Tu r b o Tech 1 0 3 ( Ex p er t )

This article is a bit more involved and will describe parts of the compressor map, how to estimate pressure ratio

and mass flow rate for your engine, and how to plot the points on the maps to help choose the right

turbocharger. Have your calculator handy!!

1 Parts of the Com pressor Map: ◊ The compressor m ap is a graph that descr ibes a part icular compressor ’s performance character ist ics, including eff iciency, m ass flow range, boost pressure capability , and turbo speed. Shown below is a figure that ident ifies aspects of a t ypical com pressor map:

◊ Pr essu r e Rat io

• Pressure Rat io ( ) is defined as the Absolute out let pressure divided by the Absolute inlet pressure.

Where:

o = Pressure Rat io o P2c = Com pressor o Discharge Pressure o P1c = Com pressor I nlet Pressure

• I t is im portant to use unit s of Abso lu t e Pr essu r e for both P1c and P2c. Rem em ber that Absolute Pressure at sea level is 14.7 psia ( in unit s of psia, the a refers to “absolute” ) . This is referred to as standard atm ospher ic pressure at standard condit ions.

• Gau ge Pr essu r e ( in unit s of psig, the g refers to “gauge” ) m easures the pressure above atm ospher ic, so a gauge pressure reading at atm ospher ic condit ions w ill read zero. Boost gauges m easure the m anifold pressure relat ive to atm ospher ic pressure, and thus are m easur ing Gauge Pressure. This is im portant when determ ining P2c. For exam ple, a reading of 12 psig on a boost gauge m eans that the air pressure in the m anifold is 12 psi above atm ospher ic pressure. For a day at standard atmospher ic condit ions,

12 psig + 14.7 psia = 26.7 psi absolute pressure in the m anifold

• The p r essu r e r a t i o at this condit ion can now be calculated:

26.7 psia / 14.7 psia = 1 .8 2

• However, this assum es there is no adverse im pact of the air filt er assembly at the com pressor inlet .

• I n determ ining pressure rat io, the absolute pressure at the com pressor inlet (P2c) is often LESS than the am bient pressure, especially at high load. Why is this? Any rest r ict ion (caused by the air f ilt er or rest r ict ive duct ing) will result in a “depression,” or pressure loss, upst ream of the com pressor that needs to be accounted for when determ ining pressure rat io. This depression can be 1 psig or m ore on som e intake system s. I n this case P1c on a standard day is:

14.7psia – 1 psig = 13.7 psia at com pressor inlet

• Taking into account the 1 psig intake depression, the p r essu r e r a t i o is now:

(12 psig + 14.7 psia) / 13.7 psia = 1 .9 5 .

• That ’s great , but what if you’re not at sea level? I n this case, sim ply subst itute the actual atm ospher ic pressure in place of the 14.7 psi in the equat ions above to give a m ore accurate calculat ion. At higher elevat ions, this can have a significant effect on pressure rat io.

For exam ple, at Denver’s 5000 feet elevat ion, the atm ospher ic pressure is t ypically around 12.4 psia. I n this case, the p r essu r e r a t i o calculat ion, tak ing into account the intake depression, is:

(12 psig + 12.4 psia) / ( 12.4 psia – 1 psig) = 2 .1 4

Com pared to the 1.82 pressure rat io calculated or iginally, this is a big difference.

• As you can see in the above exam ples, pressure rat io depends on a lot more than just boost .

◊ Mass Flow Rat e

• Mass Flow Rate is the m ass of air f lowing through a com pressor (and engine!) over a given per iod of t im e and is com m only expressed as lb/ m in (pounds per m inute) . Mass flow can be physically m easured, but in m any cases it is sufficient to est im ate the m ass flow for choosing the proper turbo.

• Many people use Volum et r ic Flow Rate (expressed in cubic feet per m inute, CFM or ft3/ m in) instead of m ass flow rate. Volum et r ic f low rate can be converted to m ass flow by m ult iply ing by the air density . Air densit y at sea level is 0.076lb/ ft3

• What is m y m ass flow rate? As a very general rule, turbocharged gasoline engines will generate 9.5-10.5 horsepower (as m easured at the f lywheel) for each lb/ m in of air flow. So, an engine with a target peak horsepower of 400 Hp will require 36-44 lb/ m in of air flow to achieve that target . This is just a rough first approxim at ion to help narrow the turbo select ion opt ions.

