Will your twin powered boat plane on 1 engine?

[Trav4011]

darbikrash - 1/27/2008 1:19 PM

I'm nut sure I understand how to relate an engine overload conditon to manifold pressure if the engine is naturally aspirated. Can you help with this?

Naturally aspirated engines by definition can see no more the 14.7 psi absolute or 0 psi gauge pressure. Most gauges read in, well, gauge pressure so all we really see is vacuum. (0 psi gauge) which is more typically callled boost. Even boost by itself does not tell you that an engine is overloaded, just that is is under boost. As maximum boost is usually regulated by either a waste gate or electronically, the fact that an engine hits maximum regulated boost (as seen by the boost gauge) in and of itself does not tell you that the engine is overloaded. Running at max boost is a duty cycle issue, not an overload.

Aircraft engines are for the most part turbocharged, and pilots use manifold pressure as a means of expressing % power output indepedent of altitude, which is important for reasons of air density. Turbochargers (but not superchargers) are decoupled from air density, as they just spin faster and compensate for the decreased air density by increasing the volume of air flow.

It is my understanding that another piece of infomation is needed to determine overload, and that is EGT. Another way of saying overload is lugging. The classic definition of lugging is when the EGT is RISING and the boost (manifold pressure) is DROPPING, which will quickly create a runaway heat cycle and fail engine parts in a hurry.

Not sure this works if all you have is manifold vacuum on a naturally aspirated engine.

Isn't the vacuum gague really just showing you throttle position on naturally aspirated engines?

We tune our race engines by EGT and A/F.. boost pressure, within reason, doesn't really matter. We usually tune for "x" amount of horsepower at whatever boost level it takes for the given atmospheric conditions. EGT and A/F guages would be nice on any engine.. marine or otherwise.

Travis

[Trav4011]

Originally Posted by kerno

If you can make true (above ambient) manifold pressure without a blower or turbo, I'd like to see it done.

A few of the LSx powered GM ram air cars do it at high speed..

No doubt though, I'm right there with you. manifold pressure doesn't really mean anything in respect to whether or not a warranty should be tossed out.

[kerno]

I own a 2006 GTO with the LS2 in it. The ram air effect is not much different from the 1973 WS6 SD Trans Am's I drag raced. Maybe 1/10 psi above ambient. I held the NHRA SS KA record along with the IHRA J-stock records on 1976.

[peptide]

Within the context of internal combustion engines, vacuum is a word used to denote a gas pressure inside (note that I haven't specified a particular area-yet) of an engine that is less than ambient air pressure. A low-pressure condition can exist in almost any cavity in an engine, but let's focus on the inlet tract and cylinder for a moment.

Most inlet tract vacuum occurs because of air restrictions at, and ahead of, the intake valve(s). The downward movement of a piston in a cylinder creates a cavity with greater volume than when the piston was at the starting-point (at the top of the cylinder), and air pressure outside of the inlet tract pushes air through the inlet tract and into the cylinder. Any obstruction prevents this from happening instantly, and thus there is a pressure differential within the inlet tract / cylinder.

With a handful of notable exceptions, most spark ignition engines have a throttle located in the inlet tract to allow the operator (directly or indirectly) to control the amount of air filling the cylinders and thus the output of the engine. The throttle, when in the lowest demand setting (closed), causes a large pressure differential and at the highest demand setting (wide open) causes much less of a differential. Pressure readings after the throttle are typically referred to as manifold pressure (even when not located in an intake manifold proper).

What is relevant to the discussion is that fuel and spark control strategies, aside from a few very rudimentary applications, use inlet manifold pressure a key indicator of engine load. Manifold pressure and RPM together allow engineers to calculate (though not measure directly, that requires a completely different sensing method) engine load as factor to be used in fuel injection quantity and spark timing algorithms. And while manifold pressure is heavily influenced by throttle angle, throttle alone is not the sole determining influence on manifold pressure. And without going into a lot of detail that I don't have the time for, all of this true for systems that develop greater than ambient pressure in the inlet.

The bottom line is that manifold pressure is actually a good indication of engine load, and I watch it on cars, bikes and boats to give me a feel of the amount of drag that I am encountering. I would be happy to point anyone interested in some interesting reference material if anyone would like to read more on this topic.

Mike

[TheFloater]

Will your truck tow your boat up a long, steep grade and maintain 55 mph in top gear if you keep it floored?

Maybe so, and maybe your gauges will say everything is OK, but I still wouldn't do it. Very hard on the engine and transmission.

Planing a twin engine boat on one engine amounts to the same thing.

And it is simply impossible to have manifold pressure that is above atmospheric pressure in a naturally asiprated engine. It takes a pressure gradiant to get air to flow into the engine in the first place. If the manifold pressure is higher than ambient, then the air will flow backwards out of the manifold through the throttle valve, and the engine won't run.

Guys mentioned ram-air boxes at high speed, but this is just a means of elevating ambient pressure. The pressure in the air box is still higher than the pressure in the manifold. Always.

[rotordriver]

I would use this in comparing twin engine to single engine performance. If my boat will get up on plane on 1 motor and run 4800 RPM WOT and I can cut it back to 4000 RPM and still stay above planing speed, is this not the same as my truck running around town empty and then hooking up my 10000 lb. trailer and pulling it down the road?

[TheFloater]

OK, if you're way overpowered and/or way underpropped, you can probably cruise at plane on one engine without doing any harm. A few boats fit this description. Most don't.

[docj]

What about tilting up the dead engine??? I keep hearing that it is very damaging to the linkage to do that...

[Trav4011]

kerno - 1/27/2008 11:34 PM

I own a 2006 GTO with the LS2 in it. The ram air effect is not much different from the 1973 WS6 SD Trans Am's I drag raced. Maybe 1/10 psi above ambient. I held the NHRA SS KA record along with the IHRA J-stock records on 1976.

It was a joke...