words by pablo deferrari

Picture
a close up of the throttle body showing you the larger air hose in the middle that goes to the air/oil separator bottle flanked by the two coolant hoses.
I recently had a discussion with a friend about the 968 and its fascinating engineering details when the topic turned to the heated throttle body found on this car and the 944 S2 that was designed to prevent the throttle body from icing. pretty cut and dry, right? 

well, because I never found an "official" statement from Porsche discussing this design in more detail, I decided to poke around and see what other enthusiasts thought.
the search revealed a thread on Rennlist simply titled "968 engine: why is the throttle body heated?" here's what some of the members had to say after a member named "mmmbeer" asked the question...

mmmbeer:

"As shown in this picture the 968 throttle body have two water-hoses connected to it (the center one is an air hose). 

This heats the throttle and intake manifold to engine operating temperature. I know that it is not optimal for performance, but why is it heated? 

Maybe to make the engine more stable during all outiside temperatures? But that mean you will not get the advantage of cold intake air.

We have bypassed this on both of our 968 engines and experienced no problems, the advantage was a cool intake manifold (during driving).

Any suggestions to why it is heated?"

951Tom:

"In colder climates the throttle can freeze up and stick open."
Picture
Steven Wade's CS' engine bay showing the location of the throttle body from afar...

scular:

"From my understanding it is to give a better vaporization of fuel lowering emissions and increasing fuel economy. Personally I don't know of any major gains from this, but I am a Floridian (anything below 75F is near freezing)."

mark parker:

"I'm speculating, but I think Tom is right. The throttle plate is the point where relatively high pressure (atmospheric)air becomes low pressure (manifold)air, and at any time air changes pressure states,(from high to low) heat will be lost. If the air which is losing heat contains moisture, the now colder air will condense the moisture out, and it will form ice crystals which could bind the throttle plate. You can see the same phenomenon if you remove the valve core from a tire. As the air goes from high to low pressure, frost will form on the valve stem. I've seen it on 90 degree days."
Picture
the components of the 968's fuel injection system.

triscadek:

"Venturi effect. 

I had an Accord that the carb. would ice on below 45'on damp days and the car would just die, then the heat from the engine would melt the ice after 20-30 min. and then it would run ok."

jim968:

"I dunno about newer stuff, but from what I've read, older piston engine aircraft often have a provision for carburetor heat; apparently the pressure transition in the venturi is enough to cause frost to form in the throat(as mentioned above). On a humid day, this can actually build up enough ice to block the flow of air into the engine. Typically happens on climb-out after takeoff, sucking lots of thick, wet air at full throttle. There have been some crashes caused by the pilot forgetting to turn on the carb heat. 

I've seen frost on the outside of an air-cooled VW intake... cool, humid day, cold engine, headers that had no provision for carb heat.

In the case of our cars, though, I think Tom L. has it right... keeps the throttle plate from freezing up. 

Jim, "Honest Officer, I didn't mean to be going that fast! The throttle plate was frozen open!"
Picture
view of the throttle body and its connections on a 944 S2

M Danger:

"Yes, Most piston aircraft have carb heat. We (in TX) always pull carb heat on decent to landing(never on take off) And the rpms drop approximately 200 rpm. So bypassing the Heated TB may have some benefits. all the other 4's dont have them, not sure about the 928s."

Roy LaZalle:

Iceing is real. I had it twice in the same spot traveling from Sun Valley ID to Mountain Home. The last time it was in a Ford van. My guess is that it was a combination of the temperature, altitude, gas with water in it and a missing intake air pre-heater hose. After stopping for he second time to let the thing thaw out. We cleverly made a hose from a Coors cold pack and never looked back. (no doubt the best use for a Coors product I have heard of) --Roy--

Danno:

I wonder why only on the 968 though? Seems like a complicated way of doing something that can be easier and simpler with a pull-pull throttle cable.

Dan Pultz:

The 944S2 also has the same coolant lines. The lines look large, but the flow is probably not very much because there is only a small size hole in the throttle body casting.

Zoxx2004:

it was actually used to speed up engine warm-up.US EPA requirements were getting tougher late 80,early 90's .its was used to pass emissions.
 


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