THE FOUR LITRE ENGINE STORY PART 2 UPDATE IN PROGRESS
By Alan C Smith
This Episode is intended to provide an update on 4 Litre matters.
For new members that wish to read the previous articles; they were published in Review 222 October 2001 page 291, Review 236 December 2004 page 362, and Review 246 August 2006 page 203.
I was very surprised to see how long ago the last article was published; time seems to zoom past once retirement age is reached. I thank readers that were kind enough to Email me asking when the next article was due.
In 2006 I was already several years over retirement age and decided that when the project that I was working on finished, I would hang up my computer and get on with the Bentley projects.
I had bought a ruin in France near Angouleme, which came with a 100 sq. metres of garage and workshop with the intention of combining my love of France with my love of Bentleys.
Although I had the experience of rebuilding six stone houses in Scotland behind me I still became a slave of the project by working all hours trying to restore the ruin just like those poor tortured folk that you see portrayed on the telly. Then I had to start work again to make more money to finance the restoration with the result that I had no time to work on the Bentley projects or the house or write any further articles because I was just too cream crackered. Lately I had to deal with bereavement in the family and the fracture of my left tibia which left me almost drained of life force.
The bulk of this article was written around 2007 but the content is still meaningful. I have updated parts of it and added more news as time does not stand still.
When I sat down to write the fascinating story of the 4 litre engine tests carried out by Ricardo & co and their attempts to improve the engine`s performance. I discovered that some of the test results that I did not have time to copy from the 4 litre file during my visit to Ricardo were essential to give meaning to this part of the story. Unfortunately there will be a delay before I can complete this section as I will have to obtain a copy of these tests from Ricardo to do so.
After part two of the “Four Litre Engine Story” was published in the Review I received an important telephone call from Toby Silverton from the USA. I was in the attic of the house I’m restoring in France and wrote the information that he gave me on one of the new joists that we were installing. Toby said that there was an article in BDC Review 187 February 1993. He told me that contained in this article was the probable fate of the 4 litre engine sent to Ricardo just before the liquidation. I should have had this Review but did not find it! However a telephone call to BDC HQ and Gordon sent me a spare copy, which I received within a couple of days.
The article is titled “Experimentally Speaking” which commences on page 35. On page 38 there is a letter from Witchell, who had stayed on as chief of the Bentley service depot after the liquidation, to Sidgreaves, the Rolls Royce Managing Director. This letter was dated Jan 14th 1932 in which Witchell informs Sidgreaves “That there is a 4 litre engine lying in the test bed which would be available for experimental work. The engine is standard in all respects with the original camshaft BM 8743 and with valve springs BM 7715 and 7716”.
Now here is the interesting part, Witchell goes on to say “That there is a modified form of camshaft BM 9290 which was fitted to those chassis sold to Jack Barclay Ltd., and Jack Olding and Co”. He adds the information “So far we have no valve springs for use with this camshaft with satisfactory results”.
This confirms the remarks attributed to Conway the Bentley Motors store man.
The article goes on with several letters going to and fro the last dated Feb 8 1933, with the result that after a year the engine was to be sent back to the service depot. No engine number was quoted so that a positive ID with the Ricardo engine is not possible.
Amongst my spares I have both camshaft BM 8743 and BM 9290 and three different types of inlet valve spring sets, although there is only one spring set listed in the parts list for the 4 litre inlet valves.
At last I had been able to do some serious research on the 4 litre drawings thanks to my old pal the late John Hunt who had all the Bentley drawings on his database. I was able to join some of the Tuesday Meetings held at his house and had the opportunity to quickly look through all the 4 litre drawings and parts list. Which resulted in invoices for drawings from the foundation totaling several hundred pounds, but it was worth it, as I had made some exciting discoveries, which I will reveal in later episodes.
