yertyat

does anyone know if the fan shroud duct needs to be vented? and if so, why?

Makmov

That used to go to the rear heater blower, however; it does do a lot for cooling the engine so I don't think you want to block it off

Shamus964

Yep - what Makmov said - it releases heat captured by the plastic shroud atop the motor. I've seen the rubber boot replaced by a piece that has a screen on it and that's what I'd use if I didn't want to run it into my heating/cooling system.

yertyat

so the purpose of a screen or a vented block off plate is for engine cooling when the engine is off?

crg53

The screen is there for 2 reasons.

1: To let the hot air out when engine is running.
2: To prevent your fingers or any other object to get in contact with the fan.

I would just drill a bunch of 1/4" holes in the plate you have in there now.

yertyat

ok, i assumed that the fan blows cool air over the top of the engine and out the bottom.

FeralComprehension

It does....

Makmov

It does, but a fair amount goes out that duct too. Anything you can do to help keep it cool and a good constant airflow will help.

And I don't know for sure but, knowing Porsche, they have probably exactly figured out what airflow was required to make it live. If you close off that duct you probably, and perhaps significantly closed off a percent of the airflow, which is very important on these cars, especially on the 964/993 where they were pump out much more HP and ran higher compression, which equates to heat.
It is one of the reasons they did go to water cooled.

Some people dont think that fan does a whole lot but I can tell you what happens when it stops and you keep driving it. You cook the engine, and melt all the plastic bits off.

Gus

kind of opposite of what I consider would be reasons to block off - If the hot air passage ( fan duct) is not going to be used for the purpose of moving ( hot or warm) air to the cabin for heat then block it off and use the air to cool the engine. By blocking it off you get more air flowing over the aluminum fins on the cylinders in the engine area and not being blown out in the engine bay as a wasted cooling air. It accomplishes noting going out the fan duct. Since the blower has been removed there is no longer a cool down air flow after shut down.

ALEX P

I figured that the holes are there to keep an even spread of air over both sides of the engine so if it is blocked off completely, presumably there would be more air going over the left bank than the right - does that make sense or am I talking rubbish?

I've got to make up one of those block offs for mine!

Gus

Not sure I really understand why there are any holes in the block off - you are just pushing cooling air for the engine out into the engine bay and this serves NO PURPOSE at all. If you re going to remove the blower and not have any heater - then the best design is to remove the fan duct itself and replace with a fabricated piece of fiberglass that conforms to the design of the fan shroud so that the air is pushed back and down to the cylinders .

yertyat

hey, thanks for all the response. i have to agree with you gus. seems to me the more air flowing over the top of the engine the better. i've put over 1k miles on the car since i removed the heater bypass pipe and have not noticed any difference in engine temp. i have noticed a big difference cabin temp. since there is now no hot air being blown into the cabin. and thats a good thing these days! i still don't completely understand why the rennline design (as pictured above)is the way that it is, or the screen on the fan shroud of a lightweight c4. unless makmov and crg are right in that it allows for hot air to escape from the top of the engine even when all of the cooling air is forced down..

altarchsa

You're right Gus. Opening up that block off plate doesn't make any sense at all! You would just be allowing the hot air above the engine to be pushed into the engine compartment to be recirculated back over the engine by the alternator fan.

More importantly, you would be reducing the pressure created by the alternator fan to push cool air from the spoiler down over the cylinders. C'mon guys, this is a basic HVAC.

The real issue is to force cool air from the spoiler down over the cylinders to cool the engine when it's running, not to let hot air escape when it's shut off! This is the first time I've ever seen so much bad advice on Rennlist.

altarchsa

Hot air only goes out that duct to the exterior when the heater fan and ductwork to the crossover pipe below the engine are connected. My comments are based on the assumption that the heater fan and duct to the crossover pipe under the engine are not intended to be reinstalled.

Makmov

Not only, depends on if the system is all working and where the heater is set. Otherwise it blows out the wheel wells.