Turbo Selection + Calculations: a lesson on how to pick your turbo size.

[EEngineer]

Now that the only thing standing between me and my turbo build is $$$, I started doing quite a bit of research on turbos and what not.

Garratt's website has this great tutorial about some critical information about turbos and I took my time and went over the info. I'm going to use the info on the website to choose a turbo that's right for my 3VZ-FE engine and my power goal.

If your setup is different, all you need to do is to plug in your own numbers and work out something different.

this is my first shot at this and i stand to be corrected if my understanding is skewed.

I copied and pasted the content below and highlight my own calculations

GARRETT CONTENT STARTS HERE:

Things you need to estimate:

1· Engine Volumetric Efficiency. Typical numbers for peak Volumetric Efficiency (VE) range in the 95%-99% for modern 4-valve heads, to 88% - 95% for 2-valve designs. If you have a torque curve for your engine, you can use this to estimate VE at various engine speeds. On a well-tuned engine, the VE will peak at the torque peak, and this number can be used to scale the VE at other engine speeds. A 4-valve engine will typically have higher VE over more of its rev range than a two-valve engine.

from my understanding, Volumetric Efficiency (VE) is the ratio between current torque over peak torque at a certain RPM.
This is my recent dynograph.



judging from my dynochart, my 3vz has peak torque of 169ftlb at 4800 RPM
What we are calculating here for is the VE at peak hp, which happens at 5800RPM.
At 5800RPM, the torque dips to approximately 152ftlb
so my VE at peak power is 152/169 = 0.899 = approx 90%


2· Intake Manifold Temperature. Compressors with higher efficiency give lower manifold temperatures. Manifold temperatures of intercooled setups are typically 100 - 130 degrees F, while non-intercooled values can reach from 175-300 degrees F.

3 Brake Specific Fuel Consumption (BSFC). BSFC describes the fuel flow rate required to generate each horsepower. General values of BSFC for turbocharged gasoline engines range from 0.50 to 0.60 and higher. The units of BSFC are
Lower BSFC means that the engine requires less fuel to generate a given horsepower. Race fuels and aggressive tuning are required to reach the low end of the BSFC range described above.

For the equations below, we will divide BSFC by 60 to convert from hours to minutes.

numer 2 and 3 are pretty straight forward! so let's look at the calculations

To plot the compressor operating point, first calculate airflow:



Where:
· Wa = Airflowactual (lb/min)
· HP = Horsepower Target (flywheel)
· A/F = Air/Fuel Ratio
· BSFC/60 = Brake Specific Fuel Consumption ( ) ÷ 60 (to convert from hours to minutes)

BSFC literally means how many lbs of fuel you need for each HP you generate consistently for an hour

EXAMPLE:

I have an engine that I would like to use to make 400 hp, I want to choose an air/fuel ratio of 12 and use a BSFC of 0.55. Plugging these numbers into the formula from above:


of air.

Thus, a compressor map that has the capability of at least 44 pounds per minute of airflow capacity is a good starting point.

Note that nowhere in this calculation did we enter any engine displacement or RPM numbers. This means that for any engine, in order to make 400 Hp, it needs to flow about 44 lb/min (this assumes that BSFC remains constant across all engine types).

Naturally, a smaller displacement engine will require more boost or higher engine speed to meet this target than a larger engine will. So how much boost pressure would be required?


for my power goal, I was 350hp to the wheels and taking the drivetrain loss of 15%, i'm shooting for 411bhp. the above calculation is for 400hp, so that's close enough. so no calculations done here. At this point i think everyone understand now that power = flow.



◊ Calculate required manifold pressure required to meet the horsepower, or flow target:



Where:

· MAPreq = Manifold Absolute Pressure (psia) required to meet the horsepower target
· Wa = Airflowactual(lb/min)
· R = Gas Constant = 639.6
· Tm = Intake Manifold Temperature (degrees F)
· VE = Volumetric Efficiency
· N = Engine speed (RPM)
· Vd = engine displacement (Cubic Inches, convert from liters to CI by multiplying by 61.02, ex. 2.0 liters * 61.02 = 122 CI)

this formula calculates how much boost you need to get that much power.