What is sure is that the chief designer Barrington had introduced a long awaited drawing change sheet system, where all changes to a drawing, which was being revised and reissued, were written down as history, the document given a number and filed in the archive. The last series of drawings produced by the Bentley design office include the sheet number of the change details. I imagine that these change sheets no longer exist; perhaps our friends at the WOBMF could look into this, as it would explain a lot of things if these sheets could be found.
We can say that the surviving parts list for the 4 litre engine was not up to date at the time of the liquidation. The proof of this is that the parts list does not include the latest camshaft BM 9290, which as we have seen, was fitted to quite a few 4 litre cars. (See later note).
The drawing for BM 9290 still exists as does the drawing for the standard camshaft.
No new inlet valve spring drawings are on the database, this does not mean that they were not drawn up; perhaps the drawings did not survive.
Possibly BM 9290 is based on the Ricardo design C305 dated June 26th 1931 which was in the Ricardo file but for technical reasons could not be reproduced for me during my visit. I will ask Ricardo’s if they can send me a copy in order to verify this. An interesting fact is that the standard camshaft is based on the Ricardo drawing C281 dated 30 / 01 / 1930 but is not an exact copy as there are slight differences.
Another exciting thing that happened was that James Stickley mailed me from the USA offering me a set of feeler gauges for the 4 litre engine I gladly accepted and received the set of feelers around Christmas, and sure enough they are the real thing but the tappet clearances are different to those quoted in the “Technical Facts of The Vintage Bentley”. There is a nagging thought that the tappet clearances for the standard cam may not be correct for the later cam. We need to look carefully into this. Is there a club member who has camshaft expertise that can analyze the drawings and recommend the correct tappet clearances and perhaps design the appropriate valve springs for the later cam?
Regarding the cylinder head gaskets, I have sourced these in Australia and have received samples of the two different gaskets but I have yet to check out the quality and dimensional accuracy. There is a gasket maker in the UK that I`ve heard about who has recently made gaskets for the early engines so I will make some enquiries into this and let you know.
Looking through the Review Index I noticed that in Review 21 June 1951 pages 30-34 there was an article by “Mac” L C McKenzie the renowned Bentley tuner and restorer. This article was first published by “Motor Sport” magazine in 1937 and copied by permission. The title of the article is “TUNING THE BENTLEY” which covers briefly all the vintage engines including the 4 Litre. And I reproduce this 4 Litre section verbatim below.
The Ricardo o.h. Inlet Engine.
About fifty of these 4-Litre cars were produced after the o.h. camshaft Bentleys had gone out of production. The bottom half of the engine is beautifully made and would permit of a greatly increased output. The head design does not allow any increase of compression ratio and increasing the port areas and fitting triple carburetors only results in an output of 150 bhp against the standard output of 120 bhp. Supercharging is the most likely development, but Mr. McKenzie installs 6.5 Litre old-type engines in the 4-Litre chassis for clients who require improved running while retaining the advantages possessed by the later chassis.
Seventy four years later the situation is somewhat different with 6.5 Litre engines in very short supply and probably only available as newly manufactured. In the light of today’s Bentley values which are now more focused on originality, this conversion is probably financially unsound as well as not being politically correct. The solution which maintains originality and is most cost effective is not to put in a bigger engine but to up rate the 4 litre engine output.
Now for some news about a 4 litre supercharged Bentley. The well known Bentley Restorer Neville Webb of “The Vintage Garage” in Australia was commissioned by Dutchman Jos Fruytier to build a rather exciting Bentley.
The basis of the car is a 3 litre chassis into which has been mounted a blown 4 litre engine with the blower driven from the front of the crankshaft as in the original 4 ½ litre blower cars. The spigot on the front of the 4 litre crankshaft facilitates this. This interesting car is described in an article by Neville Webb in Review 263 page 190
This is certainly a very tempting proposition for the budget challenged Bentley builder with cheap to buy and easy and cheap to rebuild 4 litre engines lying around unused. Just think, 180 HP in a light chassis will give around 180 BHP per ton, and will be able to show most vintage Bentleys a clean pair of heels, and be capable of leaving a nice trail of burning rubber whilst accelerating away from the lights.
