Good Alternative To Paper/Foil MFI Hoses?

[Jandrews]

I have no interest in originality...just performance and durability. I find the stock, double layered configuration of paper/foil to be unwieldy to install and keep in place, fragile and generally a pain in the a$$. Does anyone have any recommendation for these hoses that go from the heat exchanger to the engine tin, and then from the engine tin up to the thermostat housing on the MFI pump?

Details, sources and pics would be greatly appreciated.

Thanks,

JA

[wswilburn]

Aeroduct, available from Aircraft Spruce. I prefer the SCEET. http://www.aircraftspruce.com/catalog/appages/sceet.php

[jpahemi]

That's what I used after wrestling with the dark grey OEM hose which wouldn't fit over the SSI inlet (probably due to the angle it was coming from).
I also used it for the SSI outlet to the flapper valves; very durable and no unusual oder.

jp

[Grady Clay]

John,

Re: the MFI hot air hoses to the thermostat (not heat exchanger cold air hoses).

THE important issue of the thermostat air hoses is the rate of heating the thermostat. Too cool and the thermostat doesn’t turn off soon enough resulting in increased oil dilution with gasoline. Too hot and the thermostat turns off too soon resulting in poor (lean) running during warm-up. Given a choice, I prefer the latter.

The system design assumes a perfectly sealed heat exchanger, the thermal characteristics of the heavy rubber hose from the heat exchanger to the tin, the concentric flex hoses and a proper (adjusted & clean) thermostat. Yes, you can change the components. If you do so, keep the thermostat turn-off characteristics foremost and not necessarily just expediency.

Any time I have made changes in these air tubes, it has been to change the warm-up/turn-off of the thermostat. Here in the Colorado High Rockies (cold winters, thin air) it has been to turn off the thermostat sooner. Some ‘tricks’ we have found necessary are to increase the insulation of the concentric hoses, insulate the thermostat and add a hose from the thermostat to the low pressure side of the engine fan to draw more hot air through the thermostat and let the thermostat get hot. This is particularly true for a MFI 911 buzzing around Vail or Aspen.

I agree the concentric hoses are flimsy and a pain. I consider them expendable and simply replace them when they become frayed. Are there better choices? Probably but it is the thermal characteristics I would want to control.

Best,
Grady

[canamfan]

Wow, now that's some brilliant knowledge. I appreciate someone who has gone the length to find such great information on a seemingly "not so important" part, that now has significance, those factory engineers have their purpose in full. Thank you "Grady Clay" for some insight to these heater hoses, I always tended to believe factory parts had purpose & I really can appreciate HVAC in my 911 working properly living in the fridgid winters of Canada.

[dlague]

Is it REALLY that complex?

[Grady Clay]

Dick,

In the balmy hills above LA (Santa Monica ‘mountains’) I suspect you don’t even need a thermostat or cold start.

For us sots that play in real mountains; living at 5-8000 feet with 12,000 foot passes and winter mornings at -20F and lower, we need all the systems working. A big cold weather issue is diluting the oil with gasoline. Typically this is cold start and thermostat related. If that isn’t minimized and the oil changed VERY often, bad things can happen. This is not seen in SoCal unless your shop starts and moves your MFI 911 a few feet twice a day for some long period.

It isn’t that Porsche actually engineered all of this. Most of the MFI system is from fairly standard Bosch systems that pre-date 911 use by decades. That said, the Porsche engineers specified the particular parts to get the desired result. This cold-running air warmed thermostat system is unique to 911 as far as I have ever seen.

I think Porsche’s basic engineering constraint was the continued use of the ‘911S’ heat exchanger. From there they had to decide how to regulate the mixture during warm-up. The ‘beauty’ of using the hot air from the heat exchanger is that it is load sensitive. More load = more heat and the engine is warming up faster. The greater heat turns off the cold running enrichment sooner. The converse is true and it is ambient temperature sensitive. All very clever and it works.

We have to remember that the MFI system isn’t ‘smart’. There isn’t any ‘feedback’ in the system. It is built and run ‘as engineered’. If there is some issue due to external conditions, we have to become somewhat inventive.

MFI is really a racecar system, somewhat tamed for the street. That is its beauty.

Best,
Grady

[Aurel]

Pelican sells nice alternatives;
Aluminum hose for the inside:

Space age hose for underneath the car:

[JFairman]

I went to a local hose supply place "Mako hose" and matched up the diameter of the heater hoses to some black flexible corrugated high temp neoprene hose they had.
I used it and it works great, looks good, hasn't melted or become crispy, and it's inexpensive compared to oem hoses.

