Saturday, July 18, 1998

The 10,000 rpm Bomb

Several times a week we get calls from people that have just exploded their engine and, for the most part, they're looking for answers as to why it happened. On occasion, they'll send us some of the parts (or the whole mess) for evaluation hoping that we can shed some light on the problem. Well - sometimes we can and sometimes we can't.
Lets take a moment to consider where it all started and where we are now.We took a 5 hp. lawnmower motor that was designed to turn a maximum of 3,600 rpm then bored, stroked, welded, ground, filed, polished, fitted, crammed, invented and generally violated the entire book on common sense and ended up with a remanufactured 10,000 rpm BOMB and all of this was done in the name of fun. Well, of course it is.
We just finished testing an engine with a .174 overbore and 3.000" stroker crank and at 9,000 rpm this is what was happening inside that engine:1. The valves were opening and closing 150 times per second.2. The crankshaft rod journal was traveling 79.8 mph in a 3" diameter circle.3. The piston & rod moved, stopped then changed direction 18,000 times per minute (300 times per second).
Think about this:The piston is at top dead center (TDC) in a momentary stop position, we've already had combustion, the piston travels down the cylinder 1.3671" reaching a top speed of 84.3 mph. while the crankshaft has rotated 75° and all of this has only taken 1/720th of a second to happen.
Another way to look at it:The piston and rod start and stop 300 times per second reaching 84.3 mph. between each cycle.
Now, we ask the $ 64,000 question:Could anything go wrong in this kind of environment ?The answer is - Everything.It's truly an engineering miracle that this (or any) engine ever gets to 9,000 rpm just once let alone sustaining it.