Super charged 4

[Peter Cobbold]

Tony,

I'd be wary of using the bypass that Moss supply.

In Mercs etc the blower is declutched when the bypass is open to the manifold. Even so that can heat up the air in the blower and the blower:

http://www.superstreetonline.com/how-to/engine/0802-turp-supercharger-tech/

Moss dont allow for the blower to be stalled, so when the bypass isolates it ( at cruise) the air trapped inside will get hot, and the blower too.

To me, Moss got it wrong. Keeping mixture running through it at cruise will keep the blower nice and cool as the fuel evaporates with no risk of cooking the rear blower bearings.

Peter

Edited January 23, 2017 by Peter Cobbold

[Tony]

Tony,

I'd be wary of using the bypass that Moss supply.

In Mercs etc the blower is declutched when the bypass is open to the manifold. Even so that can heat up the air in the blower and the blower:

http://www.superstreetonline.com/how-to/engine/0802-turp-supercharger-tech/

Moss dont allow for the blower to be stalled, so when the bypass isolates it ( at cruise) the air trapped inside will get hot, and the blower too.

To me, Moss got it wrong. Keeping mixture running through it at cruise will keep the blower nice and cool as the fuel evaporates with no risk of cooking the rear blower bearings.

Peter

Hi Peter,

Interesting article, i see what you are saying, will have to see what happens once it is up and running, i have concentrated on trying to keep as much heat away from the blower

as possible with heat shielding and air flow. For engine cooling i have a triple cored radiator and fitted it with a 17 inch fan which covers about 90% of the radiator core.

I was thinking more about heat soak from the blower to the engine block, but have a heat shield between exhaust manifold and blower inlet maifold with a cold air supply flowing between the two.

will have to wait and see how effective this setup is.

 

regards tony

[Peter Cobbold]

Hi Peter,

Interesting article, i see what you are saying, will have to see what happens once it is up and running, i have concentrated on trying to keep as much heat away from the blower

as possible with heat shielding and air flow. For engine cooling i have a triple cored radiator and fitted it with a 17 inch fan which covers about 90% of the radiator core.

I was thinking more about heat soak from the blower to the engine block, but have a heat shield between exhaust manifold and blower inlet maifold with a cold air supply flowing between the two.

will have to wait and see how effective this setup is.

 

regards tony

Hi Tony,

Those modifications sound good, heat is the enemy of power !

.

The casing of my old Wade runs at ca 40C with all the mixture going through it all the time. And it's on the opposite side of the engine form the exhaust. The heat comes from the turbulence set up in the gas by the thrashing the rotors give it. If there was a bypass closing off the mixture flow I suspect that gas and then the casing would get much hotter as the same trapped air gets thrashed over and over. There would also be no fresh through-flow of cool mixture to carry that heat away.

There was a post about a year ago mentioning a Moss M62 on a Canadian TR6 failing after 10k miles....rear bearings cooked I think. Might be worth making enquiries.

I think John 'bjerv' on here has tested the temperature of the rear of the blower on his Moss kit ( TR6) which also has a bypass. Stick-on temperature-sensitive labels are available that change colour. eg:

http://temperature-indicators.co.uk/acatalog/Industrial_10_Level_Indicators.html

Peter

Edited January 24, 2017 by Peter Cobbold

[Tony]

Hi Tony,

Those modifications sound good, heat is the enemy of power !

.

The casing of my old Wade runs at ca 40C with all the mixture going through it all the time. And it's on the opposite side of the engine form the exhaust. The heat comes from the turbulence set up in the gas by the thrashing the rotors give it. If there was a bypass closing off the mixture flow I suspect that gas and then the casing would get much hotter as the same trapped air gets thrashed over and over. There would also be no fresh through-flow of cool mixture to carry that heat away.

There was a post about a year ago mentioning a Moss M62 on a Canadian TR6 failing after 10k miles....rear bearings cooked I think. Might be worth making enquiries.

