Selecting a Carburettor Replacement : -

Date	Discussion
Jan 2023	The inlet manifold of the Rhapsody's engine has been custom made to make the Holden car engine suitable for use in a boat. It allows the carburettor to be mounted level, (in the same plane), as the engine and it is entirely enclosed in a sealed water jacket. The water jacket has the dual function of both cooling the exhaust manifold and heating the raw river water, before pumping it into the cylinder block, to stop the engine running too cold. So, the existing manifold had to be retained. This constraint limits the types of fuel injection that can be considered. The Holden cylinder head uses paired inlet ports - which means that each pair of cylinders shares a single inlet port. As a result, there is no opportunity to install separate injectors for each cylinder as might be the case in a more modern engine. The only remaining option, therefore, is to use a Throttle Body Injector (TBI) - sometimes called Central Fuel Injection (CFI). A TBI is an aluminium casting that mounts in the same place as the carburettor and, indeed, looks a bit like a carburettor. It has the same throttle butterfly valves to control the engine speed, but "squirts" fuel into the engine rather than have the engine "suck" it in as with a carburettor. More importantly, the TBI has no float bowl and is a completely sealed system, which, barring mechanical failures, means it will not leak fuel vapour into the air, or into the bilges of the boat. It is true that a TBI needs to be electronically controlled by an Engine Computer Unit (ECU) and whilst this adds a level of complication to the whole project, it does provide the opportunity for improved tuning and, therefore, better performance and economy. TBI systems were used on motor vehicles from the mid-1980s, through to the mid-1990s, whilst the manufacturers transitioned from carburettors to the full sequential direct injection we commonly see today. This was done to meet the rising emission standards, and fuel economy targets, being set by governments in order to address pollution and climate control problems. The first step, for me, was to comb the Internet, (and, physically, in wrecking yards), to find a donor vehicle - one that was fitted with TBI as standard and that was commonly available. After much research, I selected a 1996 Holden Barina SB "Swing". It was actually made by Opel in Germany and marketed there, and in many other countries, as the Corsa "B". It's a General Motors product and was sold in Australia in large enough numbers for them to be readily available in wreckers' yards. Given that the Opel engine is only 1600cc and the boat engine is 2440cc, it might seem an odd choice at first glance. However, consider the individual size of each cylinder - in the Opel it is about 400cc which compares nicely with the six cylinder engine, at 406cc. It is true that the injector has to fire more frequently at three times per engine revolution instead of twice, but given that the Opel engine is rated at 6000rpm, whilst the boat engine will not exceed 4000rpm, the maximum operating speed required of the injector is the same. It is also interesting, and very reassuring, to note that the final throat sizes of the Opel TBI and the Stromberg carburettor, it was to replace, are exactly the same, at 35mm, which suggests that the overall fuel/air mixture volumes would be about the same. There are a number of aspects of the already installed and running carburetted engine that have to be changed, to move to fuel injection. Specifically, these are the air intake system and the fuel system. An ECU will need to be sourced and installed as well, of course, with its attendant wiring. To simplify the air intake system changes, it was decided to use the complete air-box and filter unit from the donor vehicle. This has required a new mounting frame to be made and installed in the hull. It now connects into the original trunking to access fresh air from outside the hull and is ready to connect to the new TBI using a standard size 60mm hose, (albeit a bit longer). The changes required for the fuel system are discussed, in detail, in the previous section of this document. However, it turns out that there was a naïve assumption being made, above, about the impact of a single injector fuelling a six cylinder engine as opposed to a four. Whilst the maximum engine speeds quoted are correct, the "normal" operating speeds paint a different picture. Most engines will cruise at much less than their theoretical maximum speed, perhaps only 2000rpm. So, whilst the Opel engine can reach 6000rpm, it would only have to work that hard occasionally, and even then, for quite short periods. The six cylinder engine, at the same relaxed 2000rpm, requires the injector to work at a 33% higher rate than it does in its four cylinder guise. Some testing of the ECU on the bench, (more of that later), showed that the single injector working at that higher speed not only reached worryingly high Duty Cycle percentages, but it also started getting hot, too. Therefore, the plan to use a single injector, working so hard, for potentially long periods, became a matter of some concern. One of the alternative TBI units that had been under consideration, when the conversion project was first planned, was from a bigger, six cylinder engine and it had two separate injectors in the same TBI body, instead of one. I had assumed, at the time, that it had been done to achieve the necessary fuel flow rates, but now, I suspect, the real reason is that it allows the injectors to be operated alternately, at half the speed, making twice as much time available for each unit to complete its cycle. The twin injector TBI being considered was from a 3.9litre Ford EA/EB Falcon made from 1988 to 1991, but for some reason, suitable donor wrecks are very hard to find. (Perhaps it's because they're ideal for fuel injection conversions!) However, I had lodged a query with a local wrecker's website many, many moons ago, asking him to let me know if one of these particular vehicles became available for wrecking, and then I promptly forgot all about it - until last week when he, quite serendipitously, sent me an email. A quick visit to the wrecker's yard secured not one, but two, twin injector TBIs to investigate. Back to the drawing board? No, not quite. The modifications to the fuel system and the air intake system are unchanged. The ECU just has to have one minor software setting altered, so that it knows that there are two injectors instead of one, and, subject to testing, all is well. The adaptor to physically mount the new TBI on to the engine has to be changed, of course, which is a pity after all the work that went into creating the first one, but it's a small price to pay for a system that will, ultimately, be significantly better matched to the job. The new, twin injector, TBI has dual-throats, and given that the existing manifold is only designed for a single throat (carburettor), mounting the new unit might have presented a challenge. However, in 1965, when the type of engine, now installed in the boat, was in common usage in cars, it was a fairly common practice, amongst younger drivers who wanted to improve their car's performance, to “hot them up” by changing the carburettor. One of the quite simple tuning options, at the time, was to change the original single throat Stromberg carburettor to a dual throat unit, (usually from Holley), and there were special adaptors readily available, at the time, to facilitate this change. Using a dual-throat carburettor is not relevant, but it did make me wonder whether one of those special adaptor plates, from 1965, would be of any help in mounting a very much later dual-throat fuel injector of a similar size. I contacted a company in Queensland called ReCarb Pty. Ltd. and found them most helpful. I sent them a drawing of the proposed dual-throat fuel injector body and not only were they able recommend a suitable adaptor plate that, with some modification, would do the job, but they still had them in stock! So, the “Redline” adaptor, model 10-501 duly arrived and was easily modified, as suggested, to suit the new injector body, just as they had said it would! So, for the princely sum of $79 - problem solved.

