Larger Throttle Body - Effect on Throttle

[FusiliJerry]

Chasing down every viable improvement on my 2001 since I got it 2 years ago, a larger/bored out throttle body is frequently on the list. Looking for some accounts on a stock 4.0; does it appreciably improve performance at WOT? My biggest reservation and want of opinion is whether it will make light throttle too "touchy". I actually love throttle response at the low and mid-end as-is.

 

[Fred]

I have a 90 so they are very different. I had Mopar headers and a 2.5 inch exhaust installed early on, I've had it since 92. The exhaust didn't make a whole lot of difference. I live at 6k feet, so the beast always ran rich. I had the throttle body bored, actually the same as an HO T body, and it did respond. I haven't kept numbers like gas mileage for a long time, so I have nothing definite to show you, I just remember the beast being happier. The beast on 33's, now on 35's/

 

[Anak]

Not that I have installed a larger throttle body (I do have one--it is pending, as follows...), but as a matter of general principle you need to improve the airflow all the way through the engine in order to get an appreciable improvement.

My bored out throttle body is intended for my stroker build which includes a Russ Pottenger ported head and intake, larger cam and a Borla header. All together those should move more air through the engine. If I were to just install the throttle body on the stock setup I don't think I would get an improvement.

 

[75SV1]

I have bored out the stock TB on a few of my XJs. So, 60mm. They also had/have to CAT back 2.5 exhaust and Doug Thorley headers. On my '98 it did much better on inclines in WV. So up to 9 degree inclines. I did not notice any issues with lower or midrange. I did run basically the same on my 2000 XJ. Ran good overall. Has some issue with misfires, so hard to really evaluate. I did have a 62mm (BBK ?) TB at some point on my '98 and is now on the 2000. The '98 didn't like it. I am not sure if much or any MPG improvement. I did give a few I bored out to a Guy who does work on my XJs. He seems to like them on his '96 stock XJ. Not sure if he put one on his '96 2dr 4 cyl XJ.

 

[FusiliJerry]

Interesting. Being I'm not planning to do headers or exhaust any time soon, it sounds like it may not be worth the trouble at the moment. Being the stock intake inlet is 64mm or near enough, its always made me wonder what might be left on the table by throttle body restriction alone.

 

[75SV1]

The opening might be 64mm. The diameter at the butterfly is 60mm. Then it is restricted after that.

 

[FusiliJerry]

Fair point, didn't know it was 60mm.

 

[XJlimitedx99]

I did a larger TB awhile back (62 mm, IIRC), and I sometimes don't like it because I find its hard to hold a steady slow speed, such as driving through a downtown area at 20-25 mph. Its like I accelerate too much even with the smallest throttle input I can give, then need to let off an coast down slightly again.

Not that its horrible and I totally regret it; definitely not, but I certainly noticed a difference in throttle response.

 

[FusiliJerry]

I did a larger TB awhile back (62 mm, IIRC), and I sometimes don't like it because I find its hard to hold a steady slow speed, such as driving through a downtown area at 20-25 mph. Its like I accelerate too much even with the smallest throttle input I can give, then need to let off an coast down slightly again.

Not that its horrible and I totally regret it; definitely not, but I certainly noticed a difference in throttle response.

This was precisely my worry, thanks for your account of it.

 

[Tim_MN]

I have 60mm bored TB on my 2000. The off idle and low end throttle response is bit more touchy compared to the original TB and to my 1999. This causes the bored TB to be less than ideal when on the 4x4 trails. The bored TB does make the transmission downshift sooner than stock when you floor it. If you can do one cheap, or do it yourself, try it, but don't pay more than $75 for a bored TB.

The basic rule of hot rodding is you have to have better exhaust before you can maximize the intake.

The best first mod to make is cat back exhaust, followed by a better flowing exhaust manifold.