Wow, I'm actually sitting here reading the list for a bit! And I may
get caught up again on the -request mail. Once last week, I actually
processed two requests the same day they came in.
Anyway, here's a TeX formatted piece of a booklet on Triumph performance
I was once working on, back when I was driving my TR6 around. I should
probably redo it all for Spitfires, since that is what I have been playing
with for the last few autocrossing seasons. Anyway, it certainly isn't yet
complete, there may be some cryptic notes to myself in the text, and the
TeX commands should be pretty easy to remove or ignore.
mjb.
----
\section{Heads}
Okay, so we have already had a brief introduction to the two main types of
engine to be discussed here, the Triumph 2000cc six and the 2500cc six. This
section will detail some of the changes which can be made to the cylinder head
for increased power. The main areas to be considered are compression ratio,
porting and valve configuration. There are four basic heads involved in the
range of engines under discussion.
\subsection{GT6 Mark I}
These heads had small (5/16") head studs and external pushrod tubes which
are not part of the casting. This is the "straight port" head, with spacing
that is not the same as other GT6 engines. This head is not recommended for
serious souping, as the later castings have better design. It also has
smaller valves, with 1.31 inch intake and 1.18 exhaust.
\subsection{TR5 PI}
These heads had 3/8" studs which were used for the remaining life of
the six cylinder cars, and the pushrod guides were an integral part of
the castings. Intake port spacing is the same as that on the later
TR6, with the intake openings spaced on 2.21 inch centers. On all
four types of head the exhaust ports at the manifold face are the
same. Also the valves are the same size, with exhausts measuring 1.26
across the face and intakes 1.44 inches. This head was not only used
on the TR5, but also the GT6+ and Mark III.
\subsection{TR250, early TR6}
The head for the carburated engines had intake ports a bit closer
together than the head above, being on 1.91 inch centers, only .3
difference. It is possible to swap intake manifolds from one to
another, but if you are reading this far, you probably aren't
interested in poor performance.
\subsection{Later TR6}
Around 1972 Triumph went to a PI head for all TR6 models, and made a lot of
little changes to the engine to meet stricter USA emissions requirements. It
has the wider intake ports and better flowing intake passages, as well as
having valves of a slightly larger size. These changes, coupled with a
revised cam profile and CD2-SE carbs (the ``tamper-proof'' ones with
adjustable needle height) improved emissions and actually increased the
horsepower rating a bit, from 104 at 4500 to 106 at 4900 rpm.
Of course those not fooled by horsepower ratings will notice that the figures
point to a lower torque across the usable rev range, but it is a bit flatter
in the midsection. Not a bad job, really, on the part of Triumph.
\subsection{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 to $600 for cast
pistons, we will look into the cheaper method 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 ratio, 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.
\bigskip
\begin{tabular}{|l|rrrr|} \hline
Application & CR & Thickness & CC depth & CC volume \\ \hline
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 \\ \hline
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 \\ \hline
\end{tabular}
\bigskip
[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 ratio 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.
\begin{verbatim}
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
\end{verbatim}
\subsection{Valves}
Okay, now that you have your plans for the killer engine's compression
ratio, time to think about some other items, like the valves. Porting,
intake passage size, finish. Lead-free mods, oversize valves. Selection
of racing springs and alloy retainers.
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