The 4 litre engine is actually a genuine vintage engine, the unsuccessful side valve version was designed in 1928 and the Ricardo head version, which was based on the side valve engine, in 1930 with prototypes running and production engines assembled in 1930. John Hunt told me that WO had delayed the launch date of the 4L until 1931.
This is not the first 3 / 4 to be built but it`s almost certain that this is the first 3 / 4S that has been built. The blower car has now been acquired by BDC member Dutchman Bert Degenaar whose team of Dutch engineers are now doing the development work to finish this project off and are determined to have the car up and running for BDC Silverstone, this should be something to watch out for.
Daan Rens, another Dutchman, was the first to combine the 3 litre chassis with the 4 litre engine and I had the pleasure of being driven in his car which had a very lively performance which is not surprising due to a powerful engine in a lightweight chassis.
Daan is a 4 litre guru and I believe that the engine in Jos`s car was sold by Daan to someone in Australia some years ago. Unfortunately Daan has sold his car and is no longer a member of the BDC, but he did let me have all of his remaining 4 litre spares including the pattern for casting the 3 carburettor inlet manifold.
Now some more exciting news; Chris Williams has sold his lovely 4 litre saloon and the new owner didn`t want to buy the spare engine which Chris had overhauled together with the original engine that was in the car when he bought it.
Luck has a lot to do with life because I had phoned Chris for some advice on rebuilding my 4 litre engine when he told me that his spare engine was available. We struck a deal there and then, and the engine is now mine, this will save me a lot of time as I have inserted the engine temporarily into my 3 / 4 ½. Soon I can take the car to a rolling road to have the engine settings tuned to give best performance and get a figure for power output. I enclose some of photos of the almost complete conversion and a brief description of the work required.
Installing a 4 Litre engine into a 3 Litre chassis
Please insert photos 034 036 & 037
This is fairly simple to do although probably not the thing to do on original cars; but it does make a lot of sense if building a car from parts. What I wanted to do was install the 4L engine in my car temporarily so that I could experience 4L motoring whilst my other car is under construction.
After removing the bonnet, the radiator and the engine, the hard part is chiseling off the rivets on the first cross member and the two engine rear mounting brackets and removing them. My car is a bitsa so that I didn`t feel too guilty about this. As the 4 L engine is approximately 150mm longer than the 4 cylinder engines this will push the radiator forward by this amount as the bulkhead has to remain in it`s original position in order to avoid major surgery to the bodywork.
Of course if mounting the engine in a new long chassis the bulkhead and second and third cross members will be mounted 150mm to the rear so that the radiator can be in it`s normal position to keep the proportions correct. This also gives a nice long bonnet just like a Le Mans Rep without the cost of having to add a false bulkhead. It also pushes the centre of gravity rearwards. The weight of a 4L engine is 350 Kg which is not a lot more than the 4 cylinder engines.
A new front cross member will have to be fabricated from 76mm diameter x 6mm wall thickness mild steel tube which will have to be bent to a 250 mm radius at the centre to avoid clashing with the engine timing case before straightening out again to meet the side members. I didn`t follow the 4L design slavishly because it`s over designed and the 3L side members are less deep than the 4L side members As this is a temporary installation It`s necessary to utilize existing holes in the chassis for the cross member fixing bolts. The 3 L radiator mounting brackets are bolted onto extensions welded to the cross member and the rear mounting brackets are fabricated from 6mm thick mild steel plate.
It`s important that the 4L engine is located in the chassis so that the whiffle tree fingers with the plate clutch equalizing bar installed are in the correct position to operate the 4L clutch. A 4.5 L drive shaft and flange will be needed instead of the 3L assembly. The clutch pedal will have to be removed and a new one installed fabricated along the lines of a 4.5L plate clutch pedal, this is to avoid the pedal from clashing with the 4L crankcase.
A temporary bonnet will need to be made and bonnet cills made from ash or hardwood. As you can see from the photos the installation is almost complete and I hope to get the car running this summer.
