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ALEX P
 
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Vent holes & grill in rear

My '85 has been dissassembled for a while now to fix rust repairs and repaint. When I took the rear reflecter off the car I discovered that it had been attached with glue

I am reluctrant to put it back on and am thinking of replacing it with a simple grill/mesh, something along the lines of the images below.







The problem is that I do not want a grill with no purpose or function so was thinking of opening up the rear panel with an additional couple of holes:



And then drilling a series of vent holes through the rear crossmember. Maybe something like:





I have seen similar done to 964s such as the DP Motosport one below:



Drilling through the crossmember itself doesn't look especially easy as it isn't an entirely flat surface/box section but and I would focus the holes fairly centrally near the fan.

Just wondering - Has anyone done this? I'd love to see some photos of what drilling and cutting you have done if so.

Old 04-14-2016, 05:40 AM
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I wonder how much hot air the fan would draw from the exhaust system via these holes in the rear sheet metal? Ambient air is cooler from the engine lid.

Sherwood
Old 04-14-2016, 09:31 AM
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+1 I agree with Sherwood especially while idling or slow speed.

Also, If you are running with a tail, such as a whale or duck, then you have a natural pressurized air intake (I.e. Downforce) which you want to force thru into the fan housing without any air escaping thru all those holes.
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Old 04-15-2016, 07:19 PM
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FWIW, on my 77 930 I did something similar, but drilled five 2" holes in the horizontal removable piece below the rear reflector "box" and the five more 3" holes in the rear sheet metal to vent the heat from the turbo. I fitted stainless mesh behind the holes for a more finished look. Before doing this, I cooked part of my rear fiberglass bumper. The heat that moved through the holes did not enter the engine compartment.



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Old 04-15-2016, 10:02 PM
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Quote:
Originally Posted by mmastro View Post
FWIW, on my 77 930 I did something similar, but drilled five 2" holes in the horizontal removable piece below the rear reflector "box" and the five more 3" holes in the rear sheet metal to vent the heat from the turbo. I fitted stainless mesh behind the holes for a more finished look. Before doing this, I cooked part of my rear fiberglass bumper. The heat that moved through the holes did not enter the engine compartment.



You didn't mention how you confirmed heated exh air isn't drawn into the fan. One way Is with a thermocouple positioned at the fan inlet with and without the holes blocked and at various veh. speeds. A red hot turbo would provide worst case results, and hot air radiated by the intercooler only adds to the cooling load. Any cooling air heated by the exh. system is counterproductive.

Sherwood

Last edited by 911pcars; 04-16-2016 at 01:14 AM..
Old 04-16-2016, 01:08 AM
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I wasn't as concerned with the conducted heat coming off of the inner vertical wall of the rear reflector box and into the engine compartment as much as I was concerned with allowing the hot air flowing through the rear box itself to escape rather than stay in the box to cook the paint off the rear panel and also burn my rear F/G bumper. The conducted heat from the box would raise the temperature of the engine compartment air from a physics POV, but that proved to be minimal as the oil temp never reached over 200 deg in this configuration (at idle in traffic or on the race track) and the paint no longer cooked/bubbled off the rear panel. The F/G bumper didn't burn either after that.
Although I didn't measure the fan inlet air with a thermocouple before and after the changes, the real-world results I experienced for my particular situation satisfied me. I can't do any further testing on the car b/c it sold 3 years ago. I thought I'd offer the OP an additional option to do something with the unsightly, reflector-less rear panel. I'm obviously not a physicist.
Old 04-16-2016, 08:33 AM
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An additional note, The difference between my configuration and the OP's is that I did not drill holes in the inner vertical (engine compartment side) panel of the rear reflector box as that would directly allow hot exhaust air into the engine compartment/fan inlet, effectively rendering the original engine compartment/tin gasket useless in its function of reducing the amount of heat allowed into the upper engine compartment from the exhaust side.

Last edited by mmastro; 04-16-2016 at 08:42 AM..
Old 04-16-2016, 08:40 AM
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Quote:
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An additional note, The difference between my configuration and the OP's is that I did not drill holes in the inner vertical (engine compartment side) panel of the rear reflector box as that would directly allow hot exhaust air into the engine compartment/fan inlet, effectively rendering the original engine compartment/tin gasket useless in its function of reducing the amount of heat allowed into the upper engine compartment from the exhaust side.
Additional details. Better. More helpful to others contemplating adding sheet metal openings.

Thanks
Old 04-16-2016, 09:25 AM
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Thanks for the replies guys, interesting stuff.

I did also consider putting the holes in the exhaust heat reflector plate (similar to Mikes above) as my silencer is mounted higher than usual but couldn't work out if it would be beneficial or detrimental.