EXAMPLE:

To continue the example above, let’s consider a 2.0 liter engine with the following description:

· Wa = 44 lb/min as previously calculated
· Tm = 130 degrees F
· VE = 92% at peak power
· N = 7200 RPM
· Vd = 2.0 liters * 61.02 = 122 CI

= 41.1 psia (remember, this is absolute pressure. Subtract atmospheric pressure to get gauge pressure (aka boost):

41.1 psia – 14.7 psia (at sea level) = 26.4 psig boost

As a comparison let’s repeat the calculation for a larger displacement 5.0L (4942 cc/302 CI) engine.

Where:

· Wa = 44 lb/min as previously calculated
· Tm = 130 degrees F
· VE = 85% at peak power (it is a pushrod V-8)
· N = 6000 RPM
· Vd = 4.942*61.02= 302 CI

= 21.6 psia (or 6.9 psig boost)

This example illustrates in order to reach the horsepower target of 400 hp, a larger engine requires lower manifold pressure but still needs 44lb/min of airflow. This can have a very significant effect on choosing the correct compressor.

This is where everything gets interestiong. I'm going to use the formula to find how much boost i need on my 3VZ engine and below are the variables i'm going to use.

Wa = 44 lb/min as previously calculated
R = Gas Constant = 639.6
Tm = 130 degrees F
VE = 90% at peak power
N = 5800 RPM
Vd = 3.0 liters * 61.02 = 183 CI

plugging into the formula:



i get

[44*639.6*(460+130)] / [0.90*(5800/2)*183]
= 16604016/477630
= 34.7 psia

taking away atmospheric pressure of 14.7
i get an exact 20psi.

this is kind of disappointing really...considering how much of boost i have to run to get to 400hp level...i'm most likely not going to be able to run this kind of boost on my stock block. However, this is simply a calculation...so what the heck, let's continue.


With Mass Flow and Manifold Pressure, we are nearly ready to plot the data on the compressor map. The next step is to determine how much pressure loss exists between the compressor and the manifold. The best way to do this is to measure the pressure drop with a data acquisition system, but many times that is not practical.

Depending upon flow rate, charge air cooler characteristics, piping size, number/quality of the bends, throttle body restriction, etc., the plumbing pressure drop can be estimated. This can be 1 psi or less for a very well designed system. On certain restrictive OEM setups, especially those that have now higher-than-stock airflow levels, the pressure drop can be 4 psi or greater.

For our examples we will assume that there is a 2 psi loss. So to determine the Compressor Discharge Pressure (P2c), 2 psi will be added to the manifold pressure calculated above.



Where:

· P2c = Compressor Discharge Pressure (psia)
· MAP = Manifold Absolute Pressure (psia)
· ΔPloss = Pressure Loss Between the Compressor and the Manifold (psi)

For the 2.0 L engine:


= 43.1 psia

For the 5.0 L engine:


= 23.6 psia

for my 3.0L 3VZ engine
P2c= 34.7 +2 = 36.7psia

Remember our discussion on inlet depression in the Pressure Ratio discussion earlier, we said that a typical value might be 1 psi, so that is what will be used in this calculation. For this example, assume that we are at sea level, so ambient pressure is 14.7 psia.

We will need to subtract the 1 psi pressure loss from the ambient pressure to determine the Compressor Inlet Pressure (P1).


Where:

· P1c = Compressor Inlet Pressure (psia)
· Pamb = Ambient Air pressure (psia)
· ΔPloss = Pressure Loss due to Air Filter/Piping (psi)

P1c = 14.7 - 1

= 13.7 psia

With this, we can calculate Pressure Ratio () using the equation.


