Purpose of tube in RR chassis: see pic

eapcpa · 03-26-2008, 07:25 PM

What is the purpose of the tube shown in this picture just in front of the oil filter. I am referring to the one on the frame rail in front of the filter and behind the fender brace hole. Pic is courtesy of dmcummins sorry I copied it to save myself the trouble. My engine bay is stripped and getting prepped for paint. Just curious.

Flieger · Flieger's Garage

Mine is like that, too. I don't know what it is for but now I am wondering. Maybe it is a hole for a line to vent fumes of some kind into the air intake? I have a hose attachment port on the back of my plenum.

I have a front mounted oil cooler (radiator style). The heater system is gone. I have a catch tank for oil overflow.

EDIT: the hole is hidden in front of and below the oil filter

Tyson Schmidt · 03-26-2008, 08:12 PM

The tube goes through the frame box and into the heater duct just ahead of the heater valve. What it does, is supply warm air to the engine intake anytime the heat is on. It's there to prevent carb icing in cold weather.

HarryD · 03-26-2008, 08:15 PM

It is a breather vent for the crankcase. Mine vents into the intake plenum of nmy engine.

It is part #38 in this picture:

HarryD · 03-26-2008, 08:17 PM

Quote:

Originally Posted by Tyson Schmidt

The tube goes through the frame box and into the heater duct just ahead of the heater valve. What it does, is supply warm air to the engine intake anytime the heat is on. It's there to prevent carb icing in cold weather.

Tyson,

I always thought it was a vent between my oil system and the air intake.

Flieger · Flieger's Garage

If it were to prevent carburetor icing, why would my 1971 911S have it? It came with MFI originally. The thermostat line for the MFI injection pump is from the other side's heat exchanger.

69 Soft Targa · 69 Soft Targa's Garage

Hi Harry. I don't believe the tube Ed is referring to is in your diagram.
In the picture it's to the left of the oil filter on the frame.

Quote:

Originally Posted by Flieger

If it were to prevent carburetor icing, why would my 1971 911S have it? It came with MFI originally. The thermostat line for the MFI injection pump is from the other side's heat exchanger.

A '71 911"T" would have carburetors.

eapcpa · 03-26-2008, 09:11 PM

Quote:

Hi Harry. I don't believe the tube Ed is referring to is in your diagram.
In the picture it's to the left of the oil filter on the frame.

That is correct. It is the tube mounted in the frame. My 70S targa has it with nothing connected to it.

Tyson,
If you are correct and I have no reason to doubt it, do you know when and if Porsche stopped putting it in the frame?

Tyson Schmidt · 03-26-2008, 10:05 PM

They stopped in 1972.

The '72 and '73 had an intake pre-heater with a thermostatically controlled flap for emissions reasons, so they didn't need the other one. The whole chassis changed that year as well.

As far as why the earlier MFI cars have it too, well, good question. Probably also helped with warm-up.

HarryD · 03-26-2008, 10:27 PM

Quote:

Originally Posted by 69 Soft Targa

Hi Harry. I don't believe the tube Ed is referring to is in your diagram.
In the picture it's to the left of the oil filter on the frame.

A '71 911"T" would have carburetors.

Ah-ha. Now I see, Thanks

eapcpa · 03-27-2008, 05:59 AM

Thanks for the info. So when the heat is on, warm air blows out of this tube into the engine compartment. I assume then, if I put a cap over this tube to stop the air flow, it will not have any negative consequences.

Willem Fick · 03-27-2008, 07:00 AM

I am soooo glad this thread happened - I was refitting my engine lid last night, and dropped a nut (what are the odds?!) down that exact same tube!

Gogar · 03-27-2008, 07:56 AM

Quote:

Originally Posted by Tyson Schmidt

T
As far as why the earlier MFI cars have it too, well, good question. Probably also helped with warm-up.

I imagine that they only made one chassis, so every car would have it; and maybe it would just go unused on the MFI cars. Thats my guess, anyway.

Tyson Schmidt · 03-27-2008, 09:06 AM

Quote:

Originally Posted by eapcpa

Thanks for the info. So when the heat is on, warm air blows out of this tube into the engine compartment. I assume then, if I put a cap over this tube to stop the air flow, it will not have any negative consequences.

No, there was a paper corrugated hose that ran from that frame rail tube up to an extra snout on the aircleaner trumpet.

john walker's workshop · 03-27-2008, 09:36 AM

rear defog also, on 912s at least.

eapcpa · 03-29-2008, 06:21 AM

Tyson

Quote:

No, there was a paper corrugated hose that ran from that frame rail tube up to an extra snout on the aircleaner trumpet.

Thanks, I kinda figured that was the case. I did not complete my thought obviously. I was just thinking in my car with MFI the hot air blowing in the engine bay when the heat is on serves no purpose unless it is already disconnected at it's source.