Combustion Chamber Volume tolerances

[ysmalkie]

I am preparing my head for assembly and did a few measurements today.

I measured today my cc volumes and came up with the following results:
98.6
99.4
99.5
99.6
99.5
98.0

I was wondering how significant is that difference and is there anything I can do about it?

I guess I would have to measure the block volume as well before any actions?

Tadek

[Tom W]

How significant it is depends on what you want to do with the engine. If you’re squeezing every last bit of power from the engine, and plan on driving it at full throttle a lot of the time (or going racing), then it’s more significant than if it’s a gently driven road car.

Did you measure the head before it was reworked? The depth the valves are cut to will affect the figures you get, as will anything that’s been skimmed off, though this should affect everything evenly if it’s been skimmed evenly. Given that the values are lower at the outer ends, do you know if the head was warped before it was skimmed, and has been skimmed to level it out?

You’ll also need to measure cylinder volumes, both at BDC and TDC to make sure you get the same swept volume and compression ratio. There’s more to it than just getting everything to match too, you should be working towards a specific compression ratio and compressed volume.

I forget the figures now, but there are recommendations for maximum tolerance on variation of combustion chamber volume in several books on building race car engines. Getting everything to exact factory spec is the basis of blueprinting, I.e. removing the factory tolerance and setting everything to the maximum/minimum value as appropriate.

Before doing anything, if measure again a few times and take an average reading. Your variance could just be down to measuring.

[abowie]

You're looking at a 1.5% spread across all six. I'd call that pretty good, and in fact it's probably within the error of the measurements you're making anyway.

I seriously doubt that it will have any effect on your engine at all.

[christopher storey]

[ysmalkie]

Well, I did measure it 3 times and the figures were within 0.2cc. So I guess it's not the measurements.

I have the measurements of the valve stem height (the part where the spring is, and they read as follows (in mm):
Spring height space - Inlet 29.1 29.26 29.37 29.17 29.41 29.4
Spring height space - exhaust 29.32 29.5 29.48 29.31 29.52 29.49

or in in.:
Spring height space - Inlet 1.146 1.152 1.156 1.148 1.158 1.157
Spring height space - exhaust 1.154 1.161 1.161 1.154 1.162 1.161


I did this in order to find out if I need to shim the springs.
This tells me that the 1st inlet valve sits lowest, but the 4th one is pretty low too, so there is no pattern.

In order to gain 1.5cc, I would have to cut the inlet and exhaust seat by about 0.1mm (or 0.004")


I have no way of telling how was the head before as I did not measure it, also there was work done on it before, as the head is signed by a fellow called Dan Donovan with a date 1977 and a place (San Marcos) probably for a fellow called George Sanders, as his name is also on the head.

However, if the head was warped, taking into account dome diameter of 83mm, it would have to be warped by about 0.05-0.07 mm (0.002"-0.003") which I guess is very much possible..

Since I cannot measure the bottom end as it's not ready, I thought of leaving the head as is and once I get the complete picture of the total volume, I thought of actually equalizing the cylinder head volumes by skimming the flat portion of the piston top.

What do you think??

To be quite honest, this is the first engine I am building and I would like to do it right :-)

Tadek

[abowie]

On rebuilding a head my machinist always replaces the valve seats so that, when I shim them, the range of shims is well less than 10 thou.

With simple calculation, on a 1.75" valve 1cc difference is equivalent to about 0.2mm or about 8 thou so that is probably the reason for your differences.

Again, I think it's largely irrelevant. To be honest, when I cc a head I only ever measure one chamber.

I think your head is fine.

[christopher storey]

Under no circumstances should you take any material off the top of the pistons . With all due respect, you seem to be treating this engine as though it is a RR Trent or a GE90 high by-pass ratio turbofan with sub-millimetric clearances of parts running at temperatures well in excess of their melting points . It is not : these engines, despite their sophisticated specification, are in fact quite agricultural, which is one of the reasons why they still give wonderful service more than 70 years after they were first produced . The best money you can spend when rebuilding an XK is to have all the reciprocating and rotating components professionally balanced

[ysmalkie]

So what you are trying to say (diplomatically and gently) is that I am slightly overdoing it??.. :-)

Tadek

[ysmalkie]

One more question - should I use the valve stem seals?

I can, the valve guide is prepared for it.

Tadek

[malcolm]

I'll be interested in the answers! I know some swear by them, the respected Rob Beere uses them, my cylinder head was done by CMES who are used by Angus for all his heads I believe and they told me don't use them, Thruxton Jag say no, so I'm not sure what the answer is!

[abowie]

One more question - should I use the valve stem seals?

I can, the valve guide is prepared for it.

Tadek

I always have the valve guide machined for a modern seal and use them. I can think of no advantage to not fitting them.

Note that I am referring to something like this, not the o rings that come with the VRS kits.

[abowie]

So what you are trying to say (diplomatically and gently) is that I am slightly overdoing it??.. :-)

Tadek

I think so. Make some broad assumptions on the volume of the top of your cylinder and piston at TDC. I know that what I am about to say isn't truly accurate, but I think it stands to compare the difference in volumes you have measured.

Lets guess that with the dome it makes -25 cc, that is it protrudes into your combustion chamber by 25 cc.

Furthermore let's assume the total cubic capacity of your engine is 4.2l, so 700ml per cylinder.

If you then calculate an ideal compression ratio:

CR = total cylinder volume divided by combustion chamber volume at TDC
SO for your biggest measurement CR = 700/(99.5-25) = 9.395:1
And for your smallest CR = 698.5/(98-25) =9.568:1
So the difference is 0.131 which is bugger all.

If, again making some broad and not entirely accurate assumptions,but which again I think are valid in comparison if not in fact, and you convert your CR to cylinder cylinder pressures you get 139 psi vs 136 psi.

[christopher storey]

Andrew : I think there is a small error in your calculation,. but it does not affect the principle which you are setting out. The static CR calc needs also to take into account in the pre-compression volume the combustion chamber volume. So we are not talking about 706cc but 706 + 99 cc= 805 ( in practice we should also take account of the volume between the top of the piston at TDC and the top of the block, but let us ignore that). There is also the complication of these being domed pistons, but again ignoring that the theoretical CR is 805/99 = 8.13 or 804/98 = 8.20

[abowie]

Yes.

As I said there are a number of arguable broad assumptions, but it's about comparison not absolutes.

I've allowed for the domes (-25cc which is accurate for 9:1 pistons) and the choice of dividing 4.2 by 6 or making it 4.2 by six plus head volume ( plus or minus dome volume) is again, arguable.

I've also ignored adiabatic heating, leak, and other factors that would affect true compression.

Either way, the difference is really not significant.

[ysmalkie]

Thank you all - this has been very helpful.


Tadek