The 4.5 / 4 car that I`m building is still a pile of parts and I will start putting this together when I get some time during the summer. The engine parts have been worked on with the white metalling done, the cylinder bores honed out to take a set of good used original pistons with new rings, there is really very little to do compared with rebuilding a 4 ½ litre engine.
What I will be doing is to look into ways the engine can be made to give a better performance and torque at the lower end of the rev range. The camshaft used by the later engines gave a better low range torque output so this camshaft is probably a good starting point for the search. I hired a so called specialist to help me redesign the camshaft but after giving him all the drawings, information and cash deposit up front, two years later there had been no progress made. When I phoned him to remind him again, he told me that he had other more important work to do first. It goes without saying that I cancelled our arrangement forthwith.
Is there a club member that can give me some impartial advice about getting an improved camshaft designed? I know that there are firms that offer this service, but I`m a bit wary about going to somebody that has probably just bought one of those computer programs, and has no proven track record.
Another possibility is to have the combustion chamber modified, again is there anybody that can give me some advice or recommend a cylinder head expert?
Yet another possibility that has been already tried and tested by Neville Webb in Australia is to increase the bore from 85mm to 89mm and fit new pistons. Neville reports that the modification gives an increase in performance and I have obtained a set of pistons from him which I will try out. As the wall thickness in the bores is only 6mm and boring out to 89mm will reduce that wall thickness to 4mm it could be that the wall thickness is marginal. Again is there anybody that can give me some advice on this?
I have looked into getting new blocks made to enable the bores to be 100mm which will give a capacity of 5.2 litres. There are potential problems with coolant circulation which need to be addressed before going forward with this project. There`s also a hefty cost to have the pattern made, so if any 4L owner is interested or can help, please contact me.
I will also look into fitting the three carburettor inlet manifold with appropriate modern shorter SU carburetors so that they fit under the bonnet, also a tubular exhaust manifold to shed the extra heat created by the improvement in performance, as the standard manifold, due to it`s large heat sink mass, is considered to be the cause of exhaust valve seat recession which can be experienced under long hard driving.
When I`ve built the engine the intention is to mount it in my 3 / 4 ½ in place of the ex Chris Williams engine so that a comparison can be made between the standard and modified engines.
I won`t have time to do this scientifically, to eliminate all the variables, what I’m hoping for is that the difference in performance between the two engines will be large enough to be able say conclusively that the modifications have been successful. Of course I will keep you all informed about these modifications so that you can carry them out on your own 4 litre engine. It will cost a heck of a lot less than sticking in a great lump of a 6.5 or an 8 litre engine and you will get the same or better performance and retain the simplicity of the 4L engine.
I`m happy to tell you that there is a new member building a 4.5 / 4 car, his name is Anthony Barret and he`s still collecting parts to finish his car. That will increase the number of these cars to two in the world.
Recently Jimmy Medcalf was very helpful in arranging with the WOBMF that I was able to take photocopies of the 4 litre service sheets. I`ve had a look into them but haven`t yet been able to finish cataloguing the engine faults that were recorded on the sheets. However I can tell you that the majority of 4 Litre engines were fitted with the later camshaft BM 9290 which is not listed in the parts list. As with the other Bentleys, wheel wobble was also a frequent problem, and still is according to recent letters in the Review; unfortunately there is no record of how the Bentley mechanics corrected this.
There is a clue to the solution of this problem contained in a paragraph in Review 92 April 1969 page 152. Here Mr. A E Dean from Bentley Motors fitting & testing department explains that the 4 litre cars suffered from serious wheel wobble at 75 mph, this was so bad that it could bend the steering arms and track rod, break a finger and shatter instrument glasses. Nobby Clarke and Stan Ivermee came up with a solution where by increasing the length of the front spring second leaf by 3/8” of an inch the car would respond to the normal wheel balance and steering tube adjustments.
That`s all for this time; as usual I`m always happy to receive any information, chat or give advice about the fascinating 4 litre engine.
