ammout of air for 892 735hp

[Capt. Tobb]

I have an 86 48' viking (dont hate me) with 735 hp 92's. Viking did not put vents through the side of the hull into the ER until '88 or so. The only air is from two small 12v blowers and two 12"x12" vents from the cockpit. The PO installed two 400 cfm 110v blowers, they are huge! My question is how many cfm is required to equal 725sq inches total. ( aprox 50sq in per 100 hp.) I would like to get some smaller 110 blowers if I can. Thanks....

 

[Genesis]

You can't have too much air.

You can, however, have too little, with disastrous consequences.

You might try shutting one down and pulling a manometer reading in the ER when running flat out to see what you got.

 

[krush]

You need to see if your engines are creating a significant pressure differential between ambient and inside the engine room.

You can do this with a simple clear tube and some water. Make a U and and have one end of the hose end in the engine room and run the other end outside to ambient.

make a mark where the water is with engines off and then go for WOT run and see if there is any change.

 

[TedZ]

Hi,

Try to help, but there are lots of assumptions here.

Given that an 8v92 is prox 736 cu. in. displacement and if a 2600 RPM air pump completely evacuates the "charge" 1300 times a minute (2-stroke) then it is pumping 6644 cu. ft. per minute. The space the engines are in are something like 10' x 5' x 16' as a guess or 800 cu. ft. So the air has to change about 8 times or so EACH MINUTE. To pass that much air would require a big vent. If you like Q = VA Area = 6600 / 20 or 330 square feet of venting or two 10' x 15' vents, an impractical size.

So what to do? First, you don't run wide open much and the reduction in air required is more or less linear. Second, the vents are not your only source of air but the others are hard to calculate! Third, i quessed a velocity of about 16 MPH for the incoming air. At speed there is a ram effect and the velocity is likely much more than that. Lastly, an engine is not an air pump and the volumetric efficiency is likely 60 or so percent which lowers air needs. It is only that high cuz Detroits have blowers.

The message is the same no matter how many opinions you get. Make the vents as big as is structurally and aesthetically possible and you won't go wrong.

Things you should know:

There are lots of assumptions above and the vent design always limits flow.

Omega is a good web site for air flow. Check it out.
http://www.omega.com/GREEN/pdf/AIRFLOW_MEAS_REF.pdf

I nearly flunked Compressible Fuid Flow in school!

Ted

 

[REBrueckner]

Captain Tobb,
Were I you, I'd inquire of an '88 Viking owner with 8V92's or Viking or Detroit Diesel with your question. The only practical way to answer your question is via trial designs, then manometer TEST of actual results. You may get vent sizes/ actually used from this group and that could be helpful.

The nice part about getting actual designs is that production boat manufacturers likely have the engine manufacturer make actual tests to insure engine warranty coverage by that manufactuer...exhaust back pressure, intake air flow, raw water flow, RPM to specs,fuel line size,etc.

 

[MikeP]

As several folks have said, CHECK to see if you have a problem before you fool around with changing vent size. Odds are pretty good that what you need to do is NOTHING!

I'd suggest you run the boat at WOT, and see if it is achieving its loaded WOT RPM spec. If it is, more air isn't going to help anything. If it isn't, open the hatches and see if the RPM increases. If it does, you need bigger vents.

It's true that more air can't hurt but modifying something to provide more air than is necessary for operation is just a waste of resources.

Since a 5000+HP top fuel dragster has an intake opening of around 200 sq INCHES it's clear that a lot of advertising money has been spent trying to convince boaters that a 700HP diesel needs something orders of magnitude larger!

 

[Boatsb]

Ditto for MikeP. If it seems to be working don't re-invent the wheel. I found a few boats where the air comes from other places than the side vents and usually thought. The testing with the hatches closed and open is just too simple to go wrong. FWIW there will be some negative pressure in the engine room with the hatches closed. Thats normal. How much is the issue but if it runs fine don't worry. The good thing about smaller vents is less salt spray.

 

[Boss Lady]

Here is a thread where we discussed vents. http://www.samsmarine.com/forums/showthread.php?t=6542&highlight=vent

There is some info on what I did for my 8v92 735 engine vents and Detroit shows 2000 cfm at 2300 rpm on their air flow graph. PM me your email and I will send you pictures of the ER vents we made and a comparison photo of the original airboxes.

 

[Brian Degulis]

DD has a spec for the max allowable vacum if the engines are original and the boat has not been changed then it should be OK. If you want to be sure do the maometer test you can buy one or make one as Krush suggested just understand how to read it. You can also buy a simple guage from grainger if you don't want to bother with the manometer. But first you need to know what is the allowable max. Cranking it up and opening the hatches will only show a severe problem you can be way out off the spec and still see no change hatches up or down. The affect from that will be long term but simple to check that's the first logical step.

Brian

 

[MikeP]

I spent YEARS building/dynoing performance engines and working extensively with airflow both internally (cylinder/head/valves) and externally (sufficient intake air).

If you connect a vacuum gauge to a properly sized engine airbox, it will show a vacuum. This might encourage someone to conclude that the airbox is too small when it is not. Similarly, one might find in a boat that there is some vacuum against the engine room door and then make the (incorrect) assumption that since there is vacuum, the air intake vents must be too small. People are making a lot of money selling this load of Cr@p.

The BEST way to determine if the engine is getting enough air is to ask the engine, not to measure pressure in the airbox (engine room). Since a dyno is not easily available to hook to your boat, the easiest way is to simply do the wot load test. Again, we want to know what the ENGINE needs. If it NEEDS more air, it will show that by increasing the RPM to match the excess fuel that wasn't being burned. Of course, if the fuel wasn't being burned, the ENGINE would have already told you by producing black smoke.

No black smoke at loaded WOT and she turns the spec RPM? You don't have a problem.

 

[Boss Lady]

engine "room" vents do more that just supply air to the engines. They are also supposed to provide enough airflow to evacuate fumes and heat. This is especially important to remove heat after shutdown. The engines are heat soaked and continue to radiate heat and can elevate the temps in your salon for several hours (this is good in the winter, but really sucks in the summer). I added large blowers to more quickly cool down the engine room. The original blowers were little more than toys on my boat. The Hatteras airboxes have many restrictions to smooth airflow, and we are talking about 30 year old designs. There have been many improvements in this department since the time most of our boats were made. Air handling has benefitted from computer modeling, which was not available when most Hat models on this forum were designed.

 

[Top Shelf]

Capt Tobb, I've got a 1989 48 CNV with Covington 8V92's. The only air induction this boat has is from under the covering boards in the cockpit. I've often wondered about getting enough air to the engines.

What I've found is I can get very close to WOT without smoking (planning is another issue :} ) and there's no difference or increase in speed or RPM if I run with the ER door or hatch open. Which opens to the cockpit on this boat. And there's never any vacuum or pull on the ER door or hatch. I think your Viking ER hatch opens into the Salon, right?

So I'm with the other guys, if you've got some smoke or can't get to WOT, try opening the engine hatch (keep small dogs and kids out of the way) and maybe the Salon door. If you're making RPM and no smoke, you're probably fine. Thanks..

 

[Boss Lady]

Another thing to check is the air filters. Some people have changed them to models that have less airflow than stock. This will restrict airflow, while appearing to be normal. Not all filters are created equal.

 

[TedZ]

Boss Lady is correct IMHO. Most boats do not have enough venting. As i've said before, there is a lot of inefficiency between black smoke and marginal combustion. Four bucks plus a gallon...10% adds up fast especially on long trips.


As far as a 300 MPH dragster having 200 sq. in. of intake...?? When you have an average speed profile of 150+ MPH you'll be all set with the venting you have as the ram effect will handle it just fine.

Ted

 

[TedZ]

BTW if you check the Omega web site you will see that at 150 mph (13,200 ft. per min) the dragster gets a minimum of 18,300 CFM! At the end of its run when it needs the HP it gets twice that. If you take into account the blower it is lots of air.

Ted

 

[MikeP]

Yes, it's a LOT of air and a little 200 Sq In opening is sufficient for it. To me that makes the point...

But lets look at this another way. If the induction opening to the engine is, say 25 square inches, which it is on a 8v71TI, then that unrestricted opening size will ENSURE enough air to operate the engine at WOT. That is the basis for modern automotive airbox design. The intake opening of an airbox is the diameter of the carb or throttle body air bell - that's all it needs to be. In fact, if it's made larger, you lose airbox (and engine) efficiency. The box is fairly large but the opening to the box is quite small.

