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Old 09-27-2016, 09:22 AM   #1 (permalink)
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Modding 2007 Jeep Grand Cherokee

I was looking around and haven't been able to find anyone having done work on these. I have the 3.7 v-6, with 4wd. Beside the 4wd there seems to be good potential for improvements. CD is .41 and I see other Midsize Suvs that are similiar ( Audi, BMW etc) have cd's around .35, down to .33. ?Wit highway economies about 25 to 28MPG.
The cooling air is very poorly set up, as the radiator basically hangs down in free air below the bumper, so recirculation is possible at low speeds and air flow through the grill is wasted and goes around the radiator since there is now enclosure in front of the radiator. Also with the v-6 there is a 12 inch gap underneath behind the radiator and engine bottom tray that probably creates additional drag. The bottom is the typical mess, except the smooth undertray under the motor. No air flow control around tires. There is a small built in spoiler on the top of the lift gate and the mirrors aren't too bad....they are spaced away from the body.
Stock I got about 20 to 21 running at 60 to 65 with Air on about 50% of the time in rather mild weather. Gearing appears pretty good with RPM S lower than expected on the highway, considering the small engine. It appears the v-6 gets numerically lower gearing than the v-8....interesting. Also, it is interesting that the v-6 only gets slightly better mileage on the highway than the 4.7 v-8. I suspect on the highway the v-6 is running with a more open throttle causing the engine to run richer than the v-8s. I suspect some aero work such as sealing up around the radiator to eliminate air flowing around the radiator, then closing up much of air inlet area (most of upper grill and gaps around the headlights, hood,etc) and then a front undertray and closing up of the front of the front wheel wells and channneling the air around the tires will result in major reduction in drag and probably maintain stock cooling capacities. I suspect the engine will respond to a smaller throttle opening with leaner operation...further increasing gains. Another nice feature is that the rear pumpkin and suspension bits are pretty well tucked up behind the fuel tank, so airflow should flow past these typical obstructions easily. The frame behind the axle slopes up at about 12 degrees around the spare. It should be pretty easily to close up this area with a diffuser. Angle is probably too steep, but still better than flat and airflow should be less turbulent. A steep angled diffuser should also create some nice downforce and along with the front undertray creating downforce, improve highway stability.

The stock air box is the same as the 4.7 and 5.7 v-8, so filtration capacity is probably good, but the airbox to throttle pathway looks very turbulent, so that should be changed. Also, the air inlet to the box is rather restrictive and is located at the top and just in front of the radiator, so it probably pulls alot of warm air in city traffic. I expect I can create a large cavity below the air box and open the fog lamp opening in the bumper to supply air to the cavity and then supply plenty of cool air to the filter. The cavity will allow water to drop out in rainy weather, so the filter will stay dry. K&N claims about a 7.5 HP increase with their CAI on this little motor, so the intake could obviously use some work. The exhaust is also the same as the v-8s, however, magnaflow claims about 17 HP increase and big torque gains with thier exhaust on the v-6! The exhaust must be really poorly designed. I may address this too, at some point. This engine needs to make power more efficiently while revving in order to move this heavy vehicle in city stop and go traffic. The v-6 is also a rather high compression motor, 9.6:1 IIRC, so it should produce good low RPM torque.
The nice thing about aero modding the bottom of an SuV is that there is plenty of room to work and lots of room for gains!
Hopefully this vehicle will respond as well as my 93 tracer auto wagon, going from a best of 32 MPG up to 47 mpg with these same mods. However, the Ford 1.9 goes into lean burn (20:1) , not sure about the 3.7 however.

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Old 09-27-2016, 12:56 PM   #2 (permalink)
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You don't want cold air intake of you ate trying to improve mileage on a gasoline engine.
Big over sized exhaust is the same way.
On a gasoline engine anything that increases "hp" is going to usually lower MPG.
These companies may claim there is a fuel economy improvement by adding their products but as far as we can tell it's a lie.
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Old 09-27-2016, 02:38 PM   #3 (permalink)
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Intake and Exhaust

I believe that's usually the case, but this engine needs to rev efficiently in order to move a lot of weight in the city. I am going to try both cold and warm air out over time.

On the exhaust, the magnaflow exhaust pipe isn't any bigger, just the tail pipe. Stock exhaust is 2 1/2 inch back to tailpipe which is 2 1/4. Magnaflow is 2 1/2 all the way back. The stock size smaller pipe should keep the scavanging velocity up for low end torque, but the freer flowing muffler allow more flow when needed for higher end power production. I also wonder if the freer flowing muffler may improve bottom end torque because the effective exhaust length is longer since the exhaust pulses travel all the way to the end of the exhaust more easily. I did the same thing on the Ford 1.9, kept the stock pipe size, but upgraded to the GT muffler (direct bolt in). The engine pulls very nicely at low rpm and makes much better power than stock when revving out. The 1.9 does bout 27.5 to 29 mpg in the city with no special driving techniques. In fact I want to see if I can get the auto trans into overdrive sooner because the low end torque is so good. Currently it goes into overdrive and locks up at about 37 MPH and can still accelerate strongly at those speeds. This should hep city mpg. The Jeep drops into overdrive about 30mph and tugs along all that weight at about 1200 rpm.

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