Jeep Cherokee Aero mods

[AdrianD]

Hello everyone!
I've been lurking here for quite a while and reading up on aero mods.
I drive a 96 Jeep Cherokee with a turbo diesel (European model) and at highway speeds (60mph) I'm getting around 28-29mpg. It already has EGT and boost gauges and cruising at 60mph I have an EGT of about 600F before turbo and 5-6 psi boost. Most knowledge is that the lower the EGT and boost, the better the mileage, right?
Tires are a bit bigger diameter than stock. Nothing special about the tread, they look like a highway tire.

I have a 1200 mile road-trip coming up and I plan to do some aero-mods to the Jeep:
Roof-rack cross-bar delete, airdam, grille block, mini-belly pan (gas-tank to rear bumper) and rear tire spats. In that order, depending on the time available.

All mods except the airdam seem rather straight-forward. But I don't have a good idea how to build the airdam.

How low should I make it? Right down to the point that no suspension components are visible? And a bit lower in front of the front tires?
Or something similar to a tray, which starts at the bottom of the stock lip and is angled down?

One more mod I'm doing is a 2.5" exhaust, right after the downpipe. Being a turbo-diesel I think that keeping the backpressure low would improve MPG.

Engine documentation shows lowesc BSFC is at 2000rpm. In 5th gear I should be between 60 and 65mph.

I have pictures of a front airdam used in the SCCA Jeep Comanches. As soon as I can post, I will post them.

This was a long post...

[spacemanspif]

Mods sound like good ideas for the Jeep.

Is boost needed on the highway? I'm thinking you could save some fuel if you had a way to control boost and run N/A while cruising.

Any thoughts on building rear wheel skirts or a kam back?

[AdrianD]

There's no way of running it N/A on the high-way. I can dial back fueling very easily since it has mechanical injection pump.
A big improvement on that side would be a VGT Turbo, with electronic control. That way I'd run with very little restriction on the exhaust while cruising. And lots of power when needed.

Somebody here mentioned that aero mods should start from the front and work their way back, so I'm going to try the airdam and grille block first. But a Kammback spoiler, like the BMW X3 has sounds good.

I only have about 12-14 hours of work before the trip and I need to build my exhaust, the radiator, intercooler and oil cooler ducting, airdam and remove roof rails. So I more than likely won't be able to build rear wheel skirts or a kammback spoiler.

[elhigh]

Hi AdrianD,

RE: starting at the front and working back: I agree. I think, if nothing else, it will radically improve your cooling by reducing your underhood pressure. A small belly pan that extends rearward from the air dam will improve airflow through the radiator, not to mention retard and maybe even eliminate turbulent flow forming along the aerodynamically messy underside of the engine. Ideally you want your air dam to be as low as your lowest under-car feature, but anything is better than nothing.

Obviously you would need to do some testing to determine what will have the greatest effect on your bottom line.

The crossbar removal is a no-brainer, if you're not rooftopping anything, pull 'em. No expense, no permanent vehicle modification.

Having the BSFC map for your vehicle is great.

[dirtydave]

just keep it in a high gear and keep the speed down you should be ok.

[cRiPpLe_rOoStEr]

So, is your Cherokee fitted with that 2.5L VM Motori engine or with the 2.1L Renault engine? If you're lurking about a VGT upgrade, why not eventually a whole engine swap? Anyway, I have already seen some mechanically-governed engines fitted with VGT, such as the International 2.8HS used in Argentinian-made versions of the Ford Ranger.

[AdrianD]

Sadly, an engine swap is not possible because of legality reasons. I wanted to swap in a VW TDI but hit a brick wall with the local DMV equivalent...

In regards to cooling, I can easily block a large chunk of the grille since I now have a 2.5" core radiator instead of the stock 1.5" core. Extra cooling in the summer heat helps.

The airdam will be simple, should be done in a couple of hours and removable. My front bumper is getting all kinds of work, welding and grinding to get the intercooler and oil cooler ducting built so a couple more holes to attach an airdam are nothing.

[serialk11r]

I'm guessing you don't have an electronic wastegate, and I don't know how much they cost to install, but opening it to bypass the turbine at cruise somehow would probably help a little. I doubt that your engine would be anywhere near full capacity running NA on the freeway, and pumping all that excess air through the engine in theory reduces the efficiency by a lot, even if it doesn't cost much energy to pump the air.

I have never actually looked closely at a turbocharger, but I'm guessing that the high temperatures at the exhaust mean that a spring loaded wastegate would have to have the spring be sitting at some distance from the wastegate itself, so if you could attach a cable to the wastegate actuator rod or something and have a manual turbine bypass, that could do the trick?

[AdrianD]

Your idea sounds interesting. An electronically controlled turbine should be easy to implement since it only needs to read boost and adjust to that. VGT control will need RPM and throttle position. I might just do it after I return from the trip.

As far as engine limitations go:
Maximum pressure loss over the intercooler is around 10-12psi. A LOT. This means exhaust manifold pressure is HIGH. Which in turn means quite a bit of wasted energy there. Might I add that the hot side of the turbine is small, so that means even more restriction. Choked intake, choked exhaust. Exhaust was so choked by the stock muffler that it would start to overheat on long climbs. Replaced that with something less restrictive and overheating was gone.

Yeah...that's me whining about the Jeeps bad implementation

[serialk11r]

10-12 psi is way too high, are you sure it's not clogged with something??

VGTs are really expensive, and more than anything else what one will give you is better horsepower but not fuel economy. I feel like your turbo has to have a wastegate on it already, why not just bypass it for now with a clamp or something and see if that improves your mpg.