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Old 10-17-2008, 06:36 PM   #21
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RE: Lehman Idle Speed

Quote:
RickB wrote:

*The engines do use a mechanical supercharger to deliver charge air. It is a gear driven 2-speed centrifugal blower.
You're right, I forgot that.* The power recovery blowers got all the publicity * When I moved to Hawaii the Navy had a fleet of radar picket planes on the south ramp of Honolulu Airport.* They were part of the Cold War DEW line--- they would fly out and hold station for hours and hours*over points in the Pacific watching for incoming Soviet bombers.* Whenever they fired these things up (they had the Wright R-3350 engines in them) the entire airport would disappear in a fog of blue-white smoke.* You could see it from downtown Honolulu (no high-rises to block the view*in those days).

The other thing I remember from my countless flights to the mainland and back as a kid*on Stratocruisers and DC-7s was*the ENDLESS engine warm-up before takeoff.* I think they spent as much time warming the engines up as they did making the flight.

And now before John cuts us off we should get back to talking about marine diesels.......
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Old 10-17-2008, 08:48 PM   #22
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RE: Lehman Idle Speed

Marin,
I checked my Lehman OM and parts manual and you are right. After break in 600-700 RPM is the spec. These old tachs are probably way off- I better get the phototach before I get down there with my toolbox!
Steve
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Old 10-18-2008, 12:49 AM   #23
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RE: Lehman Idle Speed

I don't know what kind of tachs you have in your boat. In our '73 GB they are Stewart Warner, and while they are reasonably in the ball park you can tap on the glass and change them by 100 rpm or so I use all the gauges on our boat as trend indicators rather than actual measuring devices. A previous owner marked all the "normal" readings on all the gauges with thin strips of colored tape. Since the boat ran for several decades at these settings before we got it, we figure as long as the needles are always near the tape marks, we're good.
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Old 10-18-2008, 05:00 AM   #24
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RE: Lehman Idle Speed

I was Power plant officer on P2's in a patrol squadron, for a couple of years , 3350 time!

IF we got 2000hours between change outs we were really happy .

But much of the damage was due to Navy stoopidity (suprise!).

Each and every TO was done at full power , as you would use getting off a 3500ft Nav Fac (a small island) even tho some runways were 8000, or 9000 ft long.2900RPM , 52.5 in of manifild pressure with alchohol and water injection, 5 min max.

Only in the air line did I learn the secret of TO by Da Book with much reduced power settings.

But they sure did sound good at TO.

The best was on Command changes , we would TO as a section , 4 planes going at once.

Of course it was illegal by Navy rules

, BUT the old CO wasn't going to NOT hand over the squadron , just to raz some junior officers,and the new CO had the problem of it" didn't happen on his watch".

Fun for the feeble minded maybe , but great fun indeed.


-- Edited by FF at 06:03, 2008-10-18
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Old 10-18-2008, 01:08 PM   #25
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RE: Lehman Idle Speed

Something to keep in mind about idle speed. Over proped boats have considerally more thrust at idle speed and consequently require shifting in and out of neutral gear at dead slow speeds much more than correctly proped boats. To make an over proped boat eaiser to live with at very slow speeds one should get the idle speed as low as possible. Thanks Marin, I remember those 3350s both While flying to Alaska in the late 50s on the Connies and in the Navy ( P2s & P5s at VP31 in San Diego ). Saw a Connie at a fly-in several years ago and it seemed tiny. I remembered them as huge .. but it didn't seem very large then. It was a restored MATS bird. As for Deisel and gas engine efficency I heard or read they are almost the same at WOT. By the way this is the first post using my new computer .

Eric Henning

-- Edited by nomadwilly at 14:13, 2008-10-18

-- Edited by nomadwilly at 14:14, 2008-10-18
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Old 10-18-2008, 09:44 PM   #26
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RE: Lehman Idle Speed

Hmm, that "same efficiency at WOT" sounded pretty suspicious, so I did a little looking around (ain't the internet a wunnerful thing).

Turns out that the gas motor manufacturers (Crusader, Mercruiser, Volvo) don't seem inclined to publish fuel consumption curves like the diesel mfrs do.

But I found a boat test on the Mercruiser site that gave fuel flows at WOT.

