Engine Grill Question

Tie couple pieces of yarn to grill and see if it sucks or blows.  My guess is it sucks in at idle (not moving) and blows out at speed (due to 1/2 moon cut out in fire wall above flywheel).  

I'm not an engineer, but here's may take on it:

-Your engine grill is for air intake.

-Your shroud fan and carb(s') draw in air, lowering the air pressure in the engine bay.

-The lower air pressure in the engine bay, when compared to the pressure on the outside of the grill, causes air to be drawn into then engine compartment.

-The inside/outside pressure differences, and the amount of air being drawn into the engine bay, changes in relation to your engine rpm.  Higher rpm means more air is being consumed, which in turn lowers the air pressure in the engine compartment and increases the amount of air being drawn into the engine compartment.

-This is where the size of the intake vents comes into play.  If the draw from the fan and carbs is greater than the needed volume of air that can pass through the vents the engine could start to overheat.

-I thing the only time air will actually come out of the vent is when the engine is at idle and the car is stationary.  At idle fan and carb intake will be at a minimum and the air pressure difference between the engine bay and outside will be very low.  Possibly low enough for the hot engine bay air to be able to rise and exit through the vent.

Ok. Currently the rain cover was cut away (PO) to make room for the single progressive. I'm putting a new engine in with dual carbs and seen some speedster owners with fans mounted directly under the engine grill. Wasn't sure if they were extracting or exhausting air with the fan in this location. Was considering to do the same of seal it up or leave as is.

I'd agree with intake if it wasn't for that large 1/2 moon cutout in the fire wall for the fan intake.  Later VWs went to vents on engine hood for added air intake but the engine is entirely closed over their flywheel so the upper grill under rear window and the lid vents are only air for them. Seems logical that airflow would create a low pressure area sucking out air.  Need one of our aeronautical guys to chime in.

I agree 100% a very good explanation, at idle speed it will draw in less air not very noticeable. If I may add it is very important to seal off the engine bay this is very important, the exhausts and heads generate a lot of heat normally if the  engine bay is not sealed correctly or missing tins, hot air is drawn from underneath into the engine bay thus adding unwanted heat.

Scott, remember you can only pump so much air into the engine compartment.  The air has to go out as fast as it comes in. 

Thing about a box with a 6" diameter hole at one end and a 3" diameter hole at the other end.  Putting a fan on the 6" end to force more air into the box won't work.

Now, if that 3" diameter hole changes to an 10" diameter hole....that's another story.  In that case, I think the fan would be beneficial.

Ron,

I attached a few pics of what it looks like now.

The hole is pretty large—cut out by the previous owner to fit the single on a stock 1600. The weber progressive single didn't have any problems drawing air directly from the outside and never saw temps go above 190, even in 90 degree traffic. So I'm not sure if my current set-up with compartment pressure even applies. Everything is sealed below. It will now be completely open once I have my dual set-up. Just wondering to keep open, mount a fan with a switch for stop and go traffic on hot days or seal up again. 

There's actually mention of using fans and opening deck lids in Rusty's recent post yesterday under deck lid screen.

If you even drive (or even worse, park) in the rain you will want to find a way to keep the water from going down your carb.

Rain hat for that one!

Scott, going with duals will be a BIG improvement.

Just for info, I have a Chesil and have no cut out hole for the fan. It relys on intake from the grill as a VW.