I'm sure everyone here knows what a CAI (Cold Air Intake) is... designed to draw cooler outside air into the engine instead of warm / hot underhood air. This is argueably desireable in summer when the under hood temp climbs outta sight. These thoughts carry over from our experience with gassers and I'm not at all sure they directly apply to a compression fired engine (diesel)... especially a turbo-charged intercooled diesel. Any rate the OEM air box is plumbed into the fender which draws cold air from behind the headlight... Atmittedly not a huge passageway but a CAI. Just a general reminder, that diesels "fire" from the heat of compression. A turbo adds temperature when it boosts the charge pressure but then in goes through the intercooler... before reaching the intake manifold. NOW I want to flip flop the scenerio... in winter the ouside air will be well below the most economical point. The IAT sensor will adjust the fuel to run righer in order to make it run. The cold fronts so many of us use block some or all of the grill raising the under hood temps above ambient. Those who are running BHAFs are already drawing engine room air... despite any heat shielding panels that may be in place. So this may be benificial in drawing the warmest winter air available. I'm still running the stock air box plumbed into the fender. Even with the cold front in place I suspect the air drawn in behind the headlight is still colder than underhood air. So my theory is that it would be better to disconnect the snorkle into the fender & block that opening in winter. Maybe even do the swiss cheese mod to the airbox. Your thoughts?

The only way t oknow for sure is hook up a live data tool like a ScanGauge II or simular... Watch the IAT temps. If the temp falls below 100*F you'll start seeing a loss in MPG. But the IAT does affect the timing directly.

High performance buffs love the colder the better because it will dump a heavier load of fuel in the cylinder for racing. But for the common Joe looking for MPG's numbers then the temps should stay above 100*F mark.

As for drawing in the heated air... kind of funny there is little to no difference seen in switching from Stock air box to BHAF with a heat shield. reason being is the intercooler is doing its job of cooling the air back to outside temps. But what gives the gain is the heated coolant passage in the manifold.

That is the boost port but just right out of site of this pic is a 1/2 pipe plug most love to pull but find out its a coolant passage. Yeap there is 190*F coolant being pumped under the manifold heating the air as it enter the cylinders.

But typically the offset of temp at full coolant temp will be roughly +40*F over outside temps. So if its 60*F outside your IAT will be right at 100*F... now with winter fronts its about 50-60*F offset over outside temps. So at 0*F it will be about 60*F in the manifold. This is because there is very little cold air crossing the intercooler.

So the whole idea of CAI is rather like shooting yourself in the foot... Being the real factor for the common Joe is how clean the intercooler is and if you got a winter front on at the time...

On a personal note I've not seen a single product that can claim a reduction in IAT temps... Think about it...

Thanks Mike. This is really interesting... In essance, the actual injection ports are so far removed from the filter element & air supply... and the air supply temp is sensored & manipulated by Cummins... that trying to vary the temp does not give measurable changes. So supplying enough air for needs (which will be increased if modded) really takes precedence. Such is the differance between naturally aspirated gas engines... and turbo charged intercooled diesel. I bet CAIs will continue to be great sellers... because consumers generally don't know this. But the members here DO!Thanks, Russ

Now if we roll back time to the 1989 to 1991 Dodge trucks I bet the CAI would do a world of good being there is no intercooler... So basically the stock filter was the best being the air was pulled from the front bumper. But now the turbo charger warmed it up, then the manifold, etc... But since the 1989 to 1991 model years had mechanical pumps it didn't change the timing or fuel rates but it did change the density of the air. This is where the nice part of the elctronics came in for performance and the VP44 was the first of its kind. Where the ECM could detect all these variables and adjust timing and other parameters to meet the road and weather conditons over the old school mechanical system that was fixed to 1 setting mostly.

And I never plugged in a block heater since the age of diesel electronics, I am not even sure if they would work if I did plug them in? So for sure with electronic every thing has been optimized.

No... The grid heaters are disable after a starting temperature of above 60*F in the manifold... Most block heaters will warm the IAT up to about 100-110*F... Once again that coolant passage in the manifold...

mopar1973man...have you ever observed an upper limit on heat for MPGs? or better yet...what is the highest iat that you've seen? with the coolant passage...i wonder if a thermostat with a higher temp would be better for winter time?

I think the rule of thumb is to be over 100F but not excessively. So 100-120F I would say is where you want it. I asked him a while back what he had maxed out at and he said 176F was the highest he'd ever seen. Pretty amazing that that's the hottest it has ever got. Higher temp thermostats do help mileage since, well look at cold oil and how it cranks over and compare it to a warm engine starting... the hotter the better, as long as its not excessive. That diesel power magazine where they got 31mpg out of a 3500 used a 200F thermostat for this very reason. If there was a way to keep coolant steady then I think I would set mine to stay at 210. If you get too hot you start breaking down your oil and things.. and you must remember that oil generally will be the same temp as the coolant since they exchange heat in the oil cooler, and the turbo bearings are cooled by the oil so that's another reason you don't want to get excessive heat. It might not be a concern without a trailer since the turbo will only be building 10psi max if your "mileage driving"..

