Engine Air Intakes
 

What is the benefit of having scoops in your sundeck? How are you able to tell if your performance will improve? Like most recreational boats, I have padded hatches with minimal intakes on the side of the boat. Just wondering how this might affect having the engine run more efficiently?

hugely. heat is the enemy of performance in all respects. ill bet that you get engine compartment temps well over 100 deg routinely and intake manifold temps much greater then that.

all that hot intake air saps power like crazy... on the dyno it was something on the order of 1 hp per 5 deg of inlet temp over STP. on a carb motor is makes the motor run rich as hell which also hurts power and everything else...

scoops will take care of the inlet air temp issue but getting the air temps down in the engine compartment is a much more important and straight forward exercise...

if you do all this, everything will get better... whether that the work and effort are justified by the increased performance is a matter for you to decide. the boat ISN'T going to go 10 mph faster...

what it WILL do is get better fuel milage, run considerably better all the time and make everything in the engine compartment last a lot longer. on my top gun w/ 502s doubling the air intake area on the sides and doing dedicated air extractors made a world of difference in all respects...

I put them on my cat to have clearance for the superchargers.
They draw in A LOT of cool air. That never hurts an SC motor.
The only problem was that at speeds over 85mph, they would draw in so much air, the hatches would start to lift.
Gary

When Phil was dialing in that 38 TG with 675 Zul's that ran 102mph he determined the boat needed some outside air. The above scenarios are 100% accurate, but another way Phil could tell the motors weren't getting enough air is that all of the PVC sawdust from building the cabin interior was being sucked back into the bilge during WOT runs. If it's pulling air from the bilge all the way up to the front of the cabin, it probably needs a little more fresh air from outside!:D

well... all that means is your exits aren't big enough... you want flow so you can drive the hot air out as well as feed th supercharger

It was on my Chris Cat. The entire side of the engine compartment had the 4" high vents pointing backwards. And there were 2 3" round bilge blower ports over the transom.
Any more holes, and the boat would look like swiss cheese.
I just added a hatch lock.
Gary

My mill is NA. Does the increased air still make a difference? Do you have to put in a new sun deck hatch? How can they be integrated into my existing set up? Does the height of the scoop make a difference? Appreciate the responses.

Gary, at the rear of my hatch, between my scoops, I have a 12"x12" hinged panel that raises to dump excess air pressure in the engine compartment. Once the boat slows if sets flat again and you can't even tell it's there. It is just hinged, and the air opens and closes it.


Darrell.

Yeah, I started seeing those on race boats.........
Right after I sold the Cat.

DMOORE
Do you have a picture of the hinged panel? That sounds like a great idea.

Honestly, unless you REALLY like the look, need the hatch clearance, and/or dont care about lost space on your sun pad, there are probably better ways to get more cold air in there.
Check in some aftermarket boating catalogs. I've seen the triangular shaped vents that could be easily added to the side of the hull above the rub rail.

Well how about this for a reason......
A 502 Cu inch motor, turning at its 5400 rpm WOT sucks in 22590 cu inches of air every second. Thats a cube of about 28" x 28" x 28"!
A four stroke motor running at 5400rpm fills all 8 cylinders with every second rotation of the crank. So 2700 times per minute is 45 times per second. 45 x 502 cu ins is 22590 cubic inches. Then add some air going through the motor compartment to transport the heat from the block and the headers, and you can start to see why cracking open the hatch at speed (apart from being dangerous) can add a mile to your top end on a warm day. A hood scoop could bring the same improvement......

welllllllllllllllllllllllllllllllll..... yes and no. certainly your arithmatic is correct... but the motor is doing all its breathing thru the two annular openings in the throttlebody ( on your 502) so, in theory, all you would need is the same annular openings to atmosphere anywhere in the engine comp for them to breath... so as long as thats the case, you aren't going to pull a negative pressure in the engine compartment... but thats not what we are talking about... your idea of cracking the hatch is exactly correct... i did that on my gun in the very beginning... dropped it on a couple of pieces of 2x4 as a test and everything got so much better so fast it was scary... and thats when i started working out how to do it for real.

my belief is that is is stricly a fresh air and air temp issue... get lots of air in and get lots of air OUT. keep the intake air and engine compartment as cool as possible and you are way way ahead of the game.

That's the great thing about the panel. You can't tell it's even there. It is part if the upolstered pad. You can walk, sit or lay on it without even knowing it's there.


