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  1. #1
    Join Date
    Apr 2007
    Location
    Westwick - Essex
    Posts
    119

    Default Petrol engines – engine compartment ventilation.

    Looking for advice please, regarding ventilation of the engine compartment.
    I have twin petrol engines on my elderly (late 80’s early 90’s) sports cruiser; it is fairly tight in the compartment so much so that the top of the carburetors silencer / air intake fixing bolts make an indent on the compartment top hatch sound deadening. This has always been the case since new however. I have blowers to extract fumes and hot air, exiting at the stern and two of the blowers are on all the time. Again all since new. I have recently upgraded the exhaust manifolds to stainless steel Hi-Tek types which appear to give a 10% performance upgrade.
    However there are no air intakes! Later design of my model shows two louvered air vents, one on each side into the engine compartment.
    Now before I start cutting a 500 x 50 mm cut out in the sides, I am looking for expert confirmation.
    Some of these boats came with Diesel engines, so I know that ventilation for these is important, but my understanding was that petrol engines also need a good flow of fresh air for them to perform as intended.
    Appreciate any advice and if the air should be “forced”.
    Thanks in advance.

  2. #2
    Join Date
    Jul 2012
    Location
    London / West Cork
    Posts
    245

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by moresparks View Post
    Looking for advice please, regarding ventilation of the engine compartment.
    I have twin petrol engines on my elderly (late 80’s early 90’s) sports cruiser; it is fairly tight in the compartment so much so that the top of the carburetors silencer / air intake fixing bolts make an indent on the compartment top hatch sound deadening. This has always been the case since new however. I have blowers to extract fumes and hot air, exiting at the stern and two of the blowers are on all the time. Again all since new. I have recently upgraded the exhaust manifolds to stainless steel Hi-Tek types which appear to give a 10% performance upgrade.
    However there are no air intakes! Later design of my model shows two louvered air vents, one on each side into the engine compartment.
    Now before I start cutting a 500 x 50 mm cut out in the sides, I am looking for expert confirmation.
    Some of these boats came with Diesel engines, so I know that ventilation for these is important, but my understanding was that petrol engines also need a good flow of fresh air for them to perform as intended.
    Appreciate any advice and if the air should be “forced”.
    Thanks in advance.
    Easy way to check is run it flat out, check your speed.
    Open the engine hatches a bit, run it flat out again and see if there's any difference.
    If there's no difference in speed the engines must be getting enough airflow.
    www.fimtra.com - Distributed software, Innovative solutions.

  3. #3
    Join Date
    Jun 2013
    Posts
    3,521

    Default Re: Petrol engines – engine compartment ventilation.

    My maths may be a bit off here, but by my calculation, a pair of 5.7 litre V8's need about 11+ cubic metres of air per minute at 4000 rpm. You may have smaller engines, of course, but even a couple of little four cylinder Volvos will get trough about 5m3 of air at 4000rpm. It doesn't need to be forced, but there does need to be a way for that much air to get in.
    If found in the Brexit forum, please return to the real world.

  4. #4
    Join Date
    Mar 2002
    Posts
    17,630

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by CLB View Post
    My maths may be a bit off here, but by my calculation....
    Forgive my bluntness, but your maths is off in so many ways that I don't know where to start.
    Let's get rid of the V8 first - the cylinders number and layout is not relevant in this context, only the displacement is.
    Back to the maths, it sounds like you reached the 5 cubic meter as follows:
    5.7 litres x 4000 rpm = 22.8k litres x 2 engines = 45.6k litres, rounded to 5 cubic meters.

    Now, there are three main reasons why this is way off the mark:
    1) Firstly, if anything, the above rounding should have been 50 cubic meters, not 5.
    2) In 4 stroke engines, the air is sucked in every other revolution, so you should have multiplied the displacement by rpm/2.
    3) In petrol engines (as opposed to diesels), the air required heavily depends on the throttle position, which at 4000rpm is bound to be not yet fully open, on those engines - and we know nothing about that, anyhow.

    That's all academic, though.
    Practically, I would second JL suggestion above: that's the easier way to check the existing situation.
    I would just check the difference at WOT in rpm rather than speed (which is just a consequence).

    Trouble is, if the engines would'n reach the same rpm with the hatch closed, then the OP question still stands.
    And I would never dare suggesting the size nor the position of the air intakes to be cut, because that's very much boat-specific.

    Btw, I am at a loss in understanding what the exhaust manifolds change has to see with the air intakes.
    If the engines worked fine so far, with whatever air intakes the boat already had, I very much doubt that a tiny rpm increase (if any) gained with the s/steel exhaust manifold could make them not adequate anymore.

  5. #5
    Join Date
    Jun 2013
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    3,521

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by MapisM View Post
    Forgive my bluntness, but your maths is off in so many ways that I don't know where to start.
    OK, let's have a look at it. Your bluntness may be justified, but I'm still not sure.

