Why you should backdate your heat on a 3.2

[bill3667]

I'm anxious to do this too on my '88 Cab. The blower only blows it's fuse so it will save me the expense of replacing it. I too would like to keep the footwell blowers working.

We do get some unusual looks driving around with the top down on a beautiful 40 deg. day but the heater does it's job.

TIA,
Bill W.

[NOH20]

A tech article on this would be great. Parts are available through Parts Heaven or any Porsche "junk yard." As mentioned there are several ways to accomplish the backdate. For the Carrera's you'll need some pre '74 engine tin from the LH and RH side. The rear engine tin on your Carrera can be used but will need to be modified since you will open a hole in the tin when you remove the blower assembly. I had a piece of sheet metal welded on and then repainted the piece.

The LH duct is also pre '74 or you can use a 930 piece. John walker modified the duct I purchased from Parts Heaven and it turned out perfect. John forms the part to allow clearance to the dizzy. There are 2 different ducts you can use for the RH side. One is a one piece design that comes from very early cars and the other is the 2 piece that most people have used, both do the job.

The heater hoses can be purchased at any Aircraft store. I bought mine from Aircraft Spruce and had no trouble installing them. They can be found at www.aircraftspruce.com

I learned a lot doing the backdate and can't thank JP enough for his help. The engine bay is 100 times easier to access and the heat works great.

Brad

[jpahemi]

Brad:
Such a change from the day you recieved Ben's muffler and all the work that was ahead of you seemed as if it would never get done; now I see a whole different engine bay in less than 6 months. I remember your frustration when you embarked on this project; all that stress for nothing, great job!!!!!
Regards,
J.P.

[mjmoran]

Now, back to the subject on the 993 resistor to allow the footwell blowers to continue functioning...who has this info? I had done this backdate a long time ago on my Carrera and have really enjoied it. It would be nice on colder days to have the footwell blowers working to keep the heat more consistant (the rear engine blower never was a good plan though) IMHO.

Cheers,

[BeauBlues]

I remember seeing a thread where someone had heater hoses running from the bottom of the engine oil cooler. Can somebody inform me more on this, and is it any good?

[KFC911]

Originally posted by NOH20
...For the Carrera's you'll need some pre '74 engine tin from the LH and RH side. The rear engine tin on your Carrera can be used but will need to be modified since you will open a hole in the tin when you remove the blower assembly. I had a piece of sheet metal welded on and then repainted the piece.

The LH duct is also pre '74 or you can use a 930 piece. John walker modified the duct I purchased from Parts Heaven and it turned out perfect. John forms the part to allow clearance to the dizzy. There are 2 different ducts you can use for the RH side. One is a one piece design that comes from very early cars and the other is the 2 piece that most people have used, both do the job....


Brad

As Brad stated, there are different ways to do this, but for simplicity, you don't have to mess with the RH side at all on a Carrera if you don't want to. His technique requires replacing both LH, and RH air ducts, and replacing (or modifing) 3 pieces of engine tin. IMO, it is much better (on the Carrera's) to simply replace the LH air duct (with the 930 part, Widebody's fiberglass piece, or a JW modified 911 part) only and route a single hose from it to the pipe protruding through the left side of the Carrera's rear engine tin.

[84CAB]

As Brad stated, there are different ways to do this, but for simplicity, you don't have to mess with the RH side at all on a Carrera if you don't want to. His technique requires replacing both LH, and RH air ducts, and replacing (or modifing) 3 pieces of engine tin. IMO, it is much better (on the Carrera's) to simply replace the LH air duct (with the 930 part, Widebody's fiberglass piece, or a JW modified 911 part) only and route a single hose from it to the pipe protruding through the left side of the Carrera's rear engine tin.

Yep thats it...

Now, back to the subject on the 993 resistor to allow the footwell blowers to continue functioning...who has this info?

I have searched the archives more than once, if someone knows they haven't let us in on it.

[Rot 911]

Originally posted by 84CAB
Yep thats it... I have searched the archives more than once, if someone knows they haven't let us in on it.

Actually I have done a bit of work on this problem and finally came up with a solution. First I tried a ceramic resistor which failed in just a couple of days, you can read all about the installation here: Heat backdate and footwell blower

And the failure of it and my next idea here: Ceramic Reisistor Failure

I installed the aluminum housed resistor in the same place as the failed ceramic resistor. The first one I installed was a 50watt, 50 ohm resistor which didn't work very well. The footwell blowers would only come on for a minute or so and then go off.



