Are Engines Being Starved of Air Intake?

[Dare2goBare]

I often though about removing one or all of the drink holders on the rear of the CM hatch just to see if there is any difference.

But, the water was too good & kept skiing

Hummmm....Might just have to try this trick this summer....just to see.....

[justgary]

Just a couple of thoughts:

1. Moving several hundred CFM of air past a 160*F engine block is not going to heat the air measurably, even in a tight space. The air is cool enough.

2. If the engine were starving for air, it would not reach maximum RPM under load. I'll make you a restrictor plate for your throttle body if you want to verify this.

3. If you think a blower will help, it needs to supply around 1000 CFM, or you will just be introducing a worse resistance than you already have. A little 50 CFM pup is nothing compared to your engine.

4. The manometer idea is good, but it could be pesky in a moving boat. Just use a vacuum gauge with a small hose stuck into the compartment to verify that you don't have additional vacuum in there.

5. You don't have additional vacuum in there.

[justgary]

I thought about it a little more and decided that I can show you my own vacuum reading by simply plotting some data I took last fall. The ECM reads absolute pressure on my engine rather than mass air flow, and in order to calibrate itself to altitude it has to read the pressure before you crank the engine. It stores this as static pressure, since the intake pressure is the same as the atmosphere before the engine moves. After the engine is running, the manifold pressure is lower than atmospheric because of the restriction at the butterfly in the throttle body. At idle, it is much lower.

In theory, when you have the throttle wide open, the throttle body does not cause any drop in pressure because it should be able to pass slightly more air than the engine can use. In other words, the intake is at atmospheric pressure at WOT. I had a hard time believing this when I first learned it, since it seems that the throttle body must pose some restriction, and therefore a drop in pressure. The attached plot from my boat shows that not only did the intake manifold see atmospheric pressure at WOT, it saw one bit more than it did when the key was turned on. You can see that the ECM took the new maximum pressure as static atmospheric the first time I went wide open.

Anyway, I understand that my direct drive boat is not the same as yours, but I will bet that my hood seals to the floor a whole lot better than your boat does. All of the air it gets comes through the bilge via the relatively small vents in the stringers. Anyway, I now know that my boat doesn't have an airflow problem.

[The Hulk]

Great input/info everyone.

my theory is in past boats WOT means probably 30mph-50+ a LOT of airflow/intake and air blowing/sucking in every crack and crevice.

1000CFM is like a standard U-Hual size worth of air every minute.

So at wot surf weighted boat only going about 10mph won't have nearly the same airflow thoughout.

2. If the engine must work to get/SUCK the air vs it being there does that result any loss in HP?

3. I know our boats are not sealed up much but 1000cfm sounds like a lot through a few 4" pipes that are not smooth and go through a few bends. As well as whatever cracks and under seats etc.

4. a hole below back center seat would require a hole and be loud.. no thanks. 4" pipes in the back vents are easy to add a blower to without any holes.

5. If you bring in more FRESH o2 it can't hurt.

6. If anything it will help air out stinky wet ballast lockers! That might be worth the $50 anyways!

7. Every boat and set up is different and some may be sealed better or worse. Add jackets wet towels gear etc..etc..and some cracks and gaps can quickly dissappear which then brings the question back for wonder..

Edited March 7, 2015 by The Hulk

[The Hulk]

I forgot full 550 '900 sacs and there is probably less than 5-10 cubic feet of airspace left in the whole engine and locker areas. And surely only couple max in the engine area with a pizza tray on top. That's means nearly NO air within this whole back is available at immediate hand for the engine To pull from

Hit the throttle WOT to pull someone up and assume roughly 1000cfm of engine needs. And full sacs let's assume 10 cubic feet of air available airspace in whole back and the engine would suck that ip in 0.6 seconds . say you have 20 cubic feet available in your back that's still used up in 1 second roughly. So by this the air must truly be getting SUCKED into that area From anywhere possible. The tighter your boat is sealed the more suction you will have in the openings. The less it's sealed then less noticeable suction.

Perhaps after you get going it works out but I can't help but believe haul shot might improve slightly?

[The Hulk]

Attwood 4" blowers only 230cfm would need 4 of these if engine area was sealed up. I'll add 2 can't hurt. Possibly a 3rd to one from front

[shawndoggy]

you are creating a problem where one does not exist. Please turn this energy to "boat brakes" or steering in reverse.

