal against the enlarged bore?
If I were trying to make bolt-on mods to an OB engine, I'd start by looking at Impreza and WRX parts (OE and aftermarket) for things that will work. The trick is having to get a custom ECM map after mods since I doubt any tuners have a ready to download map for the OB.
Of course, if you've already done the research and found nothing is available, then you are on the right path for custom mods.
 

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Discussion Starter #7
Thanks for the input guys. I'm asking because I also have a Jeep xj, and this is a pretty popular mod for the 4.0. It made quite the difference in my xj and I was thinking it would be good on my outback. What I would likely do is but one at pick n pull on one of their sale days and have a machine shop bore it out. I would still have the old one just in case it isn't a good mod. I would need a new butterfly valve plate, but I don't think that would be hard to find. If it is tapered like the xj TB (the diameter is smaller on the intake manifold side) then I would just remove the taper.

I might be comparing apples to oranges (with my xj), but it was a great mod and I want a few things to liven up my outbacks disappointing throttle response.
 

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Discussion Starter #8
I'll have to look into resetting the computer, but if it is similar to the xj you just disconnect the battery, press on the brakes or turn the lights on, and the adaptive memory is reset.
 

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Discussion Starter #9
By the way, I'm still trying to figure out this forum on my phone. Though I'm not using quotes or anything yet, I am reading all the posts and I appreciate the comments.
 

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take some pics along the way , we'd like to see how things progress.

reduce mass if possible, lighter rims and tires, don't haul around tools, maybe even leave the spare tire/tools at home if you can always call for help. Pull seats if literally no one will be sitting in them

1 hp =~12 lbs?, much better if you remove rotating mass (rims/tires, crank pulley maybe, flywheel maybe) (these calculations all depend on hp/mass rtio of the car to begin with - and of course, some mods may be 'unpleasant' in a daily driver)

user @west_minist has helped people get more performance from their whip's engines, just, not a lot of potential left in the NA engines, and of course, there's plenty of bits to help it corner and brake better.
 

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Discussion Starter #11
Yeah I'll keep everyone updated on what I do. My ultimate goal is increasing performance and fuel efficiency. It will be slow going though; I'm busy and I have my Jeep to work on as well. I'll post updates on what I do, starting with the throttle body.
 

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Discussion Starter #12
So I got a throttle body from pick n pull recently. Surprisingly the diameter was 64 mm tapered down to 60 mm. I'm not sure how much I'll bore it out but I'll keep all of you posted.
 

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Why do you believe a bigger throttle body will provide better fuel efficiency?
 

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Discussion Starter #14
It is proven that a larger throttle body (often when combined with other mods) will show an increase in horsepower and torque at a given rpm. A larger throttle body will allow more air in, which will require more fuel to burn, thus increasing power. In other words, a larger throttle body increases the efficiency of the engine; there will be more available power at a given rpm. It is more efficient because the increase in power is greater than the increased fuel consumption. Some believe that this upgrade causes the air fuel mixture to be more lean, which also increases fuel efficiency.

I know that a larger throttle body increases fuel mileage because I've done the mod on a 1997 Jeep Cherokee. It was the first modification I made that affected airflow. After upgrading to a 62 mm throttle body from the 55 mm stock TB, I noticed a large increase in throttle response and a mild increase in gas mileage (like 0.5-1 mpg). I think having that modification helped with I did other things, such as a performance catback exhaust and a custom CAI.

There can be problems if you bore out the throttle body too much, namely losing air velocity, which ideally is something like 300 f/s. I was surprised that on my outback the TB is already 60 mm. Since it is tapered (the other side measures 64 mm), I might just bore it 64 mm all the way through and see what kind of difference that makes.
 

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Master Caster
2005 XT, Mildly Modified...2006 XT Limited, Highly Modifed
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That 4.0 is a different motor. They were designed for the '78 model year and improved along the way. It left little bit on the table. They also benefit from a throttle body spacer.

This has little effect here in the 6-star world, It might be measurable, but NOT by the Butt dyno. I like the XJ reference, though.
 

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Master Caster
2005 XT, Mildly Modified...2006 XT Limited, Highly Modifed
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Discussion Starter #17
That 4.0 is a different motor. They were designed for the '78 model year and improved along the way. It left little bit on the table. They also benefit from a throttle body spacer.

This has little effect here in the 6-star world, It might be measurable, but NOT by the Butt dyno. I like the XJ reference, though.
I am skeptical of throttle body spacers... I doubt they do anything

Really the whole point of doing these things is to increase fuel efficiency. Vehicles back in 2001 weren't exactly built with fuel economy as a priority. As with most mass produced vehicles, cost was the biggest factor. If they had a choice of increasing gas mileage by 1 mpg or saving $4 per car, they would choose the latter.

Small things add up. I would be happy to get 0.5 mpg from doing this. I measure gas mileage at the pump so I would notice a difference.
 

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Master Caster
2005 XT, Mildly Modified...2006 XT Limited, Highly Modifed
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I am skeptical of throttle body spacers... I doubt they do anything
They increased turbulence, in the case of the TB-FI in the XJ 4.0

If, .5 is measurable to you i the OB, then the TB spacer on the XJ 4.0 would have been as much benefit as the boring. But who really cares here on the OB forum.
 

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2001 OBW VDC, 2004 WRX, 1999 LEGACY L (in a heap)
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It is proven that a larger throttle body (often when combined with other mods) will show an increase in horsepower and torque at a given rpm. A larger throttle body will allow more air in, which will require more fuel to burn, thus increasing power. In other words, a larger throttle body increases the efficiency of the engine; there will be more available power at a given rpm. It is more efficient because the increase in power is greater than the increased fuel consumption. Some believe that this upgrade causes the air fuel mixture to be more lean, which also increases fuel efficiency.
but.... would any of that matter with computer controlled fuel injection/ignition management?

It makes sense giving a computer controlled system more air input is going to lean it out to a degree. But it would only have a power or efficiency increase if the existing tune was running [relatively] rich, right?

If you mess with air or fuel, you really need a tune to actually capture any potential it offers is what I was always guided.
 
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