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21' Diesel Nordic in Feb. Powerboat mag

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Old 02-08-2008, 08:27 PM
  #31
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The blow off valve would be on the intake side between the compressor side of the turbo and the throttle plates.

On a gas engine, the throttle plates are downstream of the turbo compressor. When the turbo is spooled up and the throttle is suddenly closed (like when you shift the gears), the compressor hits a wall at the same time the hot side is not getting a lot of pressure, effectively slowing the turbo quickly.

The blowoff valve opens when you let off on the gas and vents the pressure to the upstream side of the compressor or to the atmosphere. It would keep the turbo spooled up between shifts.

Not sure if diesels use a blow off valve, but it would work real good on a performance boat that would see airtime or in a truck with a manual trans. It would not be a big benefit in a truck with an auto shift (turbo stays in boost between shifts) or in a boat that never needs to be throttled quickly.
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Old 02-08-2008, 09:26 PM
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Ok, but first what movie is the line from?????

7 blades - extreme!

I agree with the fall of plane theroy but why is he able with the Bravo drives to stay on plane? I'm guessing the props are small and with the hubs the blade area is not much and they are probably 4 blades?

baywatch - tell us about what the boat does now, how much HP, what size props, at what rpm can you stay on plane down to?
You two definately know your stuff. You have pretty much pegged perfomance numbers and props without me giving you the info.

First let me state that I truely don't plan to do anything along these lines for several years (realistically I would say 4-5 years out). The curreny power only has 250 hrs and that is after I put 100hrs on last season. I plan to use it and enjoy until the current power gets tired and I am hoping technology continues to gravitate towards the diesel option. I do appreciate your input as far as determining if what I want to do is attainable and what sort of a price tag the project would have on it. I certainly don't want anyone to put too much time into the numbers expecting me to do something in the next 12 months. Joe, regardless of where this thread goes, I will definately contact you personally when I am ready to pull the trigger on the project.

As far as numbers are concerned minimum planing speed without tab input is right about 2400rpms depending on air temperature (rpms run about 100 higher if it is cool and dry). Speed at that RPM is right about 31 mph on a hand held gps. I am running stock 502 mag mpi's that currently have 250hrs. They are supposed to be 415 hp at the prop and I am currently spinning non labbed 4 blade 24 pitch bravo 1's. Regular bravo 1's with 1:5 ratio.

The best I have seen for top end was 67.1mph on a handheld gps at 4980 Rpms. That was light on fuel (200 gallon tank) with me, my wife and our two year old. Temp was in the mid 60's with slight wind chop. It will do 65mph any day but I rarely run it above 60mph. With a small child it is sort of like having a caddy that will cruise beautifully through the snottiest water at 45-55mph all day long.

Most of our running is between 3,000 and 4,000 rpms which at 3,000 rpms is about 38 mph and 4,000 is closer to 55mph if neutral on tabs with slight trim.

Hopefully this helps and I appreciate the info on what is possible with the old resin bucket.

Thanks,

Josh
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Old 02-08-2008, 11:03 PM
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So, after we look at what your boat does speed wise, it all comes right back to same thing. RIK blindly came up with a speed based on diesel Hp and helped to support the theory - Hp is Hp whether it is diesel or gas, it takes X Hp to go Y mph and it doesn't matter what drives it - I've said that before.

RIK, here is where I always have a problem. Take Josh's boat and at 2,400 rpm he has a 1,600 rpm prop speed to give him 31mph. My thinking and testing (years ago and engines & props have changed since), says to me if I over-drive 15% (.85) and run the engine at 1360 rpm with that small prop, I should be able to duplicate his same speeds and be on plane(providing weight doesn't play a big factor)? I think then when I get running faster I have a larger power band on my engines compared to prop rpm because of the OD.

When I would run reduction every little change in engine rpm is magnified at prop speed.

So, I would want to vary more prop speed with less engine rpm's because I have an engine with less rpm's to work with???

6, 1/2 dozen or the other as they say I guess.

propster - I hear what your saying but, diesels don't have a throttle plate in them. Blowoff, popoff etc, all the same thing, I think you'll find they are more to prevent compressor stall which is the turbo is still making boost and there is no where for it to go. The compressor starts to shutter and make fluttering sounds which will damage it over time. They vent the pressure to prevent damage but that actually has no bearing on spooling back up. Most all gas engines run very small diameter turbo's so they spool up with very little exhaust pressure. The waste-gate let's that excess pressure escape so even when you lift off the throttle for that split second you still have ample heat and pressure left to keep it turning. Your absolutly right the throttle closing will back up boost and they relieve it for damage control.

Also consider when your boat is flying in the air your off the throttle for many more times the duration than you are during the quick shift of a gear.
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Old 02-09-2008, 06:26 PM
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Hey, I wasn't that blind when I pulled the numbers. I even got the plainning speed.

The od will put less torque to the water than the ud and this is where things will not start to go bad.

The boat is heavy, narrow with a high deadrise and the hp is low, reducing torque compounds the problem.
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Old 02-09-2008, 07:29 PM
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RIK, I meant that as a compliment meaning you didn't know what he had and when he said what he had it just worked out to be close to the same diesel hp and speed.

I gave you a BRAVO (not the drive)LOL
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Old 02-09-2008, 08:09 PM
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Don't get me wrong...Diesels are great but nothing like the sound of a healthy big block in a boat.
agreed
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Old 02-09-2008, 08:27 PM
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Habana Joe,

The movie was "Analyze This." Too easy.

Beyond that, I have nothing that could be mistaken for intelligent to add to this excellent conversation.
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Old 02-09-2008, 08:34 PM
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100 plus

Go to my site pick out a tee shirt send me a pm witn style, size & address Happy Valentines Day!
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Old 02-12-2008, 02:16 AM
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Saw the diesel Nordic in the lobby of the L.A. Boat Show. This engine has a ways to go before it is "commercial". The exhaust manifolds and crossover pipe looked like the truck parts, with just a loose heat shield over them. There was an aftermarket engine controller, water to air intercooler, but otherwise the engine looked pretty much like the truck engine. Before they could put it on the market, they would also have to pass EPA emission tests.

Michael
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Old 02-12-2008, 07:31 AM
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Michael1,

Thank you for the report, I figured as much.

I find it funny how all these stories of Duramax greatest, big Hp and long life come to the surface everytime the word "diesel" pops up!

How did these stories start, where do people hear about these? Ray from Raylar came on the other diesel thread and said (I'm not quoting him here), that it's going rather slow with Duramax developement, not as planned.

I know everyone just repeats what they read, but how/where do these "stories" get out there?

Advice - Anyone that wants to really do a diesel project use the Cummins B as a base engine, you'll go further quicker & cheaper!
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