Engine room air vent size

[Boss Lady]

I got a real education on adequate vent size to feed high HP diesels at the boat show. The 8v92 pulls over 800 cubic feet per minute for a natural aspirated engine. I am trying to calculate the air draw for turbo charged engines. it would appear that a turbo would double this requirement. Does anyone have this information handy? It will save me a ton of work if you do. I saw a recommended setup which included a monster fan unit to move enough air to do this, it also came with a $7,000 price tag (ouch!) So I will try the cheap charlie route and see if I can engineer my own system.

 

[MikeP]

Are you getting some indication that you don't have sufficient air available to the engine? As far as calculating the flow - with one engine's airflow known, you'll be close enough by just using the difference in HP rating to determine the CFM flow of the other, engine.

If a 400 HP Diesel engine takes, for example 800CFM of air, a 600HP engine would require the same percentage increase - 1200CFM.

 

[Genesis]

A turbo DIESEL actually wants about 20% more than you'd calculate this way because part of the airflow goes out the exhaust unused and keeps things more reasonable temperature-wise.

That is, a strict horsepower calculation is the lower boundary and is not really enough.... add 10-20% to that for reasonable numbers.

BTW airflow isn't the whole story - its airflow, pressure fall (that is, how much vacuum is created in sucking the air in given no forced ventilation) and TEMPERATURE of the air at the engine's air intake. If the vacuum and temperature specs are met then you're ok - if not you aren't.

 

[TedZ]

The pressure is the thing...not much you can do about the temp except provide more air.

We use $200 data loggers and a pressure transducer to measue engine room positive or negative pressure. The calculation is simple with respect to cfm. You know the displacement of one cylinder and the RPM. Volume of air pumped will be about 20% greater with turbo'd engines.

You don't want negative pressure under any circumstances.

 

[Boss Lady]

Just got off the phone this afternoon with some pros who do this for a living. The original airbox openings are 9.5" x 49", their calculations show these openings are not large enough. These engines pull 2049 CFM each for a total of 4098 CFM for the pair. This effectively changes the air in the engine room 8 times a minute! WOW With the original airboxes with the 3" holes in them there is no way they could flow even 25% of the air needed. They are sending me the info on how much larger the openings need to be and of course they want me to buy their airboxes. This is worse than restrictor plate racing in NASCAR! If your engine room is warm then it is a pretty good indication that you have restricted airflow coming in the engine room. There is no way you are getting rated HP if they can't get the air needed. I will let you guys know what I find out tomorrow when they send the drawings and prices.

 

[MikeP]

I had no idea that these diesels needed so much air - then, as I thought about it I realized, THEY"RE TWO STROKES! Doh!!!

Incidentally - I found this handy-dandy calculator for determining CFM needs based on engine size/type:

http://www.sbfilters.com/cfm_calculator.php

I can verify with a lot of experience with gas engine building that the figures are right on the money for them (and much lower than most people think). So I'll assume their two stroke diesel calculations are equally accurate.

 

[SKYCHENEY]

Mike, either that calculator is way off or Chirs is being told he needs way too much air. One of you guys has to be wrong because I can't make these two numbers come out the same.

 

[Boss Lady]

The increased CFM for the opening is to make up for the losses in the filter system. The theoretical is around 1700 CFM for the engine displacement turbocharged, but there is another 15-20% unaccounted for so the 2000 CFM is pretty close.

 

[Genesis]

The overhead on a 2-stroke Detroit is about 20% on a turbo engine, because they scavenge using the airbox "extra" air. That is, they're "wasteful" in their use of inducted air due to scavenging requirements compared to a 4-stroke.

You can get away with not having all that extra on a non-turbo engine but with a turboed motor you can't because the airflow ALSO keeps EGTs reasonable. Without the excess airflow you'll burn a 2-stroke turbo motor up.

 

[egaito]

I had no idea that these diesels needed so much air - then, as I thought about it I realized, THEY"RE TWO STROKES! Doh!!!

Incidentally - I found this handy-dandy calculator for determining CFM needs based on engine size/type:

http://www.sbfilters.com/cfm_calculator.php

I can verify with a lot of experience with gas engine building that the figures are right on the money for them (and much lower than most people think). So I'll assume their two stroke diesel calculations are equally accurate.

So now we know what the engines want....how does one go about calculating the vent size?

 

[MikeP]

Heck, I would have lost a bunch of money had I bet on this 2-stroke diesel air requirement (before finally engaging brain).

