THE 4 LITRE ENGINE STORY By Alan C Smith IN PROGRESS
Part 2 THE RICARDO FILES.
I would like to thank the Review readers who contacted me after part 1 was published ( ISSUE 222 Page 291 ) giving little snippets of information about the 4 litre and also about other Bentley related matters. Also greatly appreciated were the kind comments about the article.
Preparing such an article is extremely time consuming and there is a responsibility to the reader and posterity to get it right, not least, to make it interesting. I take my hat off to authors like Mike Hay, whose hefty tomes, must have taken extreme dedication to write.
A lot of the points brought up by readers will be covered in following installments, and instead of discussing these points out of context, it would be best to wait until these events are covered in chronological order. It will make the story flow better.
Unfortunately, there was no 4 litre technical information offered, so would any reader with any information whatsoever, however insignificant, please take the time to contact me, as it could be an important piece in the puzzle, and will help future 4 litre owners.
One problem that I have had, was that I was unable to access the 4 litre drawings which are under the custody of the Foundation. However David Rolf has promised to let me have the key drawings I need. The reason I need to pore over these documents is manyfold. They contain drawings from the sidevalve version of the 4 litre, also there are two different versions of the production engine, and we need to know which parts are interchangeable between the engines. At the moment anyone requesting 4 litre drawings from the foundation may be sent the wrong drawings for their version of engine! So it`s important that this work is carried out.
The last time I was allowed to search in the files was february 2001. In the short time I was able to look, I discovered some anomalies which I think most people are unaware of.
My first discovery was a drawing for a magneto clamp plate. This was not used on the production engine, but could prove that the sidevalve engine was actually built, as the drawings in the possession of the foundation are prints that came from the inspection department, indicating that the part had been manufactured and been checked by the inspection department. The drawing office originals were destroyed after the liquidation of Bentley Motors, reputedly by WO.
The drawing office designation for the 4 litre engine is SV 1 ( sidevalve 1 ).
There are two different drawings for crankase, block, cylinder head, inlet and exhaust valves, valve springs, exhaust manifolds etc.The reason for this was that because of the haste to get the engine into production the design had to be ” frozen ” too early in development. So the design was continued on a second version of the engine in parallel, which was to replace the first version after the initial batch of 25 laid down had been manufactured. The reason for this was that Bentleys did not consider the engine they were putting into production was good enough and wanted to continue development to improve it.
For the purposes of identification in the following articles we will call these engines Type 1. ( early ) and Type 2. ( late )
Most of our readers will have never seen a 4 Litre car, let alone a 4 Litre engine. Until 4 years ago neither had I !
Most Bentleyphiles have seen under the bonnet of a 3 or 4 ½ litre where the engine seems to fill the space, exuding a heady testosterone rich bravura. The architecture of the engine is visually exciting with a myriad of little work intensive shapes and details to catch the eye, it seems almost to be alive. It looks the business, and it delivers what it promises, delighting both the driver and the onlooker.
When a 4 Litre bonnet is opened, it is somewhat of a disappointment. The engine seems shrunken, depressed, to almost hide, it is so low. There is definitely a crisis of identity here. Everywhere there is austerity except for the wonderful pair of SU HVG5 carburettors which, unfortunately, are not interchangeable with the 4 ½ 6 ½ type due to the fixed float chamber position, different mixture control and separate slow running device, similar but not the same as the 8 litre. Compared to the 4 ½ the 4 l engine looks about as appealing as a double helping of soggy cabbage on a school dinner plate.
THE VISIT TO RICARDO CONSULTING ENGINEERS Date 31 01 01
It was with a sense of schoolboy excitement that I drove from the Midlands down to Shoreham where I had an appointment to see John Loveridge of Ricardo Consulting Engineers ltd. My quest was to look at the Bentley 4 Litre file. After the formalities at reception, John led me through a maze of passages, where I caught glimpses of machine shops and heard the muffled roar of engines under test, as we walked towards the library.The library was huge, and after being introduced to Roland Christopher, the information manager, I was able to look at the file itself, a fairly thick grey A4 folder, which lay on a desk looking rather unassuming. John parked me in a quiet conference room, where at last I was able to open the file and look at it`s contents
The first disappointment is, that the file contains no personal correspondence between WO and Ricardo or Barrington and Ricardo The file comprises some numbered sketches, some engine test results and a lot of undated reports written on plain paper. This wasn`t going to be an easy task to collate. One of the problems is that some of the Engineering drawings, although numbered, did not have the completion date written on, but were just date stamped on the day of issue to Bentley Motors. This means that there will be uncertainty about chronology. Also there is no way of knowing if the file contains the whole of the technical information that was transmitted to Bentley Motors.
