Anyone know the difference between the US and UK spec 5.3? All I can find is the compression ratio is 12.5:1 instead of 11.5:1 but does that seriously change the output by 40hp?

 

US versions had much stricter pollution controls, which rob a lot of the power.
aside from that there is of course the steering wheel on the opposite side, which was to accommodate the fact that their tea cups have a handle on the left side as opposed to US who have right handed cups.

 

Besides compression ratio my understanding is that the ignition advance/timing curve cost a few horsepower. Of course there's the cat converters as well....a few horpower there, too. Emission controls, as mentioned.

But horsepower ratings have as much to do with the advertising and marketing dept as they do with the engineering department.

The lower compression was standardized for all markets when Marelli ignition was introduced....and I don't recall that Jaguar lowered the advertised horsepower on the non-USA cars :-)

Cheers
DD

 

what does this include other than the cats? I know "removing the air rails" is a popular mod but I have looked into it and removing the air system will give a hp gain of maybe one from the pump not being driven...
So with nothing else does that mean there is 30 some odd hp trapped in the cats?

 

Correct on the air injection costing very little horsepower.

I think the compression ratio is probably most significant followed by timing and cat converters

UNCLOGGED cats don't really soak up all that much, IMHO. 5-10 hp maybe? I gutted the cats on mine and hardly felt any difference at all...a bit more at high rpm is all

Cheers
DD

 

My Aussie spec Marelli 89 runs 12.5cr according to the book.

Well flowing Cats will rob no power from the engine. I have just had mine replaced with 2.5" highflow units. I was talking to the exhaust guy who did the job and one of the restrictions in the OEM cats is their shape. If you look at all performance cats they are shaped with angled ends (unlike many OEM cats that have flat ends), he was saying this greatly increases the flow capability of the cat.

Us Aussies need a handle on each side of the cup...

 

gutting the cats can cause you to lose power if the case isnt smooth. because you want a smooth flow. the easiest way to gain that 20-30hp is to open up the exhaust and intake a little bit. since you essentially have 4 mufflers per side with the dual cats

 

Not that easy, and I seriously doubt it'll be 20 or 30 horsepower.

I removed the front mufflers, replaced the rear muffler with straight-thru, and, as mentioned, gutted the cats.

I'll grant you that gutted cats still may not have the best flow but doing so was the last of my exhaust mods and it certainly did make matters worse...or at least not enough to feel in the seat of my pants.

I'd say I picked up *maybe* ten horsepower all told. Just enough to feel. A 25-30 horsepower increase would be unmistakable.

Other mods in the "not much difference at all" category were opening up the air intakes and K&N filters. I felt an improvement over 5500 rpm....seemed to climb to 6500 more easily...but that's about it.

I experimented with a couple different iterations of the vacuum advance scheme with no noticable difference. AJ6 Engineering claims the timing curve and vac advance differences are significant. I must not have used the right one....there are several...or perhaps the difference isn't apparent on the low comp engine.

What DID make a difference was using AJ6 Engineering's Superenhanced ECU. Quite an improvement in mid-range performance. Worth the money, IMHO.

Another big improvement came from a higher stall converter, shift kit, and modified kickdown circuit. This took much of the laziness out of the 400 trans.


Cheers
DD

 

I was beginning to wonder if my humor was lost here!

 

I felt a difference in the "seat of the pants" once I removed the rear mufflers. I can't say what the change was after removing the cats because I never drove it with them on.

 

Is there any reason to replace the cats with new ones instead of just straight piping??

Personally I like the smell of unburnt fuel lol. Didn't know if it would mess with the computer though...