For the 2.0 L engine:


= 3.14

For the 5.0 L engine:


= 1.72

for my engine
=36.7/13.7 = 2.67


so now that i have my pressure ratio and my airflow i can put my motor on some turbo maps to check out their efficiency and find the right one for me.

the green dot is where my 3VZ should be at 2.67
this is on a GT3076R ...not bad but not the greatest because it's towards the right on the middle light of efficiency and falling on towards the choke line on the right.

this means that the turbo is a bit small because it's not going to flow as well.



A GT3582R would be even nicer! The turbo is further from choke lines but I think it might be too big. I'm still not that great at reading the turbo compressor maps, so someone fill me in on my readings.




at the end of the day, using this information we can put our power goal and engine setup on a compressor map of a turbo...so we'd at least some some idea whether the turbo is going to work out for the car or not...and even predict the behavior of the turbo on the vehicle.

i'll be adding to this thread whenever someone can fill me in on info i missed or wrote wrong.

happy modding

[Eye8Pussies]

looks like some solid info here- haven't had time to sit down and actually follow the calculations though- but I'm sure I'll get around to it some time lol

everybody who is thinking about turbos and turbo setups should definitely read this

[jtex2717]

This is the beginning of a serious thread.....im excited to see where this will go.....oh and Im picking up a 3vz next weekend......but its auto (FAIL).....So I will be looking for a manny tranny......(NO HOMO) lol

[chronoti]

you left out one part,

how does all this apply with my m62????



meh guess i'll just have to copy tony

[EEngineer]

after a lengthy conversation with tony, I've gotten some more knowledge out of him..

what i posted here is a very basic rule of thumb way of getting the right turbo.

there are a lot of variables here that WILL change as soon as the turbo is boosted, so actual experience is very important as well once it comes to turbo selection.

for example, as soon as the car is boosted, the Volumetric Efficiency can be changed and played around with by tuning the car differently.

variables like what type of turbo it is, single ball bearing, dual ball bearing, or journal bearing, ...what kind of turbine it is....these variables all relate to how well a turbo can flow.

So, from the first post, I calculated that I needed 20 psi of boost to reach 400hp at the crank, however this number is relative. There are ways to cheat around it to achieve that number with less boost...by playing around with turbine housing size (A/R ratio), turbine size, etc...

Also, you would have to cater the turbo build to your taste and how your car responds to power. For example, a Dodge SRT5 is boosted in such a way that it has 350hp and 400+ lbs of torque...it can shred tires, but it won't grip and won't be faster than a car with the same amount of boost...such as civic...which would probably develop 450hp and 250 torque.

this is where the driver has to decide where the power band is going to be. having a smaller turbo will achieve a lot of torque at low end. Torque is fun, but placing the powerband in a lower RPM range, you'd simply burn tires and the HP would dip as soon as you reach into the high rpm region....such as a SRT5....
having a larger turbo you lag a little, however for the camry V6, NA torque is more than enough to accelerate the car...so having a larger turbo will achieve great top end power.

overall, the best setup would be a controlled progressive boost. Having a fairly large turbo that spools quickly..such as a GT35R, and have it set to lower boost between 3K~5k and higher boost 5K+. this is ideal because you can utilize the torque safely, and by the top end, the power is great! This is the safest bet...only downside is that a EBC that can acheive this will run $500ish, but for a street turbo setup, this is a MUST.

Lastly, there's no definitive way of tuning a car as different tuners have different understandings of each car....basically the tuner has to know the car really well in order to tune it very well...that's why there are shops that exclusively specialize in a certain type of cars......but pretty much all of them can do hondas lol..

NOTE:
A lot of this info is a regurgitation of what I learned from people like Tony the Tiger and other car whizzes...however, it's good to write it down so i could come back and read this whenever I need reference info.

[Creeps228]

good stuff man, i didnt know this much info was needed fro a turbo setup. most of it was relative to me but calculating boost, was some i never thought you could come up with one out of calculations.