Just a word about drawing numbers. The normal practice in small drawing offices at that time was that the draughtsmen would sign out a drawing number from a drawing register that was kept in the office. If there were several ongoing projects in the office the drawing numbers for a particular project would not necessarily be in numerical sequence. So that the only way to see if there is a complete set of drawings is to be in possession of the drawing register or a specific project drawing or parts list. These were not available at Ricardos.
One thing we have to keep in mind is that the 4 litre was originally designed as a side valve engine and due to the poor power output obtained, the design was changed to incorporate the Ricardo patent High Power Head. As is usual with such a change of direction, and the time and financial constraints, the instructions from the board to the drawing office would have been the familiar dreaded words. ”Change as little as possible ”. The normal outcome is that the finished result usually resembles the proverbial dogs breakfast. We will see that this change produced some unusual design features in the 4 litre that I will outline in the following text.
THE FIRST DOCUMENTS IN ASCENDING DATE ORDER WERE:
INSERT DRAWING LF 778 HERE.
S.693 undated issued to Bentleys 30/01/30.
This seems to be the first drawing issued to Bentleys and comes a long time after WO had visited Ricardo in September 1929. There didn`t seem to be much urgency on Ricardos part.
The drawing is not reproduceable but luckily Ricardo sketch no LF. 778 which is based on S.693 is reproduceable and hopefully will be included with this article.
S.693 is a cross section through the High Power Head and the cylinder block showing the valve gear. Also a part plan section through the cylinder head showing the combustion chamber ( not shown on LF.778 ).
The first detail to strike the eye is that the top of the block is sloping down from the left hand side of the engine about 4 degrees. It took a long time to realise what the reason for this is. This is one of those compromises brought about by the change from side valve to the High Power Head. The camshaft was too far from the centreline of the engine for the new head layout, so that the exhaust valves in the block had to slope inwards by 4 degrees to the vertical to be in the correct position in the combustion chamber. The designers decided to put a slope on the top of the block so that the valve stems were perpendicular to it and therefore the top of the exhaust valves parallel to it. The pushrods for the inlet valves also slope from the camshaft to the rocker arm in order that they can be contained within the new rocker cover envelope which is dictated by the position of the spark plugs.
Looking at the inlet valve arrangement, the 50 mm diameter valve head is completely recessed into the cylinder head with a close gap between the side of the valve head and the block, with the valve head flush with the cylinder head, a condition that Ricardo calls ”masked”. Presumably this is to allow the cylinder head to be very close to the top of the piston in order to increase the compression ratio, which is alright in theory but gives problems in practice, as it will be difficult to vary the valve timing without the valve hitting the piston. It is a mistake to machine away this masking when re-cutting the valve seats as this has the effect of reducing the compression ratio. The adjustable ball ended tappets were not adopted by Bentleys who used a special rolling contact profile machined on the end of the rocker abutting onto a hardened square pushrod head.
The single valve spring seems very short for the size of the valve, which has a large diameter stem. The rocker itself seems massive and the pushrods although beautifully made with attention paid to lightness, are still too heavy.
Let me explain: the valve springs are not only to keep the valves seated onto their seats and gastight, they also have to restrain the mass of the valve and valve gear which is being accelerated by the camshaft. If the springs are not strong enough the result will be valve bounce or crash as it`s known, where the springs become ineffective at certain rpm. Of course stronger springs can be fitted but there is usually a penalty of heavy wear on the valvegear. In extreme cases overstressed springs can fracture, the 4 litre is in this category but more of this later.
The combustion chamber is positioned centrally over the exhaust valve and over part of the cylinder.The spark plug is positioned in the combustion chamber directly over the exhaust valve. One interesting thing is that someone has penciled in some freehand changes to reduce the throat area at the inlet valve. After the first 11 engines the diameter of the inlet valve was reduced.
