So you want more power. Are you tired of the stock M62 blower ? First of all, the stock restrictive exhaust (manifold, DP and catback) will have to be replaced by a nice flowing turbo header, a 3” downpipe, and a 3” catback. I would personnaly not go under 3” running the kind of numbers that we’ll calculate together over this very post. And do not forget to upgrade the stock clutch, possibly valve springs (if you plan to go past 7000 RPM) and pistons for 350+ WHP. And have we talk about bigger injectors ? Needed for sure.

First of all, I give all the credit to a web page I found over the internet, which was dedicated to turbo sizing for a different motor. That page is :

http://www.mrcontrols.com/primers/sizing.htm

So I got this info suited for the marvelous piece of art, the LSJ. Here we go :

To really determine what a given turbo could potentially do on the LSJ engine, you need to cut throw away the marketing and advertising claims and go straight to the heart of the matter: the compressor map. On the following link you’ll find the compressor map for a Garrett GT2871R 56 trim (0.6 A/R) compressor wheel :

http://www.turbobygarrett.com/turbob...R_743347_2.htm

Looks somewhat intimidating, doesn't it ? Let's not worry too much about that and take it one thing at a time.

The first thing that we need to look at are the numbers across the axis on the left side of the graph that start with 1 and go up. These indicate the pressure ratio at which the turbine is operating. The pressure ratio is just the absolute pressure at the outlet of the compressor divided by the absolute pressure at the intake of the compressor. Most often, we make these calculations at sea level atmospheric pressures (14.7 PSI), but if you live at altitude, you should use the atmospheric pressure representative of your location. There is a pressure drop of 0.5 PSI per 1000 ft over the sea level.

Now, to determine the absolute pressure at the outlet of the turbo, add the turbo boost pressure to the intake pressure which should be atmospheric pressure unless your air filter is very dirty or your air intake is too restrictive for your setup. Suppose we want to determine the pressure ratio for 15psi of boost at sea level. That will be :

Pressure Ratio = (15 + 14.7) / 14.7 = 29.7 / 14.7 = 2.02

So if you take a ruler and lay it down horizontally across the compressor map just a tiny bit above the "2" on the left axis scale you can see that it cuts a pretty nice line across the middle of the map. Trust me for now that that's a good thing if we plan to operate this turbo at 15psi.

Across the bottom axis on the graph we see air flow given in pounds per minute. Some compressor maps give it in Cubic Feet per Minute (CFM) which is actually better. To convert pounds per minute into CFM, you need to take the temperature of the air into consideration (the ideal gas law tells us that as gas heats up, it expands, which means that the hotter the gas, the less it weighs per cubic feet, which is why a hot air balloon rises). Fortunately, most compressor maps are taken at 85F (but I cannot confirm what is the air temperature for that particular map). One cubic foot of air at 85F weighs 0.07282 pounds. So, at 85F, convert pounds per minute to CFM by multiplying by 13.73.

So, if we take our ruler again and set it horizontal just above the "2" pressure ratio mark and then look at the range from the surge line to the end of the balloon, we have a permissible range from 13 pounds per minute to 42.5 pounds per minute. This translates to 180 CFM and 580 CFM, respectively. This is a big range. Will the LSJ with this compressor be able to flow this much air? No, we need to consider the fact that an engine is an air pump and at a given intake pressure it will only be able to ingest so much air. We’re talking for now about the stock LSJ with the stock head and stock cams.

Let’s calculate how much air will flow through the engine at 6500 RPM, which is a good point to start for now. So like said you have to start with engine displacement and an RPM point, then plug it into :

CFM for 4 stroke = Displacement in CI / 3456 * RPM * VE

The stock LSJ has a stock displacement of 122 cubic inches, so at 6500 RPM it will flow :

CFM = 122 / 3456 * 6500 * VE = 229 CFM * VE

VE is volumetric efficiency, which is a value indicating how much of the potential air flow volume actually makes it through the engine at a given RPM. If you throw in a guestimate of about a 83% VE for the LSJ @ 6500 RPM (again we’re talking about the stock head), you get :

