So let's guess that you only get 5 hp...is there any other reasons or advantages to do the swap?
Chris (Double Down)
As someone who's actually done this conversion I think I can give some feedback. Prepare for some reading ahead. :compwork:
My 92 XJ was, engine wise, bone stock when I got it except for a cheap open element air intake system and 2.5" cat-back exhaust. It was a PIG. No power at all. I just thought it was the 31" tires and wheels slowing it down. I started to change a few things to my liking and here's my impression of each one. No dyno results, but it's my DD so I noticed everything I did.
First thing I did was get rid of the open element intake system and put the factory air box, inlet elbow and a cheap Fram air filter.
RESULTS = Major difference in response and power. I had cleaned and oiled the air filter so it wasn't clogged. My theory was it was just sucking hot air from under the hood. The factory air box gave the engine some fresh air.
Next step was NGK plugs and Standard cap and rotor.
RESULTS = Smoother idle, better response and driveability.
This next step was a big one. I figured, since I had the intake off I might as well do a header too. So, I put on the 99-up intake manifold, P/S pump, new drive belt, T-Stat, APN header, 49psi fuel pressure regulator and Mustang injectors. Not the CORRECT injectors though but I got them for free and mine had 162k on them so I threw them in.
RESULTS = I couldn't believe the difference. I figured it MIGHT be noticable from what everyone has said, but it was NIGHT AND DAY different. Keep in mind that I'm running too much fuel pressure for these injectors, stock throttle body still and stock pipe from the header to the converter. But it was worth every penny.
The next step was replacing the only stock section of exhaust. I had a local shop bend up a 2.5" pipe from the header to the converter and also added a flex pipe under the oil pan. My theory is the lack of flex section combined with the constant strain on the drivers mount (mine was broken) from wheeling/torque is one of the causes of cracked manifolds. Anyway.
RESULTS = Slightly different exhaust tone but not anything noticable on driveability.
I'm almost done so stay with me.
The next step was a home ported throttle body.
RESULTS = scary throttle response! The first time I drove it all I could think about was, "How am I going to slightly blip the throttle over rocks if needed?". But now that I've driven it a while I'm used to it. But it was noticeable and worth it.
Final step, where the XJ is at this point at least, is the correct 4-hole NEON injectors.
RESULTS = Smooth idle. Noticable change in throttle response and driveability. I just did this so I can't report on gas mileage yet. But I haven't LOST any with the previous mods so I don't expect to with this one.
Would I do any of the above mods again on another Jeep? In a heartbeat. I've even put off the 4.10 gears until later this year because of the power it's got now. In fact, my buddy is putting an engine in his and we're gathering the same parts for it. Now, as you can tell, it was more than just an intake manifold swap. But like any mod you do to an engine, you have to add the support pieces or you won't see any changes.
Hope this was helpful.