S.706 dated 30/10/29 issued to Bentleys 30/01/30
This is the earliest readable date, a rather unusual drawing presumably in response to a request by Barrington due to a problem in mounting the 4 litre sidevalve engine into the chassis. Remember that the Bentley engines were all mounted on the chassis contributing to chassis stiffness and therefore to crankcase distortion in some way but for the 4 litre a different solution where the engine would be flexibly mounted, with no input loadings from the chassis, was obviously being sought after.
The drawing shows the 4 litre engine with in outline with a mounting lug at each corner. The engine is mounted in a sub frame which comprises two channel members parallel with the huge sidemembers supported by a massive lorry type channel cross member either end of the engine.
The amazing thing is that the engine is mounted on coil springs and has spring steel locating plates fore and aft and transverse to keep the engine in position, and a spring loaded arm mounted between the top of the rocker box and the bulkhead to take out the torque reaction. It`s hard not to be scathing about this design which seems to have been made by someone, perhaps a young graduate, with theoretical knowlege but little practical experience. I can`t believe that Bentleys were actually paying for this work and that it was coming from a leading consultant.
Needless to say Barrington did not use this design on the 4 litre.
C.281 02/10/29 issued to Bentleys 30/01/30
The original cam diagram.
S.693 A 25/02/30 no issue date to Bentleys.
This is an update to drawing S.693 the difference being that the inlet valve now has double valve springs, and the rocker has been changed to the rolling contact type for the pushrod end as on the production engines.
Obviously Ricardos response to the valve bounce problem.
S.870 no Dates. Probably June 31
This drawing, which has been badly traced in ink, shows the original S.693 drawing updated to the production engine configuration.
C.305 issued to Bentleys 26/06/31
Revised cam diagram with 12 mm lift.
Was this cam profile and timing ever used on the engine? Only a comparison with the 4 litre drawings will determine this. Conway, the Bentley Motors storeman, said That a camshaft was fitted to the 4 litre which used to break the valve springs. Another mystery to solve!
S.897 01/07/31 no issue date to Bentleys.
This drawing was issued a week before Bentleys went into receivership and three weeks after Ricardos had received a 4 litre engine for test.
The subject matter is an assister spring for the inlet valve which is shown mounted between the tappet at the camshaft and the upper part of the valve chest. This is a desperate attempt to get the inlet valve to work properly, clear proof that the inlet valve springs were not able to do their job properly as they were too short to give the required spring rate without being overstressed, thus making them susceptible to fatigue fracture. Possibly because a modified camshaft had been fitted to try and improve engine performance.
The engineering of this assister spring is awful, but it`s not the fault of the designer, the task he was given was not really viable. I can find no mention in the engine test notes that this assister spring was ever fitted to the test engine. Again it was just a waste of man hours to be lumped onto Bentleys bill.
The real solution was to modify the cylinder head to give longer valve springs and lighten the valve gear, but as we know, time had run out for Bentleys and this solution was not an option.
THE 4 LITRE ENGINE TEST.
On 08/06/31 the engine promised by Bentleys was delivered to Ricardo.
Allocated job No C 665 c. The engine No was VA 4092
This engine had been used for acceleration tests at Brooklands and the first test carried out by Ricardos was in this state of tune.
Ricardos carried out a series of tests firstly to try and improve the performance and then to try to cure the chronic pre-ignition. This is a fascinating insight into how Ricardos engineers tackled the job and deserves a separate installment to do it justice as unfortunately it would be too lengthy to be included in this episode.
To end this installment I will write down Ricardos engineers comments on the tests which were concluded on 20/08/31. The test engine was not used in a production car so what happened to it? Does it still exist somewhere?
REMARKS ON THE TESTS.
This is a verbatim transcript from an unsigned, undated hand written report.
The Bentley ” 4 litre ” provides a striking example of an engine which refused to give anything like it`s expected performance, and which would not yield to any treatment imposed on it. The question of the chronic pre-ignition from which the engine was suffering was never cleared up, although the disease itself was located. All the indications went to show that the exhaust valves were responsible, but no explanation is forthcoming as to why they should have behaved in this way. As the Chenard single cyl. Engine fitted with precisely the same head and having the same capacity is immume from pre-ignition while developing a much higher power output, it would appear that the reason lies in some feature of the 6 cyl. Block such as a tendency to distortion in the neighbourhood of the exhaust valve.
