Thermal Insulators for Carbs

[chris_seven]

The 1969 911T seems to have Thermal Insulators fitted between the inlet manifold and the head - Part Number 901.108.131.00.

The PET describes this as an intermediate flange.

They seem to have been fitted for use with the IDTP and then the Zeniths.

They appear to be still available and are made from Tufnol.

They don't appear to have been fitted to the 911S and early engines with carbs.

I believe that these were fitted to avoid percolation problems.

I have been thinking about making insulators to fit between the carb and the manifold instead of between the head and manifold.

I wonder if anyone has tried this and if it worked?

[Steve@Rennsport]

Hi Chris,

We've used insulators on each & every carburetor installation since 1977.

Its just critical for preventing percolation, given the US summertime temps and the RVP of our fuel.

[chris_seven]

I agree that they are important and have made suitable insulators for Solex 40PI between the manifold and the head as we want to keep the fuel in the reservoir cool and as this is in the manifold the position is important.

For Webers and Zeniths we were thinking of moving the insulator to sit under the carb to manifold as this seems a more common practice with Holleys and other carbs than fitting the isolator under the manifold to head.

[Steve@Rennsport]

The reason I prefer the plates between the heads & manifolds is to help keep the manifolds cooler for best performance (increased air density).

The only reason I can think of for placing them between the carbs and manifolds might be to help reduce carburetor icing; something that happens in cool, moist weather.

[E Sully]

I would think using the insulator between the manifold and head would be more beneficial than between the manifold and carburetor. Wouldn't this keep the manifold cooler and thus keep the heat further from the carburetor body?

[Steve@Rennsport]

Originally Posted by E Sully

I would think using the insulator between the manifold and head would be more beneficial than between the manifold and carburetor. Wouldn't this keep the manifold cooler and thus keep the heat further from the carburetor body?

Yessir,..it sure does, based on what I've observed with an IR heat gun.

[hdrockerroller]

PMO uses the the thermal insulators between the heads and manifolds in their carb kits.

[JFairman]

Thermal insulators and twelve 8x1.25mm stainless steel studs for $50 here...
Heat Insulators
They are available in different sizes for Carbs.

I put a 40mm set of these heat insulators between 40mm aluminum CIS injector blocks and the heads on my 930 motor and they fit perfectly.
The stainless studs in the plastic bag are holding my B&B headers on.
FWIW, they are all working great.

[chris_seven]

Porsche did start the practice of fitting insulators between heads and carbs and I agree that this would keep manifolds cool.

If, however, the main reason for fitting the insulators is to reduce percolation then the carbs will be cooler if you fit the insulators between the carb bodies and the manifold.

The manifolds will always lose some heat so the face at the far end of the manifold will be cooler and so the insulator will have an easier job.

If cool manifolds are important then I would question why so many cars used water heated inlet manifolds?

There is an interesting statement about Solex 40PI carbs in Excellence was Expected and it is ascribed to Ferry Porsche :We placed the fuel reservoirs in the inlet manifold to improve evaporation.

I have never understood this remark as I always believed we should avoid hot fuel.

I do realise it is custom and practice on 911s to put the insulator under the manifold but this seems out of step with many other cars and installations.

I can't seem to find a good engineering reason why the approach taken by Porsche is the best solution.

[E Sully]

I will question your theory here.
"If cool manifolds are important then I would question why so many cars used water heated inlet manifolds?" I believe in practice the "heated water" is actually coolant. I think "heated water" is relative. It is cool compared to head temperatures. While it seems to be common practice to heat the intake when engines are cold started, after warming up this changes to ambient temperature. The manifold is cooled by the "heated water" as engine head temperature rises, as is the entire engine.
"The manifolds will always lose some heat so the face at the far end of the manifold will be cooler and so the insulator will have an easier job." Yes, the insulator will have an easier job, but by isolating the intake from the head sooner, would not the entire intake stay cooler since it would not be heated as much in the first place? Even using fiber gaskets on the intake will insulate parts, although to a lesser degree than the "insulators" Is the theory for inter coolers on turbos to keep the air intake cooler and denser for more power?
"There is an interesting statement about Solex 40PI carburetors in Excellence was Expected and it is ascribed to Ferry Porsche 'We placed the fuel reservoirs in the inlet manifold to improve evaporation." This might be due to inefficiency of early carburetors to atomize fuel. I would think later more efficient designs led to isolating the fuel and keeping it cooler.
The 1973.5 cis used steel intake runners, which were then changed to aluminum. I would think the steel was cheaper, but aluminum was then used due to better cooling properties. Also, with fuel injection is the fuel not cooler due to the pressure drop as it passes through the injector nozzle into the head?

