Cam Timing, By Lift At Tdc

[Sandy 191]

 

This is probably the thing I'm asked to explain to people most often, so a rough guide is long overdue. I have based this on an XU 16v of mine, with long duration cams; so I'll detail some of the differences on other engines at the end, so be sure to read through! I don't recommend you attempt jobs like this unless you are pretty technically savvy and able to protect yourself from making errors. I cannot and will not take any responsibility for your ability to get things wrong; this is just a guide and not a failsafe, comprehensive one at that.

 

Introduction:

To attempt this, really you need to already understand the four stroke cycle and have some concept of valve overlap at TDC (Top Dead Centre, the top of the piston stroke). If not, then read up on it! But in basic terms, the fours stroke cycle takes two crank rotations to complete, but the cams only rotate once over the same cycle. If you focus on one cylinder to make it easy to understand, it will come up to TDC twice; once on the firing stroke with all valves closed and once on "overlap", when the the exhaust valves are closing at the end of the exhaust cam profile and the inlet valves opening at the start of the inlet cam profile. The "overlap" is what I'm talking about here.

The traditional way to time a cam or cams, is using the peak lift point and how that relates to the crankshaft angle, for example an inlet cam might have its peak lift set at around 110 degrees ATDC (After Top Dead Centre) and the exhaust cam a similar angle BTDC (Before Top dead Centre). That's quite an easy way to understand, but not hard to get wrong and since we're often running very tight valve to piston clearance at TDC or even valve to valve in some engines, the lift at TDC is a very sensitive value, that we often need to know, to be sure the whole lot doesn't clatter together when it's running. It's also an extremely accurate way to time cams, once you're competent at it, the engine's sweet spot can sometimes be sensitive to as little as 5 thou lift at TDC (0.127mm), about half a degree or the width of a human hair. Achieving accuracy like that with lift at TDC is much easier in my opinion.

 

Before you start

You will need a dial test indicator gauge, with around 10mm minimum stroke, a good adjustable magnetic stand for it and a nice stout piece of mild steel/iron to bolt to the head, to put the magnetic stand on. Plus all the usual tools.

Best to practice this method with an engine that's already together and working, I'll describe how to first fit cams using this method later. Bear in mind, with number one cylinder (flywheel end) on overlap, the lobes on that cylinder will point almost towards each other on a TU engine (rear facing induction) like so:

 

On XU and EW engines (forward facing induction), with number one cylinder at TDC on overlap, the lobes will point away from each other and down, as shown later on in this guide....

 

Method:

When timing a belt driven engine, it makes the most sense to only take measurements by turning the crank in the normal engine rotation, clockwise at the belt/pulley end for all modern Peugeot engines. The point of this is to put the load in the belt as mostly occurs when running. As long as you have fixed tensioners (spring loaded ones are not advisable for tuned engines), you can usually safely turn the crank backwards, but only to go back, always measure going forwards.

The first and most important reference, is a good, accurate TDC mark. Standard TU flywheels have a highly accurate locking slot for TDC, but others will need TDC to be found.

Remove both cam covers and all four spark plugs. Turn the crank until the lobes on number one cylinder (flywheel end) are both roughly equal pushing on the followers (eg exhaust closing, inlet opening). That cylinder should now be near TDC. I use a specially adapted dial gauge to find TDC, but since you probably don't have that, you'll need to initially feel for it, using say a long 1/4" drive extension down the plug bore, while turning the crank (on 8v engines etc with inclined plug bores, take extreme care not to trap/crush/bend the tool doing this, be careful!)

Once you've felt the piston is at TDC, with the dial gauge/stand solidly and closely mounted on the bit of steel/iron, bolted down; you can support the extension tool and position your dial gauge tip in it, to use the gauge to find the highest point of the piston. There will be some "dwell" at TDC, so make it more accurate by checking the slight drop either side of TDC, to confirm the middle. It is pretty tricky to do it this way, TDC finder tools can make it easier and an adapted dial gauge like mine, makes it MUCH easier!

 

Standard TU flywheels have that locking slot at TDC, which is accurate and makes it a lot easier, but still advisable to check and qualify TDC, sometimes balancing drillings can give a false position!

 

Once you're confident the crank is at TDC on number one, paint mark the flywheel. This is also useful for checking ignition timing alignment later!

 

Now, in the spirit of always measuring in the direction of engine rotation... turn the crank backwards, until the inlet cam lobe lifts off the follower on number one, as in valve closed. You now need to set up the dial gauge, with a bit of suitable bent rod screwed into its tip, on to that cam follower, with enough compression of the dial gauge, that it won't run out of travel as the follower drops when you turn the engine to TDC. Also try to get the dial gauge mounted as close to the axis of follower travel as possible, for best accuracy and to minimise skidding of the bent rod:

 

 

Be careful also, that the bent rod won't fall off the follower or foul the surrounding casting or cam itself as it goes. This might take a little patience to get right and the value of a solid and well mounted dial gauge stand, will probably be obvious now.