With regard to the power output the performance was always very poor. Max torque occurred as expected at about 2200 – 2300 rpm the best obtained being 106 lb/ sq in
BMEP.
Perhaps the most valuable piece of information gleaned from the tests is to be found in the test with the revised camshaft, see figs 8 and 9 and test 7, with which the torque was raised considerably at low speeds without seriously affecting that at high.
Authors Note: The Chenard single cyl engine results quoted in the above text were from 1926 also there was no mention of pre-ignition in the 4 litre single cyl. tests produced by both Ricardo and Bentley. I wonder why Ricardo did not draw upon the experience gained by Chenard in developing their 9cv engine, which should have been available to him via their subsidiary C.I.S, before presenting the cylinder head design to Bentleys.
The comment regarding the distortion of the cylinder block was taken from a suggestion by Bentleys, Ricardos did not prove this under testing.
Now just a word about what is happening in the 4 litre world. I just heard that an exciting project is nearing completion in Australia. This is a 4 litre engine which has been supercharged and fitted into a 3 litre chassis. Chris Williams has just rebuilt both the engine in his lovely 4 litre Saloon and the spare engine. David Smethurst is still valiantly struggling with the rebuild of his 4 litre saloon and engine and spare engine but needs pushrods, does anybody have any? My project at the moment is a 4 ½ / 4 which is slowly going forward with completion hopefully next year.
The amount of spares available to 4 litre owners is miniscule, can anyone help with a manufacturer / supplier of cylinder head gaskets in the UK, or does any body have any new old stock of 4 litre gaskets lying around in the garage.
There can be no doubt that in the future the 4 litre will become very desirable when it`s potential becomes known, but more of that in the next episode.
I will be very happy to hear from anyone that has any news or information regarding the 4 litre engine please contact me by completing the form on the Home Page.
Part 2 THE RICARDO FILES.
I would like to thank the Review readers who contacted me after part 1 was published ( ISSUE 222 Page 291 ) giving little snippets of information about the 4 litre and also about other Bentley related matters. Also greatly appreciated were the kind comments about the article.
Preparing such an article is extremely time consuming and there is a responsibility to the reader and posterity to get it right, not least, to make it interesting. I take my hat off to authors like Mike Hay, whose hefty tomes, must have taken extreme dedication to write.
A lot of the points brought up by readers will be covered in following installments, and instead of discussing these points out of context, it would be best to wait until these events are covered in chronological order. It will make the story flow better.
Unfortunately, there was no 4 litre technical information offered, so would any reader with any information whatsoever, however insignificant, please take the time to contact me, as it could be an important piece in the puzzle, and will help future 4 litre owners.
One problem that I have had, was that I was unable to access the 4 litre drawings which are under the custody of the Foundation. However David Rolf has promised to let me have the key drawings I need. The reason I need to pore over these documents is manyfold. They contain drawings from the sidevalve version of the 4 litre, also there are two different versions of the production engine, and we need to know which parts are interchangeable between the engines. At the moment anyone requesting 4 litre drawings from the foundation may be sent the wrong drawings for their version of engine! So it`s important that this work is carried out.
The last time I was allowed to search in the files was february 2001. In the short time I was able to look, I discovered some anomalies which I think most people are unaware of.
My first discovery was a drawing for a magneto clamp plate. This was not used on the production engine, but could prove that the sidevalve engine was actually built, as the drawings in the possession of the foundation are prints that came from the inspection department, indicating that the part had been manufactured and been checked by the inspection department. The drawing office originals were destroyed after the liquidation of Bentley Motors, reputedly by WO.
The drawing office designation for the 4 litre engine is SV 1 ( sidevalve 1 ).
There are two different drawings for crankase, block, cylinder head, inlet and exhaust valves, valve springs, exhaust manifolds etc.The reason for this was that because of the haste to get the engine into production the design had to be ” frozen ” too early in development. So the design was continued on a second version of the engine in parallel, which was to replace the first version after the initial batch of 25 laid down had been manufactured. The reason for this was that Bentleys did not consider the engine they were putting into production was good enough and wanted to continue development to improve it.
