I have an 04 convert with a JLT fenderwell CAI and am looking to install 75 mm throttle body. What are advantages/disadvantages of keeping stock plenum? Really like the look of the stock plenum .
Have searched throttle bodies/plenums on thissite and have not found any information about keeping the stock plenum on so I guess it is not wise?
Thanks for any help.

 

there are two problems to keepinga stock plenum with an aftermarket throttle body (especially a larger 75mm)....

1 -you're not letting the larger potential of incoming air enter the plenum.
2 - and by having a TB/Plenum combo that dont match up smoothly, you're also creating unnecissary turbulence that hinders the the air flowing as free and one-directional as it should....try picturing how traffic slows down when 3 lanes merge into two....same kinda idea.

all that being said, the difference in horsepower between a full aftermarket setup vs a 75mm TB/stock plenum is so small that you or anyone else probably wouldnt notice a difference.

then....theres always the option of porting your stock plenum from where the TB meets it to match the size of the new TB....there are a couple good write-ups on the web about it...i'll try and find one and post it....

 

So there will be enough meat on the stock plenum opening to port it out to match the 75 mm opening? After porting the opening is thereanything different on the plenum inside tofix?
Thanks for the help!

 

A 75mm TB will fit on the stock plenum but its not about the opening, its about how the aftermarket plenums are ported differently internally than the stock plenum. They are ported so that there is less turbulence in the air flow path inside the plenum to the intake manifold.

With the aftermarket plenums, more air is let in and less turbulence is present, making a smoother transition for the air to go from the TB to the intake manifold.

 

GT03 TJS did you happen to find ay write ups?

 

Here's a link to Accufab's flow data, about 1/2 way down the page you will find the 1996-2004 4.6L 2V data. The stock plenum only by itself (no TB) can only flow about 500 cfm, all the aftermarket units, except the C&L, can flow 600 cfm or better.[/align][/align]The aftermarket plenums have both larger passages, and are taller to provide a more gradual turn and higher stack velocity.A 4.6L at 6500 RPM has a theoretical intake flow of 14,950 liters/minute= 528 cfm--in the real world, 475 to 500 is all it needs. This is some 45 to 70 cfm more than the stock TB/plenum can flow without affecting performance...[/align][/align]I would alsoadd that without a custom or canned tune to take advantage of the higher airflow potential there won't be much gained except bragging rights--in fact I'd spend money on a handheld tuner (and load a premium fuel tune) before updating the TB/plenum.[/align]

 

I plan on a custom tune but wanted to change out the throttle body prior to getting the tune.
Thank you for the information.

 

You decay rate WILL be affected so there is a possibility of having Idle hang.

 

[/align][/align]The OEM plenum is the weak link, keeping that and changing the TB only will not gain much...[/align]

 

+10

If he has an automatic, it will drive the tranny crazy when you come to a rolling stop.
It can be fixed with a tune and *some people* report the ECM learns the correct parameters after a few days
of driving.

That was NOT the case with my Accufab 70mm combo so back it went.