Post
Wed Mar 31, 2010 2:10 pm
Tim, sorry, possibly I should have mentioned what this graph actually tells you. The Y-Axis is the compressor pressure ratio. That is to say, the pressure P2 (measured at the compressor outlet) divided by the pressure P1 (measured at the compressor inlet). Since you are operating at sea level, I have set P1 to 100kPA, which is typical for down in the Cape.
In other words, you can read off your boost pressure from the Y-axis. For example, at a pressure ratio of 1.5 you must be making 0.5 bar of boost if atmospheric pressure is 1 bar.
The X-Axis is the mass of air that is being inducted into your 2500cc engine. The relationship between the X and Y axis is indicated by the red line, which I calculated.
And since we know the relationship between the Rotrex speed (indicated on the graph as the bold black lines) and the engine speed (due to the pulley and Rotrex gear ratio) we can say something like: At a Rotrex speed of 70,000 rpm, which corresponds to an engine speed of 4742 rpm, your M20B25 will induct (by virtue of the Rotrex) roughly 0.18kg/s of air at a pressure ratio of roughly 1.7 (ie, at 0.7 bar boost). Have a close look at the graph and confirm for yourself that you can see this.
Now, if we know how much air your engine is inducting at any given engine rpm, we can calculate roughly (assuming optimal ignition timing angle and ideal fueling) how much torque and power your engine will output as a function of RPM.
What bothers me, however, is that just from looking at the above graph I can already tell you that your Rotrex is touching on 1.3 bar of boost at 6100 rpm engine speed (90000 rpm Rotrex speed). This is 0.4 bar in excess of the 0.9bar boost you stated. However if you are indeed running at up to 1.3 bar boost, your M20B25 engine will indeed induct, as per the above graph, roughly 0.27 kg/s or air, which, at an AFR of 13:1, will correspond to roughly 249kW or 334hp total engine power output. Please note that I say total power, since the power available at the flywheel is this figure less what it takes to drive the Rotrex, alternator and the water-pump. If you take your engine all the way up to 6774 rpm (at the Rotrex limit) your engine will flow roughly 0.32 kg/s of air at a pressure ratio of 2.6 (1.6 bar boost). That should correspond to roughly 295 kW or 396hp total power. Now while that may sound like an awful lot of power, just wait and see how that figure drops when you take into account how much power the Rotrex is consuming in order to pump the air that makes that power possible!
Tim, I am not sure where your boost gauge is connected or how high it can read, but I think you will find that my calculations are reasonably accurate (although I did have to guess at you engines VE given that you have changed the cam). If you are running to these sort of pressures it would explain why you are literally "blowing" your spark out. On the other hand, if we solve your ignition problem you may well find yourself popping your engine. A figure of 1.6 bar boost on an 8.8:1 CR does concern me. Does your Gotech-Pro include a knock sensor?
Something else to note is that these calculations are performed assuming the throttle is wide-open. At part throttle the mass of air inducted into the engine (and hence power) will be very different.
"It is amazing how many drivers, even at the Formula-1 level, think that brakes are for slowing the car down." - Mario Andretti
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