enough math to scare even a math teacher!!

[kc8oye]

I wasnt' sure if this was modifers for engine material.. so it went here

 

As you guys know, I'm workin on a twin turbo setup for my monte.. and I just discovered my note taking skills are lacking..so I had to go back and re-do all of the math involved in selcting what turbo(s) work... so I figured I'd share the math with you just in case someone might be interested..

 

What you need to know:

 

Engine Displacement: 355 cid

Target HP (flywheel): 600hp

Maximum Engine RPM: 6500 rpm

Ambient Temp and Baro: -10F to +90F ? Baro: nfc

 

 

What you need to estimate:

Engine VE: No idea. on the low side I would think.

Intake Manifold temp: (130F estimate) no idea right now, can be measured easily tho.

BSFC: Brake specific fuel consumption Again, no clue.

 

 

1) Start by Estimating Peak compressor airflow required to achieve the h.p goal.

 

AF = HP x AFR x BSFC/60

 

AF = Actual mass airflow in lbs/min

HP = Target HP

AFR = Air/fuel ratio

BSFC = Brake specific fuel consumption/hour (divide by 60 for minutes)

 

AF = 600 x 12 x (0.55/60) = 66lbs / min

 

 

2) Calculate required manifold pressure to meet the h.p or target airflow.

 

MAPreq = (Wa x R x (460 + Tm))/(VE x (N/2) x Vd)

 

MAPreq = Manifold absolute pressure (psia) required to meet the h.p target.

Wa - Airflow (actual) (lbs/min)

R = Gas Constant = 639.6

Tm = Intake Manifold Temp

VE = Volumetric Efficiency.

N = Engine Speed (rpm)

Vd = Engine Displacementin Cubic Inches. (CID = Liters x 61.02)

 

Wa = 66lbs/min (From above)

Tm = 130F (from above)

VE = 92% at peak power = .92

N = 6500 (from Above)

Vd = 355cid (From ABove)

 

MAPr = (66 x 639.6 X (460 + 130))/(.92 X (6500/2) x 355) =

(66 X 639.6 X 590) / (.92 X 3250 X 355) =

24906024 / 1061450 = 23.46psia

 

Boost = Mapr - 14.7psia (Sea level) = 8.76 psig

 

3) Correct for pressure drop between compressor and engine.

 

P2c = Map + Ploss

 

P2c = Compressor discharge pressure (psia)

MAP = Manifold absolute pressure (psia)

Ploss = pressure loss between compressor and manifold.

 

Ploss estimated at 1psi with no intercooler

 

p2c = 23.46 + 1 = 24.46 psia

 

4) Correct for Pressure drop upstream of the compressor inlet.

(typical drop thru air cleaner might be 1psi)

 

P1c = Pamb - Ploss

P1c - 14.7 - 1 = 13.7psia

 

5) calculate required pressure ratio (IIc) thru the compressor

 

IIc = P2c / P1c

 

IIc = Pressure ratio

P2c = ABS compressor discharge pressure (psia)

p1c = Compressor inlet pressure

 

IIC = 24.46 / 13.7psia = 1.785

 

6) plot the target maximum MAF and maximum pressure ratio point on cadidate compressor maps to see if the

 

compressor can meet this "worst case" requirement.

 

 

 

[kc8oye]

This is the compressor map it refers too.

Remember, I'm planning Twins, so you halve the air requirements.. 33lbs/min @ 1.78 pressureratio.

 

T3/T4 Hybrid

 

if you follow allong the bottom to about 33, and then up the left side to 1.8 (close enough to 1.78)

you'll see that I'm to the right side of that large center island. that makes this turbo just a hair too small. it would work, but it would be on the ragged edge, and would leave me no room to grow, and would problaby REQUIRE an intercooler even at my low 8psi..

 

so we look at the next bigger turbo.. that brings us to here now I'm on the left side of the center island which says the turbo a little too big.. but that's ok. Im not against increasing the boost just a little bit to put the turbo into it's most effecient region. (ideally, you want to be right in the middle of that large center island)

 

[CrazyDavey]

This is the compressor map it refers too.

Remember, I'm planning Twins, so you halve the air requirements.. 33lbs/min @ 1.78 pressureratio.

 

T3/T4 Hybrid

 

if you follow allong the bottom to about 33, and then up the left side to 1.8 (close enough to 1.78)

you'll see that I'm to the right side of that large center island. that makes this turbo just a hair too small. it would work, but it would be on the ragged edge, and would leave me no room to grow, and would problaby REQUIRE an intercooler even at my low 8psi..

 

so we look at the next bigger turbo.. that brings us to here now I'm on the left side of the center island which says the turbo a little too big.. but that's ok. Im not against increasing the boost just a little bit to put the turbo into it's most effecient region. (ideally, you want to be right in the middle of that large center island)

 

Tim, you're not even married.....

[TN454Monte]

Quote:

Tim, you're not even married.....

I want to see that personal add....!!

"Must want twins, likes dogs and Monte Carlo's a big plus!!"

[Slaman]

Thanks Tim. I think we talked about this before and I mentioned I have considered a twin turbo set-up with my TPI someday... This is some good ifo to get me started when I head down that path.

[kc8oye]

i have a really good book on the subject.. it's a necessity.

 

i'm still going to be building a lot of this on a wing and a prayer because there are just too many variables I can't account for (like the actual VE of my engine)

 

[wallaby]

Do they want the VE now, or after the turbo install?

I'm not sure what they are after there; the boost will "cover up" or fix most if not all of the VE issues....that is what it does, and that's where you get your horsepower gains.

 

This reminds me of the engineering that went into the Bugatti Veron...they decided on a target horsepower, then built an engine to provide it. They converted everything into BTUs and figured out how much fuel would need to be burned to provide that many BTUs. Then they figured how much air had to be supplied to support that fuel...and arrived at the engine displacement figure.

Same math, but worked in reverse.

[CrazyDavey]

Do they want the VE now, or after the turbo install?

I'm not sure what they are after there; the boost will "cover up" or fix most if not all of the VE issues....that is what it does, and that's where you get your horsepower gains.

 

This reminds me of the engineering that went into the Bugatti Veron...they decided on a target horsepower, then built an engine to provide it. They converted everything into BTUs and figured out how much fuel would need to be burned to provide that many BTUs. Then they figured how much air had to be supplied to support that fuel...and arrived at the engine displacement figure.

Same math, but worked in reverse.

 

I remember VE day...everyone was sooo happy......

 

 

 

[kc8oye]

you need to know ahead of time, before the turbo. because it makes a huge difference.

 

with a VE of 92% I only need 8psi to make 600hp

with a VE of 75% I need 14psi to make 600hp.

 

a turbo that can provide 66lbs/min of air @ 8psi might not be able to do it at 14...

 

[USMCBLB125]

Just get a procharger.

[kc8oye]

The problem with superchargers is the horrible parasitic losses they generate when you aren't 'on boost'

 

they can easily trash your 'cruising' mileage by 20-30%

 

Turbo's have negligble effect on the engine as a whole when they aren't being used.. and only require about 6hp worth of WASTED heat from the exhaust to spin when making boost.