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bally Samba Member

Joined: April 29, 2006 Posts: 1194 Location: Melbourne, Australia
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Posted: Mon Mar 02, 2009 3:22 am Post subject: 36hp Judson vs 40hp Judson? |
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Hello,
My late 55 Vert currently has a '61 1600 single port fitted.
I have options on a reconditioned '58 36 horse engine and a Judson to suit and I have a spare 59 40 horse engine that I could use instead, again I'm looking at a Judson to suit. I'm inspired by Vic's 40 horse old speed engine so am considering giving the '59 a bit of a tweak and then fitting the Judson.
Will a 40horse Judson work on a 36 horse engine?
Reliability is important to me. Is there much of a difference between a 40hp and 36hp Judson powered engine when it comes to reliability?
Any tips or suggestions greatly appreciated.
Many thanks,
Dave
Last edited by bally on Mon Mar 02, 2009 6:06 am; edited 1 time in total |
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John Moxon  Samba Moderator

Joined: March 07, 2004 Posts: 14183 Location: Southampton U.K.
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Posted: Mon Mar 02, 2009 5:50 am Post subject: Re: 36hp Judson vs 40hp Judson? |
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bally wrote: |
Will a 40horse Judson work on a 36 horse engine? |
No the pulleys won't line up
Quote: |
Reliability is important to me. Is there much of a difference between a 40hp and 36hp Judson powered engine when it comes to reliability? |
Depends on how you drive them and the quality of the Judson you fit. If you want top end performance and drive a Judson accordingly you could come unstuck, through the stock carburations inability to keep up and the engine's dislike for being over revved.
Another minus on a Judson driven for out and out performance is an inherent weakness of sliding vane superchargers. They mysteriously loose boost when the hard driven charger reaches a certain temperature. Back off for a while and the boost comes back up but I have yet to hear of a scientific reason for this.
However if you use the Judson for how it was intended, to improve mid range torque it changes the whole nature of the 36 and 40hp engines.
Best tip, drive it like a stock 36hp or a stock 40hp and you'll find it does much more...a delight to drive with few vices. _________________ John.
Judson Supercharger Information on The Samba
My 1958 Shorrock Supercharged Karmann Ghia
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bally Samba Member

Joined: April 29, 2006 Posts: 1194 Location: Melbourne, Australia
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Posted: Mon Mar 02, 2009 6:12 am Post subject: Re: 36hp Judson vs 40hp Judson? |
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John Moxon wrote: |
bally wrote: |
Will a 40horse Judson work on a 36 horse engine? |
No the pulleys won't line up
Quote: |
Reliability is important to me. Is there much of a difference between a 40hp and 36hp Judson powered engine when it comes to reliability? |
Depends on how you drive them and the quality of the Judson you fit. If you want top end performance and drive a Judson accordingly you could come unstuck, through the stock carburations inability to keep up and the engine's dislike for being over revved.
Another minus on a Judson driven for out and out performance is an inherent weakness of sliding vane superchargers. They mysteriously loose boost when the hard driven charger reaches a certain temperature. Back off for a while and the boost comes back up but I have yet to hear of a scientific reason for this.
However if you use the Judson for how it was intended, to improve mid range torque it changes the whole nature of the 36 and 40hp engines.
Best tip, drive it like a stock 36hp or a stock 40hp and you'll find it does much more...a delight to drive with few vices. |
Thanks John,
I'm not looking for out and out performance, just an enhancement on what was around at the time so I can keep up with modern traffic (ie within the legal limits).
I'm still unsure whether to go for the 36 engine or the 40 - I'll have to give it some thought...
Cheers,
Dave |
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Komissar Samba Member

Joined: May 18, 2008 Posts: 894 Location: Bucharest, Romania
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Posted: Mon Mar 02, 2009 11:56 am Post subject: |
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From what I understand ... you already have the 40HP Judson hardware... so, by all means a 40HP Judson+ 40HP engine is the best combo.
You have more power than the 36hp engine, and also... the parts are easier to find end cheaper...
If you like the 36HP look, (without the fresh air heather hoses) you can go for a 36HP-style fan shroud for the 40HP ( like the ones on the 40HP's made until 1962 I belive ) and stale air heating systems... _________________ 1965 Sunroof Beetle
1972 Type 3 Squareback
1978 Skoda 105S |
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a-train Samba Member

