Crazy to try this -- or crazy not to try this?

[Jack Olsen]

I did an odd little project during the kids' nap today. My exhaust is off of the 911 because I had a chain tensioner fail (the chain did NOT jump, fortunately) and have been waiting for the parts to come in for the fix. In addition to the muffler, the air duct that runs around the rear-most part of the engine, just above the muffler, came off to make it easier to get to the chain box.

If you don't know the 993, here it is in a diagram. The part that's bugging me is #8 in this exploded diagram. It's made of cast aluminum and -- being a Porsche part -- it currently costs $871.



But the price isn't the thing that makes me crazy. 'Cause I've already got it. But...

It weighs a lot.

And its location (in the very, very back of the car) makes that weight even worse. Granted, this is partly racer's anorexia. But the thing is 8 pounds 13-1/2 ounces.



That's a lot to lose, if I could get rid of it. And because my 1972 911 doesn't run the catalytic converters that were in that general location for the 993, a replacement wouldn't need to be as durable as the original.

Please note: I am never put off by the idea of throwing Home Depot parts into my car. They ought to sponsor me, darn it.

So I went down and picked out a very lightweight combination of galvanized steel and aluminum ducting. Even with the high-temp adhesive and rivets holding the pieces together, the new weight was MUCH lower.



7 pounds 8 ounces lighter, to be exact.

The whole assembly I put together is 1 pound 5.5 ounces.

Does my part look as good as the factory original? No. Is it as stout as the factory original? No way; I think I could poke a finger right through it. Will it work, though? I think so. It's not exposed to much at all other than vibration and heat, since it's sheltered between the engine sheet metal and my muffler. It should be plenty heat resistant, by my estimation. And a steady vibration shouldn't do much to it.

But does anyone know the specific function of the heat shield (#11) in the diagram?

Here are the two set next to each other, now with some paint on my version of the part.





Am I stupid to try this? I don't see why Porsche made the thing has heavy as they did. If it can be replaced with lightweight ducting, then I'd like to do that.

I test fitted it, and it tucks away easily up above my Manaflow.

[Justin@Athens]

Hmm . I go to Lowes myself, but is that heat shield not just extra shielding for the timing chain cover area?

[bgyglfr]

Cool idea. If it ends up not holding up, you could always take your prototype to the local muffler shop and have them bend some material for you. You could probably even have the heat shield incorporated to that as well.

[Tippy]

Totally what I'd do.

[HawgRyder]

If heat becomes a problem with your design...you might try PEET tubing from the aircraft industry.
It is some type of flexible silicon based stuff...good for more than 450F I think.
Bob

[porsche930dude]

hey if its tucked up in there and you cant see it then great. I just wouldnt want people looking at it and saying what the hell is that. But if its hidden then no problem. I used those rubber tubing pipes that are for routing your exhaust fumes under the garage door for my heat ducting. Works great. Looks like you could have even stretched that pipe more to save even more weight. Did you use high temp tape? Foil tape is best for that kind of piping. especially if its gonna get hot

[mjshira]

wondering if that set up can handle the heat. have to imagine Porsche made the part as thick as it is for a reason?

[Jack Olsen]

Thanks for the input. I'll take a look at what that shield is doing. I can pretty easily make a mount for it independent of the duct.

mjshira, in the stock configuration there was a LOT of heat, since the dual cats were there. But I don't run cats. And my thinking is that aluminum's melting point is pretty high, no matter what its thickness is. We'll see.

[mjshira]

Cool. Good luck Jack.

[Jack Olsen]

Thanks, guys.

The project brings up a question I don't have an answer for. I've always believed that there were two reasons for air circulating through the heat exchangers. One (of course) is heat, but the other is that there's a supplemental cooling effect from the air that's going through. I mean, the thing is set up so that there's air being blown through the exchangers when the heat and the real blower are shut off -- with the hot air spilling out of the heater valves. And I believe there was a certain temperature where the computer would turn on the heater blower when the engine was hot enough. I assume this means that it wouldn't be a good idea to simply remove all this stuff -- or to simplify it so that I was only feeding one exchanger and only heating on the driver side.

Does anyone know if I'm right about this? I know some guys run swapped engines without heat at all, and I've always suspected that they might be paying a price in terms of cylinder head temps or even engine oil temps.

[tirwin]

Jack,

You're probably aware of these guys already but I thought I'd post a link. I just bought the SCAT ducting and some clamps from them to replace my ducting in my '83.

