I would like to buy a Hess & Eisenhardt Convertible 5.3 V12 XJS (01/1988). I'm not sure how many Hp it is and if this American version had a low power compared to the European model of coupé version.
Thanks
Tito

 

The H&E version had the same BHP as the contemporary US spec coupes had. In general, the emissions equipment, and the lower octane fuel available generally in the USA at that time, meant that the US spec V12s had about 260 BHP compared with the UK spec of about 295. The US spec V12s had a lower compression ratio than the UK spec engine also.

 

My 1990 XJS convertible V12 --5.3 --261 HP and says only use supreme 91 octane.

 

Which is the same as regular fuel in Europe.

 

Regular fuel in Eu is 95.

 

Different octane scales. Europe uses RON, North America uses AKI. 91 AKI is roughly equal to 95 RON.

 

yes, as has been pointed out a thousand times before. just leads to Cson on both sides of the pond.

 

changing equipment emission and using EU fuel is it possibile to obtain the UK spec of about 295 BHP?

 

According to Roger Bywater.....

WELCOME TO AJ6 ENGINEERING / AJ6 Engineering

.....quite a bit of the HP loss was due to emission equipment and, more specifically, differences in vacuum advance controls. He says...or at least implies...that not so much HP difference is accounted for by the drop in compression from 12.5 to 11.5.

I find this a bit surprising but....what do I know?

Cheers
DD

 

Our cars had the same CR as the USA spec but my car is listed at 284hp in 1989 on 91RON so with the 98RON available almost everywhere now and strategy link installed I would expect close to Europe HP around 290hp. Although strategy link is listed for 95RON not 98 so a little tickle should release a few more hp.

Our cars differed in only having one catalytic converter after the down pipes and no air pump. This would free up the exhaust considerably allowing a more aggressive timing map (dissy or Marelli).

So it should be easy for USA folk to up power to EU/Aus spec by gutting the cats in the down pipes and re-mapping the ignition for higher octane fuel.

 

I had a 1992 Series III V12 sedan, which kept the Lucas ignition, rather than Marelli. I changed it to the Euro style downpipes with no cats at all, and I couldn't tell any difference. I didn't do any objective measurements, but the "seat of the pants" meter didn't notice an improvement.

I didn't do anything with the ignition, but looking at the parts book it appears that Australia and Sweden have the same distributor, and the rest of the world has a different distributor. So in theory a Canadian and UK car has exactly the same mechanical advance and vacuum advance capsule.

The air injection pump on my 94 XJS has a clutch like the Ac and only runs for the first 45sec or so. My 92 had a diverter valve, so it vented to atmosphere after startup, so there was no load on it. It would take virtually no power to turn with no load on it.

 

Right.

The difference, I think, is in the (complex) vacuum advance control system...which allows/disallows the vacuum advance to come into play.

But, then again.....

On my old '88 XJS with Lucas ignition I experimented with different schemes for the control system and felt no difference in the seat of my pants.

I also felt no seat of the pants difference in opening up the air cleaner inlets, gutting the cats, or removing the forward mufflers. Well, maybe a very slight difference with the mufflers at high RPM. Might've been my imagination.

I really think the difference in compression ratio makes more of a difference than Bywater implies.

The only mods to my XJS that truly gave a palpable an improvement were installing one of Bywater's "Superenhanced" ECUs and changes to the TH400 transmission: shift kit, downshift mod, higher stall speed torque converter.

On my present V12, in the Series III sedan, going to 3.31 gears made quite an improvement. I plan on the kickdown mod and Superenhanced ECU later.

Cheers
DD

 

Doug
I totally agree about the AJ6 mods. In my experience, there is no doubt at all that the AJ6 Super Enhanced ECU and large throttles make a huge difference. I have experience with three Lucas cars, including a USA spec car, and it did a huge amount for all three of them. In the case of the USA spec car, I got rid of all the vac tubes etc etc, and just connected a line from under the A bank TB direct to the vac capsule. It had no cats, air pump bits removed, but having done that vac mod, it's performance was just as lively as a UK spec car with the same mods. I did advance the timing a touch, drive-timing the car as advised by Grant Francis (turns to the Southern Cross and bows) and that is undoubtedly part of the difference.
I have not tried a car with the AJ6 intake manifold runners bell-mouth mods, but I would be very interested to hear from anyone who has fitted this extra bit.
As to the CR, after about 1988 or so, and certainly all Marelli Cars, ALL V12s were the lower CR. Mr Bywater told me this had no effect on overall power development; and that the higher UK CR was for part throttle MPG reasons - not doable in USA cars because of the octane levels then prevalent in USA fuel (which is where this discussion started, with the MON and RON confusions!).
Greg.

 

In principle, yes. However it probably needs adding the AJ6 Engineering large throttles and super enhanced ECU, advancing the entire timing a touch, and scrapping the vac tubes in favour of one vac line from under the throttle body direct to the vacuum capsule in the dizzy, to get the car really going well.

 

According to the ROM the USA/Canadian/Japanese have different vacuum regulators fitted and list different vacuum specs from the UK, the Australian car is listed as not having the regulator and all 3 have different ignition timing specs with vacuum connected.

Removing the in-downpipe cats alone may not have any effect on power without ignition/fueling changes. Although there was a noticeable difference when I added the 2.5" exhaust, but no noticeable difference when I added the large intake trumpets

 

When you say ".....all 3 have different timing specs...." are you referring to base timing? If so, I agree.

What I'm referring to is the advance curve built into the distributor. As far as that goes there are only 2 choices, and the USA cars had the same distributors as all the others with the exception of Australia and Sweden. So, as far as that goes, at least, a USA car isn't at a disadvantage.

Cheers
DD

 

Agreed. But the larger throttle bodies and enhanced ECU is going beyond "ROW" spec.

If the compression ratio truly isn't particularly significant in maximum horsepower, a USA car should be capable of near-ROW output using the correct recipe of standard parts. It's on my to-do list....and will mean searching overseas for some goodies, I reckon.

Cheers
DD

 

What I was reading lists the base timing and mechanical advance as the same, what changes is the vacuum specs, so the vacuum advance looks like it is different.

 

Pretty sloppy spec isn't it? 19 degrees BTDC ±7 covers a wide range of "acceptable".

All of the timings given overlap when the tolerance is taken into account.

 

I did notice - that's 37% tolerance and you can bet it's retarded not advanced.

I posted this mostly for the different vacuum spec at the distributor, this would have an affect on the vacuum advance curve.