Darrell.

Let's say I crack the hatch (mine splits down the middle) for a full throttle run. Will the hatch stay in place? I figure that if I notice any gains during a back to back runs, that the motors need additional air?

You are quite right about the fresh cool air issue, and also correct with the observation about how small the intake opening on the motor is, whether its a carb or fuel injection. However, now work out at what velocity the air enters the motor. It is way way faster than the boat speed, for example. The air going in through the air intakes has to speed up ALOT.
This acceleration causes heat and uses energy.
Then there are the aerodynamic qualities of the boat itself, that come into play.........

again... yes and no.... that acceleration you note happens on the other side of the venturi and is a naturally occuring phenomena. and all the other issues you note are correct and a direct result. which is why a scoop is generally preferable to no scoop in those applications where you can get away with it for whatever practical reasons. a positive pressure on the intake is always better in some degree then anything else for all those reasons.

my position would be this. in a race application or a boat where performance is everything and practicality mean nothing then a hatch mounted scoop will have to be the correct solution.

for the rest , then getting LITTERALLY as much fresh air moving in and out of the engine compartment by conventional means i.e. much larger side mounted ducting similar to stock but scaled up as much as the boat allows along with sufficient fan driven exits will bring big gains in every sense. that hinged panel deal is a great idea.

on another note, i figure a lot of the blower motor issues that show up here are related to this issue and i don't think i would ever do a blower motor without some direct outside scoop or air intake.... but thats a whole other story and not an " apples to apples " deal.

For example if on August 29 at 4 pm on a lake at 915 ft elevation the air is 90*F the dew point is 68*F and the altimeter setting is 30.1"Hg the relative hp will be 95.4%. If the air in your engine compartment is 120*F then the relative hp will be 92.5% or 2.9% difference. If the engine compartement air temperature is 165*F the relative hp will be 88.6% or 6.8% difference. If your engine hp rating is a 525hp then at 95.4% relative hp at the 90*F that would be 500.8 hp or at 120*F 485.6 hp or at 165*F 465.1 hp. The 120*F is a stabilized real temperature for my engine compartment with hatch closed and 90*F ambient. The 165*F is a real engine compartment temperature for twin 496HO engine installation fit in there with a shoe horn.

There are two choices. Try to fully ventilate the engine compartment so that the engine sucks ambient temperature air from there or direct feed the ambient air to the engine intake. My opinion is that the latter will be more successful assuming CFM air flow capability is equivalent to the engine's draw.

Ok, just for the fun of it, lets work it out.

The total surface area of the throttle body opening on my 525 is about 8 square inches (2" x 4").
The 22590 cu inches of air that it sucks in, will have to be doing 2824 inches per second, or 169425 inches per minute, or 10165500 inches per hour.

Thats 847125 feet per hour.......which works out to about 160 mph........double the speed that my boat is running at. However, as the air intakes on my 25 Outlaw are at least double the size of the throttle opening, the amount of air that the motor needs will only have to enter the engine compartment - at boat speed.

So, you are quite right! The scoop isn´t neccessary! Twins and blower motors might need much more air, but it still doesn´t appear to be the major factor.

I´m fitting a hood scoop to my Outlaw, but partly because in my opinion, it suits the Factory 1 race look of my boat.........
and just try lying ACROSS the sunpad of an offshore boat anyway....you just roll straight off:D :D :D :D :D

close but no cigar. your math is based on the assumption that the area of the scoop only double the area of the throttle opening. in fact the scoop area is routinely 4 to 6 times the throttle opening. that is a linear offset ( as you note) so the speed necessary to equivilancy would only be 1/4 to 1/6 that of the nominal air inlet velocity to begin to generate a positive pressure.

i did a lot of testing with airboxes sealed to the intake and pressure sensors and the results were always very interesting and the degree to which scoop placement was important was just amazing. but equally so is the effect of the inlet tract temp... before the extra venting i believe i could litterally have fried eggs on my intake manifold when i opened the hatch... post venting i can lay hands on it comfortably after wot for 15 mins.... and the improvements in all respects were dramatic.

Take a look at this thread. The system is being tested on a twin 496HO engine compartment with high temperatures.

http://www.offshoreonly.com/forums/s...old+air+intake

I thought I would need extra venting after installing a 540hp engine in a compartment made for 300. I loged the temp and pressure at the spark arrester, and found that the temp and pressure stayed close to ambient at all conditions.

