Cowl induction for cold, ram air for the Si?

[NitrousG35]

I have been giving a lot of thought about intakes lately, especially a source for cold air/ram air.

Currently, the only option out there is a CAI from Injen or AEM. We all know about the hassles in installing a CAI, like removing the front bumper and having to cut some of the fenderwell material away. Also, there are issues with hydolocking due to the low placement of the filter element.

Looking at the SRI's, only K&N makes an attempt to use the cold air tubing from the factory as well as a heat shield. However, without the filter element being sealed off, there is no vacuum to pull cold, dense air in.

Comptech had a great idea with the Icebox, sealing off the filter element and using a snorkel to pull cold air in. The same goes for the Type R airbox. However, neither one of them is totally utilizing a high pressure area to maximize the ram effect of the air.

I remember in 1968 when Chevrolet offered a cowl induction cold air option on the 1968 Z/28. The idea had first been applied to 1963 Chevrolet Z11 stock cars in NASCAR. Both NASCAR and the SCCA required air cleaner elements to fit under a stock hood without a scoop. There was a duct that ran from the cowl area of the firewall to the air cleaner.

Chevrolet approached the subject scientifically. They studied airflow and high and low pressure areas. They found that there were two ideal places to ingest cold, dense air: Under the front bumpers and the cowl area in the engine compartment. Forward facing scoops in the middle of the hood like found on many of the classic muscle cars did little to draw in cold air. They were mostly cosmetic.

Chrysler found when running tunnel ram manifolds on their Super Stock and Pro Stock cars that a scoop very high up above the hood could also draw in lots of cold, dense air. However, this was not practical on a street car and was not allowed in NASCAR or the SCCA.

Later, Chevrolet took this idea of pulling cold air from the base of the windshield with scooped hoods like those found on the L-88 Corvette and the 1969 Z/28 Camaro. Both were revolutionary. As they say, things go full circle. What worked in 1969 would still work today.

Here is what I am getting at: I am studying the firewall area of the Si in the engine compartment, just below where the wiper area is. That area is a high pressure area as it is at the base of the windshield. By cutting a hole in that firewall area and running a duct from the firewall area to the air filter of a SRI system or something like X has, making sure that the entire air filter area is enclosed or shrouded, cold air could be ducted into the motor that would provide a cold, dense, ram air charge.

The benefits of this would be advantageous over either a SRI or a CAI. First, there would be no heatsoak. The motor would only be drawing in air from the base of the windshield. Secondly, there would not be a problem with hydrolocking as it would not be placed where rainwater could enter in. The high-pressure area at the base of the windshield is utilized to enhance the ram effect by allowing the air filter to draw fresh cool air which creates a denser fuel charge thereby increasing volumetric efficiency and horsepower.

I know that X tried something along these lines, but he made two mistakes: First of all, the scoop he made was up in the airstream, facing forward. His idea was good, but he forgot that there is actually more high pressure at the base of the windshield pulling downwards. Besides being kind of ugly, the scoop was placed wrongly to be effective. You want something either from the firewall area or to have a scoop akin to the '69 Camaro to make it work properly.

There are different kinds of plastic shrouds and tubing that one could use to duct from the firewall to the air filter assembly of a SRI. One would also have to construct a box or greatly modify the stock box. In any event, this has got me thinking about using technology that has been around for a long time to get more performance out of my Si.

BTW, Chevrolet found that for every 11 degree drop in intake temperature, there was a 1% increase in horsepower. On the 1969 Camaro, there was a 15hp increase!

Post up your thoughts. I am trying to think outside the box. When we put our minds together, we can up with some great ideas. I have seen many people here on the forum who are innovative and brilliant in conceiving and designing parts for the Si.

