[David Holmberg]

How do you go about selecting the proper size of turbo, based on motor size and HP desired?

Too large = slow spooling
Too small = too much heat

And when is two better than one?

Just to get things started.

[Tom Vaught]

How do you go about selecting the proper size of turbo, based on motor size and HP desired?

Too large = slow spooling
Too small = too much heat

And when is two better than one?

Just to get things started.


I will try and answer Question #1:

How do you go about selecting the proper size of turbo, based on motor size and HP desired?

This website gives you a starting point:

http://www.rbracing-rsr.com/turbotech.html

They have a “Turbo Calculator Horsepower and Airflow” and a Pressure Unit Conversion Calculator

The calculations that RBRacing uses are based on the following:

a) That one pound of air flowing through the engine in one minute is the same as 12.392 cubic feet of air flowing through the same engine.

b) That one pound of air or 12.392 cubic feet of air will make 8.261 Horsepower

c) That one pound of air per minute is the same as moving .0058 cubic meters of air per second

d) That one pound of air per minute is the same as moving .0076 kilograms of air per second

The Garrett Turbo people and the Borg Warner People use different numbers

The Garrett and Borg Warner numbers are very close and are based on:

a) That one pound of air flowing through the engine in one minute is the same as 14.4 – 14.5 cubic feet of air flowing through the same engine.

b) That one pound of air or 14.45 cubic feet of air will make 9.633 HP.

c) That one pound of air per minute is the same as moving .0068 cubic meters of air per second

d) That one pound of air per minute is the same as moving .0075 kilograms of air per second

The differences are in the Temperature and Pressure assumed in the calculations.


You need to understand that deal before you mistakenly use an Internet calculator to make a calculation of horsepower from a Boosted Engine system.

The confusion also comes from the fact that different people use different numbers for a Cubic Foot of Air and what the weight of that Cubic Foot of Air is. A Pound of Air will always be a Pound of Air.

A Kilogram of Air will always be a Kilogram of Air. A Cubic Meter of Air can change as can a Cubic Ft of Air.

I bring this up as Hugh MacInnis, (the guy who wrote the “Turbocharger” book) was a big fan of using CFM in his calculations. Most Turbo Companies today use Lbs/minute or Kg/second on their Turbo maps.

SO THE FIRST DEAL IS DECIDING WHICH HORSEPOWER CALCULATOR YOU WANT TO USE.

Johnta1 posted some links in the Turbo Links section of the forum. Unfortunately some of the links do not work properly. Here are the Turbo Links (to the Garrett web site) for Turbo1, Turbo2, and Turbo3.

http://www.turbobygarrett.com/turbob...o_tech101.html

http://www.turbobygarrett.com/turbob...o_tech102.html

http://www.turbobygarrett.com/turbob...o_tech103.html

You use the Turbocharger COMPRESSOR MAPS to get a rough idea of the airflow that a selected Compressor Wheel can provide vs the Compressor Pressure Ratio capability of that unit.

The Turbo Regal website has a good page on Turbo Calculations and where they come from:

http://www.gnttype.org/techarea/turbo/turboflow.html

We are just getting started on the Boosting Forum. This Turbo Knowledge process will not happen overnight or will it be “go to this website for all of the info”. Some people have been playing with the Turbocharger for a long time. I have been dealing with Turbo stuff before 1973. I helped people build engines with turbos in the mid 70s. I worked on the Ford Motor Co. 1979 & 1980 Turbo Mustang programs years before they went into production. Many Boosted Engine programs followed those programs at Ford. The ECOBOOST vehicle program is the current program I am working on. I AM STILL LEARNING!

Read the stuff in this post and we will keep going.

Tom Vaught

[David Holmberg]

Great info Tom.

I think my problem is every time I look at a compressor map, it looks like it's written in Chinese. I get more information from cloud formations...

[Tom Vaught]

David, Go to the this turbo link again:

http://www.turbobygarrett.com/turbob...o_tech103.html

Go down the page until you find the GT3071R map, which uses a 71mm, 56 trim compressor wheel. The map will have Lbs per minute on the bottom part of the map. It will have pressure ratio on the left side of the map.

