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Kevin D · Joined: 28 Jul 2005 Posts: 152 Location: Boise, ID

Can somebody direct me to discussions on air flow mods to the radiator? I've done a few rudimentary searches and found some great discussions on everything but. ..
I have a 931 vented nose and was wondering the consequences of redirecting that air directly to the radiator. It currently just blows into the engine compartment. I currently have the sheet metal off and the mod looks pretty straight forward, but it would not be easy to reverse if it proved to be counter productive.
Currently I run with the needle at 1/2 temp on cool days, 3/4 on hot days (w/ AC). I was hoping to drop that average a bit.
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'88 924S 3L 6sp RedRat
'92 Duc 900ss
'02 Duc 998 Carbon
'72 RD 350

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

!tom · Posted: Tue Jun 11, 2013 2:10 pm Post subject:

I'm guessing the area of the opening of the baffle should be similar to the area in the radiator that air can travel through, rather than the frontal area of the radiator.
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78 924 NA
5-lug

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

The opening should be SMALLER than the area of the radiator thru which air can pass. I didn't intend to include the end tanks as part of the frontal area, as they are pretty much blocked by the chassis anyway.

This picture sort of illustrates it:

In the above pic, you can see that the bumper obstructs the top half of the rad's heat exchanging surface area. What I would do is construct a baffle that would come from the top of the rad right down to the trailing edge of the bumper. The bottom surface would come from the bottom of the rad out to the trailing edge of the chin valance. And then I would slightly angle the two side pieces to make the opening narrower, as depicted in the Max Boost diagrams above. The outsides of the side pieces would need to be blocked off to prevent any air from going that way, forcing most of the air to be pulled into the inlet or up over the hood. The air is going to take the path of least resistance, so if there is a low pressure zone created by the inlet, that's where most of the air on the frontal section of the car is going to get "sucked" into. A splitter or additional length on the chin valance would probably aid this even further.

Another way to put it is to emulate the opening that they created on the DProd with the "cow catcher" scoop up front. You can see in the following photo how much smaller this opening is than the stock opening above. The chin valance on this setup creates a natural shroud for capturing the air, but more still could be done on my particular car to seal the opening all the way from the valance back to the radiator.

Gary Ambrus had such a setup fabricated for his race car along with a lowered chin valance, but I unfortunately do not have any photos of it.
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erstwhile owner of just about every 924 variant ever made

Kevin D · Joined: 28 Jul 2005 Posts: 152 Location: Boise, ID

Thanks Ideola!
So I'll focus on fabricating the side baffles (top baffle already exactly as you described oem) and building a chin for the beast. That will be my next thread... a search for good home built chin/splitter designs.

Follow up question... I do have the OEM belly pan installed, but I have no hood venting. Any concerns with running the 931 nose without the 931 hood (which pulled even more air in right?)? Should I add some venting in the hood close to the firewall to allow some spillage? Or will that belly pan suck enough hot air out the bottom?
I do have a 931 hood laying around, but then I would have to do some painting

.

A couple other details:
It is a '92 3L NA, not a turbo
The exhaust is taped
Original baffle behind bumper that runs to top of radiator intact
Lower temp switch installed
951 radiator installed with tight shroud
AC condensor still there
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'88 924S 3L 6sp RedRat
'92 Duc 900ss
'02 Duc 998 Carbon
'72 RD 350

Kevin D · Joined: 28 Jul 2005 Posts: 152 Location: Boise, ID

BTW - that flat-black euro 931 looks very nice. More pics?
_________________
'88 924S 3L 6sp RedRat
'92 Duc 900ss
'02 Duc 998 Carbon
'72 RD 350

!tom · Posted: Tue Jun 11, 2013 3:27 pm Post subject:

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

@Kevin
Yes, I think you need to consider some means of extracting the additional air pressure that you are now allowing into the engine compartment. I'm not sure the NACA duct will do that, as it's more designed to draw air in, although some have hypothesized that it also is designed to vent air out while at a standstill. In your case, it is the high pressure air that is put there at speed that you need to extract, and 931 NACA duct hood isn't the right tool for that job.

I'm not familiar with the undertray on the late 944. Does it have louvers, or is it a solid piece? If it's a solid piece, then I think you need to add some extraction vents (louvers) somewhere: either on the underbelly or on the hood. Or you could go really radical and build a monster extraction vent for your stacked AC condensor / radiator similar to the one Bass GT built for his front mount intercooler:

Note the lip at the leading edge of the extraction vent in the above pic to create a low pressure effect for aiding in the extraction. Steve's vent mimics the one used on the rare 968 turbos, as well as famous race cars like the GTt40. I've seen this approach implemented as well on the frontal heat exchangers (oil and air units) on 911s.

More pix of my 931 Club Sport here with build thread here.

@!tom
Yes, I think you're on to something there.
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erstwhile owner of just about every 924 variant ever made

v8carreragts · Joined: 05 Sep 2003 Posts: 665 Location: Tucson, AZ

Another area to look at for relieving under hood pressure is the sides of the front fenders between the door and wheel well. Something like the extractors used on the 2nd gen Trans Am (70-81) If you look at those, you will see that the leading edge is raised as well. This created a low pressure area at the opening helping to pull air out of the engine compartment.

It doesn't help much to increase the air entering the engine compartment without adding a place to let the air out.

I have most of the front area open to the radiator on my car which includes the turbo front panel and the GTS front bumper. You would be hard pressed trying to get close to the area of the radiator on these cars. I also removed the cross member and replaced it with a new piece that does not block as much flow. The radiator will also be sealed to the opening. I also intend to build side extractors to aid in venting the engine compartment.
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84 944--924 Carrera GTS clone w/520HP Chevy 302 V8--RUNNING!
2015 AUDI Q7
2013 Dodge Charger R/T AWD HEMI
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