Dick/Shane:
Thanks for the information. I am another guy who would like the look of the
triple ZS. I read the catalogue from Goodparts and he seems to make it clear
that he believes that the result is a considerable improvement in
performance without an increase in consumption (except when being pushed of
course). I have two sets of ZS carbs (one on the car already of course) and
thought that this would be a fun upgrade using factory original parts.
Should I be planning on rejetting the carbs or what? I like the look, but
don't want to ruin the cars performance. I recall having put K&N air filters
instead of the standard air box on a 2-stroke motorcyle. I never figured out
why the bike acted so weird until much misery had been endured.
My TR6 is a street machine. Recently rebuilt engine with a slightly warmed
cam and electronic igition and headers to the Falcon sports twin exhaust,
but standard pistons etc. If this mod will only work with my foot flat on
the floor, then it's no good to me at all.
Mark Hooper
1972 TR6
-----Original Message-----
From: tr6taylor@webtv.net [mailto:tr6taylor@webtv.net]
Sent: Friday, January 10, 2003 3:12 AM
To: 6pack@autox.team.net
Subject: Air Flow, + No doze
E-mail message
From: madmax_xx@hotmail.com (Shane Ingate) Date: Thu, Jan 9, 2003,
Pack) Subject: Re: PI vs dual ZS vs triple ZS -- dumb questions
Shane writes:
A perennial sin (especially in the hot-rod world) is to over-carb an
engine. The first major step to be made is to simply examine the air
requirements of a motor to figure out how much carb one needs. How often
do we see 750 cfm Holleys on a street-driven small block V8 when they
really need only a 600 cfm Holley? One down-side of over-carbing is poor
throttle response.
I do not know what a ZS flows; I'm *guessing* it is around 150 cfm (Bob?
Vance? Dick?). If my guess is right, then two of these will flow 300
cfm. Three of them would be 450cfm, which is more than this motor needs
except a worked motor at really high rpm. For comparison, 40mm DCOEs
flow over 200 cfm, so triples would flow 600 cfm, which is WAAAAY more
than this motor could ever Shane
Dick, replying---I think Shane is right about some owners
overcarburetting their engines. We know it looks cool to hang either
larger or more carbs on an engine, as all but the most dedicated of us
spend more time looking at such an engine and bench racing, than
actually competing.
Back to the "Cubic Feet per Minute" of air engines such as the TR Six
require, I'll pass on the formula: ZZzzzz. Hang on, there will
be something here for nearly everyone!
Theoretical cfm = rpm x (cubic inches) displacement divided by 3456.
(3456 is the constant for CF) So...
6,000 rpm X 152 / 3456 = 264 cfm.
A good, stock engine operates at around 80% Volumetric Efficiency, so
the "theoretical" 264 cfm is now down to requiring only 211 cfm.
On a well prepared street engine, V.E. can go back up closer to 90%.
(238 cfm)
A race engine can actually exceed the 100% thru ram air phenomenia, so
at say, 110% it can use 290 cfm.
Forced induction, depending on boost pressure, can make this number jump
up to 350 cfm, or more.
IF the 1.75 carbs flow near 150 cfm, (I think it's higher) this would
mean that two of them could easily take care of air flow.
For those who race-shift closer to 4,800 rpm, which is a good point for
the stock, or mildly modified engine, this gear change will bring you
back close to the max. torque of 3,000. Going beyond 4,800 (in anything
but top gear) will not only push the engine beyond where power starts to
fall rapidly, but will then lose the place of max. torque when the shift
finally occurs.
Engines with cams that have their max. torque up higher, and can easily
exceed 6,000 rpm can adjust accordingly.
So, what would this mean if the TR engine had more air flow capacity
than needed? Probably on either an SU or ZS setup with more than two
carbs, it would suggest that the engine would not lift the air valves up
far enough to take in extra air, nor expose the thinnest parts of the
needles for max. fuel. These are "smart" carburetors. If the demand
isn't there, it will not overproduce. This doesn't mean that one
couldn't pour too much (or too little) fuel into the motor. The jet and
needle size still has to be right for the application. I'd say those
with Webers have their own sorting out to go through.
Coffee time...
Dick Taylor
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