Calculating compression ratio - have I got this right?

[ratwing]

It's a 4.6 Rover, the block was decked after fitting top-hat liners and the heads were heavily skimmed too (damaged through something hard getting into the cylinder) and it'll only run without pinking on 99 octane and a maximum of 21° ignition advance.
The inlet manifold bolts were a bit awkward to screw in too which I now know can be due to the decking/skimming narrowing the side to side bolt hole distance.
It's a standard 3.9 cam, I've checked that it's timed in properly and the lifter preload is .020"
It's running on SU carbs, the needles were dressed to size on a rolling road but I think the mixture is slightly on the rich side.
I've measured the combustion chambers, 28.75cc and the piston bowl/cylinder at TDC, 28.2cc. The compressed composite head gasket is 1.2mm (8.3cc) or 1.02mm (7.1cc) depending on where I look, I was using Elrings FWIW. Bore is 94.04mm, stroke is 82.00mm.
On calculating, I was expecting something over 10:1 but depending on the gasket volume I get either 9.9:1 or 9.7:1 which doesn't seem particularly high considering it left the factory at 9.35:1 so are my sums right?

[minorv8]

Your math sounds about right.

But your timing figures are a bit odd. Are you absolutely sure that your true TDC matches the markings on the pulley ?

My previous engine combo was 4.6 with CR around 10.6-10.7:1, Merlin heads, Real Steel hydraulic Tornado cam. It´s got an aftermarket ECU and I have about 15 degrees timing at cranking, over 20 degrees at idle, 30 degrees at max torque and 32 degrees from 4500 rpm upwards. It runs on 95 octane without any issues.

[sidecar]

I've just done the maths using your figures and I get 9.7:1. I run 10:1 in my 4.6 and any engine that I build I tend to go for 10:1, I have no issues with detonation on normal unleaded so something is amiss with your setup!

I run a similar set of timing figures to MinorV8.

[ratwing]

Cheers for the replies guys, something must be wrong then. I'll check everything again today. If the cam timing was a tooth out, could this cause pinking (or hotter running which I put down to the retarded ignition timing)?

[sidecar]

Cheers for the replies guys, something must be wrong then. I'll check everything again today. If the cam timing was a tooth out, could this cause pinking (or hotter running which I put down to the retarded ignition timing)?

Not sure about that but I guess if the cam timing was out it wouldn't help! I run my 4.6 with a 74 degree stat which some people might moan about but my engine does not have top hat liners. (It is a red grade block). The running temp of my engine is about 80-85.

[ratwing]

I've checked the volumes again and got the same figures, I also tried checking the cam timing with a dial indicator on a tappet but found it difficult to get reproducible readings, however taking the average I got 2° difference at the crank so 1° at the cam which makes me think it's ok.
I'm completely out of ideas now so am thinking of putting the heads back on with composite gaskets and 0.5mm spacers, this'll take the CR down to around 9.3:1 - it might not help but I don't see that it'll make things any worse either unless someone knows otherwise?

[Ian Anderson]

What did the spark plugs look like?
Were they a coffee colour or black, or shiny?

It could be that your mixture is way out one way or the other which can cause hot running.

Also have you managed to get all air out the system so make sure no hot spots which also go towards pinking?

Ian

[ratwing]

The metal outer of the plugs (NGK BPR6ES) are matt black-ish and the centre ceramic is clean and white inside with a slight pink-brown discolouration where the centre electrode pokes out.
I'm pretty certain there are no airlocks anywhere, the manifold carb tower vent is clear and I've even replumbed the heater so hot water returns to the radiator instead of back to the water pump inlet - I've no heater matrix so it didn't seem sensible to dilute cool water from the radiator with engine temperature water.

[minorv8]

Since you apparently have the heads off check the tdc marking on the pulley. Does your timing light work properly ?

Which ignition system do you have ?

[sidecar]

There is no way that the issue is because the CR is too high, if you lower the CR then you will just loose performance and still not fix the issue. I think that it will either be a carb (mixture) issue or an ignition timing issue.

You could consider fitting an exhaust gas analyizer such as an Innovate LC1 which takes the guess work out of setting up the carbs.

[ratwing]

I've checked the TDC pointer/pulley marks and they're ok, I've made another pointer so I can see them (no.1 pointer is obscured by the remote filter and oil pipes) and used no.6 cylinder for timing but this should be ok?
The timing light is new, it's just a simple basic one with no dials, knobs, adjustments or readouts so I can only assume it works ok.
Ignition is a standard p6b distributor with a Lumenition electronic kit and different springs from the Real Steel kit to give maximum advance at 3000rpm, I tried bobweights stamped 13° and found they actually gave 28 not 26° advance (probably the holes/pivot pins/stops are worn?) so used them. I don't know what the idle figure is because I don't seem to have written that down. NGK BPR6ES plugs.
The carbs were set up on a rolling road and they welded threaded fittings into the exhausts for lambda sensors, the exhausts are black but not really heavily sooty so could be a bit rich?
I know I've measured the combustion chamber and TDC cylinder volumes but still have a nagging doubt, I know for a fact that quite a lot of metal was removed from the heads to get rid of the pits yet I measured 28.75cc when I thought they were only supposed to be 28cc to start with. I don't have any standard heads to compare mine with, is it possible that the heads on my bigger valves (41.4mm inlet, 35.5mm exhaust) account for this? Maybe they're thinner or have bigger recesses etc?

[sidecar]

When you 'buretted' the piston did you put it on TDC then stick a plate of Perspex over the cylinder then fill the space with paraffin or something like that? (if so you normally have to seal the piston to the bore with vasoline)

With regards to the head volume from memory were something like 26.5cc. (Mine are the older 14 bolt heads but with the Real steel big valves).

With regards to the AFR I don't think that you can really use the colour of the tail pipe these days because unleaded fuel always makes the tail pipe black. I really don't think much of 'reading the plugs' either although in your case the white centre does sound lean-ish.

[ratwing]

Yes, that's how I measured the piston/cylinder volume, I also turned the crankshaft slightly each way when the space was full of liquid to see if the piston was truly at TDC and it was.
My head castings are HRC2479 and googling gives a chamber volume of 29cc, I don't know if this is right though.

[sidecar]

It sounds to me like you have done all the right things, some people just throw a motor together without measuring anything which is fine if everything is standard but it is not fine in my humble when things are not standard. Your issue is not the CR but unfortunately I don't know what the cause of the issue is. Are you sure that what you hear is pinking?

[ratwing]

My initial thoughts were a high CR but having measured I realise this is not the case.
I think it's a pinking noise, the rolling road guy thought so too.
The cam seems to be installed correctly, preload is right and the roller rockers (they were cheaper than a new set of standard rockers!) have left marks across the middle of the valve stems so it looks like the valve gear is ok.
I've found the rolling road printout but it only goes from 2575 to 5178rpm (looks like I've lost the first page up to 2575rpm), it shows a line for AFR starting at about 12.5 and gradually falling to 11.9 so that looks ok.
All that's left is the distributor which is the one thing I didn't look at before taking the heads off
I can't really check for advance etc until the engine is back in one piece so I'm going to put it back together with composite gaskets and have a look at the timing.
Does this sound about right when doing the timing?
mark 28° on the pulley
disconnect & plug the vacuum advance pipe
run the engine to operating temperature
strobe and adjust the idle to 10° at 800rpm
run up to 3000rpm, if necessary rotate the distributor to get 28° advance
return to idle, if the advance has altered from 10° then change the springs/weights/stops and repeat the timing check.