I have read a few books , and most say there is little to gain in fitting 2inch cards instead of the 1 3/4 carbs , unless you plan on going racing , normal driving the XK Engine will not run out of air with the HD6’s carbs !
I have both so not sure what to use , any pointers would be appreciated .
Engine is the 3.4 XJ6 unit , so has the straight port head and the like ,
At the moment the engine is running on a test bed , only down side I can see , is a cable choke .
It would be more straight forward to use the HD6 carbs
In a simple perspective, internal combustion engines are basically air pumps. How much air pumped per second can be a central factor in engine torque and power. Everywhere on the path of the air flowing into and out of the engine can influence the total air pumping rate. Back in the day, carbs were matched to the rest of the engine design and displacement to give the performance desired for the operating conditions.
Car racers operate near torque maximum as much as possible, this often corresponds to being near, or at, full throttle. Coupled to this narrower operating range than street driving is the issue that a less than one percent difference can mean significant difference in race finishing position. A tiny increase in performance in this operating window can drive a choice of carb that is not so useful for general driving conditions.
Very roughly speaking a change to 2 inches from 1.75 in carb diameter would offer an increased maximum airflow of about the square of that ratio or about 30% increase in the maximum air flow which could go through the carb. However, the rest of the intake path, the cylinders, and exhaust also controls the total airflow that can occur. These other sources and impediments to flow mean the increase is less than 30% maximum, maybe even not noticeable. And when going less than full throttle, the air/fuel ratio would stay similar regardless of carb choice and the cylinder/piston air pumping rate is not changed, so my initial estimate for part throttle is the large carb would run with a slightly smaller butterfly opening providing about the same air and fuel as the smaller carb which was designed for the engine in the first place. Under normal driving conditions I would not expect much noticeable change by going to a larger carb than specified by original car manufacturer. Going to a smaller carb could restrict airflow substantially and lead to noticeably less torque. Still, maybe there are some performance conditions where people have experience to comment on noticeable differences under regular driving conditions, rather than racing?
Just curious, where does this power-of-two relation come from?
While not exactly correct, the relative airflow is approximately in proportion to the inlet area. The area for a circular inlet is the diameter squared times pi/4. The area is increased by enlarging the dimension in one direction and multiply-increased by also enlarging in the direction perpendicular to the first dimension. The area of a 2 inch diameter circle is about 3.14 square inches (more properly called inches squared for those who object to inches not being square), the area of a 1.75 inch diameter circle is about 2.4 square inches. The ratio of 3.14/2.4 = 1.3, giving a 30% increase in area going from 1.75 to 2 inches diameter.
Thanks, Roger. I can see how you get the 30%, but it’s the idea that relative airflow is proportional to inlet area that I don’t understand. Is this something particular to carbs?
Fluid flow proportional to pipe cross sectional area is a starting point in considering most liquid and gas flow through pipes and orifices, it is not particular to carburetters. Moving fluid at constant velocity through a hole moves a quantity of fluid per unit time in proportion to the cross sectional area of the hole. In our case, the fluid is air (with gasoline added) moving through the throttle body area of the carburetter. Doubling the cross sectional area of a hose doubles the fluid quantity per second if the fluid velocity remains the same. This simple model ignores frictional issues, such as found near a wall, but oftentimes the simple model works well enough for a problem at hand.
To see how closely this simple model works in the real world of Skinners Union carburetters, refer to page A.14 of SU Workshop Manual - Carburetters and Electric Fuel Pumps (June 1956) which shows BHP ratings for SU carbs running in size from 0.875" to 2.5" throttle diameters with examples nearly every 0.125" increments. SU gives the BHP rating for each carburetter, these ratings can be converted to BHP per square inch of throttle area. Across the entire range of sizes the SU carburetters give 22.19 BHP per square inch of throttle opening with a standard deviation of about 0.51 BHP. It is unclear to me how much of the standard deviation is merely SU rounding their BHP ratings to the nearest 5 BHP since all except the 1 inch carburetter have BHP ratings ending in 0 or 5. In other words, the flow rate proportional to throttle area does apply to these SU carburetters. Also, if the engine BHP is known, the SU carburetter appropriate for the engine airflow is readily selected from the page A.14 chart. A larger throttle body does not change the rest of the engine/air pump throughput rates for general driving conditions.
Thanks, Roger, for your thoughtful answer to my question. Your data from actual SUs is much appreciated, and I suppose could be used to settle the argument regarding “improvements” made by substituting triple HD8s for dual carbs, either 2 inch or 1 3/4 inch.
I also now understand your assumptions. First, that fluid velocity remains the same. I had been supposing that it doesn’t. And second, that friction is ignored. I had considered that to be of paramount importance, except for superfluids of course. 
But the actual data you were kind enough to dig up, and your calculations, prove that your assumptions are indeed adequate.
Looks like the HD6’s will be fitted , I guess there is good reasons for fitting 2inch carbs , as Jaguar only fitted them on XJ6’s , and not 1 3/4 .
If the size of the air inlet made that much difference , funny how Jaguar nearly halved the size of the intake on the fuel injection engines , to that of the carb !!
Had a set of HS6 carbs come the other day , off a 240 Jag with a manifold , more or less as they come off the car , so has all the linkage , as the choke is more or less the same as the HS8 , and as my car was a 240 , with the slide choke control on the dash , I may go back down the HS8 road , just got a 2 inch inlet horn , the inlet to carb holes just need opening up a little , then again , the XJ6 inlet manifolds look just like the 240 , so may be a case of just swopping the stat housing over , once I get it off the 240 manifold I will know ,
Well I managed to get the 240 manifold apart, nice surprise to see the main part that the carbs bolt to , and the first part of the stat housing are very much the same , just the last part is different , so the HS8 carbs can bolt on to the XJ6 manifold , and just use the top hose connection point from the 240 !
An E-type with 4200 ccm displacement has 3 HD8 2inch carbs. Each cylinder has 700 ccm of displacement. That is the reason, why Jaguar mounted 2inch (HD8) carbs.
Your motor has 3400 displacement = 566 ccm for each cylinder. I think that the 1,75 inch carb are big enough for the limousine. There will by a small different in max power between a 1,75 ich carb and a HD 8 with 2 inch.
Regards Wolfgang Gatza
Hi Gatza , I am not fitting HD8 carbs , am fitting HS8 , if you look at the picture of the manifolds , look at all that pipe work for the choke on the bottom manifold , compared to the top one , HS8 has a manual choke , plus they have no rubber parts , and very easy to adjust , more power if any is a bonus !