For the purposes of identification in the following articles we will call these engines Type 1. ( early ) and Type 2. ( late )
Most of our readers will have never seen a 4 Litre car, let alone a 4 Litre engine. Until 4 years ago neither had I !
Most Bentleyphiles have seen under the bonnet of a 3 or 4 ½ litre where the engine seems to fill the space, exuding a heady testosterone rich bravura. The architecture of the engine is visually exciting with a myriad of little work intensive shapes and details to catch the eye, it seems almost to be alive. It looks the business, and it delivers what it promises, delighting both the driver and the onlooker.
When a 4 Litre bonnet is opened, it is somewhat of a disappointment. The engine seems shrunken, depressed, to almost hide, it is so low. There is definitely a crisis of identity here. Everywhere there is austerity except for the wonderful pair of SU HVG5 carburettors which, unfortunately, are not interchangeable with the 4 ½ 6 ½ type due to the fixed float chamber position, different mixture control and separate slow running device, similar but not the same as the 8 litre. Compared to the 4 ½ the 4 l engine looks about as appealing as a double helping of soggy cabbage on a school dinner plate.
THE VISIT TO RICARDO CONSULTING ENGINEERS Date 31 01 01
It was with a sense of schoolboy excitement that I drove from the Midlands down to Shoreham where I had an appointment to see John Loveridge of Ricardo Consulting Engineers ltd. My quest was to look at the Bentley 4 Litre file. After the formalities at reception, John led me through a maze of passages, where I caught glimpses of machine shops and heard the muffled roar of engines under test, as we walked towards the library.The library was huge, and after being introduced to Roland Christopher, the information manager, I was able to look at the file itself, a fairly thick grey A4 folder, which lay on a desk looking rather unassuming. John parked me in a quiet conference room, where at last I was able to open the file and look at it`s contents
The first disappointment is, that the file contains no personal correspondence between WO and Ricardo or Barrington and Ricardo The file comprises some numbered sketches, some engine test results and a lot of undated reports written on plain paper. This wasn`t going to be an easy task to collate. One of the problems is that some of the Engineering drawings, although numbered, did not have the completion date written on, but were just date stamped on the day of issue to Bentley Motors. This means that there will be uncertainty about chronology. Also there is no way of knowing if the file contains the whole of the technical information that was transmitted to Bentley Motors.
Just a word about drawing numbers. The normal practice in small drawing offices at that time was that the draughtsmen would sign out a drawing number from a drawing register that was kept in the office. If there were several ongoing projects in the office the drawing numbers for a particular project would not necessarily be in numerical sequence. So that the only way to see if there is a complete set of drawings is to be in possession of the drawing register or a specific project drawing or parts list. These were not available at Ricardos.
One thing we have to keep in mind is that the 4 litre was originally designed as a side valve engine and due to the poor power output obtained, the design was changed to incorporate the Ricardo patent High Power Head. As is usual with such a change of direction, and the time and financial constraints, the instructions from the board to the drawing office would have been the familiar dreaded words. ”Change as little as possible ”. The normal outcome is that the finished result usually resembles the proverbial dogs breakfast. We will see that this change produced some unusual design features in the 4 litre that I will outline in the following text.
THE FIRST DOCUMENTS IN ASCENDING DATE ORDER WERE:
INSERT DRAWING LF 778 HERE.
S.693 undated issued to Bentleys 30/01/30.
This seems to be the first drawing issued to Bentleys and comes a long time after WO had visited Ricardo in September 1929. There didn`t seem to be much urgency on Ricardos part.
The drawing is not reproduceable but luckily Ricardo sketch no LF. 778 which is based on S.693 is reproduceable and hopefully will be included with this article.
S.693 is a cross section through the High Power Head and the cylinder block showing the valve gear. Also a part plan section through the cylinder head showing the combustion chamber ( not shown on LF.778 ).
