I have tried applying Corky Bell's calculations to get some indication of CFM rates to aid reading of compressor graphs.

His calculations are as follows:

CFM Normally aspirated (N/A) = Cubic Inches x rpm x 0.5 x efficiency
=====================================
1728

CFM (boost level) = (1 bar atmospheric + boost pressure) x CFM (N/A)

The "0.5" is given as a static value and efficiency is normally 85-90% for petrol engines apparently. The 1994cc engine capacity is equivalent to 122ci.

Applying that to 1.2 bar boost shows as follows:

CFM (N/A) = 122ci x 7000rpm x 0.5 x 90%
---------------------------
1728 = 222.4 CFM

CFM (1.2bar boost) = 2.2bar x 222.4CFM = 489.3 CFM

================================================== =================
....or changing the efficiency down to 80% results:

CFM (N/A) = 122ci x 7000rpm x 0.5 x 80%
---------------------------
1728 = 197.7 CFM

CFM (1.2bar boost) = 2.2bar x 197.7CFM = 434.9 CFM

================================================== ================

These figures (if correct) seem to take the CFM outside the efficiency of the TD04H-15G map here.

Apparently, you can also approximate the CFM for an engine by multiplying BHP x 1.5 which also seems to fit ?

What do you think ?

Thanks

Gavin

[EDIT: Corrected transposed figures in formulae above]

[Edited by GavinP - 4/1/2002 1:47:36 PM]

Gavin,

Apart from using 85% for VE, the figures are right, that's why people complain about the boost dropping off at the top end. This TD04 can only flow 428 cfm, so it's well outside it's efficiency.

I've compared your bhp x 1.5, and can't get it to work. If you had a UK car, running 250bhp, it would be using 375cfm.......doesn't sound right to me.

Mark.



[Edited by R19KET - 3/31/2002 6:28:00 PM]

I am learning about this too. This is my understanding which may be flawed. The 0.5 factor I believe comes from the fact that a four stroke petrol engine only moves half its displacement per engine revolution.

You have to also incorporate the efficiency of the turbo and intercooler, as well as reducing volumetric efficiency at high engine speeds.

The turbo efficiency is the biggest problem. You want to work out the airflow at a given pressure ratio to enable you to see what the turbo efficiency is, so a circular calculation! This seems a bit odd to me, but overall we are trying estimate the flow, and unless there are leaks the flow at any point in the system is the same, although the pressure will not be.

So a pressure ratio of 2.2 becomes more like 1.8 in terms of airflow multiple compared with N/A. (1.2 bar boost * 67% efficiency + 1 bar atmospheric). Presumably this is a consequence of heating leading to lower density.

So in my car when you consider I am running about 1 bar at 6000 RPM, I can only multiply the airflow by about 1.6 compared with N/A. My TD04L-13G has a 360CFM rating at PR 2.0.

So 169 CFM at 6000 RPM N/A, then intercooler efficiency is included in the overall VE of 80%, I can multiply by 1.6 = 270 CFM.

The 0.5 factor I believe comes from the fact that a four stroke petrol engine only moves half its displacement per engine revolution.

If I have just spouted a load of twaddle forgive me, I tried my best!

Thanks for the replies guys.

The "1.5 x bhp" seems roughly accurate in that my car is apparently making 280-300bhp (estimated by PE) so 400-450CFM would fit ?

When Bob mapped my car last time, boost was tapering off over 6000rpm even whilst using the Dawes. This seems to demonstrate hitting the flow limit of the TD04H.

Using the "Power" button on the auto transmission to get the gears to hold to higher revs did nothing for performance.

I think 100% efficiency when calculating the normally-aspirated figure is zero restriction for the intake and exhaust. Although the "real life" restriction of the intake, IC and exhaust will reduce efficiency, the impression I get from reading Corky's book is that it unlikely to drop efficiency below 80% taking this into account ?

From reading the TD04H map, the stock boost level of 0.8bar puts the turbo in the "sweet spot" for efficiency and the boost increase of a third has moved it to the fringes/outside the map...

Thanks

Gavin

It's very rare that one can actually achieve what a turbo is rated to, since they a rated on a flow bench, in "ideal" conditions. These "conditions" can't, and don't take into account the engine, exhaust system, etc', etc'. That's why it's amusing to hear someone with a ***bhp turbo, expecting to get ***bhp. Sh*t, on that basis, I'd be running a 600bhp turbo :D

Same goes for flow, you're very unlikely to be hitting the map flow rates, even at a given rpm x pressure. On the other hand, all turbo's can be "overdriven", but obviously well outside their efficiency.

Gavin, remember the flow would depend at what rpm your max bhp was achieved. So if fro example it was at 6000rpm, 1.2bar, etc', the estimated flow would be 395cfm.

Mark.