Non oxygenated fuel costs more than fuel with Alcohol so it should be better, shouldn’t it?
No! Our V12’s need that alcohol. First Ethanol is 114 octane. Plus it runs cooler and keeps our engines cleaner. All while adding a trace amount of power.
The keeps our engines cleaner is the important part, and why we need Ethanol.
The V12 only has a 2&3/4 stroke. Thus the piston speed is much slower than a long stroke engine. That’s great for cylinder wear. Open up even a high mileage V12 and you won’t find any ridge.
But, that slow speed means carbon will build up behind the rings. Regular high RPM bursts ( Italian tune up ) will help reduce that. So will using ethanol. But normal driving that carbon will slowly build up until the rings are stuck.
Another thing that Ethanol does is intake valves tend to build up deposits on their backsides. Doubt me? Pour a little gasoline in a clear class and let it evaporate. The deposits you see multiply it times the hundreds of gallons of gasoline the valve will flow and now you should understand why you need ethanol.

 

Hey Man! Thanks for this...

Could you help me out?
Say a little more about the 2&3/4 stroke idea. Or, link me to something. I don't know anything about this...

 

Each cylinder of the 5.3 v12 has a piston stroke of 70 mm. Which is about two and three quarter inches. That's kind of short but it's not Ferrari short. 60mm or so. No one should get too excited about running ethanol in an XJS. The lack of any ethanol compatible rubber, steel line and tanks all makes running ethanol a bit of a Gamble without upgrading everything. Furthermore all fuels are extremely clean these days and most include some detergents. Deposits come from oil burning which is independent of fuel. Ethanol also does not have the same specific energy density as a gasoline therefore you would need to reprogram the fuel system to deliver more fuel. On modern cars that are known as flex-fuel there's a sensor that reads ethanol content and runs a modified fuel map. The only Fuel That should be used in an XJS is what it was designed for... Regular old gasoline. Sure a tiny amount of ethanol mixed with gasoline probably won't do much harm but running significant amount of alcohol is foolish without major accommodations.

 

JJJ
An "oversquare" short stroke engine is one where the diameter of the cyclinder (90mm on the 5.3 HE) is bigger than the length the piston travels up and down the cylinder (70mm taking ICSA's word for it).
The original XK 6 cylinder Jaguar engine is a long stroke engine, that is the length the piston travels up and down the cylinder is longer than the cylinder diameter.

Now, for a given number of revolutions per minute for two engines of the same capacity, the distance a long stroke engine piston will travel in one minute (cumulatively up and down the cylinder all strokes added togther) MUST be longer than the distance an oversquare engine piston will travel.
Therefore, taking Mgar's point that an oversquare engine's piston travels a shorter distance than an identically sized undersquare piston would, it MUST logically follow that the oversquare piston is travelling more slowly.
But, and here I depart from Mgar's point to a degree, I do not believe the difference is significant as far as the engine getting gummed up is concerned. The mean piston speed of a V12 engine at (say) 2500 rpm is pretty high - although I am too lazy to calculate it for you. Sadly, in this day and age, the fact is that very very few owners give their cars and thus their engines a decent caning. Partly because they feel the engine is fragile (whereas actually it is bulletproof) and partly because speed restrictions make this hard to do. This, I believe, is the reason, if they do, that V12s get gummed up and need the famous Italian tuneup!
I feel certain that the XK engine, or any engine, babied in the same way that V12s are, would gum up just as much.

 

Thanks, both ICS and Greg... Super interesting.... Really!I know/knew nothing of this - and this degree of XJS (or engine for that matter) info. 70mm seems so so so short. But 90mm I guess is what makes up for that....?

I'm going to go Google stuff about this now, lol
ICS... Maaaaan, 50hrs + per week, a Lady and Brooklyn, 3 Jaguars and a Porsche, classes - have all conspired to keep me away from you and your garage. I know we'd talked about some stuff and I apologize. Jus this city can be consuming. I'll reach out soon,,, that is unless YO MAD at me, lol...?

​​​​​​​Be well all!

 

The 12x aspect of this engine helps a lot. One cylinder has a volume of 0.44L.

 

I was just reading this...

Ok... I don't understand how a SHORTER stroke reduces piston speed, at all... It seems like the opposite would be true...

When I drive my car (XJS) I have always been concerned with how high the revs would be at say, 80mph... I know the engine will keep giving, I can feel it, but it scares (I use that word mildly) me. Always, in my experience in any car I've owned, getting over 3000 rpm,,, I told y'all I am a "Sunday" driver,,, was I thought - beating on the engine.

I have so much to learn about what I have, in this XJS machine!

