Not all Cylinder Heads are alike... There are several different cylinder
heads for the 1500. Changing out to Flat-top pistons with a
low-compression USA Head will give you about ~8.2:1. I ended up
skimming about .060 of my head to get over 9:1 compression and others
have told me they skimmed .100 to get 9.5:1.
If you have your head off, you can see some cast marks that look like
they shouldn't be there... I believe Triumph was making modifications to
the head height so they could pass US emissions. I have before and
after pictures of the cast marks on my web site:
http://webpages.charter.net/eighmy/spitfire/index.htm
Also, you have to read John Davies' excellent article on raising
compression:
http://www.totallytriumph.net/spitfire/skimming_your_head.shtml
Also, Paul Tegler has some info on non-1500 engines at:
http://teglerizer.com/triumphstuff/spitheads.htm
> Flat top pistons will give you a theoretical 9:1 compression ratio. I
> have heard that you can go up to 9.75:1 and still use pump gas. CR
will
> change a little bit depending on how much your head is milled (if at
> all), and how big of an oversize piston you will need. You will
probably
> want to cc the combustion chamber (measure the volume in the head, in
> other words) and use that together with the stroke to calculate CR.
> There should be some info in the arhives on how to do this. Or someone
> else can chime in.
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