Once again I feel compelled to write something. This one is about the myths of ram air and the effects of intake tuning in general.

There are many ways to get air into an engine. The easiest way is to just slap a filter on it and let it suck it in by itself. This is the most common method. As we all know, the more air we get into the engine, the more fuel we can add, and the more power we can get. This is the basis for forced induction. Unfortunately forced induction requires that a little bit of power be robbed from the engine to make more. Since we still come out ahead in the long run, this is acceptable. Obviously there are some systems that are more efficient than others. The goal of everyone of them remains the same though. More air is more power.

There is more to making power than just giving the engine the required amount of air. There are also other ways to increase output. The length of the intake runners makes a huge difference in where the engine makes the most power. Shorter runners make better high rpm power. Longer runners make better low rpm power. This is because of pressure waves inside the manifold being timed to help add more air to the engine just before the ports close. In a sense this is ram air. However this is not ram air in the context that I am going to focus on right now.

In a ram air intake system, air is supposedly channeled straight from the front of the car and into the engine. This high speed air supposedly forces more of itself into your engine and gives you a gain in power. Can this actually work? Yes and no. It depends how it is done.

Most systems out on the market today are "cold air". The don't so much ram air into the engine as they do just send cold air to it. By cold air I mean air from outside that is colder than the hot engine bay air. Colder air is denser air and denser air is more air. This doesn't mean it is ram air though. Let's get a little technical for a minute.

The rotary as we all know doesn't flow the same amount of air as a 1.3 liter engine. It is closer to a 2.6 liter engine interms of size but it still flows more air than this. for everything to work properly, we have to size it's airflow at almost 4 liters. That's alot more than 1.3. Since we really want our intake air to be going no faster than 180 ft/sec. through the throttlebody we need to figure out how much flow that is at max rpm. In the case of the Renesis it is about 9500 rpm. At 9500 rpm this would be about a 3" inlet pipe. It would be close. The throttlebody on the RX-8 is a little smaller than this though. Just for argument sake, we'll stick with the 3" number.

If a we were at 9500 rpm, we would need to be moving at 180 ft/sec or about 123 mph just to make the forward velocity of the vehicle the same as the airflow into the engine. You'd have to go a whole lot faster to get any usable gain as this is only the break even spot. Obviously we would be at a lower rpm at 123 mph than this so we should have more air entering the engine through ram air right? Not necessarily. If we were to put our air filter straight into the path of the outside incoming air, we still wouldn't get a ram air effect. The 3" inlet pipe is not going to take in hardly any more air. The reason is that as air is trying to force or ram itself into this pipe, it is doing so in a very small space. Maybe a little more will get in, but you won't really notice it. Luckily there is a way to get around this.

Let's say we used a 6" diameter pipe that faced forwards into the airstream. We can't just taper this down into a 3" pipe since the added air will just reverse itself. We'd actually get less air into the engine this way since we'd have to speed the air up as it enters the pipe. when we speed the air up, we lower it's pressure. That is not the goal. Less pressure is less air. If we feed the 6" pipe into an airbox or plenum chamber, we can alleviate this problem. We then feed the 3" pipe to the engine into this. The 6" inlet pipe has 4 times the area of the 3" pipe. When we bring all of this air into the airbox, the greater area or the airbox forces the air to slow down. When we slow air down, we increase it's pressure. This may confuse you when it comes to turbochargers or superchargers but I assure you this is what is happening in them too. The exception to this is the roots blower. It doesn't compress the air by slowing it down. It speeds it up and causes it to build up in the intake manifold. This is where the pressure is increased.

Back on topic. This airbox is now pressurized to some extent. Don't expect gains of supercharger proportions though. This higher pressure air is what the 3" pipe to the engine is feeding off of. Suddenly we have higher than ambient pressure entering the engine. We call this boost! Now we are ramming air into the engine. In order to utilize ram air, we must find a collection point for the air where it can build pressure. If you just have a filter in the airstream, you don't have this.

Racing Beat found that on their 1st generation Bonneville RX-7, a true functional ram air system added 3 mph to the top speed of the car. That doesn't sound like much but at 180 mph, it takes alot of power to go a little faster. This should still emphasize the fact though that you are not going to get incredible results from ram air. It is alot of effort for a little gain. If we could get even 1 psi of extra air into the engine, on a 200 hp engine that is still around 13 hp more. This is only at high speeds but it is more than you have now. If you find old pictures of the Racing Beat 1st gen. you will notice that there is a single 6" pipe that is fed air from the front of the car. It also dumps into a large plenum chamber in front of the throttlebody.

