I apologize if this rambles, but I wanted to share my thoughts and see what others think. Let me first say I'm an absolute tinkerer... so I don't claim to know what I'm doing in regards to cars or engineering. With that said, I've been mulling this over for several years off and on but never really tested this theory. It all kinda started when my MAF started going out a few years ago. Aside from the kinds of problems that causes (which we're all familiar with), I thought it was strange that it affected shift patterns in the xjr. Several months later, my plastic intake tube was loose and that too changed the feel of the shifts. Since my car currently is mechanically perfect, it does change the way it shifts from time to time. Sometimes, it's smooth as silk, totally impreceptable... other times, it's still smooth but just different. I know these cars take various inputs, and electronically changes it's shift patterns based on current conditions to make it shift smooth, but to me there are still differences from day to day, stop light to stop light, accelerating slow or fast, etc.

So I started mulling it over and realized air volume has an effect on how these cars shift. I started tinkering with the intake positioning to see if I could consistently make changes in the way it feels. I almost always drive in "normal" mode vs. "sport" just because the shifts are smoother.... it doesn't normally have that "ok I'm gonna shift soon, so I'll back off on the throttle for a split second, shift, then resume drive" feeling that sport mode almost always has. Even then, the shift "feeling" is sometimes harsher (or more pronounced) than other times.

Anyhow, experiencing all of the above, I figured if the car had more instantly available air to work with, then the computer and intake wouldn't have to work so hard to drag air into the tube, meter the volume, and figure out it's settings to make the car shift smooth. So using various fittings from home depot, I added a half-inch to the intake tube where it meets the throttle body.

Fired it up and immediately noticed it made no difference in idle quality (my car idles totally smooth to begin with)... however, blipping the throttle slightly felt more responsive... good sign. I put it in reverse and barely felt any kind of "thump" for lack of a better word... backed out of my garage and noticed the tip-in on the pedal was more sensitive, put her in drive (again, it went into gear with barely a flutter), and went out for a spin around town.

Here's what I notice most... in "normal" drive mode, it shifts through the gears and I can barely tell it shifted aside from the rpm change. In "sport" mode, the shifts are as smooth as my car used to be in normal mode aside from being slightly "faster". In either mode, I can barely feel the downshifts. When I floor it in either mode, I get quality feeling shifts that are consistently perfect every single time.

I've been driving like this for a little over a month, and I can honestly say it's made a huge difference. It's the same quality shifts day in and day out. I can only attribute this to the extra half-inch of readily available air volume in the intake that doesn't have to be metered... it's just instantly available.

Since it's been such a positive result for a month straight, I decided to make some brackets for the new intake position and finalizing the extension piece to make it a permanent "mod".

I'd love to hear others' thoughts. Maybe this is the concept behind those throttle body extensions? It does seem to have more kick when gunning the thing.

NOTE: Adding a half-inch to the head of the intake, from what I can see, is probably maximum since there's maybe at best 1/8 inch space now between the top of the intake tube head thingy and the hood when closed!

 

Let us see the mod!

 

This sounds very interesting. I too have wondered about the shift behaviour and the driving forces behind it. Just as you describe, the ever changing nature of the shift.

I would like to hear the techs chime in on this. My science edjucation isn't good enough to fully understand why the extra 0.5" in tube length would make this change? Isn't this post MAF? What sensor past the MAF would be adjusting for more volume in the air tube prior to the throttle body?

I know it may not be your permanent solution, but let's see a picture so visual people such as me can see what you have described.

 

pics, diagram and parts list!

 

@Midlife, that's the thing. I think it has to do with more volume of air swirling around right near the throttle body that's "unaccounted" for? I don't know how to describe it, but it just "seems" like the computer or it's adaptive shift feature doesn't have to "think" about pre-shift-adjustment-for-smoothness... it just "does" it.

regarding pictures: I'll post up as soon as I can... I did take some pictures a month ago just for this reason, but wanted to give the test a good run

 

So I went out this morning to mess around somemore... first start of a cool morning, didn't let it warm up or anything, just started, put it into gear immediately, and went around the neighborhood a few times.... in sport mode.... and I swear the shifts were just as smooth cold as it is warm. I don't know why this had made such a big difference, but it has. I'll post diagrams ASAP as I'd be interested to see if you guys get the same results. I also noticed that any abrubt release on the pedal from a stop light (such as in stop and go traffic) is significantly smoother than before. I gotta run to work, but will get more info up soon.

 

Look forward to see what you have done. If a before and after was done on a rolling road / Dyno I am sure the resaults will be interesting.
Regards
Gareth

 

very interested to see what you put together tech!

 

Did anybody get the info on how to do this and what he used? has any one added a TB spacer plate and see how that works? Been looking to make that simple MOD since it does increase MPG and HP...but curious as to how this one worked out....

 

Snake oil (on his part). When one washes their car it certainly seems to run better.

RE your second question, do you understand that it is almost impossible to get BOTH improved fuel mileage AND an increase in power... they are diametrically opposite each other relative to their physics.

 

LOL yes you got me there, its an increase in fuel miliage with the mods to the intake, it would be an increase in HP when combined with a freer flowing exhaust. i was more looking for the improved miliage and increase in response, which a spacer plate on the TB would do that, I just hadnt heard of anyone doing it from the front....