What is the function of the heater blower swinging weight??

[notmytarga]

I recently finished an engine rebuild and decided to keep most parts stock including the heater blower, especially since it contibutes to exhaust/engine cooling. While tearing down my engine I was able to fix a previously jammed fan cage so it all works well now.



I noticed a flapper type valve that is actuated by a weight that swings forward underneath. The flapper would partially obstruct the airfow downstream of the fan if the weight moved forward which would happen going downhill or braking. I could only guess as to why this would be helpful.

Here is what it looks like removed:




The reason I ask is that I traced a rattle to this part, so I separated the halves of the splitter and removed it. I can't imagine that its function is critical, so out it came. By the way, it weighs 63 grams. I'm going to have to learn the cornering dynamics of the car all over - back to the track!

Does anyone know the reason that the weighted flapper part exists??

[VaSteve]

subscribed....i don't think mine had that when I took it out.

[fintstone]

Looks like some type of safety device to cut off air to the cabin from the engine area after an frontal impact.

[notmytarga]

The weight swings freely and the flapper does not completely cover the area when closed. It doesn't lock out like an impact protection device would. The flapper covers the upper portion of the fan output when closed and is asymetric around its shaft.

My first guess is that the design is to reduce the airflow into the heat exchangers and over the exhaust and into the cabin when stopping fast - because the air momentum would carry the air in that direction - That is far-fetched.

My next guess is that the weight pulls the flap open for full flow at low engine fan pressure speeds and the flap progressively closes against increasing airflow at higher engine/fan RPM. I think that makes sense. Taking this mechanism out to solve my rattle will cause heater flow to increase proportionaly to engine RPM. My heater flow into the cabin may be more erratic but the concern I have is how much will this 'steal' engine fan flow that would be better sent to the cylinders and heads? Perhaps I will reinstall it with bushings for smoother, no-rattle operation.

Do other years have these??

[VaSteve]

Originally Posted by notmytarga

My next guess is that the weight pulls the flap open for full flow at low engine fan pressure speeds and the flap progressively closes against increasing airflow at higher engine/fan RPM. I think that makes sense. Taking this mechanism out to solve my rattle will cause heater flow to increase proportionaly to engine RPM. My heater flow into the cabin may be more erratic but the concern I have is how much will this 'steal' engine fan flow that would be better sent to the cylinders and heads? Perhaps I will reinstall it with bushings for smoother, no-rattle operation.

Do other years have these??

This sounds more plausible. Trying to even out the flow in some manner. I know my car gets hotter faster inside, the more fun I'm having at the wheel. Seems like an overthought solution to a problem. I just checked the stuff I took of mine, whatever you have is homegrown.

[notmytarga]

If by "homegrown" you mean that the weighted heater blower flapper is NOT stock for this 1975 911S Cailifornia motor - I'm rather sure that it is.

If the function is to increase resistance to airflow when pressure builds during increasing engine RPM then it would serve to make the heater system airflow more uniform through the engine fan speeds AND/OR to shunt more airflow over the engine (cylinders/heads/internal oil cooler) at higher RPM's. Both seem desireable.

I'll have to find a way to reinstall the flapper without the rattle. The plastic 'bore' that is formed by the upper and lower halves of the splitter when they are screwed together has likely worn beyond close tolerance. My next project!

Does anyone have an engine heater blower with this mechanism or is it limited to the 911/44(1975 California) engine in an attempt to make up for all the heat producing parts?

[VaSteve]

Originally Posted by notmytarga

If by "homegrown" you mean that the weighted heater blower flapper is NOT stock for this 1975 911S Cailifornia motor - I'm rather sure that it is.

If the function is to increase resistance to airflow when pressure builds during increasing engine RPM then it would serve to make the heater system airflow more uniform through the engine fan speeds AND/OR to shunt more airflow over the engine (cylinders/heads/internal oil cooler) at higher RPM's. Both seem desireable.

Does the 75 have (had) the 5 blade fan? Maybe it was compensation for that? (also, maybe you don't need it anymore). I have never seen such a thing in my reading here for the past few years, but I've not seen everything either.

[notmytarga]

My engine has an 11 blade fan which I believe is the stock fitment. This flapper part looks like it has always been there.

[Bill in OKC]

Originally Posted by notmytarga

My next guess is that the weight pulls the flap open for full flow at low engine fan pressure speeds and the flap progressively closes against increasing airflow at higher engine/fan RPM. I think that makes sense. Taking this mechanism out to solve my rattle will cause heater flow to increase proportionaly to engine RPM. My heater flow into the cabin may be more erratic but the concern I have is how much will this 'steal' engine fan flow that would be better sent to the cylinders and heads? Perhaps I will reinstall it with bushings for smoother, no-rattle operation.

Do other years have these??

And when the heater is off... maybe it prevents some back flow - and maximizes cooling air over the cylinders?

[tcar]

Originally Posted by notmytarga

My engine has an 11 blade fan which I believe is the stock fitment.

Nope. It's not stock.

All 74-77 911's had 5 bladed fans.

Many have been replaced over the years. Maybe even most of them have.

[fanaudical]

Resurrecting this thread because I'm rebuilding my blower fan and have the same questions. In looking at my fan, I don't see how increasing pressure forces the flapper closed - the angles are all wrong. In fact, I don't see how the weight ever does anything but hold the thing wide open.

Does anybody have any further thoughts/info on this?

Thanks for the help.

[Jerome74911S]

I backdated the heating on my '75 engined car. I assume the blower belonged to that engine, as it was attached to it.

Anyway, no such mechanism on my blower. It's an interesting gizmo, so I hope someone comes up with the correct answer.

[notmytarga]

I will have to stick my head into the engine bay and see if the weight moves with engine revs. It is either the engine fan/blower fan that moves it or it responds to deceleration - which would be weird.


fanaudical - what is the year of your car? Put it in your signature. Be proud of your children - we want to know their ages!

[Howard M]

I replaced the OEM heater fan on my '76 and had no such device. My car is a non-California car.

[Bob Kontak]

Originally Posted by fanaudical

In looking at my fan, I don't see how increasing pressure forces the flapper closed

So with at least two flapper units in play in the world, notmytarga is vindicated.

I may have to get my search on to put this to bed.

[fanaudical]

I have a '75 with the stock 2.7 (that I'm just finishing the rebuild).

I assumed that the lower duct sits horizontal with respect to ground. However, if it does sit tilted upwards as in the picture, then the flapper would swing shut with increasing air flow.

[maui44]

My car is a 77 (California car) and it does not have the part.

[porterdog]

Originally Posted by notmytarga

My first guess is that the design is to reduce the airflow into the heat exchangers and over the exhaust and into the cabin when stopping fast - because the air momentum would carry the air in that direction - That is far-fetched.

Possibly it's designed to reduce the flow of entrained exhaust into the car while stopping? (total guess)

An ex-gf of mine had an '01 VW TDI. If you came to a stop in that thing with the sunroof tilted open at the rear the entrained exhaust would dump into the cabin- nasty.

In the morning (=cold start) if I leave the heat on by mistake I get a bunch of exhaust in the cabin of my C4 when I back out of the garage...

[b rock]

Where's Grady?

[notmytarga]

I did the experiment last night. Throttling the engine to 4-5K did not effect the swinging weight, I tried it with and without the duct from the engine shroud to the blower connected. There is a GOOD increase in airflow through that duct with higher RPM - it just doesn't affect the flapper and weight.

The lower duct does sit level when in operating position. This brings me back to the silly theory that it slightly restricts flow during braking.