Engine cooling

[72SMFI]

I dont understand why Porsche used the system it did with the heat exchangers. In the summer hot air will move across the headers and exits under the car In front of the engine. This is taking air that could be used to cool the engine and dumping it rendering it useless. Why didnt they use the heating valves "flapper boxes" in the engine compartment. This would allow the fan to run air to the heat exchangers when it is cooler out, and when hot it would put the all of the air on the pistons and engine. It seems to me you would get incressed cooling in the summer without loosing the heater for the winter. It seems I am missing somthing here.. Wayne

[72SMFI]

How about a flapper box in engine compartment that you can open and close manualy so you can keep the flapper boxes under the car intack but be able to get more air to the engine in the summer? Anyone??

[widebody911]

I've never figured that out either, unless they felt that there was an excess of cooling air available for the cylinders and heads.

Having been an air-cooled guy all my life, I have a hard time accepting that.

The VW style heater box (from which the 911 system was derived, like it or not) closes the valve when the heater is not in use, and uses the extra cooling air for the engine. Even the pre-60 'stale air' heater system ran all of the air thru the cylinders before sending it to the cabin (*gack, cough*).

On my 3.6L, I have a Cup air bypass tube, but for the summer, I've fabbed a small blockoff plate for the fan shroud, so now all of my air goes thru the cylinders. Yes, I know there's a factory piece to do the same thing, but this is cheaper and faster.

[island911]

A little thermodynamics to consider. . .

Our engines make power ONLY by having a relatively cool low pressure side to move to.

The bigger the difference, the faster it can all happen.

So, cooling the headers, cools the exhaust gas. Cool exhaust makes less pressure for the scavanging action of the piston, as it uses power to push out exhaust gasses.

Many have the idea that the exhaust is just about the same as a sink drain. Of course this is a Very crude idea of what's going on.. . .but atleast it sells a lot of coffee can mufflers

[72SMFI]

Island So what don't you get. Are you saying that Heat exchangers hold more heat than headers alone and must be cooled to operate correctly.Heat exchangers would overheat without cooling and would loose horsepower because of thermaldynamics. If that was the case would SSI give a significant advantage being that the stainless would cool faster retain less heat and therefore would give more ponies than a factory system on a pre 73 car.

[steve rush]

xxxx

[Chuck Moreland]

Certainly the continuous air flow through the exchangers will cool the tubes. But I don't think the goal was reducing exhaust gas temperature for improved power production.

On the contrary, many race cars use a header wrap; an insulation layer to keep the exhaust gases hot through the tubes. The purpose is to keep the gases voluminous and moving rapidly thereby increasing the scavenging effect. Cooling of the gases reduces their volume thereby slowing them down. Scavenging effect is killed by slow moving exhaust gases.

Here is a tech topic from the Elephant Racing newsletter that explains header operation and the whole scavenging thing:

http://www.elephantracing.com/techtopic/headers.htm

The downside of the header wrap is the intense heat shortens header life substantially.

I suspect the reason Porsche keeps the air flowing through the exchangers is to cool them to extend their life. Without air flowing the heat exchanger shrouds become insulators and the inner tubes would get exceedingly hot. Good for scavenging, bad for life expectancy.

This is probably less of a problem with stainless exchangers.

[island911]

72SMFI, whether Heat exchangers (stainless or steel) hold more heat than headers alone, isn’t quite my point.
My point is more (while trying to leave out terms like enthalpy) that the temperature difference is the “big mover” in getting power out of an engine. Having appreciation for this give one insight in to the most basic consideration of engine operation.
You see, there is this basic idea in thermodynamics that hot wants to move to cold. The bigger the difference the faster it happens (read: available power)
This basic idea was so good. . . it became Law. . .and the Law is so good it doesn’t even need police for enforcement. (Although some people think they can circumvent this law using a phenomena discovered by our own board member wckrause, called “Thermal Conviction”) heh-heh. . that still cracks me up.

Anyway, consider you have 6 header pipes (1.4" ID) that lead into 2 at the muffler (1.87" ID) and go out the 1 Tail pipe (2.24" ID). (This is what I have for my SC-SSI's. ) The tubing cross-sectional area starts at 26.4 in², goes into the muffler with only 11.75in² and leaves the Tail pipe with only 7in² -gasp- So, either the exhaust is providing great thrust and someone could sell us an 8.4in² tail pipe or a great deal of cooling is taking place.
Things that make you go - Hmmm

There are times when cooling the “headers” cost power.
For instance if you have turbo chargers, you would not want to take heat out pre-turbine. (and yet on a 930 they do – makes me "go Hmmm" - oh well)

On the ssi’s vs factory system; I believe the factory system may beat the SSI’s. Stainless generally moves heat slower than plain old steel, but not by much; and considering stainless doesn’t corrode away like mild steel, the SS may be made thinner. Having less material for the heat to transfer through, the SS may shed the heat faster.
Before I bought SSI’s I thought about this. Even though I could have had a used factory system for $500 less, I concluded the balance was still in favor of the SSI’s.

