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 Marshall Supercharger Information.
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Bob Stringfield

United Kingdom
854 Posts

Posted - 05/07/2011 :  20:36:27  Show Profile
There is what appears to be an interesting 'Flight' article on the types and application of the Godfrey cabin blower on Google under 'Marshall Cabin Blower'.

I lack the computer expertise to reproduce or archive it, sadly, and, of course, it may be very old hat to the experts.

Is there a simple illustrated guide to the various motor and aircraft blowers? As these were in RAF service, there is sure to be a specification / maintenance publication somewhere.

David Allison

United Kingdom
665 Posts

Posted - 06/07/2011 :  11:50:06  Show Profile
Bob

There is plenty of information available on Marshall supechargers both on the net and published - some of it good and some indifferent.

My personal view is that you are better taking a blower to a specialist such as Derek Chinn.
He has plenty of experience in making old blowers work like new.

Roots type blowers are simple and have very few moving parts in comparison to the vane type drum compressors (Cozette, Powerplus, Zoller, Centric, Shorrock).
Developed like drum compressors from the compressed air generators the Roots blowers were more cost effective and had a longer service life than the drum compressors.

Because the Roots type blowers have fewer moving parts there is very little which can go wrong - the internal seals are pretty critical - also the drive gears must be lubricated.
The inside of the casing also needs some lubrication - either fed through the casing or added into the inlet.
It is also critical that the casing clearances are set carefully - running a steel through the casing and between the vanes to ensure that there are no tight spots (or loose ones).
The shim steels were often called "rules" because they looked like a steel rule (never a ruler in engineering) hence the terms "rule set up" or "rule assembled" etc...

They still manufacture Roots type blowers in Shoreham under the company name Edwards High Vacuum - originally this was the Marshall Godfrey factory.
Using roots blowers in two, three and four stage chains to generate vacuum pressure for the medical and pharmaceuticals industry (amoung others).
Running at shaft speeds of up to 10000 rpm and creating a vacuum of up to 50 bar.

Roots type blowers are also still used for compressors - although more commonly now the Nordec Eaton type screw is used.
Nordec was also a part of the Marshall Godfrey group - but split up in the 1950's.

The article highlighted is based on the Marshall Cabin Blower - it is not really a technical publication - more it highlights the use of blowers to pressurise the aircraft cabin allowing aircraft to fly at higher altitudes.
The roots type blower went out of favour pretty quickly though because it was relatively heavy - the development of lighter weight centrifigal blowers able to give similar output from a smaller overall package - eventually seeing the final coffin pins of the older design.

There is plenty of material available on cabin Blowers but not in RAF service format (which in my experience is of very little help and full of red tape anyway) mostly because of the Official Secrets Act - standard armed service proceedure is to replace rather than maintain because that way an un-skilled fitter can carry out the work.
Damaged or worn components are usually routinelly "returned to manufacturer" for service, maintainance or repair.

Centrifigal blowers however (as an aside) do not work so well as superchargers on car engines (except for pre-war board racers running at relatively constant engine speeds) mainly because they do not respond quickly to changes in shaft speed or throttle openings.

Supercharging is in fact an engineering blind alley - a means to an end, allowing small capacity engines to achieve high power outputs with reasonable reliability.
However superchargers give no increase in working efficiency and require a large amount of fuel to gain best effect out of them.
Before the war short stroke high revving engines were considered a blind alley - they thought that revving to 7000 rpm needless complication - what they would think of modern cars revving reliably to that figure over 100000 miles and bike engines routinely reving to double that over 50000 miles? I am convinced that our forebears would shudder!

Regards David
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Gordon

United Kingdom
608 Posts

Posted - 06/07/2011 :  13:55:50  Show Profile
Bob,
I thgink the boklet yoyu are looking for is Air Publication 1519 vol1. This covers ,amongst others, Marshall Cabin Superchargers - cabin, types MK IX, Mk XII, Mk 22.
I have a copy of this booklet which you are welcome to borrow. I live just up the road from you so suggest you get in touch by email so we can arrange something convenient.

