turbos and fuel octane.

I suggest you restrict you explanations to subjects you have some knowledge of.

Stop being picky, I was at work and im not a mechanic ;o)

Bottom line is the pressure at TDC potentially is higher aand the charge is hotter as the air has been squashed through a (not to mention very hot) turbo first then compressed further by the piston. Higher octaine fuel allows you to get more air/fuel into the chamber without preingnition and spark timing retardation!! More air in the chamber means higher pressure which means hotter charge. but the benefits are bigger bang and more power!! There are many factors that justify a turbo requiring higeher octane fuel.

I think my WRX just 'goes' better on it!!! Its the AWD in the wet thats really puts the smile on my face and a puzzled look on anyones face who is (trying) to follow!!!

Ross

Then tell us what he was wrong about.

I have a turbo Diesel - how does this effect me?

Nige

Good stuff. I love to read information like this.

OMG... I think you guys better read this, and fast especially if you are modifying a WRX for more power:

Steve. '03 Rally Blue WRX 5spd ~320 hp

1) Premium gas is universally considered to be 91+. In California that's pretty much all that's available (it used to be 92 at the vast majority of pumps). I haven't figured out where one gets the 93 octane unleaded recommended for the WRX STi. 2) The ECU of any modern fuel injected engine should prevent running lean or rich for any extended period of time as long as the engine has reached full operating temps. 3) Technically octane rating is the resistance to "knock" compared with a reference fuel composed of pure octane. Resistance to knock is really just resistance to spontaneous combustion (before ignition by a spark or flame) due to a combination of pressure and temperature. The two ways this happens are called detonation and preignition. 4) Detonation is one kind of spontaneous explosion. The initial explosion from the spark plug causes a pressure increase in the entire cyclinder as the flame expands. This pressure increase may cause a part of the fuel/air mixture to spontaneously ignite before the flame reaches it. So you end up with two seperate explosions in the same cylinder. Very bad. 5) Preignition is another kind of spontaneous explosion. It's usually caused by some residual heat source (carbon cinders or a spark plug with an improper heat range) igniting the fuel/air before the spark plug ignites. Also bad. 6) Knock sensors will detect mild knock before the driver can hear it, and long before it gets to the point where it can damage an engine. They work reliably enough that some manfacturers say it's OK to use regular fuel in an engine designed to take advantage of premium (like a Toyota Camry V6).

Thank you. But you're not "nospam".

Modern direct injection diesels work by the injector spraying a very fine mist of Fuel into the combustion chamber, this fuel ignites and burns when it hits the very got air that has been compressed by the cylinder.

Ross

Good info, thanks.

The long, needle-fine (.4mm) center electrodes of the Denso Iridium plugs can turn into glow plugs and light the mixture off early. In this case you'd have pre-ignition which would most likely result in detonation. This is one good reason why the NGK Iridiums with their shorter and wider (.6mm) center electrodes are superior.

It's easy to understand why these two terms (detonation & pre-ignition) cause confusion. There are a lot of subtle variations regarding abnormal ignition events, and many times it's not clearly one or the other which is occuring, but a combination of the two. -Danny

can turn into glow plugs

which would most likely

their shorter and wider

cause confusion. There are

times it's not clearly one

Basically you're relying on this mixture to detonate. Also - don't glow plugs help speed it up (almost an intentional preignition) when the engine or air is cold?

They help make it happen when it is cold. When the engine is cold, the pressure alone is not sufficient to cause ignition; the engine is designed to run most efficiently when warmed up. So, you generally need the glowplugs to get things happening when the engine is cold

My cummins does not have glow plugs, their usually found in smaller 4 cylinder and low output v8's (like the oldsmobile diesel and gm 6.2 "slug"). Mine does however have a "grid" that glows like a toaster in the intake manifold, i'm guessing to heat the incoming air. I never use it, besides I like the way it pours out the white smoke when its 10 degrees. My neighbors like it too :)

Uh - several members of my extended family have driven vehicles with the engines you describe. One was a 1980 VW Rabbit Diesel. Loudest car I've ever had the displeasure of riding in. The ridiculous lack of sound insulation didn't help. I guess VW did everything in their power to get 55 MPG, but forgot that actual human beings would ride in these things. My uncles bought a pair of Oldmobile diesels - a Delta 88 and a Cutlass Supreme. Eventually they passed one on to my folks after our car died. They smelled, were loud, and didn't go fast. The only saving grace was getting almost 40 MPG in a full-sized sedan.

My father leased a 1984 Mercedes-Benz 300D-Turbo. That was a nice ride, save the awful MB-Tex vinyl seats. It had a distinctive rattle at idle, but at highway speeds is was nice an smooth. I'm not sure why MB went with a 3.0L 5 cylinder turbodiesel though.

I had an '85 Audi 5000 STD (turbo diesel) FWD which only gave up the ghost a couple of years ago. It also had a 5 cylinder, but I think it was only

2.3L not 3L? Not a bad car. Great for highway cruising. The turbo made it bearable, unlike a buddy's Mercedes 240D (non-turbo) I drove a long time ago: it took several seconds to start moving from a standing start. With a turbo, a diesel feels more or less like a regular car. The 5 cylinder sounds odd, but it's a very smooth engine, with lots of torque: more than a 4, almost as much as a 6, er, I guess that's pretty obvious? However, the Audi diesel was really expensive to maintain.

These days I'm driving a '98 Subaru Legacy Brighton and I LOVE IT!

p.s. Every now and then I read the Alfa newsgroup about their turbo diesels and they sound pretty exciting, but I can't afford that now.

I'm guessing that your noted increase in maintenance expense was due to the fact of its being an Audi rather than its being a diesel. The most expensive-to-maintain car I've ever owned -- and by a very large margin -- was a 1990 Audi V8 Quattro (not a diesel).

- Greg Reed

Actually, I think it was probably both Audi+diesel that made it more expensive. Turned out that most of the parts for that car were NOT common with other models of the same year. I don't know why a diesel model needs different parts (with a much smaller production), but many of them were unique, and expensive. For example, the radiator was "special" (well, I can sort of understand that might have to be heavier?) and cost me Cdn$1000 to replace. My mechanic said that even VW diesel rads were more expensive than gas versions. Hard to determine if that's simply the result of "market economics" or "price gouging". Why should a diesel radiator cost x2+ more? I could understand 20% but 200+%?!?

p.s. Another "weird thing" with that Audi. The top radiator hose was a "3-way" hose. You'd think a standard "generic" rad hose would do? Nope! Gotta buy that special Audi hose (for big $$$) with the little "bleeder hose" coming off the middle. Ah, but that reminds me...

Gotta love duct tape (go! Red Green!). I actually made a temporary repair of that top radiator hose on the Audi diesel using duct tape. The hose split lengthwise (for about 2" length) on the highway coming into Montreal. It was a Saturday and the only thing open (no garages) was a Canadian Tire (within walking distance). Since I had nothing to lose, I bought a roll of duct tape and wrapped a "sausage" around the split part of the top rad hose. It was about 3/8" thick and extended a couple of inches either side of the split. Good enough to drive to my brother's place in Montreal. Then I thought I'd do a little experiment: after the weekend I drove from Montreal to Toronto (about 600Km), and drove it around for about a week in Toronto, before taking it to my mechanic. Unbelievable! The hose was intact and bone dry! Gotta love duct tape!