Pilot Supplies and Aircraft Parts from Aircraft Spruce

SCAT can withstand temps "greater than 550°F". I guess the question is... Is that good enough?

[Jack Olsen]

The adhesive I used says it's good to 2,000°. Aluminum's melting point is 1,221°. In fact, the lowest melting point I can find for any aluminum alloy (admittedly, only from Googling it) is 720°. I would think the paint on my fiberglass bumper would have problems at a lower temperature than that?

Next time I've got the muffler very hot, I'll try to hit it with the IR thermometer.

[Arctic Rat]

I suspect the aluminum mass of the original piece helped to scavange some additional heat from the muffler dirrectly under it to kind of "pre-heat" the air before going into the heat exchanger to "heat up" that is after all your cabin heating system correct? Might provide a more even heating of the air after you drive "x" amountof miles and heat up the muffler and cast aluminum piece. OR it could just be an over engineered P.O.S who knows

[Lapkritis]

Originally Posted by Arctic Rat

... OR it could just be an over engineered P.O.S who knows

Ding!

[Jim2]

Jack, best as I can tell that heat shield protects the chain case cover (rubber gasket) from baking due to being very close the cat converter. Remember that the right bank of cylinders and chain case sit farther rearward than the left bank of cyl.

The cast alum crossover mates directly onto the cat converter along it's entire length. My observation is this was done because the 993 heat exchangers are far shorter than the earlier 964 and Carrera models so added heat was drawn from the cat converter.

The heat fan operation - god knows whats going on. The electric fan in the engine bay draws outside air through the engine grill and the dump valves discharge below the engine sound tray so I doubt the system has anything to do with cyl head temp. Recall how the heater valves on the older cars all vented to atmosphere when there is no demand for cabin heat, essentially the same as the dump valves under the 993 sound tray. With EGTs running 1000F+ while cruising and 1500F+ at wide open throttle the headers are cherry red inside the exchanger housings. I suppose without a constant flow of air through the exchangers a demand for heat would get the driver jumping about if they had a 1500F slug of air come out the heater vents.

[scarceller]

Originally Posted by Jack Olsen

Thanks, guys.

The project brings up a question I don't have an answer for. I've always believed that there were two reasons for air circulating through the heat exchangers. One (of course) is heat, but the other is that there's a supplemental cooling effect from the air that's going through. I mean, the thing is set up so that there's air being blown through the exchangers when the heat and the real blower are shut off -- with the hot air spilling out of the heater valves. And I believe there was a certain temperature where the computer would turn on the heater blower when the engine was hot enough. I assume this means that it wouldn't be a good idea to simply remove all this stuff -- or to simplify it so that I was only feeding one exchanger and only heating on the driver side.

Does anyone know if I'm right about this? I know some guys run swapped engines without heat at all, and I've always suspected that they might be paying a price in terms of cylinder head temps or even engine oil temps.

Yes the heat exchangers do provide extra cooling even when not using the heat. When heat is off the heat flapper valves dump the extra heat just above the front of the transmission. The 84-89 cars have a heater control relay that actually activates the rear blower if engine temp goes above a threshold. This causes a lot of cooler air to circulate through the exchangers and help cool the bottom of the engine. So you are 100% correct to leave this feature in place.

Even the earlier cars always had air flow through the exchangers, this was provided air from the main engine fan.

[ClickClickBoom]

The 64K question, why would the good Dr's engineers design such a piece? Determine the reason and the answer is close. I would be reluctant to change anything to do with cooling airflow without solid reasoning. One of the problems with the heat exchanger system is that the exhaust is shielded and unable to cool itself, hence the airflow through the tube you so want to reduce in weight. Almost every corrosion resistant material becomes reactive at elevated temps, stainless, Ti ect. Why not extrude hone the tube to reduce the weight?

[cgarr]

Originally Posted by tirwin

Pilot Supplies and Aircraft Parts from Aircraft Spruce

SCAT can withstand temps "greater than 550°F". I guess the question is... Is that good enough?

I use a lot of this stuff both cars and plane pricy but works.

[Elombard]

I would consider getting an exhaust pipe 2.5" "Y" and plumb it with the scat tubing. Actually the "Y" from a 3.2 heater might work there? I like your solution but I am not sure it will hold together?

Or You could do the hidden heat trick where you plumb off the engine tin at the front of the motor?

I think the heavy factory piece is one of those compromises that Porsche had to make for a luxury sports car and you will have minimal issues losing it.

[GaryR]

Dump the whole mess and put headers on it!