By Alan C Smith
This Episode is intended to provide an update on 4 Litre matters.
For new members that wish to read the previous articles; they were published in Review 222 October 2001 page 291, Review 236 December 2004 page 362, and Review 246 August 2006 page 203.
I was very surprised to see how long ago the last article was published; time seems to zoom past once retirement age is reached. I thank readers that were kind enough to Email me asking when the next article was due.
In 2006 I was already several years over retirement age and decided that when the project that I was working on finished, I would hang up my computer and get on with the Bentley projects.
I had bought a ruin in France near Angouleme, which came with a 100 sq. metres of garage and workshop with the intention of combining my love of France with my love of Bentleys.
Although I had the experience of rebuilding six stone houses in Scotland behind me I still became a slave of the project by working all hours trying to restore the ruin just like those poor tortured folk that you see portrayed on the telly. Then I had to start work again to make more money to finance the restoration with the result that I had no time to work on the Bentley projects or the house or write any further articles because I was just too cream crackered. Lately I had to deal with bereavement in the family and the fracture of my left tibia which left me almost drained of life force.
The bulk of this article was written around 2007 but the content is still meaningful. I have updated parts of it and added more news as time does not stand still.
When I sat down to write the fascinating story of the 4 litre engine tests carried out by Ricardo & co and their attempts to improve the engine`s performance. I discovered that some of the test results that I did not have time to copy from the 4 litre file during my visit to Ricardo were essential to give meaning to this part of the story. Unfortunately there will be a delay before I can complete this section as I will have to obtain a copy of these tests from Ricardo to do so.
After part two of the “Four Litre Engine Story” was published in the Review I received an important telephone call from Toby Silverton from the USA. I was in the attic of the house I’m restoring in France and wrote the information that he gave me on one of the new joists that we were installing. Toby said that there was an article in BDC Review 187 February 1993. He told me that contained in this article was the probable fate of the 4 litre engine sent to Ricardo just before the liquidation. I should have had this Review but did not find it! However a telephone call to BDC HQ and Gordon sent me a spare copy, which I received within a couple of days.
The article is titled “Experimentally Speaking” which commences on page 35. On page 38 there is a letter from Witchell, who had stayed on as chief of the Bentley service depot after the liquidation, to Sidgreaves, the Rolls Royce Managing Director. This letter was dated Jan 14th 1932 in which Witchell informs Sidgreaves “That there is a 4 litre engine lying in the test bed which would be available for experimental work. The engine is standard in all respects with the original camshaft BM 8743 and with valve springs BM 7715 and 7716”.
Now here is the interesting part, Witchell goes on to say “That there is a modified form of camshaft BM 9290 which was fitted to those chassis sold to Jack Barclay Ltd., and Jack Olding and Co”. He adds the information “So far we have no valve springs for use with this camshaft with satisfactory results”.
This confirms the remarks attributed to Conway the Bentley Motors store man.
The article goes on with several letters going to and fro the last dated Feb 8 1933, with the result that after a year the engine was to be sent back to the service depot. No engine number was quoted so that a positive ID with the Ricardo engine is not possible.
Amongst my spares I have both camshaft BM 8743 and BM 9290 and three different types of inlet valve spring sets, although there is only one spring set listed in the parts list for the 4 litre inlet valves.
At last I had been able to do some serious research on the 4 litre drawings thanks to my old pal the late John Hunt who had all the Bentley drawings on his database. I was able to join some of the Tuesday Meetings held at his house and had the opportunity to quickly look through all the 4 litre drawings and parts list. Which resulted in invoices for drawings from the foundation totaling several hundred pounds, but it was worth it, as I had made some exciting discoveries, which I will reveal in later episodes.
What is sure is that the chief designer Barrington had introduced a long awaited drawing change sheet system, where all changes to a drawing, which was being revised and reissued, were written down as history, the document given a number and filed in the archive. The last series of drawings produced by the Bentley design office include the sheet number of the change details. I imagine that these change sheets no longer exist; perhaps our friends at the WOBMF could look into this, as it would explain a lot of things if these sheets could be found.
