Compression ratio

Raising the compression ratio is one of the quickest and easiest ways to increase power on these engines, and there are two ways to do so. The best way, of course, is to leave the head casting pretty much as is, in terms of thickness, and get custom made pistons. This avoids any possible problems with cutting too much off the head and breaking into a water passage. In general, this is not a real problem unless one tries to mill off enough to get compressions on the order of 13:1, a bit high for street use. So to avoid the expense of a set of pistons, which could run around 450 to600 for cast pistons, we will look into the cheaper methos of raising compression, milling the head. And as one will see soon, the head has a lot of metal built into it which can be safely removed, as Triumph themselves did so.

As in the intro, perhaps a detailed chart is in order here, relating the various models to engine configurations. In the chart below, ``CR'' refers to compression ration, as in 9.25 to 1. ``Thickness'' is the height of the head as measured from the cylinder mating face to the machines surface on which the rocker cover supposedly seals. ``CC Depth'' is the distance from the mating face of the head to the top of the combustion chamber, not counting valves. ``CC volume'' is the combustion chamber volume. Not having a very large variety of actual heads and engines covering all the possibilities some of these figures are calculated. Some are actual measurements from parts I have, and some figures are taken from service manuals and competition prep booklets.

Application CR Thickness CC depth CC volume

GT6 Mk I 9.50

GT6+ 9.25 3.450 0.425 34

GT6 Mk III 9.25 3.32 .47 37 (early)

GT6 Mk III 8.0 3.40 .55 44 (late)

TR250, TR6 8.5 3.55 .60 48.5

TR6 middle 7.75 3.54 .68 55

TR6 late 7.5 3.55 .70 57

TR6 race 1 10.25 3.375 .52 41.6

TR6 race 2 12.3 3.295 .44 33

GT6 race 1 10.75 3.31 .385 30

GT6 race 2 12.75 3.22 .300 24.4

Check the TR6 thickness versus CC depth, looks odd.

The observant enthusiast may notice some of the above info points to a quick way to increase the compression ration on the 2.5 liter TR6 motor: drop on a stock GT6 head. As a matter of fact, that is exactly what I might do this winter, once I get the head off the GT6 engine and dig out those oversize stainless valves...

Another way to get a slight increase in the compression ratio, steel shims, figure out difference.

Overbore to get compression, displacement increase.  Standard bore 2.94 inches.

 TR6, 2500

      Bore      10:1    10.5:1      11:1    11.5:1      12:1     Disp.
-----------------------------------------------------------------------
      Std.     42.50     40.00     38.00     36.00     34.00     2498
    +0.010     42.80     40.40     38.20     36.20     34.40     2513
    +0.020     43.10     40.80     38.40     36.50     34.60     2530
    +0.030     43.40     41.10     38.60     36.70     34.90     2548
    +0.040     43.80     41.30     39.00     37.00     35.20     2565
    +0.120     46.30     43.80     41.40     39.20     37.30     2700

 GT6, 2000

      Bore      10:1    10.5:1      11:1    11.5:1      12:1     Disp.
-----------------------------------------------------------------------
      Std.     33.30     31.30     29.60     28.00     26.60   1998.00
    +0.010     33.50     31.60     29.80     28.20     26.80   2011.00
    +0.020     33.80     31.80     30.00     28.40     27.00   2024.00
    +0.030     34.00     32.10     30.30     28.60     27.20   2038.00
    +0.040     34.30     32.30     30.50     28.90     27.40   2052.00
    +0.120     36.30     34.30     32.40     30.60     29.10   2164.00