|
And I think this is a totally reasonable position.
The proper volume of fuel for a given engine load is the
proper volume of fuel and it doesn't matter much how it
gets there, at least if you are comparing the proper amount
to an improper amount (ie improperly tuned DCOE carbs vs
properly tuned SUs).
If this was true, it would mean that when buying & installing fixed choke carburettors, you could just buy the biggest one possible. If you were going to buy some DCOE's, you'd buy 55mm ones. If you were buying a Holley 4bbl, you'd bypass the little 600cfm vacuum secondary model & go for a 1150 Dominator.
In reality fuel atomization matters a lot. This is why injection delivers more power & economy than carburettors & why SU's deliver better low to medium speed power & economy than fixed choke carbs. It's also the reason that Twin SU & Injected don't need vacuum advance, but that's another thread.
Given proper tuning and my understanding of different people's
objection, I'll propose the following summary:
weber downdraft:
CON: Reduces air velocity with the 90 degree bend.
Hard to tune. Unequal length runners not optimal.
PRO: Fewer carbs to tune. Intake pulses more evenly
distributed to carb throats.
holley on weber downdraft manifold:
CON: Reduces air velocity, unequal length runners.
PRO: easier to tune, intake pulses evenly distributed
SU sidedraft:
CON: SUs are all worn out or expensive to buy new. Hard to tune
perfectly to a non-stock motor. Intake manifold unevenly feeds
pulses to carb throats.
PRO: SUs somewhat inexpensive to refurbish. It is possible to
use volvo's tuning specs on a stock motor and get good tuning.
Better intake air velocity from direct shot into intake port.
Easy to tune to 75% of ideal.
Mikuni on SU manifold:
CON: Expensive(ish). Same manifold effects as SUs.
PRO: Easier to tune than SUs. Achieves almost ideal use of SU
manifold by virtue of carb design.
DCOE type carbs on DCOE manifold:
CON: Absurdly expensive to tune. All the bits expensive as well.
Ideal fitment may require putting a dent in the fender.
PRO: When tuned properly these carbs are nearly perfect. There
is a reason racers and ferrari use them.
Throttle body injection on DCOE manifolds:
CON: Expensive and complex to install
PRO: All the performance advantages of DCOE carbs but easy to tune.
Lynx DCOE manifold:
CON: You need DCOEs, injection, or adapt SU/Mikuni carbs to this
manifold. Unequal length runners.
PRO: Fewer carbs to tune (if using DCOE style carb). maximizes
air velocity.
D-Jet injection / D-Jet manifold:
CON: D-Jet doesn't adapt itself to non-stock motors. Hard bits
complex and expensive to adapt to a carbed car.
PRO: Near ideal on stock motors.
Programmed Injection / D-Jet manifold:
CON: Expensive (in time or money or both) to install even on a D-Jet
equiped car. It still won't have quite the throttle response of a
DCOE style manifold.
PRO: Ideal running performance on all motors.
K-Jet on K-Jet manifold:
CON: Rare. Fuel metering device reduces intake velocity. Old and
expensive if some bits break.
PRO: Adapts to wide variety of motor changes. Very very efficient
and simple design. Some bits are so common they're quite cheap.
This was the stock injection sold by volvo in some markets into the
mid 90s, for good reason.
More pro's & con's to add:
SU pro: they generally come free, they give MPG's that is only beaten by Injection.
DCOE pro: You can change the choke size to match your engine. You get extra power & torque that is inherent in Independent Runner manifolds.
D-Jet pro: You can modify it to supply 25% more fuel & create 25% more horsepower. You get extra power & torque that is inherent in Tunnel Ram manifolds.
D-Jet con: You need to use a camshaft of similar duration to the D camshaft to go with the VE curve that is built in to the ECU.
K-Jet pro: Can supply enough fuel to feed 80% more power without modifying it. You get extra power & torque that is inherent in Tunnel Ram manifolds. The power lost by the metering flap is negligible.
K-Jet con: The sensitivity of the metering flap means that long duration camshafts upset it badly causing a very rough idle. Brittle in cold weather.
Mikuni pro: Probably provides part throttle fuel economy that approaches SU standards due to it having a small venturi during part throttle like an SU.
Mikuni con: If you buy the right size to suit your current engine it will invariably be the wrong size if you make alterations to your engine.
|
