944obscene

So I got really bored today. Slow day AT&T and I had the internet at my finger tips. So I decided to start running some numbers and see what's going to work best for me at about 350whp. I'm estimating that this will leave my engine producing what, up to about 410-415whp?

400 X .15= 60 => 400 - 60= 340

415 X .15= 62.4 => 415- 62.4= 352.6

Lets look into what its going to take to reach this power output. You could calculate for CFM, as the correlation between air consumed, and power produced is as accurate as I know you can get.

I've been running numbers all day, just for fun. Both for about 350whp and for the stock setup where I'm only looking to run about 16psi. Just entertaining myself.

I looked up the CFM equation which states:

CFM= (L X RPM X VE X Pr)/5660

L= displacement in Liters
RPM= peak RPM of motor
VE=Volumetric Efficiency... obviously (Which is something I want to get, more or less accurate.
Pr= pressure ratio

Not sure what 5660 is. Isn't that the crosover point where torque and hp meet?

So let me see if I set this up right..

(2.5 X 6500 X 85 X Pr)/5660= Projected CFM at a certian boost level, based on Pr. Pr being (14.7 + lets say 16psi)/14.7= 2.08

So its: (2.5 X 6500 X 85 X 2.08)/5660= 507.59

So I'd assume that at about 16 pounds of boost, my motor is consuming ROUGHLY 507 CFM of air. Correct? As long as I know I'm on the right track, I know I can figure things out here. Just so you all know... Math has always been my WEAK POINT! Not good for an engineering major!!!

Here's what I'm getting at though. What should I calculate for, estimate at etc. to know what its going to take to achieve my goal? Obviously, all I can really do is speculate but I hope and assume that some people will be versed in this. I'm still learning a few things.

So first, I need to figure out how much my engine will be flowing to produce a certain amount of power. Then, I'll have to figure what turbo will best produce the mount of airflow I need to achieve what the motor needs for it's projected output.

What I really want are a few things regarding the turbo motor. What is the VE of these motors? What calculations will help me figure some of these numbers out, and what the hell am I doing here at work trying to do math?! I do enough of it at school!

Thanks for any help. Just having a little fun here.

User 41221

Without cracking open a text book, isn't 5660 the number that converts cc's into cubic feet? Torque and HP cross at 5250, btw...

Regards,

V2Rocket

why not just buy the LR 340 kit lol

cuz very roughly 507cfm would be around 340HP at the crank. in my bored-math the NA engine is around 88% VE and a turbo generally makes an engine more volumetrically efficient, however the compression ratio has to come into play as well in that calculation but it should still be higher than 88.

also consider, if youre using redline as your HP peak if the turbo can still make power why not raise the rev limit?

rough rule of thumb is that HP desired * 1.5 = CFM needed

xsboost90

i can tell you that stock intake, engine, header, crossover, Lindsey 3" exhaust, k27/8, 21lbs boost, Maf kit, Tial wg - you can get 359 to the rear wheels...

ArthurPE

T/HP constant = 60 sec/min x 550 ft-lb/HP / 2Pi ~ 5252.1176...

from T = P x w (Torque is usually Moment, w = ang vel 2Pi RPM)

the best equation for theoretical torque, forget power, it's just the rate of application of T, which is an instantaneous force

T = V Pm/4Pi x Ve
V = displacement
Pm = mean effective pressure ~ Cr x boost
Ve = vol eff

let's assume
V = 2.5 l ~ 153 in^3
Ve = 1.10... turbo engines >1, why? just negate friction/pumping losses by cranking boost up a bit
my 1.1 is estimated from the boost and torque peak rpm...
Cr = 8.0 ~ 118 lb/in^2 (psi)
Boost = 11 psi ~ 1 atm or a factor of 1.75 absolute

