Modified SU carb needles for L28

[Stanley]

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.

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 ?

 

 

[siteunseen]

Ztherapy puts SM needles in their rebuild kits so I assume those are good all around.  

@rossiz has SUs on a 2.8.  Maybe he can offer some insight?

[Stanley]

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).

[240260280]

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.

 

 

 

Edited December 13, 2017 by 240260280

[Stanley]

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.

 

[Mark Maras]

@rossiz Geoff, we need your input. Having done the conversion successfully, what needles are you running?

[Patcon]

Have you seen this?

http://www.sumidel.com/shop/category/su-needles

Look through some of the links at how many needles are available..

I thought about getting a needle profile chart

They list an SM needle under 0.100 fixed. No station sizes shown though

[rossiz]

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. 

 

[siteunseen]

I think that's what Stanley wanted to hear.  Thanks Rossi for replying.  You have a solid car and enjoy driving it.  I like that!

[Mark Maras]

Thank's Geoff. I'd hazard a guess that Geoff has SM's. As I recall the SM'x did run a bit richer and those are the needles that ZTherapy supplies.

[Stanley]

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 December 14, 2017 by Stanley

[Mark Maras]

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.