xe inlet cam in c20let, Motoryzacja - dokumentacja serwis diagnostyka
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TECH NOTE
MYTH BUSTING
We take a Cavalier Turbo to Jamsport in Northampton to see if swapping the inlet cam for a
20XE one will see an improvement in the output of the engine.
Words: Chip | Photos: Chip, Jason Simms
THE THEORY
If you read internet forums dedicated
to modifying Vauxhalls such as ctor.
co.uk, migweb.co.uk or vauxsport.
com, you will often see people on
there who have done this swap and
claim gains from it. Some of them are
using the XE cam on standard cam
pulleys and some on verniers, but the
general consensus seems to be that
it’s a worthwhile modification.
The only problem is that details
of the best way to go about it are
hazy, with disagreements over the
usefulness of a vernier cam pulley
when doing the swap. There seem
to be mixed opinions among those
who use the standard pulley about
whether you are better off installing
the cams on the standard marks or
moving the cam timing by a tooth.
The theory for altering the timing
is simple: remove the overlap that
using the XE cam on standard timing
marks will introduce. On normally
aspirated engines, this overlap
provides a useful mechanism
for using the pressure drop
created towards the end
of the exhaust stroke to
start drawing in gases on
the inlet stroke by having
both valves partially open
at once. Conversely, on
a turbo engine this effect
isn’t helpful and the boost
can end up blowing straight
through the engine and
down the exhaust, resulting
in a drop in both power and
economy, as the fuel and
air mix is not used by the
engine at all.
The overlap can be
removed by retarding the
intake cam, advancing the
exhaust cam or a combination
of both. But to allow sequential fuel
injection, the Motronic ECUs used
on the LET and XE engines use a
cam trigger located in the distributor
which is driven off the exhaust cam.
This tells the ECU which cylinder is
currently on the intake stroke. As
a consequence, if you start to alter
the timing of the exhaust cam it can
confuse the ECU, so we have opted
to avoid this risk by only altering the
intake cam timing.
These are the different cam options
that we are testing:
- Standard LET cams
- XE inlet cam on standard timing
- XE inlet cam retarded half a tooth
- XE inlet cam retarded a full tooth.
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XE INLET CAM IN C20LET ENGINE
THE TESTING
In order to allow us to alter the timing
by anything other than a full tooth at
a time, we need a vernier pulley. The
pulleys we ordered turned up with
the wrong tooth pattern — early XE
ones have a different tooth pattern to
LET ones, so bear this in mind if you
decide to order some. I had to remove
one from my Nova, as there was no
way of getting hold of one anywhere
else at such short notice!
The first test that we did was to
install the vernier and run the car up
on the dyno again with the standard
LET cam still installed. This was in
order to check that the results hadn’t
changed, due to any minor difference
introduced by swapping one pulley
TEST CAR
Our test car belongs to Martyn
Webster from VXON, which was
featured in issue 59, and it’s a good
example of a relatively common ‘fast
road’ spec Cavalier Turbo.
The engine specification is a hybrid
turbo running Phase II chips, an
uprated intercooler, standard cams
and standard engine internals. On
our initial dyno run it showed a
good spread of torque, with a peak
of 295 lb/ft and a healthy 273 bhp.
Martyn’s car is an all-rounder; it’s
his daily driver as well as being the
car he takes to shows. He’s more
interested in an engine that pulls
well throughout the rev range than
a peaky engine with a big bhp
figure. His current spec satisfies
that brief quite well.
Several dyno runs were
made to ensure there
were no discrepancies.
Standard cam pulleys
with cam cover removed.
for the other. The results came back
exactly the same as on the standard
pulley. We did three runs and the
results were within 1 bhp each time,
which meant that we were all set to
proceed with the cam swap.
Jamsport made short work of
swapping the LET cam for an XE
one, and then we got the car back
onto the rollers and did another
three power runs. Results showed an
increase in both power and torque in
the higher end of the rev range, but
at the expense of a loss of torque
lower down. The peak bhp figure was
up from 273 to 278 and peak torque
showed a healthy gain of 11 lb/ft to
take it from 295 to 306.
The cam timing was then changed
by half a tooth and another three
runs were completed. Results this
time showed a big difference in
bottom end torque; the boost came
in a lot stronger and the peak torque
figure was up by a whopping 48 lb/ft
from the original 295 to 337. Power
was almost the same as on the
standard timing marks, showing
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TECH NOTE
279 bhp, but it made this power
nearly a thousand rpm earlier in
the rev range. At the top end, power
tailed off a lot sooner than before
and from 5500 rpm it made less
torque than either of the cams on
standard timing marks.
Finally the cam timing was
changed to be tooth retarded, and we
did a final three runs. Results initially
looked as if they were going to mimic
the standard timing, following fairly
closely at the lower end of the rev
range, but in the mid-range a ‘hole’
appeared where the torque curve
suddenly dipped. The reason was
that the ECU had retarded the timing
as a result of detonation. Jamie was
listening in on the det’ cans and
had heard the sound of a crack of
detonation for a split second, which
then disappeared as a result of the
ECU pulling the timing out. We tried
Refitting the timing belt.
again and the results were
the same, with Jamie
signalling detonation,
literally a split-second
before the ECU pulled the
timing. His ears are finely
tuned from years of
experience mapping cars,
and both he and the
engine’s knock sensor
were telling the same
story. We were forced
to abort the test.