We can say that the surviving parts list for the 4 litre engine was not up to date at the time of the liquidation. The proof of this is that the parts list does not include the latest camshaft BM 9290, which as we have seen, was fitted to quite a few 4 litre cars. (See later note).
The drawing for BM 9290 still exists as does the drawing for the standard camshaft.
No new inlet valve spring drawings are on the database, this does not mean that they were not drawn up; perhaps the drawings did not survive.
Possibly BM 9290 is based on the Ricardo design C305 dated June 26th 1931 which was in the Ricardo file but for technical reasons could not be reproduced for me during my visit. I will ask Ricardo’s if they can send me a copy in order to verify this. An interesting fact is that the standard camshaft is based on the Ricardo drawing C281 dated 30 / 01 / 1930 but is not an exact copy as there are slight differences.
Another exciting thing that happened was that James Stickley mailed me from the USA offering me a set of feeler gauges for the 4 litre engine I gladly accepted and received the set of feelers around Christmas, and sure enough they are the real thing but the tappet clearances are different to those quoted in the “Technical Facts of The Vintage Bentley”. There is a nagging thought that the tappet clearances for the standard cam may not be correct for the later cam. We need to look carefully into this. Is there a club member who has camshaft expertise that can analyze the drawings and recommend the correct tappet clearances and perhaps design the appropriate valve springs for the later cam?
Regarding the cylinder head gaskets, I have sourced these in Australia and have received samples of the two different gaskets but I have yet to check out the quality and dimensional accuracy. There is a gasket maker in the UK that I`ve heard about who has recently made gaskets for the early engines so I will make some enquiries into this and let you know.
Looking through the Review Index I noticed that in Review 21 June 1951 pages 30-34 there was an article by “Mac” L C McKenzie the renowned Bentley tuner and restorer. This article was first published by “Motor Sport” magazine in 1937 and copied by permission. The title of the article is “TUNING THE BENTLEY” which covers briefly all the vintage engines including the 4 Litre. And I reproduce this 4 Litre section verbatim below.
The Ricardo o.h. Inlet Engine.
About fifty of these 4-Litre cars were produced after the o.h. camshaft Bentleys had gone out of production. The bottom half of the engine is beautifully made and would permit of a greatly increased output. The head design does not allow any increase of compression ratio and increasing the port areas and fitting triple carburetors only results in an output of 150 bhp against the standard output of 120 bhp. Supercharging is the most likely development, but Mr. McKenzie installs 6.5 Litre old-type engines in the 4-Litre chassis for clients who require improved running while retaining the advantages possessed by the later chassis.
Seventy four years later the situation is somewhat different with 6.5 Litre engines in very short supply and probably only available as newly manufactured. In the light of today’s Bentley values which are now more focused on originality, this conversion is probably financially unsound as well as not being politically correct. The solution which maintains originality and is most cost effective is not to put in a bigger engine but to up rate the 4 litre engine output.
Now for some news about a 4 litre supercharged Bentley. The well known Bentley Restorer Neville Webb of “The Vintage Garage” in Australia was commissioned by Dutchman Jos Fruytier to build a rather exciting Bentley.
The basis of the car is a 3 litre chassis into which has been mounted a blown 4 litre engine with the blower driven from the front of the crankshaft as in the original 4 ½ litre blower cars. The spigot on the front of the 4 litre crankshaft facilitates this. This interesting car is described in an article by Neville Webb in Review 263 page 190
This is certainly a very tempting proposition for the budget challenged Bentley builder with cheap to buy and easy and cheap to rebuild 4 litre engines lying around unused. Just think, 180 HP in a light chassis will give around 180 BHP per ton, and will be able to show most vintage Bentleys a clean pair of heels, and be capable of leaving a nice trail of burning rubber whilst accelerating away from the lights.
