Changing compression ratio

[carmadmike]

Hi I have a 2500s engine in my vitesse. It is made up from a Vitesse Mk2 block which has been decked to the top of the pistons, 2.5 crank and pistons, and a 2500s head.

I would like to change the comression ratio from 8.3 to1 to 9 to 1 to work with a new camshaft I am fiiting. the camshaft is a Chris Witter cw3021 camshaft simular to vitesse mk2 and 2.5pi but with inlet lift increased from 232 to 261.

Is it possible to do this by skiming the head.
How much metal can you take off the head before the valves will hit the pistons.
Do the inlet and exhaust valves have to be the same level as the inlets stick out more on mine
Thanks in advance for any advise Mike.

[James]

You can take off easily enough to bring it from 8.3 to 9 — shouldn't be a problem at all — it's not much.

Give it to any competent machine shop and ask them to skim it to 9.

As the pistons are zero'ed there shouldn't be anything else they need to know.

[carmadmike]

Hi James thanks for the reply.
Do  you know if the valves should be both level as the inlets stick out futher than the exhaust in the combustion chamber.
I only ask as I took the head to someone who was recommended by a local machine shop to calculate the compression ratio and he started talking about having the block decked futher and fitting high copression pistons. He also mentioned about the valves and said they need renewing at more cost.

[Nick Jones]

Mike,

Valves should be level.  If the exhausts are lower than you may be seeing the effects of seat recession from unleaded fuel.

The correct, accurate way to get the right CR is to first have any needed valve work done, then measure the chamber volume (because valve height has a BIG effect).  Then you need to work out the area of the chamber opening to enable you to work out what thickness needs removing.  HG thicknessed (compressed) and deck height also needs to be considered.

I suspect you'll need to do the calcs yourself - I've never found a machine shop that would do it!

Nick

[carmadmike]

Hi nick the head was suposed to have been reconditioned with unleaded valves and it has only done about 1500 to 2000 miles. Would the valves have seat recession in such a low mileage.

[Richard B]

carmadmike wrote:

and he started talking about having the block decked futher and fitting high copression pistons.

No such thing as high compression pistons. All Triumph 2.5 pistons are flat topped.


The CR for a PI is 9.5, If you have an S head then skim to 3.400 from 3.475 to achieve this. If you want more speak to Andy Thompson he has a lot of practical experience and I recall him talking about 9.8 or was it 10:1 Watch out for pinking though with UK fuel.

If the exhausts are receding I would have inserts fitted asap (as you have the head off). I believe you can have just the exhausts done as the inlets do not have as much of a problem.

Course then you are looking at new guides, and valves.....  


2.5 PI + TR5/6  All MG, CP, CR  9.5:1

2500 TC + S MM 8.5:1



Sorry just re-read your post. Unleaded valves on there own wont help, its the inserts that you need.

[Nick Jones]

Shouldn't have!  But does it have inserts fitted (see pic, look for the ring around the exhaust valve - that's a hard insert)? Because unleaded valves on their own won't do it - the head is the softer part and freshly cut seats are more susceptible to recession than old, well-hammered ones!  Even then I'd not expect noticeable recession in that mileage.

Nick

[carmadmike]

Hi Nick I got it wrong. its the exhausts tha stick out futher than the inlets. I cant see the ring around the valve like yours in the picture but it may be behined the vavle will have to take it out to see.

Ritchard are those figures for the over all depth of the head. 3.475 being standared and 3.400 to give a cr of 9 to1

[Richard B]

2.5 PI + TR5/6  All MG, CP, CR  9.5:1  3.400" total height of head from face to rocker cover

2500 TC + S MM 8.5:1                        3.475" total height of head from face to rocker cover



So if you wanted 9:1 (at a guess) I'd go for roughly half that.

[carmadmike]

Thanks Ritchard for the figures but going back to the valves the exhaust sticks out more than the inlet about 0.05 thou on average over the six valves will this have an effect on the cr or a just minimal effect.

[piman]

Hello Mike,

as Nick says, the only way to do this properly is to measure the volume of the chamber.
As the inlets are lower, I would check the seat area as you are looking for a performance increase with the camshaft. (Is this the cam that CW recommends his special inlet valves?) The seat should be quite narrow and it benefits airfow if the seat has a much steeper angle below the valve seat to reduce shrouding of the valve.