When I read the first post that the 8V92 needed 2000+CFM per minute, I immediately said to myself that it was impossible. Heck, I built (gas) engines that put out Waaay more HP and only needed half that much air. Again, I totally forgot the 2 stroke part. If you use that same calculator and change it to a 4 stroke (gas OR diesel) - the air requirements drop to what I experienced in my engine building days.

In our 53, IF you run at WOT, there is considerable vacuum generated in the eng room. You can easily tell this by trying to open the eng room doors and feel the pull against opening them. I've read where this is bad. However, at the same time, there is no trace of black smoke that would indicate insufficient air to engine. So it seems to me that if the engines don't show any symptoms of insufficient air, than the actual operation is ok. But perhaps I'm missing something obvious again...

 

[Genesis]

Got pyrometers? What do they read?

 

[OldHatt45]

First look at your rpm at cruising speed. Then check your speed via GPS or something else reliable. If you have Flo-scans look at the burn rate as well.
Also, check the temperature of your engine room.

Do all the above with the engine room closed as per your normal operating environment.

Write it all down.

Bring the boat back to the dock. Next day (or a couple of days later) with cold engines, repeat the above with the engine room door (and exterior door if you don't have a cockpit engine room door) open such that you get all the clean cold air your engines can suck in. If you had/have insufficient air handling, you should notice a few things.

OldHatt45

 

[doc g]

I remember seeing in one of the magazines an article that gave the direct calculation of how many square inches of opening (vent) you needed . I believe that it was based solely on HP though. Does anyone recall seeing that article ? My first guess is it was in Passagemaker but I'm not sure......Pat

 

[MikeP]

Don't have Pyrometers/gauges. I have the oem Hatt sensors on the exh that feed the 12 point alarm. They have never triggered and the last time out a few weeks ago just prior to winterizing we were at WOT for 20 minutes with no temp issues of any kind and doing 17.7 Knots per GPS - couldn't quite hit 18 .

Maybe I should have opened the eng room doors! I'll try that next year.

 

[OldHatt45]

doc g,

The formula that you are talking about is
Horsepower / 3.3 = Sq In Intake Area.

DOn't remember the book (old timers syndrome ;-) but I do remember the formula.

OldHatt45

 

[MikeP]

That formula fits 4 stroke engines intake size and corresponds with the intake opening on any performance and competition car that I ever built but since the two strokes apparently move twice as much air for the same HP, I would assume one would have to double that figure, right?

 

[OldHatt45]

As far as I know, that formula is for 2 stroke diesels.
I could be wrong, but if memory serves me, it came from a book that discusses only 2 stroke diesels.

Intuitively, if I use the formula on my 450 HP number, you get 132 Sq Inches required. The original Air Intake area on my 45 was 66 Sq Inches (Old rectangular vents. The New Vents (Slane) provide around 200 Sq Inches of area. So in any case I think in my case even if the formula needs to be doubled, the new (current) vents will provide enough area. ;-)



OldHatt45

 

[MikeP]

Yes, you are right - I went back and rechecked the size on some of the intakes we used for performance engines. as an example, a 700+ HP SS/A Hemi Barracuda had an intake opening of around 12" x 4" - "only" 48 SQ in. I don't know where I got that formula confused with what we used to use - it sounded familiar for some reason I guess. Just goes to show me not to trust the memory; look it up!

 

[Boss Lady]

When designing an air flow system (just like air conditioning duct work) everytime air has to make a turn you lose capacity due to drag, so you have to start at the point of demand (the engine) and work out to the hull. Every step that would cause a decrease in air flow must be addressed and compensated for or else you will wind up with inadequate air flow. Therefore you cannot simply calculate the point of demand flow and then say that is the opening size you need for the side of the hull. The starting hull opening will need to be considerably larger to make up for all the losses as the air flows through the hull opening, through the baffle and filter system (which I found out has to be considerably larger due to drag than I realized), throught the interior hull wall into the engine room and to the engine air intakes. Upon actual investigation this is area that considerable performance increases can be obtained from installed power. I don't think any of us have been getting the power and efficiency that is already there, our engines are starving for air. I am not a professional, but I would recommend that everyone should look at conducting a simple test. Run your boat normally and do a timed test from a standing start to max speed, then open an engine room hatch and a window or cabin door so that the engine can get more air and repeat the test. If you get a faster time to speed then your engine could benefit from a larger air induction vent.

I ran a drag car myself for several years. We tried to use a high pressure area to feed the cold air induction system, the higher air pressure provided more flow. I know that this works because it is measureable.