Mine are for interior heat from the heat exchangers on a 930, and not for AFR calibration warming the manual fuel injection.

[jpahemi]

Grady:

I never would've realized the conductivity component of the silver metallic air hose. My take was more aligned with the weather resistant characteristics, than the thermal. For my 3.2, it's not an issue, but as you pointed out, it sure would be for the warm up rate application. The rubber/silicone aircraft hose is more of an insulator, than a conductor.

Regards,
jp

[Jandrews]

I think we are mixing topics a bit here (MFI thermostat hoses vs heater hoses), but it's all good.

Grady, thanks for your excellent insights. Just to be clear, what is the specific recommended hose from the heat exchanger to the tin (i.e. outside the engine compartment)? I also like your idea of a hose on the negative pressure side. I am having a bit of a hard time visualizing that. And I like the insulation idea too...any more specific recommendations there?

Lastly, is there a good way to guage the thermal properties of these various combinations? I.e. if I went with Spruce neoperne inside the engine compartment, how would I know if it is working "better" or "worse" than the concentric jobs?

Thanks,

JA

[Grady Clay]

I think all those components were very standard German industrial/automotive items. I think the large rubber hose between the heat exchanger and tin was chosen for road debris resistance and this was the only part available. Dramatic overkill. There should still be ‘good used’ of these around.

The original hoses between the tin and the thermostat were the foil/paper type common to VW and many (every?) other German & European car. A clue here is the Porsche engineers decided to use two concentric hoses to provide some insulating characteristics.

As I recall, the plastic versions first appeared on the 2.4 (’73?). I hag initially assumed that the change was for cost and durability but the original material was commonly available at far less cost to Porsche. So, why the change and spend more? Could it be insulation factor?

In general the foil/paper version works just fine – it just isn’t very durable. If you find bulk or some longer VW piece, you don’t have to pay PCNA prices.

In a few cases I took some more foil/paper hose and ran it from the thermostat air outlet to the rear-facing side of the engine fan. This helped ‘pull’ the hot air through the thermostat. That was probably with a less-than-perfect heat exchanger.

There are quite a number of ways of insulating the air path from the heat exchanger. The issue is to decide what, if anything, you need.

There are two aspects to the thermostat function: How rich it makes the pump at a given overnight temperature and how fast it turns off during warm-up. The bi-metal disc stack is shimmed to adjust this mixture. There is a limit. Take the thermostat off the pump body and it is full rich. The same is true if there is cold clearance between the thermostat rod and the pivot lever in the pump. You can easily inspect this through the 2-screw port on top of the pump near the thermostat.

The rate with which the thermostat turns off is obviously influenced by the engine load, rpm, ambient temperature and the thermal insulating characteristics of the air path to the thermostat. Given the condition of many heat exchangers, installing a new heat exchanger may be the first choice.

The original Factory set-up is pretty good. Circumstances like tooling around Vail sometimes inspire creativity. If you think you need to tinker, find out where you are now. Very rich mixture is sometimes difficult to measure and you are measuring a transient mixture during warm-up. It is not easy to get repeatable measurements.

One approach is to force the thermostat to turn off ‘too soon’. This makes for poor (too lean) running during warm-up only. You can easily do this with a very well insulated, lightweight hose from the heat exchanger directly to the thermostat by removing the left tin.

You can remove the paper gasket that go between the thermostat and pump body to lean the basic thermosetting slightly without adjusting the shims.

One way to evaluate the system is to disable the cold start and adjust the thermostat just lean of where the engine will just try and start cold. If a cold engine starts on the thermostat only, the thermostat or main mixture setting is too rich. This is a good starting point but time consuming to find.


Always keep in mind CMA and the need to have everything working correctly before you adjust.

Best,
Grady

[Eddyvdw]

Hello Grady,

I'm in the process of getting my car back on the road (after 18 years of sleep). The MFI pump is tested and calibrated on a test bank by Mark Jung. I think I have the hoses just right but not sure about the quality of the HE. The flow from the HE to the flapper box is hot and activates the thermostat in the flapper box so should be okay. Question, how hot should the thermostat be after a 20 min test drive? It is warm but I can still hold it without burning myself. I don't want to change the settings of the pump, Mark did a great job and I don;t want to mess it up.

What are the symptoms of running rich besides low MPG, very smelly fumes and thin oil?

Thanks and cheers from Prague.
Ed