The first detail to strike the eye is that the top of the block is sloping down from the left hand side of the engine about 4 degrees. It took a long time to realise what the reason for this is. This is one of those compromises brought about by the change from side valve to the High Power Head. The camshaft was too far from the centreline of the engine for the new head layout, so that the exhaust valves in the block had to slope inwards by 4 degrees to the vertical to be in the correct position in the combustion chamber. The designers decided to put a slope on the top of the block so that the valve stems were perpendicular to it and therefore the top of the exhaust valves parallel to it. The pushrods for the inlet valves also slope from the camshaft to the rocker arm in order that they can be contained within the new rocker cover envelope which is dictated by the position of the spark plugs.
Looking at the inlet valve arrangement, the 50 mm diameter valve head is completely recessed into the cylinder head with a close gap between the side of the valve head and the block, with the valve head flush with the cylinder head, a condition that Ricardo calls ”masked”. Presumably this is to allow the cylinder head to be very close to the top of the piston in order to increase the compression ratio, which is alright in theory but gives problems in practice, as it will be difficult to vary the valve timing without the valve hitting the piston. It is a mistake to machine away this masking when re-cutting the valve seats as this has the effect of reducing the compression ratio. The adjustable ball ended tappets were not adopted by Bentleys who used a special rolling contact profile machined on the end of the rocker abutting onto a hardened square pushrod head.
The single valve spring seems very short for the size of the valve, which has a large diameter stem. The rocker itself seems massive and the pushrods although beautifully made with attention paid to lightness, are still too heavy.
Let me explain: the valve springs are not only to keep the valves seated onto their seats and gastight, they also have to restrain the mass of the valve and valve gear which is being accelerated by the camshaft. If the springs are not strong enough the result will be valve bounce or crash as it`s known, where the springs become ineffective at certain rpm. Of course stronger springs can be fitted but there is usually a penalty of heavy wear on the valvegear. In extreme cases overstressed springs can fracture, the 4 litre is in this category but more of this later.
The combustion chamber is positioned centrally over the exhaust valve and over part of the cylinder.The spark plug is positioned in the combustion chamber directly over the exhaust valve. One interesting thing is that someone has penciled in some freehand changes to reduce the throat area at the inlet valve. After the first 11 engines the diameter of the inlet valve was reduced.
S.706 dated 30/10/29 issued to Bentleys 30/01/30
This is the earliest readable date, a rather unusual drawing presumably in response to a request by Barrington due to a problem in mounting the 4 litre sidevalve engine into the chassis. Remember that the Bentley engines were all mounted on the chassis contributing to chassis stiffness and therefore to crankcase distortion in some way but for the 4 litre a different solution where the engine would be flexibly mounted, with no input loadings from the chassis, was obviously being sought after.
The drawing shows the 4 litre engine with in outline with a mounting lug at each corner. The engine is mounted in a sub frame which comprises two channel members parallel with the huge sidemembers supported by a massive lorry type channel cross member either end of the engine.
The amazing thing is that the engine is mounted on coil springs and has spring steel locating plates fore and aft and transverse to keep the engine in position, and a spring loaded arm mounted between the top of the rocker box and the bulkhead to take out the torque reaction. It`s hard not to be scathing about this design which seems to have been made by someone, perhaps a young graduate, with theoretical knowlege but little practical experience. I can`t believe that Bentleys were actually paying for this work and that it was coming from a leading consultant.
Needless to say Barrington did not use this design on the 4 litre.
C.281 02/10/29 issued to Bentleys 30/01/30
The original cam diagram.
S.693 A 25/02/30 no issue date to Bentleys.
This is an update to drawing S.693 the difference being that the inlet valve now has double valve springs, and the rocker has been changed to the rolling contact type for the pushrod end as on the production engines.
Obviously Ricardos response to the valve bounce problem.
S.870 no Dates. Probably June 31
This drawing, which has been badly traced in ink, shows the original S.693 drawing updated to the production engine configuration.
C.305 issued to Bentleys 26/06/31
Revised cam diagram with 12 mm lift.
Was this cam profile and timing ever used on the engine? Only a comparison with the 4 litre drawings will determine this. Conway, the Bentley Motors storeman, said That a camshaft was fitted to the 4 litre which used to break the valve springs. Another mystery to solve!
S.897 01/07/31 no issue date to Bentleys.
This drawing was issued a week before Bentleys went into receivership and three weeks after Ricardos had received a 4 litre engine for test.