I have just install a QuadZilla Scout and am monitoring the CAI temp. I have noticed a 40* to 45*F difference between the overhead temp to the CAI. I agree with all that has been said here. I am planning on removing my "snorkel" to the fender and possibly blocking the hole in the fender well - all while comparing the temps.

Another good compare to give...

Yesterday I was going to class in Cambridge Idaho about 90 miles from me. Ok. I started out with a cold truck and headed out the current temp here was 40*F but by the time I climbed to New Meadows, ID it was 22*F and the IAT was showing almost 60*F (floating 58-59). But would you believe it my MPG on the SGII was down to 13 MPG in 20 miles of travel... By the time I got to my class 90 miles away I barely got 16 MPG. This with conservative driving too (55-60 MPH).

Now The class is over the temp outside climbed to 65*F and was nice out with a mild temps in Cambridge ID. Now on the way home I kicked it up to speed limit (65 MPH) after completely warming up the engine. But now my IAT was 105*F and the outside temp was holding 63-66*F most of the trip... By the time I got home I pulled 20.5 MPG.

Air temp in the manifold is important...

back from the dead...I have a cowl CAI on my truck with the 3" flex line. I did lose about .5 MPG and reduced the EGT about 50* or so. I'd like to try it again while towing, maybe later this month..Ops

Do me a favor Ops if you can measure you IAT temps with a live data tool and see if the +40*F offset is different that will tell you a bunch...

It sure will. Any suggestions for a live tool? I'm on a budget but I see the value of having such a device.Ops

It sure will. Any suggestions for a live tool? I'm on a budget but I see the value of having such a device. Ops

elm 327...cheap and generic the chinese one's are knockoffs(ie not the elm chip, but a reverse engineered chip) but often work then its a matter of software:evilgrin:

...or my favorite. www.scangauge.com

The 3rd gens run a MUCH cooler IAT than the 2nd gens. Under empty cruise I am only 6-10* above ambient temps, if the outside temp is above freezing. I will get to +35* when boosting and towing hard on a hot day, and +50* on a cool day. As the temp drops below freezing the IAT's run warmer than the ambient by a lot more. There must be enough heat in the engine bay? Even doing 80 at -15*F the IAT's won't really drop below 10*, and are normally in the 20-30* range. Of course the truck has to work much harder in the cold dense air. As for the worse mileage with the cooler IAT's, I think that the other effects of the cold air are what make the mileage drop, tires are harder, fluids aren't as warm and viscous, and don't forget the density of the air is MUCH thicker. If IAT's played that much of a change on their own then I would only get 13-16mpg based IAT's of ~60 which for me is 50* weather, and that just doesn't happen. I have noticed the energy required in cold temp is MUCH higher, just watch your boost gauge. Normally at 80 I run 7-10 psi of boost, if its cold outside the boost is higher, in 10* weather the boost will jump to 18-20 psi. I know the 2nd and 3rd gen IAT's are nothing alike so its not a 100% fair comparison.

I know the 2nd and 3rd gen IAT's are nothing alike so its not a 100% fair comparison.

True... But what is weird I can climb a grade and be a 15-20 PSI of boost and still +40*F over outside temp or cruising main street Riggins, ID at 25 MPH and still be +40*F overside temp... For sure a 2nd gen thing...

True... But what is weird I can climb a grade and be a 15-20 PSI of boost and still +40*F over outside temp or cruising main street Riggins, ID at 25 MPH and still be +40*F overside temp... For sure a 2nd gen thing...

Yeah, if I am going up a grade at 15-20psi empty I am about +20, and 55 mph cruise I am +6, and city driving anywhere from +10-+20, seems to be a lot more variety than you get. Yesterday pulling Horseshoe Bend Hill, 14K GCW, 55mph (following a 1/2 gasser with a TT), in 5th gear Boost was 20-25, EGT's were 1100ish, Ambient was 92*, IAT's were ~127 at most, coolant got to 211*F. Do your IAT's go up with the exhaust brake on? I don't remember ever looking at it before the cam, but now they go up about 10* with the EB on, and drop as soon as I turn it off. I know the pressure isn't pushing exhaust into the intake as that would go up a LOT more than 10*, but I am guessing its from the lack of airflow and probably happened before the cam.

Do your IAT's go up with the exhaust brake on?

Yeah mine jumps anothe 20-25*F on top of the 40*F offset... I've seen as high as like 160-170*F while jakin for manifold temps... But as soon as the Jake is off the temp falls again. I think this is due to more valve over lap... Now a tidbit change in the temp curve... Once the outside temp reaches 100*F out (105* in Riggins, ID last night) the offset is reduced to about +30*F. Only seen this once now... I'll watch at higher temp to confirm.

Thanks for the info..I am still not sold on it being valve overlap, since the EGT's are min 400* with the EB on I would expect to see a MUCH higher temp increase if it were overlap.

I'm thinking that the Second Gen fender snorkle, CAI... are a carry over from the gas engine trucks... certainly not a special for CTDs. I've been contemplating a manual change over from warm underhood air for the winter to a CAI for summer. Why not?Russ