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Old 11-20-2006, 11:55 PM   #1
mjo
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Default Hot air intake

I'm driving only 20 miles a day, mostly city driving. The outside temperatures have been getting below freezing and it takes forever for my car to warm up. I've been getting only 30 MPG and drive lightly to conserve gas. So I decided to build a hot air intake and put it on my '95 SL2. It was extremely cheap to build and only took an hour to install right. I'll post some pictures later.

Now the car is running fantastic - the car warms up completely after only 2 miles. Engine's running very smooth and is quieter.

The materials that I used were: $3 aluminum "heat riser" tube from Carquest, $18 header wrap bought on Ebay, $3 metal screw-type clamps from Walmart, and used some of the handyman's favorite tool (duct tape).

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Old 11-21-2006, 12:47 AM   #2
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Default Re: Hot air intake

Quote:
Originally Posted by mjo
I'm driving only 20 miles a day, mostly city driving. The outside temperatures have been getting below freezing and it takes forever for my car to warm up. I've been getting only 30 MPG and drive lightly to conserve gas. So I decided to build a hot air intake and put it on my '95 SL2. It was extremely cheap to build and only took an hour to install right. I'll post some pictures later.

Now the car is running fantastic - the car warms up completely after only 2 miles. Engine's running very smooth and is quieter.

The materials that I used were: $3 aluminum "heat riser" tube from Carquest, $18 header wrap bought on Ebay, $3 metal screw-type clamps from Walmart, and used some of the handyman's favorite tool (duct tape).
Post some pictures. I'd like to see that.

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Old 11-21-2006, 02:15 AM   #3
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Default Re: Hot air intake

Also if you know post what your gas mileage was before and after the install of this.
I remember seeing someone here that has done this with some gains in mpg.



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Old 11-21-2006, 02:15 AM   #4
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Default Re: Hot air intake

Anyone else think this is strange?

Having your engine breath cool dense air would be idea...
If your car is taking a long time to warm up, you may need a new theromostat.

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Old 11-21-2006, 02:36 AM   #5
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Default Re: Hot air intake

I don't know if it is strange or not but I do wonder about the physics involved. With the big market for COLD air intakes and the advantages often touted for them, I do wonder about the logic for making a HOT air intake.

They obviously are contradictory. I have often considered some form of modified air intake but I would like to see the data and hear the physics of each to actually know which one is the way to go. Personally I would think the cold air intake would be better because cold air is more dense and would contain more air molecules per cubic foot.

That fact is also supported by the fact that many forced air, (super charged or turbo charged) intakes often incorporate intercoolers to cool the air after comming out of the compressor.

It does not surprise me that this mod to heat the air helps warm the engine quicker but I would be surprised to hear that there is an increase in all other engine parameters such as performance and fuel economy.

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Old 11-21-2006, 01:16 PM   #6
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Default Re: Hot air intake

Well if you think about it, the car takes air in by volume but uses it by the pound. So, if you are using hot air, you are getting less air by weight. Less air by weight would mean the PCM, clued in by the O2 sensor, would add correspondingly less fuel. I would think there would be a price to be paid in performance but, you would get better mileage.

The posts that claimed better mileage were by CheapyBob. His claims were pretty dramatic. I'm not so sure he wasn't unconciously changing his driving style as well.

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Old 11-21-2006, 01:29 PM   #7
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Default Re: Hot air intake

Quote:
Originally Posted by BarnOwl
Well if you think about it, the car takes air in by volume but uses it by the pound. So, if you are using hot air, you are getting less air by weight. Less air by weight would mean the PCM, clued in by the O2 sensor, would add correspondingly less fuel. I would think there would be a price to be paid in performance but, you would get better mileage. ....
ok but: to hold a car steady at any given speed on the road takes a certain amount of air and gas, and they must be present in equal ratios

if you are pulling in hot air you will need to push the gas pedal down further, so if anything the mileage would be equal.

Getting the intake to warm up faster will take the fuel injection system out of 'choke mode' quicker, but after the first 5 minutes of driving that should not make a difference.

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Old 11-21-2006, 02:28 PM   #8
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Default Re: Hot air intake

If you live in a cold climate one thing you could do to get better gas mileage is to put a block heater on your engine. Better yet if you park the car in a heated garage.

