Boatnik

Just how much CFM are we pumping through our diesels?

Clunk

It's hard to find a number under boost conditions because of so many variables like air density and temperature....the deeper you get, the deeper it gets.
This'll keep ya busy for a while:
http://www.turbofast.com.au/turbomap.html

Herrin811

best I could come up with on a different calculator was around 425cfm

apache

Somewhere I read 2500CFM @?? , the flow would vary heavily upon boost etc.. I flow tested the catalyic converter on my old toyota truck in the 80ies on our machine at work. Stock it was 390 cfm and after, ahhhhhh..... it broke well say, it flowed 690. Truck still was anemic as could be. It seemed it was better pre broken

Tuckerdee

I read last week that an HX35 was rated at 600cfm.

BC847

In terms of exhaust:

With the above, we're moving right at 1332 CFM through the exhaust system at 3000rpm.

Hope this helps.

Tuckerdee

Yes, but that's for expanded/heated gases. Cold air intake is much less. Perfectly effecient, non-pressurized intake is ((engine displacement/1728) X Max RPM)/2. You divide by 1728 because there are 1728 cubic inches for every cubic foot. You divide by 2 because because the engine displaces it's full volume every 2 crankshaft revolutions. So, for our engines, that's only 312.5 CFM at 3000 rpm. Someone will have to fill in here about how turbo boost affects this.

HOHN

CFM is easy to calculate.

(Displacement (in cubic inches) x engine RPM)/3456. The multiply by volumetric efficiency (.85 for 85% and so on).

So, for a CTD at 2000 rpm, you have 359*2000/3456*.85= 176.6CFM.

You will have a lot more CFM if you're up on boost, but the formula won't account for that. You can roughly account for it using PSIA and PSIG approximation. For example, if you live at sea level (and ambient pressure is 14.7psia), then when you have 14.7 of boost (iow, 14.7 psig), then you will have roughly DOUBLE the air flow.

In terms of formula, just take the above calculated CFM, and multiply by a ratio.

Find the ratio: (boost+ambient air pressure)/ambient air pressure.

Let's say our above scenario was at 10psi of boost, and we live at sea level. Then the ratio becomes: (10+14.7)/14.7=1.68

So now you take your 1.68 times the 176.6CFM we found earlier and get a total of ~297CFM.

So the engine is flowing 297CFM at 2000 rpm, with 85% volumetric efficiency and with 10psi of boost at sea level.





But this is the redneck math way to do it.

It's FAR more accurate to do a MASS flow calculation. Volume varies with pressure, density, and temperature. Mass is mass, so you can ignore all those variables.

Mass flow is king, and it's the "proper" way to calculate, imo. Once you find mass flow, you can easily convert to CFM for any combination of PSI, temp, and density using ideal gas law.

jmo

frostie

The problem we are having here is called volumetric efficiency. The math that we see reflects basic engine displacement times rpm. some of you with higher numbers are factoring in that efficency.
even with a non turboed engine you get a much higher number due to the fact that air has mass, which means air can gain volocity in the port, that combined with the camshaft overlap and valve timing, you get way more air into the cylinders than what the basic math shows.
did I confuse anyone well I understand it but Im not the best at spelling

Clunk

So much for the killer cfm's...thanks Hohn

pwrtripls1

I have a lb per minute flow meter I could get bored one day and use, but the I have to figure out how to convert that to cfm.

Boatnik

Thanks for the info. At the dyno, using a bourdon tube mechanical gauge, the needle pulsed badly at 22-25 psig. The calculation for a natural asperated engine I was familiar with but turbo engines I wasn,t sure of the relationship. It makes sense now that I have seen it to multiply the number of atmospheric increase to the theroretical flow rate of a positve displacement pump/engine.

jleonard

According to the specs for a 370 hp marine 6BTA, intake air flow is 657 CFM
523 CFM for the 220 hp version
599 CFM for a 270 hp version
657 also for the 330 hp
Jay