Estimating BHP from CFM and a fluke find

[S2central.net]

Went on a search for this calculation today. Nearest thing I could find was a calc for BHP on non-turbo engines based on atmospheric pressure (AP), compression ratio (CR) and cubic capacity (CID). Also needs Volumetric Efficiency (VE) and RPM. Namely -

BHP = (AP x CR x VE x CID x RPM) / (5252 x 150.8)

I tried working this backwards into a calculator for airflow in CFM based on-

Flow = (CID x (RPM/2) x VE)/12^3

But I couldn't make sense of unit conversion or some other goof I made.

Taking the original BHP calculation (clueless of where the 150.8 comes from) and multiplying that by pressure ratio (PR) I get the following estimate of BHP for a turbo engine. Thus -

BHP = (PR x AP x CR x VE x CID x RPM) / (5252 x 150.8)

I checked the figures with typical flow numbers from an RS2 and K24 turbo and found them to be about right with ~360bhp and ~280bhp respectively.

In the process, I noticed that by multiplying Corrected Flow (expressed in lbs per minute direct from the compressor map) by compression ratio I get estimated BHP values that are less than 1% lower than that horrible calculation above.

WOOT !!!!!!!!!!


Am I nuts ? If not its a great discovery for mankind - well for me anyway



Paul

[Pete_RS2]

You should get out more Paul. LOL!

[4WheelDrifter]

Nice one.
I have a copy of your spreadsheet i knocked up with help from Corky. I'll give that a go myself.
Corrected Flow x CR = BHP

[S2central.net]

You should get out more Paul. LOL!

Aye yer probably not wrong... but to be fair that was a bit of lunchtime research today.

Any more number jockeys out there to sanity check this stuff ?


Paul

[4WheelDrifter]

Yeah.
Dunno how you found that, or how it works.
Did some rough figures for the A3 on a K03. It comes out about 220BHP.

[Keeper of The Kitten]

You do realise that CID means "cubic inch displacement" don't you?!! That's 16.3871cc for every cu in (sad knowing a conversion figure, but hey we use 'em all the time at work!) Just wondering!

The other saddo fact - 5252 rpm is where the graphs for torque (lb-ft) and power (bhp) cross - so the numbers are the same for both values, whatever type of engine it is - from weedeaters to 6.0 Audi A8s...

I need some sleep!

__MDC__

[S2central.net]

Errr yes - don't worry - I know how to convert CID to cc...

I also know about the magical 5252 number that correlates power and torque. Its actually 33,000 divided by 2*Pi. The 2*Pi number comes from that being a unitless representation of one crankshaft revolution... And the 33,000 number comes from James Watt who decided that one horse could lift that weight (in lbs) a distance of one foot per minute.

1 HP = 33,000lbsft/min

Now thats proper sad !

[Keeper of The Kitten]

Even more sad, apparently the average horse actually has about four hp!

__MDC__

[martinjs]

Even more sad, apparently the average horse actually has about four hp!

__MDC__

Maybe with a mihnea remap it will have 6hp

[Keeper of The Kitten]

Yeah, and I've seen some of those race horses with the machined aluminium air intake fitted in their throats!

I wonder if the MAF always needs cleaning on a horse?

__MDC__

[Phillio]

and the citroen 2cv means deux chevaux, or two horse power.

probably the same as a small pony then.