Alec

[JohnD]

It will have an effect on CR, small but not minimal.
You can estimat what it will be by measuring the diameter of the valve, using V=Pi x R^2 x H and pulling that volume out of the CR equation.
It's your choice - get a machine shop to recut the exhaust valve seat to lower it.

As Nick suggests, NEVER rely on anyone saying you can skim X from a head, and NEVER, EVER, "roughly" anything! - you cannot know if someone has already skimmed it.   Always measure and calculate.  You do have a lot of scope.   The six cylinder heads of 2000 and 2.5 engines were basicly the same casting.  They skimmed a LOT off in the factory for the 2000, until they started to use domed pistons that allowed the same heads for both engines.

You can build a 2000 with dome tops, I'm doing that now, but you need to expand the chambers a LOT!

Skimming until the valves hit the pistons would raise the CR too high for petrol, unless you have a wild camshaft, with high lift and a very wide overlap, that doesn't suit a Triumph anyway, or dome tops.    

John

[carmadmike]

Went to a local machine shop today and they measured the head for me. It was 3.510 thou. They couldnt calculate the cr for me.
Richard you quoted 3.475 for a 2500s head it seams mine is bigger and has the number on the head is 219016 which is a 2500s head I think. Any one know why my head is bigger and it also has been skimed as well.

Alec I had the chambers measured by a enginering company they charged me £60.00 to do it. They measured the head with the old head gasket and asked for the bore size as well as the stroke. He says the cr is at 8.3 to 1. It doesnt add up if a standard 2500s head is 8.5 to 1. My head has been skimed, My block has been decked, the bores are 20 thou O/S so it should be higher than a standard cr shouldnt it. They also said about a lot of other work like
Decking the block further so the pistons are higher than the block, New high compression pistons, And new valve seats. He didnt recommend skiming the head Now I am very confused.

[carmadmike]

Just been looking at another thread headed cylinder head identification dated 20/1/2009 by markdetriomphe. My head has the same numbers on it (219016 stamped / 313247# cast / and v3297 cast   and is the same size as his 3.510. So it looks like my head is a spaires stock head and this is why it is 3.510 and bigger than a 2500s head. Am I right in thinking this.
Thanks Mike.

[Nick Jones]

£ 60.00 to measure the chambers.......  :o  I'll do it for £30!

Head height - that's odd.  There is a listing for a head that starts off at 3.535 thick but definitely an oddball (TR6 Africa or some such) and not a 219016 number.  Does the number look original - the 219016 head attracts a premium in some quarters so might get re-stamped....  :-/

Proper S 219016 should start at 3.475 as already stated, but it won't have grown!  Worth checking the measurement maybe?

see http://www.triumphclub.co.nz/?page_id=653  for head no.s  (towards the bottom of the page)

As regards how to raise CR from this point, if your block is already decked to bring the pistons flush you do not want to go any further in that direction, you've only got the compressed head gasket thickness left less rod-stretch less fear factor - not alot!  Pop-up pistons is for the racers.  Not good advice IMO - be wary.

There should be plenty of scope to skim that head - the same basic head casting was used for 2L.  I have a 219016 on my 2L Vitesse.  This is skimmed to give 9.5:1 with an undecked block and some chamber work.  Had to remove something like 4mm!  It's thin now and I wouldn't want to take any more off, but you won't need to go anything like that far with a 2.5.

Cheers

Nick

[Gt6s]

There are three depths of cylinder heads thinest is the flat top 2ltr next is the thicker 2.5 and dome top piston 2 ltr engine. Thickest head of them all is the late 2.5 (2500 S). These three heads have their specific length of push rods.

[MarkDeTriomphe]

carmadmike wrote:

Just been looking at another thread headed cylinder head identification dated 20/1/2009 by markdetriomphe. My head has the same numbers on it (219016 stamped / 313247# cast / and v3297 cast   and is the same size as his 3.510. So it looks like my head is a spaires stock head and this is why it is 3.510 and bigger than a 2500s head. Am I right in thinking this.
Thanks Mike.

