Check my CR sum assumptions
Discussion
Ive only guessed and still am at my compression ratio.
I realize there are many assumptions without proper measurements but i would appreciate comments on my sums. I am soon to be fitting 40 thou Cometic head gaskets and worried i may be heading a little too high CR wise.
My engine specifics are as follows.
4.6 landrover crossbolted block. This was fitted with top hat liners a couple of years ago by Chesmans in Coventry. I am pretty sure they said the block did not require significant decking.
So it’s a standard 94mm bore and 82mm stroke for 4552cc.
The pistons are 4.0 shallow bowl so sit at the same deck height as 4.6 but only have 13cc as opposed to 22 for 4.6 pistons.
The cylinder heads are TVR 500 big valve and extensively ported. They have been skimmed somewhat and the chambers were originally 28cc. I have polished the chambers and taken most of the ‘bump’ away to try and eliminate hot spots (seemed a good idea at the time).
I had previously used Elring composite head gaskets that I thought were 1.2mm thick fitted but some people think they could be thicker at maybe 1.28 or even more.
I have a pair of 40 thou Cometic MLS gaskets to fit and wondering if I may be taking my CR too high.
I am not sure at all how much is accounted for piston height
So here’s my Maths with a few assumptions
Base original spec would have been 9:35:1 i.e. 569.062CC swept volume per cylinder / 8.35 gives 68.151 CC. Take away 28cc heads, 22cc pistons, 8.88cc comp head gasket leaves 9.27cc for deck height clearance = 1.335mm.
Does this look feasible upto now?
If so then taking my numbers.
Previously on Elring composites.
569.062 swept volume + 28cc for the chamber, 13 for the 4.0 bowl, 8.88 comp gasket and the above 9.27 for piston height = 59.15 = Total 628.212 / 59.15 = 10.62:1
When 40 thou Cometics are fitted
569.062 swept volume + 28cc for the chamber, 13 for the 4.0 bowl, 7.05 MLS gasket and the above 9.27 for piston height = 57.32 = Total 626.262 / 57.32 = 10.93:1
I realize there are many assumptions without proper measurements but i would appreciate comments on my sums. I am soon to be fitting 40 thou Cometic head gaskets and worried i may be heading a little too high CR wise.
My engine specifics are as follows.
4.6 landrover crossbolted block. This was fitted with top hat liners a couple of years ago by Chesmans in Coventry. I am pretty sure they said the block did not require significant decking.
So it’s a standard 94mm bore and 82mm stroke for 4552cc.
The pistons are 4.0 shallow bowl so sit at the same deck height as 4.6 but only have 13cc as opposed to 22 for 4.6 pistons.
The cylinder heads are TVR 500 big valve and extensively ported. They have been skimmed somewhat and the chambers were originally 28cc. I have polished the chambers and taken most of the ‘bump’ away to try and eliminate hot spots (seemed a good idea at the time).
I had previously used Elring composite head gaskets that I thought were 1.2mm thick fitted but some people think they could be thicker at maybe 1.28 or even more.
I have a pair of 40 thou Cometic MLS gaskets to fit and wondering if I may be taking my CR too high.
I am not sure at all how much is accounted for piston height
So here’s my Maths with a few assumptions
Base original spec would have been 9:35:1 i.e. 569.062CC swept volume per cylinder / 8.35 gives 68.151 CC. Take away 28cc heads, 22cc pistons, 8.88cc comp head gasket leaves 9.27cc for deck height clearance = 1.335mm.
Does this look feasible upto now?
If so then taking my numbers.
Previously on Elring composites.
569.062 swept volume + 28cc for the chamber, 13 for the 4.0 bowl, 8.88 comp gasket and the above 9.27 for piston height = 59.15 = Total 628.212 / 59.15 = 10.62:1
When 40 thou Cometics are fitted
569.062 swept volume + 28cc for the chamber, 13 for the 4.0 bowl, 7.05 MLS gasket and the above 9.27 for piston height = 57.32 = Total 626.262 / 57.32 = 10.93:1
I'm not expecting a lot of answers to this.
Maybe this'll help.........
http://mathforum.org/dr.math/
,..... Sorry Derek.
Maybe this'll help.........
http://mathforum.org/dr.math/
,..... Sorry Derek.
Sardonicus said:
Its that piston/deck height I am struggling with Derek 
but here is what I have 10.66 compo's and 11.1 
Cometic's 
the heads I reckon I am not far off but guessing the deck is like pissing in the wind 
we are not far off though I reckon.
When i pull the heads i will feeler gauge under a flat edge to the piston at TDC. but here is what I have 10.66 compo's and 11.1 Cometic's the heads I reckon I am not far off but guessing the deck is like pissing in the wind we are not far off though I reckon. Maybe that will help.
Is there a way i can measure the cobustion chambers without the fancy equipment?
Sardonicus said:
db484bhpv8 said:
Is there a way i can measure the combustion chambers without the fancy equipment?
Not IMO if you have removed the bump and the inlet valves are un-shrouded than this can be offset from any facing/skimming you had carried out though 
Yes thats what i was thinking..... hoping.
Pack plasticine into the chamber. use a flat edge to finish. Then use displaced water method into a measure
Hi Derek, I don't think you can possibly go on assumptions TBH as you need to get the combustion chambers in a position [block face level] and measure using liquid as per Brummie on his heads picture like wot I did. For starters you begin with the idea that a 4.6 [450] TVR [RR] lump is 9.35:1 as stamped on the block but it isn't. I scratched my head when measuring my 'standard' set-up before changing pistons etc thinking I'd made a mistake and so got an engineering shop to check and it turns out I was right, the 'standard' CR as I found is closer to a shade under 9.0:1 and closer to 8.9:1 because of the valve cut outs in the pistons and the use of composite [not cometic] gaskets. Also, when my heads were skimmed it brought the cc down from 28cc to 27cc. But then you've added considerable volume to the heads by removing the 'bump' Don't really wanna be guessing mate. I think the above is a reason why bolt on FI options work so well on standard 450 lump. 
Edited by macdeb on Tuesday 16th December 21:17
Edited by macdeb on Tuesday 16th December 21:56
macdeb said:
Hi Derek, I don't think you can possibly go on assumptions TBH as you need to get the combustion chambers in a position [block face level] and measure using liquid as per Brummie on his heads picture. For starters you begin with the idea that a 4.6 [450] TVR [RR] lump is 9.35:1 as stamped on the block but it isn't. I scratched my head when measuring my 'standard' set-up before changing pistons etc thinking I'd made a mistake and so got an engineering shop to check and it turns out I was right, the 'standard' CR as I found is closer to a shade under 9.0:1 and closer to 8.9:1 because of the valve cut outs in the pistons and the use of composite [not cometic] gaskets. Also, when my heads were skimmed it brought the cc down from 28cc to 27cc. Don't really wanna be guessing mate. I think the above is a reason why bolt on FI options work so well on standard lump.
sounds about right Mac I came up with late 8's stock and I have the valve reliefs too.Edited by Sardonicus on Tuesday 16th December 21:24
Brummmie said:
My LS is 11.4:1 as I worked out from my chambers, I took the combustion chambers out to the head gasket, and de shrouded the area so flow is maximised at low valve openings.
You guys need to be over 10?
Ive done a little deshrouding but may do a little more when ive measured up.You guys need to be over 10?
Looks like my CR will be close to 11:1
Be nice to gain a few bhp. 341 bhp for 75bhp per litre
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