YT 3706 - Technical improvements
In case there was any interest in the technical improvements I've been quietly making, I thought I'd send these as a separate e-mail, including a picture. I hope this is of interest.
I should start by pointing out that my mission is not to modify my YT too far from standard. I sought out a YT wanting a comfortable 4-seater open tourer; a fun family car for sunny week-ends. YT3706 is not a concours project, but still a highly original car with just a few minor mods under the hood to help it do the job. That is, cover the slightly longer Australian distances. Apologies to the purists, but I can recommend the minor "bolt on" work described below. At around A$1,300 spent, the performance results have been good value for money.
The XPAG engine is very original and in very good order. Step one was to replace the original but slightly vague Lucas distributor with small bodied Bosch 485009 with 10 degree advance (BTDC). Point gap is 0.4 - 0.5mm. Dwell angle is 48 - 52 degree. A more precise advance curve with modern points instantly gave the engine better pick-up and more torque at lower revs. Modern HT leads also helped.
Step 2 was to replace the original oil-bath air filter and its constrictive manifold with a pair of K&N high flow filters. This is a considerable cosmetic change under the bonnet but I could not understand the logic of having those wonderful twin SU carbies sucking through such a complicated common filter/silencer. The new canisters needed to be modified to take a new rocker-cover breather pipe (via a copper pipe "T" piece). The forward canister needed a slight shaving on one corner to give adequate clearance from the right hand bonnet. Finally, the right hand radiator stay was replaced with a modified curved strut. These modifications are visible in the attached picture. In terms of performance, the increased induction response is noticeable. There is no lag in throttle response and a full air-fuel charge being delivered. Induction noise is obviously greater, but in an open touring car this is hardly an issue. In fact, I quite enjoy the sound.
Step 3 was to replace the standard cast exhaust manifold with a custom made, double coated exhaust header/extractor. This was a big decision and the biggest cost (approximately A$800) but having experienced significant improvements on other engines by changing the exhaust system I knew it could only help. The result surpassed my expectations. Firstly the engine runs noticeably cooler. The double ceramic coated mild steel header pipes evacuate the hot exhaust gases efficiently out of the engine bay. This means that the whole induction side of the engine (including the carbies) runs cooler, delivering a higher density fuel-air charge. Secondly the quicker evacuation allows the improved induction path to reach its potential. Finally, the whole exhaust system is quieter. The noticeable harshness of the standard manifold is gone, with a smooth, lower frequency, balanced exhaust note resulting. There is also a very sexy popping sound as the power comes off at higher revs.
I have not dyno-tested the car to measure the horsepower gain overall but it is noticeable. It is very quick, with lots of torque to corner in all gears. Hill climbing is greatly improved, with 4th gear coping with all but the steepest grades. On the highway accelerating smoothly to over 65mph is easy. There is power to go further but I've not pushed it too far. There is easily enough power to cope with a 5th gear, but that's not on the agenda. Under the bonnet the result is tidy and attractive (see photos). Fuel consumption is also better. All of this is without shaving or porting the head, which is a much more expensive exercise.
The XPAG is a great little engine. It needs just a little help to realise its potential. If you don't mind being a little away from the original, I can recommend the above bolt-on's.
Robert Ades. Australia.