This has all been discussed before, randomly in threads and sometimes dedicated threads to the actual technology(s).
In effort not to de-rail some other threads, I thought I'd bring this one back up, and kind of put all the thoughts together.

Here's my take. Please interject with your thoughts and any other ideas you may have. This thread is to throw it all out there and see what sticks and what runs down the sewer.

VVT - Variable Valve Timing has become a great way to operate an engine efficiently over a wide range of rpms. Traditional single timed cams had the deficiency of having one range of power. Depending on the cam design, this power could be set at a desired rpm level. The other areas of operation, however, would suffer.
VVT takes this deficiency away by adjusting the timing of the cams to open and close the valves at oppurtune times for increased charging of the cylinders at all rpms ranges.

Superchargers - Superchargers are a forced air system. Thier benefits are that they will increase volumetric efficiency of the engine to over 100%, thereby creating more power. During the compression cycle, the air is heated, but due to the abundance of cooling water around a boat, that air can be reduced in temperature rather easily with the use of an aftercooler or intercooler. Superchargers draw from the power from the engine to operate them, typically in the form of a belt connection to the crankshaft pulley. Therefore, some of the additional power made by the Supercharger will be required to operate the supercharger. The overall efficiency is hindered by this.

Turbocharger - A turbocharger is similar to a supercharger in the respect it force feeds the engine. But instead of pulling power from the engine to operate it, it uses the exhaust gases to spin it. This makes a turbo charger more efficient than that of a supercharger. But one draw back of the turbocharger is its lag. Because it is spun by exhaust, it may take a second or two to "spool up" to begin making its boost. This is typically only apparent on start-out. The other draw back to turbocharging, mostly on a gasoline engine, is the heat build-up during operation. Because the engine will have a heavier boost demand, the turbo will be operating at a higher speed constantly and may have an effect on the units longevity. Why this does not occur with diesels is beyond me. It was said diesels have a lower exhaust temperature, thereby less wear on the turbo, but I do not know.


Very basic info here, just to start a possible discussion.

The cost to get the benifit of the Variable Valve Timing in a given engine is done by GM and then sold within the package sold by GM to a marine engine builder? Or its done by the marine engine maker?

Mutt, I always thought diesel exhaust was plenty plenty hot. I was confused when I read that. Now, the only explanation I can come up with as to why marine diesels work well is that since most diesels don't turn much over 3000rpm, maybe that does keep the exhaust cooler and thus it's easier to raw water cool the exhaust? I have have often wondered why turbos on diesels are "no problem" (if not required) in regards to heat in the engine compartment, but a big problem for gas engines. Is it also possible that since diesel fuel's flash point is so much higher than gasoline that the Coast Guard regulation of under 200 degrees doesn't apply to diesels? I haven't been in a diesel engine compartment while they were running, so I have no idea how warm it gets in there.

Diesels may be built for higher temps, by making them thicker where they need to be. Most diesel engine parts that I have seen and handled are massive in their weight.

That's a good point, seabuddy. And yes, i can see how you could make a water cooled exhaust manifold very thick to cool it. But on the one marine turbo I have been up close to, (Cat 3408) the exhaust side of the housing looks just like the one on a Caterpillar Diesel semi truck engine. And I know they are hot. So how does that not radiate tons of heat in a closed compartment?

The diesel engine compartments that I have crawled around have been hot after a cruise, hotter it seems to me than a gas boat, IMO.

Plus, diesels sometimes have bigger volume air intakes and exhaust systems plus even some have fans to cool down the engine room.

Most diesels also have bigger sized thrugh-hulls for cooling water intake, too.

Okay, excellent information.

If we can't have 496s or 454s, then I think I want a supercharged 383MPI in my cruiser.

I have also heard that due to the turbo size and low rpm of the engines, the diesel turbo does not spin at a high rate of speed like some gas units.

As for the heat produced, most turbo diesels I've seen in marine applications had heat blankets around the exhaust. I can not find the information right now, but ASHRAE has a chart of BTU/hp emitted to ambient. I don't know which is higher, but i do know they are different, gas and diesel that is.

Here is something else to ponder, which is the most efficient as far as BTU of fuel per hp?
Typically a forced induction engine is about 10% more efficient than naturally aspirated, with equal compression ratios at combustion. But this information does not include VVT, and is slightly older, 2001. But for the reasons that Forced induction is more efficient, VVT does not address, so I tend to think that forced is more efficient than VVT.

Yea, the Diesel turbo's get hot. One Turbo started a fire onboard a boat while I was out about 7 miles off Newport Beach, CA.

Direct Injection?

Turbocharging also requires aftercoolers/intercoolers. Compressing air generates heat. Both turbos and super chargers require compressing air.

I'm going to jump to the VVT direction because I don't have enough time to discuss both systems at the moment.

VVT and direct injection allow compression increase on stock fuel. Higher compression=higher potential power, right? Direct injection is the secret to getting the 12/13 compression range on 87 fuel.

VVT, as Mutt mentioned, allows for complete (or higher) compression at lower rpms. This is how the low-end torque is generated with VVT. It keeps the compression high at lower engine speeds. Conversely, as the engine speeds up, VVT adjust something (depends on the particular design we're talking about) to allow more charge to enter the engine, therefore allowing for more (or same if you're at peak) power to be generated. Ya'll already knew that though.

Controlling intake valves separately from the exhaust valves extends that can be corrected for. The first VVT systems where lightyears more powerful then the traditional cam setup. The newer independent VVT system are, well, flat-out amazing.

All of the valve technology in the world won't compensate if the intake and exhaust manifolds cannot handle the volume (both in and out) required to generate X power. A 300hp engine will have to be able to move more air than a 250hp engine of the same displacement.

That factor hasn't disappeared.

Also, we cannot ignore internal strength required to generate such power increases.

More Later. I love this thread.

But VVT does not provide a means to get 100% volumetric efficiency. It gives a higher volumetric efficiency over a wide range of rpms in comparison to naturally aspirated non-VVT engines. To get big power out of limited displacement, I feel you will require 100%+.

DI will provide a more suited control for higher compression, but do not expect a lot of extra power from increased compression. But if controllable, that slight power increase is "free" so to say.

How about DI on a forced induction engine where the resulting compression ratio is high? Less cooling of the air charge would be required, but still helpful.

There's also been talk of eliminating the cams completely for a solenoid actuated valve set up. Reduced friction and full electonic control are a couple attributes. But this technology is years away from automobiles, not to mention making it to marine power.

As a kid, my father had a diesel tractor with an exhaust temp gauge. I want to say under a heavy load it would run continuously between 900-1000 degrees.

Carry on.

How much power are we talking?

Are we considering replacing the 5.7L 300hp engine with a 3.5L 300hp engine?

Are we wanting to replace the 8.1L with a 400hp 6.0/6.2L engine?

Are we wanting to make 350hp from a 2.5L I4?

I think the end power goal depends on what technology is best for the application.

That's the most insightful thing you've ever said, D-Rod! And I agree 100%.