Boring TB, Venturi effect good?

[Mopar500]

I have read that many people have had great results boring out the tb on 4.0s.
It seems to me that the tb is designed to have a venturi effect to increase the velocity of the air mixture entering the intake, and that the boring process actually removes this.

Question 1.
Why would the factory design the tb with a tapered design when it would seem that a straight design would be easier to produce? Is there some benefit that is eliminated with a bore?

Question 2.
I wonder if there is some sort of a placebo effect happening here. Has the difference in power/economy been measured?

Any ideas or thoughts?

 

[badron]

(IMO) the TB will not produce a meaningful venturi effect. There can be a use for one if an amplified vacuum signal is need or a vacuum signal needs to be maintained at WOT operation by some port. But my 01 needs no shush signal. The shape of the TB may have to do with ease of manufacturing more then anything.

 

[Mopar500]

That seems to make sense.

 

[jocko463]

I have read that many people have had great results boring out the tb on 4.0s.
It seems to me that the tb is designed to have a venturi effect to increase the velocity of the air mixture entering the intake, and that the boring process actually removes this.

Question 1.
Why would the factory design the tb with a tapered design when it would seem that a straight design would be easier to produce? Is there some benefit that is eliminated with a bore?

Question 2.
I wonder if there is some sort of a placebo effect happening here. Has the difference in power/economy been measured?

Any ideas or thoughts?

I have a 4WD '00 XJ (stock save for a DynoMax catback) and tried the over-bored throttle body route and found the transmission (AW4) had lost all of its "snap" when shifting. Rather than having the normally firm feeling when shifting, the tranny ooozed between gear shifts. The necked down area of the TB creates a higher vacuum signal that is apparently used by the transmission computer module through the manifold absolute pressure unit to determine some engine parameters (load, throttle opening, etc). If you bore out the venturi, the vacuum signal is greatly reduced fooling the TCM and boogering up the shift profile of the tranny. When I replaced the bored TB with the original, everything came back to normal with the transmission. Having had the "big" TB on for about a week, I can say I really didn't notice all that much difference in performance. There may have been a slight improvement in highway cruising (a little more punch when passing, eaier to maintain speed on hills or with A/C on) but not all that much.

IMO, the 4.0L has been designed to perform quite well in an overall, daily driver capacity. Naturally there are compromises because of the many parameters the engineers had to cover in desigining this engine. Can it be made to make more power?... certainly... but like any other engine you have to go the total route (heads, cam, flow improvements, better cooling, etc) to get any meaningful increases. More likely than not, you actually degrade performance or make no improvement at all going with the "bolt on" performance mods. Like you indicated, if small or insignificant design changes can produce significantly measureable increases in performance, don't you think the engineers would have incorporated them?

There is a thread on this topic a few months back (I started it by asking a question about my big TB experience) and I think there is also a link to a site where someone did some dyno testing with various TB mods.... will try to find it for you and link it.....


Just my 2 centavos...

mike
00XJ

 

[Fred85]

the transmission (AW4) had lost all of its "snap" when shifting. Rather than having the normally firm feeling when shifting, the tranny ooozed between gear shifts.

the shift firmness on the AW4 is controlled by the kickdown cable which is adjustable, and not linked to any sensors on the TB......i had no such problem with a change in shift firmness when i bored mine

i've read on more than one occasion that the taper is there to prevent such a jerky power surge off idle and in 4low........which makes good sense, my long pedal has become very sensitive off the line

 

[j99xj]

i've read on more than one occasion that the taper is there to prevent such a jerky power surge off idle and in 4low........which makes good sense, my long pedal has become very sensitive off the line

That's my theory.

The only purpose of that lip on the throttle body is to make the engine less jumpy. My 4.0 with the bored tb has so much throttle response that I must be more careful when someone is immediately in front of me (like in traffic) so I won't rear end them.

As we are all aware of, the 4.0 is an old school motor with a somewhat lazy cam profile that builds low end torque very quickly. Most "average" people don't want snappy throttle response. They want a friendly, smooth, and progressive application of power. But I'm sure that 99% of the folks on this forum are NOT "average."

I feel the 4.0 was originally meant for a 62 mm throttle body, because the intake hole is that size. I feel the lip was a last minute change to the design to smooth out the throttle response so the average idiot wouldn't spill their kool aid.