Chevrolet Cowl Induction duct that connects to the firewall:







1969 Camaro with Cowl Induction hood:





Here is exactly what I am talking about on a Mazda Miata:

Hole in cowl wall that will duct cold, rammed air into the intake:



Enclosed Simota filter, necessary for ram air:



Example of duct that can be attached to cowl wall area:



Finished product and facsimilie of what we can do on the Si:


On a K series motor:



On an Integra/RSX:







Article about a Miata in Australia that employed a cowl induction setup:

Here is the article that is being discussed. It is worth reading IMHO:

The Plan of Action
One popular aftermarket intake available in Australia for MX5s is the Loch Stewart airbox. (Pics of this design can be found at http://www.miata.net.au/loch.htm). Replacing the entire airbox up to the airflow meter, it picks up cold air from the cowl below the windshield through a large 3-inch tube, and feeds it in a straight path through a (stock size) flat panel filter to the airflow meter. The cowl is a known high-pressure area - notice how air is pushed through your cabin vents when driving at speed, even when the HVAC fan is switched off.

The theory behind these particular intakes is sound, though in my opinion they suffer two problems. Firstly, by being constructed from fibreglass over a mandrel, the raw external appearance is well below what one would expect from a product costing hundreds of dollars (the inside is however beautifully smooth). Secondly is of course the price - if something could be built just as effectively for less, then it would be worth doing.

Other aftermarket cold-air intake systems available from the US (eg Jackson Racing, Racing Beat) draw cold air from over the top of the radiator, but given the current value of the Aussie dollar, these are prohibitively expensive. The LHD Miata, it should also be noted, does not have the ability to be fitted with as simple a cowl-induction intake as the Loch Stewart, as the brake master cylinder and booster are located where the intake would go.

I decided to follow the same principle as the Loch Stewart system, which of course involves the painful decision to cut a big hole in the sheetmetal of your car. This panel is however non-structural, so not likely to cause any weakening of the chassis, nor cause possible rust as might a hole through the wheel guard. By keeping the end of the intake above the bottom of the cowl channel, there should also be minimal chance of sucking water into the intake.

Gathering the Parts
I also wanted to incorporate a larger airflow meter. Wreckers were searched for a cheap RX7 airflow meter, but the cheapest source turned out to be a fellow AutoSpeed member who answered the call when a Wanted notice was posted on the For Sale forum.

Working out how to fully enclose the filter (so that cold air could be ducted to it) was a dilemma, and about the only easy choice I found was a Simota Super Power unit that incorporates a plastic heat shield around a cone filter. Analysing and measuring photos from the web showed that the intake hole to this filter was approximately 100mm - coincidentally a popular plumbing size. The filter (with a carbon fibre finish) was ordered from the Autospeed Shop for $81.

From a large hardware barn, a neat PVC 100-to-65mm adaptor was found at a mere $2.50. A (minimum order) one metre length of 65mm PVC plumbing tubing was bought for another $10, the idea being to use a short length of this to go back through into the cowl. 75mm tube would have been nice, but I couldn't find any 75-to-100 mm adaptors. Given that the length of tube required was about 100mm, I figured this was a minimal compromise. When the filter arrived, it was found that the plumbing adaptor slid straight on like it was custom-made to fit! I was expecting to have a slight tolerance error that could be corrected by softening the PVC, but this was extra-good fortune.

The Simota filter uses the ubiquitous 3-inch outlet duct size. While adaptors can be bought cheaply (for about $25) to fit such a filter, I had the materials available to build my own. By notching and pressing a short piece of 3-inch aluminium tube in a vice, the end could be squashed into a match for the RX7 airflow meter's rectangular intake (the inner circumference of both pieces being almost identical). This provided a very smooth transition with no square edges. This tube was bonded to a flat plate that had suitable mounting holes for the airflow meter, and the adaptor was complete.

Installation
At last, it was time for assembly. With the stock airbox removed, a 2½-inch holesaw was borrowed and used to cut a hole through the cowl wall. This hole required enlarging, using grinding tools on an electric drill, until the 65mm PVC tube could fit through. A 2¾-inch (70 mm) holesaw would have been perfect, but I wasn't going to spend $50+ buying one for a one-off job! The bare metal was finished with touch-up paint, and a length of plastic edging strip fitted inside the sharp metal edge to prevent damage to the PVC tube.

