Unvented heat exchangers cause engine wear?

cowtown · 04-18-2003, 09:10 PM

A friend of mine who owns an '83 SC went over to Andial (a very high-end shop from the looks of it) to look around the other day.

He was talking with the guys there about installing blockoff plates that would cap the two air outlets at the fan shroud that route air to the heat exchangers (for whatever reason).

The Andial guy told him that, even if he didn't care about heat, he needed to keep the ducts hooked up to the exchangers so that air could flow through them, or engine wear would be accelerated due to heat buildup in the exchangers.

Has anyone ever heard of this before? I can understand the theory, but am wondering if this is really an issue.

I was planning to get my 930 engine running without heat ducts to the exchangers installed (and caps at the shroud outlets), but this made me think twice. Can't believe it would be anywhere near the damage caused by the original thermal reactors, but...?

Colin

82SC · 04-18-2003, 10:58 PM

don't know if this is related...but I was driving from LA to Carmel...at 3AM and cruising at 110mph (totally empty roads) for a couple hours straight...and naturally my temps slowly climbed...

I got cold and cranked the heat up...and I saw the needle dropping right before my eyes...So I do think it is possible for heat to build up in the HE's...and affect engine temps

I know even without heat going to the cabin, the hot air is supposed to vent out of the HE's...I don't know if that was too restricted, or maybe my cables are maladjusted...but it made a huge difference

and if andial says so...I'd lean towards believing them

Porschephanatic · 04-19-2003, 03:03 AM

IMHO, I'd believe Andial. I thought the blockoff's were for header use only, as headers would dissipate more heat.

Bill Verburg · Bill Verburg's Garage

You want to keep all of the holes, seams etc. on the top of the engine sealed(or hooked up to their assorted ducts) so that the maximum amount of cooling air flows around the cylinders and heads. Once the air has gone past the cylinders and heads it makes little difference where it goes. Though straight out the bottom and back is most efficient.

beepbeep · 10:18 AM

Well, if you want to keep your turbo spinning as efficiently as possible you would be interested in keeping exhaust gasses as hot as possible.

If heat is not needed, i suggest you to cut away shrouding around exchangers alltoghether, exposing the pipes. Then use exhaust bandages to isolate all pipes. That way, you will retain more heat in the exhaust gases, thus kepping volumetric flow high (= more gases to keep turbo spinning) and isolate "headers" (which they are not) from heating up engine trough convection.

Or better still, weld a decent pair of headers and thow away those pesky heat-exchangers. Even though they are better than U.S. Thermal Reactors (which are downright awfull), they are far from optimal. You want turbo to be as close to heads as possible and dissipate as little heat trough headers as possible.

And you are in it for the power, not heat...right?

Wayne 962 · Wayne 962's Garage

This has been debated many times before. The 911 heater flapper boxes have pass-through vents in them so that heated air will pass through the exchangers at all times, regardless of whether the heat is on or off.

I'm not too sure I agree with the Andial theory - as the sheet metal and air around the heat exchangers makes for pretty poor insulators. There is indeed air running through there all the time though, even when the heater system is off. I personally think that the heater system is continuously 'bled' so that when you turn on your heater, you don't get a blast of 1000 degree heat coming at you all of a sudden.

Keeping in mind that it couldn't hurt, if you are blocking off your heat anyways, then you should probably run headers regardless.

-Wayne

Rick Cabell · 04-19-2003, 10:37 AM

My view is that there should always be air flowing through the heat exchangers. The header pipes are bolted directly to the heads, and the heads are where the highest engine temps occur. The hotter those header pipes get, the hotter the heads will be, and the more quickly the engine will experience valve guide wear. Having cool air moving constantly across the header pipes will certainly cool them, and result in cooler head temps. Furthermore, if you cap the exchangers (as opposed to just leave them uncapped and unhooked), there will be zero air movement across the headers, which will quite effectively insulate them even more and cause still hotter temps.

If you don't care about having heat but don't want to have the heating plumbing hooked up, why not just have a shop cut the heat exchanger sheet metal away from the header pipes (very easy job), and just run the headers?