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DanP
11-09-2012, 12:51 PM
Hi all..

I have been mulling over the various forums and haven't been able to find a solution to my question. I have a 1981 MC and the motor box is starting to fall apart. I was wondering if it is possible to reduce the size of the box to something closer to what the 90's MC's have (which also have the same motor - the 351W). There is obviously less airflow (or rather air resistance) in a smaller box, but there doesn't seem to be any issue with that in the later model MC's. They have roughly the same airflow pattern as my boat.

Thoughts?

Suggestions?

gweaver
11-09-2012, 02:09 PM
The only problem I see would be what you pointed out- a smaller volume of air for the engine to draw from, but you're right, the newer boats seem to do just fine.
I think you could probably make a form out of thin plywood or foam-core board and use that to help mold fiberglass. Or build a wood engine box and then cover that in fiberglass. It would certainly open up the possibilities to do some custom touches- cup holders or sunglass pocket or something.
If you do it, post pics!!
Good luck!
Greg

TRBenj
11-09-2012, 02:21 PM
Air flow due to box size is a non-issue. The carb draws in several hundred cubic feet of air per minute... it will need to pull air in from outside the box regardless of its size.

Was MC still making their motorboxes out of wood in the early 80's? If so, just rebuild it any size you want out of original materials. I wouldnt glass over it on either side- that will accelerate rot. If its glass, why not just repair it?

thatsmrmastercraft
11-09-2012, 02:33 PM
Clearly there is a lot of extra space under the box that can be eliminated. My boat came with an engine box from a late start & stripes. This worked for me until I located a period correct engine box and sold the old box through the classifieds. Perhaps you can find a boat being parted out. Most any inboard box will work. I don't know what you hope to gain. There isn't a significant difference between the box my boat came with and the correct box I have now.

DanP
11-11-2012, 07:19 PM
Thanks,guys, for the suggestions. Making one out of glass would be a nice winter project.

I think I found intake volume numbers at about 170cfm. Can anyone confirm I'm in the right ballpark?

JimN
11-11-2012, 07:31 PM
Thanks,guys, for the suggestions. Making one out of glass would be a nice winter project.

I think I found intake volume numbers at about 170cfm. Can anyone confirm I'm in the right ballpark?

That would be the average- at high RPM, it's quite a bit more.

DanP
11-11-2012, 07:38 PM
How much more are we talking, Jim?

Sullivan
11-11-2012, 09:10 PM
I was thinking there are some air ducts that feed the engine. I was thinking they come from in vent in the nose of the boat. At least I know I had some air ducts that fed right into the airbox compartment in my 81 Ski Supreme and then the air vented out the back of the boat and even had a vent I think in the air box.

FrankSchwab
11-11-2012, 10:39 PM
CFM should be easy to calculate. Let's assume a 350 Cubic inch engine, which means it sucks in about half that per revolution - so 175 Cu in per revolution. At a 5000 RPM redline, we're talking (5000 * 175) = 875000 cu in / min., or about (875000 / 1728) = 500 CFM. Actual air flow will be a bit less, as the engine doesn't fully fill the cylinders at high RPM.

Hence, the popularity of 500-600 CFM carburetors for our engines.

DanP
11-12-2012, 09:43 AM
Thanks Frank. Very simple and concise explanation for what could have been a lot of bumbling in the dark. So, does that mean those little vents at the back of the box would be more of an intake vent rather than an outlet? Or would the vent system from the hood be able to easily handle that kind of airflow?

My google-ing tells me that 500cfm is a bit of a tight squeeze through two 3 inch tubes without some sort of pressurized system. Especially since the surface area at the intake in the hood is even smaller than the cross sectional area in the tubes.

Sorry if I have gone way off topic from my original post. Im just curious about the air flow direction and source now.

CCAnderson
11-12-2012, 09:48 AM
those nice little vents on the back work great for the blower but when you are running the engine hard all the air flow will be in the vents plus any other opening, through the engine, and out the exhaust.

JimN
11-12-2012, 09:53 AM
Thanks Frank. Very simple and concise explanation for what could have been a lot of bumbling in the dark. So, does that mean those little vents at the back of the box would be more of an intake vent rather than an outlet? Or would the vent system from the hood be able to easily handle that kind of airflow?

My google-ing tells me that 500cfm is a bit of a tight squeeze through two 3 inch tubes without some sort of pressurized system. Especially since the surface area at the intake in the hood is even smaller than the cross sectional area in the tubes.

Sorry if I have gone way off topic from my original post. Im just curious about the air flow direction and source now.

The air coming in at the front and exiting the rear are there so the bilge is ventilated, with the added benefit of providing air for combustion inside of the engine. If the vents weren't there, the concentration of combustible vapors in the bilge would cause major problems in the event of any kind of spark, assuming the fuel:air ratio reaches the danger zone.

The direction of air flow doesn't really matter but if the enclosure is too close to the engine and there's no vent on the cover, heat won't be allowed to escape through convection. You also don't want to have hot surfaces too close to combustible materials.

DanP
11-12-2012, 10:03 AM
Great guys, thanks...

So even though they look like outlets, they could be inlets for combustion air, but are also convection heat release.... interesting.

As for a new compartment size, I was going to take a look at the clearances of the newer models and just duplicate something like that. You're right Jim, no point to pushing the clearances for no reason.