[chris_seven]

Ed,

I am not sure I agree with your concept of water cooling for normally aspirated engines - and I am only thinking about insulating carbs.

Water heated manifolds have virtually no impact on cold running will actually increase warm up times.

I think that the main idea of the heating is to stop fuel condensing onto the walls of the manifold which will lead to some very poor running.

As the fuel is picked up from the jet it will cool rapidly and if it hits the cold walls of the manifold it will stick and you will have a wet boundary layer which is not good.

If the inlet manifold is warm this won't happen and as air is a relatively good insulator there won't be enough time for the charge to warm up as it just won't stay in the manifold for very long.

There is a SAE Paper which examined the effect of preheating fuel charges on both specific horsepower and engine acceleration and within limits preheating a fuel charge was found to be beneficial.

This is quite different to heating up the air prior to it picking up fuel which I would agree has a very negative impact.

If fuel charges are heated too much I would also expect that pre-ignition would be a problem.

The question then becomes what is the 'ideal' inlet manifold temperature.
I had the impression that the insulators fitted to 911s were only to prevent percolation and this was the basis of my question.

We have plenty of Zenith 40TINs running without insulators and they run quite well although I ould accept we may improve them by fitting isolators.

I guess we will have to try both and measure.

The Solex 40Pl is a traditional caburettor with the exception that it has a 'spill tube' to replace the float bowl.

It has an idle jet, an air corrector, an emulsion tube main jet and an accelerator pump an operates in an identical manner to a Weber or a Zenith. It certainly post dates the early Weber 'DCO' which was first introduced in sand cast form in the very early Fifties and had all of the feature we now associate with the dcoe and IDA families.

It doesn't need a fuel reservoir it just needs a constant fuel supply to the spill tube and a means of removing the fuel from the return system.

The 'head' required to allow fuel to be drawn from the main jet is governed by the spill tube which can never change.

The idea of putting a float controlled reservoir below the level of the fuel head and then pumping fuel around a weir doesn't really make sense.

If the float is removed it is possible to pump fuel directly to the spill tube and return the spilt fuel to the fuel tank. This keeps the fuel cool before it is metered and works quite well.

[Steve@Rennsport]

Chris,

Good stuff there and all true.

While I have no information and experience with European gasoline formulations, I suspect they are quite different from the current garbage we have for fuels sold the US, as mandated by our EPA. The only exception are racing fuels.

RVP's vary by region here in the US as well as by season, however our fuels are quite prone to boil when heated up very much, hence the need for insulators on ANY carbureted engine.

Cold manifolds make more power than warm, much less hot ones, due to improved air density. Back in my drag racing days, we packed the intake manifold on our V8's with ice between runs to deal with heat soak and it always made a noticeable and measurable (ET & speeds) difference when we did this. Its the same principle still used today with "air-gap" intake systems on V8 engines. You can see this on all NASCAR engines.

I can re-create this phenomenon in the dyno room on the 911 engine using a CO2 fire extinguisher to cool the intake system, if its alloy. Naturally, our carbon fibre (and any plastic) intakes are less sensitive since they run cooler.

No question that cold manifolds affect driveability so in cool, moist weather, a heated (or at least a warmed) intake system is preferable. For best performance, its not so good.

I don't have much experience with the Solex carbs, but I can say that Weber and PMO carburetors are plenty sensitive to float levels and heat; both need insulators, especially in warm to hot weather.

[chris_seven]

Our fuel is not too bad but tends to be higher Octane.

99 RON is readily available and BP and Shell fuels are quite consistent.

We seem to run our engines quite well without insulators but we do percolation mods on the Webers but not on the Zeniths.

I have an IR temperature probe so I will make some measurements as it is interesting to try to learn more.

I was trying to avoid making more than one size of insulator but it looks as if I just have to make the full set and stop being a cheapskate

We have already made our own gasket/repair sets for Solex, Weber and Zenith as I am not keen on either the fit or the materials used in the kits we find here in Europe.

[crummasel]

I think the question whether its preferable to insert insulators between carburetors and manifolds or manifolds to heads is interesting in result.
Agreeable the observations of Steve on drag engines.
What on street engines?
Its obvious, that liners should be kept cool, no question.
Inserting the factory insulators mean air loss in the area where the liners enter the air cooling shroud. Do you add manifold to airshroud gaskets here?
Small detail but a weak point IMHO.
Simpler here would be to benefit of the lip of the manifold to cover the hole in the airshroud and to use insulators then between carbs and manifold.
Just a consideration.
On fuel quality in europe: Yes, we have better octane values on gas fuel as far as i can read, but i assume the gas pressure (in its result percolation) is similarly bad as in the US. So concerning that no real benefit here.