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Discussion Starter #1
Results of Steel/Plastic CAI debate


Well I spent some time this week putting together my own simulation of air temperatures based on the analysis that GulfCoastMustang presented late last week. He may be the only person that cares about the blood and guts, so the rest might prefer to simply skip to the results below.

Method- I used a simple heat balance just like GCM did setting mCpdT = Q = UAdT

System assumptions were CAI diameter of 3” and length of 28”, air inlet temp of 75F and ambient engine compartment temp of 145F. Tube thickness of .12” (might be a little too thick but it doesn’t affect the results) and a thermal conductivity of 16 W/m*K for steel.

I took the relevant properties of air (thermal conductivity, viscosity, density, heat capacity, Prandlt number) and used cubic spline functions in MathCAD to interpolate values over a range from –10 to 260 F.

Natural convection is hard to figure for the engine compartment, because no one knows exactly how air moves in there. Typical h values for natural convection are around 5-6 W/m2*K however, but I assumed 25 W/m2*K just to be safe.

Calculating an overall heat transfer coefficient at 200 CFM (flow rate at 2500 RPM) gave us U=2.6 W/K, which is pretty small (I used Dittus-Boelter to get hin values for the inside of the pipe). The overall temperature rise at those conditions is about 1.6 degrees F. Now at a worst case assumption where convective heat transfer is SO robust that the wall temperature is steady at 145F, the overall temperature rise is 7 degrees F (hin, which becomes U, for the inside of the pipe is 70 W/m2*K).

If you look at the Densecharger, which is made of ABS plastic, the thermal conductivity drops to about 0.9 W/m*K and the overall heat transfer coefficient and overall temperature increase goes practically to zero.


What it all means. There is no significant temperature effect introduced by a CAI, no matter which brand you buy. Unless your filter is exposed to engine bay air, a stock airbox, steel CAI, or plastic CAI will all have approximately the same inlet temperature to the intake plenum. Thus the ONLY benefit you should expect to see from a CAI is the performance increase gained by having a smooth pipe instead of rough rubber as the conduit for inlet air.

Based on the numbers I see, I would recommend going with a system like C&L if you want to play with the intake tract. It doesn’t appear that the aluminum inlet after the MAF will affect temperature, the heat shield should protect you from warm engine bay air, and the benefits of their MAF tuning are well proven. I also don’t see any further benefit by ignoring their recommendation NOT to use a CAI with their system. Clearly the CAI will not greatly affect inlet temperatures, and forcing the air to make the sharp turn left just before the MAF is likely to disrupt the ability of the MAF to get a consistent signal (it takes a surprisingly long time to get steady, fully developed flow.)
 

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I didnt see any advantages as well as far as temp with the plastic or aluminum, but the fenderwell deal is a problem, thats why I trashed mine and went with the 11 inch S&B racing filter and protected it with a aluminum box ( keep the fan wash & heat away from the filter). BTW good info..Da Snake Boy!!!
 

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Not having done those specific calculations, I can say that at least intuitively the conclusion looks reasonable (as long as the units were consistent, i.e.: dimensions in m and temperatures in K). Sorry, I teach introductory physics so I have to ask about units :)

However, I am still not convinced there is anything to be gained over the factory "CAI". Other than perhaps a throatier sound on WOT. The differences in air flow between a smooth and a ribbed plastic inlet cannot be very significant, if they are of approximately the same diameter.

After fussing with CAIs and MAFs for about two years, I have re-installed the oem air inlet system and the car has never run better. I wish I had some data, but by SOTP, the factory setup does the best job in my particular 96 GT. Go figure.
 

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Discussion Starter #5
Actually the greatest part of MathCad is that it handles ALL unit conversions behind the scenes. Its pretty easy to tell when you screw up. If you are expecting a temperature output and you get some funky unit output like W/dyne * sq. inches then you know you messed up.

Actually small variations in wall friction CAN make big differences in pressure drops, but for near stock N/A power levels I don't think you will observe any effect. PowerPipes work for a reason however, and low wall roughness coefficients are the reason.

I'm not really convinced that there is anything ot be gained by ANY CAI at near stock levels, but I do believe there is some tweaking to be done at the MAF.

Bear in mind that the factory inlet is not the bottleneck in your 96 GT, the wheezing heads/intake combo is.
 

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NorthwestGT, to answer your question about the flow differences of a segmented or rough pipe as opposed to a smooth mandrel bent aluminum or steel pipe, consider this. A few years back when I owned an 86 lebaron turbo, don't laugh...... I met a guy that had a similar car and he swapped out the 18" corrugated plastic inlet tube from the fender to the airbox with a 3" smooth piece of silicone? pipe. It was blue and picked up .3 sec in his quarter. Apparently, this accordian, SP?, shaped pipe causes alot of static friction in the pipe. Refer to your static friction losses of pipe and consider the inside surfaces of cast iron as opposed to the same size id pvc. Food for thought.