Once it's all settled down, zero the dial gauge there (this is why I like to use a digital gauge, analogue gauge users will need to count rotations). Now turn the crank slowly towards TDC, until you see it reach your mark. On a digital gauge the lift will be displayed, you will have needed to count the movement on an analogue gauge.

 

 

The engine shown has long duration cams which means big lift at TDC numbers, standard 16v cams will usually be less than 1mm, fast road cams maybe 1-2.5mm, depending on spec. I recommend you turn the crank back until the inlet valve is closed again, check you zero point is still zero (adjust if necessary) and repeat tow or three times, so you know you have a repeatable value and get a feel for it.

Once back at TDC for the last time, moved the dial gauge over to the exhaust follower opposite only compress it slightly (to allow room for a rising exhaust follower) and zero it.

 

 

Now continue turning the crank beyond TDC until the exhaust cam lobe comes off the follower and the valve is closed, the dial gauge will read the exhaust lift at TDC (just backwards). Again, it's advisable to go back before TDC, forward to TDC (to keep rotation correct) and repeat this several times, to get a figure you're confident in.

 

All being well, you now have a set of inlet and exhaust lift at TDC figures!

If you want to change the timing, I advise doing it slightly away from TDC, so that risk of valve to piston contact is reduced. With vernier pulleys, slacken the bolts on the pulley you wish to adjust and very carefully turn the cam by the centre bolt if necessary or spanner flats on the cam if it has them:

* Inlet cam is turned with engine rotation to increase lift at TDC (Advance), or vice versa.

* Exhaust cam is turned with engine rotation to reduce lift at TDC (Advance), or vice versa.

 

How much lift at TDC?

The main thing you need to know, is how much safe room is there, in terms of valve to piston contact or valve to valve contact.

Valve to piston (VTP) clearance I'll cover fully in another guide, but you really ought to check during assembly, how much valve vertical travel there is, with the piston at TDC, then subtract safe VTP clearance from that, to give you a maximum safe lift at TDC figure on each side. Typically, 1.5mm inlet and 1.8-2.0mm exhaust clearance over the lift at TDC, is the minimum. That clearance will be reduced when running, dramatically and the consequences of contact are dire!

Valve to valve (VTV) contact again needs to be checked at the assembly stage and around 1.5mm clearance between the valves as they pass, is very close, since they wobble in the guides, don't faithfully follow the cam profiles and grow in size when the engine is running at speed. TU engines aren't that close really, but it's a great way to wreck high spec XU and EW engines.

Cam timing is a good way to tune the delivery of the engine and a great way to ruin it too. Lift at TDC is not going to keep making the engine better as it increases. Most cam manufacturers will give advised settings as a guide, but be aware that it may not mean they are safe and rarely optimum. Also be aware that some quoted figures are off the base circle, so the valve shim clearance with solid lifters in that case, needs to be subtracted.

 

New cam fitting, timing lift at TDC

In this case, it's advisable to start without cams fitted, mark TDC before you start, turn the crank so the pistons are all at half height in the bores with 1 and 4 rising with engine rotation, cambelt and rollers ready, but belt off. Best to fit the inlet cam first, with the lobes on number one just touching the followers with engine rotation, as if starting to open them; bolt it down, then turn the cam back until the lobes on number one come off the follower slightly, so those valves are closed. Now fit the exhaust cam with lobes just touching the follower on number one, as if rising off with engine rotation (mirror image of inlet on that cylinder), bolt it down, then turn the exhaust cam forward, so the lobes on number one are just off the follower. Fit the belt back cover (if used) and pulleys without moving the cams. On TU engines, the number one inlet and exhaust lobes should point towards each other and up slightly; on EW and XU engines, point away from each other and down slightly.

Now set the dial gauge up with sufficient compression on the number one cylinder inlet follower and zero it. turn the inlet cam forward until the gauge shows the desired lift at TDC, best to turn it back to zero again, then forward to settle the bent rod and check you've done it right, it should then happily sit where you've left it. Move the dial gauge to the exhaust cam and zero it, with enough compression to give range for the lift at TDC, turn the exhaust cam back until it nearly shows the desired lift at TDC, but leave it a bit short, because it tends to overshoot when the belt is fitted and tensioned. Again a good idea to repeat, to be sure.

