Constantin Supercharger (French Roots type)

[Pelle_Racing]

Hi Guy,

To accompany the TC Spider in the garage I bought a 1938 Citroën 11BL historic race car (Light 15, in the UK a think) last September.

Racing at Sturup Raceway, Sweden September '08

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Now to the question (first a little background):
It should be running with a vintage french supercharger from the company "Constantin", which supplied after market superchargers for Peogeuts and Citroëns beginning in the 40's.

The car now runs with an supercharger a few "years" to new (for the race series regulations).

With the car I got a very nice (and quite huge) Constantine supercharger. Unfortunately the previous owner had a "Nitro-Dragster-Specialist" put teflon strips on/in the rotors. This was done very poorly and the car only went ~200meters before total blower brakedown.

I have taken the blower apart and all but the rotors looks ok, and I'm quite sure that I have fund a way of remaking two new rotors for the blower (I have modelled them up in CAD using geometrical formulas from some Roots patent and smooth-talked the machinists at the company toolshop to wire-cut them!)

How to the question:
Now that I'm having new rotors made, i might as well get the clearances perfect. Do you know what the suggested running clearance should be for a blower, both rotor-rotor and rotor-casing? I have searched Google for hours and hours, with no result.¨
Another thing, how do you time up these things? There a no "keys" on the shafts or gears, they appear to be press or shrink fit.

Below you can see a picture of a similar supercharger (off the internet).

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Best regards

Sune

[robert kenney]

I think you are looking for an absolute clearance spec. Take these numbers only as a referance and not a rule. I have inspected non stripped blowers and specs for rotor to housing range from .004 - .006, mesh clearances when taking out gear lash were .006-.008. These were older roots 4-71 styled units designed for very moderate boost levels under draw through apps. IIRC 5 - 10# boost.

Every blower and operating regime is going to require different setup clearances. At boost pressures the rotors deflect and can touch. That is where the experience comes in.

You need to take into account the rotor rigidity (a function of diameter and length) and the expected boost and charge temperature for rotor thermal expansion and the material CTE. Your clearance requirement is compounded by the fact that you have designed your own rotors.

I run teflon inserted MoonyHam 6 and 8-71 blowers on the street all of the time. The last was a 6-71 running 15#. After 2 years it still develops the same rpm/boost profile. I would recommend stripping the rotors and always run fuel through the blower. It helps lubricate the Teflon strips and reduce the rotor and charge temperature. The stripping runs 0 or real close to 0 clearance to the housing so it develops the best pressure profile and is the most efficient.

Many blower manufacturers apply a abradable surface to the rotors that allows the blower to mate during the first 100-200 miles as the rotors touch.

What is your specific application? Expected max boost.

I can't give you exact values for rotor clearance but I wanted to post my blower experience and knowledge gleamed from conversations with major blower builders in the US.

Guy I hope I have not over stepped my boundaries posting to this question in GC Q&A.

Robert

[Pelle_Racing]

Hi Robert,

Thank you very much for the knowledge-sharing!

As I haven't done any of the modifications to the car myself, I don't have a lot of "insight", YET.
(The history and know-how of the stock car is well known, as my father is a long time ensusiast of these cars)
But I'll try to list as much as I know:

The car is basicaly only for racing. Completely stripped out with racing seat, Dunlup Vintage Racing tyres etc.

The basic application (apart from the car in the picture) is a four cylinder OHV 1911ccm engine with a bore of 78mm and a stroke of 100mm.
Stock (factury) output was 45-60BHP and this engine is quite heavyly modified with ported head, bigger valves, all balanced, lightened flywheel. All the "usual" stuff, and has been rolled at 105HP at the front wheels, which should correpond to about 120-125 at the whywheel (with supercharger).

The "redline", I guess is around 6500-7000 RPM, but the engine clearly peaks around 4500RPM as it is now.
With regards to the boost (by the way, the blower is placed between the carburettor and the intake, so fuel is going through the blower) it is fitted with a bolw off valve, in order to be able to adjust the boost.
Is this at all a good idea?
The previous owner has run it at about 1.2-1.5 PSI boost.
I have no idea if these numbers all add up to make sence(?)

