Can I post in two different forums? I may post this in the X308 forum, as well.

I have a question for somebody who’s knowledgeable in engine design, or has read about this subject before.

The 4.0L straight six AJ6 engine in my ’94 XJS is factory rated at 219 HP (or BHP, whatever)

The 4.0L V8 in my old XJ8L was factory rated at 290 HP.

So, how can two engines with the exact same displacement differ by 71 HP or 32.5% in the power they can make? Is it because of the number of cylinders, or is it because one has the cylinders in-line and the other is in “V” arrangement? But, if the number of cylinders made that much difference, then the extra four cylinders in the similarly V arranged XJS’ V12 engine, along with its 1.3 L extra displacement, should easily generate power in the mid 400’s HP, which couldn’t be farther from reality. So, then, it’s not about the number of cylinders.

Interestingly enough, however, the torque figures difference between these two engines is only 12 Lbs/ft, with 290 Lbs/ft for the XJ8 and 278Lbs/ft for the AJ6 in my XJS. So, perhaps the answer should be in finding how can the AJV8 engine build up rpm’s fast enough to make superior HP out of similar available torque? How does this happen? Maybe it is not just one major design improvement, but rather a combination of things, like intake manifold design, or compression ratio, or fuel injection and/or ignition new technology tricks, etc. But, whatever they did, it sure made one heck of a difference. Your comments are most welcome. Thank you.

Cheers,

 

Improvements in ECU, variable valve timing on the V8 NA (supercharged cars retain static cam timing) plus 6 years further on in technology, that’s off the top of my head.

 

The 4.0l AJ6 is rather detuned. My former AJ16 3.2l had 211 hp... My current 3.2l AJ26 has 237 hp. Smaller engines, nearly as much or mkre power.

Why is the AJV8 power powerful? Basically because the it was a fresh piece of paper. The AJ6 is basically an evolution of the XK6 engine from the 50ies. Just made of aluminium and with some ore odern tweaks (4 valves per cylinder etc.). Alone the change from AJ6 to AJ16 introduced something like 20 hp in 4.0l form. But the engine not as revy as the AJV8, which is square (bore and stroke are at 86 mm) in 4.0l form. It has VVT, a more modern multipoint injection, ...

The V8 is the more modern engine. But it also isn't as long living. Has a lot of built in faulta. Plastic galore. Been there, am still suffering.

 

There are so many variables that affect engine power.

Firstly engine power comes from torque and power is when the torque is put to use. So for HP power = torque ft'lb's x RPM / 5250 or kw = torque Nm x RPM / 9588.

So if the the AJV8 has a similar torque curve to the AJ16 but revs more it will make more PEAK power, this will not necessarily make the car quicker. If the power band of the AJV8 is the same to the redline of the AJ16 the cars will be similar in performance, the AJV8 may have more outright top end speed.

 

My old AJ16 would rev to 5500 rpm. Thd AJ26 will do 7000 rpm...

AJ16 (3.239 cm3)
211 hp at 5100 rpm
301 nm at 4500 rpm

AJ26 (3.244 cm3)
237 hp at 6350 rpm
310 nm at 4350 rpm

Only 5 cm3 more... But 26 hp and 'only' 9 nm more torque.

Compared to:
AJ6 (3980 cm3)
233 hp at 4700 rpm
366 nm at 4000 rpm

AJ16 (3.980 cm3)
241 hp at 4800 rpm
375 rpm at 4000 rpm

AJ26/27 (3.996 cm3)
284 hp at 6100 rpm
375 nm at 4250 rpm

So basically the torque figure has stayed the say over time on both engine displacements. No matter if AJ16, AJ6 or AJ26/27. There is no real difference. The big difference actually comes from the transmission. The 5 speed attached to the V8 performs a lot better than the 4 speed on the I6. It also has of course a further gear and better ratios. And of course the fuel economy is an awful lot better than the I6.

 

Combustion chamber design and manifold design also play a very large part in engine power, as well as the associated electronics. Thanks to computer power the 2.0 4 cylinder engine in the F type can now make 296HP. The 2.0 is also supercharged, which helps too.

Consider how far the design software has come too, the flows through the manifold and in the combustion chamber can be much more accurately modelled now, previously it was simply impossible to do.

I remember reading an article a few years ago about the development of the Ford 5.0 V8 "Coyote" engine and they said that each simulation of the flow in the combustion chamber took about 2 days for the computer to do. Consider that at that time Ford had the second largest computing power in the world, after the US government. They were putting serious computing power into these simulations and it still took so long to do.

In the early 90's when Jaguar was working on the AJ16 it was not possible to do design to that level.

 

displacement vs displacement is mostly about efficiency, otherwise its differences in air/fuel in and out. So aside from turning more energy into heat than fuel, it makes a difference how much air/fuel they can take in and put out to begin with.

The number of cylinders doesn't matter much, and both are 4 valve designs.

