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1998 2.8GU wagon
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Forcing more air in with the turbo gives the required cylinder pressure using a lower compression ratio than required with an NA engine.
 

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MK Wagon, and an SD33T in pieces...
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652 Posts
Is there a specific advantage to a lower compression ratio in a diesel? They also seem to start and idle (no help from the turbo there) much better than older high-compression diesels.
 

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Y2KGUII ZD Wgn
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Is there a specific advantage to a lower compression ratio in a diesel? They also seem to start and idle (no help from the turbo there) much better than older high-compression diesels.
Define l'ow compression'.
 

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MK Wagon, and an SD33T in pieces...
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Define l'ow compression'.
Well, I dunno, maybe I’m just used to the old mechanical engines, that’s mostly what I research obviously, so I probably (well, definitely) don’t know as much as I’d like to think I do, but I’m used to hearing 20:1, up to 22:1 thrown around, so when I hear of modern engines having 16:1 and less sometimes compression it intrigues me.

I know petrols run anywhere from around 8:1 compression to 12:1, but they don’t need to bring the flash point of the fuel down like a diesel does.

Maybe I’m misguided? I dunno, it’s interesting to me anyway.
 

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Y2KGUII ZD Wgn
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I'll check tomorrow but from memory a DI is 17.9-1 which is not 'high' in comparrison to many other diesels, particularly non turbo.
Diesels do have a wider range of efficient compression ratios than petrol engines, I'm not sure where it stands today but when I was building racing engines12.5/13-1 was peak efficiency.
 

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1989 GQ TD42 wagon
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4,284 Posts
Basically lower compression + turbo is more efficient than higher compression.

BTW, old Land Rovers ran 7:1, and I have seen old petrol cars with 6:1 compression. Says a lot about the quality of the fuel available back then that they had to go that low to avoid pinging.
 

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I Have Imaginary Friends
Patrol Hybrid.
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21,850 Posts
While we are here discussing technical things in a non technical thread, can someone explain to me why doesn’t a diesel engine create a vacuum. They still work on the same principle as a petrol engine of suck, blow etc. air doesn’t just fall into the cylinders. And none of that malarkey that the fuel is injected in as there are fuel injected petrol jobbies.
Educate me!
 

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1989 GQ TD42 wagon
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While we are here discussing technical things in a non technical thread, can someone explain to me why doesn’t a diesel engine create a vacuum. They still work on the same principle as a petrol engine of suck, blow etc. air doesn’t just fall into the cylinders. And none of that malarkey that the fuel is injected in as there are fuel injected petrol jobbies.
Educate me!
Simply put, because diesels run wide open throttle (or in many cases, no throttle at all) at all times.

In a naturally aspirated petrol car, the throttle plate(s) will be closed at idle. The engine is still trying to draw air in through the intake, but can't get much in past the throttle. This causes a vacuum to form in the intake. The more you push your foot down (especially on a car with a cable or rod actuated throttle, as opposed to fly-by-wire), the more the throttle plate(s) open, allowing air to flow into the engine more easily. This allows air to enter the engine more easily, reducing the level of vacuum in the intake.

At wide open throttle, there will of course still be some level of vacuum -- but negligible. Now if you remove the throttle mechanism entirely, you effectively have wide open throttle at all times. As diesels rely on the heat generated by compressing air to ignite the fuel in the cylinders, restricting the amount of air entering the engine will soon result in no ignition of the fuel at all. No throttle = no (or negligible) vacuum.

Some diesels do have a throttle, but it's only used for shutting the engine down more smoothly. It stays in the full open position while the engine is running.
 

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I Have Imaginary Friends
Patrol Hybrid.
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Thank you a very good explanation, that is easy to understand.
 

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1989 GQ TD42 wagon
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Thank you a very good explanation, that is easy to understand.
You're probably old enough to remember vacuum powered windscreen wipers. The vacuum level dictated how fast they went. Hence why going up a hill, the wipers barely moved and you couldn't see chit.

Coasting down the other side the damn things would go so fast you'd wonder how they didn't fly off!
 

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I Have Imaginary Friends
Patrol Hybrid.
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21,850 Posts
Old FC wagon, dads falcoon, yep also remember fitting the fuel economy gauge, just a differently labeled vacuum meter. Plssing down rain going up a hill, blind, going down, wipers are doing their helicopter gunship impression. Then we had the first generation electric wipers, one speed only. A vw I had, difficult to drive at night in the rain as there wasn’t enough power for lights and wipers. The horn was barely a groan. Old days was not the best days. Give me a modern vehicle any day, well, to a point. There is too much reliance on electric switching for basic functions. Want low range, turn a dial, nah, I want to grab that lever and feel it engage.
 

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1998 2.8GU wagon
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3,920 Posts
I'm going to miss the 4WD lever and manual gearbox when I get rid of the Patrol. Auto is good for town and highway driving, but I enjoy the manual on back roads & off road. At least the PS has flappy paddles to play boy racer.
 

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Y2KGUII ZD Wgn
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While we are here discussing technical things in a non technical thread, can someone explain to me why doesn’t a diesel engine create a vacuum. They still work on the same principle as a petrol engine of suck, blow etc. air doesn’t just fall into the cylinders. And none of that malarkey that the fuel is injected in as there are fuel injected petrol jobbies.
Educate me!
Let me clarify this, there is technical and then there is technical, if the question had related to specific technical details for repair or mod to a patrol (or non Patrol for those that own something else), etc, something like 'I am losing boost, what do I do'. I would have moved it to the correct section, but seeing as it was for more generalised information, I let it go.
 

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Rust is just natural weight reduction.
1986 SD33T SWB
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Well I dragged another MK home today. Didn't buy it, just collected it from one of my storage facilities.... I.E. The misso's parent's house.
As is standard the white shorty was the tow vehicle, and for what it is, it does pretty good. Happy enough to sit on 90-95km/h on the freeway, aside from a couple of hills where I backed off to 80 to keep EGTs in check.
The brakes suck though, even with trailer brakes, they leave a little to be desired. I guess that's why the tow rating that I definitely didn't exceed is 2400kg.

Moving the other Patrol on and off the trailer and into my property with no clutch or power steering and half flat tyres was a bit of fun. Had to put it in gear with the transfer in neutral, then slam the transfer into gear. Not ideal when you're rolling down a hill you want to be driving up lol.
 

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Rust is just natural weight reduction.
1986 SD33T SWB
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11,501 Posts
I also had the trailer from 8am-midday today. Midday being when the hire joint closes on Saturday, and they are not open Sunday.
I returned it at 12:02pm. Bloody lucky the gate was still open. I'm away for work crap all week next week, so if I didn't get it back I would have been stuck with a hired trailer that doesn't fit through my gate for a week when I'm not around. Lucky lol.
 

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Nuffin now
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I also had the trailer from 8am-midday today. Midday being when the hire joint closes on Saturday, and they are not open Sunday.
I returned it at 12:02pm. Bloody lucky the gate was still open. I'm away for work crap all week next week, so if I didn't get it back I would have been stuck with a hired trailer that doesn't fit through my gate for a week when I'm not around. Lucky lol.
You could have left it there and chained it to the front gate. :unsure: 🤣 😇

Foo
 
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