From: Michael Cogswell Michael_Cogswell@gtsi.com
Kirby:
Do I remember correctly that you were installing the AJ-6 engine mods?
If so, which ones have you installed? Are you happy with the results?
How about posting a report on the XJ-S list for all of us to enjoy?
Yeah, yeah, yeah… I suppose it’s time to report.
It has taken several months for all the bugs to get worked out, and I
was determined not to report until the project was completed. Well,
it’s now completed, so here goes.
As you all know, I have a 5-speed in my car. Turns out that the
stock ECU includes a feature called overrun cutoff, which means that
when the throttle is dropped to idle at elevated RPM, the fuel is
completely shut off until the RPM drops to somewhere near idle. This
apparently causes some problems with a manual transmission – cars
that come from the factory with EFI and manual transmissions
universally have no overrun cutoff.
I was having some problems, mainly engaging the clutch from
standstill in 1st gear. It would tend to shudder something fierce,
and after considerable fiddling I finally figured out that it had
nothing whatsoever to do with my E-type SIII flywheel/Chevy 10.4"
clutch plate/Kevlar clutch disk/homemade pedal/etc., etc. The
biggest clue was that it didn’t shudder – at all – when the engine
was cold; it would engage perfectly smoothly, just like I wished it
would do when hot. My conclusion was that the problem had something
to do with the closed-loop operation that kicks in when the engine
warms up, and perhaps with the switching from open-loop to
closed-loop when the throttle comes off idle (the '83 XJ-S EFI is in
open-loop at idle because the Lambda sensor output is unreliable at
low power).
So, I asked AJ6 Engineering if they would delete the overrun cutoff
for me. Roger Bywater said he’d be happy to delete the overrun
cutoff gratis if I paid for the “enhanced” or “super-enhanced” ECU
mod – but felt that neither mod was likely to affect the problem I
had. Still, I knew I needed to do SOMETHING to address the problem,
so I opted for the “super-enhanced” mod and shipped the ECU off to
the UK.
Also note that my ECU had a failed circuit in the fuel pump control.
I had had it jumpered since I got the car, didn’t really bother me.
However, Bywater agreed to fix that too.
It took one hell of a long time to come back. Bywater and I
established e-mail communications, and I told him that I wasn’t in
too much of a hurry; since he was busy with people who were left
walking by ECU failures, he put mine on the back burner and didn’t
get back to it until I started complaining.
When it did come back, I incurred a C.O.D. charge of nearly $100 –
cash only – to receive it! This P.O.'d me considerably, since it
had only cost me $40 to ship it, air mail and fully insured. I had
sent it there via US Postal Service (highly recommended – the price
has since gone DOWN, since they now have a special bargain price for
anything under 5 lb and the ECU weighs 3). However, Bywater has more
trouble using US Postal service sending back, uses UPS, and they rip
you off something fierce for “customs brokering charges”.
Anyhow, the revised ECU made its presence known immediately. The
most noteworthy change, IMHO, was that the car was a LOT more
powerful! This despite Bywater’s insistence that the mods should not
affect HP, only throttle response and mid-range torque. Whatever,
where the car formerly “ran out of gas” around 5000 RPM, it would now
scream until I chickened out and shifted. And, as hoped, the
engagement of the clutch worked much better. My initial reaction was
that, soon enough, I would be posting a recommendation to this group
something along these lines: forget the GM400 mods, forget tweaking
this and tweaking that, just ship that ECU off to AJ6 Engineering and
call it a day! Easily the biggest performance boost the car has had
– INCLUDING the installation of the 5-speed!
Unfortunately, everything was not OK. Had a very unique problem:
when the throttle was increased, the engine would run “flat” for
about a half second, then suddenly take off. Bywater & I still
haven’t agreed on what caused this, but my own feeling is that it has
to do with the “super-enhanced” mod.
The “enhanced” mod supposedly changes the fuelling maps to provide a
little more mid-range torque. The “super-enhanced” mod goes beyond
this in adding a separate accel map that is based on the throttle pot
rather than the manifold pressure sensor. Since the MPS is in the
ECU in the trunk and there’s a long vacuum line from the engine all
the way back there, the response is sluggish at best; when the
throttle is opened, the manifold pressure rises immediately and
therefore the airflow into the engine increases immediately, but it
takes some time for the increased pressure to get to the ECU and
therefore for the ECU to start sending increased fuel (beyond the
accel injector pulses), so the response is lame. The
“super-enhanced” switches to the throttle pot map for a half second
or so after a throttle change in order to provide instant response,
returning to the MPS control after enough time has passed for the
pressure sensor reading to have stabilized.