The 4 litre engine is actually a genuine vintage engine, the unsuccessful side valve version was designed in 1928 and the Ricardo head version, which was based on the side valve engine, in 1930 with prototypes running and production engines assembled in 1930. John Hunt told me that WO had delayed the launch date of the 4L until 1931.
This is not the first 3 / 4 to be built but it`s almost certain that this is the first 3 / 4S that has been built. The blower car has now been acquired by BDC member Dutchman Bert Degenaar whose team of Dutch engineers are now doing the development work to finish this project off and are determined to have the car up and running for BDC Silverstone, this should be something to watch out for.
Daan Rens, another Dutchman, was the first to combine the 3 litre chassis with the 4 litre engine and I had the pleasure of being driven in his car which had a very lively performance which is not surprising due to a powerful engine in a lightweight chassis.
Daan is a 4 litre guru and I believe that the engine in Jos`s car was sold by Daan to someone in Australia some years ago. Unfortunately Daan has sold his car and is no longer a member of the BDC, but he did let me have all of his remaining 4 litre spares including the pattern for casting the 3 carburettor inlet manifold.
Now some more exciting news; Chris Williams has sold his lovely 4 litre saloon and the new owner didn`t want to buy the spare engine which Chris had overhauled together with the original engine that was in the car when he bought it.
Luck has a lot to do with life because I had phoned Chris for some advice on rebuilding my 4 litre engine when he told me that his spare engine was available. We struck a deal there and then, and the engine is now mine, this will save me a lot of time as I have inserted the engine temporarily into my 3 / 4 ½. Soon I can take the car to a rolling road to have the engine settings tuned to give best performance and get a figure for power output. I enclose some of photos of the almost complete conversion and a brief description of the work required.
Installing a 4 Litre engine into a 3 Litre chassis
Please insert photos 034 036 & 037
This is fairly simple to do although probably not the thing to do on original cars; but it does make a lot of sense if building a car from parts. What I wanted to do was install the 4L engine in my car temporarily so that I could experience 4L motoring whilst my other car is under construction.
After removing the bonnet, the radiator and the engine, the hard part is chiseling off the rivets on the first cross member and the two engine rear mounting brackets and removing them. My car is a bitsa so that I didn`t feel too guilty about this. As the 4 L engine is approximately 150mm longer than the 4 cylinder engines this will push the radiator forward by this amount as the bulkhead has to remain in it`s original position in order to avoid major surgery to the bodywork.
Of course if mounting the engine in a new long chassis the bulkhead and second and third cross members will be mounted 150mm to the rear so that the radiator can be in it`s normal position to keep the proportions correct. This also gives a nice long bonnet just like a Le Mans Rep without the cost of having to add a false bulkhead. It also pushes the centre of gravity rearwards. The weight of a 4L engine is 350 Kg which is not a lot more than the 4 cylinder engines.
A new front cross member will have to be fabricated from 76mm diameter x 6mm wall thickness mild steel tube which will have to be bent to a 250 mm radius at the centre to avoid clashing with the engine timing case before straightening out again to meet the side members. I didn`t follow the 4L design slavishly because it`s over designed and the 3L side members are less deep than the 4L side members As this is a temporary installation It`s necessary to utilize existing holes in the chassis for the cross member fixing bolts. The 3 L radiator mounting brackets are bolted onto extensions welded to the cross member and the rear mounting brackets are fabricated from 6mm thick mild steel plate.
It`s important that the 4L engine is located in the chassis so that the whiffle tree fingers with the plate clutch equalizing bar installed are in the correct position to operate the 4L clutch. A 4.5 L drive shaft and flange will be needed instead of the 3L assembly. The clutch pedal will have to be removed and a new one installed fabricated along the lines of a 4.5L plate clutch pedal, this is to avoid the pedal from clashing with the 4L crankcase.
A temporary bonnet will need to be made and bonnet cills made from ash or hardwood. As you can see from the photos the installation is almost complete and I hope to get the car running this summer.
