Sean,

Ok that makes sense - atleast, as much as its going to until I go pull apart and SU *coughnevergunnahappencough*.

But it does require the piston to move in the cylinder BEFORE any extra fuel gets through? If this is the case then the webber setup should give more fuel quicker shouldn't it? And hence be more responsive?

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Nah I know how the webber accell pump works, well, in theory. Its what physically happens when the SU varies its venturi effect that I'm interested in. Something moves yes? Piston in cylinder which moves needle up/down in throat?

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Here are two examples of SU's Vs 40mm Webers.

Example 1:
"Damo666---My 180B rallycar recently got 62rwkw with;
- Stock L18 bottom end
- Twin 40mm Webbers on shitty Lynx manifold
- Mild ported head & unknown cam
- Bluebird elec dizzy
- factory exhaust manifold & 3.9 diff "

Example 2:
"Datsss73---180B Auto, making 59rwkw;
- Stock L18 bottom end
- Twin 45mm SU's
- Matchported head with 25/65 cam
- Bluebird elec dizzy
- factory exhaust manifold & 4.87 diff"

Example 1 has a total intake surface area of 160mm
Example 2 has a total intake surface area of 90mm

Considering example 2 is using a automatic trans and example 1 has a manual gearbox, example 2 would have a greater amount of KW @ the flywheel than example 1.

As example 1 has 70mm more total intake surface area but makes less KW @ the flywheel than example 2, and only 3 more rwkw, I fail to see any advantage in using the 40mm webers.

This was the basis of my "40mm webers blow" comment

Makes sense to me. So to sum up:

SU:

When we crack the throttle open we see a decrease in vacuum at the barrel, this will cause the piston to drop back down under gravity as a specific rate (dependent on the viscoscity of the oil in there). As the piston moves down the fuel supply to the engine is increased.

Webber:

Mechanical accel pump uses the movement of the throttle to actuate a secondary jet and squirt fuel into the barrel.

It makes sense that there will definitely be more lag in the SU as things have to move in response to the change in throttle before any fuel delivery is changed, whereas with the webber the fuel delivery is changed during the change in throttle. That being said, given two rebuilt correctly setup carbies, one webber and one SU, I would hope it would be minimal.

Other differences is that the SU then only has one main jet to fiddle with? Probably an advantage to useless carby people like me, but the webber will offer you more tunability (great for racers).

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Datsss73,

That may well be the case but there are FAR to many variables in there to say that the carbies are responsible for the different results. Experiences are valuable, but only if you can explain why.

Also, what are your 45mm SUs choked down to? twin 45s on a stock 1.8L sounds a tad large to me.

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

No chokes on a SU mate.

sean- I really dont agree with your argument, I do not experience any "lag" with the SU's. They give instant throttle response.

Ok so I'm a moron. SUs aren't twin barrel are they?

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Nope definately not.

i cant be stuffed reciting lines from my "How to power tune SU carburettors"... But they can produce similar power figures to similar weber applications.

however i will be putting a single 45mm dcoe on the l18 i mentioned soon.

Also, i wouldnt bother with the "twin su's" that come off our SSS models, as they are hitachi clones and as my carby specialist says. you cant get needles for them anymore. but you can get needles for genuine su's.

no needle = make your own = good luck!

All from http://teglerizer.com/sucarbs/techtip6.htm#STRU



DESCRIPTION
The model HJL38W6 carburetor is of a horizontal, variable venturi type, which is used in the L16 and L18 engines. This carburetor is designed to keep constant flow of intake air through the venturi under all engine speeds. That is, the venturi opening is automatically adjusted by sliding the suction piston in accordance with the change in volume of intake air.
Metering calibration is accomplished entirely by the jet needle fixed into the suction piston. Then, the relative position of the taper jet needle and nozzle gives the correct air-fuel mixture covering all operating speeds.

When starting the engine, the nozzle is lowered by pulling the choke knob. Consequently, an enriched air-fuel mixture is obtained. Under normal running, the vacuum in the suction chamber lifts the suction piston which slides the jet needle up and the proper mixture is supplied.
This carburetor has the following characteristics:
1. Air flows fast in the venturi when the engine runs at low speeds. Therefore, fuel is fully turned into spray, so that good driveability can be obtained.
2. As the venturi opens wide at high speed running, high output can be provided to reduce fuel intake resistance.

