Mark,
So for my 500cc motor, if I have calculated correctly, my intake flow at 8000 RPM would be around 69ft^3/minute, but what would my exhaust flow be? I think the big shop vac will probably move 200ft^3/ minute, but it is hard to check.
Theoretically, I have just one breather on the B50 BSA so I should be able to measure crankcase pressure, assuming that I had the proper gauges, which I don't. I say theoretically because most BSA's have a bunch of unscheduled breather points. I have a manometer which will read up to 10" H2O, so I could hook that up. Unfortunately the motor is sitting on the floor, the frame is waiting to be painted on a warmer day probably next June, etc. etc. And the nearest dyno is about a 6 hour drive, but it is on the way to Loring, ME, so that will be the first chance I get to try anything new. So for now, it is just bench racing, trying to set up some things for the dyno run.
Tom
Tom,
Since you cannot duplicate the pressure in the exhaust pipe when the exhaust valve first opens at the blowdown period (pressures probably in excess of 100psi) without a 50/75 hp electric motor and a very large rotary vane blower, I would do the testing through the exhaust pipe with the shop vac wide open. I would then test angles & shapes of the insert (vacuum adaptor) for the highest depression. (vacuum extraction) Make a water manometer out of some vinyl tubing, a yardstick or tape measure and a wooden board to support everything.
The majority of dyno testing I have done uses calibrated gauges (in cfm) to measure & quantify whether the crankcase produces either blow-by (positive pressure) or vacuum (negative pressure). These are now done digitally, but originally were an analog measurement. If your dyno facility is not capable of measuring crankcase vacuum/pressure, a 5/10 cfm gauge (.5 cfm graduations minimum, .1 cfm is better) is cheap enough. (If they do not measure this, it shows that they are not serious about development.) You can then connect the gauge to your crankcase breather via Tygon or other clear, heat-resistant tubing. Your first efforts should be to minimize or eliminate, blow-by. Your engine should have a max. of 1.0 cfm blowby. More than this indicates either very poor ring sealing or excessive windage in the crankcase. A hot cylinder leakdown test can identify poor sealing of the rings, and often on bike engines this identifies poor cylinder wall prep and/or concentricity. If you can't get the rings to seal, you can never control crankcase pressure effectively,
and, it is lost horsepower.
Have you ever done any hot leakdown checking to evaluate ring to cylinder wall, valve to seat sealing? If so what were the leakage rates? Hot leakage rates in excess of 1/2% (at TDC) indicate that those basic issues need to be corrected. BTW, thinking that excess leakage rates will "wear-in" is optomistic. It's like playing 5 card stud with 4 cards.........
Crankcase vacuum, on the other hand, presents opportunities for bhp gains. Less windage = more bhp. Crankcase vacuum allows the initial ring tension to be lowered (on all rings) reducing parasite drag for a free net bhp increase,
as long as effective ring seal can be maintained. But do not be deceived, achieving higher levels of crankcase vacuum, and their attendant benefits, is a difficult process.
Access to a dyno that already has these test capabilities would be helpful. Let me know if you need more info on this.
Fordboy