Don:
The reason PI performed so much better than ZS:
1. Better mixture balance between cylinders by virtue of
one injector per intake runner. Carbs are subject to
the vagaries of mixing and air flow in the plenum.
2. PI cars have higher compression (9.5:1) compared to
a max of 8.5:1 for Carbs (later dropped to 7.75:1).
3. The PI cars always had the dual exhaust system, which
gave better breathing than the single pipe on the
carb cars. Triumph later added the dual pipe to the
US spec cars to make up lost performance as emission
limits were steadily tightened.
4. The PI cars had a better cam for most of their life.
The carb cars had a wimpy cam for most of their run.
Once again, the cam was upgraded later as emission
controls took their toll on the carb'ed cars.
5. PI cars had lower gears, which gave better acceleration.
7. Although it is less certain, the PI cars had a higher
rocker ratio giving more lift and duration for a
given cam (1.5:1 vs 1.45:1). This point has been
disputed.
8. The PI cars had cold air induction, the carb'ed cars
never did. Cold air is denser, which allows more
fuel to be burned, which makes more power.
So you see, it is much more complicated that just bolting
n PI and expecting 150HP "because PI is better". No doubt PI is
better, but it is just part of a whole package that gave better
performance on the European spec'ed cars. The PI cars
got TERRIBLE mileage relative to their US counterparts. From
memory (always dicey with me) it was about 9mpg worse on PI
cars.
Cheers,
Vance
------------------------------
1974 Mimosa Yellow Triumph TR6
Cogito Ergo Zoom
(I think, therefore I go fast)
-----Original Message-----
From: Don Malling [mailto:dmallin@attglobal.net]
Sent: Tuesday, January 07, 2003 6:58 PM
To: 6-Pack
Subject: PI vs dual ZS vs triple ZS -- dumb questions
Some say that dual ZS are sufficient until I run over 5000 rpm for over
one minute, because that is where I will run into fuel starvation with
dual ZS. Some say triple ZS will add to performance at any RPM.
What is it about PI that provides a performance increase over
carburetion?
Is it a case where a theoretically very small carburetor could never
provide enough fuel air mixture to a theoretically very large engine
without running too rich -- I assume running too rich means that the
fuel is not being atomized or vaporized correctly? On the other hand, a
theoretically very large carburetor could not provide correctly mixed
fuel/air to a theoretically very small engine because the theoretically
very small engine can not produce enough air flow to make the
theoretically very large carburetor work correctly?
Hmmm.... Is that it? There has to be a balance between the "size" of the
carburetor and the "size" of the engine for things to work correctly?
And even when working correctly (the best it can work), the carburetor,
by its nature, still can not generate as much correctly mixed fuel/air
as can the PI?
If this is the case, then were does triple ZS fall in the scheme of
things -- on a scale of 1-10, where 1 is dual ZS and 10 is PI? Maybe
the answer is at low RPM triple ZS is at a 2 and at high RPM triple ZS
is at 7?
Thanks,
Don Malling
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