CFM (LSJ itself) = 229 * 0.83 = 190 CFM

If we take that 190 CFM divided by 13.73 (cubic feet per pounds) then it translates to 13.8 pounds per minute. This appears to be just at the limit of the surge area on compressor map. Surge is caused when the engine cannot ingest enough air to keep the compressor inside its map. It is not quite the case, however, because this is only telling you what the engine can flow in a naturally aspirated mode. To determine what it will do under boost, you have to determine what density ratio the compressor and intercooling system you have will give you. To do that we need to take our boost point and determine how hot (T = Temperature) the compressor is going to make the air at a that boost :

Tout (in F) = (((Tin (in F) + 460) * (Pressure Ratio exp 0.283)) - 460)

So, let say you set the boost controller for 15psi of boost at sea level at an ambient temp of 85F (85F in this case so that our computed CFM ends up matching that of the compressor map).

Tout = (85 + 460) * 2.020.283 - 460 = 205F

This assumes an ideal, 100% efficient compressor. The round circles in the compressor map tell us how efficient the compressor is going to at a given pressure ratio and flow level. Since most of turbos are at least 70% efficient or better, we'll use that figure for now (which IS conservative) and double check later to make sure we were either close or underestimating a little. Our real outlet temperature is going to be :

delta T actual = delta T ideal / efficiency

For our example, the delta T ideal is 205F - 85F or 120F :

delta T actual = 120F / 0.70 = 171F

171F is how much the compressor is going to heat the air above the inlet temp, so the real outlet temp is 171 + 85, or 256F. What happens when this air mass hits the IC ? Two things: first, a pressure drop and second, a temperature drop. The pressure drop is going to be about 0.5psi for a good sidemount IC (or probably the stock air to water IC too) and we will assume a 65% efficiency number which is probably quite close to the reality :

T IC drop = (T IC in - T ambient) * IC efficiency

So we get :

T IC drop = (256 - 85) * 0.65 = 111F

Therefore the IC will drop the turbo outlet temp by 111F, turning the 256F air into 145F air and dropping the pressure 0.5psi to 14.5psig. What does this do to our normally aspirated engine? Well, the density of the air is increased by a ratio :

density ratio = ((Tin + 460) / (Tout + 460)) * (Pout / Pin)

For out example, we get :

Density ratio = ((85+460)/(145+460))*(14.5+14.7)/14.7 = 1.79

This density ratio means that you will get 1.79 times as much air flowing through the engine with this compressor and intercooler combination at this pressure point and this ambient temperature than you would in normally aspirated mode.

Going back to our 190 CFM value (for the LSJ itself @ 6500 RPM), we multiply that by the density ratio (1.79) to get 340 CFM (when divided by 13.73 converts to 24.8 pounds per minute). This is still inside the compressor's map so we have a reasonable value (if it weren't, you wouldn't be getting 15psi out of the compressor, your actual pressure would have dropped). Additionally, this is right in the compressor's maximum efficiency range, so our manifold temperature will probably be a little lower than we calculated with our 70% efficiency value and our density ratio just a tad higher. This means we are close enough to the money to make it work for our purposes. No real need to go back and try to get the value to be more accurate, since we are already guessing on a number of other things (such as VE) which is having a bigger impact on our actual flow.


Given what we have calculated, we can approximate how much horsepower we will produce. The basic crank HP formula is :

Crank HP = MAP (in absolute psi) * Compression ratio * CFM / 228.6

The compression ratio for a Stock LSJ is 9.5. So, we plug in the real numbers into our HP formula and get :

Crank HP = (14.7 PSI ambient + 15 PSI boost -0.5 loss by IC) * 9.5 * 340 / 228.6 = 412 BHP

Now the turbo itself will take about 22 HP to be driven so 412 – 22 = 390 BHP.

Throw in 15% drivetrain loss and you have a result of 330 WHP at only 6500RPM (Dynojet dyno), which again is a conservative number. And that’s at 15 PSI of boost over the ambient air pressure. Not bad for that small 2.0 powerplant !

