Smaller Throttle Body Beneficial at lower RPMs?

[StellaTheZJ]

Hello folks, I've got a 1996 Jeep Grand Cherokee with the 4.0l I6 engine. I've been considering replacing my throttle body with one from a Jeep 2.5l I4. They are externally identical, including linkage, but the 2.5l TB has a much smaller bore.

Anyone have experience with this? The Jeep runs at about 1800RPMs on the highway, so I'm not worried about lack of power with the smaller TB. Am I likely to notice any positive change at all before the smaller TB runs out of air at really high RPMs?

[pete c]

I wouldn't mess with it. All it is going to do is lower your max power and a little extra max power is nice to have when you need it. Also it will mean you are running with the throttle open further for a given power output which could cause issues with the FI system.

[RobbMeeX]

Not worth the money it costs to buy the tb. Mostly just fail.

[kennybobby]

Originally Posted by StellaTheZJ

Am I likely to notice any positive change at all before the smaller TB runs out of air at really high RPMs?

The smaller bore means you will apply higher throttle angle for the same power level--and that is a good thing because it increases the volumetric efficiency of the intake. Most cruising is done with the throttle just barely cracked open. Most folks don't drive using the maximum horsepower of the engine--how often do you need it?

[thomason2wheels]

I bet the smaller throttle body will makemore torque at a lower rpm and will make a measurable and significant improvement in fuel economy....same principal of changing a 2 bbl carb with a spreadbore 4 bbl....gets better mileage in every case i know of. Id be interested to know how much of an improvement was realized.

[nbleak21]

Originally Posted by pete c

Also it will mean you are running with the throttle open further for a given power output which could cause issues with the FI system.

Exactly. Ideally you would want to run a larger throttlebody so that the same amount of air gets in while the TPS is showing a lighter throttle reading...

Keep in mind though, that this will likely change the VE at varying rpms and throttle positions... could benifit, could hinder your efforts.

If you want to give it a run for your money without the added spendings, you could get a$5 piece of lexan and run a restrictor plate (the same diameter as the 2.5L) in front of your 4.0L Throttle body.

Though the results will be different whether your system runs off a MAF or MAP system, too.

[ps2fixer]

My vote is on worse MPG... Yes the TPS and the butterfly will be more open, but in intake restricts more air. I beleive a similar effect would be seen with a dirty air filter, which according to the 1st thing to do to your car is a bad thing (tip top running shape). I have also seen posts of people replacing the factory air boxes with different types of intakes that restrict the air less and gain mpg.

Remember the engine is basically an air pump, the harder it is to get air into it, the less efficent it is. I could be completely wrong, but that is my thinking on it.

If the swap did end up in better mpg, I would assume having a restrictor plate would be more efficent since the air would have more area to move in the larger tubes.

Either way you go with this, keep us informed .

[wungun]

Interesting idea...
Typically, a smaller diameter, longer intake tract is beneficial to low-end power, at the expense of high rpm torque/horsepower.
Exactly in rpm range you'll be cruising at.
If your Jeep uses a MAF setup, be careful your smaller TB assembly isn't part of the intake where your MAF is located. In otherworldly, if you use your MAF sensor in a smaller diameter housing, you will seriously mezs up your fueling, as the ECU determines the mass of the air flowing thru the sensor based on the fact that the air is also traveling thru a certain, and known, intake diameter.

If you're going to go this far, I'd also check the intake manifold for velocity stacks...
You can also shift some power down low by using longer stacks.
My one car runs a ECU controlled, duel stage intake...at 5000rpm, it switches from long tube stacks to shorter ones...and you should hear the difference when it happens To WOT!

[Honda100]

As people have said, since it's FI, you are going to have a Throttle position sensor. So at a certain opening, you're going to have the computer thinking a certain amount of air is going in (it doesn't know that your TB is smaller) so it'll be calculating for the larger TB bore. That, and while i am not sure if the Jeep uses a MAF or a MAP, either one's readings are going to be off of what's actually going on. I'd say, just don't do it.

Also, I had a Mitsu Lancer a while ago which used the variable intake runner "thingy" which broke. It got stuck on "low speed" setting, and the mpg only went up maybe by a half an mpg, and it's questionable if it was down to the intake or not.

[wungun]

Originally Posted by Honda100

Also, I had a Mitsu Lancer a while ago which used the variable intake runner "thingy" which broke. It got stuck on "low speed" setting, and the mpg only went up maybe by a half an mpg, and it's questionable if it was down to the intake or not.

If it was stuck on the low speed setting, then I'd think it wasn't that, that affected your MPG...Unless you're typically cruising in 3rd gear at 70mph