That way you are not running the car with an ice cold engine in the morning, and you should get better effeciency.

The cost of heating a garage, or even of running a block heater, may not offset your savings in gas, but at least you are paying the money to your electric company instead of your gas station.

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Old 11-21-2006, 03:23 PM   #9
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Default Re: Hot air intake

In the mods section there has been lots of talk about warm vs cold. The warm air helps gas mileage, and engine breathability, while the cold air hurts gas mileage a bit, and loses low end, but gains upper end hp. Since turbo's take time to spool up, the rpm's ahve to be kept up on them, anyway, so the cold air only helps.

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Old 11-21-2006, 03:57 PM   #10
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Default Re: Hot air intake

Quote:
Originally Posted by KCW
ok but: to hold a car steady at any given speed on the road takes a certain amount of air and gas, and they must be present in equal ratios
You're correct. It takes a fixed amount of energy to keep the car going down the road at a fixed speed. Therefore there would be no gain if you are talking about highway driving for either a cold or hot air intake. OTOH there might be a gain to be had at the cost of accelleration in city driving with a hot air intake.

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Old 11-21-2006, 04:42 PM   #11
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Default Re: Hot air intake

this sounds like a job for the MythBusters!

The temp of the air itself entering the combustion chamber should not affect mileage significantly. The fuel injection system is going to measure the temp of the intake air, and the engine block, and pump exactly the right amount of fuel into the engine. If the air is warmer the computer will see that and know it is less dense, and put less gas through the injectors

but! that means you get less power out of the engine for any given throttle position. We dont care about gas consumption vs throttle postion, we care about mpg driving the car. I think you will still have to push the pedal down further if you are pulling in warm air, and it will be a wash.

To get good data on this you have to start with a car that is running properly. Its possible the thread starter has a bad air intake temp sensor, and he has compensated for that by forcing the air across the hot manifolds.

That doesnt mean a properly running Saturn engine will see any benefit by doing the same thing.

If you have an engine with a carb, and the mixed air/fuel has to travel from the carb to the intake ports, then cold metal will make the fuel condense out, and you will have to run more-choaked until things warm up. I can rationalize a benefit with a carbed engine, but not with a fuel injected engine like the ones in our Saturns.

I have reservations, but this sounds like an interesting area for experimentation, to see if anyone can carefully document their cars consumption in cold weather, make the mods, and see if any benefit results.

Then you have to take the mods back off, and see if the performace returns to the original levels (ie, it wasnt caused by something else, like... you happened to pump your tires up after you made the mods...)

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Old 11-21-2006, 04:47 PM   #12
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Default Re: Hot air intake

Unfortunately for this one we only have annecdotal evidence and I'm not really convinced either. I am pretty sure that it will warm the engine a little faster but, that's all.

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Old 11-23-2006, 01:24 AM   #13
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Default Re: Hot air intake

Well, the heat riser tube only has a 1.5" diameter. I found that I can't just plug the original air inlet completely otherwise the engine doesn't get enough air. I drove with the main inlet stuffed with paper towels and the car didn't like that. It ran sluggish at high speed, the temp dropped and I was smelling gas out the tail pipe. I replaced the paper towels with duct tape which I covered only half of the main air inlet with so that half the air can come from the main inlet and the other half can come from the hot air tube.

So even after putting the car under some hell and getting the ECU out of whack the fuel tank indicator seems full for the miles I put on so far. For those who are curious the needle passed the halfway full mark at around 160 miles. Before this mod I was getting only 30 MPG max (all city driving).

I'll post pics within a couple of days and also post the first fuel economy measurement. I also have lots of ideas for upgrades so stay tuned...

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Old 11-23-2006, 11:22 AM   #14
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Default Re: Hot air intake

you cant get an accurate mpg number by looking at the gas tank gauge. You have to top the tank up all the way, reset your trip odo, and then carefully refill the tank to the same level after 200 - 300 miles.

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Old 11-23-2006, 07:26 PM   #15
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Default Re: Hot air intake

Pictures of the heat riser installation

Put one end into the opening for the resonator
The other end extends between the block and behind the exhaust manifold. It is supported by
a couple of cables that attach to the engine dipstick tube

In order to suck in as much heat as possible while retaining the correct flow rate,
I only blocked half of the main intake tube that enters the airbox:

The air box is insulated

using this stuff

To get more of a lean burn I'll have to drill a hole into the air filter box
and tap it 1/8" NPT. That way the sensor will read a warmer (correct) air temperature.
But first I'll wait until my MPG results are calculated at my next fillup.


