Now someone who is more knowledgeable may be able to help more here but this is how I made it work.
OK Guys as promised... How I picked my Turbocharger/s
At first, when I was being sensible I was looking at turbocharging my stock engine. After much deliberation I choose a pair of GT28/76's, basically as large as you can go in a Garrett GT28 size frame. The compressors flow 440hp worth of air each and being realtively small ball bearing units would see full boost below 3000rpm, making 5 or 6 pounds at about 2300rpm. In other words a massive lower/midrange but only very slightly tapering off towards 6250rpm when they start to lose there eifficiency.
I sized these turbos up on the large cubic inch motor, and talk about come on boost hard. Anyway, these turbos where pretty much aperfect match for a 346-360ci engine but where starting to struggle on the larger cubic motor. If you can keep your turbo's efficent and your charge temp down, you can run more timing less fuel and make more power. A gentleman I know is using these on his forged 360ci. Stock heads, good sized turbo cam, made 522rwkw. Great little turbo's. Though he was running exotic fuel, so on pump roughly a 450rwkw pair of turbos.
So the hunt was on for another pair of suitable turbos.
No for those of you who want to cheat, and size your turbos the easy way I suggest this website. Though you will need to know your VE of your engine and a few other bits a pieces.
http://www.turbofast.com.au/javacalc.html
http://www.buicks.net/shop/reference/carb_cfm.htm
I would just like to state this is not my work... I found it on the internet but served me pretty well until I bought a decent book on the subject. The text in bold is mine and most likely has the errors.
The purpose of this little paper is to show the reader how to calculate the volume and mass of air moving through his engine, and how to size a turbochargers' compressor to move that quantity of air. It should also offer some enlightenment of the effects of temperature, pressure, and intercooling on the engine's performance.
Engine Volumetric Flow Equation
This equation is for finding the volume of air going into the engine. The displacement on our cars is 231 cu.in. We have a four stroke engine; the intake valve on a cylinder opens once every 2 revolutions of the engine. So, for every 2 revs the engine takes in 231 cu.in. of air. How many pounds of air is that? That depends on the pressure and temperature of the air in the intake manifold. But the volume is always 231 cu.in. every 2 rpm.
volume of air (cu ft/min)= engine rpm x engine cid / (1728 x 2)
So therefore (cu ft/min)= 6500rpm x 408ci / 3456 = 767CFM @ 100% VE
Now at this point its easy to go one step further and times this equation by your VE and you will have your calculated CFM, you can either do this with an educated guess IE 80% or work it out via this next method
- worth noting for you guys with maf's still in place i believe you can work out your VE by simply calculating your CFM @ 100% efficency then comparing actualy CFM as measured by the MAF then work it out like so
actual CFM Measured / Calculated CFM x 100 - This will give you a VE %
So therefore at my guesstimate of 85% VE It works out as such
(cu ft/min)= 6500rpm x 408ci / 3456 x .85 = 652CFM
Ideal Gas Law/Mass Air Flow
The Ideal Gas Law is a handy equation to have. It relates the air pressure, temperature, volume, and mass (ie, pounds) of air. If you know any three of these, you can calculate the fourth. The equation is written:
PV=nRT
where P is the absolute pressure (not the gauge pressure), V is the volume, n is related to the number of air molecules, which is an indication of the mass (or pounds) of air, R is a constant number, and T is the absolute temperature.
What are absolute temperature and pressure? Do we care? Of course we do!
Absolute pressure is the gauge pressure (measured by a gauge that reads 0 when it is open to the outside air) plus atmospheric pressure. Atmospheric pressure is about 14.7 psi at sea level.
Example: a boost gauge reads 0 psi before it is hooked up. Hook it up, boost the car, and it reads 17 psi. 17 psi is the gauge pressure, the absolute pressure at sea level is 14.7 + 19 = 33.7.
A pressure reading is marked psia or psig. The "a" stands for absolute, the "g" for gauge. (The psi stands for Pounds per Square Inch). As we just showed, 17 psig = 33.7 psia. A perfect vacuum is 0 psia, or -14.7 psig.
The absolute temperature is the temperature in degrees F plus 460. This gives degrees Rankine, or deg R. If it is 80 deg F outside, the absolute temperature is 80 + 460 = 540 deg R.
The Ideal Gas Law can be rearranged to calculate any of the variables. For example, if you know the pressure, temperature, and volume of air you can calculate the pounds of air:
n=PV/(RT)
That is useful, since we know the pressure (boost pressure), the volume (which we calculate as shown in the first section "Engine Volumetric Flow"), and we can make a good guess on the temperature. So we can figure out how many pounds of air the engine is moving. And the more pounds of air you move, the more power you will make.
Here is the Ideal Gas Law rearranged to the two handiest forms, with the required constants:
To get pounds of air:
n(lbs/min)= P(psia) x V(cu.ft./min) x 29 all divided by / (10.73 x T(deg R))
To get the volume of air:
V(cu.ft./min) = n(lbs/min) x 10.73 x T(deg R) all divided by / (29 x P(psia))
Celsius to Rankine Conversion http://www.metric-conversions.org/te...to-rankine.htm
So for lbs a minute in my case hoping to run up to 20psi n = (14.7+20) x 652 x 29 all divided by / (10.73 x 549) = 111.4lbs/min
Volumetric Efficiency
If life was perfect, we could fill the cylinders completely with air. If we had 17 psi boost in the intake manifold, we would open the intake valve and get 17 psi in the cylinder before the intake valve closed. Unfortunately, this doesn't usually happen. With some exhaust remaining in the cylinder and the restriction offered by the intake ports and valves the actual amount of air that flows into the cylinder is somewhat less than ideal. The amount that does flow divided by the ideal amount is called the volumetric efficiency.
