Thread: Ford 302 DynoSim, street motor

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Originally Posted by slepper63

i hate to ask but what does bmep is high on this build mean? what bmep mean?

It's an indication of cylinder pressure and stands for "brake mean effective pressure". The figure is determined by several factors, but the main players are the efficiency of the intake manifold and cylinder heads, static compression ratio and the closing point of the intake valve in relation to crankshaft and piston position. If cylinder pressure exceeds the detonation resistance of the fuel being used and there are no mechanical interventions such as squish, then the motor could detonate and disassemble itself. You may have heard an engine "ping", also called "spark knock" or "dieseling". That is detonation and is like someone swinging a sledgehammer down onto the piston crowns for each ping you hear. Broken rings and broken pistons or worse won't be far down the road if you allow detonation to continue unchecked.

You'll see that I specified a very tight "squish" dimension of 0.038", so that by the time the piston gets a little taller from combustion heat, the crankshaft bends a little and the rods stretch a little, there will be a near collision of the piston crown with the underside of the cylinder head. This "jets" the mixture from this area and across the chamber, homogenizing the mix so that the possibility of detonation is reduced. David Vizard has done extensive testing of the squish dimension. He said that the tighter he ran the squish, the more power the motor made, down to a squish of 0.026" on a SBC, where he encountered interference between the piston crown and the cylinder head. He is a no-nonsense writer and you may gain some insight from reading his material.