Petrol engines – engine compartment ventilation.

[moresparks]

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.

 

[James L]

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.

 

[CLB]

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.

 

[MapisM]

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.

 

[CLB]

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.

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

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

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

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

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.

 

[Grubble]

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.

 

[moresparks]

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.

 

[James L]

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.

 

[CLB]

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.

 

[moresparks]

“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.

 

[MapisM]

You are misquoting me.

Ops, guilty as charged.
Maths still wrong though, see next point (on top of throttle) ...

A four stroke engine will suck in every four revolutions

I don't think so.

Regardless, I just couldn't resist challenging your math, but as I said all this is just academic, and not helpful for the OP.
If she were my boat, the check suggested by JL still would be my first course of action.

 

[MapisM]

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.

There are actually different schools of thought on this.
I used to own a US speedboat, big block powered and pretty fast.
Both her blowers extracted air from the bottom of the engine compartment, which is obviously how they must work, in a petrol boat (@ CLB: they would be useless for preventing an explosion, if they would push air into the e/r).
But while they were not wired to run permanently when the engine was on, the manual recommended to keep them running not only before startup, but also whenever idling for some time (I can't remember the exact wording, but this was the sense).
The rationale is that even if the air sucked by the engine normally grants enough circulation, petrol vapour are heavier and could cumulate at the bottom whenever running for some time at very low rpm, also because the engine air intake is usually much higher, so a little air exchange at the top of the e/r could be insufficient to prevent explosions.

 

[James L]

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.

I'm no expert but that seems like a strange arrangement. 1 should be plenty to remove fumes.
Are you sure of the original direction of the 2 permanent blowers, any chance they were blowing fresh air into the engine bay, if as you say there is no other ventilation this would seem like a more sensible arrangement. 2 blowing air in all the time the engines are running and 1 to run before startup to remove any petrol fumes.

 

[MapisM]

Agreed, the opposite would make more sense.

 

[Rocksteadee]

Maths being my strongest weak point.
2.5l 4 stroke fires every 2 revs. Every 2 revs the engine has fired every cylinder.
5500 rpm / 2 = 2750 cycles x 2.5 l = 6875 l x 2 engines = 13750 l = 14 m2 per min. Add to this air suction fans

Take of engine cover at tick over and feel how easy / hard it is to lift.
Take off cover at flat out and feel if it has resistance ie being sucked on and if revs increase
Look for tell tale black streaks at edges / joints of engine cover where air is being sucked in.

The “bilge fan” or the (incorrectly named) “engine blower” is normally (if pipe is at bottom of engine bay or bilge) a suction fan to remove petrol fumes from the bottom as they are heavier than air.
If pipe is around the air intake or may blow from front to back of engine to give cooling air as well, and it blows in this is then forced air for engine consumption

 

[CLB]

Ops, guilty as charged.
Maths still wrong though, see next point (on top of throttle) ...

I don't think so.

Regardless, I just couldn't resist challenging your math, but as I said all this is just academic, and not helpful for the OP.
If she were my boat, the check suggested by JL still would be my first course of action.

Yep, fair cop. I was thinking of the four strokes, but that is, of course, achieved over two revolutions. Basically double my figures.

 

[Grubble]

“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.

You can always fit a petrol vapour alarm - I think I would on an older boat, especially if I was planning to sleep on it.

 

[moresparks]

Hmm... a lot to think about, many thanks for your suggestions.
The blowers that are on all the time are connected via one of the engines ignition switches, so it seems that the builders made a conscious effort to ensure the blowers work effectively.
All the louvered vents are at the stern; to have both “suck and blow” from the same area I thought would have been dangerous as you may be pushing extracted vapours back into the carburetors?
I can check next time I am out and prop up the hatch a little and test.
I will also look into a petrol vapour alarm specifically for the locker. I currently have a CO alarm near the helmsman position which is next to the main cabin door.

 

[Momac]

. 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 engines are water cooled so dont rely on air for cooling.
At slow speed , after a fast run , the engine temperatures should fall.
I am surprised there are no air intakes .

 

[Portofino]

Petrol sources with old engines are from in no particular order .....
The lead carb plugs seeping , carb gaskets leaking seeping , poor fuel lines deteriorated. Poor adjustment of the idle jets or stuck choke not quite closing .
So it makes sense with an aged carb boat to run the bilge extraction fan all the time when the engines running and what ever ( suggest 5/10 mins ) before start up .
Hot starts too .Because the residual block heat will evaporate off any float chambers into the air filters and out into the ER .

The OP ought to fit some sort of fwd facing ram air scoops, the faster ( more rpm ) the boat goes the more air is stuffed in to feed it .
Vetus do some nice SS air vents
A row of theses for example https://www.vetus.com/en/ventilation/shell-ventilators.html facing Fwds on top of the cover ?