◊ Su r ge Lin e

• Surge is the left hand boundary of the com pressor map. Operat ion to the left of this line represents a region of f low instability . This region is character ized by m ild flut ter to w ildly f luctuat ing boost and “barking” from the com pressor. Cont inued operat ion within this region can lead to prem ature turbo failure due to heavy thrust loading.

• Surge is m ost com m only exper ienced when one of two situat ions exist . The f irst and m ost dam aging is surge under load. I t can be an indicat ion that your compressor is too large. Surge is also com m only exper ienced when the throt t le is quick ly closed after boost ing. This occurs because m ass f low is drast ically reduced as the throt t le is closed, but the turbo is st ill spinning and generat ing boost . This im m ediately dr ives the operat ing point to the far left of the com pressor m ap, r ight into surge.

Surge will decay once the turbo speed f inally slows enough to reduce the boost and m ove the operat ing point back into the stable region. This situat ion is com m only addressed by using a Blow-Off Valves (BOV) or bypass valve. A BOV funct ions to vent intake pressure to atm osphere so that the m ass flow ram ps down sm oothly, keeping the com pressor out of surge. I n the case of a recirculat ing bypass valve, the air f low is recirculated back to the com pressor inlet .

• A Ported Shroud com pressor ( see Fig. 2) is a feature that is incorporated into the com pressor housing. I t funct ions to m ove the surge line further to the left (see Fig. 3) by allowing som e airf low to ex it the wheel through the por t to keep surge from occurr ing. This prov ides addit ional useable range and allows a larger com pressor to be used for higher flow requirem ents w ithout r isking running the com pressor into a dangerous surge condit ion. The presence of the ported shroud usually has a m inor negat ive im pact on com pressor eff iciency.

Fig. 2 Fig. 3

◊ Th e Ch ok e Lin e is the r ight hand boundary of the com pressor m ap. For Garret t m aps, the choke line is t ypically def ined by the point where the efficiency drops below 58% . I n addit ion to the rapid drop of com pressor eff iciency past this point , the turbo speed will also be approaching or exceeding the allowable lim it . I f your actual or predicted operat ion is beyond this lim it , a larger com pressor is necessary.

◊ Tu r b o Speed Lin es are lines of constant turbo speed. Turbo speed for points between these lines can be est im ated by interpolat ion. As turbo speed increases, the pressure rat io increases and/ or m ass flow increases. As indicated above in the choke line descr ipt ion, the turbo speed lines are very close together at the far r ight edge of the m ap. Once a com pressor is operat ing past the choke lim it , turbo speed increases very quick ly and a turbo over-speed condit ion is very likely.

◊ Ef f i c ien cy I s lan d s are concentr ic regions on the m aps that represent the com pressor efficiency at any point on the m ap. The sm allest island near the center of the m ap is the highest or peak efficiency island. As the r ings m ove out from there, the eff iciency drops by the indicated am ount unt il the surge and choke lim it s are reached.

2 . Plo t t i n g You r Dat a on t h e Com p r esso r Map I n this sect ion, m ethods to calculate m ass f low rate and boost pressure required to m eet a horsepower target are

presented. This data will t hen be used to choose the appropr iate com pressor and turbocharger. Having a horsepower target in m ind is a vital part of the process. I n addit ion to being necessary for calculat ing m ass f low and boost pressure, a horsepower target is required for choosing the right fuel inj ectors, fuel pum p and regulator , and other engine com ponents.

◊ Est im at i n g Req u i r ed A i r Mass Flow an d Boost Pr essu r es t o r each a Ho r sepow er t a r ge t .

· Things you need to know: · Ho r sep ow er Tar g e t · En g in e d i sp lacem en t · Max im u m RPM · Am b ien t con d i t ion s ( tem perature and barom et r ic pressure. Barom et r ic pressure is usually given as inches of m ercury and can be converted to psi by div iding by 2)

· Things you need to est im ate: · En g in e Vo lu m et r ic Ef f ic ien cy . Typical num bers for peak Volum etr ic Eff iciency (VE) range in the 95% -99% for m odern 4-valve heads, to 88% - 95% for 2- valve designs. I f you have a torque curve for your engine, you can use this to est im ate VE at var ious engine speeds. On a well- tuned engine, the VE will peak at the torque peak, and this num ber can be used to scale the VE at other engine speeds. A 4-valve engine will t ypically have higher VE over m ore of it s rev range than a two-valve engine. · I n t ak e Man i f o ld Tem per at u r e. Com pressors with higher efficiency give lower m anifold tem peratures. Manifold tem peratures of intercooled setups are typically 100 - 130 degrees F, while non- intercooled values can reach from 175-300 degrees F. · Br ak e Speci f i c Fu el Con su m p t ion ( BSFC) . BSFC describes the fuel f low rate required to generate each horsepower. General values of BSFC for turbocharged gasoline engines range from 0.50 to 0.60 and higher. The

unit s of BSFC are Lower BSFC m eans 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 descr ibed above.