A short piece of the tube was cut, and the inside of one end chamfered slightly to hopefully smooth the flow of air into it. The trick of heating and flaring out the tube into a bellmouth could not be used since the tube could only be fitted through the hole by inserting it from the engine bay side. The tube was fitted to the PVC adaptor, and both were given a coat of Penetrol (penetrating oil - to help the paint stick without chipping), followed by a coat of silver spraypaint. I thought silver would look the best purely for aesthetics, though theoretically it should also help keep as much heat out as possible.

The whole intake contraption (snorkel tube, PVC adaptor, shrouded filter, airflow meter adaptor and the standard [see breakout box below] airflow meter) was bolted together and fitted to the car. Two small metal brackets were used to support the airflow meter to existing mounting holes, now vacated by the stock airbox. The only other matter was to relocate the windscreen washer bottle to the wheel guard, since the hole in the cowl goes right through where the bottle originally sits.

Testing
Back on the road, the most immediately noticeable change was the loud induction roar - definitely a more angry sound than stock, and quite pleasant in an aggressive way at WOT. Seat-of-the-pants testing confirmed more urge in the midrange, and also a willingness to run right out to the engine cut-out at 7200 rpm. Fitting the manometer showed that the total pressure loss had dropped from 42cm to 26cm H2O (a 38% decrease). This meant that the restriction upstream of the airflow meter had improved from 25 to 9cm H2O. Potentially more could be gained by fitting the RX7 airflow meter, but as explained in the breakout box, that setup was not tested.

Using a stopwatch, and hand-timing some acceleration runs was carried out to compare the new system with the stock one. To avoid the launch variations of a 0-100 km/h run, I chose to start at a cruise at 2000 rpm in 2nd gear, punch the throttle wide open, and run out to 6000 rpm. Redline would have been nice, but I didn't want to break the speed limit by too much... With the standard intake, over 8 runs back and forth I had averaged 6.1 seconds. Fitting the new intake system dropped the average time to 5.8 seconds, a 5% improvement. While the 0.3 second improvement may not seem much, it was highly repeatable.
Temperature measurements were made with the probe fitted at the entrance to the snorkel. Under nearly all driving circumstances, the intake temperature was within 5 degrees C of the ambient temperatures - obviously a small amount of heat-soak from the engine bay was still occurring. However, the improvement was still typically about 30 degrees C cooler than before. Using the rule of thumb that a 4 degrees C temperature drop is worth an extra 1% power, this would imply that I could expect up to 7% extra power over the standard system. Parked at idle, the temperature got closer to 10 degrees above ambient (hard to judge since I didn't have a reference thermometer, but still well below the stock setup), and immediately dropped when moving again.

Later, the opportunity arose to dyno test the car at SelectMaz in Epping, Melbourne, with the MX5 Club of Victoria. The graphs show that there is a moderate torque improvement in the midrange at about 4000 rpm, and also a good top end improvement above 5500 rpm. Peak rwhp went from 75 to 79hp - a 5% increase. I can tell you that the graph also compares very well with the aforementioned Loch Stewart system which was fitted to other cars tested on the same day, giving almost identical power outputs (very similar exhaust systems were also used on those cars).

Note also that the bonnet was 'popped' to the safety latch for both runs, giving slightly cooler air to the engine bay - if the bonnet had been fully closed, I would expect the stock intake to have sucked hotter air and given worse results, and the corresponding improvement to look even better.

[melomood]

wow thats a great idea +1.

[Slowroll90]

it works for the Sti why not the Si lol
no but it would def have benifits but a lot of work to do it right
props if you do it though

[melomood]

i know how you can do it, it will just look like **** though. but who cares, performance over looks, imo

[honda_man12]

That is a great idea for sure...my only issue is making it look as clean as possible

[qwerty :)]

X already did this

[I Vtec, do you?]

Originally Posted by qwerty :)

X already did this

As Bill already pointed out, he has the cowl facing the wrong way. Apparently there is a denser source of air at the bottom of the windshield. Basically, it seems like X's idea would work perfectly if it was just flipped around.

Sadly.. I don't see how you can ever test this kind of intake since you wouldn't see any of these gains on a dyno.

[KidnKorner]

Originally Posted by NitrousG35

there would be no heatsoak

There goes that word heatsoak being misused again. Heatsoak has nothing to do with the temprature of the air an intake system ingests. It has to do with the heat transfer components under the hood are subjected to by the engine.

