Heat exchanger 101

[beaker]

Are there differences between SWB and LWB heat exchangers? Do the exchangers differ from a carb car to a car with MFI? What are the differences if any? This question is specific to up to 1973 911's.

Beaker

[sithot]

I believe the early 911 heat exchangers were the same up until the 911-S appeared.

They were prone to cracking and Porsche had a "campaign" to replace them for obvious reasons. Of note is the fact that you could update your car to "S" style heat exchangers for a small up charge or get new original style for free. There was a horsepower increase according to the bulletin. The standard exchanger had step-downs (for lack of a better description) which the S did not. I cannot imagine why anyone wouldn't pay this additional charge to get the better exhaust.
These early heat exchangers had NO take off provisions for anything in the engine compartment.

In 1969 the MFI cars had a heat supply take off for the pump (A). This utilized a heavy duty (steam type) hose that routed off the back of the heater box on the left side. It connected to a metal tube which passed through the left side sheet metal attached to the cam tower. Another paper type hose connected on the top side of this tube to feed heat the pump thermostat. The purpose was to control the mixture. A thermostatic assembly on the pump moved the rack.
This take off was carried throughout the entire run up until 1974 although it was blocked off on CIS cars. Carburetor cars used the old style exchangers. As for current production I don’t believe you can get anything from Porsche for the left side that doesn’t have this attachment. SSI does make them with and without. IMHO, SSI still makes one of the finest aftermarket products for our cars to be had.

In 1972 they added an additional chamber on the left side (B) that was used to move pre-heated air into the intake. One end of this chamber was an intake and the other connected to the pre-heat assembly via a heater-type hose. FWIW; the sheet metal on the left side for a ’72-’73 car is probably the hardest to source if you are going for a 100% original setup. The associated pre-heat valve assembly is not easy to find either.

I've attached a photo of the pump thermostat heat for all MFI cars and intake pre-heat (’72-’73)

Many of the pre-heat assemblies have survived but many are gone. My ex-'73 did not have one. According to John Daniels at SSI this was something that he did not incorporate into his design as Chuck Stoddard nixed the idea. Apparently there was some concern over ingestion of foreign matter into the engine through this intake. Some time ago Roger Grago and I had a conversation about Porsche recommending removal of this assembly in a technical bulletin but he couldn’t recall when it was published.
The 2 drawings are of the left side exchanger. The one with (A) & (B) marked is the '72-'73 style with the additional "heat chamber" for intake pre-heat.
(A) in both photos is for the heavy duty rubber (steam type hose) for the MFI regulator.
The left side "A" exchanger was used all the way up to and including 1974 models although the heat tube will be plugged on CIS cars.

As for availability I’m not sure that the 100% correct left side ’72-’73 exchanger is made any longer although Stoddard has a part number for it (911-211-021-14) It is quite expensive.

YMMV.

Best,

Tom

[beaker]

Sithot -
Thank you for this very detailed explanation w/photos to boot. This is why I enjoy this site so much.

Beaker

[HughH]

Hi Tom
excellent explanation

I was surfing through a parts supplier's site in Europe the other day and saw Dansk 72 /73 LHS exchangers for sale - at a significant premium to the standard MFI one

Reading your explanation I wonder why anyone except for a restoration to concorse standard would want to buy one
the link, for those who want to window shop, is
http://cgi.ebay.com.au/ws/eBayISAPI....=STRK:MEWAX:IT

[sithot]

That is a correct left and yes, it is expensive compared to the '69-71 style.
The intake (C) does get its air from the engine compartment.

Attached is a photo to illustrate the additional intake pipe for the regulator housing. (911.108.015.00 72-73 only) This is attached to the intake on the chamber shown at (C) via a section of steam hose similar to what comes off the rear for the MFI pump. These hoses must be able to withstand the temperatures of the exhaust so they are robust in nature.

The rear tray assembly for the '72-73 IS different since as the pre-heat hose is piped from (B) through it in 72-73 only.
Many of these will have been blocked off with piece of metal if the pre-heat system is removed. Otherwise you are looking at the ground!
My ex-’73-S had what appeared to be a factory type plug although it could may have well been a well engineered can because it was cut and slipped over the hole in the tray perfectly!

Number 13 in the diagram is the sheet metal used on all cars from 69-71 with MFI. Look at the photo and take away the intake pipe for the '72-73 cars and you have the correct part (901.106.083.02) for a '69-'71 car.

Good luck!

Tom

[beaker]

So, am I correct to decipher my spare exchangers to be 1969-73 MFI?

Beaker

[sithot]

The left one has the cap in the output for the pump thermostat (previously shown) which you can remove for MFI and install the proper hose to the fitting on the left side sheet metal. It does not have the extra chamber for pre-heat but if you are not utilizing that (as seen in the photo) it's fine.

These will also work on any car from 1965 to 1974 with carbs or CIS.

Tom

[sithot]

The "Steam Hose" that Stoddard sells. This one runs from the heater box to the tube that passes through for the MFI thermostat. At the top side of that junction it utilizes a paper/foil hose (different part) as seen in the previous photos. Whatever you utilize on the bottom side remember that it has to be able to live in a very harsh environment.

Tom