SU carbs WILL work perfectly on my modified L28 and give DCOEs a run for their money!!! | The Z Club of Great Britain
  1. The Z Club relies on the support of it's paid members - join the club or renew your membership here.

SU carbs WILL work perfectly on my modified L28 and give DCOEs a run for their money!!!

Discussion in '240Z / 260Z / 280Z' started by AliK, Jun 11, 2019.

  1. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    As the title suggests the gauntlet is down!!!

    Don’t get me wrong, I salivate like the next man at the mere sight of the DCOE carbs but this is something I want to crack!

    This will be a long post / thread as it will be the musings of a madman refusing to give into conventional wisdom that “them SUs just won’t flow”.

    There is a wealth of information out there about how to modify your L series engine, but very little on AFR, needles and carb set up for modified engines using SU carbs.

    So I set about working it out from first principles and I’m doing this write up in case it is helpful to anyone else. I will keep updating it as I experiment; I humbly ask for kindness in your commentary and feedback however dumb my attempts appear.

    Having had the car RR tuned and the SM needles re-profiled professionally, the car is producing great power at the top and a has very bumpy power curve in the low to mid range.

    This is primarily a road car and needs to be more than bragging rights on headline figures so the carb setup has to work for more than just the top end power.

    The AFR on the modified needles is rich everywhere minus the very top end (over 6000 rpm in 4th) where it goes lean (13.5ish); I was told I need triple DCOEs to get a richer top end with less compromise on being rich low down.

    The challenge:
    I will ABSOLUTELY keep my SU carbs. After-all, there are people with them who are racing and making more power than I do + some running 12 second quarter miles on SUs. So it is very possible, it just needs patience, data and analysis + a rather sizeable learning curve!

    The goal:

    To maintain the top end WOT AFR of between 11.5-13.0 across the rev range and between 14.0 - 16.0 cruising.

    [​IMG]

    The engine is an L28 with compression of 10.5:1, 270 degree cam on an E88 head with large valves, sucking through Z therapy SU carbs, blowing through a Z Story Race/Sport full system and producing over 232BHP.

    I wrote to Z therapy to see if I need bigger nozzles and this is what came back, I love those guys ....

    [​IMG]

    Onto the carbs.

    Firstly, I needed to understand the principles of SU carb operation.

    Lots of info on the links below and the internet on the topic.

    https://www.mgexp.com/phile/1/492119/SU_idiot_guide.PDF

    https://sucarb.co.uk/technical-su-carburetters

    https://zparts.com/index.php/resources/su-carburetors-explained/

    In short for those new to this: the SU is a variable choke carb where the choke size increases with engine load and raises a tapered needle out of the jet allowing more fuel flow.

    The greater the load, the greater the suck and the higher the piston / needle raises, opening more area in the jet. The other key factor to consider is that the air flow velocity across the "bridge" / jet affects fuel drawn, so a piston high up with the needle pulled up may still cause a lean condition as air flow velocity (read vacuum) across the bridge is reduced compared with the piston at a lower position.


    [​IMG]

    There are very few things you can adjust:

    Float level
    · Height of fuel within the jet tube and float bowls. Most important and the first thing to set at around 10mm below bridge as a rule of thumb.

    Needle profile
    · Over 700 needles and profiles available, note: the taper is not always linear
    · 14 stations, each station is 3.2mm: stations 1&2 = idle, 3-6 = Cruise and Acceleration, 7-9 = top speed - last 3-4 stations not used

    [​IMG]


    Jet Height

    · Each full turn = 6 flats; 1mm

    Spring Rates
    · Controls the height of piston hence the Venturi choke size and therefore the air velocity across the bridge / jet. Stronger springs = Faster air velocity = more fuel but more choked airflow and richer operation in general.

    Damper oil:

    Thin = sluggish throttle response,
    Thick = snappier response.
    Note the damper oil only affects mix between piston height transitions but not the ultimate AFR once the piston has fully risen/dropped. SAE 20-30 oil is recommended depending on what you read; I use SU official oil.

    Still with me and reading?

    Then onto the needle work next. I will write it up the on my commute tomorrow ...
     
    Last edited: Jun 11, 2019
    slaphead, tyroguru, Woody928 and 2 others like this.
  2. atomman

    atomman
    Z Club Member

    Morning Ali ,

    Its always a balance between smaller choke size gives better response down low and bigger gives more up top , as well as trying to keep the AFR in, I'm sure you will find a happy place,

    And if you come up with a new profile you want to test I will have a go at machining some up for you if needed,
     
    AliK likes this.
  3. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Great advice and thank you sir that is a very very kind offer!!
     
