How much air density benefit from CAI vs warm air under hood

[sciguyjim]

I've seen several questions recently about whether a restrictive CAI would be better than an open filter sucking in warm air under the hood. Depending on the restrictions, of course, It seems to me that the small % inc in density of cold air vs warm will not be as beneficial as an open filter assy sucking freely at the warm under hood air. This could be a much larger % change so it could be more beneficial. What do you think?

 

[evelrod]

Well, Jim, "I THINK" that if you were reading those posts you MUST have completely missed the message! Under hood temps can exceed 200f quite easily even on cooler days. The cooler 'outside' air is more preferable even if ambient is 100+ ! Trust in the fact that if it were NOT a benifit, the mfgrs world wide would not be using cool air intake systems on EVERY vehicle made these days! As to the restriction in air flow by the use of a couple of feet of hose---negligable as to automotive use. Keep in mind ---it is not WARM air your 'sucking'---it's HOT air. I had assumed we had gone past this 1950's idea---guess not!


Rod

 

[sciguyjim]

Ok, I guess I'm wrong. There's a lot of details I don't know yet about how everything works. It just seemed more likely that more air, even if hot, can be better than less cold air. On my car for example, the snorkle on the air filter canister was positioned less than 1.5" from a wall. That doesn't allow much room for airflow so I removed the can and have an exposed filter in its place. It can still get whatever cold air comes in but it also has as much warm air as it needs. Are you suggesting I replace the can and find a way to pipe in cool air instead of just exposing the filter?

 

[evelrod]

YUP!

Rod

 

[GregLocock]

Jim, do the math(s). Under hood temp=80 deg C, P= 100 kpa, so density is around 1.2 *(273/(273+80))*100/101

worst case F(ilter)AP at WOT is say 97 kPA absolute, even on the worst intake I have ever seen, short of the one that squashed flat, and say it warms up to 40 on the way in to the filter (which would be very bad indeed) so density is 1.2*(273/(273+40))*97/101

In practice you'd have to be nuts to lose 3 kPA in the intake pipe itself, but you could be unlucky with the position of the orifice. Try and mount it where all the bugs get squashed. Cheers

Greg Locock

 

[sciguyjim]

Greg,
Unfortunately math isn't my strong subject. I haven't seen the exact equations you used but have seen similar. I did a similar thing but with density figures I got from a reference book:

Temp=30C, (86F); density of dry air at 30C=1.1649 gm/liter
Temp=40C, (104F); density of dry air at 40C=1.1277 gm/liter

Decrease in density due to temp rise = (1.1649-1.1277)/1.1649 * 100 = 3.19%

I don't know if the engine power is exactly linear with air density, but this at least shows a 3% drop in density (oxygen available) with air only 10°C warmer. So, 3% of a possible 200hp=6hp gain with 10° cooler air.

Is this what you mean?

 

[GregLocock]

Yes, but I'm guessing that the ambient air is around 40 degrees cooler then the underbonnet temp.

So, using 3% per 10 degrees, that would be a 12% loss, but the pressuure is possibly 3% higher, for a net reduction in air density of 9% for the underbonnet system.

To a reasonable approximation the power of a street engine is proportional to the air mass flow rate, which is the air density times the speed times the volumetric efficiency times the swept volume /2.

So if you don't change any of the others then then power will be proportional to the air density. Cheers

Greg Locock

 

[sciguyjim]

Thanks for taking the time to explain this Greg. I looked at the space under my hood again today and there was even less room than I remembered. I can't even turn the air filter can in the necessary direction, much less put a big tube on it and try to route it to a cool area.

 

[evelrod]

You did not mention what car you have but, I'm guessing it's a V-8 with a carburetor. If so, it might be possible to duct outside air in via the plenum at the base of the windshield. You must cut and fab a bit, though.

I think you and Greg are being a bit conservative on the under hood temps. The only test(early 70's) I ever did on this subject using a mechanical pyrometer (primarily used for tire temps) indicated a lot higher temps. on a Lotus Elan. (we had changed to 45mm Webers and the 40mm airbox would not fit) Very limited space in a very small car---ran a duct through inner fender panel to special K&N filter housing. Not perfect, but a bit better. Still had to deal with under fender heat.(I used the same method on my Lotus Cortina except I went to the headlight bucket) I don't recall the exact horsepower numbers, but It was about 3 hp at the rear wheels. If I am remembering the hp figures correctly thats about 3% increase. Well worth the effort! (The testing was on a portable chassis dyno with large squirrel cage blowers to simulate road speed so the hp numbers were for comparison purposes only.)

AND---as I reread this it is entirely possible that a small change in jetting may have been needed. That means the test(?) is not exactly 'scientific method'!


Rod

 

[GregLocock]

I'm guessing there's some heat soak in a long intake tract... but you will go and throw real life experience in!

The intake side of our engine is generally around the 80 mark, but the exhaust side would be any silly number you want, obviously. We used to burst hydromounts due to heat soak from the downpipe - glycol everywhere.

Cheers

Greg Locock

 

[sciguyjim]

Greg,
I have an '89 Pontiac Trans Am, 5.0L, tuned port injection. If I remove the air conditioning I can easily run an intake tube down to the underside of the engine compartment. Even a very flexible plastic tube has to be squashed to fit between all the hoses and wires in the way now.

Evelrod mentioned going to the headlight bucket. That is the closest access to outside air that I have. My intake snorkel is about 4" diameter but the hole in the wall behind the retracting headlight is only about 2" diam. I can see a narrow space between the wall with this hole and another wall in front of it (headlight mechanism maybe). My initial thoughts were that trying to suck air through these narrow gaps would be too restrictive, that's why I opted for the hot under-hood air.

To use the holes behind the headlight I'd have to find a way to construct a slanted, cone-shaped device that would connect to my snorkel on one end and butt up to the wall with the holes at the other end. So far I haven't gotten any good ideas. I'm not the type to tear things apart if I don't have to. In fact, I'll settle for hot air if I can't get the cold easily enough.

During my experiments with modified air filtering mechanisms, I saw how the car would act if I pushed it too hard for the amount of air it could inhale. This is the only way I know of to tell me if it's able to breath freely enough. If it turns out that it can't breath freely enough through the holes behind the headlight, then I'm back to the choice of restricted cold air or lots of hot air.

Time to wander through the hardware stores again with a head full of ideas looking for stuff I can use. Thanks to everyone for helping me approach the problem from the right direction.

 

[sciguyjim]

P.S. If you want to see a couple photos of the car and engine, goto:

http://www.geocities.com/sciguyjim/index.html

and click where it says "My Car" at bottom left. (There are a bunch of other photos there too you might get a kick out of.) Enjoy.

 

[carnage]

so if you were to fabricate a CAI system for a vehicle, would it work best if made out of a non thermaly conductive material?

 

[ivymike]

Unless you ran into other problems (breaking, etc), then yes.

 

[JayMaechtlen]

of course, you could go with two slightly smaller or non-round ducts... suck from the smaller holes, one on each side of the car - can you go under the radiator?

you might prefer an insulated/non conductive material, I suspect- but a long tube with no fins is a lousy heat exchanger anyway, right?

(how do you past pictures, again?) Jay Maechtlen

 

[sciguyjim]

Jay, I considered using 2 or 3 smaller tubes to bring in at least some portion of cool air, but again I was put off by the amount of restriction I'd introduce into the airflow. After a couple failed ideas I finally hit on an easy way to run a 3" diam tube into the fender for cool air, and I didn't even have to squash the hose! I'm sure the new air is cooler than it was before and I still have more than enough airflow. Thanks everyone for explaining this to me, I hadn't realized it made such a difference.