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TR2/3/3A latest with my H6Ss

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TexasKnucklehead said:
So did the TR4a have better performance? Hp? Torque? Gas mileage? There must be some reason for the change.
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There were modest increases in rated torque and power. IIRC the TR4 was rated 100hp at introduction, while the TR4A was rated 105hp. Whether those numbers are "real" or just marketing-speak is open to debate. Supposedly the changes also produced better midrange power and throttle response. But the change was not just carbs, the camshaft & manifolds changed as well. And there were some changes to the cylinder head and valves during the TR4 run that likely improved breathing as well. (In particular, the exhaust valve stem got smaller, leaving more room in the smallest part of the port for the exhaust gases to go by.)

If I recall the history correctly, the switch to ZS carbs was originally made because one of Triumph's competitors (BMC) merged with Skinners Union. To avoid the hassles of relying on a competitor for parts, Triumph (who was by then part of Leyland) turned to Zenith (who were controlled by Solex in France). Then later, when BMC was teetering on bankruptcy, the British government pressured Leyland into going back to buying carbs from them.
Or something like that.

Anyway, the switch away and back to SU carbs was not for technical reasons, but political ones. By the time they switched back to SU, the H-series carb was obsolete and the HS-series had replaced it. The performance difference is small and mostly (I think) unintentional, but the HS was an improved design that addressed several weak spots of the H-series design (like eliminating the jet and bowl seals that perpetually leak).

Then in 1968 they switched back to ZS carbs again, and that change was for technical reasons. Not for better performance, but because the Lucas fuel injection planned for the TR5 was found to not be capable of meeting US emissions requirements. I've never heard if the SU carbs were considered or not, but supposedly they had a last-minute program to use the "emissions" version of the ZS carbs and create the TR250 so they could sell something to the US.
 
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