` Pipe Sizing Calculator

INTAKE PIPES AND EXHAUST PIPES SIZING CALCULATOR

In keeping with the general tone of the Daihard calculators, this calculator will ballpark pipe selection to make your Charade the fastest supadupa racer on earth. Don't blame us if things go pear shaped, you should do your own homework to validate our figures. You may find the estimated max power RPMs ambitious, but the calculator doesn't care whether or not there are mechanical constraints. You need to input the RPMs you are comfortable with to size the pipes.

Tech Tip: The best heads tend to have the smallest port sizes for a given volume of air, which simply means sometimes it's best to trade of some pressure drop to keep the air velocity high. If you have ever over ported a head you will know what we mean.

Full Screen Mode

OK before you go plugging in figures and shelling out money, here's the first small tutorial:

The Intake Pipes

You've read it all before, the intake pipe diameter and length is very important because it affects volumetric efficiency, impinges on pumping losses, mean effective pressures, etc, etc. You have also heard of rarefaction waves, reversion pulses, syphoning, blah, blah, blah. Well guess what we actually incorporated a lot of this stuff in the calculator to make out we know what we are doing. This will also hold truw for the exhaust side of things where we claim a proper sized exhaust will suck harder than the piston itself.

Alrighty heres what we have thrown into the soup to come up with the intake pipe sizer:

Long inlet pipes increase volumetric efficieny at low speed, but decrease VE at high speed (VE is volumetric efficiency);

Mean effective pressure increases proportionally with volumetric efficiency;

The amplitude of pressure waves increase as pipe length increases;

Pressure peaks occuring around the inlet closing occur later as the pipe length increases;

Cylinder vacuum comes on later in the stroke as the pipe length increases, until the number (frequency) of pressure waves per engine cycle changes and it reverts back to the original angle and starts coming on later as the pipe continues to be extended.

Later inlet valve closing is better with long pipes and earlier with short pipes. This is because the arrival of the pressure wave is later with long pipes

Which when translated means you install long small bore pipes when you want power and torque at low to medium revs, long large bore pipes for midrange torque and short pipes when you want performance at higher revs (at the expense of losing some of your low end performance).

So what about all this pressure waves stuff you ask? Well that's simple too, because some else spent a load of some government's money when dinosaurs walked the earth and came up with some observations that you can get a whopping 15%-20% increase in mean effective pressure by just making the pipe a length that encourages a third harmonic based on the speed of sound and engine rpm. And better than that the fourth and fifth harmonic suck too!

The Exhaust Pipes

Learn a lot from the previous section? Probably not, but we aren't here to study for you. So onto the exhaust pipes. What do we know about exhausts you ask. Well we know that there lots of guys who swear by their own secret sizing method and guess what, we probably have it as an option here too!.

The object of sizing the exhaust is similar to the inlet, but in this exercise we want the exhaust pipes to scavenge as much waste gas as possiblel and also induce fuel mixture into the cylinder. If we are clever we can create enough vacuum on the exhaust port to pull extra fuel/air mixture in. We know that the piston is going to pull nearly it's own displacement into the cylinder, but with some extra help from the exhaust escaping we should be able to fill the clearance volume as well.

So the same principles apply to the exhaust as the inlet, longer pipes suit low and midrange performance and short pipes are good for high end performance. We know that a vacuum does appear on the exhaust port if the system is designed well, otherwise car makers have wasted a lot of money on PAIR systems and the like.

Once again sonic waves, pressure drop, gas velocity, etc come into play. And before you start getting cute about our pipe sizes, consider that there are dedicated engineers out there who spend most of their time imperically testing the best fit for particular engines with more equipment than the average enthuisiast. We don't have that luxury.

Just a note on intake plenums.

When dealing with air movement there is a latent or potential energy Static Pressure (Sp) and a kinetic energy Velocity Pressure (Vp). Both pressures combined make up Total Pressure Tp. When you put a gauge on you invariably measure Tp unless you are smart enough to install a pitot tube..

So reading off a gauge can be a little deceptive in giving a true indication of what the air is air doing. But lets assume the air is travelling at say 30m/s open throttle into the plenum. Well you know that some of the air will continue as flume until it hits the opposite wall of the plenum, but a large percentage of the air will slow down. The plenum is therefore a cushion box and is designed to proportionaly raise Sp in relation to Vp which is especially desirable for when you have a manifold of pipes all requiring change in direction of common supply flow at different times, like intake pipes do. Of course the plenum will tend to knock out a lot of the pulsations coming back up the intake pipes too.

If you haven't twigged yet, it's the difference in static pressure regions that causes the air to flow and it's friction losses and dynamic losses (flow separation) that reduce the flow.

Now here's something worth pondering. A lot of plenums have a throttle body coming in on one end and the intake pipes lined up from front to back. As the air streams into the plenum the Vp starts to reduce and the Sp starts to rise accordingly, but this will favour a higher static pressure approaching the end of the plenum. To overcome this you really should taper the plenum so that Vp is relatively higher than or equal to that at the TB end, thus making the Sp more uniform throughout. The ends of the plenum should also overhang the first and end intake pipes to provide some cushioning and thereby helping the first and last intake pipes.

What volume should the plenum be you ask? Well a factor of two (2) times the engine displacement is freely bandied about, as is 65%-80% for three cylinders (use two plenums for a six), 50%-60% for fours cylinders and 40%-50% for V8s, but really the best size is going to be determined empirically and I can really see a 6 litre plenum on top of a3.0 litre six using the 2 times rule of thumb LOL..

Some people swear by small capacity plenums to provide smooth engine response, others swear the opposite. The pundits will say the plenum should increase in size as you chase higher rpm, but this will make throttle response worse and low end power will suffer.

Obviously decreased lag should occur with smaller plenums. At first glance a smaller plenum should offer higher plenum/cylinder differentials (P1V1= P2V2), thus better flow and more impedance to blow back during valve overlap, an advantage when using long duration cams. But also consider there is less volume and the suction stroke cylinder may be grabbing air from the other pipes/runners, thus reducing VE overall.

This calculator actaully has some thought behind rather than rules of thumb. Mind you it was only a fleeting thought. Boundary layers ppfft who needs them.

So what have we done? Well you can work that out.