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Acura RSX Type S - The Great Intake Test

We Discover What Makes An RSX Intake Tick

May 1, 2003
0305_impp_z+aem_intake+engine_view Photo 1/1   |   Acura RSX Type S - The Great Intake Test

We thought this would be easy. Gather up as many new RSX Type-S intake systems that we could find, locate a Type-S we could use as a guinea pig, bolt 'em on and let 'er rip. Take a few pictures, post the dyno numbers and, voila, we have a nice juicy story.

Boy, Were We Wrong.As it turns out, it took several months for us just to get the intakes in one place. The RSX is still a very new car, and not everyone had their intake systems ready to go when we were. As of press time, only Advanced Engine Management, DC Sports, Injen, Jackson Racing and K&N responded to our query. AEM and DC Sports sent two intakes each, a short-ram and cold-air system. Injen sent its short-ram, and K&N sent its own short-ram and a stock replacement filter for the RSX.

Jackson Racing provided us with a mostly finalized prototype version of its intake. However, the elbow bend for the pipe was revised before there was time to bend a new one, so Jackson simply modified the bend with short lengths of hose. Oscar Jackson assured us that, although it wasn't pretty, it was the final shape for the intake and would provide the same numbers as the production version. In addition, DC Sports had provided us with its finalized prototypes, which were the models for the production version. We loaded them into the car, and hit the testing room.

OK, we know what you want to see, and if you just can't wait to find out what the final power numbers are, turn to page 95 and look at the chart there. When you're done, come back here.

Finished? OK. Beyond just finding out peak power numbers, we wanted to answer a few other questions about these intakes, and about the RSX in general. For example, how much do the systems flow? How much of a difference do the cold-air systems make over the short-ram systems? How can one get consistent dyno numbers on a car that is constantly switching between six different fuel and spark maps?

To answer these questions, we devised a rigorous testing regimen to evaluate the intakes not just on sheer horsepower numbers, but to determine exactly how that power was being made.

DesignsThere are two basic designs for RSX intakes; the short-ram, which places the filter under the hood, and the cold-air intake, which puts the filter outside the engine bay. However, both look "backwards" in the engine compartment. The K-series engine in the RSX sits on the opposite side of the engine bay than we are used to seeing in a Honda or Acura. As such, the intake manifold is on the front of the engine, so aftermarket intakes are routed differently.

Short-rams invariably place the filter next to the driver's-side firewall. These intakes take minutes to install, with the whole procedure taking less than a half hour using only simple hand tools. The most difficult part was removing the bulky stock airbox, and if that gives you too much trouble, you should master that whole "chewing gum and walking" thing before doing this install.

During our installation procedures, we found the AEM short-ram was the easiest to install; however, we should remind you some of the other pieces were still in prototype stage, so the mounting points might have been revised by the time you read this. The only exception to the plug-and-play nature of the short-rams was Injen's piece, which includes a stamped-steel heat shield. But this hardly adds any complexity to the installation.

The cold-air intakes are a different story. Because the intake is on the front of the engine, the pipe for a CAI has to make an abrupt turn downward to the front of the vehicle. If you're installing an intake for aesthetic purposes, skip the CAIs; all you'll see is about 3 in. of the first bend before the intake system burrows through the nose of the car. Installation is not nearly as easy as the short-rams, either. The windshield washer bottle is in the way. Both AEM and DC Sports have different solutions to that problem. AEM provides a relocation kit, complete with new hoses, straps for holding it in place and other hardware needed. DC Sports, on the other hand, offers a new, reshaped bottle that will fit in the stock location, saving a bit on the installation time. Both kits require the driver's-side fender liner be cut to accommodate the new intake pipe and the filter placed in front of the left front wheel. Plan on at least a couple of hours to install one of these cold-air systems, and have a lift handy, or at least a strong floor jack and jackstands

FlowThe theory behind an intake system-be it short-ram or cold-air-is it sucks more air into the engine, allowing for more fuel to be burned and, therefore, creating more power. But just how much does it flow? In other words, how much air is an intake capable of moving from point A (the outside edge of the filter) to point B (the end of the pipe)? To measure flow, you use a flowbench, and to measure something the size of an intake, you need a big one. Luckily, Branch Flowmetrics in Cypress, Calif., had one that met our specs. We strapped all nine of our aftermarket systems to the bench, as well as a stock air intake assembly from an RSX Type-S, and tested them all at 12 in. of water.

