spazzytroy

iTrader: (1)

I'm now to the point where I need to choose a turbo to continue with my build. I followed the Turbo 103 formulas from Garrett (seen here http://www.turbobygarrett.com/turbob...o_tech103.html) and they generally seem to jive with what Zero R and Howard posted in their guides.

My goals for the project remain mostly the same, albeit a slightly higher power level:
~500rwp @ 7-7.5k
Excellent transient response for the power level
Daily driver capable

Now for the fun part, using



Where:

· MAPreq = Manifold Absolute Pressure (psia) required to meet the horsepower target
· Wa = Airflowactual(lb/min)
· R = Gas Constant = 639.6
· Tm = Intake Manifold Temperature (degrees F)
· VE = Volumetric Efficiency
· N = Engine speed (RPM)
· Vd = engine displacement (ci)

And guessing that 68lbs/min will get me just over 500, a 130* intake charge, 90% VE (restrictive stock porting and quietish exhaust?), and a 4L displacement:

We get (68 * 639.6 * (460 + 130)) / (((.90 * 7000) / 2) * 244) = 33.3863544psia / 14.7 = 2.27(18.68lbs boost, wtf?)PR while I'm here at my relatively low altitude.

Then I guessed at an optimistic 3700rpm min rpm for full boost, and solving to get airflow from pressure & rpm we get:



(((33.38 * .90 * 3700) / 2) * 244) / (639.6 * (460 + 130)) = 35.9360162lbs/min at 3700rpm.

Plotting all of that onto a few maps, I tried to find an optimal compressor and frame, with just a smidge of room if 68lbs doesn't get me there.



Why is my PR so far off from what I've seen with other builds? (https://www.rx7club.com/showthread.p...light=20B+TiAL)

Also, I'd like to also figure out how turbine selection works, as the information around is a little less plentiful than compressor selection. In what order should you go about deciding the size of either variable when optimizing response? (I've read Howard's comparison based on area, and ZeroR's suggestion of larger wheels w/ smaller ARs. More of a why does this happen question.)

Finally, I had initially decided on a BB series guessing that it would help with my desire for transient response. I am open to other suggestions.

Apologies for the wall of text, and thanks in advance for the help

spazzytroy

iTrader: (1)

Awesome, took my edit button away...

Anyways, I think I found the reason for the PR error. I'm guessing that the "/2" term in the denominator (that is unlabeled btw, thanks Garrett) is for the fraction of revolutions in which intake events are happening. 1/2 is incorrect for a rotary, however 1/1 doesn't make the numbers match.

Not fully certain if this is the solution, but substituting the *(1/2) with a *(2/3) yields numbers more in line:

(68 * 639.6 * (460 + 130)) / (((.90 * 7 000 * 2) / 3) * 244) = 25.0397658 (~10lbs boost) = 1.70 PR

BigJohn'sSon

2/3 seems to be correct:
The rotor turns 1/3 turn for each shaft rev, (1/3)
The actual displacement is twice the advertised displacement. (2/*).

A NA rotary has a VE of around .70
Of course with FI the VE can go above 1.00.

spazzytroy

iTrader: (1)

Hmm, I thought that by multiplying the displacement (2.0L = 122ci * 2 = 244ci), I had taken care of the rotary displacement already.

arghx

iTrader: (3)

. n/m