If I hadn't done that first, I would have spent a lot of time creating ducts for no gain. A thermocouple to go on your multimeter and a sensitive vacuum gage are cheap.

i saw that thread and while interesting, i thought that cold air box arrangement average at best for a solution to the overall problem. what it does is address the throttle air intake issue and stalls the air in the engine compartment competely.

as for your temp logging, if you can keep your engine compartment at ambient at long wot runs... and i do 15 or 29 mins each way to lunch once or twice a week, that is fantastic and your system must be amazing because you are the single only example of that that i have heard of or seen. i quadrupled the area of my intakes and exhausts and use extractors and with box stock 502's i can just barely get to ambient and thats with heat shield tape around the exhaust...

This spring, I built what you could call
call a "cold air box".

I have 575SCi's, I built a box that surrounded
the flame arrestors that attached to the engine hatch,
and was ducted over to the side of the boat,
where it mated to a plenum where the external vents draw air from.

Being that I just took away one of the major sources
of turning the ambient air in the engine compartment,
I added 2 4" bilge blowers on each air inlet to keep
the engine room air from becoming stagnant ( and excessively hot).

I ran these blowers off of a relay that I have one of the
hour meters trigger, so whenever the ignition is on,
the blower is drawing in outside air pushing it into the engine
compartment.

I usually run with the standard bilge blowers running,
so there will be some air movement in the engine compartment.

This spring I took a temp reading from right
inside the box, right near the flame arrestor, and another
reading from the engine compartment.

The outside air temp was 77*

Temp measured inside the box.....77*

Temp measured in the engine compartment 103*

Here's what I did notice performance wise.

I did not gain any RPM's, but before the mods,
after you would be at WOT for say over 30 seconds,
you would see the RPM's drop 100-200 RPM,
now, they don't fade away.

So, was it worth doing it ??? I think so.

Plus, it's a hell of a conversation piece.

Rebel Heart, Your open hatch idea is a good one, before you go crazy cutting holes see if you really need it. You could open your existing intakes easily without much trouble. The dump gates on transoms become nec when you have scoops and run in the 100's, with out them the engine hatches swell up and can even blow off from excessive pressure. You need air in and air out. If you want to play it all the way you could install temp gauges in and out of engine compartment to measure air temps. It's just a matter of how far you want to play it. Step one see if you need it. Good luck.
Jim

I ran about 10 minutes at various loads ranging from cruising to brief WOT blasts, and the temp showed 89deg max (ambient was 87). The thermocouple was taped to the mesh of the flame arrestor. I still haven't gotten it jetted right, and am still having drive issues, so I'll try the experiment again once all is 100%, but with engine temps that never go over 150 deg., and water cooled exhaust that never gets hot to the touch, there's just not much to heat up the air since it's getting pulled through by the engine.

I'm all for cold air intakes if they work, but I still haven't seen anyone say they actually went faster after all this work. You'd think the boat manufacturers would put in some pretty neat ducting if you could actually get some signifcant speed from it. (how many thousands of dollars does it cost to get a couple mph from internal mods?)

i don't think you are going to see ANY increase in MPH from original max. the drag numbers are so big that you would have to start taliking about 50 hp gain per side to see it on the speedo and thats unlikely to happen unless you had serious problems to start with...

in my case it was simple. i came from an industry where this sort of exercise is not only standard but where the formal engineering theory and practice are well known. only the specific execution changes.

i looked at the boat when i got it and said to myself, " this can't possibly be right... 850 hp in what is effectively a sealed 75 or 100 cubic ft space with 4 giant exhaust pipes ( albeit water cooled but not cold or ambient by any stretch of the imagination) and the motors BREATHING this air... all fed by 4 - 2 1/2 holes and existing thru only 2 of the same...

so once everything else was all sorted out i went and ran it as fast as it would go... and repeatably , in very good conditions it was 65 mph at 5000 rpm all gps all within such a small degree of difference as to be statistically meaningless. the other significant observation was the inability of the oil temp to stabilize.

so i open the engine cover and the intake manifold had to be 150 degrees and the heat was unbelieveable...

so... i take two pieces of 2x4 and drop the hatch on it so i now have a 240 sq inch opening on the leaing edge and the sides rough the same...

same test...

