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Modified SU carb needles for L28


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280 engine will move more air and so need more gas. The question is how much to reduce the needle diameter for stock N-27's. Did some math but wondering how helpful it is.calc for SU needle mod 280Z.jpg

So it would seem like you could just polish the needles down from stock dimensions at each station by one tenth. Not that easy though. When I installed K&N's and a 2 1/4" tailpipe on the stock 240 engine it was flowing more air and had to richen the mix. When I installed a better-breathing skyline Y-70 head it was flowing more air. Had to modify the needles and install stiffer springs to keep from running lean.

The new engine is not a wild build but will have a 280zx head and other mods. It will have 3-2 headers but same 2 1/4" pipe. Is my math useless ?

 

 

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Station 1 is 0.099 for (British) SM's vs. 0.095 for N-27's etc. (Hitachi). SM's wouldn't work on my L24. Too lean at low end under load. Had to set mix nuts 3 1/2 turns down or it would go lean and stall out leaving a stop sign. Then it would be too rich at mid-range and foul the plugs.  Might be OK for L28 though. Maybe OK for L24 with oversize nozzles (if stock needles are too lean).

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This may be helpful. (no units).

You will need to map air flow at rpm to vacuum then map vacuum against needle height (annular ring exposure) for various throttle openings. Then map the fuel flow from exposure and vacuum.

 

image.png

 

 

Edited by 240260280
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Thanks for the graph.  I need to think about it before commenting. Would like to hear from others with L28 daily drivers using SU's (not race cars since they don't have similar idling and low RPM performance requirements).

I saw at MSA site if you use early nozzles you need early needles. Called them this morning but didn't get a definite answer about the difference. Called Bruce Palmer, asked if the distinction was bogus, he said yes. He also said the stock Hitachi SU nozzles were 0.100 inside diameter, same as the British SU's that use SM and related needles. So if that's true, then why are SM's and similar British needles, listed as .100 which refers to the nozzles, all listed as 0.099 at station 1, while the N-27's are 0.095 (miked by Captain Obvious and verified by me and my Japanese micrometer ? I expect the other stock Hitachi needles, like N-54's and N-58 (US spec. emissions) are also .095 at station 1.

I wonder why the stock non-emissions needles were changed from N-27 to N-54 for '72.

I was thinking of getting some oversize nozzles but Bruce says they would be too rich. I remember reading a thread about SU needles on an AU site. Boring out the nozzles to use British needles like SM's was discussed but no one had tried it.

The R&R mechanic, a reliable source IMO, says it can't be running rich or the rings won't seat. Plan to wake up the new engine in the next few days.

 

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I’m afraid I don’t have much to offer here, as I’m nowhere near the level of dialed-in performance that you’re trying to hit. My 280 is a DD that I have a great deal of fun with, but it’s not a track day weapon. I have a set of round tops that I rebuilt with a ZTherapy kit and it runs great for what I need. It was a couple years ago, so I don’t recall specifics on the needles - I believe they are a bit richer than stock. 

At some point, my plan is to swap out my 6-1 header for a 6-2-1 so I can install dual O2 sensors to measure & map (probably when I do the cam) but I’ve been largely distracted by other projects since the Z is running well enough as is. 

 

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Here's part of the needle profile book, just the .100 needles so is easy to print. It's an old book so there might be newer needles that aren't listed.

Needle charts pp 47 to 49.pdf

I don't think there's a one-size-fits -all SU needle for Z cars. Not that I know very much, but the official SU tuning book page 7 says you need different needles if you change the exhaust or air cleaner for example. Here's a PDF of the whole book, it also has the needle profiles. Tuning_SU_Carburetors.pdf

If the needles are too lean they can easily be polished richer. If they're too rich you're stuck,  pun intended.

Edited by Stanley
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One of those old SU articles said that if you're going richer in any stage, .001-.002 is the amount to remove. When leaning out a stage, they said .001 increments are best. More danger in harming the engine with a lean condition. They also recommended not changing the idle stage because if you richen or lean the idle stage, it will do the same to the entire range. I understand the wisdom in modding needles in small increments but I can't figure out why changing the idle stage would affect everything else. It must be true, it was printed in an old book and not the internet.

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I don't get it either but last summer I tried richening stations 1 & 2 on the SM's thinking it would allow me to raise the mix nuts and have a good idle without going too rich at mid-range. Didn't work, it ran way too rich; don't know why.

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4 hours ago, Stanley said:

why are SM's and similar British needles, listed as .100 which refers to the nozzles, all listed as 0.099 at station 1, while the N-27's are 0.095 (miked by Captain Obvious and verified by me and my Japanese micrometer ? I expect the other stock Hitachi needles, like N-54's and N-58 (US spec. emissions) are also .095 at station 1.

