SU Bridge Evolution: Round Top vs. Flat Top (Comments welcome)

[240260280z]

Top view looking down piston bore (flat top is on right)

Flat top:

1. Bridge is wider (front to back).
   
2. Jet sleeve is fixed and jet depth is set to correct height in factory (no choke up/down movement).
   
3. Front and rear ramps are different geometry.
   
4. Piston diameter is smaller
   
5. Has 3 circuits


1. Main (jet and needle like in a round top)
   
2. Choke (see the small forward spraying nozzle upstream of main jet)
   
3. Power Valve (off idle transient to WOT...see the hole on right side of ramp at back of bridge)
   


4. Has air bypass around main jet for setting idle mixture.
   
5. Has integrated fuel bowl below jet.
   
   
   Front view looking into throat (flat top is on right)
   
   Flat top:
   1. Bridge is lower in throat.
      
   2. No stepped edge on front of bridge. It blends into ramp.
      
   3. Choke nozzle is seen in foreground in front of main jet protruding from the floor.
      
   4. Floor of throat from front of carb blends into ramp.
      
   5. Choke valve (in open position) is seen in foreground near top.
      
   6. Tube bringing ambient air to push up on piston is seen in foreground on right.
      

Rear view looking into carb (flat top is on right)



Flat top:

1. Bridge is lower in throat.
   
2. Higher vertical dropping edge "cliff" at back of bridge
   
3. Narrower vertical dropping edge
   
4. Rear edge of bridge is further back in carb and closer to throttle valve
   
5. Power valve fuel outlet is seen as hole on lower right of photo below the bridge level.
   
6. Idle air outlet is as hole on middle right of photo above the PV fuel outlet.
   
7. Bushing for throttle valve (removed) is seen on side wall of throat
   

Edited September 6, 2011 by Blue

[SteveJ]

Unfortunately, at the time I am viewing this, there are no pictures.

[Zs-ondabrain]

I'll back that up Steve. No pic here either.

Great info but pics would be awesome.

Dave

Also, the bottom pic links give me this........

"Invalid Attachment specified. If you followed a valid link, please notify the administrator"

Edited September 6, 2011 by Zs-ondabrain

[240260280z]

Weird, let me see if I can fix.

How is that now?

Edited September 6, 2011 by Blue

[SteveJ]

Okay, I can see the pictures now. Of course, I need to get a lot smarter about carburetors to really understand the post.

[Zs-ondabrain]

Totally makes sense now. I actually understand 90% of it. So does this mean the FT is a better flowing Carb with less intrusion of the center ramp? Does the smoother floor to ramp layout improve flow over the Round Top? Better power or MPG?

Basically, what does each change do for the mixture and power from one carb to the next?

Dave

[steve91tt]

Great information. Thanks for posting.

[240260280z]

Does the smoother floor to ramp layout improve flow over the Round Top? Better power or MPG?

We will find out.... with data over the next month or two.... Tony D already knows.

I presently am planning a "diluting circuit" for a round top to make it a 2 stage carb... an accelerator circuit would just be needed to make it 3....thinking on that now.

The fact the 2nd gen also draws fuel lower in the throat (like a British racing mod), it has a narrower throat (like Mikuni bike CV ), a big fuel bowl sits right under the bridge, and the fact its bridge geometry is much different means that it was changed for either performance, fuel efficiency, emissions, reduced manufacturing costs, or a combination. As mentioned, exploration, data gathering and comparisons are needed. There are many other interesting differences that will be shared too.

Remember that Nissan engineers seemed to always fight emissions performance losses that hurt the base L24 building block by increasing horse power (stroke & bore) so it is reasonable to assume they also looked elsewhere for improvements.

Probably heard in the halls of Nissan HQ in the early 70's:

Engineer "The !$^& US pollution and safety regulations are turning the car into a fat lazy pig."

Bean Counter "If the power disappears, the customers will disappear and we will disappear."

Boss "Work the problem and give me solutions."

Edited September 6, 2011 by Blue

[Captain Obvious]

Nice documentation. Thanks for taking the time to put that together!

Couple small corrections...

The tube seen on the right side foreground of the flat top in this pic is the ambient pressure vent to the float bowl, not the high pressure side of the suction piston:

There are two holes down inside the flat top throat that vent to the bottom of the suction piston, but you cannot see them in any of the pics. We didn't talk about them, but they are above the choke plate.

Also, I'm really not sure about the "off idle transient" effectiveness of the power valve. Unless there's some sort of transient second order transient "uber reduction" in manifold vacuum when you change the position of the throttle plate angle, it appears like the power valve does not act like an old school "accelerator pump". It simply has a vacuum threshold above which it starts to supply additional fuel.

I know I've heard that belief before and I'm no expert, but my analysis of the power valve operation does not support that concept.

[240260280z]

Nice documentation. Thanks for taking the time to put that together!

Couple small corrections...

The tube seen on the right side foreground of the flat top in this pic is the ambient pressure vent to the float bowl, not the high pressure side of the suction piston:

Philip: Argggh sorry. I tried to figure that out at the last minute last night when posting. I was squinting at the FSM drawing to chase and it looked like it simply went under the piston. I assumed it replaced the two holes in the round top front flange.