The results were very interesting. The lowest number, not surprisingly, was for the stock airbox with the stock filter, posting a corrected number of 237.0 cu. ft per minute (CFM). Replacing the stock paper filter with K&N's oiled cloth filter raised that number to 243.9 CFM, not much of an increase, really, but it demonstrates the biggest impediment to flow in the stock intake isn't the filter, but the intake assembly itself.

One look at the stock intake, and it's easy to see why. The path it takes is tortuous, sucking in air from the nose of the car, but immediately bending it through different sized pipes, routing the air to the back of the engine compartment, altering the path 180 degrees as it passes through the filter, then finally shooting it over to the throttle body. Plus, the intake itself is filled with baffles and anti-resonators to keep the noise levels down. It works, but it also significantly impedes flow.

From there, we moved on to the intake systems themselves. When all was said and done, Injen posted the highest number on the flowbench, at 351.4 CFM (see graph "Flowbench Results"). Interestingly, the cold air intakes posted the lowest numbers, with DC Sports CAI coming in at 322.1 CFM and AEM flowing 323.2 CFM. The K&N Typhoon short-ram system flowed the lowest number of all the short-rams, barely beating out the cold air intakes at 323.3.

Why the disparity in numbers? Well, the K&N Typhoon quandary is easiest to answer, as it uses a smaller pipe than the other short-ram systems. From there, it's a little tricky. Cold-air intakes, in theory, result in more power, however, they do this through filter location, not flow. Long pipes and multiple bends are anathema to good flow numbers, and cold-air intakes have plenty of both, with pipe lengths at least twice as long as the short-rams.

Imagine sucking milk through a straight, standard-length straw. Easy, right? Now, imagine that straw was twice as long and curls all over the place. You need more effort to suck the liquid up to make it to your mouth. The same basic principle applies here. The Injen intake, on the other hand, was not only the shortest of the group, it boasted a big pipe and only one easy bend, a roughly 60-degree turn that it made where it attached to the intake manifold.

Then we got curious. With the Injen pipe clearly the best-flowing design, we decided to find out who was using the freest-flowing filter. Although they were all of the familiar cloth-soaked-in-oil variety, there are differences you can't see just by looking. We removed the Injen filter and flowed the pipe with the AEM, DC Sports, Jackson Racing and K&N-supplied filters (see graph "flowbench Mix and Match"). For the most part, not much changed with the AEM, DC and K&N filters returning flow numbers almost the same (and in the case of the K&N filter, exactly the same) as the Injen filter. However, when we attached the Jackson Racing filter, we got a reading of 359.7, notably higher than the Injen number. So the flow king was a hybrid of Injen and Jackson Racing. OK, you can't actually buy that combination in a store, but it's interesting to note.

TemperatureBut there's more to intake efficiency than just flow. Cold air is denser, and if an intake inhales cooler air from the outside world, rather than the hot air from under the hood, then it would stand to reason that CAIs would make more power in the real world.

We wanted to test this, especially with the RSX. There is a camp that says because of the large cutouts behind the wheelwell in the RSX, short-ram systems will be just as efficient as CAIs. The thinking is enough air circulates under the hood when driving that the advantage of the CAI is taken away, and the deficits of the poorer-flowing long pipes will come into play.

We set about to discover if this was true in a couple of ways. First, we tapped into the stock computer using an OBD-II tool. This allowed us to monitor the temperature the engine itself was seeing via the intake air temperature sensor located in the intake pipe. We also mounted thermo-couples on the car's roof to measure ambient air temperature, under the front bumper on the driver's side to measure the air temperature at road level, and inside the engine compartment near where all the short-ram systems placed their filter to measure underhood temps.