For the equat ions below, we will div ide BSFC by 60 to convert from hours to m inutes.

To plot the com pressor operat ing point , f irst calculate airf low:

Where: · Wa = Air flowactual ( lb/ m in) · HP = Horsepower Target ( flywheel)

· = Air / Fuel Rat io

· = Brake Specif ic Fuel Consum pt ion ( ) ÷ 60 ( to convert from hours to m inutes)

EXAMPLE:

I have an engine that I would like to use to m ake 400Hp, I want to choose an air / fuel rat io of 12 and use a BSFC of 0.55. Plugging these num bers into the form ula from above:

of air.

Thus, a com pressor m ap that has the capabilit y of at least 44 pounds per m inute of air flow capacity is a good start ing point . Note that nowhere in this calculat ion did we enter any engine displacem ent or RPM num bers. This m eans that for

any engine, in order to m ake 400 Hp, it needs to flow about 44 lb/ m in ( this assum es that BSFC rem ains constant across all engine types) . Naturally, a sm aller displacem ent engine will require m ore boost or higher engine speed to m eet this target than a larger engine will. So how m uch boost pressure would be required?

◊ Ca lcu lat e r equ i r ed m an i f o ld p r essu r e r equ i r ed t o m eet t h e h o r sepow er , o r f low t ar g e t :

Where: · MAPreq = Manifold Absolute Pressure (psia) required to m eet the horsepower target · Wa = Airf lowactual( lb/ m in) · R = Gas Constant = 639.6 · Tm = I ntake Manifold Temperature (degrees F) · VE = Volum et r ic Eff iciency · N = Engine speed (RPM) · Vd = engine displacem ent (Cubic I nches, convert from liters to CI by m ult iply ing by 61.02, ex. 2.0 lit ers * 61.02 = 122 CI ) EXAMPLE:

To cont inue the exam ple above, let ’s consider a 2.0 lit er engine with the following descr ipt ion:

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

= 4 1 .1 p sia ( rem em ber, this is absolute pressure. Subt ract atmospher ic pressure to get gauge pressure (aka boost) :

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

As a com par ison let ’s repeat the calculat ion for a larger displacem ent 5.0L (4942 cc/ 302 CI ) engine.

Where: · Wa = 44 lb/ m in 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

= 2 1 .6 p sia (or 6.9 psig boost)

This exam ple illust rates in order to reach the horsepower target of 400 hp, a larger engine requires lower m anifold pressure but still needs 44lb/min of airflow. This can have a very significant effect on choosing the correct com pressor.

With Mass Flow and Manifold Pressure, we are near ly ready to plot the data on the com pressor m ap. The next step is to determ ine how much pressure loss exists between the compressor and the m anifold. The best way to do this is to m easure the pressure drop with a data acquisit ion system , but m any t im es that is not pract ical.

Depending upon flow rate, charge air cooler character ist ics, piping size, num ber/ qualit y of the bends, throt t le body rest rict ion, etc., the plum bing pressure drop can be est im ated. This can be 1 psi or less for a very well designed system . On certain rest r ict ive OEM setups, especially those that have now higher- than-stock airf low levels, the pressure drop can be 4 psi or greater .

For our exam ples we will assum e that there is a 2 psi loss. So to determ ine the Com pressor Discharge Pressure (P2c) , 2 psi will be added to the m anifold pressure calculated above.

Where: · P2c = Compressor Discharge Pressure (psia) · MAP = Manifold Absolute Pressure (psia) · ∆Ploss = Pressure Loss Between the Com pressor and the Manifold (psi) For the 2.0 L engine:

= 4 3 .1 p sia

For the 5.0 L engine:

= 2 3 .6 p sia

Remember our discussion on inlet depression in the Pressure Rat io discussion ear lier , we said that a typical value m ight be 1 psi, so that is what will be used in this calculat ion. For this example, assum e that we are at sea level, so am bient pressure is 14.7 psia.