The 4.5 / 4 car that I`m building is still a pile of parts and I will start putting this together when I get some time during the summer. The engine parts have been worked on with the white metalling done, the cylinder bores honed out to take a set of good used original pistons with new rings, there is really very little to do compared with rebuilding a 4 ½ litre engine.
What I will be doing is to look into ways the engine can be made to give a better performance and torque at the lower end of the rev range. The camshaft used by the later engines gave a better low range torque output so this camshaft is probably a good starting point for the search. I hired a so called specialist to help me redesign the camshaft but after giving him all the drawings, information and cash deposit up front, two years later there had been no progress made. When I phoned him to remind him again, he told me that he had other more important work to do first. It goes without saying that I cancelled our arrangement forthwith.
Is there a club member that can give me some impartial advice about getting an improved camshaft designed? I know that there are firms that offer this service, but I`m a bit wary about going to somebody that has probably just bought one of those computer programs, and has no proven track record.
Another possibility is to have the combustion chamber modified, again is there anybody that can give me some advice or recommend a cylinder head expert?
Yet another possibility that has been already tried and tested by Neville Webb in Australia is to increase the bore from 85mm to 89mm and fit new pistons. Neville reports that the modification gives an increase in performance and I have obtained a set of pistons from him which I will try out. As the wall thickness in the bores is only 6mm and boring out to 89mm will reduce that wall thickness to 4mm it could be that the wall thickness is marginal. Again is there anybody that can give me some advice on this?
I have looked into getting new blocks made to enable the bores to be 100mm which will give a capacity of 5.2 litres. There are potential problems with coolant circulation which need to be addressed before going forward with this project. There`s also a hefty cost to have the pattern made, so if any 4L owner is interested or can help, please contact me.
I will also look into fitting the three carburettor inlet manifold with appropriate modern shorter SU carburetors so that they fit under the bonnet, also a tubular exhaust manifold to shed the extra heat created by the improvement in performance, as the standard manifold, due to it`s large heat sink mass, is considered to be the cause of exhaust valve seat recession which can be experienced under long hard driving.
When I`ve built the engine the intention is to mount it in my 3 / 4 ½ in place of the ex Chris Williams engine so that a comparison can be made between the standard and modified engines.
I won`t have time to do this scientifically, to eliminate all the variables, what I’m hoping for is that the difference in performance between the two engines will be large enough to be able say conclusively that the modifications have been successful. Of course I will keep you all informed about these modifications so that you can carry them out on your own 4 litre engine. It will cost a heck of a lot less than sticking in a great lump of a 6.5 or an 8 litre engine and you will get the same or better performance and retain the simplicity of the 4L engine.
I`m happy to tell you that there is a new member building a 4.5 / 4 car, his name is Anthony Barret and he`s still collecting parts to finish his car. That will increase the number of these cars to two in the world.
Recently Jimmy Medcalf was very helpful in arranging with the WOBMF that I was able to take photocopies of the 4 litre service sheets. I`ve had a look into them but haven`t yet been able to finish cataloguing the engine faults that were recorded on the sheets. However I can tell you that the majority of 4 Litre engines were fitted with the later camshaft BM 9290 which is not listed in the parts list. As with the other Bentleys, wheel wobble was also a frequent problem, and still is according to recent letters in the Review; unfortunately there is no record of how the Bentley mechanics corrected this.
There is a clue to the solution of this problem contained in a paragraph in Review 92 April 1969 page 152. Here Mr. A E Dean from Bentley Motors fitting & testing department explains that the 4 litre cars suffered from serious wheel wobble at 75 mph, this was so bad that it could bend the steering arms and track rod, break a finger and shatter instrument glasses. Nobby Clarke and Stan Ivermee came up with a solution where by increasing the length of the front spring second leaf by 3/8” of an inch the car would respond to the normal wheel balance and steering tube adjustments.
That`s all for this time; as usual I`m always happy to receive any information, chat or give advice about the fascinating 4 litre engine.