[Bill Verburg]

I believe that Chuck is right. In addition the % of air flowingm through the heat exchanger heating ducts is tiny. The % of thermal load is probably not measurable

[Wayne 962]

I certainly don't agree with 90% of the discussion here. The heat exchangers are not there to cool the exhaust...

The original question was regarding the flapper boxes. In an ideal system (where there are no leaks) the flapper boxes will close and not allow air into system. Therefore, when the boxes are closed, it will theoretically create a mostly sealed system all the way up to the engine fan. This has the same effect as having the flapper boxes up in the engine compartment. The flappers do bleed off some of the air...

Put another way, with the flapper boxes closed, the air from the engine compartment is not going to flow into the heat exchangers. This is because with the flappers closed there is no place for the majority of the air to release to.

Think of it as if it were a sealed system that used water instead. Fill a straw with water, then block off one end of the straw with your finger. Then place the open end of the straw under the faucet. The straw will fill up with water. Then, any extra water will flow around the opening of the straw because you have the end blocked off. The 'valve' is at the end of the straw, however, no water enters in because it has no place to go. Even if there is a small crack where your finger is attached, not much is entering the straw.

The straw idea is very similar to the heat exchangers on the 911. The only exception is that you're dealing with air, which has a tendency to escape out cracks and crevices, rendering some minute air loss, in addition to the small bleed from the flapper boxes. The air loss leaking out does indeed remove cooling air from the engine. Howevever, in a properly maintained heating system, this air loss should be small.

If you are working on your engine, you should never run it with the heater system disconnected at the engine. The heater system itself (even when open) has back-pressure that will divert air over the cylinders. Running the engine without the block-off plate or air adapter to the heater system closed off will indeed rob your engine of cylinder cooling air.

As for the air from the engine cooling the exhaust pipes, that makes no sense to me whatsoever. Firstly, the air flowing past the pipes is very miniscule. Secondly, the heat exchangers actually work to insulate the pipes from a huge source of flowing air - airflow underneath the car.

Bottomline is with a properly installed and maintained heater system, very little air is actually flowing through the heat exchangers when the flappers are closed...

I guess the question to all is why is there that bleed valve in the flapper boxes?

-Wayne

[island911]

Originally posted by Bill Verburg
I believe that Chuck is right. In addition the % of air flowingm through the heat exchanger heating ducts is tiny. The % of thermal load is probably not measurable

Seems that the amount of heat going to my toes is more than "measurable" my toes have not ever been calibrated but they say it's in the range of kilowatts. . .where is that electric heater thread?

[Chuck Moreland]

Actually the math on header and exhaust isn't just that easy.

Exhaust is moving through the header primaries in pulses. Each pulse comes only on the exhaust stroke. This can be approximated as running a 25% duty cycle.

The tailpipe is running a 100% duty cycle.

Using your numbers, the 26.4 sq in primaries in 25% duty cycle are effectively 6.6 inches. About the same as the 7 inch tailpipe.

Headers are all about scavenging and that requires velocity in the primaries. If it were simply a matter of minimizing back pressure then you can bet race cars would vent their exhaust valves directly into the atmosphere with no headers at all. But they don't.

Due to scavenging, headers make even more power than no exhaust system at all.

[island911]

Originally posted by Wayne at Pelican Parts
I certainly don't agree with 90% of the discussion here. The heat exchangers are not there to cool the exhaust...

The original question was regarding the flapper boxes. In an ideal system (where there are no leaks) the flapper boxes will close and not allow air into system.

1st, Who said "The heat exchangers are there to cool the exhaust" ?

2nd, with the flapper boxes closed, air *is* allowed into and though system. The flappers are diverters in the 911; either in to the cabin or out to the atmosphere.

"don't agree with 90% of the discussion here". . .your just trying to be contrary, aren't ya?

[72SMFI]

I am trying to reduce engine temp at the track. I am pushing my 2.4S with stock S cooler to its limits and on a 75degree day I am seeing 220 to 225 degrees. Whats going to happen when its 88 degrees outside? I was thinking that, cooling is be wasted by going through the exchangers. I was going to fab a shroud blockoff plate to keep all of the cooling to the engine. Bad idea? It seems better than a cool collar! I really dont care about my exchangers they are relatively inexpensve compared to Valve repair. I could replace the block off when not at the track. I realize my last name is not Porsche, and hell Im Irish!! But now I am thinking I might loose ponies. That is not good considering I only got 190 and my next event is Viper day! "Good thing for Porsche brakes.

[Wayne 962]

[QUOTE]Originally posted by island911


1st, Who said "The heat exchangers are there to cool the exhaust" ?