The Lbs per minute will go from 5 lbs per min to 60 lbs per minute

The pressure ratio will go from 1.0 to 3.5

There will be 3 oval "islands" in the center of the "map". The smallest island will be the 78% efficiency island. The next larger island will be 77% efficiency. The third island will be 76% efficiency. The 0.76 number that you can read means 76% efficiency.

The far left line that has a dog leg to it is the "surge line". IF the compressor is operating to the left of this line it will not be happy and will make noise and eventually break.

The second line on the left is the 68% efficiency line. The lines then go up by 2% each time until we get to the largest island which we said was the 76% efficiency island.

On the right side of the map we start with the 76% efficiency island line and go down by 2% each time until we get to the 66% line. As you move farther to the right on the islands you get closer to the "Choke Line". The compressor is not happy in this area either.

The lines that start out horizontally and then curve down sharply to the bottom right of the map are the compressor speed lines. The speeds on this map are hard to read but are approximately 58,000 rpm, 83,000 rpm, 102,000 rpm, 111,000 rpm, you can't read the next speed line, then you have 127,000 rpm, and finally 140,000 rpm.

The dotted line is the highest efficiency path (assuming no wastegate).

If we draw a line from 3 pressure ratio, (30 psi of boost) to the highest point of the 77% efficiency island and then downward to the lbs per minute line we would se at that intersect point we would be moving 40 lbs of air per minute or about 9.663 times 40 or 385 horsepower for the one turbo. Two turbos would make 770 hp at a 30 psi intake pressure. The compressors would be turning 127,000 rpm.

Not to bad for a little 5.0L engine with two smaller turbos to make 770 hp.

Tom Vaught

[Tom Vaught]

Looking at the Lbs per Minute line on the turbo map link, I provided, you will see that a larger engine will have more mass flow per revolution of the engine. If you have more mass flow, you will be farther away from the "surge line" for any given Turbo map you are looking at. Surge is a major issue on smaller engines at street part throttle conditions (with large turbos). The small engine does not have enough mass flow for the turbo to stay on the right side of the Surge line.

Speaking of the Surge Line. The left most vertical line on the turbo map is called the surge line BUT the surge line area is actually a larger area. Think of the Berlin Wall in Germany where the Russians planted mines a hundred yards before the actual wall. The turbo might actually go into surge the next line to the right on that given turbo as the mapping process is not perfect. It is done on a test stand and test stands are not engines in vehicles. Just like computer simulations are not real drag cars going down the track.

More on turbos later. Please provide questions and I will try and answer them or search for links to the answers on the web that are easy to understand.

Tom Vaught

[johnta1]

How to figure using 2 turbos?

[johnta1]

Unfortunately some of the links do not work properly.

They work fine for me?

[David Holmberg]

Thanks Tom, much more clear now.

[70 bird]

Here is a calculator thats fun to play with.....

http://www.squirrelpf.com/turbocalc/index.php

This is a good place to see what others have already built. The database will give you ideas on everything from routing pipes to track times and dyno numbers.

http://www.theturboforums.com/smf/in...?topic=50928.0

The key to boost is learning through lots of reading!

[Tom Vaught]

Originally Posted by johnta1

How to figure using 2 turbos?

Two turbos of the same size, (not "Sequential", or "Series/ Sequential"
as on some diesel engines) is pretty easy.

Say you want to make 1000 hp.

Divide the the 1000 Horsepower by 9.5 (easy pencil math) or by 9.633 (using a calculator) and you get 105 lbs/min or 103.8 lbs per minute depending on the way you do the math. Some people (without a calculator or a pencil) use 10 hp per lb of air so 100 lbs per minute and then add 10% to the number so you get for a 1000 hp engine a requirement of 110 lbs per minute. Whether it is 103 lbs per minute or 110 lbs per minute is not a big deal, as with a normal twin turbo system, you divide the total air flow (lbs/min number) by 2.

