Modifying flame arrestor

[JasonB]

I have an extra used flame arrestor for my Mercruiser 888 (Ford 302, 2bbl carb). Knowing that the engine is sitting in a very hot engine bay sucking hot air through a very restrictive flame arrestor, I've been working on ways to get more cool air to the engine. What if I attached some form of fitting to the top of the arrestor and attached a sizable hose, via clamps, that could get air from outside the comparment? It would have to be at least a couple inches in diameter to do much good, but would there be any safety implications? Could it hurt air intake? I would have to filter the air somehow, but that would be easy enough.

 

[bob58]

Re: Modifying flame arrestor

but would there be any safety implications?

the purpose is to arrest the flames from a backfire through the carb so... the hose could create an ignition source and routing it remotely gives greater opportunity for contact with other ignitable parts of your vessel.<br />Modifying it could also be a problem with boardings and inspections by the coast guard or local authorities as well as void your insurance policy on the vessel if it were damaged by fire.

 

[affa]

Re: Modifying flame arrestor

Hi<br />I have made a hose that just let the cold air come up near by the intake, I found out that there should be no reason for attaching it to the carb, since it sucks the air that is around it.<br />Ive just done this now while Im winterizing, so I havent tried it, but I think this must be better then it was, the hose size is almost 3 inch.<br />Regards<br />Affa

 

[navigator336]

Re: Modifying flame arrestor

This is an interesting concept. I like Affa's method. Before you fabricate, you might try running the boat with the hatch or engine cover open to see if in fact the denser air makes a noticeable difference in performance.

 

[Fishermark]

Re: Modifying flame arrestor

Aren't most, if not all engine compartments already vented thru with fresh air? I fail to see any benefit to any additional modifications. A car's engine compartment gets far hotter than a boat's and the performance in a car is fine. I understand the theoretical concept of cooler air equals denser air which results in more theoretical power. But unless you have a VERY restrictive engine compartment, I believe the real world application of that theory is not going to be helped by adding a little more fresh air.<br />Just my humble two cents.

 

[airman]

Re: Modifying flame arrestor

One of the boats I work on has several 3 inch hoses attached to bilge blowers to force fresh air into the engine compartment and towards the engine air intakes.

 

[JasonB]

Re: Modifying flame arrestor

The compartment on this boat is very restrictive and only has one blower (upgrading to two while I'm in there). I was thinking along the lines of a blower pulling cold air it and dumping it around the intake as well.

 

[DJ]

Re: Modifying flame arrestor

JasonB,<br /><br />Modifying the flame arrestor itself is not such a great idea for the reasons of:<br /><br />Insurance issues.<br /><br />USCG compliance.<br /><br />However, getting more air into the compartment is a good idea. As long as that air is not from an area that houses a fuel tank or fuel tank vent.<br /><br />Running two blowers, and leaving them on while running, is one good idea.<br /><br />The other may be additional exterior venting.

 

[santa666]

Re: Modifying flame arrestor

Modifying the flame arrestor is not a good idea. <br /><br />It sounds like you're trying to make a flamethrower. (all humor aside), Perhaps through the judicious use of screens and other baffle material you could achieve a greater airflow whilst keeping safety in mind.<br /><br />Again, there's no way of telling what you've concocted is working until you'e got a flame to arrest, and is that such a good time to find out?

 

[santa666]

Re: Modifying flame arrestor

Modifying the flame arrestor is not a good idea. <br /><br />It sounds like you're trying to make a flamethrower. (all humor aside), Perhaps through the judicious use of screens and other baffle material you could achieve a greater airflow whilst keeping safety in mind.<br /><br />Again, there's no way of telling what you've concocted is working until you'e got a flame to arrest, and is that such a good time to find out?<br /><br />Also, leaving a blower on while running would feed oxygen to a fire if there were one from a faultily engineered flame arrestor.

 

[Dhadley]

Re: Modifying flame arrestor

Take a close look at todays high perf boats. They all go to great lengths to get fresh air into the engine compartment.