Next time you see a product that is marketed as "ram air" ask yourself, is it really?

 

Do any TRUE ram air systems existyet for the Rx8?

 

just the one that came on the car ^^

 

What do you do for a living man? This is a great write up....

Thankx for the info. I have never seen you so enthused about an intake before as you are with Racing Beats.. They must have a great idea on their hands for you to endorse it.

 

Hehe. Simple thermodynamics. Good write-up rotarygod.

Basically, increased pressure leads to a higher mass air flow rate, which means more power.

Come back to UH and teach Thermo II.

 

Very technical and makes for a good read Rotarygod.
Unfortunately the ricers out there won't be able to comprehend it and will still believe what the marketing department tell them.

 

the other thing about "ram air" bullshit is that when you're actually trying to create a very high pressure point on the front of the car, you're in essence enhancing the drag your vehicle has through the air. at the kinds of velocities you're talking about, this is probably the absolute least efficient way to make horsepower (which is at the same time costing you an awful lot of energy to continue to push the car through the air) even at best.

besides, when you're making a serious amount of horsepower (like RG is talking about here, it's like 200... what about 400?? or 600?? or 2200??), this is a fool's game. ram air is bullshit.

 

When are you putting together all you material and submitting to an editorial?

 

I'm a real estate agent.

This was actually written so the mass marketed ricer bs doesn't make it way here successfully. It wasn't intended to promote the Racing Beat intake. I mentioned them because they did this on the Bonneville cars and proved it works.

Saying that however, their new system I do believe to be the best out there. After visiting Racing Beat, meeting the people, and seeing how they test things, I have nothing but confidence in them and their abilities. They are some of the most honest people out there and don't lie to people with large claims. They are realistic in their claims whether it's what the public wants to hear or not. You've got to respect that. Remember it is numbers not real world results that sell product. They give you real world results at the risk of hurting sales. I'll take honesty any day! This new intake system of theirs is very nice and very well designed. Your car will not be loud and will not idle rough either! This shows proper engineering rather than just slapping a filter on a tube. Although I speak against them frequently, I'm not entirely against cone filters. I am against the way most people utilize them in kits though.

I'm sure there are alot of aftermarket people and companies that wish I would go away. :D

 

Great detailed write-up as always RG... thanks!

 

RG: I could conclude, (based on what you stated), that changing out my stock airbox and putting in a k&n typhoon intake system in all could have decreased the hp of my engine, because there is no longer a box to build pressure in?

Thanks for the info

 

At the very least, you are not fully protected from hot engine bay air, even with their heat shield, the inside of the air filter does not have a nice smooth radius like the stock intake box does, and your intake tube after the filter is too long. Otherwise it's fine! The stock airbox doesn't work as a ram air box. It does tune the intake though and this offsets the purely flow based cone systems out there. Wait until the world finds out that some of the aftermarket exhaust systems out there actually make less horsepower than the stock one!

 

ouch! I'll be pretty dissapointed because I have a B&B racepipe (love the extreme sound of the rotary) and the B&B catback, have yet to dyno it. I do notice a power loss at lower rpms because opening the exhaust up and releasing some backpressure caused the loss of whatever torque I had. I do feel a difference though when I'm up at 7-8500 rpms, or it could be in my head.

 

Not all lose power over stock. Not all gain though. The race pipe will definitely give you a gain though.

 

Oh ok thanks for a minute there I thought that I was just imagining things, and that all the "fun" I had putting it in was a waste. Thanks for the info.

 

Thanks rotarygod -- as always, exceptionally educational for me. And much better written, too; the spacing makes it much easier to read.

I know we have the stickies, but I think we should highlight and collect some of the key posts such as this one as some sort of definitive collection somewhere on the site. This way, if someone wants to look up intake engineering and physics, he can just check out the section on intakes and find a couple of key posts, rather than filtering through the search function sometimes hopelessly.

Maybe even come up with a rec system?

 

no, backpressure is bad, you don't lose low end horsepower because you have less backpressure (in any system, ever, under any circumstance).

 

You lose power because you have too much flow not too little backpressure.

 

I got the point of this thread, and entirely agree with it. However, I have two questions (No2 might feel like "deja-vu"!):

1) Why are you comparing the Renesis air flow requirement as a 4L engine, whereas we know it breathes more like a 2.6L? Or do you mean that a 2.6L engine running at 9000 rpm can be compared, in term of air consumption, with a 4.0L engine running at 6000 rpm?