[saxton15]

Not an engineer by any stretch, but I have a lot of friends with speed boats who are constantly tuning their rigs. One of my friends with a 38' Fountain, running twin 575 Supercharged mercs, re-tuned to 700HP, always slightly lifts his engine hatches. While running almost WOT, it makes a noticeable difference in power. When closed they both run without issue, but that extra bit of cool air coming in adds a little boost of power.

[The Hulk]

1400 hp!! Now that sounds fun!

[saxton15]

1400 hp!! Now that sounds fun!

It's a beast. The boat is a poker run edition, so it is really light. We hit 101 MPH this summer. Once you get 5-8 speed boats all running together, you instantly become a motorhead.

[justgary]

Not an engineer by any stretch, but I have a lot of friends with speed boats who are constantly tuning their rigs. One of my friends with a 38' Fountain, running twin 575 Supercharged mercs, re-tuned to 700HP, always slightly lifts his engine hatches. While running almost WOT, it makes a noticeable difference in power. When closed they both run without issue, but that extra bit of cool air coming in adds a little boost of power.

In that case, I would say that the air flow to the engines is definitely insufficient. Once a boat gets to about 60 MPH, aerodynamic effects begin to get significant. Above 75 MPH, the air flow can cause low pressure areas behind relatively small bumps in the surface. For a boat like you describe, I would get an aerodynamics engineer to look at the vent inlets and hoses to suggest ways to keep slight positive pressure at the air filter intake. Removing coiled dryer hoses and going to smooth walls would be the first step. Limiting the length would be a close second. Having hatches open at 100 MPH doesn't sound like a good solution at all.

Again, the temperature of the air is going to be only slightly different from ambient because of the very tiny time it has to heat up near the engine before it is ingested. If you are pulling 1000 CFM through 10 CF of space, all of the air in the space will move through the space in 1/100th of a minute, or 1.67 seconds. If you put 10 CF of air near a 160*F heater for 1.67 seconds and then take it away, the air won't get very warm.

[saxton15]

In that case, I would say that the air flow to the engines is definitely insufficient. Once a boat gets to about 60 MPH, aerodynamic effects begin to get significant. Above 75 MPH, the air flow can cause low pressure areas behind relatively small bumps in the surface. For a boat like you describe, I would get an aerodynamics engineer to look at the vent inlets and hoses to suggest ways to keep slight positive pressure at the air filter intake. Removing coiled dryer hoses and going to smooth walls would be the first step. Limiting the length would be a close second. Having hatches open at 100 MPH doesn't sound like a good solution at all.

Again, the temperature of the air is going to be only slightly different from ambient because of the very tiny time it has to heat up near the engine before it is ingested. If you are pulling 1000 CFM through 10 CF of space, all of the air in the space will move through the space in 1/100th of a minute, or 1.67 seconds. If you put 10 CF of air near a 160*F heater for 1.67 seconds and then take it away, the air won't get very warm.

They are open half an inch at best. I've sat right by them and you don't see them move at all (they are electric). I'm not doubting that you are scientifically accurate. But in motion it is noticeable.

[The Hulk]

10mph surfing and if downwind means nearly no air pressure being forced through the front intake IMO... meaning it has to be SUCKED in at surf speeds. Bow rise creates a dead spot much like right behind a semi trailer. I won't go into aerodynamic details ..we call this Drag... but back to the boats MO is that if your sealed up tight their might be a very tiny difference. . With enough gaps probably not. But I'll try 2 4" blower fans or 3 to see if any difference and at least be airing out the wet lockers and gear while we ride that's worth more than any engine loss if any IMO.

[Bill_AirJunky]

..........s that if your sealed up tight .

Except that we're not.

It'll be interesting to see what difference there is. So I'm curious, several posts here with ways to test this without actually installing anything. I guess their not valid tests for some reason?

[justgary]

10mph surfing and if downwind means nearly no air pressure being forced through the front intake IMO... meaning it has to be SUCKED in at surf speeds. Bow rise creates a dead spot much like right behind a semi trailer. I won't go into aerodynamic details ..we call this Drag... but back to the boats MO is that if your sealed up tight their might be a very tiny difference. . With enough gaps probably not. But I'll try 2 4" blower fans or 3 to see if any difference and at least be airing out the wet lockers and gear while we ride that's worth more than any engine loss if any IMO.

