“IT NEEDS TO BE SORTED OUT BEFORE ALL HELL BREAKS LOOSE”
the pin and f lap with no appreciable dif ference in performance. I’ll eventua lly get around to modif y ing the second hand DISA I bought online and f itting it. Good hunting. Laurie Floyd Waranga Shores, Vic
AH, LAURIE, so it was you that wrote to me some time back on the subject of pins and valves in BMW sixes. When I got Gary’s letter, I went back through my emails to try to find your letter, because somewhere in the dim, dark recesses of what passes for my mind, I recalled that somebody had mentioned a pin that could fall into the inlet manifold and, if you were unlucky, into the engine proper where it could do untold damage to those lovely little straight-six innards.
Typically (and if you could see my desk, you’d understand) I couldn’t find your email, but here you are: Popping up all on your own. So thanks for educating me again. I’m not familiar with the wee pin you’re talking about, but if there’s a design flaw that allows anything to drop into the inlet manifold, then it needs to be sorted out before all hell breaks loose.
Anyways, armed with your info that it’s called a DISA valve, I managed to find out a bit more about it. It seems the unit is a control valve to alter the effective length of the intake runner. At low engine speeds, the valve forces air through a more circuitous route and allows for a shorter, straighter shot at the intake valves at higher engine speeds. The idea is not exclusive to BMW, but it helps make a smaller capacity engine behave like a bigger one with a broader spread of grunt.
When used in conjunction with variable valve timing (VANOS, in BMW-speak) the end result can be a 2.5-litre six that performs and feels like a three-litre. Some BMW engines actually have two DISA valves, I’m told.
You can tell that your DISA valve is heading for the end of the road if the engine starts rattling from somewhere in the intake tract. Apparently, replacement is no big deal, but while you, Laurie, have elected to run without the valve, I’m told that the engine will, indeed, run better with the valve in place. Apparently, rough running is a symptom off a dud (or missing) DISA valve, and without it, you won’t have the benefit of that variable intake-tract length thing working for you.
I’m not sure what the default tract length is without the valve fitted, but I’d say that you’re currently running with either the long tract or the short
tract exclusively. And if you can’t feel any difference, that’s probably because you either never rev the engine beyond 2500rpm (so, never need the shorter tract) or you redline it in every gear (and never need the longer tract). It’ll be interesting to see if you notice a difference when you fit your new DISA valve. Let us know.