The Triumph TR6 Engine Guidelines

by Mark Bradakis. September 1989

Squeezing a few more horsepower out of the TR6 engine is a perennial question among TR6 owners, particularly when considering the rebuild of a tired engine. This page is intended to provide some information, to help guide you in deciding what, if any, engine modifications make the most sense for your driving style and your TR6.

For those of you who do not read, I am in the process of telling everything I know about Triumphs. Here's the first article.

This is the first in a series (2 or 3 more?) of articles on some basic stuff one can do to get more grunt out of a Triumph straight 6 engine. Hopefully I will remember to use "Triumph" in the keywords so those of you who give neither diddly nor squat about such drivel can kill the probably much too lengthy notes. Those of you who ARE interested would do well to have the following on hand before planning any serious work:

Basic info about the Triumph six cylinder engine

The Triumph 6 is a cast iron block, cast iron head pushrod engine of 2 or 2.5 liter displacement. The 2 liter (1998cc) engine was found in the GT6, as well as the Triumph 2000 sedan. An earlier version of 1600 cc or so was fitted to the Vitesse, I believe. This 2 liter 6 has a bore of 2.94 inches and a stroke of 2.992 inches. Basically square. The 2.5 liter engine, actually 2498cc, uses the same bore but increases the stroke three quarters of inch to 3.74 inches. The early TR-250 and TR5 cars even used the same block as the GT6, but later the larger displacement cars got a slightly different block with a bit more clearence at the bottom for the crank throws.

The early GT6 Mark I engine (66-68) had a head with pushrod guides being pressed in tubes, while later cars of both displacements used the same head casting with integral guides. In the long stroke motor the head was used as cast, but the 2 liter got it trimmed down to maintain good compression. [Actually some early cars had different port spacing than later cars.] This will be covered in more detail in the section on heads and intake systems.

Rather than write a lot of confusing numbers, here's a chart in which all the confusing numbers are well organized. I will use the US of A market and model designations and specs unless noted.

           GT6 Mk. I   GT6+   GT6 Mk. III    TR250      TR6a     TR6b

Years      66-68      69-70       71-73       68       69-73     74-76

disp.      1998       1998        1998       2498      2498      2498

stroke     2.992      2.992       2.992      3.74      3.74      3.74

CR         8.5:1      9.25:1      9.25:1     8.5:1     8.5:1     7.75:1

bhp        95@5000    95@4700    95@4700   111@4500  104@4500   101@4900

torque    117@3000   117@3400   117@3400   152@3000  142@3000   128@3000

There is some discrepency as to when the low-compression TR6 came about, and what the actual power output is for that motor. Maybe I should have thrown in the specs for 9.5:1 injected TR6 just for fun. Anyway, later cars (75-76?) had 106 bhp but same torque and compression. Look at page 43 of the Bentleys for some further info about which year did what. 72 and 73 TR6s were actually different enough to warrant their own column above, but the chart is wide enough as is. Also, the GT6 Mk. III was manufactured up until december of 73, so you may actually come across a car labeled as a '74 model year car.

The remainder of the articles will consist of discussions on the various head configurations, intake and exhaust systems, camshaft specs and tuning considerations. I probably won't belabor you just yet with any chassis preparation stuff.

So given that basic info, send away for some catalogs and think about what sort of performance and usage you REALLY want out of the car. Feel free to email or post any specific questions, comments or complaints. The chances of me being always right are pretty slim, as you might surmise. I never worked for Triumph, I don't have Kas Kastner's autograph and I only talked to Mike Cook once years ago about Rotoflex couplings for the GT6.

Mark Bradakis



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