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Discussion Starter · #1 ·
Okay so I have an '89 Laredo, it's a little tired and could use some help. I recently got the intake of a 1993 4.0 HD. Is this where the power comes from in these? Or is it other parts or the head? Or a combination of parts? Doing a cold air intake this weekend and did spark plugs and wires today.


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HO (= high output), not HD ;) It's basically the intake manifold and cylinder head that does it.

There really isn't much real-world performance difference between a Renix and a HO 4.0 engine.
 

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The torque is about the same on these. I think only 8 hp difference. Also, have to consider at what rpm they are making the max HP. I thought my '88 Renix had more power or as much as either of my '98 4.0s. There are upgrade option with the RENIX as with the HO. If your looking for an upgrade in a manifold, you might look at the 99+ (00-up) intake. I do not know if it will fit. I'd only expect 5 hp gain with that. Think more exhaust upgrade. I will have to say my '88 was manual. In that era a manual usually felt like it had more power.
 

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There is absolutely no point in doing that. Also keep in mind Renix 4.0s have the peak torque at aslightly lower RPMs, so an older 4.0 as long as it's in good shape will actually feel a bit more powerful under normal driving.

I had a stock 4.0 Renix on both my XJs, and I could fairly easily smoke 35 x 12.50 Maxxis Creepy Crawlers in dry pavement - with a Detroit locker, on 4.88 gears. If you want more power, either stroke your current engine or swap in an LS- series V8.
 

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So really not worth all the work involved to put the new one in huh?

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If you replace the header, then maybe. Some have loss HP with the latter intake. It looks good in there though. I just installed one in a '98 XJ. Can't confirm anything. I made a bunch of changes. The '95 intake, No. To my understanding, replacing the down pipe is one of the more effective HP increases. Getting rid of the dent in it. Can't confirm as I've upgraded to a Thorley header and 2.5 exhaust. Haven't done the down pipe to 2.5. Next week maybe.
Also, what cold Air intake? The stock box is suppose to be able to supply enough for this engine. A lot of 'Cold Air' intake are taking in engine compartment air. Not the best. I am looking at the Thor or Spectrum. Down the road a bit, and don't know if they would do anything. I'd look at the +25 MPG thread or Dinos 4.0L page for ideas. Also, see Cruisers54 tips on the Renix, for Ideas and maintenance. Don't expect huge gains. I think Renix might get a bit better MPG than HO's and latter ones.
 

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Stock airbox with a good filter will supply plenty of air to your engine. Original intake manifold should be able to outflow the requirements of the engine, even a stroker. Throttle body plate may be a slight restriction, but not really an issue.

Downpipe dent shouldn't actually cause a very drastic horse power loss. Ee. Motortrend channel did an exhaust manifold dyno-test where they smacked in the primaries and collector REALLY badly (on a V8 engine though), and they had to very severely dent them to actually loose any noticable power. Downpipe is a bit further down on the exhaust, but it's not really that far from the collector.
A 2.5" exhaust with a free-flow cat and a good muffler that flows well will give you better readings. It sure won't hurt to get rid of the downpipe dent.


Many of the cold air intake setups are waste of money; they either don't suck in cold air at all, or they won't increase hp much at all.
 

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It depends on what you are trying to achieve. The OP said it seems 'tired'. I'd start with basic maintenance. O2 sensor, TPS, plugs, wire, spark plugs, brass insert cap and rotor. Possibly see the condition of the distributor. Check the timing chain's condition. OP didn't say if manual or auto. Still, check the condition of the tranny. The engine could be good, but loosing power at the tranny. I think the power for the 4.0L is good for size and weight of the XJ/MJ platform.
 

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Discussion Starter · #9 ·
It's a auto. But I just replaced the transmission. And I just did spark plugs and wires the other day. Rotor seemed fine as well


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My view is if it isn't running right or normal, Performance mods won't help or not much. I'd check for vacuum leaks, also, check the trans cable and adjust it. Do a check on the TPS. I might unplug the tans computer and see if the performance improves. If that does anything, then it would be electrical for the trans. I do suggest you look for Cruiser54's tips. They are RENIX based tips.
 

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"cold air intakes" and "performance filters" are an unknown quantity. Even K&N filters are criticised because they do not stop dust very well (how do they get greater flow - guess) and if you use your XJ very much the ordinary stock paper element will keep your engine running for years without harm at a low cost.
 

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I might look into replacing the injectors. Do a search for Volvo injectors. I think there was a thread a month or so back. The Renix are suppose to run a bit better with the Volvo injectors. Do a compression check. Then look at more basic maintenance. If you replace your TBS, only use MOPAR part. Other more knowledgeable people here recommend that. My limited experience confirms that.
 

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First off as others have hinted at:

HO myth buster

Renix in 90 made 182 HP. HO in 91 made 190 HP. That's 8 HP difference.

HO only made more HP than Renix at higher RPMs and not a bit more torque. HO had a 58 mm throttle body versus a 52 mm throttle body on a Renix. That's 20% more air available through the HO throttle body. The HO also had a better design header. See where I'm going with this?

It's only a 4% horsepower increase…..

The whole 8HP was not mostly from the head, but from the bigger TB and better exhaust manifold.

Put a 60mm TB from www.strokedjeep.com on your present manifold using the Renix head, eliminate the "crush" in your headpipe with proper re-routing, and go for it.

HO stands for Highly Overrated.

Secondly, here is a handy dandy guide to do what needs done to put that 93 engine in your 89.

http://cruiser54.com/?p=100
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Discussion Starter · #15 ·
honestly dont know how long its been. I got the jeep used, have done a ton of work to get it the way it is and I guess I was expecting more out of it since my last truck was a 4 cylinder. It also has a rough time starting now. almost like it isnt getting fuel but i know it is because the fuel rail release has fuel that comes out.
 

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You can only verify fuel pressure with a pressure gauge - both with the fuel regulator vacuum connected and disconnected. Pressing on the fuel rail schrader valve doesn't really tell you much of anything.


Like I wrote, you should easily be able to smoke stock size tires with your 4.0 - I could smoke the rears with 35s & Detroit locker, 4.88 gears. You're not gonna feel the Jeep being fast or powerful, but it has enough low-end grunt to move such a light vehicle fairly well.
 
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