My 1992 XJS V12 hasn’t been running, and lately I’ve been throwing parts at it… and came across a bargain. The USA version of the Marelli ignition ECU for XJS with catalytic converters is DAC6614, and is unavailable except used on eBay for around $300. The non-USA ECU for with catalytic converters is DAC7889, and is $91 from SNG Barratt. That could mean $91 for a few horsepower. I’m working under the assumption that the non-USA version is basically the same but has an ignition map that doesn’t meet US emissions laws.
It’s presumably for a different compression ratio?
I think you are confusing ignition (Marelli) with fueling (Lucas) ECU’s.
Me thinks – most power gains will come with fuel, not so much with timing. Of course I could be wrong, but if you don’t change anything in the air/fuel delivery systems (and/or exhaust), you may not get any measurable power gain by just swapping the ignition ECU for the European version (likely offering slightly different advance due to the different octane fuels).
The ignition map definitely could give a performance bump… because I know that there’s a logic jumper that really blunts the performance if removed. It is only wired to the Marelli ECU so it doesn’t touch fueling at all. I don’t know if the non-US Marelli ECU has a different map, but figured for $91 brand new it was cheaper than $300 used for the “right” part.
My understanding is that all Marelli cars are 11.5 compression, regardless of market.
Cheers
DD
And engine size, 5.3L vs. 6.0L?
True? I dunno. I know all 6.0’s are 11:1. I suppose they could have gone with 11.5:1 everywhere with the introduction of the Marelli, either to have only one timing map or because the ROW was heading in the same direction with available pump fuels that the US had gone years before.
If that’s the case, though, why the different part numbers for ECU’s? I suppose it could be because of a different cold running map to warm the engine up fast enough for the US EPA.
FWIW – The Model Update publication for the 1994 XJS Range states the 6.0 liter engine is introduced in this MY and lists its compression as 11.0: 1. It makes no distinction of North American versus other markets.
True. Google “Andy’s bracket” for the AJ16 engines. It is an attachment that moves the CPS a few degrees and fools the system to run a bit more advanced timing.
The jumper (aka strategy link) for the Marelli cars is there to allow retarding the timing and is to be used in case gasoline with the specified minimum octane rating is not available and lower grade fuel is poured in the tank.
If the ECU SNG will sell you runs at timing that is set for higher octane fuel and you’re using (R+M/)2 grade 91 or 92, how do you think power gains will materialize?
I don’t know the 1992 set-up, but in the 1995 V12, all computers “talk” to each other, albeit not via CANBUS. For example, timing is slightly retarded when the transmission ECU sends a signal to the box to shift gears. Fueling and ignition also interchange signals.
I retained the original bracket on my AJ6 XJ40 and modified the reluctor wheel when I was in there replacing the front seal.
There is no inter computer communication in the pre 4L80E/6 litre cars other than engine speed and trip computer functions.