I think John 'bjerv' on here has tested the temperature of the rear of the blower on his Moss kit ( TR6) which also has a bypass. Stick-on temperature-sensitive labels are available that change colour. eg:

http://temperature-indicators.co.uk/acatalog/Industrial_10_Level_Indicators.html

Peter

hi Peter,

 

Have you read this? http://www.3800supercharger.net/how.html

 

It details how the by pass works, and the way i under stand it is the the unwanted air is dumped before the rotors, what do you think?

 

regards tony

[Peter Cobbold]

hi Peter,

 

Have you read this? http://www.3800supercharger.net/how.html

be

It details how the by pass works, and the way i under stand it is the the unwanted air is dumped before the rotors, what do you think?

 

regards tony

Hi Tony,

Useful site, thanks !

They dont use a clutched pulley so the rotors keep spinning. And its a factory design so it must be right. Maybe the bypass butterfly allows mixture that has been through the blower to be dumped back into its intake. That would heat the blower less than closing iff its intake. Apologies for raising the red herring.

That said, for decades blowers fed by a carb worked fine with no bypass. So I dont see what advantage a bypass gives, bearing in mind the extra complexity.

Peter

[Tony]

Hi Tony,

Useful site, thanks !

They dont use a clutched pulley so the rotors keep spinning. And its a factory design so it must be right. Maybe the bypass butterfly allows mixture that has been through the blower to be dumped back into its intake. That would heat the blower less than closing iff its intake. Apologies for raising the red herring.

That said, for decades blowers fed by a carb worked fine with no bypass. So I dont see what advantage a bypass gives, bearing in mind the extra complexity.

Peter

hi Peter,

I read some where that the SU is also modified to allow the piston to drop back onto the bridge on over run, other wise the vacuum could hold the piston up. I think it was on an American site , wish i had saved it now,

 

regards tony

[Peter Cobbold]

hi Peter,

I read some where that the SU is also modified to allow the piston to drop back onto the bridge on over run, other wise the vacuum could hold the piston up. I think it was on an American site , wish i had saved it now,

 

regards tony

Hi Tony,

Thats a new one on me. I spent a lot of time 'reverse engineering' the SU to see how it works. The air flow to the blower will be shut off by the SU butterfly and I very much doubt the blower could cause a butterlfy to hang open. Mine never has a problem like that and it is never mentioned in Allard etc books. Once the SU butterlfy closes, air can still pass from the filter across the bridge as the seal with the piston is not complete- your HS8 should have two circular nylon inserts ( projecting ca half mm) on the underside of the piston to maintain a small gap. That flow kills the 'constant depression' needed to hold up the piston. Mine is an ex-Rover HS8 and has the inserts, its not a special blower mod.

Some carbs have a small bore drilling from near the jet mouth to the engine side of the butterlfy- it richens the over-run mixture. It might be possible for on over running blower to suck air than way, but the piston/bridge leak would I think still kill any creation of depression sufficient to lift the piston.

I have read that an over-running blower ( no bypass) can create greater depression in the blower intake than the engine will. So a limiting valve to limit the extra vacuum getting to the brake servo was used. Maybe that is the reason for the bypass, it will leave the overurn vacuum set by the engine, as in a normally aspirated engine. On the other hand my servo works fine with no bypass and no valve...maybe I've been lucky.

Peter

Edited January 24, 2017 by Peter Cobbold

[Tony]

Hi Tony,

Thats a new one on me. I spent a lot of time 'reverse engineering' the SU to see how it works. The air flow to the blower will be shut off by the SU butterfly and I very much doubt the blower could cause a butterlfy to hang open. Mine never has a problem like that and it is never mentioned in Allard etc books. Once the SU butterlfy closes, air can still pass from the filter across the bridge as the seal with the piston is not complete- your HS8 should have two circular nylon inserts ( projecting ca half mm) on the underside of the piston to maintain a small gap. That flow kills the 'constant depression' needed to hold up the piston. Mine is an ex-Rover HS8 and has the inserts, its not a special blower mod.