I have to confess that I re-measured that head again with some verniers instead of the wooden stick I used first time!  It is actually 3.475" - so spot on for an S head.  So I'm afraid I can't confirm the theory of spare stock heads - sorry about that  :(

[JohnD]

carmadmike wrote:

Alec I had the chambers measured by a enginering company they charged me �60.00 to do it.

Jesus, mike, no wonder you are confused.  You're suffering from lightwalletitis.
Burette - £10 (or a 20ml and a 2ml syringe much less, all from eBay)
Square sheet of glass (1/4" plate, drilled and chamfered from scrap by a glazier) - £10?
And you are equipped to measure as many chambers as you like, now or in the future.

Did the machine shop tell you the sizes of all the cylinders, or just one?
You need to measure them all.

JOhn

[Richard B]

Nick_Jones wrote:

£ 60.00 to measure the chambers.......  :o  I'll do it for £30!

Head height - that's odd.  There is a listing for a head that starts off at 3.535 thick but definitely an oddball (TR6 Africa or some such) and not a 219016 number.  Does the number look original - the 219016 head attracts a premium in some quarters so might get re-stamped....  :-/

Proper S 219016 should start at 3.475 as already stated, but it won't have grown!  Worth checking the measurement maybe?

There are some low compression heads for the US (etc) market TR6's but as Nick says the should have a different part No.

Where abouts are you, I promise to under cut Nicks £30. Are you sure they measured the correct distance head face to rocker gasket face.

Get one of these, not as posh as a micrometer, But I use it far more than my micrometers

http://www.ebay.co.uk/itm/IMPERIAL-DIAL-VERNIER-/250777977620?pt=UK_Measuring_Tools_Levels&hash=item3a63884314

[Clive]

Ahem.....
http://www.ebay.co.uk/itm/6-150mm-ELECTRONIC-DIGITAL-CALIPERS-VERNIER-LCD-INC-HARD-CASE-FREE-P-P-/170711951973?pt=UK_Measuring_Tools_Levels&hash=item27bf39aa65

Saves all that faffing with reading dials and stuff. Got one a few years ago, been invaluable. Also got a couple of £shop cheapy non-digital ones, useful for measuring pipes, hoses and all sorts of things where you don't need to be so accurate (ie within a mm)

Still, I digress.

Nick,are you saying I shouldn't have given my proper 2500S head away :o

[carmadmike]

Hi Nick I thourght the same about pop up pistons and maybe he was trying to get more money out of me by doing the job the long way round so made a quick retreat with my head. They quoted me £40 to start with but when I said no more work he put the price up with vat and extras. The stampped no looks original and it also has the no 313247 cast in it as in your link. As to rasing the CR the bloke in the machine shop said it was 8.3 to 1 at the moment. So if I was to have it skimed to 3.4375 in theroy it should give me 8.8 to 1 cr. To get a CR of 9 to 1 should I skim it to 3.4225 would this sound right or is my maths wrong.

John the machine shop only measured 1 camber for £60 so its no wonder I was on my toes out of there.

Ritchard I am in Poole dorset

Clive I have a set of electronic digital calipers very simular to yours but they are out and reading my head at 3.363 so mine cant be trusted.

[James]

I don't trust digital ones — I've seen a few go mental — the old school ones are hardly inaccurate and take a fraction of a second longer to read.

[carmadmike]

Anyone know what the clearence is on a standard 6 cylinder engine between the top of the piston and the top of the block when the piston is at T.D.C.
Thanks Mike

[Nick Jones]

IIRC it's around 25 thou plus the compressed gasket thickness - about 40 thou on a decent Payen one but they can vary.  You will note that some areas of the chamber are the same height as the head face - that's why pop-up isn't a great idea.

lengthy thread on a similar theme here
http://sideways-technologies.co.uk/forums/index.php/topic/712-deckingrecessed-block/page__p__9145__hl__%2Bhead+%2Bgasket+%2Bthickness__fromsearch__1#entry9145

Nick

[carmadmike]

Thanks Nick
I shound be able to get the head skimed to somewhere near 9 to 1 that im happy with.
Mike.