Rebuilding 4.0, releastic HP??

I think I am going to be rebuilding my 89 4.0. I don't think I have the time to get the parts together for a stroker.

What would be a realistic goal for more HP from this motor. Where are the best places to spend the money?

Thanks,
Paul

My current motor has 267K on it and a SERIOUS valve clatter, and it overheats.

If you want to keep the displacement the same and you want the best performance, spend more money on the cylinder head/top end.

-get the highest quality piston rings you can afford (many people cheap out in this department and buy rings that don't seal as good as they could)
-get the highest quality valve job you can afford
-enlarge the intake and exhaust ports to match the intake and exhaust manifold gasket (either yourself or a by a shop)
-polish the combustion chambers
-get ceramic coated pistons (keeps the heat in the combustion chamber where it can do useful work instead of being wasted into the cooling system)
-raise the compression ratio by machining the head surface
-get a full exhaust system from header to tailpipe (use 2.25" for more low end or 2.5" for more top end, your choice)
-bore the throttle body
-don't waste your money on a k&n, just enlarge the airbox hole and remove the silencer (K&n just lets dirt in to destroy you new piston rings)
-get a new stock cam, unless you want to tear your engine down again to replace a "hotter" cam in a few years

If I had to guess these mods on a freshly rebuilt and broken in Renix engine should give you at least 200 hp for about a 20 hp gain over the stock 177. But I could be grossly underestimating because I've never acutally seen mod by mod dyno numbers on a 4.0. (but I have seen percent gains on big v8s and such, but as a lot of us know, what works on a v8 may or may not work as well on a 4.0)

Remember that you will probably want to invest in larger injectors as well.

I just had my 90 RENIX rebuilt by a local engine shop. The only thing I re-used was the block, crank, head and valves. The cylinders were bad so it got punched 40 over and because the head gasket went and it overheated the head had to be milled and the block decked, 20 thousanths each.
The crank was polished and the head reworked with a standard valve job, I went with a stock grind cam. Everything else was new. $1800 and 1 year warranty, they assembled and primed the motor.

I have 4 XJ's, the 90' and also a 94', 96, and 00'. By seat of the pants the 90 is much stronger on the low end than the HO's, but not as much power on top end.

The rebuild did more to increase torque than hp, but the torque just makes it feel stronger.

Not true, K&N filters better than most filters.

Blaine B. said:

Not true, K&N filters better than most filters.

Click to expand...

It's all a compromise, the paper filter, filters better, but also looses flow volume fast from fine dust, pollen, moisture, oil and whatnot. The paper filter gets plugged fairly easy. The Renix has a habit of spitting oil onto the paper filter. When the paper filter gets oiled, it is effectively plugged. A little oil on the K&N doesn't seem to hurt much.
The Renix has some room to tweak, but really not that much. More air and a little more fuel. Intake and injectors are probably going to give you the most bang for the buck. Getting the heads to flow better is likely the next step.
IMO tweaking the 4.0 is tricky, the air to fuel ratio is just about as lean as she will stand. Adding fuel isn't going to increase HP, if at all. Adding a little air flow and fuel while keeping the ratio constant (near stock) is likely the answer.
The formula is fairly simple, the higher in the RPM band the motor hits peak torque the more HP. Figuring out how to do that without reprogramming the fuel management system is the trick. Just adding air and fuel without messing with the management system isn't going to get you any dramatic increases.
A Paxton pump, an intercooler and larger injectors would likely pump up the horses dramatically, the question is how long would the motor last. The brute force approach.
If I was looking for horses, I'd call Paxton and ask if anybody has installed a pump on a 4.0 and ask how it turned out and how long it has been running. If Granatelli is still around, he always seemed fairly customer friendly and answered all of my questions.

K&n filters are not worth it at all.

I used to think so, but that was before I became wiser on the subject. (and I found dirt in my intake manifold)

IF you didn't oil the K&N's their filtering wouldn't be as good, and I believe airflow would actually be less.....although they do have some new oil-less filters available now.

K&N claims their filters are the best.......Are you sure yours was sealed up and all?

If so I'd be interested to see you bring your case to K&N and see what they had to say about it.

I'm not stupid. I know you have to oil K&n filters.

Yes, mine was sealed up with that grease they supplied around the edges.

Anyway K&n doesn't give a crap what I think, they got their money.

I used to think the K&n increased horsepower and gas mileage. Turns out it was all in my head, and I woke up to reality one day.

K&n filters aren't suitable for off roading or daily driving.

They might be fine for a street rod that gets driven 2 days a year, and has a sheet over it the remaining 363 days a year. Even then I probably wouldn't waste my money.

Funny VW did a test a decade or so back and decided the old 50's era oil bath filters did the best job.