Adjustable CAM Pulleys

13th July 2004

About 2 weeks back I helped a friend to set his adjustable CAM pulleys in his Proton Wira Sedan - converted to MIVEC engine. The decision was to retard the intake and exhaust all together half-teeth.

His adjustable CAM pulleys were self-made and he only made provision for half-teeth alteration.

During installation the "mechanic" said NO, so my friend trusted him instead, when test drive, there was completely no power, regardless of what RPM. The engine sounded loud and all and everyone could see it's all show but no go.

Hahhaahha, told him.

After that, my friend (herein after called "Ah Ming") reduce the intake back to original, retaining exhaust advance half-teeth. Power come back a bit.

Next retard the intake half-teeth as I advised. More power.

Then retard the exhaust to original. More power.

Finally retard everything all together. Allot more power.


Last time used to loose to another friend's car by 2 cars gap, now shorten to 1 car or less.


Why? What makes adjustable cam pulleys work? Why in retard? My limited understanding tells me.

ANS = When intake is retarded, the piston gets to go downwards but intake valve does not open.


What happens now?

ANS = a vacuum condition is generated. 

Then the intake valves open and the air rush in. Air is stupid, when the front group rush in, the back group also rush in. There is a small delay..... negligible for (low or high) rpm, forgot which one, because air is compressible like a sponge and is elastic, which can be stretched also.

When piston reached the bottom, there are no more suction effect. The air from the rear / last / far end, is stupidly "still" traveling, pushing into the combustion chamber, even though it is filled.

Just imagine 1% more air going into the combustion chamber before the inlet valve closes.

Then combustion ignite.... that's why more power.

Advancing the intake does the opposite.... air does not get to "rush" in. Air also does not go in at all since the piston just started moving down, remember the stretch effect? And before the piston reached the bottom, inlet valve already closed.... perhaps only 99% filled.

That's intake.

Exhaust is easier to explain. Retard so that the combustion gets to push the piston down "longer" period of time before opening exhaust valves.

There are allot of other variables regarding combustion, timing of air-flow starting IN, duration of air-flow, initial velocity of air-flow, partial steady-state air-flow and finally, last surge air-flow, they are also variables over different RPM. Intake and exhaust port design passages also plays a part. CAM timing & duration, valve lift opening area, valve lifts ......

I don't know in so details. This is the best I can provide for now.


Please be warned that it does not always be "retard". Too many variables in play. This is especially true for ultra high rev engines like over 10k rpm and super powerful type like turbos beyond 300bhp. They are a completely different beast all together, requires different level of understanding of internal combustion. Perhaps the one most important criteria to concern is the CAM duration..........

This is all relative...... it all depends...... so, watch out.

This pair is for 4AG 16v - incomplete process - pictures were taken many days ago.

shiny material is AL7075 - as light as 6061 but even harder.


  1. Adjustable CAM Pulleys
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