You have a good point about the gear lash. I had completely forgotten about that.
I'll see what I can make out of it and post whatever progress I'll do here.

Guy might be able to help me coat the new rotors with an abrassive coating. I know that he has been doing that to Volumex blowers.

Best regards

Sune

[robert kenney]

Sune,
I didn't mention this before but I love nostalgia racing of all kinds. Your car is great!!! What a great pass time.
My racing back ground may not be road racing but I enjoy seeing the determination and sportsman ship in it all.

With regards to the boost (by the way, the blower is placed between the carburettor and the intake, so fuel is going through the blower) it is fitted with a bolw off valve, in order to be able to adjust the boost.
Is this at all a good idea?

On a draw through blower application like you have the so called blow off valve is a safety valve that is designed to relieve the plenum pressure in the event of an intake back fire. During the backfire all of the compressed air/fuel mixture in the intake manifold would be ignited and it needs to be relieved. With out the valve the blower would/could be blown off of the engine. All current day blown drag race applications are mandated to use a burst plate/panel to protect against blowbacks.


The boost pressure is solely controlled by the engine to blower fixed drive ratio. There is no other way to control boost pressures on a draw through setup. Other items like valve timing and engine VE (volumetric efficiency) will play a part in peak boost pressures.

Relieving the manifold pressure below the blower on your application with the carb above the blower, will allow the explosive compressed air fuel mixture to be vented. This is not a good thing. I advise against that. Except in the event of blow back

See attached picture of my current project (funny car) and the 2-year-old application (black roadster) I referred to earlier. The funny car photo shows the burst panel on the front directly under the blower. The roadster has no burst panel or relief valve but is attached using aluminum studs that are designed to fail under the high stress of a blowback. Thought I would share a bit of my own addiction.

Robert

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[Pelle_Racing]

Hi Robert,

That too (the need for a blow back releive valve), I did not know! Getting so much smarter here.
So the conlusion is that it is good to have a relieve valve, but it should not be used to adjust the boost by venting the fuel/air mixture into the engine bay (as the current setup is!).
I allready have the engine out of the car, so I'll see what I can do.

As there is very little information to be found about the original applications with the Constantin blowers I'm abit in the dark with regards to basic setup, like recommended maximum speed.
Any ideas on this, from your experiance with "older" superchargers? The engine itself is safe up to 6500-7000 RPM.

It is one massive blower you have on the roadster! Almost bigger than the engine... Thanks for sharing the pixtures. I'll try to get some pictures in from the workshop.

Best regards

Sune

[robert kenney]

Sun,
On many older vintage blowers the blow off valve was a spring loaded aluminum plate in a location similar to the burst panel on the funny car blower I posted. Many guys added extra strong valve springs to the plate to prevent early plate release. Simply relieved the excessive pressure and shuts again.

I have played with the idea of adding a recirculating air valve to a street blower much like the modern day units used with EFI. Don't know how the carburetors would act under those circumstances, so never tried.

Max rpm for a straight cut rotor roots blower is typically 8000-8500 rpm and it's efficiency goes away beyond use. I am making the assumption that because you are going to wire cut the rotors they won't have any helix in their geometry.

Blowers with any helix can exhibit varying efficiency maps. For example our blown funny car engine had a straight rotor 14-71 on it and we ran it 10% under driven and had a peak boost pressure of 30-32 pounds @ 8500 engine rpm. The long rotors needed constant re stripping and chasing of the blower pressure and fuel curve to match the air volume. A big pain in the neck in bracket racing.

We are now running a 10-71 high helix Littlefield blower 30+% over driven producing 24 +/- pounds @ 8500 and the car is faster due to a higher adiabatic efficiency which equates to lower charge temperatures. After some 80 passes the blower is still producing the same air and pressures but has not yet been restriped.

The Roadster is a real fun car to drive with 700 horse power and 2300 pounds. It needs better headers for sure to wake it up.

Sorry to ramble......Robert