The holes would allow some flow to let hot air out which is clearly a good thing however the reflector plate is there (presumably) to encourage the heat to vent underneath the car instead of potentially in an area where it will then be drawn straight back into the engine bay?
Old 04-18-2016, 05:38 AM
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Thanks for the replies guys, interesting stuff.

I did also consider putting the holes in the exhaust heat reflector plate (similar to Mikes above) as my silencer is mounted higher than usual but couldn't work out if it would be beneficial or detrimental.

The holes would allow some flow to let hot air out which is clearly a good thing however the reflector plate is there (presumably) to encourage the heat to vent underneath the car instead of potentially in an area where it will then be drawn straight back into the engine bay?
I'm not aware of the exhaust heat reflector plate on these cars. Would that be the typical sheet metal engine surround or something else?

Radiated heat from the exhaust system isn't healthy for air cooled engines. Here's one method to prevent (more) license plate panel paint from bubbling:





The rolled sheet metal could be continued further over the top to help reflect heat away from the engine. Vent holes in the rear body work (without a path into the engine compartment) also allows hot air to escape from this area.

Sherwood
Old 04-18-2016, 09:38 AM
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Quote:
Originally Posted by 911pcars View Post
Vent holes in the rear body work (without a path into the engine compartment) also allows hot air to escape from this area.

Sherwood
Like this?:



This was done to vent muffler heat which was building up with track and a/x use. Seemed to help some.
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Old 04-18-2016, 10:01 AM
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Interesting thread. I've never been aware of any sort of bottleneck for airflow to the engine fan with the stock deckled grille. I have always assumed holes back there were to improve under-car airflow from getting scooped up by the rear bumper.
Old 04-18-2016, 10:44 AM
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I guess I've always thought of the rear of our cars as having quite a parachute affect in comparison to more modern cars with undertrays so maybe giving the air a bit more of an escape path whilest helping cool the exhaust is the way to go.

On top of this the plate above the exhaust is a whole lot more replacable if it all goes wrong or doesn't work. Drilling the rear crossmember is a bit less reversable!

What I'll do is put some holes through the panel and some heat reflective material on the exhaust side.

The other smaller vent will probably be adding a grill/mesh where the licence plate sits and mounting the licence plate onto this. Hopefully the plate won't melt or go up in smoke at the first set of traffic lights!!



Old 04-19-2016, 05:36 AM
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Here is what I did with mine to protect the fibreglass bumper from heat.

Just a layer of this self adhesive stuff from the hardware store.
No heat issues with paint or fiberglass, and that even includes quite a few miles before I finished and installed my muffler tips to redirect it out the back.

Having said that, I like your idea of removing heat.

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Old 04-19-2016, 09:01 AM
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I would also keep the pressurized air in the engine compartment and force it through the engine for the cooling.

I did some cooling vents for the hot muffler in the rear bumper. Im trying to keep the appearance clean, thus this kind of solution:


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Last edited by Jaskas; 04-21-2016 at 12:23 AM..
Old 04-21-2016, 12:21 AM
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GT3 Rear Vent?

So does anyone know what purpose the vent on the GT3 served?
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Old 04-22-2016, 08:57 AM
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ALEX P
 
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That's quite a neat solution as ever Jaskas. Must have been fun cutting the ally bumpers, they're really quite thick!!
I also coated the inside of my bumper with space suit so will do the same again but cut out the numberplate section.

Acme911 - No idea what that vent is for, good spot though. Maybe it is also vent for the exhaust?
Old 04-22-2016, 10:07 AM
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Did a little more digging and found this site and after market product:
https://www.horsepowerfreaks.com/moshammer-rear-bumper-air-vent-porsche-991-turbo-%7C-turbo-s-14-16-p-151359562.html

Explanation:
"The Moshammer Porsche 991 Turbo rear vent is taken straight from the Porsche GT3 to vent hot engine air from the vehicle. Heat soak is an issue in any vehicle and Moshammer knows how to give your Porsche 991 Turbo the best in styling and functional components. With air flow coming in from the top of the decklid, this rear vent also helps pull away the hot air that can collect. "

Wonder if this is Marketing hype or mirrors the reason Porsche installed grill on original GT3?
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Old 04-22-2016, 10:46 AM
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Very interesting.

No reason why the 991 is any different in principle to an air cooled car as far as venting the engine is concerned.

So is that back to square one then or just accept that either route may have some marginal benefits?

Stumbled across this old thread but it doesn't really give any definitive answers

Removing rear reflector - cutting airholes
Old 04-22-2016, 11:22 AM
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Very interesting.

No reason why the 991 is any different in principle to an air cooled car as far as venting the engine is concerned.

So is that back to square one then or just accept that either route may have some marginal benefits?

Stumbled across this old thread but it doesn't really give any definitive answers

Removing rear reflector - cutting airholes
Does water cooling come with this kit/procedure?

Sherwood

Old 04-22-2016, 02:08 PM
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