Time bring the crank round in direction of rotation, so that number one reaches TDC, best to check that piston is rising as you do, because otherwise the pistons will hit open valves on cylinder 2 and 3! Slacken off the bolts on the vernier pulleys and feed the cambelt on (without moving the crank off TDC). Try to position the belt on the pulleys, so they are near the middle of the adjuster slots, to give room to trim it later. Tension the belt, check the crank is still at TDC, then tighten the vernier bolts sensibly (not over-tight, 7-8lbft is usually advised on M6 bolts with thread lock).

Now go back to the start of the guide and check/adjust lift at TDC as described for a built engine.

 

Variations!

8 valve: Assuming the cam chosen is a sensible choice for the engine spec and sufficient valve to piston clearance has been established; concentrate on inlet timing as the priority (it's more critical as a rule), but check the exhaust lift after as on 16v engine above, so that you have the figures for both inlet and exhaust. When the cam is "split", if the cam has identical inlet and exhaust profiles, lift at TDC will be equal. That's a good starting point and with a rounded spec engine, best performance will generally occur with one or two degrees advance ahead of that. Things can get weird with mis-matched spec engines! On TU 8 valvers, you'll have to use a flat section of upper valve retainer to register the dial gauge rod on, which can be tricky and might take a few passes to get right.

 

Forward facing or rear facing induction: I have tried to make it clear above, to pay attention to the relative cam lobe angles you'll see with each engine type. Getting that wrong will do harm, so be sure!! Forward facing engines, cam lobes point away on overlap. Rear facing, cam lobes point towards each other (roughly).

 

Hydraulic lifters: With bucket tappets, no problem. You are dropping the dial gauge on a solid and faithful component of the valve train, so you can proceed as normal. Many modern engines now have hideous pressed steel roller rocker followers, with a hydraulic lash adjuster on the pivot end. There is no practical way to set up lift at TDC in the normal way on these engines, because the hydraulic part will relax as you try to do it, softening the readings. The way I set up these engines,is complex and beyond the scope realistically, of DIYers.

 

VVT systems: Generally engine builders either junk cam phaser pulleys, lock or machine them to limit the maximum swing to a safe range and start with standard locking pins etc. VTEC style systems with more than one profile require bespoke solutions to time this way and are beyond the scope of this guide.

 

I hope that makes some sense, but if in doubt at all, speak to someone who knows what they are doing, before you proceed to the scene of the accident. Remember: Assumption is the enemy and precision is your friend.

[boldy205 75]

Interesting stuff! I've had a few beers now, makes sense almost, will try it with out beet tomorrow!

[johnnyboy666 195 1 Cars]

Excellent write up, that's cleared up a few things for me. Appreciate you taking the time to put this together for us

[petert 586]

Excellent work! Are you going to cover the LCL method next? Interesting bent pointer tool. I use a cut off, small, long series allen key, silver soldered into a cap screw. It's thin and rigid, so it slips down the side of the lobe.

[Sandy 191]

Cheers I will try and detail later how to measure cam profiles, to get the necessary data, so you can work on angles by this method. I keep meaning to make a better pointer, alloy welding rod has served me well so far!

[petert 586]

Here's some of my tools. The TDC tool screws into the plug hole. The long, large diameter rod helps to eliminate rocking. As Sandy said, you need to verify TDC by measuring the lift either side of your "guessed" TDC. On XU engines I use a flywheel to do my cam timing, as per this:

 

http://www.taylor-eng.com/xu9j4/finding_tdc.html

 

If you can count and divide by 6, the large diameter minimises error that may otherwise occur with a small protractor. Plus everybody has one!

Edited August 3, 2015 by petert

[ALEX 98 1 Cars]

Useful article. Always fancied playing around with the timing on mine ( Even tough it's currently stock). After reading this its given me confidence to give it a go.

Maybe you can advise further?.

My engine (1.6 xu 8 valve) has a machined head (light skim. the machinist said it was just a clean up,) but saying that there's no way of knowing if it's had a light skim in the past on top of that.

As measuring compression ratios isn't easy and listening to advice from Dave Baker? (IIRC Pumaracing) an increased comp ratio adds drivability and performance. Which sounded feasible, so I fitted the head with the original spec gasket (Not a thicker one).

Now thinking about the mechanics and the importance of TDC and Cam timing relation, a machined head would affect the cam timing as the centre distance of the belt is shorter. Am I correct or is this insignificant to cause the symptom I'm about to describe.

The engine has a rorty sound, as if combustion is happening in the exhaust. also the engine runs hot. the fan keeps things cool, but it was close to overheating at the weekend sat in traffic.

Tick over is just over 1000 rpm, this is the sweet spot I need (even with a brand new AFM fitted) to stop it from stalling,

Adjusting the advance by retarding the distributor still gives the rorty sound in the exhaust i.e. ignition advance seems unrelated to the problem.

Edited August 3, 2015 by ALEX

[petert 586]

As a guide, a 1mm skim from an 8V head requires the cam to be advanced approx. 4 degrees (crank), to re-establish the original position.

