Here we go again. Forchetto opens a new thread with the words "Free power" to make us look at it and swallow some farcical theory or impractical mod. 
This is just for those few of us left with our airboxes intact and not ripped out or butchered and thrown over a hedge. About half a dozen judging by the posts...Just think that in 50 years time some sad bastard that has dedicated the last 6 years of his life to restoring a Bonnie to an absolutely original and concours condition, will curse you for losing that airbox. They will become collectors items and will go for 1000's on ebay or probably in antique shops and archeologists...
Some time ago I started a thread trying to show that a small proportion of the power gains noted by removing the airbox could have been due to the enforced re-routing of the crankcase breather so that it didn't vent its foul and greasy fumes into the intake. If you want to read that its here.
This thread will try to show that a considerable proportion of the gains come from exposing the air intakes and filters to the fresh air rather than the enclosed and superheated atmosphere inside the airboxes, not just from the improved efficiency in airflow into the carbs or throttle bodies.
To start with we must remember the fact that hotter air is less dense and contains less oxygen by volume. It's desirable that the intake air temperature is as cold as possible to optimize combustion efficiency. This is well-known and it's not open to question. An old rule of thumb states that for every 5ºC (about 10ºF)increase in intake air temperatures we lose at least 1% in power/torque.
A further less well-known power loss that will affect EFI bikes, is that the IAT (Intake Air Temperature) sensor informs the engine management of this increased temperature and the computer "pulls" or retards the ignition advance to avoid detonation. This is because hotter air encourages pre-ignition or detonation, very damaging to an engine. The threshold when this takes place varies with different systems but can be as low as 30ºC (86ºF), and the total ignition retardation is typically 3 degrees.
Some of you with Tuneboys and other software that can display FI maps, can confirm these specific figures for our EFI bikes by looking for the map of Intake air temperature plotted against Manifold absolute pressure and letting us know. An example of this map on another vehicle is shown here:
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I have long suspected that this problem was acute on our bikes by just feeling the temperature of components, including the airbox, that live behind the engine and noting how high it was. I reason the airbox acts as a low temperature oven diluting the incoming air charge excessively. To make matters worse the standard snorkel pipe faces forward and is forced to breathe hot engine cooling air which is rammed behind the left-hand side panel. This is obvious as soon as you feel how warm this panel gets, even after a short run.
I've taken some measurements to show this, as follows:
I have chosen two cold days, 14ºC (57ºF), to carry out tests using a dual-probe thermometer. One probe inserted inside the airbox, the other in open air and even after a relatively short run (10 and 25 km) over the same ground at the same speeds, and starting from cold the readings were:
After 10 km the airbox air was at 9.5ºC over ambient, after 25 km 15ºC over, meaning a possible 3-4% power loss.
The next day I replaced the snorkel with the famous Polaris bellmouth to see if this was any better:
After 10 km the airbox was at 5ºC over ambient, after 25 km just 10ºC over, better, but still a 2-3% power loss.
This confirms what we all suspected, the Polaris bellmouth does improve things, not just because of any airflow properties, but because not facing forwards it draws in less hot air than the OEM snorkel. Once again, this was done on a cold day and for a shortish distance. Imagine the readings on a hot summers day and a longer run. The power and driveability losses must be considerable.
If we don't fancy airbox removal, what can be done to improve this and avoid losing significant performance due to overheated intake air?.
In cars, ducts are used to bring cold air from the front of the vehicle, or in some muscle cars from the top of the hood. This is impractical on our bikes because of lack of space.
Ideally we could do with one of the comercially available bellmouths to be extended through an opening in the side panel and exposed to outside air flow. It would have to be carefully shaped or faired to avoid a sucking out effect due to the air rush past it. Or perhaps a hole cut-out of the same panel to enable the Polaris bellmouth to draw air through it and perhaps a wire mesh guard to stop bugs, etc from diving into the airbox.
I think it's Pieman that has done this, I've seen a photo somewhere.
A small improvement could conceivably be made by fitting a deflector to stop engine hot air from sneaking past the side panel into the intake and even fabricating a heat proof bulkhead to insulate the airbox forward wall from the hot air blast.
The power and driveability loss through retarded timing by the IAT will be dealt with in a future post, after I've gathered some more data from the maps, specially the IAT v Manifold pressure one.
