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The Importance of the Cambelt

1996 Mitsubishi FTO GPX

Nov 2002 - Jan 2003

Article by:
Mark Allanson - Website

This story starts back in March 2001. A man takes a final look around the airport before heading onto the Boeing 747 for the flight to Canada. Qantas Flight QF52 departs at 11:05pm. Earlier that day he had accepted an innocent enough sounding proposal from a gentleman well known to himself. Little did he know that this proposal would lead to 18 months of heart wrenching grief...

Well maybe thats a bit dramatic. But there is a time in any FTO owner's life that the well documented and wallet threatening cambelt change must be made in order for your car to remain healthy and reliable for years to come. Before we get started, I apologise to all GR and GS owners. I use references to the MIVEC engine all the time, but you can ignore this fact as the article has nothing at all to do with MIVEC (except at the end), and everything to do with timing.

The cambelt can also be the bringer of an unknown and little thought about surprise. The lack of power. Having a stretched or misaligned cambelt can be the cause of a hard-to-notice but significant loss in power across the entire rev range of the 6A12 engine. I will explain how the function of the cambelt and how it works in the following section.

What does the cambelt do?

In short, the cambelt makes sure that all the camshafts (there are 4 in the DOHC V6 6A12 engine, 2 for each section of the V) spin at the same speed, at the same time. No matter what the RPM of the engine, no matter what the speed of the car, each cam is always in the same degree of rotation relative to all other cams in the engine.

6A12 Cambelt Diagram

The cambelt runs around the crankshaft sprocket, located at the bottom of the above diagram. As the crankshaft sprocket spins at the same speed as the crankshaft (the current RPM your engine is running at) it is this that dictates the speed of rotation of the camshafts.

To look at the importance of cambelt tension and installation you have to think about exactly how camshafts work, and what their function is. This is too long of a discussion for this article, however to cut things short, camshafts are lengths of steel that contain a series of lobes - the cams. The lobes effectively determine how long the inlet and exhaust values are open for, letting the fuel/air mixture into the cylinders, and the burnt gas out of the cylinders.

How can this effect the power of the engine?

Easily. To work properly the cambelt has to align all 4 camshafts perfectly. This means that the cambelt has to slot into the teeth of each camshaft and crankshaft sprocket perfectly. If an error was made misaligning one of the camhafts by 1 sprocket tooth, suddenly the rotation of the cams on that shaft will be out by a certain number of degrees in relation to all other cams in the engine.

Since the cams determine the opening time of the inlet and exhaust valves, misaligning in one direction has the effect of opening the intake valve of one set of cylinders before the intake valve of the second set of cylinders. The Air/fuel mixture in the first set of cylinders is then sitting inside the cylinder for longer before it is ignited by the spark from the spark plug.

On the exhaust side, this would mean the valve is then opened earlier than the other set of cylinders. Due to the precise timing of the engine, the air/fuel mixture almost definitely won't be fully burnt yet. This means that some of the combustion power of the mixture is not being used to drive the piston down, generating less power.

If you inadvertently misalign the teeth in the opposite direction, the air/fuel mixture is being let into the cylinders late, so less mixture will be in the cylinder before the spark is ignited. On the exhaust side, the mixture is then left in the cylinder longer before being let out by the exhaust valve.

But won't this damage the engine?

Yes, and No.

If we were running an old engine with no computer, this would lead to heating problems, and ultimately lower the lifespan of the engine.

Thankfully the 6A12 engine of the FTO is a nice modern marvel and it relies on a computer to determine a large range of operating parameters. In this case, the FTO's computer will retard or advance the timing of the spark (ie. fire the spark plug earlier, or later than optimal) to account for the problems in cambelt timing. Ultimately this measure taken by the computer is to limit the damage that can occur. Whenever something moves into damage control mode, you are effectively producing less power.

The amazing thing about all this is that because the FTO's computer deals with the timing problems, the car runs like a dream, as if it was a normal car. No misfiring, no audible problems. The car just seems low on power, no bite, a lack of throttle response.

Well thats all fine, but prove it please.

No Problem. Its time for a story. Not so dramatic this time, I promise.

In late March 2001 my cambelt was changed. After this cambelt change the car was low on power, it lacked bite, it lacked throttle response. There were no glaring symbols of failure, no engine warning lights, no explosions (did I say no drama?), nothing.

Before the cambelt change was done the car was dyno'ed to obtain a power reading (it was pure co-incidence that the dyno was done only weeks before the cambelt was changed). The following is the power graph, produced by a BOSCH dynometer. This dyno run was done in early March, 2001, with a K&N POD filter attached to the car. Note that the small spike @ 60km/h is a gear change from 2nd to 4th gear.



