Best engine upgrades for the cost?
- Thread starter Rob T
- Start date
Every so often I have to re-hash on this subject, so here it is again.
Cold air intakes have been proven to not work on the 4.0 engines and will actually hurt your engine more than they will do any good (see this thread: Cold Air Intake FAQ). Throttle body spacers are the same as snake oil on a fuel injected engine (see this thread: Do throttle body spacers work on Jeep Wrangler TJs?), and a larger throttle body will not do anything at all, because in order for it to actually work, you'd need to open up your intake manifold and port the head (as well as put in some bigger valves likely) so that you can open the entire airflow path, not just the throttle body.
A real long tube header will give you substantial performance gains, up to 20whp in fact (and even more with proper tuning). Unfortunately there are only two companies out there I know of who make actual long tube headers for the 4.0. AEM and ARH, the latter being the far better design of the two:
https://americanracingheaders.com/collections/jeep/products/jeep-1998-2006-tj-4-0l-long-system
Adding a catback exhaust will do little to nothing on it's own (without the header), as the factory exhaust manifold is the restrictive point in the exhaust system. Sure, you might pick up a few horsepower, but that's negligible at best.
The 4-hole fuel injectors some people like to pedal on other boards are complete hocus pocus as well. There is no power to be gained from them.
If you want the best bolt-on modification you can get that will give you a huge guaranteed power increase, you need to invest in a turbo or supercharger, something like this:
https://magnumpowers.com/product/ornare-et/
Do not waste your money on the little things as mentioned above, as they won't do anything for the 4.0. If you want power, you need full headers, a proper tune (see here: 97-06 Jeep Wrangler TJ ECU / PCM Tuning Options), and forced induction. Either that or you need to do a stroker conversion or a V8 swap.
End of story
Cold air intakes have been proven to not work on the 4.0 engines and will actually hurt your engine more than they will do any good (see this thread: Cold Air Intake FAQ). Throttle body spacers are the same as snake oil on a fuel injected engine (see this thread: Do throttle body spacers work on Jeep Wrangler TJs?), and a larger throttle body will not do anything at all, because in order for it to actually work, you'd need to open up your intake manifold and port the head (as well as put in some bigger valves likely) so that you can open the entire airflow path, not just the throttle body.
A real long tube header will give you substantial performance gains, up to 20whp in fact (and even more with proper tuning). Unfortunately there are only two companies out there I know of who make actual long tube headers for the 4.0. AEM and ARH, the latter being the far better design of the two:
https://americanracingheaders.com/collections/jeep/products/jeep-1998-2006-tj-4-0l-long-system
Adding a catback exhaust will do little to nothing on it's own (without the header), as the factory exhaust manifold is the restrictive point in the exhaust system. Sure, you might pick up a few horsepower, but that's negligible at best.
The 4-hole fuel injectors some people like to pedal on other boards are complete hocus pocus as well. There is no power to be gained from them.
If you want the best bolt-on modification you can get that will give you a huge guaranteed power increase, you need to invest in a turbo or supercharger, something like this:
https://magnumpowers.com/product/ornare-et/
Do not waste your money on the little things as mentioned above, as they won't do anything for the 4.0. If you want power, you need full headers, a proper tune (see here: 97-06 Jeep Wrangler TJ ECU / PCM Tuning Options), and forced induction. Either that or you need to do a stroker conversion or a V8 swap.
End of story
To add.
If you have bigger tires. Regear is another option.
If you have bigger tires. Regear is another option.
ErikOffroad
TJ Enthusiast
Stroker motor...extra 50 horsepower for about $200 over the cost of a complete rebuild of the engine. I am down in north Ogden, if you want to go for a drive in a stroker jeep, come on down. I am going to sell it but until then I still occasionally drive it. Tim
Brian83
TJ Enthusiast
For 35's with your 42RLE automatic transmission, you definitely need 5.38 gears. Not even 4.88 is enough. Yes I know 5.38 may seem like too much gear and it would be for most other transmissions but it's what your particular transmission needs. I had 4.88 in my TJ with 35's with the same transmission and it was a dog. I regeared from that 4.88 to 5.38 last year and it does much better on the highway now. Even my mpg went up after installing the 5.38 gears from under 12 mpg to over 14 mpg.
Honestly, making sure that your engine is functioning well may be the best bang for your buck. If the service history is unknown, work your way across wear items and fluids.
I'd start by changing the oil and sending an oil sample to Black Stone Labs, then run some engine cleaners in a bottle, seafoam the engine, and lubricate with marvel mystery oil. Change spark plugs, and flush the coolant. Do that all over the course of 4k miles before the next oil change. Right before the next oil change run sea foam in the engine oil. When you do change the oil make sure it's after a drive so it's hot and well mixed — drain while it's still hot so the junk in the oil stays in suspension. I personally wouldn't send that oil sample to be tested, you know it's not going to be good, but start sending in samples again on the next oil change after. After that, work your way to the transmission and give it a fluid flush — fresh fluid in the trans will make sure it's not causing additional horsepower loss.
I'd start by changing the oil and sending an oil sample to Black Stone Labs, then run some engine cleaners in a bottle, seafoam the engine, and lubricate with marvel mystery oil. Change spark plugs, and flush the coolant. Do that all over the course of 4k miles before the next oil change. Right before the next oil change run sea foam in the engine oil. When you do change the oil make sure it's after a drive so it's hot and well mixed — drain while it's still hot so the junk in the oil stays in suspension. I personally wouldn't send that oil sample to be tested, you know it's not going to be good, but start sending in samples again on the next oil change after. After that, work your way to the transmission and give it a fluid flush — fresh fluid in the trans will make sure it's not causing additional horsepower loss.
I personally believe Sea Foam is today's snake oil. Rarely of benefit due to virtually all US gasolines having more than enough cleaning additives to keep things clean.
I assume you are talking about down the throttle body? I do side with your opinion here, same with all of the cleaners in the bottle I suggested using, however for me it comes down to that they won't harm the engine and might help especially if you don't know how the PO treated it. I had one of these engines with a uneven idle that isn't a huge problem but annoying enough that I've been trying to fix here and there. After Seafoam it ran a little more consistent, not perfect, but better. It might be placebo or the effect of something else I put in the system finally kicking in as I have no way of measuring.
What do you want to do.The 4.0L is an excellent engine completely stock unless you want to cruise all day at 80mph. In which case forced induction or a V8 are the only answers.
Try googling Jeep Speed. Those Cherokee guys know all about making the 4.0L faster.
Try googling Jeep Speed. Those Cherokee guys know all about making the 4.0L faster.
Maintaining 80mph is doable as long as you're not dealing with extreme elevation.
Thanx for the suggestions. The first thing I did when I got it home was change all the fluids ( tranny, diffs, engine, and flushed cooling system) checked out the differential gears and greased every zert I could find.. I also changed the plugs and installed a K&N air filter. It seems to run good, I was just wondering if there was anything else I could do to squeeze a little more out of it.
The K&N filters don't filter fine particulate as well as a paper style filter. ACDelco's paper filter is what I'd recommend since it fits good. FRAM stopped fitting after they made some change to the design a few years ago.
tquig01
TJ Enthusiast
You took that way differently than I intended. Yes, yes, that is extreme elevation. I hope he has a ladder for is momma to get in.
By extreme elevation I meant mountains. Even with some hills, 35s with 5.38s you can maintain 80mph.
Best way to maintain 80 up here in Colorado at Altitude is by chaining it down on my trailer and dragging it behind my Truck