 

If an engine is turning at 1000 rpm the piston must go up and down 1000 time a minute. Right? Lets imagine the engine is doing 1 rpm, it will make the description easier, and lets make two engines, one with a 1" stroke and one with a 10" stroke. In both engines the piston must go down and up in 1 minute, 30 seconds down, 30 seconds up. On the first engine the piston only needs to move 2" overall, 1" down, 1" up, so its average speed is 2"/min. Still make sense? The second engine the piston needs to move 20" total in the same time, so its average speed is 20"/min. Now if we speed the engine up to 1000 rpm, we can just multiply those speeds by 1000 and get 2000"/min and 20,000"/min. That's quite a big difference. Now obviously 1" is a very short stroke and 10" is a very long stroke, but the principle is the same for all engines. If we look at the 5.3L V12 specifically with its 70mm (2.75") stroke we find that at 1000 rpm the piston has an average speed of 5.5"/min.
The 4.2L XK engine has a 106mm(4.17") stroke, so at 1000 rpm the pistons are doing 8340"/min.

These may not seem like high speeds, but they're average speeds. The piston has to change direction at both the top and bottom, which means it must come to a complete stop. That's the primary reason that short stroke engines can rev higher.

 

Interesting, Interesting...!
I'm on the move right now (subway in good ol Manhattan) so imma really need to sit down and digest that, better. But I think I'm picking up what YO putting down,,, Thank you for that.

 

It isn't made of glass. Take that baby right up to the 6500 RPM redline. You're missing part of the fun of you don't . Plus it keeps the combustion chambers clean, which is a good thing.

Cheers
DD

 

You have to let all of them kiss the Red Line once in a while or they get the idea you don't love them.

 

you should drive a manual version of an XJS and you will see that 3k RPM is barely enough to shift. Dont be scared. Its tougher than you think it is. Its not a fragile engine by any means. I like to rev em up and give em a nice carbon cleaning once in a while.

 

my how stupid are we colonials. For 40 years we’ve been using ethanol without trouble.
If fuels are so clean try my little test put some in a clean class bowl and let it evaporate.

 

remember the piston stops at the top of the stroke and stops again at the bottom of the stroke so it’s accelerating from stop to stop and the longer the stroke the faster it has to accelerate per revolution.
Please understand the revs are the amount of times the crankshaft completes one revolution in a minute.
The factory is satisfied 6500 revolutions per minute will not cause the engine harm. A long stroke will have a piston speed of 3500 ft per min at 5500 revolutions per minute. A short stroke will have a piston speed of 2200 feet per minute at 6500 revolutions per minute.

 

very few Ferrari people toddle along at the low revs we Jaguar owners do. Second. Jaguar themselves blew it. Not once but twice on the fuel lines. Read John Egan’s book Saving Jaguar. That’s right the factory recognizes they made a mistake with the fuel lines and orders them recalled and new fuel lines installed. The trouble is the fuel lines they replaced them with were also defective!
Porsche came to Jaguar’s aide and showed them which fuel line was actually designed for the pressure of EFI.
But ethanol does not affect EFI fuel line. (Which isn’t rubber).
As far as the sensor in Flex fuel equipped vehicles. That is required when 85% of the fuel is ethanol and only 15% is petrol.(E85) Yes an adjustment must be made at that level.
In the 5or10% commonly used today. The cooler running, higher octane (114) 10% doesn’t need adjustment. Yes you might lose a mile per gallon or less, the offset is the engine won’t have the same build up of deposits and will have slightly more power ( maybe another part of a horsepower more)
it’s well known that E85 will add 10% more power. More if you advance the timing to take advantage of the ethanol.

 

8 POINT 340" per minute, Gareth?

 

A further point in favour of oversquare engines is that there is a larger diameter in which to fit multiple valves, lighter individual valves and actuators, and a pentroof combustion chamber. Thus, in principle, enabling better cylinder filling, higher revs, and more efficient combustion. This is the reason the AJ6 engine is more efficient than the V12.
JJJ, if you are really interested in this subject, buy Roger Bywater's book, or drop broad hints to the Lady in Brooklyn, as it is Christmas time:
ENGINE TECHNOLOGY in the Modern World / AJ6 Engineering

You might not easily find it, but worth emailing Roger to ask if he is selling it in pdf form.

 

The V12 was designed for high rpm operation that is why it can cruise at such high speeds. Well over 100 mph. There was an article in Octane magazine a few years back where an XJS was driven to Monte Carlo. The author describes how the engine really doesn't start to come to life until it's going over 80 mph. With twelve smaller oversquare cylinders as opposed to the typical V8 it needs to spin to make the power and produce speed. The speed comes from opening the throttle, not from a bunch of transmission gears. Since the majority of XJS cars are equipped with automatics they rarely hit higher revs unless they are run at high speeds. These cars were not designed for typical American driving conditions which puts them at a disadvantage compared to other American performance cars. Of course American speed limits are low, though in reality cars are driving at 80 mph. and higher all over California. The GM Turbo 400 transmission was designed to be used behind large displacement low revving V8s, Clydesdales, if you will, not high winding Thoroughbreds! During low speed urban and highway operation the engine doesn't get enough "exercise". What's worse is that many like myself are afraid to over stress their old motors and are afraid to run them hard. If we won't run them hard we should at least run them often!

 

No, they travel 8.340” every revolution. The mistake I think I made was saying the V12 has an average piston speed of 5.5”/min at 1000 rpm. It should be 5500“/min. At 1rpm the XK engine has an average piston speed of 8.340”/min.

 

The piston speed varies from dead stop to around 2200 ft per sec.