Proposed intake (induction)

[XJ Dreamin']

You may know from a recent post that over the next year I will be doing some mods to my stock 2WD '93 XJ. One issue will be opening up the intake. I'm looking for improved streetability without sacrificing rough gravel/native surface performance. I'll be looking at boring the TB to 60mm and porting the intake manifold to match (or is it already 60mm? I forget). There will most likely be a TB spacer as well. To my mind, I like the shortest possible distance between the filter and the TB. Also, I don't like pulling air from the engine compartment. So, while some high flow filter will be involved, I don't really like the air tube FIPK's that I've seen. I understand they are a worthy improvement over stock, but I'm just not satisfied with that setup.

So, I've been thinkin'. The shortest possible distance from the filter to the TB is to stick the filter right on the top. Of course, you need to connect the CCV pipe and the canister pipe, and NO I'm not going to have a K&N cone filter sticking out the top of my hood. But, here's what I'm thinking. Adapt a round filter, carb-top style intake housing to the top of the TB. The CCV and canister pipes can attach to the bottom of the filter housing. Cut a hole in the hood just for the filter. Cut down the filter housing top to seal the top edge of the filter but not cover the sides. Constuct a seal between the filter base plate and the underside of the hood (hood up - no seal: hood down - filter is sealed off from the engine compartment). Now, cover the filter and the hole in the hood with a scoop open to the base of the windshield. It's minimal sheet metal mod. The trickest parts are mating it to the TB and arranging the underhood seal. It would have to be as open as any other system at idle and off the line, plus you get induction at speed and it's all cool, clean air. I know the snorkle folks claim some induction as well as cool/clean benefit but, frankly, I won't be seeing any water that a hood induction won't handle. Maybe I'll mount a snorkle blank anyway, just for the poser effect (Beezil fishing :laugh3: - no harm intended).

I'm pretty serious about this. Tell me why it won't work. Tell me I'm an idiot. I've worked on MOPAR all my life (my Dad maintained his own fleet of them - he had five drivers in the family). I am currently without welding capability unless I borrow but I don't see any need for welding here. A nice aftermarker or pull-ur-part rivet-on scoop would be fine.

XJ Dreamin'
stock 2WD '93 XJ, 4.0L HO, AW4, LT 235/75R15 A/T's

 

[Dr. Dyno]

It's not a good idea to mount the cone filter directly to the TB. If you want as much low/medium rpm torque as possible, the best arrangement is to have the filter mounted to the longest airtube that you can fit under the hood without any bends in it, which means you'll basically have to copy my homebrew FIPK.
http://www.jeep4.0performance.4mg.com/FIPK.html

The intake manifold plenum opening is 62mm so it's already bigger than the bore of your TB and you don't need to port it. Buy a TB spacer, bore that to 62mm, and mount the TB on top of that. The spacer will raise the TB 1" further away from the manifold plenum floor and increase airflow velocity as the air enters the manifold (good for torque).

 

[XJ Dreamin']

It's not a good idea to mount the cone filter directly to the TB. If you want as much low/medium rpm torque as possible, the best arrangement is to have the filter mounted to the longest airtube that you can fit under the hood without any bends in it,

Dr. Dyno! Thanks for the interest. I have reviewed your write-ups extensively and admire your work a great deal. You certainly have the results to back it up. My experience has been with large displacement V-8's: specifically Mopar 360, 383, and 400 cid. I'm a newbee with the I6 4.0L and I bow to your expertise. If I could take a bit more of your time, though, I would like to pick your brain a little further. Please don't think that I am argueing. Think of me as a really pesky acolyte who won't shut up.

As few bends as possible I think we all understand. I assume that you want the longest tube possible to provide the greatest volume possible behind the filter for immediate off-idle response. Of course there is no greater volume than the great outdoors, and if we didn't have to worry about dust we wouldn't use any filter at all. Having to deal with the restrictions inherent to filters is the tricky part. I see turque and horsepower in terms of manifold pressure. When the throttle plate is closed, the manifold is at low pressure. The engine is at idle and torque and HP are low. With the throttle open, the manifold pressure increases as air falls into the intake and torque and HP increase. The closer to atmospheric one can get the manifold, the more power can be produced, provided fuel supply and exhaust match the intake performance. If one wanted the manifold pressure to be atmospheric or greater, one would have to augment induction. I am not ready for that kind of $$$$$$$$. Distance between the filter and the TB would provide a volume that would be immediately available for throttle response. After that, performance would be largely dictated by the flow rate of the filter itself. At higher RPM, the flow rate of the filter becomes a limiting factor. However, if the filter flow rate were high enough, it could provide the volume needed for quick throttle response even if the distance were shorter. If no such filter exists, then I will have to add volume by increasing the filter/TB distance. I still would prefer to take advantage of the cooler induction off the windshield. I'll have to think about that. I wasn't thinking of using a cone in this application, but possibly a larger round filter common to older large displacement carburator applications. I haven't reseached flow rates on those but assume they should be fairly large.

The intake manifold plenum opening is 62mm so it's already bigger than the bore of your TB and you don't need to port it. Buy a TB spacer, bore that to 62mm, and mount the TB on top of that.

Of course. My mis-type. I meant to indicate that I would insure that the transition from TB to spacer and spacer to manifold was as clean as possible. Of course one would bore the TB and spacer - not the manifold.

