Exhaust Cam Timing

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Plentywahalla
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Exhaust Cam Timing

Post by Plentywahalla »

It's been a few years since I started on the Excel resto (too many other projects). But she finally hit the road a month ago with a shiny new MOT for the first time since 2007. She was glorious until the engine trashed itself again. It started as a sudden change in engine note and slight loss of power. I slowed looking for somewhere to stop when it lost power completely with a tinkling sound. I banged the clutch in and rolled to a standstill. After a tow home I gave it a thorough check over. She would turn over freely but a compression check showed zero on all cylinders.

The exhaust cam could be seen to have slipped three teeth. Intake was still aligned perfectly. I whipped the head off and sure enough has eight S bent exhaust valves. Fortunately as I was only doing about 20mph when it finally let go there was only light marking to the piston crowns and no signs on the valve seats. I checked the piston heights and all appeared on spec. This was apparently the second time the exhaust valves had had an interference problem as I discovered when I stripped the engine initially.

The engine is not exactly standard. The car had been used for track days and has a gas flowed head, an equal length 421 stainless steel exhaust, L14 inlet cam and a re-profiled exhaust cam and vernier cam pulleys. I had clocked the exhaust cam and the curved almost exactly matched the L14 profile. After rebuilding the engine I took it for tuning to an 'expert' to set up the cam and ignition timing as I didn't have a dial gauge that would read into the cylinders. I said that I wanted to make best use of the set up, but to retain some driveability. The cam timing they gave it was 2 degrees advance on inlet and 2 1/2 degrees advance on exhaust.

The timing belt was slightly over tensioned which initially gave a whine but which eventually went away. I couldn't check the ignition set up without a running engine but the engine had felt strong, although I did think it sounded a bit tinny above 4,000 revs. So I am concerned about what has caused this belt slip and what to do different to avoid a third failure.

It does occur to me that the L14 profile has a steep ramp up and long duration. Combined with 2 1/2 degrees of advance the cam is trying to punch the valves open whilst the cylinder pressures are still very high way before BDC. Is it just asking too much for the relatively poor engagement between the belt and pulley. Close examination of the belt (Powergrip) showed a row of teeth that were abraded on the driving side of the teeth with raised fibres. I am tempted to revert to a 107 cam with zero timing, but presumably the valves interfered on the and of the exhaust stroke so effectively retarding exhaust timing make interference more likely if the problem actually lies elsewhere.

Richard
1988 Lotus Excel SE - newly on road
2004 MG TF - my sunny day driver
1991 Mercedes SL - owned 28 years
1968 Triumph Vitesse - Griffon Kit Car
1971 Europa type 54 - Type 47 restomod
2010 Volvo XC90 mobile dog kennel and tool box

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Lotus-e-Clan
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Re: Exhaust Cam Timing

Post by Lotus-e-Clan »

Sorry to hear.

Are you sure that the cam belt actually shifted, and it's not the vernier exhaust cam pulley that shifted on its bolts (or was not set correctly by the tuners)?

Interested to know what revs you were using (rather than road speeds) because at high rpm there is some stretch in the reciprocating train AND the pistons tip at TDC too which adds to the issue of valve head to valve pocket clearance.

The piston deck height is another variable, but assuming the piston deck height is standard (amount poking above or below liner tops), has the head been skimmed, which will effectively reduce the valve to piston valve pocket clearance, thus limiting how far advanced the cam can be set?

I always use "cam bucket lift distance at TDC" (in thousands of an inch) when tuning my Clan's race cam rather than degrees advance, so I'm not familiar with the method. But I guess those valve heads were possibly set with too much lift at TDC for the current clearances you have?
Peter K

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Re: Exhaust Cam Timing

Post by Pete Boole »

Sorry to hear this Richard. The L14 is still a pretty "mild" cam - barely any difference to the factory "104" and a couple of degrees either way will still leave it way off the pistons normally. Is there any evidence of anything being trapped in any of the pulleys? Small stones can get down to the crank pully fairly easily. Do you run a full belt guard? Did you measure the belt tension before you removed it? Struggling to think of anything that might cause the belt to jump 3 teeth other than a damaged belt.

Pete

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Plentywahalla
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Re: Exhaust Cam Timing

Post by Plentywahalla »

Thanks for the replies.

