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Performance

 This is the second in a two-part series of articles where Dave Waplington explores the basics of vehicle handling and performance.

Just recently I took the decision to replace George’s (George is my 1967 GT6), 35 year old engine. I guess this says something about Triumph’s engineering expertise. George’s engine has been used HARD all these years and is only now showing signs of wear (a little smoky and an oil leak or two). Many years ago, I bought a low mileage MK2 GT6 engine. It had been removed so that the owner could fit a 2.5 PI motor for racing. I got the motor home and re-read my book by Vizard, “Tuning Triumphs Over 1300cc” It’s a bible for Triumph enthusiasts, detailing modifications to improve all Triumph models other than the Spitfire and Herald range.

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Modifications.

 Off came the head, and all the knobbly bits of left over casting were removed from the inlet and exhaust ports, the valve guide ends were chamfered and the combustion chambers contoured. There are some straight edges and flat surfaces in a Triumph 6 cylinder combustion chamber, these were ground to shape before polishing to a matt finish. The head was put back with the valves lapped in and a new gasket fitted.

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Camshaft.

 The camshaft profile determines how fast the valves are opened and closed, how far they are opened and when. Everyone can understand that the more fuel, the bigger the bang, lets face it, you get more power from a bigger engine, all things being equal. The other way to get a bigger bang from the same size engine is to leave the valves open longer, ie open them sooner and open them more. The cam profile I bought does just this. For the technically minded I fitted a 33/71 cam with a lift of 0.395 inch.

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Extractors.

If allowing fuel gasses into the engine is important, getting the exhaust gasses out is just as important. To make full use of a re-profiled camshaft, the gas flow through the motor has to be taken as a whole, a restriction of any sort will cause a problem. Let’s look at the exhaust manifold, basically a tube bolted to the side of the engine, with a pipe out the bottom connected to the silencer. Now that our valves are staying open longer, and all the exhaust gasses are pushed into the manifold tube, there is a real danger that the exhaust gasses from one cylinder may force themselves back into another cylinder, before that cylinder’s exhaust valve fully closes. To stop this we remove the exhaust manifold and fit a modified manifold called a set of extractors. Here the tube is replaced with a set of short pipes that join into the exhaust pipe. The set that I fitted to George has two lots of three pipes that then form one pipe, somewhere under the gearbox. The idea is that the exhaust gasses from each cylinder are physically separated from the other cylinders. I might mention that in cases where absolute maximum power is required from a racing engine, these tubes are tuned to encourage maximum gas flow in a certain rev range.

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Carburettors

Now that we have clear airways and a lessened possibility of letting the exhaust back in, what is left? BIGGER carburettors! The Triumph tuning suppliers in the UK can sell you a set of adaptors that allow 1.75 inch carburettors to be bolted on to GT6 inlet manifolds. The standard size for a GT6 is 1.5 inch. The measurement being the diameter of the hole in the carburettor that matches the hole in the inlet manifold where they are bolted together. All that has to be done is to enlarge the inlet manifold to match the adaptor size, and to do it smoothly. Fitting 3 carbies would be even better, just like an E type Jag. I bought all my tuning accessories many years ago, and all from the same manufacturer, that way, I was able to get the correct needles for the carbies that suited the cam profile. This saves endless work later, since profiling needles really needs a dynamometer and many hours of work.

PLEASE NOTE. The Vizard book was published in the 1960s using the technology of the day. Someone has since found out that a step here will cause the air/fuel mixture to trip over itself. The resulting turbulence ensures a better mix and therefore a more uniform “bang”, I have absolutely no idea how big the step has to be, so my engine is step less. In fact, if I’m truthful, I only found out about the step idea while researching for this article.

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Results.

 Well, does it work? To quote the lines from the movie When Harry Met Sally,….. “ YES YES YES YESS OHH YESSSS! “. Luckily I had the opportunity to try it out at the TSOA Wanneroo Sprint Day last December The car drives like a normal GT6 until you pass 3500 rpm, then things start to happen. The exhaust note changes, and you are pushed in the back, all the way to 6000 rpm, and probably beyond. This is not a new engine, so I don’t go there, in fact with the superb GT6 gearbox, changing at 5500 drops the revs in the next gear to around 4000. This is well in the power band already, and the engine will push some more, then into the next gear and the next etc. You get the picture? My little brown book says that I should expect about a 30% increase in power, it feels like 300%. I resurrected the lap times from my last trip to Wanneroo in 1999. My times for the short track were 1min 11.02 secs and for the long track 1 min 27.5. The best I have ever done on the long track was 1 min 26.1 back in 1976. Imagine my surprise when the times came up on the timing screen: 1 min 08.82 secs short and 1 min 23.98 long. Several whole seconds quicker, not just tenths. I was very pleased to say the least, I can think of a whole raft of excuses as to why I didn’t go even quicker still, i.e. wrong lines on some corners, braking too early, etc, so I was surprised that my times were so good.  To put this into perspective, I found that the original engine allowed me to reach 90 mph (145 kph) just before the breaking marker down the hill, and somewhat less than that on pit straight. In contrast the new engine reached 5500 rpm in top over drive down the hill and 6000 rpm in top (no overdrive) at the end of pit straight. This equates to 115 mph (185 kph) and 101 mph (162 kph) respectively. In fact the original engine would only run to 107 mph (173 kph) regardless of how long the straight road was. What a difference the modifications have made.

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A Word on Gearboxes.

Of all the gearboxes that Triumph have designed, the one fitted to GT6 cars has the closest ratios. That means that when you change gear, the revs only drop by a small amount allowing the engine to stay in the power band in all gears. In the majority of sedans, 1st gear is very low, allowing the car to move off from standing, smoothly, but 2nd is much higher geared, meaning that the engine revs drop considerably, making acceleration a more leisurely affair. It is a pity that the GT6 box is a little fragile. It is essentially a Spitfire box with bigger bearings and a higher 1st gear.

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Footnote.

Let me stress, that all this extra pushing power has to be matched with good pulling up power. A couple of months before the sprint day I invested in a set of race pads for the front disc brakes, I also use silicon brake fluid. The pads were not cheap, a tad under $300 a set, but they handled the conditions and were obviously engineered for far greater demands than I could put on them. The guys at the Triumph Centre offered me a modification that they are marketing for GT6 cars, ventilated front discs. Something I’ll save up for in 2003. Better to be safe than up side down at the end of the straight.

To Dave & Paul at the Triumph Centre, who timed the cam and tuned the carbies for me. Thanks guys. 

Dave Waplington. 

P.S. If you plan to modify your car, or are just thinking of it, have to talk to one of your club’s technical people, you can even write (email) the editor and they will be only too happy to answer your questions and give advice. DW. 

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