Any thoughts?
This is just for those few of us left with our airboxes intact and not ripped out or butchered and thrown over a hedge. About half a dozen judging by the posts...Just think that in 50 years time some sad bastard that has dedicated the last 6 years of his life to restoring a Bonnie to an absolutely original and concours condition, will curse you for losing that airbox. They will become collectors items and will go for 1000's on ebay or probably in antique shops and archeologists...
Some time ago I started a thread trying to show that a small proportion of the power gains noted by removing the airbox could have been due to the enforced re-routing of the crankcase breather so that it didn't vent its foul and greasy fumes into the intake. If you want to read that its here.
This thread will try to show that a considerable proportion of the gains come from exposing the air intakes and filters to the fresh air rather than the enclosed and superheated atmosphere inside the airboxes, not just from the improved efficiency in airflow into the carbs or throttle bodies.
To start with we must remember the fact that hotter air is less dense and contains less oxygen by volume. It's desirable that the intake air temperature is as cold as possible to optimize combustion efficiency. This is well-known and it's not open to question. An old rule of thumb states that for every 5ºC (about 10ºF)increase in intake air temperatures we lose at least 1% in power/torque.
A further less well-known power loss that will affect EFI bikes, is that the IAT (Intake Air Temperature) sensor informs the engine management of this increased temperature and the computer "pulls" or retards the ignition advance to avoid detonation. This is because hotter air encourages pre-ignition or detonation, very damaging to an engine. The threshold when this takes place varies with different systems but can be as low as 30ºC (86ºF), and the total ignition retardation is typically 3 degrees.
Some of you with Tuneboys and other software that can display FI maps, can confirm these specific figures for our EFI bikes by looking for the map of Intake air temperature plotted against Manifold absolute pressure and letting us know. An example of this map on another vehicle is shown here:
I have long suspected that this problem was acute on our bikes by just feeling the temperature of components, including the airbox, that live behind the engine and noting how high it was. I reason the airbox acts as a low temperature oven diluting the incoming air charge excessively. To make matters worse the standard snorkel pipe faces forward and is forced to breathe hot engine cooling air which is rammed behind the left-hand side panel. This is obvious as soon as you feel how warm this panel gets, even after a short run.
I've taken some measurements to show this, as follows:
I have chosen two cold days, 14ºC (57ºF), to carry out tests using a dual-probe thermometer. One probe inserted inside the airbox, the other in open air and even after a relatively short run (10 and 25 km) over the same ground at the same speeds, and starting from cold the readings were:
After 10 km the airbox air was at 9.5ºC over ambient, after 25 km 15ºC over, meaning a possible 3-4% power loss.
The next day I replaced the snorkel with the famous Polaris bellmouth to see if this was any better:
After 10 km the airbox was at 5ºC over ambient, after 25 km just 10ºC over, better, but still a 2-3% power loss.
This confirms what we all suspected, the Polaris bellmouth does improve things, not just because of any airflow properties, but because not facing forwards it draws in less hot air than the OEM snorkel. Once again, this was done on a cold day and for a shortish distance. Imagine the readings on a hot summers day and a longer run. The power and driveability losses must be considerable.
If we don't fancy airbox removal, what can be done to improve this and avoid losing significant performance due to overheated intake air?.
In cars, ducts are used to bring cold air from the front of the vehicle, or in some muscle cars from the top of the hood. This is impractical on our bikes because of lack of space.
Ideally we could do with one of the comercially available bellmouths to be extended through an opening in the side panel and exposed to outside air flow. It would have to be carefully shaped or faired to avoid a sucking out effect due to the air rush past it. Or perhaps a hole cut-out of the same panel to enable the Polaris bellmouth to draw air through it and perhaps a wire mesh guard to stop bugs, etc from diving into the airbox.
I think it's Pieman that has done this, I've seen a photo somewhere.
A small improvement could conceivably be made by fitting a deflector to stop engine hot air from sneaking past the side panel into the intake and even fabricating a heat proof bulkhead to insulate the airbox forward wall from the hot air blast.
The power and driveability loss through retarded timing by the IAT will be dealt with in a future post, after I've gathered some more data from the maps, specially the IAT v Manifold pressure one.
Any thoughts?