3. The fuel control mechanism is simple in construction because of single nozzle,
thus affording troublefree operation and smoother acceleration.
4. Engine output and accelerating characteristics are greatly improved by the use of two parallel synchronized carburetors. This means that the fuel is fed to two engine cylinders by the front and rear carburetors evenly.

Float chamber
The float system is basically the same in operation as that used in the conventional type carburetor. The level
of fuel is controlled by the float system. Fuel fed from the fuel pump enters into the float chamber through the needle seat. Flow continues until fuel level raises the float to the position where the needle closes the valve seat. As the float drops, the needle moves down and opens the valve seat. Then, fuel enters the float chamber, thus keeping the fuel level constant.

Venturi control system
The suction chamber is mounted above the venturi, and the suction piston slides vertically within the suction chamber, changing the venturi opening area.

Venturi vacuum pressure operates on the upper surface of the suction piston through the suction port, and atmospheric pressure is applied to the bottom of the suction piston through the air hole from the air cleaner. The difference between the upper vacuum pressure and lower atmospheric pressure moves the suction piston up and down. The suction piston stops as a balanced condition exists between the pressure difference and the piston weight plus spring tension. The vacuum pressure is produced by the air flow velocity. For instance, when the throttle valve is opened by depressing the accelerator pedal, the flow velocity of the intake air increases. This also increases vacuum pressure in the venturi, and the suction piston is lifted until the piston is balanced, and the venturi opening area enlarges.
When the throttle valve is closed by releasing the accelerator pedal, the flow velocity of the engine intake air in the venturi is reversely decreased. The piston goes down and the venturi opening area becomes small. The intake air flow velocity recovers as the venturi opening decreases. The piston stops going down because of a balance between the upper and the lower forces operating the suction piston.

Thus, the opening area is adjusted automatically to keep the flow of the
intake air at constant velocity in the venturi. Consequently, the venturi opening is optimum for any engine operating conditions. In addition, the suction piston rod is equipped with an oil damper to prevent the piston rising quickly as a result of sharp throttle opening. As the plunger rod positioned in an oil well operates as a fluid brake on a rapidly rising stroke but exerts no restriction on its fall, it provides an approximate degree of enrichment for acceleration.
The oil reservoir in the suction piston rod in which the oil damper plunger is equipped should be topped up to the level periodically every three months or 5,000 km (3,000 miles) with thin engine oil of preferably SAE 20 but not thicker than SAE 30.

Engine speeds can be done by changing the nozzle opening. (Hence the term "Constant Vacuum" for this type of carburettor ....Pat)
The suction piston rises or falls under the influence of engine suction controlled by the degree of throttle opening. Accordingly, nozzle opening (clearance between the nozzle and needle) changes, and optimum air-fuel mixture can be obtained for any engine speed.

If you can be bothered to read this, it will explain all.

Datsss73,

Its probably not a fair comparison to use my figures in this argument.

The webbers I'm using came off a VW Golf, and its running waaaaay rich down the bottom end (so much so, it runs off the bottom of the mixture chart on the dyno printout I was given....). I'll see if I can scan it, and i'll post it up.

And as Sean points out, even these badly jetted webbers feel like they have a heap more midrange punch & response compared to the SSS SU's they replaced.

Some questions/comments about that datsss73,

So the piston is actuated on pressure, not vacuum, fine. The main point of contention for me is this:

``In addition, the suction piston rod is equipped with an oil damper to prevent the piston rising quickly as a result of sharp throttle opening. As the plunger rod positioned in an oil well operates as a fluid brake on a rapidly rising stroke but exerts no restriction on its fall, it provides an approximate degree of enrichment for acceleration.''

Doesn't this mean that the piston is slower to rise upon throttle opening and that the venturi area enlarging is slower than it could be, and that as a consequence the mixture is lean until it is open?

I guess I'm having trouble with the `prevent rapid piston riding' bit. Isn't the rapid rising what we want? Either that or i'm not understanding the `plunger rod' bit. What is a plunger rod? And how does it affect the `suction piston'?

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD