N/A theory

this is not my work, but the work of a guy on another forum. it is posted specifically for our cars since it is from a mazda forum, and it is all theory on naturally aspirated tech. i recommend that you read this whole post and not just skim over it. the guy that wrote this is an n/a god.

Disclaimer: As with any type of Automotive Upgrade, N/A is full of Grey, it's never as easy as a Black and White answer. There is always an exception to any example. The information presented in this thread is just that, basic information. It is possible to go into much further detail on the subject, which is up to you in the future. When types and styles of upgrades are compared it is under the condition that they are all of similar capability. It is always possible to make statements false by taking them out of context or changing the basic parameters of the discussion.

Ok, so I thought I'd go over the basics of Natural Aspiration theory for anyone on the forum considering NA mods. There are a few stages in which to make power when you're going the all-motor route. Most people are familiar with the basic bolt-on mods, Intake, Header, Exhaust. These are the most basic and commonly done to give you a small boost in power, throttle response and even gas milage. With a good system, designed to work with what you have, you should see 5-10% gain in power, with the only real downside being noise. The primary way an IHE set-up give you power is by freeing it up. It takes energy for the engine to suck in, and pump out air. So by minizing pumping losses, by installing a free-flowing intake and exhaust system, you free up power.

Some power is also achieved by some more complex actions. A header's gains are mostly by reducing pumping losses. But there's more to a good design than that. A 4-2-1 design, aka Tri-Y, is a space saving header design, designed mostly to minimize pumping losses, but can also give gains in power by maximizing low-RPM velocity. Velocity is important for low-RPM power and throttle response, to help pull spent exhaust gases from the combustion chamber, and to aid in scavaging. Scavaging is the action of spent exhaust gases creating a partial vacuum in the cylinder, which when the intake valve opens, pulls in a fresh air/fuel charge. This is the reason it's recommended to keep a relatively small diameter exhaust(2-2.5") with an NA combination- to maintain velocity. With a larger bore exhaust, you have no velocity at low RPM and lose this effect. Unlike turbo exhaust, bigger isn't better.

The other way to go, is a 4-1, or tuned length header. As well as minimizing pumping losses, and maintaining velocity, you can tune a 4 into 1 design to create even more power over a somewhat narrow RPM range. The key, is that all four primaries have to be stacked two on two to work. When an exhaust valve opens, it creates a pressure wave. Every pressure wave has both a positive and negative side. When this pressure wave first enters the primary at the speed of sound, it travels down the primary and into the collector , and back up the other primaries towards the exhaust valves. As it hits the closed valve and bounces back, the exhaust valve opens and the negative side of the wave helps draw out the spent exhaust gases. Because the speed of sound is the speed of sound(it can change with gas density though), it can only happen over a limited RPM range, say 1000RPM. So if a 4-2-1 design gets you 10whp overall, a tuned length header can get you those same results, plus another 3-4hp at it's tuned RPM, usually at higher RPM, since the physical length of headers tuned to low-RPM are not viable in most applications.

Headers in general are affected by both the primary length and diameter. Like I mentioned before a smaller primary diameter favours lower RPM velocity, but can limit high-RPM airflow; thin diameter(say, 1.5") piping can be a restriction when you're running high-RPM, high-power levels, say 170whp and 8000RPM where you'd want at least 1.75" primaries, but should be fine for most low-tune, general applications.

Although a header itself will give you the single best bolt-on gain from your IHE set-up, the exhaust from the header back is important too. There are the basic cat-back exhaust bolt-ons, which like I mentioned, mostly minimize pumping losses. A high-flow catalytic converted can also free up a couple hp. The same theory applies, where smaller diameter piping will aid in low-RPM velocity. A well designed 2" exhaust would be great maintaining torque, but would choke off power above 6000RPM. 2.25" exhaust is a great compromise, and should give you the best of both worlds. Decent torque and decent power potential up to at least 8000RPM and 150whp. I chose 2.5" exhaust because of my future goals of 170whp and 8500RPM redline. I actually lost torque below 3000RPM over my factory exhaust, but will allow for all my future mods to give me maximum gains up top.

