direct comparison updraft FZB vs sidedraft Ruby carb?

[JonE]

Has any one got any data/feedback of a direct comparison of these carbs/manifolds on a standard Low Compression engine please with gravity feed? My gut feel is that the Ruby units are going to give an increase in power and are worth hanging onto if presently in situ. But I'd prefer to revert to originality if there isn't a difference or the earlier is indeed more reliable or better.

[Bob Culver]

As I have mentioned before (to great scorn) the old road tests for RN cars claimed performance generally better than obtained with the RP and early Rubies. 50 mph with ease was claimed yet later cars strugged to reach this at all. RN owners also claim notable performance. It is possible the origianl road tested cars were discreetly non standard. The commonly quoted bhps may not have been maximum. To confuse the issue the cr was raised late 1933 part way through the RP. With reduced breathing  the high rpm bhp may not have been improved.
It seems problem arise with gravity feed and high mounted carbs.

[Dave Mann]

I've tried the 26Va on our RN and the only obvious improvement is a slightly reduced fuel consumption, the downside is the vacuum wiper stopped working so I'm back on the 22FZB.

[AustinWood]

My 1929 RK performs well on its 22FZ. I abandoned the FZB because of Mazak rot. Fuel consumption is about 35mpg driven enthusiastically. It will certainly reach 50mph with no problem and I've had it off the clock (60+) on downhill stretches. Speedo agrees with my sat nav. It has the post 1933 cylinder head which I suspect has been skimmed.
Under the bonnet all looks original which is a bonus.
In the mid 60s I had a 1930 RK fitted with an SU which certainly had better performance. Would reach 60 quite readily and cruise at 50 happily. The windscreen peak was missing which I'm told contributes to drag. It also had the HC head. Fuel consumption was about 40 mpg.
In the 70s I had a 1934 4-seat tourer with standard set up, 26VA & LC head. It performed much the same as my current car but fuel consumption was markedly better. A journey in 1967 was following a friend who was towing a trailer with my 1937 Ruby on it from Dundee to Cambridge via Liverpool. Filled up before We left, filled up at the end. 10 Gallons total and a bit over 500 miles, so better than 50mpg. But it was a slow journey, seldom over 40mph, steady cruising on an open road.

[Hedd Jones]

I went from a side draft zenith to the bronze version.

It doesnt suffer from fuel starvation any more when hot, and Ive not had any blocked jets since.

Its also easier to start on the handle.

Cant say that it goes better wither way

[JonE]

ok, thanks all - will test with present arrangement and then look for a bronze carb to compare things myself. It seems the raw difference is change from 15mm to 17mm choke.

Out of interest, old Motorsport article mentions the Ruby integrated manifold as having a warm up hotspot constriction that can really do with relieving/removing. Does that chime with anyone - is it worth doing for airflow?

[Robert Leigh]

ok, thanks all - will test with present arrangement and then look for a bronze carb to compare things myself. It seems the raw difference is change from 15mm to 17mm choke.

Out of interest, old Motorsport article mentions the Ruby integrated manifold as having a warm up hotspot constriction that can really do with relieving/removing. Does that chime with anyone - is it worth doing for airflow?

When using the standard sidedraught manifold I have been getting busy with the angle grinder for years, cutting through the central connection between the exhaust part and the inlet. With modern volatile petrol it reduces vaporisation problems. I also enlarge the inlet passage in the manifold. I never understood why it is originally smaller than the carburettor bore. Can a venturi be any advantage at that point?

Robert Leigh

[AustinWood]

I had forgotten one modification on my 1929 RK saloon. Robert's post reminded me that the inlet manifold has been modified. The vertical section directly above the carburetor has been reamed to the same diameter as the carburetor. Like Robert I can see no reason why there should be a restriction there. A word of warning: Don't take it out larger than the carburetor. Flowing gas does not like to slow down, it creates turbulence.
All gaskets in the inlet are aligned to avoid intrusion into the gas flow.
I bought my 1930 Saloon that went so well from Robert in 1964. Maybe his work was the reason for its good performance!

[Robert Leigh]

I had forgotten one modification on my 1929 RK saloon. Robert's post reminded me that the inlet manifold has been modified. The vertical section directly above the carburetor has been reamed to the same diameter as the carburetor. Like Robert I can see no reason why there should be a restriction there. A word of warning: Don't take it out larger than the carburetor. Flowing gas does not like to slow down, it creates turbulence.
All gaskets in the inlet are aligned to avoid intrusion into the gas flow.
I bought my 1930 Saloon that went so well from Robert in 1964. Maybe his work was the reason for its good performance!

I seem to remember a 1" SU and a late head as well!

Robert

[Bob Culver]

Would expect reduced wiper vac to correspond with more power!
Like thousands of others I slavishly followed the Williams book and labouriously filed the venturi out of the side manifold.
But it is not a restriction, just a proper transiton to the small manifold. Without cutting them open I get the impression that the horizontal part of updarught manifold may be slightly larger. Perhaps someone can comment.
On my side draught car I found an inlet manifold same style (made out of pipe fittings!) but little over an inch i.d. no advantage. Probably too large.