66 mph over and over 5050 or so revs... BUT it got there in half the time and would stay there as long as i wanted and the oil temp got to 220 and never moved beyond.

and when i open the hatch all the way... everything is just warm to the touch.

its not magic or mythology. its thermodynamics and execution specific to your application.

the simple fact is that if you are ignoring this issue or not dealing with it you are simply driving a boat that is both slower than it could be and wearing everything out faster than it has to.

the air going into the intake needs to be ambient. the engine compartment needs to be ambient in all conditions. anything other than that is a mistake to some degree.

I am not saying that you shouldn't ventilate the engine compartment, but it isn't practical to ventilate it down to ambient. I did a rough calculation using some data from emergency generators in small rooms and scaled it up for a 400 HP engine. I too was thinking of adding bilge blowers. In a nutshell the results were something like this:

bilge blower - 60 CFM
engine needs - 600 CFM
heat removal needs - 6000 CFM

So instead of trying to get 12,000 CFM to flow through a twin engine compartment, let's get 600 CFM of ambient air for each engine and get whatever we can for heat in the compartment. It' wont matter if the air in the compartment goes to 150F, the engine, oil and water are that hot anyway. As long as the combustion air is ambient you're ahead.

We will post results including scan tool datalogging when complete.

your math makes no sense at all and your premise is simply wrong. it is not how many cfm you move as a total number. it is how often you can exchange the air after it reaches saturation temp. with nothing other than your bilge blower at 60 cfm, you would completely change the air in engine compartment every minute and a half or so... and that is ignoring the existing hard vents. that is barely good enough as long as the hard vents can supply the motor AND the exits allow 60 cfm OUT.

your premise that ambient air into a 150 degree manifold is somehow fixing the problem is absolutely absurd.

you can rationalize all you want but that doesn't change what the truth is.

Here is a twist on this subject and little of my own recent testing for anyone interested in the idea of running a innercooler on a naturally aspirated enigne. I recently spent alot of extra time and money when I dynoed my motor to set up a complete fuel map/ecu program so that I could run it N/A or blown by simply removing the blower belt (procharger mounted like a accesory),swapping ecu's,map sensors and removing the blower discharge to innercooler piping. So I can run my motor N/A or switch to blown in 10 minutes. My innercooler is mounted approx 10" from the throttle body and is connected by a 3.5" tube, I have often wondered if a N/A motor would benefit from running with a innercooler pre-cooling the intake air charge. I made a bell mouth to smooth the transition of air flow into the innercooler. After several testing sessions I can tell you in my application the added restriction of the innercooler made less power then pulling hot engine compt air directly into the throttle body,there was a 100-200 drop when running the innercooler vs not running it,the test was easy enough and now I know it doesn't work but at least I tried and got to see. I did not have a merc scan tool to see what my intake air temp sensor was reading but the fact I lost 100-200 rpm's and 2 mph told me enough.
I do like the idea of ventilating the engine compt better on my boat but I have yet to come up with a workable solution as my boat has no place above the rub rail that is flat or uniform to mount any extra venting,any ideas? (2000 Baja 272),Smitty

You would never believe it if you saw it but I made my bell mouth from a pvc 3 x 4" adapter:cool-smiley-011:

I like Tomcats theory about seperating the intake air from the ambiente engine compartment air. He´s right that the intake temp that will improve performance is lower than the required temp for the rest of the motor (160 - 200°F).
Arcticfriends experience also highlights a problem that at the intake venturi any type of channel can work like a restrictor, even when it looks like it should do the right job. I´ve seen ideas like running ducting from outside the cockpit, in a high pressure area back to the intake. The problem is that the length of the ducting will already act like a restrictor.

@Smitty Difficult to find a good place to pull in air on the 272, and the engine compartment is tight, too. You don´t want to put a scoop on it, I suppose, because itwould ruin the sleeper character of a 90+ mph 27 footer:cool: :cool:
But what about fitting spacer blocks between the hinges and the sunpad (which would raise the rear of the pad - it sits really deep on a 272 anyway, maybe it can go up 2") and then adding a filler panel (maybe 3" high) between the front of the sunpad and the lower seatback area. This pad could have slots running across it
(2" x 12" x 3). You could use standard vent grills like the rear ones on the 25.
You would only be raising the sunpad up to be a little over flush, and would increase the space in your engine compartment!

Smitty,
On my Baja I have spacers to keep my sundeck cracked about a 1" to allow more air in ...

Randy