xe inlet cam in c20let.pdf

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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