You seem really hung up on yelling SUCTION at us. Like I and others said before, if you think your boat SUCKS, put a vacuum gauge on it and measure how much it SUCKS. You will be very disappointed at the noise created by blowers that you don't need, especially when you discover that they don't do anything.

I have shown that my boat doesn't SUCK. I don't think your boat SUCKS either, and I think that as an engineer you owe it to yourself to find out before you redesign the whole (already working) system.

P.S. I think you are also ignoring the fact that you already have a great big blower capable of pushing air into the engine well above atmospheric pressure. Does it have a pressure relief valve? What pressure is it set for?

[Wayne]

I have another ponderance for The Hulk. Take a look at the distance from your raw water inlet to the raw water pump. Do you think the pump is rated to handle the suction losses?

Is the low power due to the actual air flow to use for combustion or the temperature? Your SC is water cooled, it generates a lot of heat and compressing the air raises its temperature a fair bit. Even with the air/water intercooler you can still have high combustion air temperatures.

Improved airflow to the engine certainly will help with the combustion process but I bet the temperature of the air used for combustion is a bigger issue than the actual available air volume when faced with an underperforming engine that seems fine when the dealer checks it.

Edited March 7, 2015 by Wayne

[justgary]

Good point on the SC heating. Now that *will* raise the air temperature, especially at moderate to high boost.

On the raw water inlet, you will get decent force feeding from forward motion. Probably still a net suction at the inlet at surf speeds, so it's a good point.

[Wayne]

Justgary, I have a coworker that has been testing the raw water intake path on some inboards and v-drives. The results were very interesting. Our summation was that even while under way at low speeds (think surfing, moderate to high RPM with low boat velocity) a hose longer than 3 feet can cause cavitation on the raw water pump. The sea strainers are terribly restrictive.

[Kalamazoo]

Hood scoop.

Gotta add a sun deck hood scoop.

With flames on it.

Edited March 8, 2015 by Kalamazoo

[justgary]

Justgary, I have a coworker that has been testing the raw water intake path on some inboards and v-drives. The results were very interesting. Our summation was that even while under way at low speeds (think surfing, moderate to high RPM with low boat velocity) a hose longer than 3 feet can cause cavitation on the raw water pump. The sea strainers are terribly restrictive.

I could see that happening, I suppose. The pump at high RPM wants to pump 60 GPM. I'm pretty sure it's hard to feed that pump fully, even at barefoot speed.

[justgary]

Hood scoop.

Gotta add a sun deck hood scoop.

With flames on it.

Might as well get some high-rise headers to go with it, because I'm sure the exhaust has some unwanted restriction also.

[WakeGirl]

Except that we're not.

It'll be interesting to see what difference there is. So I'm curious, several posts here with ways to test this without actually installing anything. I guess their not valid tests for some reason?

He's an engineer. We like to overthink everything.

[williemon]

Normal aspirated Engines suck. Anytime the engine is running, it creates a negative pressure in the engine compartment that will be replaced with the air sucked from any air tube or opening. The intake at the windshield does not provide positive pressure to the engine compartment. Positive pressure is not needed for the engine. As long as the square area of air intakes taking into consideration friction loss is sufficient to replace sucked air, its good. Superchargers and turbos are a different animal

but they just suck on the outside too. If anything, just add more ports to the rear or put holes or slots in the side locker partitions.

That's just my 2 cents worth but I thought Id add it anyway.

Edited March 8, 2015 by williemon

[Michigan boarder]

Normal aspirated Engines suck. Anytime the engine is running, it creates a negative pressure in the engine compartment that will be replaced with the air sucked from any air tube or opening. The intake at the windshield does not provide positive pressure to the engine compartment. Positive pressure is not needed for the engine. As long as the square area of air intakes taking into consideration friction loss is sufficient to replace sucked air, its good. Superchargers and turbos are a different animal.

That's just my 2 cents worth but I thought Id add it anyway.

Bingo.

[JoeA]

So it looks like we have a "perceived problem". Engineer some proof to show a problem exists........then engineer a solution.

Share the results.