 

[navigator336]

Re: Modifying flame arrestor

I still think the benefits can be easily tested with the hatch open while running. However, after giving it more thought, I'm betting it won't make an noticeable difference for the following reasons:<br /><br />A super charger develops something like 6 pounds of boost. Atmospheric pressure (at sea level) is 14.7 psi. So a super charger is giving in excess of 40% increase in air density. I don't know what the coefficient of thermal expansion is for air, but I do know that a car runs well in 60 degree weather or 100 degrees. A moving car is fed an enormous amount of outside air. Therefore, I'm assuming that 40 degrees variation in air temperature doesn't change density very much. A 700 cfm carb at wide open throttle is sucking 700 cubic feet of air per minute. 700 cubic feet is a space 7 feet by 7 feet by 10 feet high. Which would be a pretty monster engine compartment. So I'm guessing that the engine running wide open will turn over the air at least every 20 seconds or so. Therefore, there's already alot of fresh air coming in and the relative temperature increase can't be too great. Now, all that theory said, I still think it is an interesting concept and look forward to hearing what happens in the real boating world.

 

[jlshields]

Re: Modifying flame arrestor

Having a 700cfm carb doesn't necessarily mean its 'sucking' that much. The amount of air passing through it depends on the engine. A naturally aspirated engine can only take in so much air, depending on condition, cubic inches, valve size, lift, etc. If the engine can only suck in 500 cfm, a 700 cfm carb offers no advantage. <br />Navigator, your measurements equal 490 cu ft. 700 cu ft would be 10X10X7. <br />john

 

[navigator336]

Re: Modifying flame arrestor

Jlshields, sorry about the typo, I saw it right after the post (opps). I definitely agree with you that a normally aspirated engine won't likely draw the full cfm of a carb. A 350 would draw about 450 cfm while a 454 would be just under 600 cfm at 4500 rpm. That's why I was very conservative on the "turns of air" at once every 20 seconds. Let's say a single V8 engine compartment is 6 x 6 x 3.5 feet or 126 cu. feet. Then you take out the volume of the engine, drive, and anything else in the compartment and you are well under 100 cu. feet or an air turn every 13 seconds with a 350. My point was that there is alot of fresh air entering the engine compartment. Once things thaw out in Norway, Affa will tell us what happens in the real world!

 

[edwardh1]

Re: Modifying flame arrestor

you would think the flame arrestor would work just as well if it was inside a air tube. I thought all it did was in case of a backfire the metal wicked off the heat so no flame came through>????<br />I have often wondered why there was no intake silencer on boats - even small 4 cyl cars have them, and they really work - like on a Honda. <br />Boats are too noisy and part of the noise is intake noise from the carb/intake system.

 

[jlshields]

Re: Modifying flame arrestor

navigator, I'm with you. I think if the air intake opening for bilge ventilation is as large or larger than the carb or fuel inj air intake and your flame arrester isn't causing any restriction you're doing as good as you can without adding some boost(super or turbo charger).<br />john

 

[Ralph 123]

Re: Modifying flame arrestor

K&N makes high performance CG approved flame arrestors which are highly recommended by Moore in his book Small block Chevy Marine Performace. Not only do they allow unrestricted flow but also shape the flow to increase mixing and therefore efficiency and HP<br /><br />Check this out:<br />http://www.martelbrothers.com/customer/pages.php?pageid=37&xid=467c44394894823e 89fa2186884ad7d8<br /><br />Most engine compartment have positive ventillation - vents cut into the sides of the compartment which force air into the compartment as the boat moves - the lovers face forward.<br />I think there are mandatory for safety to prevent fuel vapors from building up in the comparment

 

[JasonB]

Re: Modifying flame arrestor

I have looked at the K&N, but hate to spend the cash on in in case I find a deal on a 4bbl carb.<br /><br />I can attest that the boat picks up 200-300rpm with the engine cuver removed, so there is definetly some potential if I can get more air to the carb. Oddly, the engine also seems to idle better that way, likely the cooler air as the engine bay gets very warm.

 

[DJ]

Re: Modifying flame arrestor

JasonB,<br /><br />I think you answered your own question. The K & N is not going to do anything for you unless the air is available. In your case, you proved that your engine compartment venting may be a bit on the marginal side.

 

[navigator336]

Re: Modifying flame arrestor

Now this is getting interesting. Is it air that's too hot or not enough air that allows a 200 - 300 rpm increase with the cover off? I'm assuming from your description Jason that your engine sits partially in the bilge and partially above deck level with a fairly "form fitting" engine cover which maximizes your interior space. You could test if it's restricted air flow which restricts your rpm's by taking a piece of vacuum line long enough to route from the carb base out of the engine cover and check vacuum at WOT with the cover down and with the cover open. If vacuum drops with the cover open, then you don't have adequate venting into the area. If it remains the same, then it must be air temperature/density that restricts the rpms. Either way, installing a couple of nice stainless steel louvers into the bow facing portion of the engine cover at about the spark arrestor height ought to give you the rpm gain.