[20strong]

What if the induction ran from the sealed airbox through the plastic cover at the base of the windshield. But you put the hole on the windshield side of the plastic sinse it doest sit flush. You could probably remove a considerably sized hole there and no one but the driver would really see it.

[Zeuceone]

im working on something like this at the moment. it will only work with the x intake but not to sure if with any other sri. xfactorwrx idea is still a good one.

[I Vtec, do you?]

I'm wondering.. would it be possible to just create a heatshield for the K & N that would completely cover the entire filter, forcing it to only draw in air from the cold air inlet?

[qwerty :)]

all the examples you gave are for carburated engines(of the ram being backwards from X's).. i think that the way X had it was the right way, the best way even though i dont think itll make a difference either way.. and it works well for the STi because it goes straight to an intercooler, not just a filter

[Zeuceone]

yup. thats where he got the idea from. you guys are stuck on the oldschool which doesnt work on the k20

[Stealthcivic]

great idea man! new ideas ... glad you came forward with some thoughts....this is similar of course to what I have been saying in my thread wanting an intake with rubber/ plastic tubing to avoid the heated tube of aluminum....you mentioned the icebox and mugen intake they both have this setup , although they run down to the front of the engine bay not the firewall, I like the idea....

[F-1 Si]

This is a good idea. If you could construct a cowl style cold\fresh air vent to work under the wiper assembly that would be ideal. Take it a step futher and wrap the K&N intake pipe with exhaust\header wrap to futher decrease the intake air temp. I think the benefits would be well worth the time.

[NitrousG35]

Originally Posted by qwerty :)

all the examples you gave are for carburated engines(of the ram being backwards from X's).. i think that the way X had it was the right way, the best way even though i dont think itll make a difference either way.. and it works well for the STi because it goes straight to an intercooler, not just a filter

Wrong. You are ignoring the laws of physics. Aerodynamic tests that have been done show that sticking up a scoop at the base of the windshield facing forward draws in less flow. It has to do with laminar flow. The critical area is where the air hits the windshield at the base. Flow separation would happen if a scoop is placed up in the airstream at the base of the windshield.

There are two ways to access that air: Cut a hole in the firewall and run a duct there, or run a hood scoop like on the '69 Z/28 and bring the scoop edge as close to the windshield as you can.

BTW, if you look at NASCAR, they all use this setup. It works because of its aerodynamic efficiency. At Daytona for example, they can make a lot of free horsepower this way. It has nothing to with a motor being carburated.

[NitrousG35]

Originally Posted by qwerty :)

X already did this

No he did not. I pointed out in my original premise in the first post that he had the scoop in the wrong location. It was on top of the wiper cowl area facing forward. His theory was good, but his placement was wrong. It does show however how clever his mind is because he is approaching things from a scientific point of view. I still feel X is a genius when it comes to having the idea of changing the MAF vanes.

[MutantCheese]

How would you stop rain water from making it way down the tube to the intake? I don't think that hydrolocking would be an issue, but large amounts of rainwater going into your engine bay still can't be good.

[NitrousG35]

Originally Posted by 20strong

What if the induction ran from the sealed airbox through the plastic cover at the base of the windshield. But you put the hole on the windshield side of the plastic sinse it doest sit flush. You could probably remove a considerably sized hole there and no one but the driver would really see it.

You still need to run a duct to the firewall to access the high pressure area right by the windshield wiper area. Some of the holes in the black plastic are already drilled out in the wiper cowl area. A guy could just open a few more up. Then, the duct could pull air from the cowl area. I think the less cutting on the outside of the car, the better. This mod would be under the hood and would be inconspicuous if a duct was attached to the hole.

[NitrousG35]

Originally Posted by MutantCheese

How would you stop rain water from making it way down the tube to the intake? I don't think that hydrolocking would be an issue, but large amounts of rainwater going into your engine bay still can't be good.

I have already thought about that. On the bottom portion of the lip, you could put a rubber seal, or in the hole that you would cut, a rubber seal like is used in the engine compartment could be trimmed to fit the base of the hole.