Engine Room Ventilatio

[yachtsmanbill]

...AND IN THIS CORNER...
When I'm running at my modest 1500 and ready for sea, i.e. ports and windows closed, theres just enough vacuum to hold the lower salon door
closed, just. I will admit the engine room is about 4-5 degrees above ambient though. Even the manifolds are about 100 F. Shut them down and it rolls up to 150 fast !! I need to find one or two more 32V blowers for a cool down helper. The one is barely adequate. One will be hosed low for fumes, but the others for hot air.
BTW, we always ran the boiler room at a negative pressure. It keeps the fire "in" where its supposed to be Even with forced draft fans on the boiler, it too was under neg. pressure. If you'd open an inspection port (gun port) it would upset the fuel/air mix enough to warrant an adjustment... and these guys were 20 stories tall. ws

[Boss Lady]

I do love this forum! Ok guys, I posted this to show what we did to my boat and the reasons for it. I brought up some food for thought, on area that I, and a lot of airflow "engineers" discussed at great length. The concensus was that the original vents were in fact way undersized for the original power, along with the restrictions of the inefficient dryer hose ducting. The blowers were rated at like 175 cfm each, which is no where near what Detroit specifies as max airflow required of 2000 cfm for each engine at wot for a total of 4000 cfm. Any a/c duct calculator will do, just figure up the amount of vent area to flow 4000 cfm. I wanted positive cross flow ventilation of outside air across the engine room to the exhaust vent in addition to eliminating air restrictions to feed the engines. We did some overkill but not much, about 10 square inches. Negative pressure in the ER "is" caused by lack of vent size (or duct size). Your air filter is rated to flow x at so many inches of water (suction pressure). This gets the job done but it is not the most efficient, if you want that then you need a much larger filter that will flow the same amount of air at near zero negative pressure. We live in a world of compromises. size and cost being the two that normally drive the train on design. Most engines never reach their volumetric efficiency due to restrictions in the induction system. A tuned runner induction system will give the most volumetric efficiency at a given rpm or resonance, and will suck at all other rpms. A naturally aspirated gas engine operates at around 32 inches of vacumm and wide open throttle is around 10 inches. there in lies your answer. A turbo engine has a mechanical pump so the restrictions are not as noticed, so you wind up having to use instruments to measure these parameters. I paid for 735 HP per side and I intend to get them. If the airflow is restricted then there is no way that I can burn all the fuel since the engines will not have enough airflow(oxygen) to do so. What to test your theory, then just put some cardboard over one vent and then see what happens to performance. You can't feed one of these babies with a soda straw.

[Trojan]

Tomorrow I'm getting out my ax and going to the boat. Going to check this out. I want that extra 3 MPH If you can't open your engine room doors. Then yes you do have a problem. Through all this crap there has been 3 different objectives. I think that if the 3 original objectives were spelled out. As requested in an early post. There would not have been all this discussion. Air feed, air exchange and air cooling have been all rolled into one. All of the info has been discussed in general. Everyone seamed to be on a different track. Damn I hope we all got it straight.

BILL

[luckydave215]

A tuned runner induction system will give the most volumetric efficiency at a given rpm or resonance, and will suck at all other rpms.

That's exactly right, and why most engines have different intake runner length for each cylinder. This spreads out the torque peak, giving a wide usable powerband.
Casual observation would suggest ease of packaging and cost reduction is the real reason, but it's usually not. Think of what the typical intake manifold looks like for a gas V-8. The "middle" intake runners are short, and the "outer" ones long. It's done on purpose.
When peak torque over a small rpm band is desired, (a generator comes to mind) the intake runners may all be the same length, and sharply tuned for the target rpm. This increases the power and efficeincy at that rpm by optimizing for it.

[MikeP]

I know I said I wouldn't post again on this subject. I lied. Just this one little post...

For those of you reading all the way to this point on this thread - Congratulations on your persistence!

But please, PLEASE, PLEASE, ignore everything you have read here - Yes, ignore mine as well - and do your own research on this subject SOMEWHERE ELSE.

Just do some searches, find some sites that discuss engines and CFM airflow, inlet opening sizes, and decide from there. You might also look at inlet/exhaust tuning to further your knowledge. Subjects such as equal length headers/ equal length intake runners might also reveal some interesting facts. Try to find sites that inform re these issues, not sites that want to sell you something.

As far the opposing views on this thread, as the diplomats correctly say, we will have to agree to disagree.

[luckydave215]

Mike, equal length intake and exhaust runners are an engineering choice you would typically see on a high performance engine, where maximum output is more important than the wide (but weak!) powerband you'd like to have pulling, say, a family station wagon driven by a soccer mom talking on a cell phone.
Engineering choices are always a compromise. You give up one thing to gain another.
I've spent an awful lot of dyno hours building motorcycle roadracing engines (thousands of hours, I raced for 20 years) and won 7 national championships to show for it.
For my money, the best tome on engine resonance is still "The High Speed Internal Combustion Engine" by Sir Harry Ricardo. I recommend it to anyone interested in engines.
It deals with fighter plane engines (written in the 30's) but engines are engines, the physics of things like sonic wave activity aren't going to change anytime soon.