Having had a chance to use Baggy Joe several times on the road since the Matra meeting on December 6th the initial impressions are very good!
Damp or wet roads have limited the opportunities to apply full boost but the main limiting factor has been the absence of an explosion pressure relief valve. If a backfire occurs in the inlet manifold the resulting explosion pressure can get up to 7.5 to 8 bar (118psi). Although the pipework and the manifold itself can probably take that pressure, for a few milliseconds at least, the effect on the blades in the supercharger could be very damaging.
To minimise the risk of explosion damage I have not fitted the last hose clip on the inlet system with the expectation that the pipe will blow off if the pressure gets too high. This happens at around 0.2bar (3psi) And it does, frequently! On Sunday in a drive of only 2-3 km the pipe came off three times! My passenger got tired of jumping out and pushing it back on again. We took a risk and fitted the last hose clip and then allowed the boost to get up to 0.5bar (7.5psi) and the car flew down the road!
The risk of a backfire is very real! While unloading the car from the trailer on Sunday evening after the meeting, in the dark and wet, I had two backfires while trying to start the engine. The first was unexpected and the subsequent noise while cranking was quite different. With a torch I could see that the inlet pipe had blown off as intended. I pushed the pipe back on, got back in and tried again to start it. This time I watched the engine through the mirror and saw a large blue/yellow flash as the backfire escaped through the open pipe, once again blown off!
So the focus is now on finding or making a relief valve. The conventional pneumatic pressure relief valve as fitted to superchargers is only 6mm bore (Norgren 1/4" type). All my instincts say this is too small to relieve the explosion pressure fast enough. The water industry has valves much larger than this and the RS catalogue (
www.rswww.com) lists several 3/4" valves at less than 25% of the price of the 1/4" Norgen pneumatic version. Worth a try perhaps?
But my real wish is to make a 30 or 40mm relief valve like the one we patented in 1976 for our company's use. It was designed to act as an automatic outlet valve for a control cabinet that had to be purged with clean air before the electricity was turned on. The walls of the cabinet might be 2m high x 900mm wide and deep so there was a very large surface area for the pressure to act upon. Our valve was 50mm bore, fitted on the outside and the valve disc was suspended on a stainless steel leaf spring. It was held closed by a permanent magnet inside the cabinet with the air gap adjusted to give the right opening pressure typically about 10mbar (0.15psi). When the purging started the cabinet pressure rose to 10mbar and the valve popped open! We detected the opening action with a reed switch and that was calibrated to give us a purge air flow rate. The patent ran out some years ago so we could use the same principle again on the blower! More thought needed.
Maybe there is a new market here!
My other concern it that the blower installation is noisy! Very noisy by most commentators' opinion and I wonder if it will pass the noise tests imposed by the FIA and MSA at race meetings. I am arranging for one of the official scrutineers to check the noise level as 110 dBA is the maximum permitted for our class in Sprints and Hill Climbs. This should happen during the next 2 weeks hopefully. At the last meeting, pre-blower, the exhaust noise was measured at 91 dBA, well below the level allowed.
Happy Christmas to all members and sincere thanks for your continuing support in this project.
Andy Owler