Re: 2.5L power improvement
FNG-I followed a link (I don't have it anymore) to a guy's page who installed the TC bored out throttle body on a 4.0...he said that it "whistled"......and that TC instructed him to "gouge" the horn near the manifold........they said that the more of a "rough gouge" the less the "whistle" has a chance to form (I guess you're giving it chapped lips [img]/wwwthreads_images/icons/smile.gif[/img])....this equated to "the more of a butcher job you do on it ....the better.
Again the TC airhorn mod is an air related improvement. That is, it gives the engine breathing capacity beyond the design specs....as does the Borla headers and cat backs. What I believe (that is, my opinion) is that ..since all the fuel adjustments are computer controlled and compensate for air flow (MAP manifold absolute pressure sensor and MAS mass airflow sensor)that the only time you'll see any improvement or power enhancement is when you're at such a high rpm that you exceed the manufacturers intake/exhaust design flow ........that is, there is still "reserve" capacity for computer compensated fuel delivery... IF you have the added air flow............which means.......the stratosphere of the rpm range. This is what makes me doubt the real usefulness of such costly improvements......hence my critical skepicism of them. Just like when my old high school pal put an Offenhauser 4 bbl manifold and a Holley carb on his Plymouth slant 6 .........without a cam change it was just a fancy looking slant 6 ..........he couldn't get anything out of the added set of lungs.........with the stock cam it could only draw so much air/fuel.
Now a super/turbocharger ......that's a different story.......it, without regard to the intake capacity/valve size/combustion chamber shape-size/exhaust restrictions/camshaft grind, is going to cram more combustable intake charge into the system. You're kinda "reindexing" the engine .......that is, you're manipulating it to have an intake charge of a higher rpm at a lower rpm. Not that improving those other things would'nt help.......but since we can't do ALL those things (mainly the cam grind) and maintain our states "seal of environmental approval".....we're pretty well stuck with what the factory gives us..............or spend the $3000. I don't think that is too much to spend if it gives you what you want. I find it curious that most of the Jeep owners will be downright radically ingenious at figuring out how to get 2mm more clearance on their rigs ......but go the simple route when it comes to power (mainly, to quote dorfs, "there's no replacement for displacement" or throw in a V8).....again....I seek the untread path.
I'm, of course, open to altenate schools of thought on headers and such... but for right now......the weight of the evidence that I've seen (mainly the LACK of real world dyno tests) leaves me with little to put faith in.....and more importantly my money.