All of those needles are called ".100 needles" because they are "designed to be used with .100 nozzles." That's just the way they are classified.

There are ".090 needles", ".100 needles", and ".125 needles." That's just the family characteristic.

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11 hours ago, Stanley said:

Here's part of the needle profile book, just the .100 needles so is easy to print. It's an old book so there might be newer needles that aren't listed.

Cool. I guess the real problem with selecting needles is knowing what needle station is producing what AFR reading. I don't know how you know at what height the piston is in the dome at any given RPM?

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 Another one of those old SU articles talked about removing the dampers (screw top with the jiggly bits), inserting rods (pencils?) with 1/8" graduations into the piston stems, starting the engine and monitor the height of the pistons at any given RPM. The part that confuses me is, will the pistons go higher than normal with no dampener. The article didn't mention it but t seems like it would due to the fact that different wt. oils will change the mixture over the entire throttle range. A possible way around this, IMO, would be to mark the side of the piston that is visible with 1/8" graduated lines (Sharpie pen) and watch the piston as it moves up through the RPM range. The article also mentioned that if the rods are equal length, the carbs are balanced when the part of the rods are equal ht. Makes sense.

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37 minutes ago, Mark Maras said:

The part that confuses me is, will the pistons go higher than normal with no dampener.

Maybe this will help... They wouldn't go any higher. They would just get there faster. The dampers don't control the height of the pistons, they just slow down the speed of travel.

Wood sticks temporarily in place of the dampers would give you a good idea of what's going on inside the carb, but I've got two issues with that technique. First, when you remove the caps, you create a small vacuum leak. The same vacuum that pulls the dampers up is now vented to atmosphere. Might be OK above idle, but I'd be worried about that leak at idle. And second... You won't really get much useful info with the car sitting still under no load conditions. What you really need to do is drive the car with sticks in place and see what the pistons do with a load on the engine.

Since the amount of piston rise is linked to engine output, with no load you won't ever get much piston rise. A transient when you blip the throttle yes, but no steady state synchronization info to be gleaned from that scenario.

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1 hour ago, Patcon said:

Cool. I guess the real problem with selecting needles is knowing what needle station is producing what AFR reading. I don't know how you know at what height the piston is in the dome at any given RPM?

Camera or measure vacuum from nipple drilled into top of plastic plunger .....remember our discussion many moons ago of mapping vacuum in carb dome against load and rpm then replicating on a flow bench to "find station"?   We have been there and almost done that :)

 

https://www.youtube.com/watch?v=i2I5QKDDEDc

 

Another:

 

 

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I think the vacuum method is important  as it can be replicated on a test bench.

One can wet-flow water safely on the bench with the reference needle and old carb then replace against the modified needle as it is worked to measure differences in fuel flow for each station(vacuum reference).  As mentioned, CO and I talked about these methods many years ago :)

Another method is to have non-tapered needles of different diameters then drive with these to map a/f against rpm and vacuum.  This will work nicely for optimizing the higher stations of idle and cruise. Again... this was from many moons ago. Note that each straight needle will suck in most load/rpms except for some sweet spots so the goal would be to find these sweet spot stations and assign the diameter of the test needle to these.

 

 

Edited by 240260280
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So yes, the math in post #1 was useless (except at station 1). The smaller the stock needle area at a certain station, compared to nozzle area, the more would need to be be removed from the modded needle to make 1/6 more gas surface area. The graph in post #4 shows this. Eventually the needles get very pointy and getting 1/6 more gas area than stock becomes impossible.

There's another problem with the math though. The larger engine makes more vacuum at a steady rpm, raising the piston and putting the needle at a richer station as discussed above. Maybe stock needles would work on a larger modified engine, except for acceleration.

 Bought a pair of stock SM's and a pair of ZH's (slightly leaner than SM's) today. Also have the modded SM's and N-27's. Don't know if I should stay with the 4 ounce red springs or move up to 8 ounce yellow. Nothing in between. Got to think about fuel economy. Yellow springs might make it a real guzzler.

 

 

Edited by Stanley
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  • 3 years later...

My early 71 came with yellow springs. I think they all did. I'm not an expert on springs but logic tells me that a stiffer spring would raise the piston a bit at idle and richen the mixture unless one raised the nozzle to compensate. Thicker oil will increase the mixture during acceleration. It acts like an accelerator pump on a normal carb.

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SUs don't have a function like an accelerator pump on a Weber, which is one of the reason Webers have a slight edge on immediate acceleration.

The spring and oil are about damping the piston to keep the engine a bit smoother instead of always micro fluctuating with engine speed changes. Spring and oil work together to set how much it takes to get going and at what rate it goes once it is going, respectively.

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