Taken from UK jeep club, asked themthe same Q, what do USA think.
Grand Cherokee (ZJ) 4.0L Laredo
posted 23 November 2006, 22:13:34 Standard intake system takes cold air from the grill and it travels through the plastic air box and tube to the throttle body, plastic being a good insulator the air charge reaches the manifold with little heating from the under bonnet heater (engine), the injectors spray atomised fuel into the air flow and it goes into the cylinder hit's the piston crown & cylinder, it takes some of the heat from them and vaporises, just before reversal the inlet valve closes the piston goes up compresses the vapour to approximately 10% of it's cylinder volume which raises it's temperature greatly, the plug fires, the flame front advances and the burning mixture expands rapidly forcing the piston down turning the crank and producing a certain amount of torque and power. The k & n intake system with it's terminal under the bonnet takes warm or hot air from under the bonnet (already rarefied & vitiated) along a metal tube putting more heat into the already thin air to the throttle body, then the sensors take over, ats see's a rise in temp then maf see's a decrease in density, the coolant temp sensor will no doubt have it's little dabble and then the final decision gained from the lambda's and the ecu will weaken the mixture to match the low level of oxygen in the mixture producing less power from the engine than with the standard intake system.
K & N filters, FIPK kit well if you look at K & N's own site you'll see their advertising and their DYNO (rolling road) test of a 1997 Cherokee 4ltr 6 pot
http://www.kandn.com/dynocharts/57-1520.jpg (click on the graph to enlarge) before & after the K & N fitting you'll see an amazing 2 BHP increase @2750 (93MPH) and topping out at 7.72BHP @ 4684RPM (never get there with auto unless you pull the stick back) I don't know who would drive in that range, (get the neck brace ready type surging power), and it only cost's £250 (if you fit it yourself) .. blimey I'll have two of 'em. Of course N-E-X-T The K & N advertising "IN THE K&N TRADITION OF ENGINEERING POWER, K&N PERFORMANCE KITS ARE DESIGNED TO DRAMATICALLY REDUCE INTAKE RESTRICTION AS THEY SMOOTH AND STRAIGHTEN AIR FLOW" the important bit here is (designed to) not (it does do) ALSO "THE GENERATION II FIPK® IS K&N'S ULTIMATE PERFORMANCE PRODUCT, DESIGNED TO INCREASE POWER BY AS MUCH AS 10%. Again "designed to" and "as much as 10%" that means 0 to 10% and that's at maximum power rev's not all thru the range as their own dyno test shows. I haven't seen a dyno test for the std FLAT filter replacement as this would probably show "0%" increase. Also dyno tests are usually conducted with the bonnet up and a large 3 foot diameter fan about 6 foot from the radiator blowing cool air through it and under the bonnet. K & n don't show torque curve graph's as they would probably be lower than with the standard system at certain rev's.
The problem with having air intakes terminals in the engine compartment is that the air has already expanded somewhat and when it gets into the cylinder things like volumetric & thermal efficiency come into effect, when the plug fires the mixture it burns at a rate relevant to the octane rating & mixture of the fuel dependant on additives like tetra ethyl lead or other burn rate retarding compounds and expands quickly to push the piston down the bore, cold air is more dense and therefore has a greater ability to expand in the combustion chamber producing more power. If you like the sound it makes or you want to use the space for another battery that's a different kettle of fish. If you actually want a performance intake system there are a few worth looking at. Some are application specific others are universal and adaptable if you search there are loads
http://www.blueflameperformance.com/car/inforceair/index.asp
http://www.potn.co.uk/-_c5206_8675_8686_p1543125.htm
http://www.europerformance.co.uk/pages/products/unavailable_online/index.mhtml?category=jetexairforceinductionkits
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=190053803012&ih=009&category=36680&ssPageName=WDVW&rd=1
http://www.directcarparts.co.uk/buy-car-accessories/product_info.php?products_id=7975&ref=232
