Re-spacing throttle body

I want to bore out the throttle body on my 2000 XJ, but after reading some of the posts on the subject have a few questions.

- Is there any noticeable advantage to 62mm straight through vs. 60mm with a taper?

- Assuming I do go to 62mm, can anyone recommend a source for replacement butterflies?

- Has anyone in California had problems getting the vehicle smogged after doing this?

FWIW, I'm not looking for better top-end performance, just a little better mileage around town and a small torque boost.

from what I hear the 62mm is only better for a stroker or non-natuarally aspirated 4.0

noresttill said:

from what I hear the 62mm is only better for a stroker or non-natuarally aspirated 4.0

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From what I've read the optimal size for a stroker is 68mm, but that's neither here nor there since I'm running a fully-stock 4.0.

I guess my question really boils down to this: is the extra expense of taking the throttle body out to 62mm worth the gain it gives, or is 60mm adequate? I found the butterflies on xj-armor's site (warnings from others noted), and if I'm going to have the shop machine it out to 60mm it'd be just as easy to go out to 62mm.

Other than that, I still need to know if anyone here has had issues with passing smog with a bored-out TB. If I end up facing an emissions nightmare, I'll look into other avenues.

From a smog standpoint....If they can't see it, they will only know it exists if you tell them.

Looks Identical on the outside, right? Should not make any difference in the tests.

casm said:

but that's neither here nor there since I'm running a fully-stock 4.0.

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Then I'd stay at 60mm all the way through...

DR

Bored my stock TB to 60MM straight thru. No need for a new Butterfly. 1" TB spacer also at 60MM and Rusty's Flowmaster 50 and mandrel bent exhaust pipe.

RESULTS:
Gained HP and acceleration, motor responds sooner off the line and revs higher before shifting the auto tranny. Downshifts sooner and pulls harder for passing. No whistling. NO CHANGE IN MPG if I am easy on the pedal.
.
The restriction on the stock TB blocks the lower 1/2 of the butterfly until it opens to more than 20% open.
.
I would definitely recommend doing the mod to 60MM. 1" TB spacer is optional but I found it at the junkyard and it was free.
__________________

I recommend the Fastman TB. Easy installation and swap. You can search for others' recommendations.
http://www.thefastman.com/Throttlebodies40.asp

JohnX said:

Looks Identical on the outside, right? Should not make any difference in the tests.

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It's more the latter I'm concerned about than the former. Having said that, I don't suspect it to be a problem, but smog tests and I have never got along.

Tim_MN said:

I would definitely recommend doing the mod to 60MM. 1" TB spacer is optional but I found it at the junkyard and it was free.

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Yeah... The spacer's not really on my to-do list, but if I ran across a freebie I'd probably use it anyway.

steveareno said:

I recommend the Fastman TB.

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Thanks for pointing that out - I may end up going this route rather than having the TB machined in order to keep downtime to a minimum.