Joined: October 11, 2003 Posts: 702 Location: Issaquah via Parkland, WA
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Posted: Mon Mar 02, 2009 12:22 pm Post subject: |
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You may already know this or you may be making the proper adjustments, but I don't believe that a judson will fit under a Convertible decklid due to interference with the rain catcher and the aircleaner (I think?). _________________ I like VWs |
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bally Samba Member

Joined: April 29, 2006 Posts: 1194 Location: Melbourne, Australia
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Posted: Mon Mar 02, 2009 1:39 pm Post subject: |
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Komissar wrote: |
From what I understand ... you already have the 40HP Judson hardware... so, by all means a 40HP Judson+ 40HP engine is the best combo.
You have more power than the 36hp engine, and also... the parts are easier to find end cheaper...
If you like the 36HP look, (without the fresh air heather hoses) you can go for a 36HP-style fan shroud for the 40HP ( like the ones on the 40HP's made until 1962 I belive ) and stale air heating systems... |
I've not got either Judson yet, but have located sources for both models. I do prefer the 36 horse look and already have a spare 36 horse fan shroud that I was going to use when building up my 1600... I'm not after awesome power, just a nice old speed remedy if I can do it.
As for the vert engine lid issue, I wasn't aware of that. Does anyone have a suggestion for getting round that issue of the air filter fouling the engine lid without removing the rain catchers please?
Cheers,
Dave |
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John Moxon  Samba Moderator

Joined: March 07, 2004 Posts: 14183 Location: Southampton U.K.
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vwhelmot Samba Member
Joined: May 04, 2006 Posts: 635 Location: united kingdom
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Posted: Mon Mar 02, 2009 2:56 pm Post subject: |
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I modified my cabrio lid as it was the air cleaner that was fouling the left hand rain catcher. It took a bit of working out, but it looks as though maybe the factory may have installed it. |
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John Moxon  Samba Moderator

Joined: March 07, 2004 Posts: 14183 Location: Southampton U.K.
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Shorrock Samba Member

Joined: January 02, 2005 Posts: 433
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Posted: Mon Mar 02, 2009 6:56 pm Post subject: |
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You can also lower the Judson by reducing the vertical height of the intake. I think I remember seeing one or two such modified "Judson Intakes" for sale over the years. You would need to use one of the new idlers on it to take up the slack and/or fit a smaller belt. I would recommend the idler anyway. I would also recommend the 40hp setup. |
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Shorrock Samba Member

Joined: January 02, 2005 Posts: 433
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Posted: Mon Mar 02, 2009 9:55 pm Post subject: Re: 36hp Judson vs 40hp Judson? |
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John Moxon wrote: |
Another minus on a Judson driven for out and out performance is an inherent weakness of sliding vane superchargers. They mysteriously loose boost when the hard driven charger reaches a certain temperature. Back off for a while and the boost comes back up but I have yet to hear of a scientific reason for this. |
Hi John,
With the Judson you have vanes that are in constant contact with the walls of the blower housing causing significant heat build up due to friction. Running the blower too hard (high rpm under load) too long will heat soak the housing. The phenolic vanes have great dimensional stability at elevated temperatures or heat resistance properties (low coefficient of thermal expansion) where the housing does not. The housing will heat up and expand until the clearances (now gaps) at both ends of the 4 vanes grows to the point that there is too much blow by to produce pressure. Back off and let the housing cool and presto boost is back. The Shorrock does not have this problem as the vanes are not in contact with the walls and the unit as a whole has similar thermal expansion properties. Cheers, Howard |
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John Moxon  Samba Moderator

Joined: March 07, 2004 Posts: 14183 Location: Southampton U.K.
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Posted: Tue Mar 03, 2009 1:39 am Post subject: Re: 36hp Judson vs 40hp Judson? |
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Shorrock wrote: |
Hi John,
With the Judson you have vanes that are in constant contact with the walls of the blower housing causing significant heat build up due to friction. Running the blower too hard (high rpm under load) too long will heat soak the housing. The phenolic vanes have great dimensional stability at elevated temperatures or heat resistance properties (low coefficient of thermal expansion) where the housing does not. The housing will heat up and expand until the clearances (now gaps) at both ends of the 4 vanes grows to the point that there is too much blow by to produce pressure. |
Hmmm, without getting too far off topic I'm not convinced that is the whole of the story Howard. Actually with the Judson the vanes aren't constantly in contact with the outer walls producing boost. The vanes slide out to seal when the engine is under load...back off and so too do the vanes. If you drive the Judson with a boost gauge it's easily illustrated...foot down, positive boost...back off, negative vacuum. If you're driving a route of ups and downs it's surprising how little of the time you drive with positive boost in the intake manifold.
For the dramatic reduction in boost it seems like the vanes refuse to slide out to make contact with the chamber wall. As we said before the effect is transitory but sudden and marked. If it were solely due to gradual expansion and blow-by you would think the reduction would also be gradual. Perhaps we need a seperate topic on this.  _________________ John.
Judson Supercharger Information on The Samba
My 1958 Shorrock Supercharged Karmann Ghia
Stop dead photo links! Post your photos to The Samba Gallery! |
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Shorrock Samba Member