Date

Discussion

Jan 2023

The inlet manifold of the Rhapsody's engine has been custom made to make the Holden car engine suitable for use in a boat. It allows the carburettor to be mounted level, (in the same plane), as the engine and it is entirely enclosed in a sealed water jacket. The water jacket has the dual function of both cooling the exhaust manifold and heating the raw river water, before pumping it into the cylinder block, to stop the engine running too cold. So, the existing manifold had to be retained.

This constraint limits the types of fuel injection that can be considered. The Holden cylinder head uses paired inlet ports - which means that each pair of cylinders shares a single inlet port. As a result, there is no opportunity to install separate injectors for each cylinder as might be the case in a more modern engine. The only remaining option, therefore, is to use a Throttle Body Injector (TBI) - sometimes called Central Fuel Injection (CFI).

A TBI is an aluminium casting that mounts in the same place as the carburettor and, indeed, looks a bit like a carburettor. It has the same throttle butterfly valves to control the engine speed, but "squirts" fuel into the engine rather than have the engine "suck" it in as with a carburettor. More importantly, the TBI has no float bowl and is a completely sealed system, which, barring mechanical failures, means it will not leak fuel vapour into the air, or into the bilges of the boat. It is true that a TBI needs to be electronically controlled by an Engine Computer Unit (ECU) and whilst this adds a level of complication to the whole project, it does provide the opportunity for improved tuning and, therefore, better performance and economy.

TBI systems were used on motor vehicles from the mid-1980s, through to the mid-1990s, whilst the manufacturers transitioned from carburettors to the full sequential direct injection we commonly see today. This was done to meet the rising emission standards, and fuel economy targets, being set by governments in order to address pollution and climate control problems.