There are two holes down inside the flat top throat that vent to the bottom of the suction piston, but you cannot see them in any of the pics. We didn't talk about them, but they are above the choke plate.

Philip: I'll put the FSM drawing up on this thread and eventually try to add colour to it for better clarity

Also, I'm really not sure about the "off idle transient" effectiveness of the power valve. Unless there's some sort of transient second order transient "uber reduction" in manifold vacuum when you change the position of the throttle plate angle, it appears like the power valve does not act like an old school "accelerator pump". It simply has a vacuum threshold above which it starts to supply additional fuel.

I know I've heard that belief before and I'm no expert, but my analysis of the power valve operation does not support that concept.

Philip: I was scouring the net at 2 am reading about it and trying to get more details of it's design and function. When we investigate further, we can put a web cam window in the carb and watch it as well as run a tube from its output out of the carb to capture fuel and see when it operates. When we do the O2 runs we can compare two runs with it inservice and out of service to see when and where it kicks in

Dang that fuel bowl reminds me of a weber! Could you do me a favour and post the volume/capacity of the flat top and round top carbs if you have a chance? That would be good info to float (pun) on the net for racers.

[Captain Obvious]

If I get the chance, I'll take some pics of the vent holes leading to the suction piston. Being as how they are hidden up on the roof of the flat top bore above the choke plate, you don't normally see them unless you're specifically looking for them. Speaking of those vent holes... At the risk of getting kicked out of the SU section for heresy, I consider this another improvement with the flat top design.

You see... On the round tops that vent hole is completely on the other side of the intake horn in the air cleaner, while on the flat tops, it is a true differential pressure right across the suction piston. Probably doesn't matter, but it's just "purer" with the flat tops.

Could you do me a favour and post the volume/capacity of the flat top and round top carbs if you have a chance? That would be good info to float (pun) on the net for racers.

You mean the volumes of the respective float bowls, right? If that's the case, then yes, I should be able to measure that (crudely) and let you know what I find out.

[240260280z]

Nice documentation.

Also, I'm really not sure about the "off idle transient" effectiveness of the power valve. Unless there's some sort of transient second order transient "uber reduction" in manifold vacuum when you change the position of the throttle plate angle, it appears like the power valve does not act like an old school "accelerator pump". It simply has a vacuum threshold above which it starts to supply additional fuel.

I know I've heard that belief before and I'm no expert, but my analysis of the power valve operation does not support that concept.

I found this on Zcar.com. It matched my interpretation of the drawing of the power valve on paper but exploration of the air and fuel lines is critical to understanding its function. My thinking is that during a throttle-up transient, the spring unloads and squirts fuel into the air stream as opposed to being to be sucked into it.... but I have no details of the control circuit yet....but I bet you are correct with the vacuum threshold I should have brought my mighty vac yesterday.

Logged

Al 260Z

Daimyo

Posts: 1719

Flat-top Owners Unite!

« Reply #2 on: September 14, 2001, 09:52:17 pm »

Regarding the notorious powerjet.

I believe how this works is as follows:

The chamber under the outer cover is ported to the downstream (intake manifold) side of the throttle butterfly. This chamber is sealed by a flexible diaphram so that it should normally be free of liquid gas. A spring under the outer cover pushes the diaphram towards the carb body and is counteracted by engine vacuum when intake vacuum is high.

On the other side of the diaphram is the 'pump' chamber with one port to the upstream (air cleaner) side of the butterfly. This chamber is normally filled with liquid gasoline which enters via a weird valve arrangement from the fuel float bowl. On high manifold vacuum, the diaphram is sucked against the spring and gasoline is drawn from the float bowl into the 'pump' chamber. When manifold vacuum decreases on throttle-on, the spring pushes the diaphram and injects fuel via the upstream port.

I think that if the diaphram were ruptured, the symptom would be excessive rich mixture at idle and at part throttle operation due to gas sucked through the diaphram and into the manifold via the downstream port.

Should the power jet diaphram fail, I believe it's possible to defeat the whole power jet by taking it off and either rotate the whole assembly or else disassemble the jet and rotate the inner gasket so that the ports no longer line up and are plugged off. The '75 FS Bulletin has some obscure comments on this.

Remember, Adam, carbs are the LAST thing to tweek on. You gotta have good settings on your valves, proper ignition timing and advance, and NO VACUUM LEAKS before you begin thinking about final settings on carbs. Get that Mighty-Vac and test ALL your vacuum auxiliaries. Plug any that are in question. Start the engine and thoroughly test for vacuum leaks all around the manifolds and balance tube. You may be able to hear some by probing around with a hose with the other end stuck in your ear (hisssssss!). Some suggest spraying carb cleaner on suspect joints or searching for leaks using unlit propane torch... Leak will be indicated by change in engine speed due to change in mixture when you get fuel source close to leak.

By the way, just heard that SCCA 'C-Stock' Auto-X national championship was taken a couple of years by stock 260 w/ Flat-tops! Could he a done that w/ some really f-d up carbs, d'ya suppose??

Good luck!

-------- Al

Edited September 6, 2011 by Blue