Engine Starving for air?
 

I have an issue with my new set up, and I have really not found any other topics about this so here is my issue question!

With my new 509's with the hatch off I gain 200 RPM's and 5 mph when compared to when the hatch is on. If I crack the hatch a small amount I will gain the RPM's back but this is hard to do, because the pressure will push the cover closed. If it is just barely cracked, and the hatch bounces open closed, you will feel the boat accelerate and then decelerate. It's very noticeable so this tells me I'm losing a lot of power when the hatch is sealed off.

I'm really baffled that engine bay is that sealed that I'm seeing that big of a fluctuation. Any suggestions here? Would changing carbs, jetting solve anything, or is this just a simple case that I need to get more air to the engine?

The basic engine specs are 509's, Dart Pro 1 heads ported by JimV, 10:1 cr, dart intake, 741 cam, Hp 950 carbs, and I'm running 34 degrees of timing.

Tuning to lack of air, gives you a good tune for lack of air, so it willbe better than not tuning,but you still have lack of air, therefore you will be down a good amt of power no matter what.

Get yourself some air.

magnehelic gauge is what you need to check pressure under hatch. But it sounds like you have not enough air.

Get a digital kitchen thermometer with a flexible probe and put the probe in the engine compartment, preferably close to the flame arrestor, run the boat and have someone monitor the temp

started this thread based off of the direction the drive depth thread was going a week or so ago

http://www.offshoreonly.com/forums/g...rculation.html

I had an issue with my Formula 255 Liberator with twin 355 Vortecs. When I first got the install dialed in I noticed that neither engine would spin over 4200~4300 RPM...either with 21" or 23" Mirage props. I checked timing, fuel pressure, took the Anti Siphon valve out of the fuel line, replaced the fuel line from the tank to each engine and a new tank vent line & screen.

One day while I was out running with a friend on a smooth day offshore, he went to the back and cracked open one side of my engine hatch while running WOT. Suddenly both engines jumped to 4800 RPM. He let the hatch down and within a few seconds the RPM dropped !!! I slowed to an idle, took the rear seat base out and stowed it forward. This revealed a space between the floor and the bulkhead at the front of the bilge. I cranked up again and was able to push 5000 RPM !!!

My fix was to cut a 2" hole on both sides of the bulkhead beneath the side coaming on each side...problem solved.

I guess the side vents weren't enough to provide the air both engines needed plus the deck hatch set up sealed too well !!

I've got a remote probe for my grill. I'll use that, never thought about using that for my boat. Good suggestion!

Did you put the 2" holes in the bulkhead where the side storage are is? That's great you gained that much, I thought 200 RPM's was a lot but it sounds like you gained 700.

it's not only the lack of air but air temp, remember all closed up air get really nasty in the there, you should run the blowers whenever the engine is running, one thing I did was to take one of my blowers and turn it around, so I have one drawing out(hose below engine) one bringing air in above...I know before somebody looses it that this in not CC approved, but you have to get air in for a engine(s) to breathe. years ago I read a TSB from merc as they found several boat did not have enough vents to support the engines to run at WOT they actually have a formula for this..

The blowers don't have enough air flow to do anything at WOT, Idle and low cruise yes, WOT NO.
I had the same problem with the Scarab. I tried to find a solution but couldn't come up with anything that looked good.
The Daytona has the opposite problem. TOO much airflow - IF the hatch wasn't strapped down it would blow open. The previous owner did that a few times by the looks of the old hatch. I am going to seal the scoop to the flame arrestor.

Mike you have the cowl like vents on each side where windshield ends correct. And then a single side vent on each side. Any way possible to get any other slim billet ones across the rear at same height as side ones or otherwise a little lower if need be. CP has a number of shapes and sizes as I'm sure you know.

Along with that is it possible to enlarge the forward cowl type ones even if you need to fabricate some custom vents. If I recall you don't have an over abundance of area there to enlarge but even a little could go a long way. I honestly believe your taking in no where near enough air and also not getting rid of what's coming in while obviously building lots of unwanted heat making tuning a nightmare as well.

Next time out pull all of your vents/cowls right out altogether. Ck out defender.com just to get an idea of some possibilities or ideas.

http://www.defender.com/category.jsp...139&id=2290141


Not sure if you'd have any use for any of these however gives you some ideas. Get some dryer vent hose and duck tape also. Lol. All chit aside you need to get more air in and out of there.

Really ??? anyway to move air thru the engine room and remove heat is a plus...

I just bought 4 nice 2" x 7" stainless vents that are used in the Donzi ZR boats. I think I'm going to evenly space them across the back of the boat. I can't really mount them on the sides without making the boat look cobbled up. When I come up next weekend, I'm going to pull the cowl vents and see how many holes are in the box's that the air initially goes into. I may be able to add an additional hole there.

