eyecandy

iTrader: (19)

Does anyone know the CFM or lbs/min rating of the stock twins? Or how I can calculate it? I am sure there is a differnce when taking into account the psi and rpm. I would assume that you would need the turbos pisi, not the manifold pressure.

I guess what I am looking for is a base line, say stock 10psi (whats the psi at the turbos, stock configuration), at varing rpm ranges.

Does the CFM of the engine have anything to do with finding it out the CFM or lbs/min of the turbos?

Project84

I don't know what the output of the stock FD twin turbocharger is, but you can use the formulae below to calculate.

RPM CFM lbs per min @ ISA conditions
1000 46 4
2000 93 7
3000 139 11
4000 185 14
5000 231 18
6000 278 21
7000 324 25
8000 370 28

Pressure ratio= 14.7+boost /14.7
CFM boosted= CFM unboosted X pressure ratio
LB/min= CFM boosted X 0.07
LB/min= hp/10

eyecandy

iTrader: (19)

Whats ISA conditions? And what are those numbers based on?

How do I use the following in and equation?

Pressure ratio= 14.7+boost /14.7
CFM boosted= CFM unboosted X pressure ratio
LB/min= CFM boosted X 0.07
LB/min= hp/10

eyecandy

iTrader: (19)

Do I also need to know the efficency of the turbo? Going from Comp inlet (cold flow) to Comp outlet (hot flow)... Or is that somehting else?

eyecandy

iTrader: (19)

Where did you get the CFM numbers in your table? Is that of a 13b rew with no boost?

eyecandy

iTrader: (19)

Anyone???

eyecandy

iTrader: (19)

Using

( CID * RPM * Pressure * Ve ) / 1728 = CFM

-CID would it be 80 or 160?
-Where does the 1728 come from? I thought it was 3456
-I assume a good Ve to use on a stock port would be between 70-75% and on a street port 85-95%?

Does that give me the CFM of the exhuast, comp inlet, comp outlet or what??

I also read taking the RWHP and multipling it by 1.9 gives a good CFM, but is that COLD or HOT flow at the compressor? But I also know that taking the Cold flow CFM and dividing it by 1.5 gives a good FWHP number, then if you take that number and remove 15% for drivetrain loss it comes out about 10% higher than the original starting number. Here is an example:

300 RWHP

* 1.9 = 570 CFM (cold or hot) If cold then;
/ 1.5 = 380 FWHP
15% loss = 323 (*.85) and 330 (/1.15) Which is it *.8 or /1.15 for the correct number?

If you change the 1.9 to 1.73 (dividing the FWHP by 1.15) the calculations come out about even. Or if you change it to 1.77 (multipling the FWHP by .85) the calcualtions come out about the same.

I hope some one can help me figure this out......

KevinK2

Turbo compr maps, and Spearco charts by implication (contrary to my pm), are based on cold ambient flow. When spearco rates an intercooler at 900 cfm and 1.5 psi drop, and then also rates it at 600 hp, they are talking cold flow into a turbo.

Best CFM estimate for twins is based on max rwhp dyno'd. 370 rwhp is typical practial limit (although as high as 400 has been claimed).

370x1.15 = 425 at engine.

using very standard factor of 1.5, cold ambient CFM into aircleaner/turbo will be

425x1.5= 640 ambient cfm.

The formula(s) to actually calculate this flow is more complex than posted, check this calculator for cfm's (not hp) and drop VE about 10% below na values. Input 4 cyl, 4.35" bore and stroke.

http://www.turbofast.com.au/tfcalc.html

For spearco IC charts, IGNORE the CID values and assume the cfm is cold flow into turbo, and check pressure drop and efficency.

Howard Coleman

iTrader: (6)

since there has been no compressor map posted for the oem turbos it is correct to reverse into the airflow...

various people have their methods as to equating rotary hp w airflow and i have spent considerable time on the subject for the last 4 years.

i have settled on 2 methods which produce similar results.

let's assume 370 max rear wheel hp from the stock turbos.

the first method is to take lbs/min airflow from the compressor map multiply it by 10 and derive piston engine rear wheel hp, then divide it by 1.3 to get rotary rear wheel hp.

using the above formula the turbos each put out 24.05 lb/min or 348 cfm

another formula is cfm times 1.9 equals one rw rotary hp.

370 rwhp times 1.9 equals 703 cfm/2 equals 351 cfm

my guess is that the compressors are very inefficient at these flow rates, as is the manifold, and if you were to plot a compressor map the airflow might be 10% more but a great deal of oxygen is being lost by the amount of heat generated as the compressor spins, probably, around 140,000 rpm.

these turbos are not happy making hp in the 370 area.

note: the above formulas take into consideration all the usual slippage due to pumping loss thru the intercooler etc..

howard coleman

eyecandy

iTrader: (19)

Keivn, those numbers from the site seem really high.... even with making the twins VERY inefficient as well as the intercooler....


Howard, So the estimated max CFM and lb/min on the stock twins is the 348 or 24.05 lb/min per turbo. How do compute those figures to "hot flow"? Or is that where you need the compressor maps?

"the first method is to take lbs/min airflow from the compressor map multiply it by 10 and derive piston engine rear wheel hp, then divide it by 1.3 to get rotary rear wheel hp."

-whats the whole formula? (piston rwhp?)

Where/how does this 1.9 come about? The reason I ask is because of the difference in the following two:

#1 Best CFM estimate for twins is based on max rwhp dyno'd. 370 rwhp is typical practial limit (although as high as 400 has been claimed).
-370x1.15 = 425 at engine.
using very standard factor of 1.5, cold ambient CFM into aircleaner/turbo will be
-425x1.5= 640 ambient cfm.

#2 another formula is cfm times 1.9 equals one rw rotary hp.

370 rwhp times 1.9 equals 703 cfm/2 equals 351 cfm


One last thing, my guess would be the max efficiency of the trubos would be from about 3500-6500rpm (65-70%)?? And above that propably drop to 50% at 8k??

KevinK2

I get about the right cfm from the ray hall site, consistent with my previosly posted values. But that site has some flaws .... ex: increasing intake temps has no effect on hp or mass flow rate.

I made a full excel spreadsheet where all variables can be dialed in. Just keep in mind the VE, as used in these cases, must be dropped from na values due to back pressure to the exh ports that exceeds the boost pressure, when maxing out stock turbos. You or Howard can pm me your address if you want a copy, and are familiar with excel.

eyecandy

iTrader: (19)

I will have to give it another try. What is a good assumed vaule for Ve with stock ports, street porting, etc?