We will need to subtract the 1 psi pressure loss from the am bient pressure to determ ine the Com pressor I nlet Pressure (P1) .

Where: · P1c = Com pressor I nlet Pressure (psia) · Pam b = Am bient Air pressure (psia) · ∆Ploss = Pressure Loss due to Air Filter / Piping (psi)

P1c = 14.7 - 1

= 1 3 .7 p sia

With this, we can calculate Pressure Rat io ( ) using the equat ion.

For the 2.0 L engine:

= 3 .1 4

For the 5.0 L engine:

= 1 .7 2

We now have enough inform at ion to plot these operat ing points on the com pressor m ap. First we will t ry a GT2860RS. This turbo has a 60m m , 60 t r im com pressor wheel.

Clear ly this compressor is too sm all, as both points are posit ioned far to the r ight and beyond the com pressor ’s choke line.

Another potent ial candidate m ight be the GT3076R. This turbo has a 76m m , 56 t r im com pressor wheel:

This is m uch bet ter ; at least both points are on the m ap! Let ’s look at each point in m ore detail.

For the 2.0L engine this point is in a very eff icient area of the m ap, but since it is in the center of the m ap, there would be a concern that at a lower engine speeds that it would be near or over the surge line. This m ight be ok for a high- rpm -biased powerband that m ight be used on a racing applicat ion, but a st reet applicat ion would be bet ter served by a different com pressor.

For the 5.0L engine, this looks like a very good st reet -biased powerband, w ith the lower engine speeds passing through the highest eff iciency zone on the m ap, and plenty of m argin to stay clear of surge. One area of concern would be turbo overspeed when revving the engine past peak power. A larger com pressor would place the operat ing point nearer to the center of the m ap and would give som e addit ional benefit to a high- rpm -biased powerband. We’ll look at a larger com pressor for the 5.0L after we figure out a good st reet match for the 2.0L engine.

So now lets look at a GT3071R, which uses a 71m m , 56 t r im com pressor wheel.

For the 2.0L engine, this is a m uch m ore m id- range-or iented com pressor. The operat ing point is shifted a bit towards the choke side of the m ap and this provides addit ional surge m argin. The lower engine speeds will now pass through the higher efficiency zones and give excellent performance and response.

For the 5.0L engine, the com pressor is clear ly too sm all and would not be considered.

Now that we have arr ived at an acceptable com pressor for the 2.0L engine, lets calculate a lower rpm point to put on the m ap to bet ter get a feel for what the engine operat ing line will look like. We can calculate this using the following form ula:

We’ll choose the engine speed at which we would expect to see peak torque, based on exper ience or an educated guess. I n this case we’ll choose 5000rpm .

Where: · Wa = Airf lowactual ( lb/ m in) · MAP = Manifold Absolute Pressure (psia) = 35.1 psia · R = Gas Constant = 639.6 · Tm = I ntake Manifold Tem perature (degrees F) = 130 · VE = Volum et r ic Eff iciency = 0.98 · N = Engine speed (RPM) = 5000rpm · Vd = engine displacem ent (Cubic I nches, convert from liters to CI by m ult iply ing by 61, ex. 2.0 liters * 61 = 122 CI )

= 34.1 lb/ m in

Plot t ing this on the GT3071R com pressor m ap gives the following operat ing points.

This gives a good representat ion of the operat ing line at that boost level, which is well suited to this m ap. At engine speeds lower than 5000rpm the boost pressure will be lower, and the pressure rat io would be lower, to keep the com pressor out of surge.

Back to the 5.0 L engine. Let ’s look at a larger com pressor ’s m ap. This t im e we will t ry a GT3582R with an 82m m , 56 t r im com pressor.

Here , com pared to the GT3076R, we can see that this point is not quite so deep into choke and will give bet ter high- rpm perform ance than the 76m m wheel. A fur ther increase in wheel size would give even bet ter high- rpm perform ance, but at the cost of low- and m id- range response and dr ivabilit y.

Hopefully this has given a basic idea of what a compressor m ap displays and how to choose a com pressor. As you can see, a few simple est imat ions and calculat ions can provide a good basis for com pressor select ion. I f real data is available to be subst ituted in place of est im at ion, m ore accurate result s can be generated.