Now EACH turbo has to be able to move (using the 3 different methods): 51.5 lbs per minute, 52.5 lbs per minute, or 55 lbs per minute. I usually use the 55 lbs per minute number, again for easy calculations in my head.

So you go to a series of maps that have a 55 lb/minute point just to the right of the 2nd or 3rd maximum efficiency island and you see what the pressure ratio is. Some maps have broad (left to right) islands which work well with street cars and lower Pressure Ratios (1.5 to 3.0) and other maps (for Bonneville type engines might have maps that have narrow efficiency islands but with high pressure ratios: 3.0 to 4.5).

You need the higher Pressure Ratios at places like Bonneville where you need to run 45 psi to make the power of 30+ psi at sea level due to the much less dense air at density altitudes like Bonneville (8,500 feet).

Not too hard really, John.

Tom Vaught

I apologize if this isnt the right place to reply to this but..

Tom, I want to thank you. I think I finally understand 'surge' now as well as reading the charts. I have MacInnis book but much of it escaped me....

Just to be sure, is surge essentially a pressure wave bouncing back and forth between the throttle plates (on a carb) and the impeller blades because the turbo is pushing more air than the engine is consuming?

[Tom Vaught]

Yes, but you can have surge a couple of different ways:

The turbo being the wrong size so that at low mass flows you are in the surge area. Course a bit more engine rpm/ mass flow and then you move into a "happy area" on the map.

The second way that you can have surge is if say you are at 40 lbs per min of air flow and you suddenly restrict the air flow by closing the throttle. NO MASS FLOW! YOU ARE IN THE SURGE AREA. But if you have a properly sized by-pass valve that dumps the excess pressure as soon as you close the throttle, the mass flow still stays up and the excess mass flow from the supercharger or turbocharger either goes to atmosphere or back to the inlet side of the boosting component (turbo or belt driven Centrifugal Supercharger).

Tom Vaught

Tom,

While looking at some Seadoo components (another area of interest) I noticed
a 'supercharger surge protector'. Being curious went to look further. They too
explaioned surge much the same as you did and then described the valving...

It seems that they use an automotive vacuum assisted blowoff valve that dumps
pressure at rapid throttle closure.

Do automotive systems employ such a device? Ive never seen any references to one.

All of this has cleared up things that have puzzled me for years. Ill bet that surge
problems were the reason for the 'glottal stop' in the airbox (57 Stude Silver Hawk
McCullogh installation). lol

[Tom Vaught]

If you have a properly sized By-pass valve(s) plumbed properly into the Supercharger or Turbocharger system, the supercharger/ Turbocharger will not see the Pressure Spike/ Surge as the valve(s) will dump the airflow before the pressure can build up in the total volume of the system (From S/C or Turbo to Throttle blade).

A good description of a "By-pass valve" vs a "blow-off" valve in in this link: http://en.wikipedia.org/wiki/Blowoff_valve

Vortech used to use Bosch By-pass valves on their lower HP S/C kits.
They later increased the valve size and allowed higher production vehicle HP numbers with efi. The very high HP "speed density" race set-ups (that do not need mass air meters) use race blow-off valves that vent into the engine compartment. Those will not impact on the efi calibration.

The valves used by Robert P. (Paxton) McCulloch in his bonnets/ boxes were "GULP" Valves that allowed airflow INTO the carb Bonnet/ Box on quick tip-in of the carb even though the supercharger was not up to speed. They CLOSED once the boost built up in the system.

Tom Vaught

Tom,

You know Im filing all of this good stuff away for the future....

That more exactly explains what I had always heard called the 'glottal stop'
I had wondered if it somehoe let air out - didnt seem to but.....

I guess a belt drive would slow down and reduce boost quickly enuf that any surge
would be short lived and not cause damage to the impeller blades ???

I recall the McCullogh and Paxton blades were more substantial than what I see in
ebay pics of turbo impellers (also ?????) They look very thin, light and fragile.


Going back to the orig question here isnt it always better to have two smaller
rather than one large to reduce spoolup time?? (much less mass vested in the
impellers as well as less centrifugal force involved in the smaller diameters)