KCW, thanks for the tip. Good thing I reset my trip odometer at the last fill-up

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Old 11-23-2006, 11:01 PM   #16
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Default Re: Hot air intake

It sounds like a good idea. Basically you are reducing the density of the air so you have less pumping losses. Also at cold temperatures fuel doesn't atomize as well so you have to increase the mixture - a choke in the old days. The warm air will get you out of that mixture enrichment phase. I had an old Mazda GLC with a carb on it. It had a summer/winter switch on the air breather that redirected warm air up from the exhaust in winter. If you didn't do it the carb would freeze up.

Of course cold high density air gives more peak power but I've always had low power cars so I've learned to manage my power.

I think I'm going to try something like this too.

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Old 11-24-2006, 09:29 AM   #17
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Default Re: Hot air intake

Back in the pre EFI days, had a car ('73 Century) that contained thermostat controled preheat on the carb intake air snorkle. When cold it pulled air past the exhaust mainfold, and when warmed up, through front of the snorkle. This improved cold engine driveability and probably emissions.

Faster warn-up to normal operating temperatures is good for fuel economy I see that in my fuel economy changes during the year. With air temperature in the 30s it takes an additional 4-5 miles (give or take) of driving to reach normal operating temperature compared to when the air temp is in the 60s. That is a hurts net fuel economy with a 9 mile drive to work.

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Old 11-24-2006, 11:16 PM   #18
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Default Re: Hot air intake

Just filled up today: 232.7 miles / 6.60 gallons = 35.26 MPG all city driving.

I think there's a problem with the IAT sensor being in the original air inlet. My opinion is that it's changing the ignition timing at the wrong temperatures. I'll have to relocate it to a place in-between the two inlets.

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Old 11-25-2006, 07:51 AM   #19
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Default Re: Hot air intake

Quote:
Originally Posted by mjo
Just filled up today: 232.7 miles / 6.60 gallons = 35.26 MPG all city driving.

I think there's a problem with the IAT sensor being in the original air inlet. My opinion is that it's changing the ignition timing at the wrong temperatures. I'll have to relocate it to a place in-between the two inlets.
AFAIK the IAT is only used during closed loop. You should do the IAT but, it should only effect the mixture during warmup.

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Old 11-28-2006, 10:09 AM   #20
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Default Re: Hot air intake

BTW, BarnOwl thanks for the tip.

Continuing my perilous experiment...
Since my last post I've added insulation to the sides of the airbox.

It's been very humid the past few days and ambient temps have been in the 50s. After insulating my airbox more I've noticed that the engine warms up even quicker. Then the car became very sluggish.

I was concerned that maybe the car adjusted the ignition timing to too much spark retard. The coolant temps seen from the dash would jump up to 3/8 and then fall to 1/4. If there's too much spark retard the exhaust valves and guides can become flame-kissed by the retarded, I mean late, combusting gasses leaving the cylinders. I'd rather not turn my engine into a puddle. So being cautious I removed the tape covering half of the cold air inlet. I also removed the cardboard that's covering half of my radiator . The car warmed up even faster today but coolant temperatures more or less stuck at 3/8 this time. I'm also getting better acceleration. I guess the hot air doesn't need assistance in being sucked into the engine because it is less viscous and rises into the airbox. Gee, maybe that's why they call it a heat riser tube .

I've done a few thermodynamic calculations and at first glance it looks like the theoretical air temperatures when the engine is compressing air are higher than I first thought. I'm planning on writing and running a script in Matlab that would calculate the air temps at each crank angle based on inlet temperature, mixture, fuel type, etc.

The Chilton's manual states that you can run spark plugs with a colder heat range if you are driving long distance trips. Assuming that they mean that a colder plug can be used in a warmer engine - I'm considering changing my spark plugs.

I'm also considering doing a few experiments with Toluene. From what I have found Toluene has a self-ignition temperature 100 degrees Fahrenheit higher than Gasoline and has a high octane. You can get the stuff from a hardware store.

And a wireless barbecue thermometer is going in my engine compartment as soon as I can grab one.

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