For your information, I’ve made the same kind of calculation for the stock Eaton M62 blower boosting 15 PSI on a 3” pulley and it gives a maximum of around 280 WHP (Dynojet dyno) at the same boost ratio. Plus the fact that it will need a lot of cooling mods to support it (additional Heat Exchanger, dual pass cooled intake and methanol injection) to be consistent. Clearly unefficient design ! Potentially some detonation inside the engine too. So have you ever seen a Staged 22 Cobalt SS, 1.5” pulley (OK that’s unreal) and throwing some 7’ long flames with it ? If you want a nice ice melter, you’re setted up ! Just kidding …

Coming back to the Turbo calculation. Therefore, because the stock head flow is efficient up to about 600 CFM, we’ll have some loss there, but at this horsepower level (330 WHP) it will be compensated by the fact the turbo efficiency is higher than the one we calculated … remember ?

So, what makes it a little tough to predict what you really are going to get is getting an idea of what the final VE of the system will be (which is not constant, but changes across the RPM/Manifold pressure range) since the turbine housing and wheel themselves are going to have an effect on the VE map. For example, a tiny turbine and turbine housing can be so restrictive that it drops the engine VE a lot (also known as "choking" the engine).

One other item we should check since we have the numbers calculated is whether the compressor will not be forced into the surge line. We saw that at a 2.02 pressure ratio, the surge line is around 13.8 pounds per minute or 190 CFM. Now, let's assume that the turbine and turbine housing we will choose can power the compressor to reach 15psi by 3500RPMs. We keep the density ratio the same, but we have to re-compute the flow for the engine at 3500RPMs. The VE at this point should be better than at 6500, so we'll use a value of 90%. At 3500RPMs, the engine will be ingesting :

CFM = 122 / 3456 * 3500 * 0.9 = 111 CFM

That's in normally aspirated mode. Multiplying the density ratio, we get :

111 CFM * 1.79 = 200 CFM

This is near the surge limit for this compressor. Granted the VE might be even better, but we could be also off. We could fix this problem on most turbos by putting in a turbine housing with a larger A/R which would slow down the spool time to bring the compressor up to this pressure ratio when the engine is revving a little faster and thus ingesting more air. The larger A/R also allows more exhaust to flow and thus improve VE to also increase air flow and move the system even farther into the compressor map away from the surge line.

Finally, for those who want more mad power, you’ll need a good head porting, a higher (7500 +) RPM limit, a higher boost ratio and some better octane fuel. It might translates to somewhere well over into the 400 + WHP range. So make your own calculation !

 

good read

 

someone did their homework.LOL!!!

 

Interesting. It would be interesting to see these calculations w/ a Super 20 G like Hahn will be using in his kit...

 

Bump )

 

IF i ever go with a turbo kit. A nice GT30R would be used on my LSJ.

 

yah def. good read!

 

Worthy enough for a sticky ?

 

X2 GREAT READ

 

The Garrett GT2871R is a nice piece. It's gaining a little popularity on the srt side too. It just gets overlooked because it's not a big turbo.

 

Digging up an old bone.. Why isnt this stickied somewhere? possibly in the FI section?

This is very very good information!

 

Is there a way we can get this stickied somewhere please ?

 

so it takes an engine some odd horsepower to run a turbo? interesting

 

From:

Squires Turbo Systems - Turbo vs. Supercharger

"Turbochargers are very similar to the centrifugal supercharger compressor, however they are driven from spent exhaust leaving the engine. They are not 'free' horsepower but they are way more efficient and cause much less of a parasitic loss than the belt-driven alternatives."

 

Compressor Flow Map Calculator

 

some people told me a gt3071 was too small, but thing had great spool, put out around 375whp depending on boost levels and put down nearly the same torque as hp - and got nearly 45mpg on the interstate with the .63 trim. it was an all around win.

if i had to do it over though, i would definitely upgrade to a gtx, cause those push a lot more air out through the same trims - they just didn't have them when i turbo'd the car

 

either way it goes, i still don't know why people waste money on turbos though, cause superchargers are FTW!