Also, since from what you are saying above, metal conducts heat better than plastic? Would icing the metal piping with bags of ice have any great effect? Better than Plastic?

Again, food for thought.

Basic theorem, chromed metal looks better!!!
 

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Aaron,

I've been looking into this as well and came to about the same conclusion as you. However, I think your "worst case" scenario is closer to reality with steel CAIs. You assume a simple heat transfer from the ambient air in the engine compartment through the tube to the intake air. I thought of it more along the lines of....
The main source of heat transfer is not from the ambient engine air through the tube, but instead is from conduction from the intake/throttle. i.e. the main heat source will be the intake/throttle->tube->air around.

Second, you can simplify the calculation even further...since the air temp of the engine is significantly higher and is near the temp of the intake, most of the heat will be transfered to the intake air at a rate based on the conductivity of your material.

lastly...I have several more reasons for this, such as the ambient air temp staying nearly the same as engine temp, and the engine not being cooled much from air flow around it....blah blah blah....

Ok I would go on but I have work to do...simply stated...
Just take the throttlebody/intake temp (the metal), and use that as a heat source of 175°F. Then you can use your conductivity values to see the propogation of heat down the tube, and the heat transfered to the forced convection air in the tube. Once steady state is reached, you can see a much more realistic value.

I'd like to know what you think about this and if you do any calculations let me know. I would do them now but obviously you have the model already and should be easy to change, and I have about 12 hours of homework to finish in 8 hours time :)

But even with these assumptions the change in temp is not as dramatic as some would think.

-DSal


P.S. If ambient air was the source of the heat..then no matter what, your pipe could never get more than 145°F...Anyone who has accidently rubbed a metal intake pipe after driving for an hour or a few runs on the track, knows that it is a hell of a lot hotter than that.
 

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sorry about the poor wording in the last post...I'll fix it later if confused...was in a rush :)
 

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Discussion Starter #10
I don't know... of course my 2V car isn't going to transfer a lot of heat with that plastic intake manifold. Not to mention that convective heat transfer tends to be much faster acting than conductive heat transfer. You will never be able to reach a steady state pipe wall temp that approaches engine operating temp because the intake charge will sweep that heat away as soon as it makes its way from the intake.

While that total amount of heat coming thru conduction might be significant with a metal intake (like on your Cobra) you are moving a large enough volume of air to take away that heat without significantly heating the intake charge. (200 CFM is at only 2500 RPM, at 5000 RPM its obviously going to be 400 CFM... that's a pretty big cold sink compared to your heat sink)

I feel pretty decent about the 145 ambient temp. My car runs dead nuts at 195, and whatever is happening in that compartment, when you are going 60 down the road you are pulling some draft-induced vacuum at the bottom of the car. That hot air must be displaced by cooler outside air. Even if the ambient temp creeps up to 160-170... the result is still not significantly changed because the heat transfer coefficient is still so damned low.

Bottom line... a product like the Densecharger is absolutely not going to allow the air to warm up at all. CAIs by MAC or BBK might allow about 5-10 degrees of heating (prolly more like 2-3) but not enough to cause a significant effect.... and in any event the only way these products are better than stock is in providing a smoother path for airflow.
 

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What do you think of this intake? It is called a RIPP (roots intake performance pipe) from Chicane. I have one on my car and by SOTP it feels stronger up top.

Dyno results:
Ripp kit tested against MAC cold air induction on 3-26-02 at Swanson Performance in Torrance California. Conditions 75 degrees ambient at 3-400 feet above sea level.

3000 rpm- gain of 0.5 hp /0.9 lb ft tq
4000 rpm gain of 5 hp / 6.8 lb ft tq
5000 rpm gain of 8.1 hp / 8 lb ft tq
 

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Very interesting analysis, Aaron. I'd definitely concur with the results, since they are on the same order as the Static conditions I did my forced convection heat transfer calculations at. The answer is interesting, though, and I hope that, even if stangers don't bother with the degree in engineering to understand all this, that they walk away with the understanding that a significant increase in intake temperature is not going to happen with either a steel or plastic intake tube. Performance increases lie in the smooth, unrestrictive flow through a well designed system.

Now, performance LOSSES are possible, if the system you use draws any air from the engine compartment. The factory system is a good one. It is a "cold air" system by design, and while slightly restrictive, is pretty hard to beat on most cars.

Obviously, when you're moving as much air as Scott's blown Mustang, these flow losses are far more critical.

On this note, I also wanted to point out that if someone considered "icing" the intake tube, or somehow diverting airflow to cool the intake tube, you are unlikely to see any temperature decreases in the incomign air stream. And that for the same reason you will not see temperature increases - airflow rate is too high to see significant heat transfer, in either direction.
 
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