Flow Testing a 2007 Civic Ex (R18) Intake

Today we're going to look at the 8th generation civic, specifically the R18 powered Ex model. We're going to take a look at the restriction currently being caused by the stock intake system and measure it exactly.

The R18 as you probably know is a 140hp power plant that is extremely fuel efficient but lacks some sense of urgency, especially when overtaking on the highway. Part throttle performance seems immediate, but it's simply a trick that Honda is playing. Push the throttle down a little further and you'll find that there's not a lot of extra umph left to give.

The EX trim civic in our possession takes around 8.5 seconds to go from 60-80mph (yawn!), and that's with the AC off and being fairly generous about the start and stop points on our stop watch. Granted, it's an automatic, but we did measure the acceleration time AFTER the car had downshifted (so that time doesn't take into account the actual downshift). This can be a little frustrating when trying to quickly overtake someone on the highway. To compare, project Lexus (IS300 e-shift) did the same sprint in a around 4.3-4.5 seconds. Granted, there's a huge difference in the two cars and comparing the two is completely unfair, but it does give some perspective.

A Look at the Stock Intake System

To get an idea of how restrictive the stock intake system is, we simply need to get out an inexpensive vacuum gauge, preferably one that measures vacuums of 0 to maybe 27 inches of water. We've talked a little about this in a previous article here.

I do not have a vacuum gauge as I do a lot of this sort of testing and I find that a digital differential pressure meter is better for my purposes. It's around $130 but it's worth its weight in absolute gold for modifying your car. It can tell you where to put hood scoops, brake cooling ducts, intake snorkels, measure exhaust back pressure and much more.

Anyhow, I popped the hood and found the PCV vent tube that runs from the valve cover to the intake arm. The PCV vent on this car isn't right at the throttle body, but it's far enough along the intake system to give me a very good idea of the total intake restriction.


I unclamped the two clamps and temporarily let the crankcase vent to the atmosphere. I then connected my vacuum hose to the intake arm and ran the vacuum hose into the cabin to my differential pressure meter. I could also have drilled different tap points and inserted a nipple to measure at a different point in the system. After measurement you can cover these small holes with silicone or similar.

After the meter was in place I took the car out for a test drive and measured a PEAK intake restriction of -8.0 inches of water. However, that was only seen once on the test drive and I was unable to duplicate it. I was however able to duplicate a pressure drop of -7.8 inches of water.

Project Lexus' original intake measurements were -9.1 inches of water but after we fixed the hood seal that was blocking the intake's snorkel a bit, we saw a measurement of just over -8.0 inches of water. The IS300's intake we said was VERY good, so the -7.8 measurement we're seeing on this Civic is a double edge sword. It means there's not much room for improvement in the stock intake and it's quite good. The downside to that of course is that there's not a lot of easy horsepower to be found here.

But let's not get too caught up on the peak values as there was a very interesting story at part throttle and especially at cruising speeds. Peak horsepower is far from everything, especially in day-to-day driving.

While in project Lexus we saw very little restriction in the system at part throttle (even positive pressure most of the time), in the civic, it seemed to ramp up pretty quickly even under what should have been small throttle openings. My theory is that the drive-by-wire system in the Civic, intended to give the impression of more power to the user, opens the throttle further than you think you are opening it when you press the throttle to accelerate. This is pretty standard practice these days with drive-by-wire systems.

The MOST interesting observation, however, was that even at 70mph, there was only a mere +0.6 inches of water positive pressure in the intake, again at cruise throttle positions. In project Lexus,we saw between +2 and +3 inches of water at that speed. That's a significant difference. That being said, it's not that the Lexus has such a better intake as it could very well be that the Civic needs more throttle opening to achieve a similar cruse speed and that's accounting for the difference. The only way to find out of course is to do some experiments.

What I suspect without any further information is that while the IS300 takes advantage of the positive air pressure available to it by taking it's air from just above the radiator via a forward facing intake snorkel, the Civic is actually taking air in from a shielded location, a location that also happens to be rather warm.

At most speeds, the Civic doesn't take advantage of that free potential horsepower (and actually a theoretical gain in gas mileage too - thanks to a reduction in pumping losses, but more on that in another article). The IS300, does.

Oddly, Honda has used this same sort of design for decades. Almost all Hondas have an intake arm that leads to an airbox with the air filter. Then, they usually have a snorkel that leads to a large resonator box that has a convoluted system of air passages in it designed to reduce intake noise.

In all these designs, the air pressure where the intake system actually takes in air, is usually atmospheric or just a hair above due to the snorkel being shielded from oncoming air.

Nissan, VW, Toyota and many others often take their intake air from just over the radiator (basically using the hood lip to shield the actual inlet from dust/water/etc) in order to take advantage of the build up of air pressure on the front of the car as the car moves down the road. I

The engine bay has plenty of places where we could change this design however and potentially pick up some power, especially while moving. As an added bonus, the upgrade should be completely reversible, and nearly completely stealth.

We'll explore some of these options and do some before/after testing in the next article. Stay tuned!



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