Some carbs have a small bore drilling from near the jet mouth to the engine side of the butterlfy- it richens the over-run mixture. It might be possible for on over running blower to suck air than way, but the piston/bridge leak would I think still kill any creation of depression sufficient to lift the piston.

I have read that an over-running blower ( no bypass) can create greater depression in the blower intake than the engine will. So a limiting valve to limit the extra vacuum getting to the brake servo was used. Maybe that is the reason for the bypass, it will leave the overurn vacuum set by the engine, as in a normally aspirated engine. On the other hand my servo works fine with no bypass and no valve...maybe I've been lucky.

Peter

Hi Peter,

 

Found the page about SU 's on super chargers, here's the link, the paragraph is at the bottom of the page,

https://www.mossmotors.com/forum/forums/thread/9344.aspx

 

regards tony

[Peter Cobbold]

Hi Peter,

 

Found the page about SU 's on super chargers, here's the link, the paragraph is at the bottom of the page,

https://www.mossmotors.com/forum/forums/thread/9344.aspx

 

regards tony

Hi Tony,

The post 11922 outlines supposed problems that old school supercharger experts never came across. Allard and SU showed a 2" SU is OK for 220hp with none of the mods mentioned in that post. Allard did sometimes add weight to the piston to increase downforce on the piston. Yet the post suggests the piston is failing to lift! Allard also used stiffer dashpot oils to slow the piston lifting The post makes intepretations that are not supported by measurements of either pressures in the choke-tube/bridge, piston lift or AFR. I take their interpretations with a very big pinch of salt. And I reckon the source of the problems lies with the bypass:

My interpretaion of the problem they describe is that sudden flooring of the throttle allows boost to pass back through the still-open bypass into the SU,past the wide open butterfly and into the choke tube. The 'constant depression' in the choke tube that is absolutely fundamental to SU metering turns into positive pressure.** The boost then enters the SU dome above the piston and pushes it down !! Their corrective drillings then allow the dome pressure to escape into the narrow gap above the bridge, pushing the piston up. Its all the fault of the bypass, thus.Upon flooring the throttle boost will build to max in less than one tenth of a second. The bypass must inactivate faster than that to stop the SU seeing boost: but does it really work that fast ?. If the bypass has not closed it only needs a bit of boost to screw up the SU. Allard , Norman etc never fitted bypasses so the problem never arose. Moss seem to have tried to cure a problem inroduced by the bypass transiently allowing retrograde boost, by fettling the SU - they had not traced the issue back to its root cause.

 

Their suggestion that sudden fast air flow will create too much suction and stop the piston lifting is plain wrong. But forgivably so, as it is often said the piston/ bridge gap creates a venturi effect and hence a depression. But an SU does not do that, it is not a Weber! The sharp leading edge to the piston and the steep leading angle of the bridge are both designed to kill laminar flow and create turbulence, and so to deliberately abolish the venturi effect. Even the SU tuning books says dont try to chamfer the corner on the piston and dont smoothe the bridge with a ramp of plastic metal. But they don't say why not.The reason why those mods fail is because they keep the air flow more streamlined and so a venturi depression will build in the bridge/piston gap and that sucks the piston down. The ventrui depression counters the 'constant depression' that lifts the piston and screws up air metering.

 

Could you take a look at your SU piston to see if those drilling mods have been done ? That post is 10 years old and they might have revised their approach since then. I'd be interested to know the spring rate and the needle profile as it will simplify my SU retuning. I use around 32oz total downforce ( lots of lead on the piston) and intend to remove it

 

The kit for the TR6 seems to have been killed by Moss during a switch form a Holley to SU carb. If they still have similar conceptual struggles as in that post I'm not surprised. But the answer would be to ditch the bypass and implement the blower installation as Allard and Norman would have. Keep it simple!

