Turbo matching help

[Canada]

I'm a bit confused here on how to calculate mass air flow.......I've done it two ways now and am coming up with two different sets of numbers.

 

The first set is friend Quincy's way.......

 

((CID * RPM * 0.5 * VE * Air Density) / 1728) * Pressure Ratio = lb/min

 

I'm using the 3.4 DOHC Vin X motor.......

 

((204.4 cu in * 7000 rpm * 0.5 * .80 * 0.076 lb/ft^3) / 1728) * 1.68 = 42.3 lb/min

 

This assumes a lot.........VE can't be this high.....even being a DOHC motor, we just picked 80% as a good average accross the rpm range....also air density.........it pry never reaches that here, but its a pretty save assumption.

 

If you use a VE of 0.6 you get 31.7 lb/min.

 

Mike Kojima wrote an article on how to do this in the June 2003 issue of SCC. His way...

 

(((Boost + 14.7) / (53.3 * 12 * (460 + Post Intercooler Temp))) * CID * RPM) / (2 * VE)

 

130 for Post Intercooler temp.........thats a sheer rectal number provided by SCC....everything else is the same. Works out to 58.5 lb/min..........not that much of a difference I though beings there are so many varibles. Until I entered 60% VE.......78 lb/min.......it INCREASED? Whats up with that? Did I screw up my algebra?

 

Here is the excel file I am using.....

 

http://www.berettaspeed.com/canada/mass_air_flow.xls

 

Here at some maps I have graphed using Q's math.......3.5k, 7k, 80% VE, and 10 psi......

 

TO4B 60-1

 

TO4B 62-1

 

GT35R

[rudefyet]

i'm going to try and calclate this out...but first...how are you calculatin the air density

 

Also i think 80% VE is actually a little low for a DOHC engine

[BIGBULS]

i'm going to try and calclate this out...but first...where'd you come up with the Air Density #? Not quite sure how you got it...i was trying to calulate using CFM not lbs/min...if your using Relative density it should be more like .9xx...but apparently i'm missing something

 

Also i think 80% VE is actually a little low for a DOHC engine

 

80% is a good average across the rpm range though (it'll be a bit less at the rev limit, more at peak torque).

 

And here's a good webpage to calculate air density: http://members.aol.com/robogeek/airfuel.htm

 

I've also looked up a few other conversions, and they all agree that 0.076lbs/cuft is DRY sea level density (ideal). So, I believe we used that as a baseline, but for us (~1000ft above sea level), we've found 0.071-0.073 to be more "real world". All that it means is that you'll have to rev a little higher to make peak boost/efficiency.

[rudefyet]

i'm going to try and calclate this out...but first...where'd you come up with the Air Density #? Not quite sure how you got it...i was trying to calulate using CFM not lbs/min...if your using Relative density it should be more like .9xx...but apparently i'm missing something

 

Also i think 80% VE is actually a little low for a DOHC engine

 

80% is a good average across the rpm range though (it'll be a bit less at the rev limit, more at peak torque).

 

And here's a good webpage to calculate air density: http://members.aol.com/robogeek/airfuel.htm

 

I've also looked up a few other conversions, and they all agree that 0.076lbs/cuft is DRY sea level density (ideal). So, I believe we used that as a baseline, but for us (~1000ft above sea level), we've found 0.071-0.073 to be more "real world". All that it means is that you'll have to rev a little higher to make peak boost/efficiency.

 

my bad i was thinking peak efficiency

 

so what are you using for the temparature here? not sure what average underhoood temp is..also...i'm just not getting the same numbers...maybe i don't undestand the calc...give me the temp and psi you used

 

just curious...the book i'm studying doesn't give a good equation to calulate air density

[BIGBULS]

I just noticed....Canada...........I think you drew the Y axis line at ~1.73 for a PR on the first two (rather than 1.68 )..........might want to redo that

[rudefyet]

wait nm...i was using the wrong psi...sorry i'm a loser

 

i got .073 with that calc...about like what bigbuls said

 

what temp did you guys use?

[BIGBULS]

70-80 degree's..............a pretty average "warm day".

 

And 14.2psi...........it's about right for our elevation.

[rudefyet]

from what i came up with...you first equation checks out pretty good so the problem must be in the second one

 

I modified the charts a little to see if i am any good at this

 

I got 40.6 lb/min at 7000rpm and assuming your wanting the highest compressor efficiency at 50% of the redline i got 20.3 lb/min for 3500rpm

 

The diaginal line i drew is to make sure each turbo is above the surge limit on your engine

 

first off

 

T04B 60-1

 

This turbo will give you around 70-75% efficiency at 7000rpm but only around 60-65% at 3500rpm...this turbo will hit peak efficiency at around 5000-5500 rpms

 

next:

T04B 62-1

 

This turbo will give you 75% efficiency at 7000rpms and around 67-70% at 3500rpms...it'll hit peak efficiency around 5500-6000 rpms

 

last

 

GT35R

 

7000rpms will permit 72% efficiency from the tubro in this case and approx. the same at 3500rpms...peak efficiency should set in around 4500rpms

 

While i may be stating stuff you already know...i'm just trying out some stuff i learned...

 

Overall as for effciency goes....my choice would be the GT35R sicne it maintains a 70% or higher efficiency through most of the rpm range

 

but then agian that has nothing to do with the actual size of the turbine..boost threshold etc....or your preferences here...i'm really curious as to what you want to achieve with your turbo setup

[Canada]

To make power............duh!!!!

 

Yep I was 0.05 off on the PR............darn.......MS Paint is a very difficult program to operate.

 

My plan is to only make 6 psi initally.....then get all of the little things tied up (tuning, getting the motor and trans able to highstand higher power levels) and jump to 10 psi.

 

The GT series does look pretty cool.......the only thing is the cost....I'm pretty sure I can pick up a TO4B much cheaper than a GT35R.........however, I have not priced them yet......so I will still be looking for them and if the added cost is minimal (under an additonal $200) I'll go for it.

 

Everyone (I posted this on 3-4 other boards) seems to agree Kojima was on crack when he published his SCC aqrticle.

[rudefyet]

LOL...of course it's to make lots of power

 

I was just curious what size turbine you were going for...just if you wanted boost down low, in the middle or up high

 

I figure you're looking for a medium size turbine so you get boost at midrange rpms and a little lag...of course big turbine means more lag but gobs of power at high rpms...and small turbine means no lag and boost at low rpms but it'll run out of steam at high rpms...whcih i assume you all ready know

[BIGBULS]

I think making full boost by 4000rpm and peak power in the 5500-6000rpm range would be ideal.

 

Likewise, I think 250-270hp (crank) on 6 psi, and 300+hp on 10psi are worthy goals................that should yield low-mid 14's on low boost, and mid to high 13's on high boost (or so one would hope).