The first step, for me, was to comb the Internet, (and, physically, in wrecking yards), to find a donor vehicle - one that was fitted with TBI as standard and that was commonly available. After much research, I selected a 1996 Holden Barina SB "Swing". It was actually made by Opel in Germany and marketed there, and in many other countries, as the Corsa "B". It's a General Motors product and was sold in Australia in large enough numbers for them to be readily available in wreckers' yards. Given that the Opel engine is only 1600cc and the boat engine is 2440cc, it might seem an odd choice at first glance. However, consider the individual size of each cylinder - in the Opel it is about 400cc which compares nicely with the six cylinder engine, at 406cc. It is true that the injector has to fire more frequently at three times per engine revolution instead of twice, but given that the Opel engine is rated at 6000rpm, whilst the boat engine will not exceed 4000rpm, the maximum operating speed required of the injector is the same. It is also interesting, and very reassuring, to note that the final throat sizes of the Opel TBI and the Stromberg carburettor, it was to replace, are exactly the same, at 35mm, which suggests that the overall fuel/air mixture volumes would be about the same.

There are a number of aspects of the already installed and running carburetted engine that have to be changed, to move to fuel injection. Specifically, these are the air intake system and the fuel system. An ECU will need to be sourced and installed as well, of course, with its attendant wiring. To simplify the air intake system changes, it was decided to use the complete air-box and filter unit from the donor vehicle. This has required a new mounting frame to be made and installed in the hull. It now connects into the original trunking to access fresh air from outside the hull and is ready to connect to the new TBI using a standard size 60mm hose, (albeit a bit longer). The changes required for the fuel system are discussed, in detail, in the previous section of this document.

However, it turns out that there was a naïve assumption being made, above, about the impact of a single injector fuelling a six cylinder engine as opposed to a four. Whilst the maximum engine speeds quoted are correct, the "normal" operating speeds paint a different picture. Most engines will cruise at much less than their theoretical maximum speed, perhaps only 2000rpm. So, whilst the Opel engine can reach 6000rpm, it would only have to work that hard occasionally, and even then, for quite short periods. The six cylinder engine, at the same relaxed 2000rpm, requires the injector to work at a 33% higher rate than it does in its four cylinder guise.

Some testing of the ECU on the bench, (more of that later), showed that the single injector working at that higher speed not only reached worryingly high Duty Cycle percentages, but it also started getting hot, too. Therefore, the plan to use a single injector, working so hard, for potentially long periods, became a matter of some concern.

One of the alternative TBI units that had been under consideration, when the conversion project was first planned, was from a bigger, six cylinder engine and it had two separate injectors in the same TBI body, instead of one. I had assumed, at the time, that it had been done to achieve the necessary fuel flow rates, but now, I suspect, the real reason is that it allows the injectors to be operated alternately, at half the speed, making twice as much time available for each unit to complete its cycle.

The twin injector TBI being considered was from a 3.9litre Ford EA/EB Falcon made from 1988 to 1991, but for some reason, suitable donor wrecks are very hard to find. (Perhaps it's because they're ideal for fuel injection conversions!) However, I had lodged a query with a local wrecker's website many, many moons ago, asking him to let me know if one of these particular vehicles became available for wrecking, and then I promptly forgot all about it - until last week when he, quite serendipitously, sent me an email. A quick visit to the wrecker's yard secured not one, but two, twin injector TBIs to investigate.

Back to the drawing board? No, not quite. The modifications to the fuel system and the air intake system are unchanged. The ECU just has to have one minor software setting altered, so that it knows that there are two injectors instead of one, and, subject to testing, all is well. The adaptor to physically mount the new TBI on to the engine has to be changed, of course, which is a pity after all the work that went into creating the first one, but it's a small price to pay for a system that will, ultimately, be significantly better matched to the job.

The new, twin injector, TBI has dual-throats, and given that the existing manifold is only designed for a single throat (carburettor), mounting the new unit might have presented a challenge. However, in 1965, when the type of engine, now installed in the boat, was in common usage in cars, it was a fairly common practice, amongst younger drivers who wanted to improve their car's performance, to “hot them up” by changing the carburettor. One of the quite simple tuning options, at the time, was to change the original single throat Stromberg carburettor to a dual throat unit, (usually from Holley), and there were special adaptors readily available, at the time, to facilitate this change.

Using a dual-throat carburettor is not relevant, but it did make me wonder whether one of those special adaptor plates, from 1965, would be of any help in mounting a very much later dual-throat fuel injector of a similar size. I contacted a company in Queensland called ReCarb Pty. Ltd. and found them most helpful. I sent them a drawing of the proposed dual-throat fuel injector body and not only were they able recommend a suitable adaptor plate that, with some modification, would do the job, but they still had them in stock! So, the “Redline” adaptor, model 10-501 duly arrived and was easily modified, as suggested, to suit the new injector body, just as they had said it would! So, for the princely sum of $79 - problem solved.