zenith_versus_su

Zenith versus SU carburettors.

Anecdotal evidence suggests that A7s which were fitted with the Zenith 26 VA carb `go better` if the Zenith is replaced with an SU carb of 1” or 1,1/8” bore. If you are not an originality purist then proper evidence is now to hand which confirms the view that changing to an SU will improve the performance of your car.

In the October edition of the PWA7C magazine is a dynamometer graph showing the comparative torque and BHP output of an A7 with both the Zenith and an SU, it clearly demonstrates that the SU gives better torque and BHP, albeit at slightly higher engine speeds. It also clearly shows that there is no point in revving your Zenith carb`d engine at over 3,300 RPM or an engine with SU at over 3,650 RPM because at those figures the torque figures “drop off the cliff” ! So throw away your double valve springs, all they are doing is absorbing power and increasing the wear rate on your camshaft and its bearings! I cannot reproduce the actual graphs which are the dyno output and so have produced the table below to illustrate the difference in the two types of carbs.

Engine	Approximate speed	Torque		BHP at wheels
RPM	MPH in top gear *	Zenith	SU	Zenith	SU
1500	21 - 24	12	<8	no data	no data
2000	28 - 32	20	20	7.5	12
2500	36 - 40	17	23	7.5	13
3000	43 - 48	14	19	7.5	13
3500	50 - 55	<5	19	7	13.2

My own experience is that the limitation of the Zenith is in the actual carb whereas the SU performance is affected more by the manifold, the side draft `as cast` manifold has an inbuilt restriction just behind where the carb bolts on, boring this out to match the carb bore is probably the most cost effective performance mod you will ever make!!

This article first appeared in the PWA7C and Devon A7C newsletters with many thanks

(*) key;- Actual road speed at a given rpm is dependent upon the back axle ratio and the size of the wheels, which all vary across models of A7. Note that gears other than top are available, with some drivers having more to choose from than others. The slowest cars at a given rpm are those from 1935-36 with a 5.25:1 back axle and 17” wheels,, and the quickest are the 1923-1932 cars with a 4.9:1 back axle and 19” wheels. Figures calculated from Bill Williams book. Nick.