16V Cam Sets & Idle Characteristics

[jdbubb at ix.netcom.com (Dan Bubb)]

The reason for the rough idle with big cams has a lot to do with the large amount to overlap. i.e.
the period at close to TDC at the end of the exhuast cycle and beginning of the intake stroke were
both valves are open simutaneously.
At low engine speeds the intake charge and the exhaust are both moving very slowly, so they don't
have much inertia and can change direction very easily.
So, you have both valves open, the intake and exhaust are both moving pretty slowly. Under these
conditions the actual amount of air entering the cylinder will be inconsistent because the intake
valve opens while the engine is still in the exhaust stroke and initially it sees a pressure pulse
instead of a vacuum pulse. Also, the scavaging of the exhaust will also be inconsistent because the
exhaust valve is still open during the beginning of the intake stroke, so the vacuum of the intake
stroke tends to suck exhaust back into the cylinder. The amount of air and fuel varies and the
amount of exhaust contamination varies. So, the amount of power from each cylinder can jump up and
down over a fairly large range and that's the rough idle.
Applying a little throttle ups the air speed and the exhaust speed and the direction of flow become
more stable and cylinder filling more consistent.
The same basic issues apply to and engine "coming on the cam". With very long duration cams the
valves are open so far into the wrong engine cycle that the flow reversal continues, to some extent,
all the way to a fairly high RPM. The speed where the intake flow becomes high enough that it's
inertia overcomes the pressure from the rising piston on the compression stroke and there is no flow
reversal is the speed where the engine see a big jump in power. "Coming on the cam".
Switching to carbs or MS won't prevent the rough idle. I would think the 50mm intake would make
things worse if anything, but doubt the effect would even be noticable. Lightened flywheel will let
the engine speed jump around more, so if the idle's rough a lightened flywheel won't help..
I've got a 274 (not real wild) in my 2.0 8V w/Haltech. Lumpy idle, but any throttle at all smoothes
it right out.
I kind of like the sound of the lumpy idle!
Well, I hope all that was clear!
Dan

Sent: Tuesday, December 21, 2004 7:52 AM
Subject: 16V Cam Sets & Idle Characteristics


> The more aggressive cam sets for the 16V list rough idle, on fuel
> injected cars, as a side effect.  Other than switching to carbs or MS
> type management does anyone know of a solution to the idle problem?  Is
> it vacuum related or is the fuel current signal causing the poor idle?
> Does a larger, i.e. 50 mm intake help or hurt the problem?  Lightened
> flywheels help/hurt the problem?
> Finally, anyone with different cam sets want to comment on how bad the
> idle is?  The worst idling car I have ever driven is my brother's built
> to the hill 69 Chevelle 454.
> Rick.
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