Austin Branson has a forerunner to the Moss TR6 kit , the Vespertini. It works fine with no bypass:

https://supertrarged.wordpress.com/2013/05/14/austin-bransons-tr6-verspertini/

 

 

Peter

 

** The constant depression in the choke tube- ie between butterlfy and piston - is small, maybe 0.25psi depending upon the spring. See top page 3:

https://supertrarged.files.wordpress.com/2015/05/the-operating-limits-of-an-su-carburettor12.pdf

So it only needs a tiny fraction of the boost to leak back through the bypass to the choke tube to kill that depression, and then to raise the choke-tube pressure a bit above atmospheric,forcing the piston down.

Edited January 25, 2017 by Peter Cobbold

[Tony]

Hi Tony,

The post 11922 outlines supposed problems that old school supercharger experts never came across. Allard and SU showed a 2" SU is OK for 220hp with none of the mods mentioned in that post. Allard did sometimes add weight to the piston to increase downforce on the piston. Yet the post suggests the piston is failing to lift! Allard also used stiffer dashpot oils to slow the piston lifting The post makes intepretations that are not supported by measurements of either pressures in the choke-tube/bridge, piston lift or AFR. I take their interpretations with a very big pinch of salt. And I reckon the source of the problems lies with the bypass:

My interpretaion of the problem they describe is that sudden flooring of the throttle allows boost to pass back through the still-open bypass into the SU,past the wide open butterfly and into the choke tube. The 'constant depression' in the choke tube that is absolutely fundamental to SU metering turns into positive pressure.** The boost then enters the SU dome above the piston and pushes it down !! Their corrective drillings then allow the dome pressure to escape into the narrow gap above the bridge, pushing the piston up. Its all the fault of the bypass, thus.Upon flooring the throttle boost will build to max in less than one tenth of a second. The bypass must inactivate faster than that to stop the SU seeing boost: but does it really work that fast ?. If the bypass has not closed it only needs a bit of boost to screw up the SU. Allard , Norman etc never fitted bypasses so the problem never arose. Moss seem to have tried to cure a problem inroduced by the bypass transiently allowing retrograde boost, by fettling the SU - they had not traced the issue back to its root cause.

 

Their suggestion that sudden fast air flow will create too much suction and stop the piston lifting is plain wrong. But forgivably so, as it is often said the piston/ bridge gap creates a venturi effect and hence a depression. But an SU does not do that, it is not a Weber! The sharp leading edge to the piston and the steep leading angle of the bridge are both designed to kill laminar flow and create turbulence, and so to deliberately abolish the venturi effect. Even the SU tuning books says dont try to chamfer the corner on the piston and dont smoothe the bridge with a ramp of plastic metal. But they don't say why not.The reason why those mods fail is because they keep the air flow more streamlined and so a venturi depression will build in the bridge/piston gap and that sucks the piston down. The ventrui depression counters the 'constant depression' that lifts the piston and screws up air metering.

 

Could you take a look at your SU piston to see if those drilling mods have been done ? That post is 10 years old and they might have revised their approach since then. I'd be interested to know the spring rate and the needle profile as it will simplify my SU retuning. I use around 32oz total downforce ( lots of lead on the piston) and intend to remove it

 

The kit for the TR6 seems to have been killed by Moss during a switch form a Holley to SU carb. If they still have similar conceptual struggles as in that post I'm not surprised. But the answer would be to ditch the bypass and implement the blower installation as Allard and Norman would have. Keep it simple!

Austin Branson has a forerunner to the Moss TR6 kit , the Vespertini. It works fine with no bypass:

https://supertrarged.wordpress.com/2013/05/14/austin-bransons-tr6-verspertini/

 

 

Peter

 

** The constant depression in the choke tube- ie between butterlfy and piston - is small, maybe 0.25psi depending upon the spring. See top page 3:

https://supertrarged.files.wordpress.com/2015/05/the-operating-limits-of-an-su-carburettor12.pdf

So it only needs a tiny fraction of the boost to leak back through the bypass to the choke tube to kill that depression, and then to raise the choke-tube pressure a bit above atmospheric,forcing the piston down.

hi Peter,

that was an interesting read, i will have a look at the SU when i have time, i know that Moss supplied a small bottle of "special " oil to be used in the dash pot.

 

regards tony