So what did we learn? The most
important thing was that swapping
the cam on your engine can make a
huge difference. What was especially
surprising was that when running a
full tooth out, the engine experiences
potentially damaging detonation.
It really goes to show that when
modifying your car at home, it’s well
worth a visit to the rolling road to
make sure that everything is working
as you think and check that nothing
nasty has happened.
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COMPARING THE CAMS
The XE cam varies from the LET cam in two ways. Firstly it has
more duration, which means it holds the valves open for longer,
and secondly it has 1 mm more lift. The combination of these
factors means that it allows a lot more air flow into the engine.
TEST DRIVE
Without Martyn having seen the
graphs, we sent him out for a test
drive, first on the standard marks
and then with the cam retarded
half a tooth.
On the standard marks, he came
back saying: “It seems to have
lost a bit at the very bottom end, but
I’d say that it pulls harder from about
3800 rpm onwards”. A quick look at
the graph showed how well Martyn
knows his car; he was right that
3800 rpm is the crossover point for
where the cam starts to work better.
Martyn is clearly not a man convinced
by marketing jargon that something
works on his car if it doesn’t!
When we swapped the cam timing
to half a tooth of retard, this was
the setting that gave the massive
torque increase on the rollers. This
time Martyn came back grinning and
said that the power was coming in
noticeably sooner in the rev range,
and a lot harder too. The result of
this is that the car is much more
usable, with Martyn not having
to keep the revs up as much to get
back on the boost out of corners.
When driving hard up through
third and fourth gears, it now
stays in the boost range when you
change gear, whereas before it
would drop below after the
gearchange and pause briefly
while it came onto boost again.
This makes the biggest difference
of all and unsurprisingly, it is
the setting which Martyn decided
to keep and which he has been
driving with since!
On the road, the modification has improved
performance across the rev range.
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TECH NOTE
CONCLUSIONS
So do we reckon it was worth
doing? Without a doubt! Both
the graphs and Martyn’s
experience on the road clearly
show that this is a lot of gain
for a relatively small outlay.
With a pair of verniers costing
well under £200 — it’s worth
splitting the cost with one of
your mates, because you only
need one each —and an XE
cam costing around £10
second-hand, it can end up
costing as little as £100!
Results will vary according
to the exact specification of
the vehicle but as we have
demonstrated, the gains are
potentially huge. In the case
of Martyn’s Cavalier, the swap
provided almost 50 lb/ft of
extra torque on an engine
which was already healthy.
We reckon you’d have a hard
job finding many modifications
to match that in terms of bang
for your buck!
The stats don’t lie
— there are gains
to be had from an
XE cam and vernier.
TORQUE LB/FT (PEAK FIGS IN RED)
BHP@FLY (PEAK FIGS IN RED)
RPM
STD
XE STD
XE retard half
tooth
XE retard
tooth
STD
XE STD
XE retard half
tooth
XE retard tooth
1500
160
140
160
150
45.696
39.984
45.696
42.840
1700
170
147
173
151
55.026
47.581
55.997
48.876
1900
180
155
186
152
65.118
56.073
67.288
54.988
2100
185
158
195
154
73.971
63.175
77.970
61.576
2300
190
161
205
155
83.206
70.506
89.775
67.878
2500
195
173
217
168
92.821
82.349
103.293
79.969
2700
201
185
230
180
103.332
95.106
118.240
92.536
2900
216
203
252
191
119.268
112.090
139.146
105.464
3100
230
220
274
203
135.757
129.855
161.728
119.821
3300
255
237
321
221
160.224
148.914
201.694
138.861
3500
280
254
337
240
186.595
169.268
224.581
159.939
3700
286
277
333
258
201.485
195.144
234.596
181.759
3900
293
300
330
276
217.574
222.772
245.049
204.950
4100
294
303
320
284
229.512
236.538
249.809
221.706
4300
295
306
308
292
241.527
250.533
252.170
239.070
4500
288
303
310
220
246.763
259.615
265.613
188.499
4700
281
300
308
210
251.466
268.469
275.628
187.928
4900
275
287
297
200
256.568
267.764
277.094
186.595
5100
270
265
288
190
262.185
257.330
279.664
184.501
5300
260
259
274
187
262.376
261.367
276.504
188.709
5500
250
252
260
180
261.805
263.899
272.277
188.499
5700
235
250
240
189
255.045
271.325
260.472
205.121
5900
220
248
215
253
247.143
278.598
241.527
284.215
6100
200
226
200
236
232.292
262.490
232.292
274.105
6300
180
205
170
210
215.917
245.906
203.922
251.904
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14/9/07 11:24:57
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