To answer the question we have to look at the
purpose of the TBS. Which is to lengthen the intake runners.
Lengthen the intake runners and get and increase in
HP/torque and MPG. That said this follows the rule of diminishing returns.
There is an optimal length for any given engine and cam profile.
In the case of the Jeep 4.0L those with engines that
were made prior to about '01 (on the exact year I am not positive) you can add
a TBS and see some gains OR you can add the intake manifold from a later model,
which has longer runners.
A company called 'Hilborn Fuel Injection' sold FI
systems for the racing crowd back in the 60's. It came with 8 (V8) rather LONG
stacks or pipes that fed the air thru the injector system and into the
cylinders. The stacks were removable and you trimmed them to length to adjust
HP/torque to suit your cam profile and car.
Adding a TBS to a Jeep that already has the improved
intake manifold will also make a impact...in the weight of your wallet.
For those of you with a earlier model 4.0L you can
add a TBS for an overall improvement.
While we are on the subject of improvements, if you
REALLY want to see an increase in HP/torque and MPG then go to a COLD air
intake system. Sucking in engine bay air is a waste of your time and will ONLY
provide a decrease in performance.
Adjusting the length of the intake in front of the
butterfly valve does have an impact but not as much as the in the runners.
There is the air turbulence and the effect of a moving consistent body of air,
which sans turbulence will gain in momentum, thus packing the cylinders with a
denser charge.
It is my guess that somebody whose fuzzy memory told
them that if the 'carb' spacers of yesterday was a good idea then why not a
spacer under the TB.
That is the problem with memory; we do not always
remember all that was contained in the reason why. In this case not only did
the carb spacers increase the length of the runners but also they served to
(and I suspect this was the most import item) reduce the temp of the
carb-gasonline and incoming air. Colder is better which is why my 'TrashAir'
works so well, in fact I would put it against nearly every CAI system I know of
on the market.
So if you want performance then just buy a $3 buck
plastic trash can at Wally World and a aluminum foil windshield shade and save
yourself about $295 bucks.
That said I believe the origin of the spacers used
back in my racing days was primarily as an insulator. Why I think that is that
many V8 engines back then ran exhaust ports that opened into the INTAKE
manifold and led directly under the carb. This made a lot of sense for those
cold winter mornings. In addition, dual exhaust (and maybe even single exhaust)
cars had a butterfly valve on one side that that had a spring that when heated
would cause the butterfly valve to open. This too served to heat the engine
more rapidly in winter.
I promptly removed the exhaust baffle and I pulled
the intake manifold and replaced the gaskets with ones that had the exhaust
port blocked off. I later added a thicker asbestos-aluminum sandwiched carb
gasket. It was maybe 1/8th or 3/16th at the most thick, been too long and I
have forgot.
As I remember (leastwise in Chevy small block circles)
the talk of spacers was always of heat reduction. I do remember some talk of
improved performance to to added height, but it seems that was on engines that
ran the Q-jet carb. Not the Holley's we ran.
I just think that time has blended heat reduction
with longer runner improvements that resulted in increased performance due to
heat reduction, NOT longer runners.
As carb stacking grew in the ranks it was not
unusual to see guys with spacers in multiple inches of height. Of course those
were always made out of aluminum, which defeated the purpose.
We see this same phenom here in off road circles. It
was only a few short while ago that lifts of 6 in and beyond were the norm. In
fact I asked one of the Jeep GURU’s, well known in the circles why he ran the
nosebleed lift (10 in as I remember). Of course those where the days when I was
called in circles as 'Low-Rider' for my 3 in lifts, which I still run today.
Bottom line: Adding a TBS to a newer Jeep is sure to produce as much results as taking your accordion to go deer hunting.