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so i did the searches here and a few google searches and still a bit confused. I have a 2005 ex and an injen rd cold air intake on it. i need/want a bypass valve. now to the question, how big do i get it? 2.5 or 3 nches? what i found looking on this site is people are saying it's a 3 inch diameter BUT i did some crude math and found that the injen rd cold air intake is 2.5477707006369...inches in diameter (8 inch circumference divided by 3.14, yay middle school math!) Sooooo.....

(cliff notes)

...does my 2005 Civic EX (auto) with Injen RD Cold Air Intake need a 2.5 inch OR 3.0 inch bypass valve???

 
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i say bigger is better!!

 
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is that an option? to choose one over the other? that doesn't sound right, maybe i'm just a stoopid head but shouldn't i get the same size diameter bypass valve that is the same size as my cold air intake piping?

 
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bigger is not better. Get the one that fits your pipe. A 2.5" diameter pipe needs a 2.5" bypass valve. Thats what the 2.5" means, it's for a 2.5" pipe.

btw, why the math? just measure the diameter directly.

 
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it is already on my car, that's why the math, got it on 3 days ago. and yea, i figured a 2.5 needs a 2.5 but some posts i found on this site say it's a 3.0 piping, so i was a little confused

 
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um, a bypass valve is not always 100 percent effective. be careful, ive known alot of people who have hydrolocked their motor.

 
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hydrolocked with the bypass valve? well I plan on getting the AEM bypass valve. can't hurt right?

 
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the whole point of the bypass vavle is to make it into a SRI when the filter at the bottom of the bumper is submerged in water. the bypass valve is like the extra air filter. so instead of water being sucked into the motor, the air is being sucked into the intake pipe throught the bypass valve. gotta remember your motor is a air sucking machine. its always suckin air into the TBI. so if you have the motor on and the filter is submergered in water with no bypass valve, then the motor will suck in the water, literally.

 
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also, the AEM bypass valve says "for use with AEM only" is this true? Can I use an AEM bypass valve with my Injen cold air intake? If I had to guess I'd say yes I can, but I'm not here to guess, I'm hear to ask you guys who may KNOW! thanks again guys

 
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yea you can

 
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Its liability protection. They're exonerating responsibility for you breaking your car not using it with their pipe.

Honestly I think the thing is bullshit. Just don't drive through 2 feet of water, simplest solution of all. You'd need to back fill the bumper cavity to cause the filter to submerge, if you're still reving the car after figuring out you're sunk, you're pretty dumb.
But use the 2.5" one, cuz thats what fits the pipe. the 2.5 or 3" is the pipe diameter, so the clamps won't fit if you buy the wrong one.