The first difference was obvious. Even at speed, the temperature under the hood was significantly warmer than the outside temperature. On the day we tested, it was a nice, comfortable 78F day. However, under the hood it was positively Phoenix-like, averaging 110.1F (ranging from a high of 116 to a low of 105). Note these temperature readings were taken at 80 mph, with plenty of air circulating around the engine bay, but they were still far above the ambient outside temperature. What's more, when the car was at idle (say, at a stoplight), underhood temperatures would skyrocket, sometimes shooting well over 130F.

How did this translate to the intake temperature itself? Well, pretty much as you'd expect. We tested a representative sample of the short-ram and cold-air intakes (AEM's short-ram and DC Sports CAI, respectively). The designs of the intake systems vary only in details, and we were more interested in the temperature of the air they inhaled, not the power they created at this point, so testing each system would have added needless complexity to the test. The AEM short-ram drew in air at a two-way average of 123.5F, 7.5 warmer than the underhood air itself. The DC Sports CAI, on the other hand, had an intake air temperature of 98F, 10 cooler than the underhood temperature for those runs. In addition, the CAI was much more consistent, running 98F on both runs, while the short-ram varied wildly, averaging 127F on one run and 120F on the second, a 7 difference.

We also tested Injen's intake system because it comes with a stamped-steel heat shield designed to block heat from the nearby exhaust manifold. We bolted it in place, made our passes, then quickly removed it and made two more passes. We were fast enough with the wrench that the outside temperature didn't change at all. However, the results were interesting. The underhood temperature reading was actually higher with the heat shield in place, but the average intake temperature itself was lower by 3F.

So, what do all these temperature readings mean? Well, it turns out cold-air intakes actually do draw in cooler air than underhood short-ram intakes do. So what? Well, as air cools, it gets dense. Denser air means more air getting to your intake manifold. Even with the effects of heat soak on the intake pipe itself, the air from the cold-air intake was still significantly cooler than the short-ram. Remember, we are reading the intake air temperature from the car's sensor, so we are seeing what the car sees. Clearly, as far as intake air temperature goes, the cold airs have an advantage.

However, the short-rams were actually capable of inhaling more air during our flowbench tests. Will this greater flow ability offset the cooler air of the CAIs? Only the dyno will tell.

On The DynoLet it be said here: Testing the RSX Type-S on a chassis dyno is a pain in the ass. Consistency is very difficult to achieve, with the engine computer constantly switching between six different fuel maps, six different ignition maps, and who knows how many different maps for the VTC part of the i-VTEC mechanism. Integras with a B18 were easy: Just strap them down, hit the go pedal, and out pops your reading, time after time. With an RSX, you can be up one minute and down the next without even touching the car.

It is for this reason that so much variation is seen in intake tests. Some systems have been reported to have astronomical numbers; however, the testing procedure itself must be called into question. Like we said, this isn't the same as the B18. If you're going to test an intake system, for example, you can't do your baseline a week in advance. If you have no choice, you must take the RSX's fickle brain into account if you want anything even close to accurate numbers.

We enlisted the help of Doug MacMillan from Hondata, a man who has more experience with the RSX's brain than anybody outside of Honda itself. He informed us of two things. First, the car is very sensitive to temperature, and will switch programs if it detects just a slight variance in the engine's temperature. Second, the car will rapidly change its long-term fuel correction, pulling out fuel (and reducing power) as intake temperatures rise.

Our handy OBD-II reader came to the rescue again. We were able to monitor the engine's coolant temperature down to the degree, and ran all of our tests with the computer reading 204F. In addition, Doug showed us the easy way to reset the long-term correction by unplugging that part of the computer briefly. By doing that, we were able to achieve repeatable dyno pulls, with numbers within a horsepower or so.

Our on-road temperature testing taught us another thing: Dyno tests need to be done hood-open to achieve anything like real-world conditions. Although the engine compartment doesn't get nearly as cool as the outside air, it does benefit from underhood circulation. Unless your dyno is in a wind tunnel, it's impossible to replicate high-speed airflow on a chassis dyno, even with fans blowing all over the place. However, opening the hood cools off the engine bay temperature to about the same levels as they would be on the road, and this is how we conducted our tests.