Again, I appreciate your attention, even if only to make sure my ignorance and mis-tpying don't lead anyone else astray.

XJ Dreamin'

 

[Dr. Dyno]

Ideally I would have liked my cone filter to be closed off in an airbox with an opening only at the front and an inlet tube pointing forwards behind the grille. That way I'd get more ram effect and create some positive pressure inside the airbox at high speed, further aiding cylinder filling and torque production. Unfortunately space is very limited. I took the airbox from a 4.0 ZJ and modified the lid to accept a large cone filter and fits inside the box very easily, but unfortunately the box itself is too wide to fit in the corner behind the driver's side headlight housing. I'll need to find a way to make it fit.
The stock intake manifold is definitely a weak link in the performance chain especially when its attached to a stroker. Though the '99+ curved runner manifold improves on the original design, it still has three inherent shortcomings; runners are too short, runners too small in diameter, and TB mounts on the top. Ideally the manifold should have had longer curved runners, a larger plenum, and the TB mounted on the side to give a more direct route for airflow instead of it going round two 90* bends. There also wouldn't have been any hood clearance issues and you could have built a thicker TB "spacer" into the manifold. The problem, as ever, is a lack of space under the hood so inevitably the manufacturer has to compromise somewhere.

 

[JONNYD]

Has anyone built custom intake manifolds for the I6? It seems you could fab one similar to a header design and resembling tunnel-ram intake manifolds typically made for high performance V-8's. It seems long runner length would be easy, and you could go at least as long as the older 98-earlier intakes had. Size would only be limited to the dia. of pipe used. If the TB mounted to the end, there would be no bends, just straight in (on various angles of course). I know that there isnt enough room for the ultimate solution, but it seems someone who can weld custom headers could do the same for an I6 "tunnel ram" intake. Is this possible? Any ideas? I bet if you could get one of these things built and get it set up for others, they would sell like hotcakes. I'd buy one.

Thanks Dr. Dyno, you are the basis of my newfound and growing 4.0 knowledge. Keep up the great website!

 

[Cjmartz2k]

So what's the concensus on the '99+ intake vs. the older one--which is better for low end/mid range/high end torque and hp?

 

[NCSUcherokee]

Getting a 99+ intake may not help all that much in the lower end of the power band, but it isnt going to hurt it. For the top end the 99+ has a clear advantage.

the 99+ is better hands down

 

[FUNKYTEE5]

Getting a 99+ intake may not help all that much in the lower end of the power band, but it isnt going to hurt it. For the top end the 99+ has a clear advantage.

the 99+ is better hands down

By looking at the 99+ intake it's clear that the runners are smaller in cross sectional area providing better intake charge velocity, hence better low-end torque. And also providing better top end with the equal length runners similar to a V-8 dual plane. I hear some say that the larger plenum kills the low-rev torque but regardless of the plenum volume, the velocity will always be the same through the runners right? That's what makes torque They didn't redesign that manifold for nothing. It's clearly a better and more efficient design.

 

[Dr. Dyno]

It IS a better design but I think DC redesigned it with emissions in mind rather than performance. The larger plenum and curved runners allow for more even air distribution to all six cylinders so there's less tendency for the inner cylinders (3 and 4) to run lean while the outer cylinders (1 and 6) run rich.
As for performance? There's a small improvement in midrange torque mainly because the smaller diameter runners curve into a large plenum and this promotes some inertial supercharging effect. I don't think the performance improvement is enough to justify the cost of buying the manifold ($100+) and installation, especially if you own a pre-'97 XJ, YJ, or ZJ and need to swap in the later-style PS pump as well to make it work.
I like the idea of a custom manifold and something similar to the one below would be close to ideal.

The only problem might be a lack of underhood space, hence the factory compromise. Here are the factory manifolds for comparison with the '99+ intake on the right (thanks GoJeep ):

 

[FUNKYTEE5]

It IS a better design but I think DC redesigned it with emissions in mind rather than performance. The larger plenum and curved runners allow for more even air distribution to all six cylinders so there's less tendency for the inner cylinders (3 and 4) to run lean while the outer cylinders (1 and 6) run rich.
As for performance? There's a small improvement in midrange torque mainly because the smaller diameter runners curve into a large plenum and this promotes some inertial supercharging effect. I don't think the performance improvement is enough to justify the cost of buying the manifold ($100+) and installation, especially if you own a pre-'97 XJ, YJ, or ZJ and need to swap in the later-style PS pump as well to make it work.
I like the idea of a custom manifold and something similar to the one below would be close to ideal.

The only problem might be a lack of underhood space, hence the factory compromise.

Agreed! But in my mind I think DC had room to improve from the older design. Equal length is good I'll find out the difference soon! And post my findings. Is that thing an Offenhauser?

FUNKYTEE5

 

[MudDawg]

Take a piece of steel plate and make a intake "header" plate...pipe it up with
steel tubing and build a plenum with a side mounted bored TB. Or use two plenums and two TBs. (the plumbing for the tubing would be easier). For that matter, you could put spigots on the plate and plenum and use hi temp tubing for the runners ala Ramchargers from the 60's.

That Clifford manifold will work best only at WOT. Of course it is also an ancient carb'd design. It has a large plenum but because it must use a vertically mounted carb, the cross section at the plenum/runner junction is small.

The ideal but cost prohibitive design would use individual throttle bodies on properly sized individual runners.