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2500S head onto a PI block


Richard B

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Standard 132bhp cam and CW performance exhaust. Normal road (fast) use.

Has anybody done this?

Can I just get the head skimmed from 3.475 to 3.400?

Do I need bigger valves?

Should I lower the comression (considering the crap fuel) skim 60 to 65 thou off?

Should I raise the compression (because I can) 85 thou off?

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Hi Richard.  Not done it, but I have an old CW article on cylinder heads which appeared in Six Appeal - funny I was just reading it the other day.

He reckons skim to 3.430".  Seems the main reason is to prevent running on if you were using carbs.  But I guess you will be fitting PI?

He also goes on about "squish area" and his preference is for raising CR by first decking the block until the pistons are flush or even proud of the block (6 thou), then giving the head a skim.

CW says he prefers the smaller exhaust valves you have for road use.

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I am having my TR6 rebuilt, seriously re-worked, ported, skimmed, guides and seats. I have been advised that standard sized valves are fine, and this is what the race cars they build use (TR Enterprises)

These 6 cylinder cars they built won the championship last year, too.


Cheers

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Hello Richard,

I skimmed my P.I. head to give about 10:1, and it has been fine (On injection). The camshaft, as I'm sure you know, alters the effective compression ratio as opposed to theoretical.
Apart from Chris, I've never heard anyone recommend having pistons proud of the block face, quite the reverse, but he must have proved it many times over?

Alec

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Richard - Skim it down to 3.400"  - wouldn't worry about decking the block - it's a good idea with the 2000 (maybe flush or 0.005" down the bores) but riskier with a 2.5 due to the floppy crank. NO need to fit bigger inlets - if anything the S head needs bigger exhausts - seems that Toms friends at TRE concur with something that GT says in his book. I have a PI head on my bench with standard sized valves  (but not standard valves) which flows 89cfm (inlet) and 63cfm (exhaust) at 0.400" valve lift

http://www.tr6.org/cylheads/index.html

That means my head man has got the exhaust on mine flowing as much as the standard inlet   - mind you about 10-12% of that gain was achieved by deshrouding the inlet and exhaust as the head is going to run with a 77mm bore and larger head gasket - but it just shows that a bigger valve in a smaller bore will be worse - (more shrouded)


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Hello all,

going off at a slight tangent, has anyone read David Vizard's book on modifying these engines.

Being a Mini man originally, he used the same combustion chamber shape as the Mini, but interestingly he proposed boring new inlet tracts such that they were inclined and came out of the top of the head, i.e reducing the angle into the valves. I don't know if he ever tested them or if anyone else had a go at it?

Alec

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piman wrote:
Hello all,

going off at a slight tangent, has anyone read David Vizard's book on modifying these engines.

Being a Mini man originally, he used the same combustion chamber shape as the Mini, but interestingly he proposed boring new inlet tracts such that they were inclined and came out of the top of the head, i.e reducing the angle into the valves. I don't know if he ever tested them or if anyone else had a go at it?

Alec


Reckon  that was consigned to the "too hard" bin of history - would have needed new inlets - probably a new bonnet - lots of patching up water jackets - and ultimately it is the exhaust valve and port that limits ultimate bhp

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Just as a matter of interest as we are discussing valves being shrouded by cylinder walls.  

Bill Blydenstein when preparing (with Chris Lawrence) his VX4/90 that came third in the the 1963 European Saloon car 'Championship' (the Challenge Cup), had just this problem due to an inherent design fault with the Vauxhall head.  I was discussing the VX4/90 rally cars with him back in the mid 1980s and he gave me some fascinating information that showed just what an imaginative tuning engineer he was.

The inlet valve on the VX4/90 engine was located so close to the edge of the combustion chamber that while it functioned satisfactorily in 'standard' form, the moment he started to work at getting the ports to flow better he hit a major problem with cylinder wall shrouding of the outside edge of the inlet valve. Relieving the top of the cylinder bore gave little help because there was not much material he could remove so his solution ....... elongate the cylinder head stud holes and slide the head across a bit!

This one engine lasted for all the races in the 'championship' (there were not that many luckily) and then he moved on from Vauxhalls at the end of the season - he commented to me that it was pointless to continue with it as the Cortina GT was coming along and the engine gave much the same power in standard trim as his VX4/90 did in its tuned form, and a lot of off the shelf tuning goodies were also becoming available.

He did however make the point that to get the head to seal properly they had stuck it down with brown (hard setting) hermatite ....... and he pitied anyone who had to try to remove it .....(whistle)

Ted

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