This dyno produced a result of 118kw (157.3hp) at the wheels of the car (The bottom line on the graph above). The curve of this graph is also much as expected, quite linear with a small pull when MIVEC takes over. For the purposes of this article we will ignore fanciful prediction of at-engine horsepower as it is far too inaccurate a calculation. With this in mind ignore the top line above.

2 weeks after this dyno was done, the cambelt was changed, and replaced incorrectly. Each cam gear (as we were to find 18 months later) was replaced one tooth out. Jump forward 18 months. I had been driving the car low on power for that long, not realising what was wrong with the car (or really noticing the lack of power as the car was running as smoothly as the next).

A sudden urge led both myself and Richard to the doors of Myaree Dyno Tune, in Myaree, Western Australia. they use a Dyno Dynamics Dynomometer for their power runs. Richards FTO was driven onto the rollers and spat out a nice 161.4hp at the wheels. My car was then placed on the dyno, and to everyones dissapointment and surprise, ran only 144.8hp. The following is the dyno from this run. Notice the totally erratic power curve, pitfalls and rises in the curve constantly over the power band.



There had to be an explanation somewhere, and both RichardH and I set about formulating a plan to track it down. The only logical conclusion we could make was that there was a fuel supply problem somewhere that was causing the computer to alter timing to cope. Blocked injectors were all but ruled out as these would cause a bad idle and audible misfire as sporadic fuel supply generally does. We decided that the culprit could be an old and blocked fuel filter flowing at a constant but less than optimal level.

Before any action was taken on these items, a chance conversation between RichardH and Mal Crockenberg (Crockenberg Motors, WA RALLIART supplier and mechanic) led to Mal exclaiming that he had 2 similar cases to this appear in recent times. The first was a Mitsubishi Magna, which ran perfectly but was lacking bite. The second was an EVO Lancer with a similar problem. Both these cars exhibited the same symptoms as my FTO. The solution in both their cases was a cambelt change went wrong, a couple of teeth out here and there.

With adrenaline running high on this news, I promptly booked the FTO into Crockenberg Motors for a "power hunt" and valve clearance adjustment. A few weekends before the car went into Crockenberg Motors we did some dyno runs using the home dyno software described in Measuring Engine Torque and Power. Power graphs from these runs were produced and saved, with the intention of comparing these to the post-fix power graphs. The below graph shows the power curve with a K&N Panel filter. Its important to note that the car most likely wasn't generating 149hp on this run, these home dyno runs were taken to produce a graph for comparison with later. (Ignore the automatic cutoff message - it does not effect these graphs).



On January 8th 2003 the car went into Crockenberg Motors and it was found that each cam gear was 1 tooth out. The power was back and the car ran beatifully (thank you valve adjustments). Before we could be sure though, there was one final step to take. Bring on the dyno graphs. An entire morning and afternoon was spent doing runs with varying intake configurations (refer to next article Comparing POD and Panel Filters). The following graphs show comparisons between the before graph (as shown above) and the after graph.


The above graph shows the comparison of before (blue) and after (red) torque.


The above graph shows the comparison of before (blue) and after (red) power.


Although the car may not produce exactly 167hp and 134 lb/ft, its obvious that there has been a substantial power and torque gain from fixing the cambelt timing issue. What else should we look for in these figures. Well first of all I guess is how smooth the curves are compared to their before counterparts. Changeover to MIVEC is a smoother affair and power delivery is very much a linear experience right up to the redline.

Another important thing to be revealed from these graphs is the peak torque outputs from the before and after graphs. With the cambelt realignment the maximum torque is produced by the engine a staggering 1000rpm lower. This is a significant change to the output of the engine as the extra torque is most useful as low in the rev range as possible.

The final thing to notice is that the MIVEC system really is a life saver in this situation. Notice how large the flat spot in power there is right before MIVEC kick-over point in the before graphs (this is extremely evident in the Myaree dyno graph), if we didn't have any MIVEC I doubt that the car would have produced much more than 120hp at its peak, with a steady decline from there all the way to 8000rpm. This may be what a GR owner might experience in this circumstance (anyone with a GR willing to put their cambelt on wrong for us to do a few dyno runs? I didn't think so).

I will be getting another Myaree Dyno graph soon, I will append that to this article, so check back.

Talk about this article in the forums.


Cambelt Diagram: This is sourced from the English translation of the FTO Workshop Manual.
Home Dyno Graphs: http://www.charm.net/~mchaney/homedyno/homedyno.htm


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