Gordon
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Bob Stringfield

United Kingdom
854 Posts

Posted - 06/07/2011 :  17:56:41  Show Profile
Many thanks, Gordon.

Now that we have all been told that a supercharger is both a blind alley and best left to professionals, I feel more inclined than ever to work towards replacing the works-fitted Marshall on my PA ( see correspondence in MMM Year Book 1982 ).

I must say that the supercharger's present-day equivalent on my workaday Golf 1.8T estate makes for a fun drive; turbochargers and superchargers are certainly well-used by present-day makers.

I had thought the 'Flight' article might be interesting background for those who come across the different Marks of cabin blower, but I stand corrected.

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RacingSnake

United Kingdom
62 Posts

Posted - 06/07/2011 :  21:55:56  Show Profile
Hey Bob, you can do it yourself, just take care!
David a "...vacuum of up to 50 bar." is not going to happen, as "vacuum pumps" work to reduce the pressure not raise it.
A couple of other general comments in the name of balance of opinion.
A roots blower of the Cabin blower era did not have any seals other than on the bearings (I believe) so there is no need for lubrication of the internals, save the drive-gears. This non-contact aspect and the constant displacement made them reliable and popular.
Vane blowers do require internal lubrication as the vane tips slide in trunions and rub on the inside of the case. Their upside is that they have internal compression so can offer higher boost than the roots blower, though that then brings additional joys such as pre-ignition. High boost was familiar territory for the Allison's "NO" racer, so Mike and David are well-placed to advise for those keen on boost.
On older vehicles a roots blower offered a relatively simple way to achieve low-end boost and sharper throttle response, by nature of the fact they are positive displacement and directly coupled to the engine.
The direct response is still important today, though in one of the high-fuel situations that David mentions...not many turbocharged dragsters...but lots with monster superchargers!

As with most things, Superchargers have their place (and are still worthwhile on MMM cars I think), but modern materials and control systems mean that most modern cars have a turbocharger setup as it is more fuel efficient, lighter, easier to package, and sounds better in the marketing blurb!

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ags

United Kingdom
275 Posts

Posted - 07/07/2011 :  00:14:28  Show Profile
Hi David, Gordon et al.,

Slightly Off Topic, but David is quite right when he says that supercharged engines, though not turbocharged ones, are less efficient than atmospheric engines. This is because supercharged engines do not follow one of the assumptions of the Carnot cycle, which is the theoretical cycle describing most internal combustion engines. This assumption is that the compression ratio and expansion ratio of the working gas are equal when the gas passes through the engine. This is obviously true for atmospheric induction, and is usually nearly true for turbocharged induction, but is not true when the engine is fed by a supercharger. The pressure rise across the supercharger is not compensated by an equivalent pressure drop when the pistons descend inside the engine. The laws of thermodynamics then indicate that the maximum thermodynamic efficiency of a supercharged engine must be less than a Carnot cycle engine so less of the fuel chemical energy is converted to useful mechanical energy or work (ie HP).

This difference is distinct from the need for many blown engines to run very rich mixtures, or have blow through valve timings, to provide internal engine cooling. Both of these settings represent further losses of fuel energy of course.

As anyone will confirm who has been close to the exhaust of a highly boosted engine being used in anger much of this wasted thermodynamic energy is dissipated as sound, because the combustion gases exit at a higher pressure than for Carnot engines.

Incidentally this thermodynamic difference between turbocharged and supercharged engines is the only real basis for the oft-repeated claim that turbocharged engines are more efficient than supercharged ones.

Even more than usually abstruse,

More technical ramblings from

Andrew Smith MMM571
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Richard Hardy

United Kingdom
1151 Posts

Posted - 07/07/2011 :  08:10:45  Show Profile
David, there are no shims in the Godfrey Marshall Mk22 Cabin blowers.

Bob, they are a relatively simple blower to dismantle and providing the front axis nuts are not over tightened then this is a relatively simple blower to sort without the need of a specialist, far easier than the Volumex too as it is self timing. With the Mk22, there will be around 12 holes in the case to tap and plug before you can use effectively.