T = 153 in^3 x 118 lb/in^2 x 1.75 /4Pi x 1.1 ~ 2765 in-lb (the units 'work', ie, dimensional analysis checks)

converting to feet 2765 in-lb / 12 in/ft ~ 230 lb-ft...very close to the actual range

let's say that's peak T and it droops 10% at HP peak 6000...208 lb-ft
HP = 6000/5252 x 208 ~237, still close, basically between the normal and S motors

let's say we need 420/237 ~77% more HP, ie, torque...
T = 5252/6000 x 420 ~ 367 + 10% droop ~ 405 lb-ft peak T

well assume:
V stays the same 153
Ve goes up to 1.2 (we know boost is increasing, therefore so will Ve)
Me = 118 x (1 + B) we'll solve for boost B in atm

405 x 12 (lb -in) = 153 x 118 x (1 + B) / 4Pi x 1.2
B = 1.8 ~ 26 psi

does that seem about right?

the best would be to increase revs a bit sya 10% to 6600
increase V, a 3 liter would be a 20% increase
so the required boost increase would be lower

now obviously, this is all bench racing and an excercise in mental masturbation lol

95ONE

Oh god.. LOL
My Book "Maximum Boost" by Corky Bell has so many scribblings in it.
I can't tell you how many times I've done that kind of number crunching. Numbers work really well when you calculate 155hp at the crank at 0 boost. But when you want to boost it, turbo efficiency / size is king to know what power at what boost. 21.5lbs of boost gave me 416hp @wheels with lame ignition advance. STOCK cam/head/bottom end. Free flowing exhaust and intake. a lot more efficient at the higher boost pressure than xsboost. But Im sure mine wasn't efficient as his at lower. So.. It's all still a crap shoot in the end!

V2Rocket

95one when you say 0 boost do you mean that manifold pressure was atmospheric, not under vacuum like an NA car?

944obscene

Okay. Two things I don't fully understand here. I don't get "Me", "2Pi" or "4Pi". Is Pi actually 3.14? meaning 2 X Pi, or 4 X Pi? And how did you get the numerical value for the Cr? I understand that 118 is the numerical value for 8.0:1, but I'm still trying to figure that bit out. I could look around for it.

I see the variances between the math and real-world application. What I'm trying to do is amateur blueprinting, and number crunching. Starting this next quarter, I get 4 bonuses a year, which means I should be getting closer to my project with a bigger turbo.

I want to make sure that I'm going to have all the right equipment for the job. Sure I could call a few people up, and talk to Mike, or John etc. but I want to do some of my own homework. I wouldn't call it bench racing as much as planning lol. But either way, it is a bit of mental masturbation. I'm still in foreplay, so I haven't gotten to the good stuff.

So lets play with the numbers here for a second. Say VE goes up a tad, because I toss a cam in. Better breathing up top, plus boost, is going to skew the numbers a bit. Also, lets bump the Cr to 9.0:1, because I'm thinking I can get away with it, on better rods and pistons, which will further increase the efficiency of the motor. Technically speaking, this is just a 2.5l air pump, and I'm just trying to get it to perform better, without having to throw copious amounts of boost and gasoline at it.

Ignition timing, is a factor, as well as AFR's, which is why I have taken the advice of a few on here, and plan on a WI system as well. I think, with the right turbo (not even a huge one), and EVEN on the stock 8:1Cr bottom-end, I could hit my goal with relatively low boost. I'm going to scour the boards a little and see what other people make, with what mods, turbo's, and what boost. That way I can see what the REAL WORLD has in store for me.

Thanks for the help man.

951Porschiste

Sorry to hijack but this is the only math I know:

V2Rocket

the higher CR will give you higher VE and better off-boost but you will make more power with low CR and more boost.

944obscene

I do agree. General rule is it's easier to make big power on low compression and high boost than it is on high compression low boost. I'm just playing with numbers though.

ArthurPE

yes Pi = 3.14159...2Pi = 2 x Pi

8:1 means it squeezes 1 atm (at perfect voleff) 8 times
1 atm = 14.7 psi, 8 x 14.7 ~ 118 psi

944obscene

Oh... Next time I'll just pay attention lol. That actually makes a lot more sense!