    Quote Originally Posted by MapisM View Post
    Let's get rid of the V8 first - the cylinders number and layout is not relevant in this context, only the displacement is.
    I agree, I just said V8 to clarify my example

    Quote Originally Posted by MapisM View Post
    Back to the maths, it sounds like you reached the 5 cubic meter as follows:
    5.7 litres x 4000 rpm = 22.8k litres x 2 engines = 45.6k litres, rounded to 5 cubic meters.
    You are misquoting me. I said 11+m3 for the 5.7 and 5m3 for a small 4 cylinder (2,3 litre in this example) engine

    Quote Originally Posted by MapisM View Post
    Now, there are three main reasons why this is way off the mark:
    1) Firstly, if anything, the above rounding should have been 50 cubic meters, not 5.
    See above

    Quote Originally Posted by MapisM View Post
    2) In 4 stroke engines, the air is sucked in every other revolution, so you should have multiplied the displacement by rpm/2.
    Surely that's incorrect. A four stroke engine will suck in every four revolutions, ergo I divided by four. 5.7l x 4000 / 4 x 2 = 11,400

    Quote Originally Posted by MapisM View Post
    3) In petrol engines (as opposed to diesels), the air required heavily depends on the throttle position, which at 4000rpm is bound to be not yet fully open, on those engines - and we know nothing about that, anyhow.
    Interesting, and not something I gave much thought to. I would say that throttle position would dictate how much fuel is mixed in, but each cylinder will still be full, so the whole swept volume must still surely be taken into account. There is a little to be deducted for the fuel mix, but I ignored that for simplicity.
    If found in the Brexit forum, please return to the real world.

  6. #6
    Join Date
    Oct 2010
    Location
    London no more
    Posts
    2,708

    Default Re: Petrol engines – engine compartment ventilation.

    Why not pull your spark plugs and see if they look normal or have signs of carbon build up (indicating it's running rich)?

    If the plugs look good I would have thought it's getting enough air.

  7. #7
    Join Date
    Apr 2007
    Location
    Westwick - Essex
    Posts
    119

    Default Re: Petrol engines – engine compartment ventilation.

    Many thanks for your responses – very interesting. They say a little knowledge is dangerous!!
    A little background.. The engines are Volvo Penta straight 4’s DOHC around 170HP each. They have only been lightly used and over the last couple of years I have been going over them to run as intended.
    Two of the blowers which should have been on all the time, were disconnected. When connected they were very noisy and inefficient, so I have replaced them with quieter and higher performance ones. With these blowers on all the time extracting air from the compartment which has no other ventilation just got me thinking (dangerous I know). I understand that petrol engines do not perform as well when they are hot and can lead to shorter lived engines. With that in mind I thought it may be better to improve the ventilation. The plugs look o/k but the engines have not run much since the blowers were replaced.

  8. #8
    Join Date
    Jul 2012
    Location
    London / West Cork
    Posts
    245

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by moresparks View Post
    Many thanks for your responses – very interesting. They say a little knowledge is dangerous!!
    A little background.. The engines are Volvo Penta straight 4’s DOHC around 170HP each. They have only been lightly used and over the last couple of years I have been going over them to run as intended.
    Two of the blowers which should have been on all the time, were disconnected. When connected they were very noisy and inefficient, so I have replaced them with quieter and higher performance ones. With these blowers on all the time extracting air from the compartment which has no other ventilation just got me thinking (dangerous I know). I understand that petrol engines do not perform as well when they are hot and can lead to shorter lived engines. With that in mind I thought it may be better to improve the ventilation. The plugs look o/k but the engines have not run much since the blowers were replaced.
    I don't think you need these blowers to run all the time, you need to run them for a few minutes before starting the engine to get rid of any petrol fumes that could be ignited by sparks when you start the engine.
    Once the engine is running, it is doing the job of the blowers.
    www.fimtra.com - Distributed software, Innovative solutions.

  9. #9
    Join Date
    Jun 2013
    Posts
    3,521

    Default Re: Petrol engines – engine compartment ventilation.

    Quote Originally Posted by moresparks View Post
    Many thanks for your responses – very interesting. They say a little knowledge is dangerous!!
    A little background.. The engines are Volvo Penta straight 4’s DOHC around 170HP each. They have only been lightly used and over the last couple of years I have been going over them to run as intended.
    Two of the blowers which should have been on all the time, were disconnected. When connected they were very noisy and inefficient, so I have replaced them with quieter and higher performance ones. With these blowers on all the time extracting air from the compartment which has no other ventilation just got me thinking (dangerous I know). I understand that petrol engines do not perform as well when they are hot and can lead to shorter lived engines. With that in mind I thought it may be better to improve the ventilation. The plugs look o/k but the engines have not run much since the blowers were replaced.
    Mapis hasn't got back to me, so I'm going to stick with my calcs for now and say that your engines (2 x 2.5 litre) will require around 6.8m3 of air per minute at max rpm (5500). If your blowers are set up to extract, you need to add their capacity onto the above figure and ensure that you have sufficient ventilation for that. I suspect that if the blower vents are the only way to get air into the engine compartment, they would be better set up to suck air in.
    If found in the Brexit forum, please return to the real world.

  10. #10
    Join Date
    Apr 2007
    Location
    Westwick - Essex
    Posts
    119

    Default Re: Petrol engines – engine compartment ventilation.

    “I don't think you need these blowers to run all the time, you need to run them for a few minutes before starting the engine to get rid of any petrol fumes that could be ignited by sparks when you start the engine.
    Once the engine is running, it is doing the job of the blowers.”


    I agree … but the blowers were designed that way by Fairline, I am guessing they felt it was necessary so I am reluctant to rewire. The Third blower is switchable.

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