However, we also conducted tests with the hood closed. This gave us a chance to see how the different systems reacted to heat soak. With the hood closed, all of the intakes suffered a tremendous drop off in power, with intake temperatures skyrocketing quickly. Even the cold-air systems were seriously affected. This means that in the real world, you would want to open your hood and let the engine cool for a while if you're trying to get consistent quarter-mile times.

A word about some of the photos. You will notice that the AEM and DC Sports cold air intakes were tested with the filter sticking out of the hood, not mounted near the ground. We did this for the sake of convenience, since removing and replacing the cold-air systems was quite difficult. Although this would seem to be an unfair advantage, we checked to make sure the intake air temperature was the same at the nose of the car as was the ambient temperature where the filter was placed. They were identical. Also, note that the intake air temperature numbers for the dyno test match the numbers from our on-road tests. In other words, despite the strange positioning, having the filter mounted in this way made no difference in our temperature reading, and we conclude it didn't make a difference in our final testing.

So, how'd they do? The only modification that didn't make any power was the K&N drop-in filter, which in fact lost a statistically insignificant 0.1 hp. Considering the low flow numbers from the flowbench testing we conducted, we didn't have very high hopes for the drop-in anyhow. As a money saver, it's worthwhile since you'll never have to replace it. Just don't expect much power from a drop-in.

On the other hand, all of the short-ram and cold-air intakes made power. Of the short-ram intakes, Injen came out on top, bumping power up 9.2 hp to 166.9. This tied DC Sports' short-ram in power, but Injen had a slight edge in torque. However, we were shocked by the numbers produced by the cold-air intakes. We're going to call it a draw between AEM's system and DC Sports. AEM's CAI added a whopping 20.7 hp, 1 horse more than DC Sports. However, DC's system added a statistically similar amount of torque.

ConclusionsYou can't go wrong with any of the systems. All of the new pipes made power, to varying degrees. The short-rams are generally much easier to install, less expensive, and will provide a good dose of horsepower. The cold-air systems are much more difficult to install, but the extra effort is well worth it considering the tremendous power gain. Read the individual tests to see our impressions of each system. Finally, note that the prices listed are MSRP; shop around and you'll be able to find almost all of them for less.

On-road Temperatures
  DC SPORTS CAI AEM SR INJEN W/HS INJEN W/O HS
2 RUN AVG., INTAKE TEMP (°F) 98 123.5 116 119
2 RUN AVG., UNDERHOOD TEMP (°F) 108 116 111.5 105
2 RUN AVG., OUTSIDE TEMP (°F) 77 76.5 77 77
2 RUN AVG., INTAKE/UNDERHOOD. DIFF. -10 7.5 4.5 14
2 RUN AVG., INTAKE/OUTSIDE DIFF. 21 47 39 42
Dyno Data
  OUTSIDE UNDERHOOD COOLANT LTC INTAKE POWER OVER TORQUE OVER
TEMP. TEMP TEMP   TEMP   STOCK   STOCK
(F) (F) (F) (%) (F) (HP) (+) (lb-ft) (+)
STOCK 87 116.1 204.0 - 0.11 122.0 157.8 0.0 119.9 0.0
STOCK W/K&N 87 115.8 204.0 -0.11 121.0 157.7 -0.1 119.9 0.0
AEM SR 86 110.0 204.0 -0.11 111.0 163.1 5.4 124.8 4.9
K&N TYPHOON 87 116.0 204.0 -0.89 112.5 165.1 7.4 123.0 3.1
JACKSON R 87 118.3 204.0 -0.11 114.5 165.5 7.8 124.1 4.2
DC SPORTS SR 87 108.0 204.0 -0.89 113.5 166.9 9.2 122.4 2.5
INJEN 87 113.5 204.0 -0.11 113.5 166.9 9.2 122.9 3.0
DC SPORTS CAI 87 120.5 204.0 -0.11 97.0 177.4 19.7 125.6 5.7
AEM CAI 87 120.8 204.0 -0.11 96.5 178.4 20.7 125.2 5.3

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