If you have to dismantle to replace the seals or generally clean out, then when you remove the main body section from the rotors, you need to very lightly tap the front of the rotor shaft, any harder and you can damage a thin integral lip inside the front of the case.

If you need some guidance then do let me know as I have rebuilt many.

Richard

Vintage MG Parts

Edited by - Richard Hardy on 07/07/2011 08:17:43
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David Allison

United Kingdom
665 Posts

Posted - 07/07/2011 :  10:06:51  Show Profile
Dear all - I made a number of errors mainly due to typing through my lunch hour!
I am aware that my diatribes are too long as well.
However I do endeavour to be technically accurate - perfect I aint.

Racing Snake - I meant negative bar - Edwards actually measure -50 bar as a scale of measurement rather than measuring atmospheric pressure (because in a vacuum as you point out - there isnt any).

Although Vane blowers do need lubrication (much more so than roots type) - centrifigal blowers tend to need little or no lubrication plus are lighter construction. I was trying to make a point too quickly and didnt qualify my argument correctly.

Richard - you are correct there are no shims in a Godfrey supercharger - however you measure the clearance between the rotors and casing using a piece of shim material (commonly called a "rule" because of its shape and length) I thought I had qualified this but obviously not.

Gordon - Bob my apologies I wasnt trying to teach or preach just to exchange information.
The flight article is based on small cabin blowers of similar construction to superchargers but it has little technical information because the "services" do very little maintenance themselves on such components.
The point about superchargers being a blind alley is based upon personal interest in the development of the internal combustion engine.
From a pure engineering point of view the supercharger has, on the petrol engine at least "had its day" - there are benifits of using forced induction but to make it work efficiently requires a great deal of computer power.

Racing engines are different - they are searching for the Nth degree of power and in this case forced induction is indeed worthwhile.

Diesel engine development is one area touched upon and because the diesel is a more efficient user of its fuel than the petrol engine, it benefits more easily from forced induction without losing too much of that efficiency.
However forced induction diesel engines are less fuel efficient than their normally aspirated forebears.

The new formula for grand prix racing will use a 1600 cc turbocharged V6 engine with restricted boost pressure and fuel tank size.
The idea is promote the development of more fuel efficient petrol engines - this is a comendable idea but I cant help but remember the last turbo F1 cars and their quest for ultimate horsepower - BMW and Honda both achieving close to 2000 HP from a 1500 cc engine.

Superchargers are as Richard points out very easy to dismantle and re-assemble.
However they often have "issues" lurking inside the casings such as, damaged rotors, damaged casings, stripped threads, damaged gears which can make them tricky to re-assemble.
The point of seaking proffesional help (like Richard among others) is that they often have access to special tooling, spare parts and expertise - which make re-assembly of these units much simpler and more cost effective.

Personally I would use a modern blower rather than a type 22 Marshall as in my opinion there is considerably less work involved in fitting the unit and because the componentry is newer the new unit will always be in better nick!
You can go really up market by fitting brand new 4 rotor roots blower - these are more efficient than the pre-war type design and lighter too, choose a blower of around 1100 cc for a 750-1000 cc engine and you will get around 7-10 psi boost at 5000 rpm.
For more boost - simple use a bigger blower - the NO special ran a 2000 cc blower on an 1100 cc engine and this gave us 20-30 psi boost at 5000 rpm.
Being brand new you get lots of shiny aluminium too.

The newest Marshall Godfrey blowers are now 60 years old and most have been used - even cabin blowers have worn casings and rotors.

But hell what do I know?
Giggles and regards David


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Gordon

United Kingdom
608 Posts

Posted - 07/07/2011 :  10:31:45  Show Profile
In the booklet I referred to, there is a picture of the internals of a MK 22 which shows a selection of "Inner ball race shims". Whether any were actually needed in the build up of a specific unit I don't know but obviously the designers recognized there might be a need. Richard, when you say there were no shims in the MK 22 were you referring to some other location?