There is a lot of misinformation and myths with intake systems. Most intakes work by minimizing pumping losses(again, by making it easier for the engine to suck in air), but there are other variables. The Cold Air Intake theoretically takes cold air from either a fender or behind the bumper. Cold air is denser, and denser air has more oxygen molecules per weight, therefore more potential to react with fuel to create power. The effect is debatable, as most CAIs are made of aluminum, which gets quite hot and heats the intake charge quite a bit. The wrong CAI on the wrong engine can have the wrong results too. Just as with exhaust, intake air velocity matters. Air has mass, which has momentum. As the piston moves down the cylinder, it pulls in the fresh intake charge. Even as the piston starts to move back up the cylinder, the intake valve doesn't close right away, and the air wants to keep moving into the cylinder, since the mass of the air behind it doesn't want to slow down. With a small sized CAI, you get that same low-RPM velocity, but just like exhaust can choke off power at higher RPM. If the CAI's diameter is too large, you lose that low-RPM velocity, but may be able to gain it up top.

The other option is the Short Ram Intake, which take air from inside the engine bay. The advantage of this set-up, is that it allow you to run a smaller diameter, say 2.5" versus the 3" of most CAIs, to allow for torque gains. But because the piping is shorter, pumping losses may not be as bad at higher RPM, than a CAI which is longer and has more bends. Air doesn't like to change directions, so the less bends you have, the better. That goes for exhaust too. The downside of course, is that it draws in hot air from the engine bay. From most dynos I've seen, the SRI has outperformed the CAI. What works in theory, doesn't always work in practice, and some cars will like one more than another. Another factor is resonance tuning. Just like tuned headers, pressure waves are created when intake valves open. Those waves can disrupt the intake flow at certain RPM. So what makes one intake work over another, could have to do with intake length, just as much as diameter, and where it pulls in air.

Filter choice can also make a difference. If you stick with a stock airbox, don't bother with the 'K&N' filter upgrade. Our engines don't flow enough air to require a K&N filter, in their stock running condition. I dynoed the K&N back to back with a Fram paper filter and then with no filter. The power was the same on all of them. Where the filter matters is on SRIs and CAIs...the pod filter. Bad designs or poor quality filtering material cam limit power. Look for ones with an intake horn to allow air to flow smoothly into the piping, such as the APEXi unit.

What matters most, is what works on your car. Learn from others who have done the work before you.

Well, that's the end of part one. Theory 102 will<

once again, not my work. the guy that wrote this is an n/a god

The first part, I covered general NA performance, and now I'm going to cover bottom end modifications that would be useful for an NA build-up.

The first thing I want to talk about is rod ratios. This is the ratio between the connecting rod, and the engine's stroke. In my case(with the BP-ZE), my connecting rods are 133mm and the stroke is 85mm, giving me a rod ratio of 1.56:1. Or just 1.56 as it's normally expressed. The FS-DE has a 135.2mm rod and a stroke of 92mm, giving it a rod ratio of 1.47. An EXTREMELY low rod ratio, and I'll tell you why. Normally in the performance world, a 'short' rod ratio is between 1.6 and 1.8, and a long ratio is between 1.8 and 2.0. As you can see, both of the Mazda engines have very short ratios. What does this mean? It means that our engines see EXTREMELY high average piston speeds, and acceleration, ie G-forces. With a short rod and relatively long stroke, the piston accelerates very quickly towards Top Dead Centre, and accelerates very quickly away from TDC. What it means is that engines with short rod ratios don't like to rev, and ones with long rod ratios do like to rev. Typically, a short ratio is good for torque and a long one is good for high-RPM power.

Here's some numbers to help you understand. The FS, with it's 1.47 rod ratio, has a maximum piston speed of 7017ft/second at 7000RPM, and maximum piston acceleration of -3378Gs. The BP, with it's 1.56 ratio has maximum speed of 6454ft/s, and maximum acceleration of 3080negative Gs. So on our engines, when the piston is moving to and away from Top Dead Centre, it's doing so very quickly, and changes directions very abruptly. On the other hand, and engine with a rod ratio of 2.0(an Indy car might have a rod ratio of 2.) would have a maximum piston speed of 5670ft/s and maximum acceleration of only 2612 Gs.

What this means, is that the FS and BP don't like to rev extremely high. Or if they do, they need better internals to be able to handle the increased loads of high RPM life. RPMs will kill an engine much faster than boost. Forces increase at the square to engine speed. ie, a 10% increase in RPM creates a 20% increase in forces. The funny thing is, plenty of Miatas see 9000RPM or more. Why? Forged internals, and some nice, strong lightweight aluminum rods(forged steel is stock). The rod ratio can be changed too. Either destroke your engine(custom cranks are very expensive, although an FP crank would fit in an FS), or lengthen the rod, by moving the piston pin higher up in the piston. This of course would be limited to a 3-4mm at most, but every little bit helps. Of course, you would need customs rods and pistons.

