Engine Compartment Ventilation

10-03-2002, 03:17 AM

This is a topic I have to pound into some peoples heads for some reason, but it's quite simple really and I love to enlighten those who have never thought about it... If you are out runnin wide open across the water, not only do your motors build up considerable heat from working and makin you happy, but 2 other things happen as well... We have all had the Temp, elevation, humidity discussions, and pretty much agree that cooler, moister (<my own words>) and denser air make more power. Well I have found that quite a few of the boats running high dollar power, just don't seem like you got whatcha paid for (dollar$ spent vs MPH gained)and guys end up spending good money after bad... and going nowhere. Well, 70% of you are running under ventilated engine compartments, and or too small of Spark arresters (too restrictive). I have seen, made, and watched up to and in a few cases more than 75 HP gains on the dyno by increasing the amount of area able to pass air to the motors thru the arresters, and God only knows how many MPH are to be gained by meeting the demand for fresh, cool air entering and exiting your engine compartments that not only keep the temps down preventing detonation and various other problems... but think of it like if you decided to put a set of 2 1/4" tube headers on your motors with a 2 barrel carbs, or Dart intake manifolds with a 1050 Dominators on motors with stock exhaust! One affects the other and one will do no good without the other, and I have also seen gains of 1000 to 1500 RPM on the top end by increasing both. A good way to check and see if you have something to gain, is to lift the back of your hatch up a couple inches to allow un-turbulent air to enter and exit smoothly, without time to preheat the inside. If you merely open the hatch from the front, it disrupts the air flow and turbulance becomes a problem as most engines work better drawing undisturbed air from a stable resource, rather than to force air into, and around the spark arresters. If you take any twin setup and increase the HP of both motors by 200 HP or more (100 ea.) I can guarentee that the factory inlets and ventilation on 97% of the boats out there is not sufficient to support the increased demand of the motors. This has to be one of the single most overlooked, restrictive, and heartbreaking problems Offshore boaters face when modifing their motors... and we're talking free horsepower!! Now when is the last time any of you got some of that? It don't happen in our world, but if you think about it, it's not rocket science... it's so simple it makes you wanna puke when you discover 5-10 MPH by giving your engines an ample supply of air IN and OUT. There is a way to check the pressurization (sp?) of your eng compartment, and if I can find the outline, I will post it.. but this is real, good, easy and effective for polishing your hard earned dollars into speed... check it out, ya never know... Im sure someone here has crossed this path before, and if you have, please share. The charts below are from the Coast Guard, and Mercruiser, to give you an idea about fresh air and it's benefits...

formula31 · 10-03-2002, 07:01 AM

Absolutely correct. I have found this out many time before. When I built my engines last winter, I went from 2-3" in and one 3" out to 3-4" in and one 4" out per side. I also do a run with the engine hatch off and flame arrester off too see if I have enough.

I also stick some stagegically placed temp stickers around the egine compartment to see what my ambiant temps get too under WOT.

dyno · 10-03-2002, 07:56 AM

I've been toying with the same idea's all summer too! i'm leaning twards adding two Harwood scoops to add flow (to my wifes dismay)Its damn hot in my bilge! there was a post a while back that some one was making nascar style vents for the side of your boat but havent heard anything......what are those vents called? tear drop shaped?

Turbojack · 10-03-2002, 09:16 AM

I had 2 problems, 1 compartment would go to negitive pressure when WOT & undercover temps to high. On a 95 degree day temps were 120-125. I have since added 2 atwood blower to take air from seating area to inside bilge. This lowered temp to aprox. 110 & 1 picked up 5 lost mph. I know I need to get air in from outside the boat which I think is probably cooler then the aire from seating area but have not seen something I really like.

Looking at this vent but looking to see if anything better. eddie marine vent

Anyone with different vents, or scoops?

Iggy · 10-03-2002, 10:18 AM

Turbojack,
How'd you know you were pulling a vacum in the engine compartment at WOT?
What instrumentation was used to measure it?

Inquiring minds want to know.

Turbojack · 10-03-2002, 11:20 AM

I could have done it Tomcats way with some high dollar instruments but all I took was a vacuum pressure gauge I got down at autoparts with some extra vacuum hose. Ran hose into compartment under lid. Gauge would read 0 with boat just sitting there. I did not check with bilge blower on or off or with motor running. When I would get going & go WOT the gauge would read 2" of vacuum.

Looking at chart Findme posted I do not have near enough air vent area.

FloridianSon · 10-03-2002, 03:25 PM

Formula 31 you did not say what you engine box temps.were .just wondering as next time out I am going too take some readings myself.Turbo listed his and I was just going to compair with a hand full of outhers.thanks

10-03-2002, 03:59 PM

I added two bilge blowers that suck air from my cabin area and blow it directly onto the flame arrester. They are routed to regular bilge blower circuit so I get extra ventilation prior to startup or whenever I feel the need for a few extra ponies. I see a 1.5mph gain.

Griff · 10-03-2002, 04:21 PM

Quote:

Originally posted by dyno
I've been toying with the same idea's all summer too! i'm leaning twards adding two Harwood scoops to add flow (to my wifes dismay)Its damn hot in my bilge! there was a post a while back that some one was making nascar style vents for the side of your boat but havent heard anything......what are those vents called? tear drop shaped?

dyno, those are naca ducts. Vonbongo has them glassed in on his new Pantera. There are some pics in the Pantera section.

formula31 · 10-03-2002, 06:49 PM

Quote:

Originally posted by FloridianSon
Formula 31 you did not say what you engine box temps.were .just wondering as next time out I am going too take some readings myself.Turbo listed his and I was just going to compair with a hand full of outhers.thanks

I did that in 93 I think so my memory isnt too good but I think it was around 130 or even higher. I remember it shocked me. It really shot up after you stopped and shut the engne off.