So:* a Mercruiser 8.1HO is a 425 HP 8.1 liter gasoline engine (I don't know who makes the 8.1 block - GM I think - but it's the same big block for all three of the gas motor manufacturers).

The Cummins Mercruiser QSB5.9 is a "modern" high output 425 HP diesel motor of 5.9 liters (interesting that it's smaller displacement than the gas motor).

At WOT, the diesel motor is pulling in 22.5 gal/hr.* This is in line with the rough number of 1 gal/hr per 20 hp on a diesel motor that I've always heard as a rule of thumb.

Now hold on to your wallets.* The 8.1HO gas motor pulls 33*gallons per hour at WOT.* This is a bit better than the old rule-of-thumb that I've used of 1 gal/hr per 10 hp on a gas motor - probably because the new motors have EFI, electronic spark control, and such.* This is running closer to 1 gal/hr per 13 hp.

It is true that the diesels are better at low output - especially idling.* Gas motors have to run rich at idle to allow the spark to initiate combustion, while the diesel motor quite happily burns everything at idle because it's waaay oxygen rich.*

So I dunno where you heard or read that they were similar efficienty at WOT - tho it might be true that they have the least difference at WOT.
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Old 10-19-2008, 04:20 AM   #27
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RE: Lehman Idle Speed

Some of the diesel /gas difference is simply the weight of the fuel choice.

The rest is due to the compression ratio, the diesel piston gets to recieve the power harder and longer on the power stroke.

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Old 10-19-2008, 07:30 AM   #28
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RE: Lehman Idle Speed

Absolutely.* Diesel is about 7lb/gal vs 6 lb/gal for gasoline.* Proportionally more carbon and less hydrogen in each molecule - and ripping apart a carbon-carbon bond produces more energy than a hydrogen-carbon bond.
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Old 10-20-2008, 06:48 PM   #29
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RE: Lehman Idle Speed

While there is a difference in the weight per gallon of diesel and gasoline, the heating value per pound is fairly close for both fuels at around 18000 to 19000 btu/lb. The percentage of carbon by weight in diesel and gasoline is within 1 or 2 percent.

To make a useable comparison you have to calculate the amount of power produced per unit of weight of fuel burned. That figure is called Brake Specific Fuel Consumption (BSFC) and for the 425 horsepower engines cited, the diesel is the clear winner with a BSFC of .38 pounds per horsepower per hour (lb/hp-hr) while the gasoline engine burned .48 lb/hp-hr.

Those figures are based on average weights per gallon for diesel at 7.3 lb/gal and gasoline at 6.2 lb/gal.

-- Edited by RickB at 20:43, 2008-10-20
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Old 10-20-2008, 11:07 PM   #30
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RE: Lehman Idle Speed

Chris,
Probably should have kept my mouth shut .. been there before. I looked but I couldn't find it. Found a book ( Power for the Small Boat - Tod Mead - 1947 ) where I thought I'd seen it but could'nt find. Interesting .. the cost of gasoline then was .20 gal and diesel was .08. put another spring line on my boat tonight as winds are expected to reach 60 knots tomorrow.

Eric Henning
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Old 10-20-2008, 11:21 PM   #31
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RE: Lehman Idle Speed

Rick and Chris---

Another question..... I understand (basically) the bit about a gasoline engine needing the correct fuel-air mixture fed to it while the diesel "creates" the correct fuel-air mixture in the cylinder because of the way fuel injectors work and the way diesel fuel burns.

So.... what about fuel-injected gasoline engines? Do they operate in a similar way to a diesel? In other words, in my car am I (or the computer) regulating the fuel flow to the injectors while the cylinders simply get all the air they need all the time to support whatever "burn power" occurs as a result of the metering of the fuel to the injectors?
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Old 10-21-2008, 05:23 AM   #32
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RE: Lehman Idle Speed

"So.... what about fuel-injected gasoline engines?"

All the fuel burned in a diesel is injected at the end of the compression stroke, the intake valves are closed, and the air is under high pressure.
*
In a fuel injected gasoline engine, the fuel is mixed with the incoming air during the intake stroke. The fuel may be injected at the throttle body or the intake ports. In*either case*air entering the cylinder contains the fuel mixture before the start of compression.