Now lets look at the engine room - it is like an airbox. The opening for air to supply the engine ONLY needs to have the same area as the engine intake opening. If that wasn't correct, airboxes would hurt power instead of help it. People are viewing this subject as if that hull opening was a long tube to the engine air intake which WOULD be very restrictive. But it's not - it's simply the intake opening for the airbox.

 

[Brian Degulis]

Yes, it's a LOT of air and a little 200 Sq In opening is sufficient for it. To me that makes the point...

But lets look at this another way. If the induction opening to the engine is, say 25 square inches, which it is on a 8v71TI, then that unrestricted opening size will ENSURE enough air to operate the engine at WOT. That is the basis for modern automotive airbox design. The intake opening of an airbox is the diameter of the carb or throttle body air bell - that's all it needs to be. In fact, if it's made larger, you lose airbox (and engine) efficiency. The box is fairly large but the opening to the box is quite small.

Now lets look at the engine room - it is like an airbox. The opening for air to supply the engine ONLY needs to have the same area as the engine intake opening. If that wasn't correct, airboxes would hurt power instead of help it. People are viewing this subject as if that hull opening was a long tube to the engine air intake which WOULD be very restrictive. But it's not - it's simply the intake opening for the airbox.

That makes no sense at all that would put the engine room at the same vacum as the air intake hell if the engine space isn't sealed from the interior you would suck the AC or heat out. Just take a look at any engine manufacturers instalation guide the minimum allowable vent size is always considerabily larger than the engines air intake. And that's the minimum more is always better. Under ideal conditions there would be 0 vacum in the engine space. The boats we build have sealed engine rooms the larger ones have blowers and ex ducts to create a slight positive pressure at WOT that's the best but that only works if the engine space is sealed.

Brian

 

[Maynard Rupp]

As several folks have said, CHECK to see if you have a problem before you fool around with changing vent size. Odds are pretty good that what you need to do is NOTHING!

I'd suggest you run the boat at WOT, and see if it is achieving its loaded WOT RPM spec. If it is, more air isn't going to help anything. If it isn't, open the hatches and see if the RPM increases. If it does, you need bigger vents.

It's true that more air can't hurt but modifying something to provide more air than is necessary for operation is just a waste of resources.

Since a 5000+HP top fuel dragster has an intake opening of around 200 sq INCHES it's clear that a lot of advertising money has been spent trying to convince boaters that a 700HP diesel needs something orders of magnitude larger!

You are right on Mike. Keep it simple stupid,(KISS), and don't do stuf that is not needed. Our fuel dragsters could suck lots of air and added lots of fuel. Nobody has ever tried to make the air intake larger. I really like the idea of operating the boat wide open, recording RPM then open the engine room door and see if the RPM increases. I will bet that it won't. Don't forget the other huge source of engine room air is the open gap above the cockpit sides and below the deck cap. That area is surely your largest air opening.

 

[MikeP]

Thanks Maynard...

Brian, re: "That makes no sense at all that would put the engine room at the same vacum as the air intake hell if the engine space isn't sealed from the interior you would suck the AC or heat out."

Actually, what you describe is exactly what you want and exactly how an airbox works. The BEST setup for the eng room would be for it to be totally sealed against any air getting in EXCEPT through the eng room vent. Example, if you remove the airbox from a Porsche 911 so the engine can "breathe better," you will find that it doesn't and the power will drop. I use the 911 as an example because I put mine on a dyno a couple of years ago fooling around with various setups and lost 8 HP when the box was removed.

The proof that an intake opening does not need to be larger than the airhorn size of the engine exists in virtually every road vehicle produced today. The proof that these boats somehow require huge vents to supply their oem engines exists mostly in advertising and marketing.

Here's another test option...If your boat engine will meet it's WOT specs then sufficient air is being provided to sustain WOT. BUT If you are still concerned about airflow, then do a time-to-WOT RPM test, first with eng room doors closed, then with them open. This is a more stringent test because it's harder to GET the air moving than it is to keep it moving. So this test might show that a larger vent COULD improve acceleration though it wouldn't affect top end power. If so, slap some larger vents on 'er!

But for gosh sake, TEST first; correct it if there is proof you have a problem. I'll bet you don't!

 

[bigbill]

hey capt tobb, didn't know we had so many drag race fans here on the forum. do any of ya'll go to houston in march for the spring nationals? me and capt tobb do. bigbill