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Thread: 4.0L throttle body on 2.5L

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    Newbie kegrill's Avatar
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    Default 4.0L throttle body on 2.5L

    Hey guys, I have a 2.5L and was curious if anyone has done the 4.0 throttle body/spacer swap and if you thought it made a difference? I'm in the process of upgrading to 4.88s and though I dont run it on the highway much, it would be nice if I could keep up a little better when I did.

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    Zombie Slayer... JoeySnow70's Avatar
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    The gains are negligeble if any....... Ran it for about 3 months with no noticeable diference other than idle was a little more quicker, no extra power felt(never dinoed so dont have any numbers to back it up)..... CAI, TB spacers or big TB wont produce power gains without a some sort of tuner or tuned ECM. The fuel management system will adjust it self to bring the system to the set parameters..... The 2.5 is a good engine, but to me is not worth to dump any money to bump up the power output. Regearing(as you already planing) is the only mod worth the money to keep the 2.5 happy......

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    Winning! friskeljr's Avatar
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    i have an 02 w/2.5 I did the swap and i noticed a big difference. It felt to me like brand new.
    Guns dont kill people..... gaping holes in vital organs kill people...

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    Senior Member SavageSun4x4's Avatar
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    JoeySnow is pretty much dead on. Generally you won't see any gains. If you do it will come on at higher rpms like highway speeds.

    The engine is a air pump, no more no less. Increase the volume of air in AND OUT and benefits can be seen. BUT you can not put a wash tub or a VIPER TB and expect to have any gains...WHY? Joey said it: "fuel management system will adjust it self to bring the system to the set parameters" However the key word is parameters. There IS room for movement, not much, but enough so that with a larger TB you can see some gains. Here we get into how much bigger. NOT MUCH or you just bang against the parameters. There is also the issue of gases OUT. Not the issue here but putting a wash tub TB on and keeping the same size outlet (exhaust system) does no good at all.

    I would say this: Will not hurt to try it, you make see some limited gains above about 2k RPM, but down where we rock crawl not likely.

    I ran a supercharger on my rig and a larger TB. I would state that any gains I had were neglible below about 1500 rpm. Above that it really woke up the engine and it did for me exactly what I wanted it to do. Haul me and my heavy metal Jeep with 37's and bead locks down the highway at 65 mph with cruise control on and the ability to pass a truck.
    Don

    '15 Jeep Unlimited Rubicon Hard Rock...OLD SCHOOL 6 Spd Manual Trans
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    Senior Member jackhammer's Avatar
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    I did it back when i first started fiddeling with her. Noticed a bit of difference. But still 2.5. Regear is where its at. Or motor swap. I have a 5.2 now. Have u checked www.4bangerjp.com yet. Good group of guys over there.

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    Newbie kegrill's Avatar
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    Thanks for the input. Pretty much what I was expecting to hear. Cant wait to get the gears and see what happens.

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    Member CJ7nvrstk's Avatar
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    The TB spacer won't do you any good.

    Is the 4.0 TB larger than the stocker? If so, then yes, you will see some gains.

    The gains you will see may be minimal but it is just the same as putting on a free flowing intake and works by getting more O2 into the combustion chamber. Increased O2 in the combustion chamber allows for a better burn, thus giving more power.
    Last edited by CJ7nvrstk; 02-03-2011 at 08:32. Reason: can't spell
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    Senior Member SavageSun4x4's Avatar
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    Throttle Body Spacers (TBS) Yes or No?


    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. Albeit small at best.

    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 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.
    Don

    '15 Jeep Unlimited Rubicon Hard Rock...OLD SCHOOL 6 Spd Manual Trans
    17 Oaks Ranch Companies LLC

    www.savagesun4x4.com

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    Senior Member Cole's Avatar
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    Adding a TBS to a newer Jeep is sure to produce as much results as taking your accordion to go deer hunting.
    yes.

    The noise WILL chase the deer away.
    Keep it slow. I can't afford spare parts.

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    U.S. Paratrooper GOVT1911's Avatar
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    I had read tons about this subject on several different forums. I ended up getting a 4.0 TB for free from a friend, so I went ahead and swapped it on (I had to change the sensors from my 2.5 onto the 4.0---different connectors) Unfortunately, I did it at the same time I regeared to 4.88. I noticed a HUGE difference, but I suspect it was most (if not all) from the gears. LOL

    I don't see how it could hurt to get more air in, so if you can get a 4.0 TB cheap, why not???

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