Here are the pictures of the vents I bought and I'm going to build these over the winter for the hatch. I think the rear vents will help both the heat escape and allow more air in.

EZSTRIPER - Those fans exhaust the engine compartment they don't suck air in except by causing a negative pressure inside the engine compartment. Even if you installed them to pull air into the engine compartment all they would do is restrict the flow because the engines draw a LOT more air than what those fans will pull in. The open hole with the fan removed would allow more air flow. What they need is more or bigger holes to let air in.
You may not realize that I am an air flow specialist.

I guarantee you that at WFO even IF they had the fans on blowing out air would be going in instead. I tried everything on the SCARAB.

Steve-
What do you think about what I am considering to increase the flow of air. I'm not sure if the 4 vents in the back will do a lot, but I think the large cowl vent on the hatch will work well, plus the sun pad will still be usable.

I don't remember what's behind your rear seat bottom and side but like other post mentioned that may be a possibility also. Take a look and see unless you already know however that's an area you could use something similar to link. Anything to get more air in there. I like your idea mentioned above and can only help. Might have to put a 12" sub grill in the middle of your back seat. Lol. Will be interesting to see what possibilities you may have along the sides of rear seat as well as enlarging the opening behind cowls.

I was thinking exact same. Open speaker holes with grilles.

Area of a 12" hole = 113.09734
Area of 2 x 6" holes = 56.54

^^^He does know what he is talking about^^^

Most 3" bilge blowers are 'rated' at 140cfm. Most 4" are rated at 240 cfm.

Each of your motors are demanding around 900cfm at WFO and top rpm. This is a very close accurate guess from many dyno's sheets measuring airflow on comparable motors. Therefore, your engines combined require 1800cfm ! That is a ton of air !

BTW: This subject is great and extremely impressed the OP tested this and reported it. Imagine how many people are running around with a performance boat and losing performance just like the OP ! And some, will perform more engine mods, drive mods, and etc, and still be choked down.

Many years ago, boat manufacturers started to become more aware of NVH (noise, vibration, harsheness) and started instituting methods to reduce thismore and more as time went on to make the boating experience even better. What's the easiest way ? Not just sounds insulation materials, but reducing the # and size of engine compartment openings. Ooops !

Remember, there is a reason the big old Apaches had big raised decks with holes or screens all over the place. Air !

http://media-cache-ak0.pinimg.com/73...a4227e90ab.jpg

http://www.luxuryatch.com/wp-content...-custom--3.jpg

http://www.rrbaonline.com/wp-content...acker_idle.jpg

http://www.offshoreonly.com/forums/a...ess__1988_.jpg

Like Getrdunn said - look EVERYWHERE to see where you can get in air without cutting holes that show. I hate to see a boat cut up with NO thought on how it will look. Panels covered with vinyl are easy to fix - fiberglass is not.

SB - the ratings on those fans is without ANY hose.With 4 ft. of hose the air flow is probably in the area of 50 and 75 respectively.

That was the reason for my single quotes surrounding the airflow ratings . I do not know what the actually move for air when they are hooked up and used in actual conditions, and I can actually assume much less. Just like fuel pumps ratings are at free flow, most of us know this isn't real world.

This is why it is great you are involved in this thread, with your vast experience in your field, you have a better handle on this than 99.9% of us.:Score-101010:

Thanks for all the input guys. Yes I agree, that it is better to make holes that won't be seen and are easily repaired vs in the side of the boat. If I'm remembering correctly the combing vents boxes have only 1 3" exit hole. The actual vents are good sized, so this will be the first place I will dig into. This is going to be the first thing I do next Saturday morning and report back.

Speaking of venting- when I noticed I had this issue, I started to look at every boat I could locate. Some had huge power and only a few vents. As SB posted, Apache and from what I found Cigarette vent fairly well. However, I was shocked at how many boats appeared to be even worse than my Formula.

My 242 has 2 ea. 4" holes in the vent box, each side, with plenty of area for flow improvement.

Is this in the side rear vents or the cowl vents just behind the windshield? It's been a while since I have looked into these.

I'm talking about the rear side vents

Im going to agree with EZstriper on this, you need to look at inlet air temp with temp probe and engine vacuum at wot, a 550 hp engine on dyno flows about 700/800 cfm when measured on dyno, your engine compt prob leaks 5 time that at seams, unless your engine compt is really sealed well I would lean towards air temp vs volume, real easy to test. When you dyno a motor at J607 with 0 humidity at sea level going from 60 degree ambient to 120 is a 6% loss. Goes up from there. If you ran a vacuum gauge to manifold you would know for sure, if motor draws .5" vacuum at wot with hatch open and goes to 2 or 3 inches with hatch closed you have a supply problem, if vacuum stays same you have a temp problem so getting a supply of cooler air will require more effort than just ventilating engine compt, fwiw, Smitty

On a side note a while ago I did a experiment on my Baja 272 thinking air temp was slowing me down, I built the last 540 it had to run blown or NA, I thought what IF I supplied the motor innercooled air when running NA? So I built a inlet venturi to mount on my innercooler and ran it NA with and without air going thru the innercooler. It was slower with the innercooled air, by a mile. So in that case the innercooler posed a restriction and it wasnt air temp slowing it down. Boat would loose 2-3 mph pulling thru innercooler even with a inlet venturi smoothing air. Motor pulled 93/94 map (about 2" vacuum) at wot restricted by innercooler and 98.5 map un-restricted (about .5"), at least I knew, Smitty

I suffice to say, if you have an engine room temp problem, you have an engine room (from outside air) breathing problem too.

