I am in the process off boring my AJ26 throttle body DIY style. I am hoping to post back with a comprehensive DIY procedure with photos etc. once I have it back in the car and I know it works.

Right now I have it set up in a Haas CNC with a boring bar. I can essentially take passes until my heart's content.

What is the largest possible ID I can bore to before the ECU will be unhappy? I have seen a thread from a while ago where someone basically redesigned the throttle housing from scratch to go larger than the OD on the stock throttle body. The project had to be abandoned due to the ECU throwing an error for idle flow out of range etc.

The biggest I have seen from all of the Maxbore threads was 82.5mm Can you go larger and have it still work?

 

Speaking only about the AJ27S 4.2 engines with a std. 75mm TB. I think 83mm or less is the max you can enlarge them to as it depends on the variations in wall thickness. Some could only go to 81mm IIRC.

The Sechtor 83mm ones were troublesome I've heard but never heard of a Maxbore one giving issues.

 

MWC, what is the stock size of the TB?
I wouldn’t go past the 82.5mm for the AJ26/27 if that’s what you read from other members of X308/X100 4.0 V8 only! Do not take measurements from a 4.2 TB as it’s completely different than ours.

 

@MWC

Have a good root through waterdragon's posts, he carried out a lot of modifications and pretty much stretched the platform to over the limits on a couple of engines. You can get a lot more info on these units by searching within this forum, it's all been done before. Twinscrew is the only real way of adding true BHP, the rest is keeping the whole thing cool - a 4L heatpump!

https://www.jaguarforums.com/forum/s...rchid=15410613

 

So after a few passes we landed on 82mm. We probably could have gone another .5mm, but we achieved a very nice finish on the 82mm pass. Trying to take another .5mm could have resulted in chatter and at that point you're running out of throttle body.

Going from 75mm to 82mm is a 19.5% improvement in overall area. I'm pretty happy with that.

Making the butterfly shouldn't be that difficult. The stock butterfly is .080" thick. Thankfully I was able to order a 12"x12" sheet of C260 brass at that .080" thickness off the shelf which was very helpful. When completely assembled and with the butterfly closed, the tolerance of the stock unit is very tight. Only a .001" piece of shim stock can fit between the plate and the inner wall of the throttle body. The stock plate is also an oval. It is .005" larger in diameter 90° to the axis of rotation. That's very easy to program, and probably crucial in order to achieve the tight fit of the stock butterfly at the slight angle it has when fully closed. I have enough plate to try 4 times. Hopefully we can find success.

 

His threads have been very helpful. That's where I kind of landed on 82.5mm as being the max. I didn't want to go farther than there was precedent for the ECU not rejecting the modification.

 

The stock butterfly to TB clearance when closed was how small?

The reason I ask is because since you have more surface area of the butterfly, you’ll likely have a higher idle and throw a code. So you’ll have to close the new butterfly more so because you have more air passing through.

Also, you’ll need to replace all those throttle shaft sealed bearings. I believe there’s 5 of them. I replaced 3 on my 98’ XJR years ago because the car was a complete dog until warmed up when cold. After the 3 bearings, it opened up promptly when cold.

If you get that TB going with no issues, I should send you an AJ27 TB and see if you can bore it out also. I have a 3.5” CF air intake that could take advantage of that 19% increase. The reason I went with 3.5” was because Maxbore was still around but I missed him by a year when his old machine broke and couldn’t get a bored out TB.

 

The TB's are both 75mm so the comparison to a 4.2L TB is valid.