Injector Sizing: REALLY dumb question -Reply

[Clare Snyder]

Bruce Robertson wrote:
> 
> Just butting in here with my two cents worth...
> 
> I thought the whole idea of having the timing overlap was to take
> advantage of the momentum of the exhaust gas passing from the
> combusion chamber into the exhaust port to "scavenge" some intake
> charge. The momentum of the gas is such that once the piston reaches
> TDC it tries to keep going  hence creating a depression if the inlet valve
> were not opening. Because the inlet valve opens the depression is
> satisfied by sucking in some fresh charge. If the overlap was too large,
> and the inlet opened way too soon then I suppose the exhaust gas into
> the inlet would be possible - but doesn't sound too flash to me!
> 
> Bruce.
You have the principal right.
The most common cause of exhaust gas recursion into the intake,
particularly on GM engines, is FCS, or Flat Cam Syndrome. If the cam
lobe wears down, which is not at all an uncommon event, the engine will 
exhibit a cloud of fuel above the carb (or throttle body) on
accelleration. A plugged exhaust (bad cat, collapsed inner section of
dual wall exhaust pipe, kinked tail-pipe, etc.) will have the same
effect. Ditto for an improperly indexed cam. If it is happening, you DO
have a problem, as the engine is designed to work the way Bruce has
stated.
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                           Clare Snyder
                         Waterloo, Ontario