I want to put a cold air intake on my 86 xj6 4.2 but there is a square air box I’m guessing to control air going into the engine is this doable if so what setups have any of you used

 

The square box is the AFM---Air Flow Meter. It's the primary signal input to the ECU to control fueling.

Eons ago some guys installed various open/cone air filters and/or K&N filters. The power improvement reported was very small...perhaps imaginary. I don't recall anyone trying an actual cold air system. Others might chime in on that.

Long ago AJ6 Engineering offered some performance enhancers for the 4.2 with the most notable (and effective) being a modified intake manifold....but those products have been out of production for many years. If you go to the old AJ6 website there might still be pictures of the manifold, intake, and exhaust systems. AJ6 would recalibrate your AFM to make the other changes effective, as I recall.

Let's hear what others have to say.

Cheers
DD

 

How would you recommend getting more power out of the standard 175 hp as it’s a American car it’s defined with lower compression I have deleted the cats out of the car

 

My recommendation? If you want a Series III with more power buy one with a V12. . Or do a V8 conversion. Or, as I did, do a V12 conversion. The latter two choices are a bit of a commitment, obviously.

Sorry about the sarcasm but, seriously, there isn't much you can do with the 4.2 as installed in the heavy-ish Series III body. There are performance camshafts out there but they're more appropriate for an XKE.

Opening up the intake and exhaust flow might get you 15 horsepower on a good day. Maybe only 10. You won't feel much difference but, OTOH, they're easy and not expensive.

Increasing the compression would help, naturally. Replacing the pistons is a big job but, as an alternative, the head can be milled quite a bit to give more compression.

Advancing the cam timing is easy. Won't give more total horsepower but with give a bit more power at low and medium RPM ranges.

The 2.88 differential blunts what power is available. A change of gearing and/or going with a TH700 trans swap would probably provide the greatest increase in acceleration.

Cheers
DD

 

IMHO, A 700r4 swap would make the most difference (I have one), a v8 swap next, and a rear gear change after that. If you're sold on keeping the 6, the 700r4 or even better a manual trans swap would give the most seat of the pants results. You already have the big valve head and fuel injection, you could go for better fuel injection (thread on here about that) or for bling 3 x SU's (expensive and not much power difference). Or a turbo which has also been done with proper results. The XJ6 is a slug and that's just the way it is. A v8 swap makes it feel like you want it to but it doesn't look like a twin cam six when you open the hood. Other JDM swaps are cool, and a 4200 gm log with mods. If you manage to get more power from the L6 for cheap I'd sure love to see a build thread. Many experts already built the hell out of this motor over the years. Turbo and a trans may be in your future.

Link to a fuel inj ecu upgrade...
https://forums.jag-lovers.com/t/seri...ade/416690/176