Rod ratio affects other things. It sounds bad on paper, but there are plenty of advantages to a short rod ratio. One, is the extreme resistance to knock/ping. Since the piston dweels for such little time at TDC, any nasty detonation has very little time to affect the piston before the piston is already heading down the cylinder. In fact, with the FS and BP, you can run some awesome ignition advance and get huge power out of it. In my case, I'm running 8 degrees more ignition advance than stock, and saw 7hp at the wheels. Ignition advance is the main reason an MP3 has more power than the stock FS. With this resistance to detonation, a short rodded engine can run some pretty large compression too. With forged aluminum pistons(which disipate heat better), there's no reason you can't run 12:1 compression ratio on pump gas, using some nice fat cams to take advantage of it.

As I mentioned, a short rod ratio means very little time is spent at TDC. This means there is plenty of bottom dwell. This means that you can open the exhaust valve sooner to help evacuate the cylinders, since most of the work will be done in the first 90 degrees of crank rotation. You can also leave the intake valve open longer during the compression stroke, since the piston spends more time at the bottom of the cylinder. This means less reversion, and more time for the cylinder to fill at high RPM.

Since our engines are short rod engines, I won't discuss issues with long rods, but you're welcome to further research it.

http://www.stahlheaders.com/Lit_Rod%20Length.htm

As with other internals, things are a bit different than how you'd prep the bottom end for boost. With an NA build-up, it's about keeping things as light as possible. The block needs no modifications, other than a possible overbore, where 1 or 2mm is the max you would want to go with either engine. The crank should be lightened and balanced. One way to lighten the crank is to 'knife-edge' it. The leading edge of the crank is cut into a knife-edge to lose weight, and cut windage losses, and allow it to 'cut' through oil and oil vapour in the crankcase. The crank is then balanced, to allow it to spin upwards of 10,000RPM in a stable and balanced fashion. At high RPM, harmonics are amplified and anything out of balance can pull and engine apart.

Strong rods are important for high RPM speeds, since they take the brunt of the forces. But they also have to be light. The best way to go is to get aftermarket rods designed for an NA application. H-Beam rods are too heavy and would be overkill. In the case of my BP, the rods are already forged steel. They're not super light, but they're strong and beefy, and there's plenty of material that can be removed to lightened them. If you're going to massage stock rods, they need to be lightened, deburred, polished and shot-peened. Shot-peening removes surface cracks, and takes care of potential problems before they occur.

All pistons should be forged aluminum for high-performance applications, as they disipate heat very efficiently. With cast pistons, hot spots can occur, which lead to detonation. Also being much lighter, forged aluminum pistons will decrease accelerative forces. In the case of the FS and BP need the recipricating parts as light as possible.

So the basics of an all-motor bottom end is being lightweight, and rev friendly. Since you can only really increase torque by increasing displacement or adding boost, you spin it faster. Since horsepower is really just how fast your engine can make torque. If you can't make more of it, make it more often. A serious NA build will have you seeing 8-9000RPM, and your bottom end should be up to the task, by minimizing forces creating by heavy recipricating and rotating parts.

I know it's been a while since NA Theory 201, so this is long overdue. This lesson is going to concern camshafts. First, I'll just recap the Otto Cycle engine ie 4 stroke. Suck, Squish, Bang, Blow. Intake stroke(drawing in fresh air/fuel), Compression(compressing the air/fuel mixture), Expansion/Power(igniting the mixture and forcing the piston downward), Exhaust(pushing the spent gases out of the cylinder). Here's a diagram showing the cycle. TDC(Top Dead Centre is when the piston is at it's upper most position), BDC(Bottom Dead Centre is when the piston is at it's lowest point of travel)



It's the job of the valves to allow air/fuel to enter the cylinder, then seal it for compression and power, then allow the gases to exit the cylinder. If the PCM/ECU is the brains, the camshafts are the personality of the engine. They control when the valves open, and how far they open and for how long. The camshafts have lobes on top of a base circle, which while turning push down on lifters(hydraulic in the case of the BP and solid in case of the FS), which in turn push down on the valves. Ideally, an instantly opening and instantly closing valve would be the way to go, but because of how harsh this would be on the valvetrain, it's pretty much impossible. So camshafts have ramps which control the valves opening and closing speed. A higher ramp rate means the valve will open quickly. This necessitates stiffer valve springs, which hold the valve to the seat and prevent the valve from launching(valve float) during valve opening. Too stiff a valve spring and a fast closing ramp rate could even cause valves to bounce of the valve seat, causing damage. It's all a delicate balance. When you rev an engine to higher RPM, because the speed of all valvetrain is higher, you have the potential for valve float and bounce too. Which is why when you get more aggressive cams, you need to upgrade to stiffer valvesprings.