Then I read this article by Bill Gregory Bill Gregory Tech Article regarding the backdate of a 964 and he recommended a 51 ohm 15 watt resistor.

Then it got warm out so I haven't actually switched out the 50 watt resistor for a 15 watt. I have no reason to believe it will not work. Here is where I purchased my resistors: http://www.action-electronics.com/resist10.htm

Did some digging and found the 993 resister #.

part # 993 616 521 01

My local dealer wants $61.15 for it.

I only did the left side backdate on my Carrera bcause I'm using the stock heat exchanger crossover pipe ("Y" shaped pipe at the front of the motor).

[widebody911]

I'd be more than willing to host a 'howto' page on backdating heaters.

[chgrec]

Originally posted by widebody911
I'd be more than willing to host a 'howto' page on backdating heaters.

Thom, that would be great for us rookies.....(and a perfect fit for me since I seem to spend a lot of time at your sites anyways....)

Are you still selling the LH side duct? If so, let me know....

Thanks
Chris
chgrec@yahoo.com

[jmarenas]

Brad, I have gone through 3 tanks of gas since I saw you, when are you going to grace the streets??? John

[KTL]

Originally posted by Superman
I've seen no more dramatic transformation than mine.

Before:




And After:



Plus, I like to tinker, and having the blower, a mile of hoses, and A/C compressor and condenser and hoses and brackets out of the way........priceless.

That's because the exposure in your first picture sucks.



Just messing with you Super.

Thanks to all for the great backdate advice. I think i'll pursue the wiring at the rotary switch. I don't feel like playing resistor games with the nimrods at the local parts store.

[Mysterytrain]

Kurt,

ok, lets talk about those resistors..50 ohms is 50 ohms...15 watts and 50 watts is a whole different ballgame. I installed footwell blowers on my SC and did some calculating to find out how high a wattage resistor I needed to use to control the blower speeds, low-med-high. I found out that the blower had a resistance of about 1.5 ohms. I also found out that the factory used a relay to send 12 volts to the footwell blowers for the high speed setting..why? ..well because the motor can draw almost 10 amps (based on 14 volts) or 13.5 amps (based on 12 volts)...that turns out to be a lot of heat ( 196 watts). On my setup I felt that full voltage applied directly to the footwell blowers made them run too fast and made them too noisey. So I have a 1 ohm aluminum cased resistor in series with the motor that is rated a 100watts..slight over kill but it doesn't get too hot to touch. Ok, back to the auto heat issue...which I have little first hand knowledge..but my suggestions would be: go with the 50 watt 50 ohm resistor, If it gets too hot go with a higher wattage; add the second resistor as Bill mentions in his link to trick the computer ( this might be why your first attempts failed, and lastly try to locate the resistors away from any addition heat.

[chgrec]

Love the new look & the easy acccess.....I was able to change out the plug wires & actually see the engine.....Nice duct Thom, it fit like a charm...

Gotta love this board....




Chris

[widebody911]

Originally posted by chgrec
Love the new look & the easy acccess.....I was able to change out the plug wires & actually see the engine.....Nice duct Thom, it fit like a charm...

Glad you liked it!

Now you just need to remove that big hunk of metal and hoses to the right of the fan...

[chgrec]

Sorry, it was 90 here in Atlanta on Friday....even with the top down, in this traffic you have got to have A/C.....

Chris......

[Tim Hancock]

My '79 SC does not have footwell blowers. If I remove the fan in the engine compartment won't my heat output be severely reduced? I drive in the spring and fall and I do not want to sacrifice heat just for tidiness sake. Will I still get acceptable heat w/out the helper fan?

[masraum]

Originally posted by Tim Hancock
My '79 SC does not have footwell blowers. If I remove the fan in the engine compartment won't my heat output be severely reduced? I drive in the spring and fall and I do not want to sacrifice heat just for tidiness sake. Will I still get acceptable heat w/out the helper fan?

I can't say for sure, but I think if you keep the RPM's up then you'll get decent heat. It depends on how much heat you want.

More RPMs mean more heat in the exhaust and higher fan speeds which would mean more heat.

[widebody911]

Heat volume is a function of engine RPM. At idle, you won't have much heat, but when you're underway, you'll have plenty - assuming everything else in the system, ie heater boxes and valves - are in good working order.