Immediately you'll notice the difference in compression, the AJ6 has 9.5:1 and the AJV8 has 10.75:1. We see similar torque at different RPM, this is mostly a function of the bore and stroke of the engines. The "same" engine with difference bore/stroke values will behave very differently (this is why LBCs with A-series and B-series engines are so fun).

Obviously the 5.3 V12 has more displacement and a high comp ratio of 11.5:1, big differences in efficiency there. Single overhead cam, older catalytic converters, air pump, relatively small exhaust pipes, much less sophisticated fuel/ignition system, 2 valves per cylinder, etc. You see loads of XKs and V12s used in racing car and boats, where they put out way more power than factory, so a lot of the issues are with factory additions, intake, exhaust, air/fuel controls etc.

 

ALL of the above , old designs VS new designs, technology is constantly advancing! along with the costs.

the Chrysler HEMI V8, was designed in the mid 1940's, it is the only engine with many small mods, that is even close to the original engineering.

they say are making over 7000 HP + in top fuel racing.

so that begs the question what does technology really do, or were other designs just simply not good enough!

ordinary mass produced engines are far from being a serious HI-performance engine.

all were made for the masses of people to simply make money for business's.

Ron

 

look at our low power Jag V12 (both preHE and HE) , terrible power output for what we think they should do. to get serious power costs a small fortune and some serious RE engineering.

take the original Jag V12 4.99L, 4 cam very well engineered ports and + many other great design features.

testing it made easy over 500HP back in the 1960'S.

but to produce them would have put the cars in the Ferrari class of money.

 

Ron, I get your point, but the development has been shifted towards what people more so care about: economy and pollution.

Look at how many cats, EGR, filtration, etc. Systems cars have. Diesels are choked to death with emissions crap like particulate filters, SCR cats, AdBlue crud. Heck, Euro 7 Emission class petrol engines are to get particulate filters here soon! Benz already has them in some of the upper models (S Class). Problem: every Form of exhaust gas filtration costs power and economy. That is why modern BIG engines will have big consumptions. But small engines are efficient and clean as hell. At least on the paper.

If you ditch the lot, you'd be puthing out significantly more power. My former C30 2.0l NA Duratec would act like a Honda but with torque. Rev to infinity. People have them putting out 280-300 hp - without a turbo! Without a supercharger! 9000 rpm redline, different cam and intake ans done. It requires dumping the emissions crap though. And that makes it illegal in most countries on this planet.

 

Hemi is a very bad combustion chamber design. It requires quite a bit more octane than other designs to prevent detonation at a given compression ratio. Because of the poor progression of the flame front it also tends to have higher HC emissions that other designs.

As we know, emissions drives everything these days. The good side is that low emissions also means efficient combustion, and efficient combustion means more power from the fuel used.

 

Jagboi, that may be true!

BUT what other engine i the world ,small enough to fit in a modern car, that can be modified to make better than 7000+ HP.

latest performance , 338 MPH, in just a 1000FT, that has to be incredible using 2 wheel drive, no rockets etc.
having its roots in the 1940's YIKES, you know those dumb Hot rod Americans.

they just dont know any better!!!

 

So the AJ16 makes 156kw at 4000rpm and the AJ26 166kw at 4250rpm based on those numbers. Without the torque graph those numbers look almost the same and the AJ126 only makes more power due to higher rev limit

 

I'l see if I can find the graphs at home...

 

I disagree the 5.3 is "low power", it was making some of the most power for its displacement at the time. It was basically a decade ahead in those terms.

The problem you see with people most of the time is they hear "V12" and automatically think 500+hp.

I also disagree, in some senses, that modern engines are far from high performance. That is totally relative, and its incredibly apparent just what technology has done. The average 4 cylinder today, at around 2.0 L, puts out around 200 hp. We saw that kind of power in engines 3x that size in other decades. Maybe they aren't putting out 7000HP or whatever and aren't made to be redlined all day, but they are much more efficient.

 

OK the jag V12 in 1971 put out around 295HP.

the 1970 Chevy Camaro with option engine 454 dynod over 475- 500hp !, the factory had to fudge the socalled 425hp ,to satisfy the insurance companies.

and most any Chrysler Hemi of that era were in the 525-575hp. factory rated 425hp,LOL,OH yeah!

believe that one , lest i remind you i was in that era with the money to enjoy it!

a quick remember ; i went 200MPH in a 1/4 mile in 1965 at 7.20 ET.

 

I agree.

When released in 1971 the 5.3 V12 was rated at 314 SAE Gross horsepower...nearly 1 horsepower per cubic inch. While not astounding by any means, that was still respectable. The SAE Net rating was 272 hp...which put it in line with many medium sized USA V8s of the time. As non-USA engines go it was one of the most powerful of the era. It kept the elderly XKE viable for a couple more years and in the XJS and sedans it gave more performance than many other Euro-sedans and GT cars of the period.

The real let-down was the early V12s drank gasoline like the biggest, most powerful V8s ever made. It was a bit like getting %$#@-ed without being kissed

Cheers
DD

 

We all bow to your greater knowledge of the subject, Doug!