Joined: January 02, 2005 Posts: 433
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Posted: Tue Mar 03, 2009 7:38 am Post subject: Re: 36hp Judson vs 40hp Judson? |
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John Moxon wrote: |
Hmmm, without getting too far off topic I'm not convinced that is the whole of the story Howard. Actually with the Judson the vanes aren't constantly in contact with the outer walls producing boost. The vanes slide out to seal when the engine is under load...back off and so too do the vanes. If you drive the Judson with a boost gauge it's easily illustrated...foot down, positive boost...back off, negative vacuum. If you're driving a route of ups and downs it's surprising how little of the time you drive with positive boost in the intake manifold.
For the dramatic reduction in boost it seems like the vanes refuse to slide out to make contact with the chamber wall. As we said before the effect is transitory but sudden and marked. If it were solely due to gradual expansion and blow-by you would think the reduction would also be gradual. Perhaps we need a seperate topic on this.  |
John, The law of centrifugal force dictates that the vanes in a Judson are in constant contact with the walls. It is opening the throttle "choke" that allows more air to enter that causes boost. Opening the throttle causing more boost and higher RPM forces the vanes to press harder and harder against all surfaces and along with the heat generated from compression increases the heat rise/load dramaticly. It does not take long relatively speaking for the housing to expand under full load. There is a point where the tolerances will open up not allowing the fuel/oil mixture to seal the gap, once that occurs blow by can be dramatic. The vanes would not stick as the retaining slots in the rotor would also grow larger causing even more blow by. In my humble and respectful opinion it is thermal expansion that causes the Judson to lose boost. Enough my good friend as I too could be just blowing hot air. Howard |
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John Moxon  Samba Moderator

Joined: March 07, 2004 Posts: 14183 Location: Southampton U.K.
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Posted: Tue Mar 03, 2009 12:23 pm Post subject: |
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Yes in normal circumstances sliding vanes when rotated will through centrifugal forces be thrown outwards to wards the chamber walls making the seal with increasing force as the rotor speed increases. However if you look at the Judson, the vane slots are cut not radially in line with the shaft but at an angle to the shaft. This acts to considerably reduce the effects of centrifugal forces. If I remember correctly Judson claim this almost cancels them out altogether, thus the speculation that the vanes aren't always in contact.
I have had long conversations with Alejandro Martin, I'm sure you have too, about this trying to understand what goes on inside the Judson in normal use but there are still issues that to me defy logic.
Any theories on this one Howard?:
The rotor rotates clockwise...which side of the vane slot wears? Logic says the shorter left side of the slot will suffer the greater wear by action of the vane against the chamber wall...in reality it's the right side.  _________________ John.
Judson Supercharger Information on The Samba
My 1958 Shorrock Supercharged Karmann Ghia
Stop dead photo links! Post your photos to The Samba Gallery!
Last edited by John Moxon on Wed Mar 04, 2009 1:26 am; edited 1 time in total |
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Shorrock Samba Member

Joined: January 02, 2005 Posts: 433
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Posted: Tue Mar 03, 2009 8:49 pm Post subject: |
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John,
From the Judson Patent "allows each vane to retract into its slot as it rides along the inner wall of the case as projected by centrifugal force". Centrifugal force is the essence of the sliding vane design. There is nothing within the design to counter act this constant force ever causing the vanes to retract away from the wall. They are indeed always in contact.
We would both agree that the vast majority of the Judson life is non-boost condition. With that stated the wear on the right side of the rotor slot is due to the vacuum being drawn during the intake stroke against the vane as it enters compression through discharge when not in a boost condition. During non-boost the vanes are pulled against the right side of the rotor during the greatest amount of travel (full to minimum extension). The blower when not in boost is an additional restriction to the intake flow with the engine trying to suck the vanes into it as it were. An example would be when you throw a belt on a well sealed blower it continues to spin right along with the engine. With the throttle nearly closed during cruise it is the engine doing the work not the blower. Open the throttle up removing the restriction air rushes in and boost is created which reverses the load on the vane (left side wear) but this is only a very short time compared to normal operation.
Again my humble and respectful opinion. Howard |
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John Moxon  Samba Moderator

