Triumph Repair: TR 6 cylinder head modification, line calculator, combustion chamber

Question
QUESTION: Scott,

I am rebuilding the engine on my 1976 TR6.  The engine is stock and has had no prior mods.  I plan to install a new, hotter cam, new tappets and springs. All bearings and rings will be replaced as well. I would like to increase the compression from the stock 7.5 to b/w 8.5 and 9.0 CR.  Please confirm the optimal amount to mill from the head to accomplish this.  I believe that b/w .060 and .080 should get me within that range. Is this correct?  Other than new pushrods, will this require any other mods?  Thanks.

ANSWER: To do it right you need to measure the head's combustion chamber cc, and then do the math to determine how much to mill the head.  Attached are a few links on how to, and an on-line calculator.  Unless you know the cc of that chamber, you're taking a stab in the dark about how much it will increase your compression.

You are correct, the only real change that would be required is changing the pushrod length; once you know how much the head is to be milled, then you can order custom tubular pushrods;


http://www.imperialclub.com/Repair/Engine/compression.htm

http://www.rbracing-rsr.com/compstaticcalc.html

http://www.sacoriver.net/~red/uccr.html


---------- FOLLOW-UP ----------

QUESTION: Thanks Scott,

But given that everything is stock, it doesn't seem to make sense to have to make all of these measurements.  The block is already at the machine shop being cleaned and dipped so that is not an option anyway. I would have thought there was a chart or something that would give the change in compression from stock at various mill amounts.  Oh well.  

Mark

Answer
Unless you know for sure that your car is 100% original, and it has the correct casting # for your year, and the overall head thickness is unchanged (never warped & milled to flatten), you would be taking a stab in the dark.  You don't need the block, just the Head cc.  You can use the spec'd bore & stroke in the on line calculator, and measure the head gasket you intend to use with a micrometer or caliper.

One reference to the low compression head has it at 59.3
http://www.topshamautoparts.com/tr6/superengine.htm

Another 54 cc
http://www.british-cars.net/triumph-tr6-bbs/compression-ratio-200706010423231145

Found you a table, too.
http://www.goodparts.com/tech_docs/TR6_Compression_Ratio.html


Good luck.