• Automotive Performance Diy's

    Automotive Performance Diy's

    Performance Diy is an automotive blog covering diy jobs from simple repairs,modifications to full restomod/restorations.If its on wheels its on Performance Diy

    Wednesday, 21 December 2011

    NA(all motor) theory










    Hello,
    I have been working and collecting data for the NA engine tuning guide and this is the data came up with feel free to point out any problems/issues with this analysis and add relevant information to it


    we all put boltons on our beloved naturally aspirated engines and when it doesn't perform upto the mark we take the turbo route or go for the more displacement or the racing engine option (B16a, B18c etc in hondas )

    but we don't fully realize the concept and actual means of tuning NA engines

    and what the rewards of a good naturally aspirated engine are.

    firstly onto the basics as most people are familiar with the I/H/E option but why? since it requires energy to suck air into an engine and push exhaust gases out of the engine , reducing these losses known as pumping losses you can free up some power.
    a good intake setup will
    1. let the engine breathe easily on high rpms.
    2. let in cold air which is denser, and denser air has more oxygen molecules per weight, therefore more potential to react with fuel to create power.

    a good header setup will help extact the exhaust gases faster without obsturctions and will increase the exhaust velocity aiding in the scavenging effect to literally pull out the exhaust gases by the partial vacuum that is created in the cylinder by this high velocity.

    and a good ignition setup will help to completely and effectively burn all of the fuel to maximaze the power.

    but these are the basics to get into serious power you need to look at the serious theoretical data on the Naturally Aspirated engine building

    firstly we know that weight is bad. especially rotating mass such as the crank, flywheel, piston and rod mass and valvetrain mass
    Why because all this mass takes up energy (vital energy to speed up and slow down) sacrificing horsepower and the blistering response of a good NA engine. secondly balancing of engine parts ,(now I could go into a whole debate on what balancing is but just take this as an example that if a steel weight is attached to a matchstick and a lesser weight of a cotton ball attached to an equal length match stick and both are spun in circular motion which on will be having more stresses on the weak points of the matchstick , the one with the steel weight

    and which on could be spun faster , the cotton weight right

    now imagine if you have 3 sticks with cotton and one stick with steel working together in a system , this system will not be balanced while spinning known as an unbalanced system

    on the internals of the engines simmilar stresses are created if a system of rods and pistons are not balanced (because these things are cast in the factory and no 2 parts are the same and the company cannot waste precious mony and labour check these minute details) (it is good for a daily driven engine but on a racing engine this thing kills power drastically
    so engine balancing is vital.

    now we come to the important upper part of the engine the cylinder head
    where everything from the path and opeining and closing of the intake and exhaust valves are determined.
    and valves and camshaft first
    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 millimeters in our case(thousandths 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 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°
    so a higher lift and duration camshaft is needed.

    beacuse a camshaft with high lift and duration will let in more air/fuel mixture resulting in a more violent burn resulting in more power being created also the on high rpms these camshafts provide the engine with the right supply of air fuel mixture and esure that the valves close in time to prevent back surge (air/fuel mix being pushed out of the mix by a opened valve as the piston comes to TDC and not sealing the combustion chamber properly for combustion at these high rpms same is the case with the exhaust cycle and is known as overlap, 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 favors high RPM operation, as you need more time with savaging. At low RPM, you can get away with no overlap at all.

    next we need to bump up the compression ratio (CR) of the engine but why?
    it is all down to the molecular level physics with High compression ratio engines and how the achieve more efficiency by high CR's and to understand engine efficiency and how it relates to compression ratio, a look at the correlation between thermal efficiency and the mechanic energy achieved from a given mass of air/fuel mixture.

    A higher compression ratio will promote a more efficient burn of the air/fuel mixture, due to the oxygen and fuel molecules being closer together.

    The air/fuel mixture in the higher CR engine is being forced into a smaller space, causing the oxygen and fuel molecules to be closer together. This promotes better mixing of the air/fuel and as the air increases with heat due to the compression, it will also result in improved evaporation of the fuel.

    Fuel molecules that are more concentrated, will result in increased expansion of gasses, and will produce more energy when ignited.
    its just that simple

    so a collection of these things can be classed as the NA Theory and how NA engines extract power.

    Discuss away!

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