Joined: March 07, 2004 Posts: 14183 Location: Southampton U.K.
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Posted: Wed Mar 04, 2009 1:23 am Post subject: Rotor wear |
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Shorrock wrote: |
John,
From the Judson Patent "allows each vane to retract into its slot as it rides along the inner wall of the case as projected by centrifugal force". Centrifugal force is the essence of the sliding vane design. There is nothing within the design to counter act this constant force ever causing the vanes to retract away from the wall. They are indeed always in contact.
We would both agree that the vast majority of the Judson life is non-boost condition. With that stated the wear on the right side of the rotor slot is due to the vacuum being drawn during the intake stroke against the vane as it enters compression through discharge when not in a boost condition. During non-boost the vanes are pulled against the right side of the rotor during the greatest amount of travel (full to minimum extension). The blower when not in boost is an additional restriction to the intake flow with the engine trying to suck the vanes into it as it were. An example would be when you throw a belt on a well sealed blower it continues to spin right along with the engine. With the throttle nearly closed during cruise it is the engine doing the work not the blower. Open the throttle up removing the restriction air rushes in and boost is created which reverses the load on the vane (left side wear) but this is only a very short time compared to normal operation.
Again my humble and respectful opinion. Howard |
Don't be so humble Howard...I'll go along with your explanation, I know I can always rely on you for a good answer.
I don't feel too bad about the highjack either as Dave has his answer plus a lot of info that's taken me over 10 years to understand.
What's next? _________________ John.
Judson Supercharger Information on The Samba
My 1958 Shorrock Supercharged Karmann Ghia
Stop dead photo links! Post your photos to The Samba Gallery! |
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bally Samba Member

Joined: April 29, 2006 Posts: 1194 Location: Melbourne, Australia
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Posted: Wed Mar 04, 2009 1:50 am Post subject: What's next? |
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John Moxon wrote: |
...Dave has his answer plus a lot of info that's taken me over 10 years to understand.
What's next? |
What's next?
As you say John, I'm 99% sure I'm going for the 36 horse engine and Judson. What other period tweaks should I be looking at for this setup to optimise the engines power? As you say, I've had the benefit of you vast experience along with the other contibutors here - why stop now
Cheers,
Dave |
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bally Samba Member

Joined: April 29, 2006 Posts: 1194 Location: Melbourne, Australia
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Posted: Wed Mar 04, 2009 1:52 am Post subject: Cabrio engine lid mods |
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vwhelmot wrote: |
I modified my cabrio lid as it was the air cleaner that was fouling the left hand rain catcher. It took a bit of working out, but it looks as though maybe the factory may have installed it. |
Thanks Lyndon - would you be able to post (or send me) some pictures of these mods? Where are you based in the UK? I'd like to have a look at these mods in the flesh please.
Cheers,
Dave |
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276995 Samba Member
Joined: February 20, 2008 Posts: 32 Location: AUSTRALIA
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Posted: Wed Mar 04, 2009 1:56 am Post subject: |
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Wow, all that info and I am still trying to get my carb dialled in so my 40hp runs with the blower on it.
Steve |
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John Moxon  Samba Moderator

Joined: March 07, 2004 Posts: 14183 Location: Southampton U.K.
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Posted: Wed Mar 04, 2009 10:13 am Post subject: Re: What's next? |
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bally wrote: |
What other period tweaks should I be looking at for this setup to optimise the engines power? |
I'm one for "toeing the party line" and the "party line" laid out by the men at Judson was that each Judson model was tailored to the characteristics of the engine to which it is to be fitted. They did the development work for each kit on a dynamometer along with thousands of miles of road testing. The kit supplied included everything required for the smooth and reliable running of the supercharged car.
You'll find their thinking here: http://vwjudsonregister.tripod.com/few_facts_pg1.htm
If you have limited experience of the workings of the flat 4 and or supercharging, I recommend you also stick to the party line. As Judson says it's done the work for you already...fit it and enjoy it.
If you're an experienced aircooled engine builder and are familiar with the strengths and weaknesses of the flat 4 and the effects of supercharging on it...then the stock Judson set-up will just be the starting point... _________________ John.
Judson Supercharger Information on The Samba
My 1958 Shorrock Supercharged Karmann Ghia
Stop dead photo links! Post your photos to The Samba Gallery! |
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