Slightly off topic:
On the Wade supercharger that was used as a air blower by the Royal Navy there are fewer holes to seal up! Just 2 breathers either end of the blower and the oil filler caps to make pressure/suction tight. All very simple in practice and no need to machine the casing to install seals.

Gordon
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David Allison

United Kingdom
665 Posts

Posted - 07/07/2011 :  11:35:32  Show Profile
The shims reffered to are to adjust the end float on the drive gear bearings - these are critical in that it is possible to over tighten the securing nuts if they are not set up correctly.

The Wade blower is very similar to the Marshall (manufactured by the same group of companies) but later in design. The breather holes and oil filler caps should have one way valves on them - a ball valve with a spring behind is normal practice which should be sealed up if used as a supercharger.

The Wade is quite a bit heavier in construction to the earlier Marshall and often has a secondary drive housing at either end of the pressure casing which contains gears at either end of the rotor shafts.

David
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John Reid

United Kingdom
609 Posts

Posted - 07/07/2011 :  12:02:47  Show Profile
My engine is fitted with a side-mounted Marshall IZ87 supercharger (kindly rebuilt by Racing Snake!), a model which was fitted to many 6-cylinder MGs.

I also have sitting on the shelf a Marshall IZ97. It has a nose piece for twin v-belt drive which is identical to the IZ87. The inlet and oulet ports are identical in size and shape, as are the stud holes. However the casing is about 10mm longer, and in the body casting you can see the lines where an insert has been put in the mould. The rotors are identical in cross section to the IZ87, but longer by the length of the insert. My blower is one of a pair which came off a Ford V8 engine.

I have been unable to find any reference to the IZ97 - does anybody know anything about them?
John R
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David Allison

United Kingdom
665 Posts

Posted - 07/07/2011 :  12:57:05  Show Profile
IZ 97 is a larger blower - I think the model numbers were something to do with the swept volume.
David A
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Richard Hardy

United Kingdom
1151 Posts

Posted - 08/07/2011 :  00:40:05  Show Profile
Gordon, there are no ball race bearings in the Mk22, they are brass caged roller bearings and rather nice they are too. As mentioned, there are no shims whatsoever anywhere in the blower. As David says, you need to set the rotors equidistant between the case ends and can use a feeler gauge

Regards

Rich

Vintage MG Parts

Edited by - Richard Hardy on 08/07/2011 00:41:25
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Bob Stringfield

United Kingdom
854 Posts

Posted - 08/07/2011 :  11:05:53  Show Profile
There was a large blower commonly available in the 'sixties and 'seventies from scrapyards, I think from Rootes ( no connection with Roots ) Commer TS3 two-stroke lorry engines. Friends in the vintage world were happy to buy them.

Bob.
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Gordon

United Kingdom
608 Posts

Posted - 08/07/2011 :  13:28:26  Show Profile
Richard,
Let me quote from the document I have which is for the Marshall MK 22 cabin supercharger:
"Rotors
12. Each complete rotor assembly is dynamically balanced, and mounted in the supercharger housings by a ball bearing at the rear, and a roller bearing at the front end."
"Body
7. Between the body and the intermediate plate, gaskets of varying thickness control the end clearance of the two rotors."

My understanding is that to control the axial end float of a shaft you cannot do this with roller bearings at both ends. You need a ball race at one end to provide axial location and properly shim inner to outer tracks to provide the correct end load and a roller bearing at the other end to take care of differing thermal expansion between casing and shaft.

The drawings and explanation are quite clear and I can only think that your blowers are very different to the information I have.

Incidentally the Mk IX has the same bearing arrangements.


Gordon
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Richard Hardy

United Kingdom
1151 Posts

Posted - 08/07/2011 :  16:51:06  Show Profile
Very interesting Gordon

Of all the many Mk22's I have rebuilt, I have never come across any with ball race bearings, all have been rollers at each end. Yes, I agree, there are sometimes extremely this paper gaskets used between the main housing and gear housing sections.

Maybe the method of manufacture changed through production. I have had several new old stock blowers in the past too which have been wired with lead seals, so maybe production technique did change. Who knows

Rich

Vintage MG Parts
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