value of inlet trumpet on air box?

[ChetSzy]

Hello fellas - I have a good understanding regarding the use of velocity stacks of varying lengths on carburetors and the like. What I am not sure of is the value of a velocity stack/inlet trumpet on the vehicles air box itself.

On this application (97-up, Jeep Wrangler, 4.0L, I-6) I personally know many drivers who have simply removed this piece and left a 3" diameter hole on the side of the air box. The group is in search of more low-end torque and they claim seat-of-the-pants improvements.

Can anyone provide some detail regarding these intake trumpets placed on the outside of the air box?

Please see pic here:

http://members.cox.net/cszymecki/Image1.jpg

Thanks in advance!

 

[ivymike]

yeah, fluid flow into a pipe is improved if the pipe doesn't have a sharp edge.

 

[Retracnic]

The intake trumpets fitted to factory airboxes typically serve two purposes:

1. To provide a more laminar flow into the airbox. This helps reduce the pressure differential associated with the airflow experiencing a sudden change in volume.
2. For NVH (Noise Vibration Harshness) reasons. A properly designed trumpet can help attenuate annoying resonance in the intake tract.

It is doubtful, that removing the trumpet, would produce any change in power output that would be detectible to the driver.

Regards,

Bryan Carter

 

[patprimmer]

The perceived performance improvement might be a result of the increased induction noise. Noisy feels faster, even if it's a little slower

Regards
pat

 

[Marquis]

I would normally wholeheartedly agree with what Retranic said.
In fact I was adament until I saw some data of an old engine of ours-actual measured dyno data.
The engine was a Jaguar AJ16 straight six 4 litre. There was a reccommended pre-production change where a smoothed over lip entry into the intake spout of the Airbox was implemented because test bed data showed quite a lot of improvement in performance [ of the order of 2 % peak power and 1% peak torque-repeatedly]

I was ready to dismiss this as test to test variation despite the consistency of the results during repeated tests.

On further examination of the data it was shown that the intake manifold depression changed with the spout design change. [ Intake manfold depression is the delta of the pressure measured post throttle compared to ambient-therefore the loss]

Conclusion; the original spout design was so abrupt that there was a 3-D effect which caused entry flow seperation INTO the spout/airbox that actually constricted the orafice area! This lead to the greater loss. Intake manifold depression has a HUGE effect on performance (numerically much greater than exhaust backpresure)-especially supercharged engine –which this wasn’t. So in effect I’m not really contadicticting retranic but pointing out a special case.

Chetszy, my “kinda”-girlfriend who I’m going to visit next month in Ohio, has just bought a Jeep Wrangler with the S6- you’ve got me intrigued, I’m going to have a peek under her bonnet/hood!

Now I have a question which I’ve seen a few times on dyno test but have never been able to explain:
I’ve seen throttle size reduction on an engine running at Wide Open Throttle where the region of peak torque is effected MORE then peak power!
Is this a tuning phenomenon? I’ve seen it both on a 4 cylinder and a V8-in both cases which generously proportioned plenums where you’d expect any tuning pulses to be damped out. It certaining can’t be a restriction effect to effect peak torque region more then the potentially more flow limited peak power region!