Since the “super-enhanced” obviously relies a great deal on the
throttle pot, a known problem in the XJ-S, Bywater immediately
suggested that my pot was FUBAR. Since I have already replaced my
throttle pot not too long ago, I went ahead and typed up a response
that that couldn’t be the problem – but before sending the message,
decided it was easy enough to check again. Sure as hell, the new pot
was FUBAR. However, I managed to fix both my pots and the problem
remained.
IMHO, the problem with my ECU was that the MPS fuel map was fine but
the throttle pot fuel map was bad – perhaps too rich. Everything
seemed to work, there was no misfiring or anything during the flat
running, it just wouldn’t pull. If it was too lean, I suspect it’d
be misfiring. So, when the ECU operates for the half-second or so
after a throttle change, the engine growls but doesn’t go. As soon
as the ECU switches back to MPS control, the car goes like a scalded
cat.
Bywater apparently doubts this explanation; he claims the two maps
are supposed to be the same. Perhaps he will enlighten us, perhaps
after he gets that ECU back (it’s en route now) and can check it out.
In Bywater’s defense, apparently I had given him an unusual set of
requirements. Most of the ECU’s he works on are UK spec, and
therefore have no closed-loop circuitry and were for the 12.5:1 H.E.
engine. Most of the US spec ECU’s he’s modified were for customers
who couldn’t care less about emissions requirements. OTOH, I
requested that the closed-loop capability be maintained in case I
have to pass emissions requirements in the future.
Anyhow, the problems I was having were definitely unacceptable. The
response lagging and then coming on like gangbusters made it REALLY
difficult to shift the 5-speed; riding in the car was like riding
with a driver-in-training. Each shift would jerk the passengers
repeatedly.
In the meantime, Bywater had also included a copy of his book on EFI
(excellent – highly recommended). Reading the book, I found out
that the 16CU ECU was interchangeable with the 6CU. Bywater had
mentioned that the 16CU was much easier to make these mods on. So,
in an effort to get my problems corrected, I asked Bywater if we
could trade my 6CU for a modified 16CU and see if that worked better.
He agreed that that would be a good idea, and sent me a
“super-enhanced” 16CU. So, on one illustrious day, I drove the car
with the 6CU one last time to clarify the symptoms in my memory, then
swapped in the 16CU and drove it again. The car ran perfectly, all
problems gone. I can now shift like I’ve done it before, and the car
still hauls ass.
Also note, when I got the 16CU via UPS, I incurred a C.O.D. of $19.
I thought this was a big improvement – until I found out that I
wasn’t supposed to have to pay ANYTHING, Bywater had prepaid the
entire shipping cost at his end. UPS is definitely to be avoided for
trans-Atlantic shipping whenever possible.
So, I have sent the 6CU back to Bywater for his analysis. All said
and done, I am more than pleased with the results. In hindsight, the
only complaint I can voice is that all of this took so damn long; I
can’t fault AJ6 Engineering for having to take two attempts to get it
right, especially since Bywater did such a fine job of getting the
problem corrected.
For those interested in getting their XJ-S to perform like it should,
these mods – enhanced or super-enhanced – are highly recommended.
The idea that the OHC V12 doesn’t pull as well as a pushrod lump
evaporates, along with all respect for the weenies at Jaguar that
failed to provide this type of performance to begin with.
Whether or not you go with enhanced or super-enhanced might be a
tougher decision. Unless Bywater can honestly claim to have figured
out the problems, those with the 6CU might be well advised to opt for
the enhanced only – or to opt to trade up to a 16CU. Of course,
having the stock GM400 makes throttle response less important than on
a 5-speed, but the half-second delay I experienced still wouldn’t be
desirable.
Also, I dunno how much the stock ECU was improved over the years. If
the newer ones (26CU, 6.0 litre, etc.) are really good to begin with,
obviously AJ6’s enhancements won’t cause as dramatic an improvement.
I am cc’ing this message to Bywater. With any luck, he’ll respond to
the group with his side of the story.
– Kirbert | If anything is to be accomplished,
| some rules must be broken.
| - Palm’s Postulate