[unariciflocos 49 1 Cars]

Peter, I have one of your stage 2 cams with a no. 4 pulley and my block is decked 0.6mm, but I've done the timing by using the standard locking for the pulleys and crank. Would I be correct to assume that in this config I wouldn't need to adjust the cam timing?

[petert 586]

A very good question. You'd assume that the two pulley system of the 16V engine would equalize the tension on each side, but it's not the case. Whilst not as bad as an 8V, you still need to advance the timing slightly to compensate for any reduction in head gasket thickness, skimming or decking. A #4 pulley is advanced considerably however and it will have the inlet timing near the correct position. The only way to confirm is to measure the lift @ TDC as described above, or use the lobe centre method. You should be aiming for 0.105" at TDC, which equates to a lobe centre line of 106 degrees.

[ALEX 98 1 Cars]

That Much!! That could explain a lot!

I always assumed the higher compression was causing the problem.

Time to buy a vernier pulley I think.

[scubacal 0 2 Cars]

very useful article, thank you for sharing

[wicked 103 3 Cars]

Method:

When timing a belt driven engine, it makes the most sense to only take measurements by turning the crank in the normal engine rotation, clockwise at the belt/pulley end for all modern Peugeot engines. The point of this is to put the load in the belt as mostly occurs when running. As long as you have fixed tensioners (spring loaded ones are not advisable for tuned engines), you can usually safely turn the crank backwards, but only to go back, always measure going forwards.

The first and most important reference, is a good, accurate TDC mark. Standard TU flywheels have a highly accurate locking slot for TDC, but others will need TDC to be found.

Remove both cam covers and all four spark plugs. Turn the crank until the lobes on number one cylinder (flywheel end) are both roughly equal pushing on the followers (eg exhaust closing, inlet opening). That cylinder should now be near TDC. I use a specially adapted dial gauge to find TDC, but since you probably don't have that, you'll need to initially feel for it, using say a long 1/4" drive extension down the plug bore, while turning the crank (on 8v engines etc with inclined plug bores, take extreme care not to trap/crush/bend the tool doing this, be careful!)

Once you've felt the piston is at TDC, with the dial gauge/stand solidly and closely mounted on the bit of steel/iron, bolted down; you can support the extension tool and position your dial gauge tip in it, to use the gauge to find the highest point of the piston. There will be some "dwell" at TDC, so make it more accurate by checking the slight drop either side of TDC, to confirm the middle. It is pretty tricky to do it this way, TDC finder tools can make it easier and an adapted dial gauge like mine, makes it MUCH easier!

 

...

I hope that makes some sense, but if in doubt at all, speak to someone who knows what they are doing, before you proceed to the scene of the accident. Remember: Assumption is the enemy and precision is your friend.

 

Regarding finding TDC; because of the "dwell" at TDC, I do use a timing discs next to the gauge to find the TDC.

To get a more accurate TDC, I try to find the point where the piston is, let's say 20mm before TDC, using the gauge with an extention on the pistion. Mark that point at the timing disc.

After that you try to find the point where the piston is 20mm after TDC. Mark the point at the disc again. Make sure that you turn the crank backwards to go from 20mm before TDC to 20mm after TDC, so that you don't need to remove the gauge when going past TDC. Turning backwards is not a real issue in this case, because the timing belt is not involved in the measurement. When you're close at 20mm after TDC, go a bit beyond that point and try to find it while turning the crank in the normal direction.

 

Now you can calculate from the markings on the disc where the TDC is. (x is (reading2-reading1)/2 )

 

​

 

From the picture you can see that you'll get a smaller error in the rotation/degree error if you measure on a steeper edge iso measuring on the top of the 'hill'.

 

Same reasoning applies when you measure lift of a cam @ TDC and don't try to find the point where the lift is max.

 

Great article!!

[ALEX 98 1 Cars]

Makes sense.

 

Anyone know the best place to buy a vernier pulley?

Tried buying a BBM one, but hes out of stock untill September.

[welshpug 1,657]

for which engine?

[ALEX 98 1 Cars]

8valve xu

[Sandy 191]

Piper in stock last time I checked.

[petert 586]

I'm not which one I'm talking about, but one on the market has a hole for the 10mm timing pin. It makes it dead easy to get the cam in the right zone before you start to tweak it.

[ALEX 98 1 Cars]

Bought myself a DTI and went for a second hand Mitutoyo one for £30.

There's loads of cheap ones on ebay (with stands) for £15 -£20. I'm assuming these are no good?

I did buy a cheap stand for the DTI for about £10. the quality looks good but it was a gamble.

Would you guys recommend a branded stand or you think I'll be ok?

 

 

Just need the pulley now .... and some free time.!