The belt has definitely slipped on the pulley. The timing mark for the exhaust is now way out and the vernier is on it's original setting. The belt was a Gates Powergrip and I know that there has been some discussion about cheap belts, but I thought that the Gates belts were the ones to have. I did have the standard belt guard fitted and I don't think there was any risk of trapped debris. The engine did sound a bit 'tinny' at high revs which could have been the start of interference. The rate of closing of a valve is limited to the rate of recovery of the springs so eventually the cam will lift off the bucket at high revs unless it's a Desmo type. I was trying to keep the revs below the critical point intentionally and was actually on my way back to the tuners to discuss and adjust the timing.

I do think that head skimming may be a contribution. I believe it has been skimmed twice and when I first stripped it I checked the combustion chamber volume with the sheet of perspex and the syringe method and, I can't remember the numbers of CC, but it was right on the limit. I've also measured the head thickness at 81.2mm and the combustion chamber depth at 12.35mm to 12.40mm. all of which shows that it has been skimmed to within a thou' of its life. However, I understand that this is all based on the original gasket thickness and the only head gaskets currently available are 0.5mm oversize. So I was confident that all would be OK. It was probably not a good idea to advance the exhaust valve timing given the loss of clearance from the head work.

Richard
1988 Lotus Excel SE - newly on road
2004 MG TF - my sunny day driver
1991 Mercedes SL - owned 28 years
1968 Triumph Vitesse - Griffon Kit Car
1971 Europa type 54 - Type 47 restomod
2010 Volvo XC90 mobile dog kennel and tool box

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Plentywahalla
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Re: Exhaust Cam Timing

Post by Plentywahalla »

Just a quick update on my bent valve drama. I think I have found the cause.

I had decided to junk the reground exhaust cam and got a new standard 107 cam, new valves and new valve guides. When I came to install all of it I discovered the old cam was almost seized in the cam carrier. It could only just be turned by hand and needed my hydraulic puller to get it out. The housings were OK in the bearing surfaces but badly scored opposite the annular oilways in the cam. There was plenty of oil in the cam carrier so apparently it was not a simple case of oil starvation, but appeared to be something abrasive in the oil.

I had all the alloy parts vapour blasted so all looked pristine clean when the engine was built up. I had pressure washed all the head and oil pump housing and cam carriers prior to re-assembly but I suspect that a plug of blasting media had been lurking around somewhere in the head and eventually got into the exhaust cam. I have re-dressed the housings and the new cam is now a good smooth fit. I hope it is going to be OK as it appears that it was only where the oil ways run that was damaged.

So maybe previously I have been over intellectualising the problem thinking about timing and clearances. The cam had probably just been seizing in the head and causing the belt to jump. I think I will just stick to the hot wash from now on and not worry about a concourse winning engine bay. Vapour blasting is not exactly a vapour but is still a fine silicate particulate that cause all sorts of problems in the wrong places. I am now worried about my Europa Twin Cam engine as that is now built up and looking very clean!
1988 Lotus Excel SE - newly on road
2004 MG TF - my sunny day driver
1991 Mercedes SL - owned 28 years
1968 Triumph Vitesse - Griffon Kit Car
1971 Europa type 54 - Type 47 restomod
2010 Volvo XC90 mobile dog kennel and tool box

Pete Boole
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Re: Exhaust Cam Timing

Post by Pete Boole »

Glad you seem to have got to the bottom of it. I get stuff vapour blasted and it takes ages to remove all the glass beads afterwards. Still a worthwhile process though - so long as you're prepared for the clean-up!

Let us know how it goes.

Pete

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bash
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Re: Exhaust Cam Timing

Post by bash »

Has anyone tried laser cleaning yet...no debris afterwards and the rubbish gets vaporised.

Bash
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Pete Boole
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Re: Exhaust Cam Timing

Post by Pete Boole »

bash wrote:
Sat Nov 19, 2022 12:11
Has anyone tried laser cleaning yet...no debris afterwards and the rubbish gets vaporised.

Bash
Not heard of that one Bash. Needs research!

Pete

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Alan_M
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Re: Exhaust Cam Timing

Post by Alan_M »

bash wrote:
Sat Nov 19, 2022 12:11
Has anyone tried laser cleaning yet...no debris afterwards and the rubbish gets vaporised.

Bash
I saw that on one of the car resto shows, either Mike Brewer or Tim Shaw doing it. Looked quite impressive.

Another option is soda blast which is water soluble so can be cleaned easily with a simple wash or more thoroughly in a sonic cleaner bath.

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bash
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Re: Exhaust Cam Timing

Post by bash »

Ive got a soda blaster and it really works, but the dust goes everywhere so I do it in the garden. It disappears with the hosepipe tho.
Bash
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Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.

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