-- Edited by RickB at 06:25, 2008-10-21

-- Edited by RickB at 06:26, 2008-10-21
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Old 10-21-2008, 06:18 AM   #33
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RE: Lehman Idle Speed

In most fuel injected gas engines, the mixture ratio (fuel to air) is controlled by the computer by increasing or decreasing how many miliseconds the fuel injector is turned on (pulse width). It determines injector time by input from the Mass Air Flow sensor, Throttle position Sensor, and Oxygen sensor. (older units use a vacuum reading instead of a MAF sensor). Spark timing is also controlled by the computer using input from Knock sensors, crankshaft sensors and cam sensors.
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Old 10-21-2008, 03:54 PM   #34
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RE: Lehman Idle Speed

Rick, Sloboat,
Do*some FI 4 stroke engines have injectors that inject the fuel directly into the cylinders? Marin, are there aircraft engines that are directly injected? Is my Evinrude " E- Tech " OB directly injected ? They call the Tohatsu " DFI " direct injected. Or do they all just squirt the fuel into the intake ports?

Eric Henning

-- Edited by nomadwilly at 17:00, 2008-10-21
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Old 10-21-2008, 10:22 PM   #35
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RE: Lehman Idle Speed

Hey Eric,
Funny you should mention the 300 SL .. My friend Larry had one of those Gull Wings and at the same time I had a 140 MC Jaguar roadster. I had complete strangers ( girls ) jump in the car at a light. Must have been the car that caused the strange behavior*as it never happened with any other car. I drove the Gull Wing and to my supprise it was not fun .. unless you were driving really hard. Even at slow speeds it took both hands on the wheel to negociate a corner .. then you could shift. Both cars could make 150 mph .. fast for the late 60s. Cheers and thanks.

Eric
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Old 10-22-2008, 09:52 PM   #36
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RE: Lehman Idle Speed

So should this thread get the award for "simplest question that turned into the most technically involved discussion" on TF???

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Old 10-23-2008, 05:03 AM   #37
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RE: Lehman Idle Speed

I don't own a Lehman, I kept watching this thread grow and finally when it hit 4 pages, I couldn't imagine what you guys could be talking about, said to myself, "How in the hell could it possibly take 4 pages to explain how to set the speed on what I have always thought of as a small diesel?"
And I'm damned if you guys didn't suck me in too!!
Have a great day!!

PS We are leaving this morning to start about a 6 month cruise, will keep in touch!!
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Old 10-23-2008, 07:50 AM   #38
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RE: Lehman Idle Speed

OK- so I think I understand now.... If I want to speed the idle up I go righty tighty on the screw!!
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Old 10-23-2008, 11:28 AM   #39
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RE: Lehman Idle Speed

Forklift---

If you are trying to figure out how to adjust the idle speed on a Lehman 120 and don't have an operators manual (which explains how to do this) you can download the FL120 operators manual from the Grand Banks Owners website. http://www.grandbanksowners.com/index.php *There is a "Manuals" section in the overall menu on the left side of the homepage. I believe that in order to access the Manuals section you have to join the owners forum, but there is no charge for this. The Manuals section also contains manuals for other engines and transmissions like the ubiquitous BW Velvet Drive.

-- Edited by Marin at 12:29, 2008-10-23
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Old 10-23-2008, 05:35 PM   #40
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RE: Lehman Idle Speed

Great information but now I have a curve for you. I have an old Volvo diesel in our boat (MD-47 series) and the manual shows the engine having both a "Throttle Body" on the intake manifold, containing the usual butterfly valve, as well as a "Control Arm" on the injector pump. My particular engine does not have the "Throttle Body" on the intake manifold and I can not understand what it would do if it was there as in my way of thinking there would have to be a very complex array of levers etc in order for the air throttle and the injector throttle to operate together efficiently. Unfortunately there are no diagrams showing how this all hooked up and my engine works just fine thank you using only the injector pump control like every other diesel I have ever seen.
Anyone care to hazard a guess as to what the "Throttle Body"would do if it existed? It does not appear to be a "Shut-Down" device as it has several holes in the butterfly valve.

Confused

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