 

[jocko463]

Consider the year of your XJ and the emissions package it has (federal 49 state vs California).... the earlier models and/or Federal emissions vehicles may not be affected by the increase in TB diameter. They also may not be as affected by TB's that have been modified by simply removing the taper. My XJ is a 2000 model, it is fitted with the California emissions package and considered a "low emissions vehicle" (the friggin' thing has *3* cats on it and 4 O2 sensors).... so maybe this may have had an influence on my outcome.

My experience was with a TB that had been completely bored through; it was oversize and bored out to match the hole in the intake manifold (62mm I believe).

My TV cable was adjusted correctly, and rechecked after the TB swap following the Chrysler FSM procedures.

Here's the link to the previous thread I started on this subject.....it even has a post with a link to a dyno test.

http://www.naxja.org/forum/showthread.php?t=93187

Hope it helps...........

mike

 

[j99xj]

I think the throttle body has very little to do with emissions.

 

[Mr. Notorious]

I've read a couple other threads about this and I noticed some talk about it adversely affecting automatic transmission shift points and what not. Now if one has a manual transmission, can a bored out throttle body still have some negative effects? Does the computer communicate with the manual transmission at all?

 

[Hellbent]

i've read on more than one occasion that the taper is there to prevent such a jerky power surge off idle and in 4low........which makes good sense, my long pedal has become very sensitive off the line

this is true. a taper immediately behind the throttle plate will make for a smoother transfer from idle to wot. the pedal is somewhat touchier in my renix now that the bigger tb is on there, but i'll take that trade-off. i'm pretty sure that is it's only purpose. i can't see that a taper would be of any other use on an mpi vehicle, after all it doesn't have boosters and idle bleeds like a carb. but maybe i'm dumb......:gee:

 

[Powerman]

We need to stop confusing the talk here with venturis. The TB is not a venturi. A ventrui is made of not only a constricting tube, but the expansion therafter as well. This crates a low pressure area at the necked down point. If you atatch somthing to that you can have a pump by passing a gas through the venturi chamber. A venturi needs a certain constriction of the opening and a certain lenght of expansion, but I don't recall what. A throttle body is no such animal. It is nowhere near such perameters. I'm no expert, but other have simply told me the necked down part is matched to the TB plate to improve throttle response.

 

[Hellbent]

i apologize. as soon as someone mentions tapers i run off and start yelling about carbs and the venturi effect. the xj is only the second vehicle i have ever owned with that new-fangled whizbang e-leck-tron-ick fuel infection. the taper is there for smooth throttle response. the end.

 

[jocko463]

We need to stop confusing the talk here with venturis. The TB is not a venturi. A ventrui is made of not only a constricting tube, but the expansion therafter as well. This crates a low pressure area at the necked down point. If you atatch somthing to that you can have a pump by passing a gas through the venturi chamber. A venturi needs a certain constriction of the opening and a certain lenght of expansion, but I don't recall what. A throttle body is no such animal. It is nowhere near such perameters. I'm no expert, but other have simply told me the necked down part is matched to the TB plate to improve throttle response.

The tapered, smaller diameter area of the stock TB on the HO 4.0L is in fact a venturi (the Renix may have a similar configuration). Rather than using it as a pump (such as in a carb to draw and atomize fuel), it is used to create a strong vacuum signal for the MAP sensor...one that is readily affected by throttle opening. The expansion area you talk about is provided by the plenum of the intake manifold (as it would be in an engine with a carb on it). Also, a venturi will increase the velocity of the intake air (TB) or fuel charge (carb) and this can have and influence on low speed driveability, create intake turbulence to better homogenize the air/fuel mixture and other parameters best described by an engineer.

 

[j99xj]

When you bore out a HO throttle body, your increasing the amount of air that can rush into the engine when its running. (duh)

More air leans the air/fuel mixture, however, since the pressure is higher on the map sensor (the venturi is bored out), this will signal the computer to spray more fuel through the fuel injectors.

More air=More fuel=More power

Granted a bored throttle body doesn't make a huge difference, but it is noticeable. Off idle torque is very good.

 

[Harlee&Tahoe]

Header on order. Rest of 2.5in exhaust already installed
I think this is going to be my next renix upgrade.
Sounds promising from the responses.
Extra little kick sounds great.

Thanks

 

[Mr. Notorious]

If driving habits same the same, and I add a TB spacer at the same time, how much would this increase gas milage?