How far the camshaft lifts the valve off the seat is known as lift, and is expressed in millimetres in our case(thousanths of an inch with domestics and old iron). Generally, the higher lift the more air can enter the combustion chamber, although there's normally a point in which more lift does not flow more air. The problem with higher lift is the ability to get the valve to that high in a reasonable amount of time that does not want to launch the valve off the lifter. The higher the lift, the less time you have to get it up to any given valve lift. The other problem with high lift is the possibility of valve to piston contact. The BP is what's call a non-interference engine. Which means even at full valve lift and the piston at TDC, there will be no contact. On the other hand, most Honda engines will interfere at the least amount of valve float or if the timing belt breaks.

The amount of time or the duration the valves or open(or more correctly, the amount of time the cam is acting on the lifter) is expressed in crankshaft degrees. ie 205° duration. Since every cycle takes two full revolutions of the crankshaft, it's even possible to have more than 360° of cam duration(in EXTREME race engines). Also remember that the camshafts spin half the speed of the crankshaft. Now, any duration figure is useless unless it's expressed at a certain lift. ie 205°@.050" lift. Which means the valve will be open for 205 degrees of crank rotation at 50 thousanths of an inch lift, or about 1.25mm. Sometimes a camshaft is measured in total duration, or advertised duration. The is the total time the camshaft is acting on the lifter. ie 230° cams. This can be misleading however, because no air flows at 0 lift and it doesn't tell us how quickly the valve opens. Air doesn't really flow at a measurable level til .050" lift, so that's the standard to compare one camshaft to another. How long the valve is open obviously dictates how long air will be entering the combustion chamber. This is one case where long is good, longer is better, but longest isn't necessarily the best. Before I continue, check out these two links showing B-series cam timing, and Edwin Man's page of Proteges, which shows all Mazda 4 cyl cam specs(click on Engine info).

http://members.aol.com/solomiata/cams.html

http://web2.airmail.net/theman/protegefaq/




So you've figured out that all 4 strokes take 720° of crank rotation to complete one cycle. So each cycle takes 180°, but FS has 230° of cam rotation during the intake stroke. What gives? Well, because air is elastic, it needs time to do it's thing. The intake valve actually opens during the exhaust stroke and closes during the compression stroke. It uses the velocity(and negative pressure) of outgoing spent gases to create a vacuum in the cylinder, pulling in fresh air/fuel during the end of the exhaust stroke. And because air is elastic and has momentum, it was to continue filling up the cylinder even when the piston starts to travel up the cylinder during the compression stroke.



Similar things happen during the exhaust stroke. Most work is done during the first 90° of crank rotation, so the exhaust valve opens early, using what's left of the explosion to speed the spent gases out the cylinder. This is also what you hear coming out your exhaust pipe.



The exhaust valve continues to stay open during the first parts of the intake stroke, like I said, pulling a fresh intake charge into the cylinder. This is known as the overlap phase. This diagram shows all the valve events as well as crankshaft rotation and piston direction.



Now, the faster the engine turns, the less time all these valve events have to take place. This is where you hear of the trade offs of cams and cam timing. Remember, at 6000RPM it takes 1/25th of a second for the piston to do one stroke. At lower RPM, the air has plenty of time to enter the cylinder. So if there's too much duration as the intake stroke turns into the compression stroke, the air/fuel charge will escape back out into the intake manifold. And if the intake valve opens too soon into the exhaust stroke, the exhaust gases will linger in the combustion chamber and contaminate the intake charge. On the exhaust side, if the valve opens too soon into the power stroke, you loose valuable expansion forces straight out the exhaust. Or if it's open too long on the intake stroke, fresh air/fuel charge goes straight out the exhaust port.

But as engine speeds climb, these issues are no longer a problem. This is where people build for performance. Since everything has less time to take place, you need more cam timing to allow it all to happen. During the intake stroke at high RPM, the intake charge travels at a high velocity and wants to continue filling the cylinder well into compression stroke. Also, during overlap there is less time for the spent gases to pull in a fresh intake charge. During the power stroke and since most of the work done in the first 90°, opening it sooner and having the gases blow themselves out the exhaust allowing the engine to work less hard during the exhaust stroke expelling gases. Leaving the exhaust valve open longer during overlap has the same general effect as opening the intake valve sooner. In general, more overlap favours high RPM operation, as you need more time with scavaging. At low RPM, you can get away with no overlap at all.

A lot of people get cam gears and talk about advancing or retarding cam timing. For the most part, retarding both the intake and exhaust cam will raise the p