The subject matter is an assister spring for the inlet valve which is shown mounted between the tappet at the camshaft and the upper part of the valve chest. This is a desperate attempt to get the inlet valve to work properly, clear proof that the inlet valve springs were not able to do their job properly as they were too short to give the required spring rate without being overstressed, thus making them susceptible to fatigue fracture. Possibly because a modified camshaft had been fitted to try and improve engine performance.
The engineering of this assister spring is awful, but it`s not the fault of the designer, the task he was given was not really viable. I can find no mention in the engine test notes that this assister spring was ever fitted to the test engine. Again it was just a waste of man hours to be lumped onto Bentleys bill.
The real solution was to modify the cylinder head to give longer valve springs and lighten the valve gear, but as we know, time had run out for Bentleys and this solution was not an option.
THE 4 LITRE ENGINE TEST.
On 08/06/31 the engine promised by Bentleys was delivered to Ricardo.
Allocated job No C 665 c. The engine No was VA 4092
This engine had been used for acceleration tests at Brooklands and the first test carried out by Ricardos was in this state of tune.
Ricardos carried out a series of tests firstly to try and improve the performance and then to try to cure the chronic pre-ignition. This is a fascinating insight into how Ricardos engineers tackled the job and deserves a separate installment to do it justice as unfortunately it would be too lengthy to be included in this episode.
To end this installment I will write down Ricardos engineers comments on the tests which were concluded on 20/08/31. The test engine was not used in a production car so what happened to it? Does it still exist somewhere?
REMARKS ON THE TESTS.
This is a verbatim transcript from an unsigned, undated hand written report.
The Bentley ” 4 litre ” provides a striking example of an engine which refused to give anything like it`s expected performance, and which would not yield to any treatment imposed on it. The question of the chronic pre-ignition from which the engine was suffering was never cleared up, although the disease itself was located. All the indications went to show that the exhaust valves were responsible, but no explanation is forthcoming as to why they should have behaved in this way. As the Chenard single cyl. Engine fitted with precisely the same head and having the same capacity is immume from pre-ignition while developing a much higher power output, it would appear that the reason lies in some feature of the 6 cyl. Block such as a tendency to distortion in the neighbourhood of the exhaust valve.
With regard to the power output the performance was always very poor. Max torque occurred as expected at about 2200 – 2300 rpm the best obtained being 106 lb/ sq in
BMEP.
Perhaps the most valuable piece of information gleaned from the tests is to be found in the test with the revised camshaft, see figs 8 and 9 and test 7, with which the torque was raised considerably at low speeds without seriously affecting that at high.
Authors Note: The Chenard single cyl engine results quoted in the above text were from 1926 also there was no mention of pre-ignition in the 4 litre single cyl. tests produced by both Ricardo and Bentley. I wonder why Ricardo did not draw upon the experience gained by Chenard in developing their 9cv engine, which should have been available to him via their subsidiary C.I.S, before presenting the cylinder head design to Bentleys.
The comment regarding the distortion of the cylinder block was taken from a suggestion by Bentleys, Ricardos did not prove this under testing.
Now just a word about what is happening in the 4 litre world. I just heard that an exciting project is nearing completion in Australia. This is a 4 litre engine which has been supercharged and fitted into a 3 litre chassis. Chris Williams has just rebuilt both the engine in his lovely 4 litre Saloon and the spare engine. David Smethurst is still valiantly struggling with the rebuild of his 4 litre saloon and engine and spare engine but needs pushrods, does anybody have any? My project at the moment is a 4 ½ / 4 which is slowly going forward with completion hopefully next year.
The amount of spares available to 4 litre owners is miniscule, can anyone help with a manufacturer / supplier of cylinder head gaskets in the UK, or does any body have any new old stock of 4 litre gaskets lying around in the garage.
There can be no doubt that in the future the 4 litre will become very desirable when it`s potential becomes known, but more of that in the next episode.
I will be very happy to hear from anyone that has any news or information regarding the 4 litre engine please contact me by completing the form on the Home Page.
SORRY FOR THE DELAY IN ADDING THE DRAWINGS. THESE WILL HAVE TO BE SCANNED AND ADDED WHEN I CAN FIND THE TIME. THIS ARTICLE WAS FIRST PUBLISHED IN THE BDC REVIEW.