I would assume that since there is a difference of about 21 cubic inches
between a 1142cc and a 1493cc engine blocks, that the CFM requirements are a
pretty minimal change between the two engines.
HS2's and HS4's both have external float bowls and similar operating
characteristics, but a differing CFM at Wide Open Throttle.
At 1.5 HG (inches of mercury), a single HS4 (1.5 inch throttle bore)
carburetor has a value of 142 cubic feet per minute at WOT.
At 1.5 HG an HS2 (1.25 inch throttle bore) carburetor has a value of around
98 cubic feet per minute at WOT.
With a set of dual carburetors, that would bring the values to 196 CFM on a
set of HS2's and 284 CFM on a pair of HS4's at peak wide open throttle.
(I don't know what rating a single ZS150cd carb is, but because of it's
integral float bowl design, I would assume it would be similar to a Skinners
Union 1.5" HIF4 which has approximately 160CFM).
When you take an engine's "theoretical air consumption" or TAC at brake
horse power, into account, we should be able to figure the optimal CFM rate
for that particular engine. If my understanding of this calculation is
correct, the TAC should be equal to Engine Displacement in cu in. x peak HP
RPMs divided by 3456.
The european (9.0:1 compression) 1500 had a peak horsepower (of 71) at 5500
RPMS.
91 (ci displacement) x 5500 (rpms) divided by 3456 = 144CFM
(which means you need atleast two 72 CFM carbs or one 144CFM carb.)
The federal (7.5:1 compression) 1500 had a peak horsepower (of 57) at 5000
RPMS.
91 (ci displacement) x 5000 (rpms) divided by 3456 = 131CFM
(which means you need atleast two 66 CFM carbs or one 131 CFM carb.)
(Actual air consumption is also affected by intake and exhaust timing,
cylinder head combustion chamber design, intake port design, intake
manifold, barometric pressure and temperature and exhaust restrictions and
all that it includes.)
To put it into perspective: according to an MGB carburetor web page, a 6500
(peak hp) RPM modified 1924cc engine should run a TAC of 221cfm, which
should be more than adequately handled by a set of HS4's with a good intake
manifold, air horns and air filters. And what's more, should have a good
high velocity density of charge.
And to cut and paste an interesting quote from the page;
"If we can provide adequate CFM through these carburetors, when combined
with the intake manifold and air cleaner assembly(s), to satisfy engine
demands, then we are better off fitting these sized carburetors (meaning
HS4/HIF4). Remember, by sucking air through the smaller diameter HS4/HIF4
carbs, as opposed to the larger diameter HS6/HIF6 carbs, the engine has to
suck harder and potentially the velocity (FT/SEC) of the mixture charge
should be higher."
I'm running HIF4's (approximately 300-320 cfm max), and am paying for it in
gas consumption. (I'm running around 23mpg in the city. And I get less then
30mpg on the highway (I don't have a hard number because I can't go a
quarter of a mile in any direction in MD without hitting a stop sign, stop
light or traffic.) but I'm also running very rich needles (AAU biased) which
contributes to my gas ineconomy. (According to one of my mid 1970's 1500
dealer brochures, an American 1500 should be running around 29.8 mpg city
and 35.4 mpg highway.)
So, I guess what my opinion would be is that a 1493 cc engine with high
compression would be well within reason for a set of well assembled HS2's
assuming the porting on the head, the intake manifold and the air filters
are also upto snuff. And a set of HIF6's (480 cfm per pair) would be some
large overkill on an unmodified engine of that same size and compression.
The HS4's and HIF4's would leave a great deal of room for modification on
the stock engine and still be utilizable.
Terry L. Thompson
'76 Spit 1500
Maryland
>
> So... what's the consensus...
> which runs better on a totally stock 1500 Spit? HS2's or 4's
>
> Paul Tegler wizardz@toad.net http://www.teglerizer.com
>
>
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