Two engines (even one) is a serious air mover in of them selves.

I will be documenting what I do and what the results are, including air temp at the air cleaner. However I think it's way more than a temp issue. It would take a little time to have a temp issue cause a problem, but when the hatch would bounce open and closed the engine would instantly react to the change. It was change you would really feel, and at WOT you need a lot of power to feel any difference. I certainly wasn't creating a 15 hp difference.

The amount of air flow you need to remove heat given off by the engine is much higher than the amount of air needed for combustion. You can get a feel for this by looking at performance data from marine engine builders like Cummins.

http://www.sbmar.com/cummins-marine-...rmance-curves/

As an example, scroll down the page and open the pdf for the KTA19 which is rated at 600 HP. On page 2 under Intake System it tells you the engine needs intake air of 1140 CFM. It also says that heat rejection to ambient (the engine compartment) is 1309 Btu/min. Let's say that outside temperature is 90F and you would like to keep engine compartment temperature no higher than 120F. This temperature rise of 30F is a typical design target for boat builders but it isn't easy to do.

This equation gives you the CFM required to remove the engine heat:

CFM = Btu/min
0.0175 X temp rise(F)

In this case the answer is 2493 CFM. Add the 1140 CFM you need for combustion air and you are at 3633 CFM per engine. Getting over 7,000 CFM into and out of an engine compartment isn't easy, which is why you can see air temps under the hatch much higher than 120F.

I agree with the posters above advising air temp measurements in the engine compartment. When you can keep the temp rise at a max of 30F you are doing well.

Very old , but very accurate, easy math we did at the track for years was 10*F air temp change will be extremely close to 1%hp change.

I say it's time to call the heating and cooling man up the street and have him start bending some sheet metal up or better yet get some theraflex hvac duct and fabricate some hoods to go around flame arrestors. Ram that fken air in any which way I could without being to ghetto. Ha.

Hey mike what is the size of the cowl vents behind the windshield. They are isosceles/trapezoid shape correct? The actual opening size?

I'm not sure the actual size, but if you want to stroll down to the boat and check it out you are welcome to do so. The boat is finally on the lift out on the peninsula. If I had to guess, I'd say 3" at the top 5" at the bottom and maybe 6" tall. Also make some time to get away from the course Saturday morning and you can hop on as I start drilling holes to go faster!

If you want to find out if it temp issue just route you vents right to the flame arrestors. You will be semi ram air but you will gardener that you dropped your inlet temp without having changing the amount of air. It your do need more air and not temp then I would route some additional vents directly to flame arrestor and leave the factory ones for cooling/ air exchange.

The engine hatch popping open and you getting more power doesn't point to more air or to hot of air as the issue. As it pops open a but you let X cfm of outside air in. It comes in right on top almost funneled to the intake. So you bypass the hot air and give it more air in the compartment. Think of a building. At 1" of w.c a 6" round duct carries 100cfm. A door way half way open at that same pressure is 1500-2000 cfm. You moving at 60 is way more pressure then 1"w.c. so you for a lot of cooler air into the compartment.

Many V boats with a windshield I've been in, the air moves backwards (towards the frt) at the rear seat/ hatch.

The shorter peoples hair blows towards the windshield, the taller people may have their hair go towards the rear.

If you sit on the very corners of the rear seat and stick your head out, your hair will go backwards.

And Yeh, I got geaky enough once on a certain boat (and a few cowl hooded cars) to use those air flow direction ribbons .

Do you mean old cassette tape "ribbons"? I've thought about using air flow ribbons to see exactly what is really going on.

John, to really get some good info on this, I may need a partner to observe while I'm driving.

K. I get the wheel and case of beer and tequila and you do what you gotta do under the hatch. Dont worry I'll be easy on it. Right. Lol. Will be around all weekend and ready to take a break from flat landers so let's go. We'll get right.

Laugh all you want but I actually built a hood around the flame arrestor on one of my boats and ran a 4 inch rigid hose to it and ran it to open air at the side of the rear bench seat. Worked great untill it sucked up a pop can. I then installed two 4 inch pipes to it and ran well with 100% outside air without being preheated. I work with a lot more than ductwork.. Did I log into Yellowbullit?