2) Please explain this : "we really want our intake air to be going no faster than 180 ft/sec. through the throttlebody". Why??

Thanks,

IKN

 

I was saying that you loose torque with an open exhaust I just phrased it wrong. But I also thought that a lose of power on the low end was a cause of less back pressure when freeing up an exhaust system, maybe I was getting something confused.

 

more good info from the g man, will an intake by itself help out at all or does it require an exhaust system etc. to make any use of it. I'm just curious, my guess is that it won't do a whole lot until you aid the fow with a better breathing exhaust but for those of us that can't really spend a lot at a time what would be the better first choice?

 

not especially. and i don't know what RG is saying about "too much flow", it's too little flow through a path which is oversized for the application. if the speed is too low, the static pressure builds and what you end up with is a lower pressure diffference across the exhaust system, effectively raising your 'backpressure' (when in fact i believe backpressure is the static pressure of the system, so the reading would actually drop with the lower velocity). brounelli's law, and all that.

and just fyi (since this is an educational thread) torque as you describe it isn't the "torque" at all, but just low end horsepower (the same as high end horsepower), and yes, this motor doesn't make as much horsepower off idle as a much larger engine would (duh). without significantly changing the characteristics of the engine's function, you don't actually lose any low end horsepower (and with a more efficient exhaust system might even gain a couple of joules), it's basically the butt dyno placebo effect.

IKN, the 180ft/sec number is an arbitrary number chosen by Corky Bell as a good number to try and keep intake velocities close to as a maximum (as in, 182 or 185 would still be ok as a maximum if you knew what was going on, but 210 is obviously not) because of usual observable build up of a boundary layer in velocity stacks with a polished surface, which of course at that point is beginning to hinder the movement of your air charge rather than enhance it. if you were doing some tricky things like beautifully scoring your intake tract with intersecting spirals, or some slight dimpling, or any of that sort of thing, you could concievably get velocities even closer to theoretical (at the same limit of force (pressure difference times area acted on) you get more air moving, so higher than his 180 number but without changing anything else to increase that velocity).

 

oh don't worry about that overgeneralised hooey, "more air in means more gas out", please.

no, your motor is still moving about the same amount in and out (unless you're actually changing the mechanical properties of the system with a priniciple like ram air, which in this thread has been all but defeated as something which isn't of merit) and using things like 'less restrictive' intakes and exhausts only 'add' power by allowing the motor to move gasses in and out with less energy (increased efficiency). so you have this left over energy which can me used to motivate the vehicle instead of the gasses in and out of the motor.

so, if that's all it is you're doing, something of benefit on one side or the other is still going to make a difference on its own, and yes there would be even more benefit if they were 'working together'. i don't really think that one is better to do before the other.

 

Thanks for the breakdown Wakeech.

I'll have to say that I had the K&N on before I replaced the cat with a straight pipe and added the catback system. The car did idle rough when I put the K&N on alone. After adding the straight pipe and catback system, the idle seemed to run alot smoother unless of course I was running the car hard then the idle was giong nuts. I don't know if that is just all in my head as well or just a coincidence.

 

As we all know every time a rotary engine eccentric shaft rotates around a total of 2 times or 720 degrees, it fires 4 of our 6 chambers. This number is relevant because a traditional piston engine that we compare displacement to fires all of it's cylinders in 720 degrees. Since 2 chambers desplaces 1.3 liters, 4 chambers must displace 2.6 liters. You must also remember though that the entire intake, compression, ignition, exhaust cycle of the rotary is 50% longer than in a piston engine. We have over 50% more intake timing and 50% longer exhaust timing. This give us far longer to take in more air. When you run tesets on these engines, it appears that the rotary is very inefficient in how it uses air. The rotary actually takes 3 complete revolutions of the eccentric shaft to fire all of it's chambers which is 50% longer than a piston engine. All of these numbers add up to the 1.3, 2.6, 3.9 liter argument. Remember that each rotor overlaps the other one in terms of breathing. When the engine makes 2 complete revolutions, some air is still in the last 2 chambers since we don't have valves to close them off. When you combine all of these strange effects, the engine actually flow about the same amount of air as the larger displacement number we can give it which is 3.9 liters. I just round it up to 4 to make it easy. These engines move alot of air for their size.