You did:

[QUOTE]Originally posted by island911


So, cooling the headers, cools the exhaust gas.


Unless that's not what you meant by the above statement.

I do disagree that that heat exchangers are there to cool the exhaust. I also disagree that air is flowing through them in order to cool them. The 911SC has a heater blower motor which is off except when the heater system is on. This increases the backpressure on the system, allowing less air to flow through when it's turned off...

-Wayne

[Wayne 962]

From Chapter 3 of the Engine Rebuiding Book:

Air Block-off Plate

The 911 engine is an air-cooled engine. As such, it receives much of its cooling via the flow of air from the large upright fan mounted on the engine. Improving the flow of the air helps to keep the engine running cooler, which translates into increased longevity. Since the 911 uses its fan to supply air for the passenger compartment heating system, some of this air is diverted away from cooling the engine. With air cooled cars, the passenger compartment heat is derived not from the engine itself, but from the exhaust temperatures.

If your heater in your 911 is not working or is disabled, then you might want to install what is commonly known as an air block-off plate on the side of the engine fan. Towards the left side of the engine, there is an air outlet that drives the heating system for the car. On the early cars, this flow of air was the only mechanism to push air into the passenger compartment. This arrangement didn’t work very well, and in later years, Porsche installed an additional blower motor in the engine compartment. If your heater system is not working or disabled (as it usually is on a track car), then a good upgrade is to replace your heater air-flow plate with a block-off plate. This block-off plate does exactly what its name implies. It blocks the flow of air, and thus diverts it back over the cylinders, where it can cool the car.

Along the same lines, you should never drive the car without the heater system hooked up properly, or without the airflow blocked off. Allowing the fan to blow air into the engine compartment from an opening in the heater system will rob the cylinders of precious air that is needed to cool them. Hooking up the heater system or blocking it off will create back pressure against the air flow that will force it back over the cylinders.

When re-installing your 911 engine, it’s common for people to leave their heater system temporarily apart during the break-in period. This is because the heater system typically gets in the way of valve adjustments, visual oil leak checks, and the tightening of the head studs. If you don’t hook up your heater system, make sure that you at least use some tape to temporarily block off the flow of air from the left side air duct.


-Wayne

[Chuck Moreland]

Originally posted by Wayne at Pelican Parts
The original question was regarding the flapper boxes. In an ideal system (where there are no leaks) the flapper boxes will close and not allow air into system. Therefore, when the boxes are closed, it will theoretically create a sealed system all the way up to the engine fan. This has the same effect as having the flapper boxes up in the engine compartment.

Well, no... the flappers never cut off the flow of air. They either direct it to the cabin or out under the car. That was the oddity that sparked 72SMFIs question.

Secondly, the heat exchangers actually work to insulate the pipes from a huge source of flowing air - airflow underneath the car.

I can imagine a discussion that went on back at Porsche....

Marketing: We have to have heat for the cabin.

Engineering: We can't have heat because heat exchangers kill header life expectancy, and those Webastos catch fire faster than Richard Pryor.

Marketing: Well you guys are the friggin geniuses, figure something out. The American's won't buy a car without heat.

Bright young Engineer: How about if we design the flappers so that a small amount of air is always flowing through the exchangers, even when the heat is off. That will cool the headers just enough to get good life expectancy.

[72SMFI]

Wayne, do you sell a block off plate. They cant be too expensive. Will it help my car at the track.

[Wayne 962]

True, the flapper boxes bleed air. However, there isn't enough air flowing through the system (IMHO) to create enough of a free-flow...

Life expectancy of the heat exchangers? Other than rust, I can't say I've ever seen one worn out?

Okay, I just called an 'expert friend' to try to figure this one out. He said something that triggered a thought in my mind...

If there is no air flowing through the heat exchangers, then where is the heat going to go? Nowhere. That's great for cooling the engine, but what happens when you need some heat? You pull the flapper box, and air flows into the heater system.

Okay, so if the heat hasn't been moving and flowing (at least slightly through the flapper boxes), what has happened to it? It's been heating up in the heat exchangers. So when you pull that flapper box, this air now is pushed into the passenger compartment system. After sitting idle in the heat exchangers, this air is likely to be at 1000 degrees or so? So without bleeding off the heat through the outlet in the flapper box, the air would become so hot when turned on, that it would melt your toes.

This, is just a guess...

-Wayne

-Wayne

[Chuck Moreland]

Originally posted by Wayne at Pelican Parts
True, the flapper boxes bleed air. However, there isn't enough air flowing through the system (IMHO) to create enough of a free-flow...

Life expectancy of the heat exchangers? Other than rust, I can't say I've ever seen one worn out?

-Wayne

I guess that means the cooling works.


The idea of extending header life is speculation on my part, but it seems reasonable. I know that header wraps reduce header life big time.