For your basic stock small block chevy, I think this number is around 0.85 (or 85%). Things like big valves, big cams, ported heads, tunnel rams, etc... get this number closer to 1.0 (or 100%). With tunnel rams some normally aspirated cars can get over 100% at certain rpms due to the ram effect.
To take this into account when we calculate flow into the engine, we multiply the ideal amount of air by the efficiency to get the actual amount of air:
actual air flow = ideal air flow x volumetric efficiency
Example
Time for an example. Lets calculate the pounds of air flowing into an engine for two different cars, an intercooled '87 and a nonintercooled '85. For both cars we will use a volumetric efficiency of 0.85. For both cars the engine is turning at 5000 rpm. What is the volume of air it is using?
volume, in cu.ft per minute = 5000 x 231 all divided by / 1728 x 2 = 334.2 cfm
This holds true for both cars, both intercooled and nonintercooled will be moving 334.2 cfm of air into the cylinders at 5000 rpm. As we will see however, the mass of air flowing is not the same.
Suppose the car an '85, so it isn't intercooled. The temperature in the intake manifold is about 250 deg F. The car is running 19 psi boost. What is the mass of air the engine is using?
Absolute temperature = 250 deg F + 460 = 710 deg R
Absolute pressure = 19 psig + 14.7 = 33.7 psia
n (lbs/min)= 33.7 psia x 334.2 cfm x 29 all divided by / 10.73 x 710 deg R = 42.9 lbs of air per minute (ideal)
lbs air per minute actual = lbs/min ideal x vol. eff.
= 42.9 x 0.85
= 36.4 lbs air/minute
What if the car is an '87, it IS intercooled, so the temperature in the intake manifold is only 130 deg F. This car is running 17 psi boost.
Absolute temperature = 130 deg F + 460 = 590 deg R
Absolute pressure = 17 psig + 14.7 = 31.7 psia
n(lbs/min)= 31.7 psia x 334.2 cfm x 29 all divided by / 10.73 x 590 deg R = 48.5 lbs of air per minute (ideal)
lbs air per minute actual = 48.5 x 0.85 = 41.3 lbs air/minute
Notice that the '87 car is getting MORE lbs/min of air (41.3 for the '87 to 36.4 for the '85) even though the boost pressure is lower. This is because the intake manifold temperature is so much lower. And more pounds of air means more power!
Compressor
The compressor is the part of the turbocharger that compresses air and pumps it into the intake manifold. Air molecules get sucked into the rapidly spinning compressor blades and get flung out to the outside edge. When this happens, the air molecules get stacked up and forced together. This increases their pressure.
It takes power to do this. This power comes from the exhaust side of the turbo, called the Turbine. Not all of the power that comes from the turbine goes into building pressure. Some of the power is used up in heating up the air. This is because we lowly humans cannot build a perfect machine. If we could, all of the power would go into building pressure. Instead, because of the design of the compressor, the air molecules get "beat up", and this results in heat. Just like rubbing your hands together will warm your hands due to the friction between your hands, the friction between the compressor and the air and between the air molecules themselves will heat up the air.
If you divide the amount of power that goes into building pressure by the total power put into the compressor, you get the efficiency of the compressor.
For example, if the compressor is 70% efficient, this means that 70% of the power put into the compressor is used in building air pressure. The other 30% of the power is used heating up the air. That is why we like high efficiency compressors; more of the power is being used on building pressure and less is used heating up the air. Turbos, Paxtons, and Vortechs are all centrifugal superchargers. The are called this because the centrifugal force of flinging the air molecules from the center of the housing to the outside edge is what builds air pressure. The maximum efficiency of these kinds of superchargers is usually between 70% and 80%. Roots blowers, like the 6-71, work differently and have much lower efficiency, like about 40%! With those, when you try to build lots of boost you have to put in a lot of power and more than half of it gets used heating up the air instead of raising pressure.
If the temperature goes up a lot when you increase the boost you can end up with fewer pounds of air going into the engine, so you lose power. That's why a Roots blower is bad if you want lots of boost. Screw compressors, like the WhippleCharger for the 5.0, have good compression efficiency. That's why the Top Fuel guys are starting to try them out, and getting good gains from them.
So? How Hot is the Air Coming out of the Compressor?
Well, I'm glad you asked. The equation used to calculate the discharge temperature is:
Tout = Tin + Tin x [-1+(Pout/Pin)0.263]
efficiency
Example: the inlet temperature is 70 deg F, the suction pressure is -0.5 psig (a slight vacuum), the discharge pressure is 19 psig, and the efficiency is 72%. What is the discharge temperature?
Tin= 70 deg F + 460 = 530 deg R
Pin= -0.5 psig + 14.7 = 14.2 psia
Pout= 19 psig + 14.7 = 33.7 psia
Pout/Pin = 33.7/14.2 = 2.373 (this is the compression ratio)
Tout = 530 + 530 x (-1+2.3730.263 ) all divided by / 0.72 = 717.8 deg R - 460 = 257.8 deg F