jks67ss396

iTrader: (1)

Hey guys

im looking at different turbo maps for my 6.0L (or 408) build and doing all the turbo math (pressure ratios and airflow requirenments). But something is not adding up.

Ive been on the Garrett turbo website quite a bit and they give an example of selecting a turbo for a 2.0L engine. they calculate airflow and pressure at 7200 rpm. Then they turn around and calculate airflow at 5000 rpm to show that a particular turbo is adequate.

This is where i get confused. An engine doesnt make the same power at a lower rpm if airflow is less (less air = less power), so why is that line between 5000 and 7200 a straight line? at a constant pressure ratio??

I buy that airflow/hp would increase with rpm, but so would boost/pressure, right?? So that Pressure ratio value (straight line between 5000 and 7200) should curve down as airflow/rpm decreases?

The reason i say this is because some of my calculations show that i need 96 lbs/min of air at 7200 rpm, but a lot less at lower rpm (say 2000).

The problem is that if the pressure ratio stays constant at the same value it is at max boost/power, then im way in the surge area of the turbo....

Also, can someone explain how boost/airflow relates to rpm in turbos? Id love to be able to see "boost vs rpm" for some of these turbos..

thanks guys!

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

71 chevy

iTrader: (18)

try this

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

jks67ss396

iTrader: (1)

thanks!!!

thats a great tool. i still wish i knew the equations behind the expected boost/rpm though.

heres my plot of what id like to do

http://www.squirrelpf.com/turbocalc/...&product_id=68

my only concern is that using the stock 317 heads, i thought that max boost you could push was 15 psi before water starts flowing.

So, this calculation shows that i can expect 18 psi for my desired power of 800.

i thought that id be able to get 800 at 15psi. But, this calculator uses a bsfc of .55. i was using .6, which shows an even higher boost.

Am i missing something?

Twin 61s seems like a good match, but what happens at the bottom of the graph? i assume that at low airflow requirements, the turbos arent spinning relatively fast and anything going on in them is neglegible??

thanks guys

jks67ss396

iTrader: (1)

heres one with a single turbo...

http://www.squirrelpf.com/turbocalc/...product_id=123

again, what happens at the lower airflow and boost reqs? is that ok to operate in that region?

71 chevy

iTrader: (18)

can you link to your config data?

i assume that 15 psi will almost double NA output and then adjust the VE % for that.

it looks like your turbo choices are pretty much perfect for what you are trying to do. what happens at low rpms is no concern . the turbos are not yet working in that region. what you could be worried about is surge and neither of your graphs are anywhere close to the surge line

jks67ss396

iTrader: (1)

yes, i guess 15 psi would double the NA power... seeing as how the 317 stocker heads dont hold much past that (or so ive read), then i guess id be maxxed out at 700 or so hp. makes sense cause i doubt a 9.0-9.5 CR stock head/cam 350 ci motor aint gonna get much better than 350 hp. guess ill get used to the 15 psi and then pop for some better heads. better heads and a cam would increase VE, right? by how much?
where would the better VE be input into this calculator?

thanks

71 chevy

iTrader: (18)

ive seen some guys on here go way past 15 psi with stock heads.

a cam on these engines really wakes up the motor, I know cammed ls1s have no problem making 375 wheel hp so that should be 750 or more wheel hp at 15psi with a cam.

where it allows you to enter rpm, to the right of it is vol. efficiency.

jks67ss396

iTrader: (1)

thanks for pointing that VE out to me. i thought that was an output of the program.

Do you know what the max boost is for these stock heads with mls gaskets and arp bolts is?

I thought it was 15...

thanks

AMRennsports

At 9.5:1 it would possible to see 100% Volumetric Efficiency, but it's very difficult with that compression. I've seen as high as 108% on the dyno with a good H/C/I combo and 11.5:1. You'll likely see about 94-97% with a decent h/c/i setup.

jks67ss396

iTrader: (1)

So with a stock cam, stock 317 heads and stock truck intake, what VE do you think id see? 85?

and, whats the VE look like at max boost vs max power?

thanks

Nitroused383

iTrader: (10)

How do you calculate volumetric efficiency? I've searched the web a bit and still haven't figured it out