  4. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    You can profile needles in a drill using 600 grit wet and dry. Or there is the old racers' trackside method of filing flats like my tuner has done.

    Here’s a good read ...

    https://www.terryhunt.co.uk/tech/picsb/pics.htm

    [​IMG]

    Looking at my modified needles under a magnifying glass / iPhone microscope attachment, I am not fully convinced by the finish, especially given that a few thou here and here has a noticeable effect.

    [​IMG]

    Here's the untouched part of the needle at the same magnification for reference.

    [​IMG]

    I marked the stations on both modified needles and with a micrometer measured each; 3 times each. Interestingly the pencil marks are impervious to petrol and don’t wash off!

    [​IMG]

    As can be seen they aren't in synch and the front carb is going much richer before coming back to trend. (RC needle used here as a straight line reference).

    [​IMG]

    Given that there are hundreds of needles and £12.50 each + delivery, experimentation gets expensive and frankly, a royal pain! I built a spreadsheet to do the hard work in finding the right needle. There are other resources out there but I built my own to look up needles against my specific profile and wrote some formulae to model the jet opening (anulus area) at different jet height settings.

    Great site for comparison in a chart
    http://www.mintylamb.co.uk/suneedle/

    Great site for data on any needle
    http://www.teglerizer.com/suneedledb/

    Comparing my needles' modified profile against others is a little misleading as the "segment" cut out of a circle is not the same cross sectional "area" as one reprofiled round to the same taper. What is a direct comparison is the annulus area i.e. The size of the jet hole with the needle in it at various stations. The diagram below better shows what I'm trying to convey. Note the needle on the left is an equivalent needle with the same measurable width at the same station as the modified needle.

    [​IMG]

    Knowing only “h” height of area left after filing and needle radius “r”, the maths for calculating the (yellow) area removed gets a little complex. Never fear for everything there is a google search.

    I used this calculator to save me the maths and worked out each station's cross-sectional area removed.

    https://planetcalc.com/1421/

    Then I subtracted it from the unmodified area to get the cross sectional area of what is left of the needles at each station. Then I took that area away from the jet cross section. That way my chart will show how "open" the jet is at each station.

    ZT carbs come with .100 jets which is 2.54mm diameter in new money; meaning a cross sectional area of 5.067 square mm.

    The easiest way to identify alternatives was to put in a reference needle like the SM on mintylamb, then select any second needle and use the down arrow on the keyboard to speed scroll until one looked like a good fit. I went through over 400 needles in under 5 mins using this method and came up with a shortlist of 5.

    [​IMG]

    Remember the goal is leaner low down and richer up top.



    Interestingly a lot of the guys with 3.0L+ strokers use SB needles. Here's one guy who did a YouTube video of his.



    Here’s the SB in green set a little leaner compared with my modified needles ...

    [​IMG]

    It looked like a pretty good fit.

    And to prove the carbs can flow and provide enough fuel here’s a really rich needle (KV) for comparison.

    [​IMG]


    Needles ordered from Burlen and arrived within a day

    [​IMG]

    So that is the prep: Next up, the real world tests ....
     
    Last edited: Jun 12, 2019
    toopy likes this.
  5. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    Ali, in my case you are preaching to the converted. I have been singing the praises of my 2" SUs (HD8s) on my track car for years.

    https://zclub.net/community/index.php?threads/sus-v-triple-twin-choke.15930/

    They were set-up on a rolling road by a very practical 'old boy' who has died now. He used Emery paper on the needles not a file!

    I have triple 40s on my Samuri and they seem to work well but I know it took Jonathan a while to find someone who could do it properly.

    I have standard Hitachi on my blue car 2.8, that also runs well but has a fairly mild cam in it (DJ Roadsports?).

    240Z 004.JPG 240Z 005.JPG
     
    Last edited: Jun 12, 2019
    slaphead and AliK like this.
  6. Farmer42

    Farmer42
    Z Club Member

    Ali

    I don't know if you are aware but Burlen do a Needle Profile Chart (Part No. ALT 9601) that give all the stage profiles and advice on alternative needles to give you the maximum power with the best mixture & airflow. Probably a lot cheaper than buying needles on a trial & error basis.

    Incidentally, my HS6 1.75 inch SUs have 0.100" jets with SM needles and they run lovely although it did take a bit of patience & tinkering to get it smack on. I haven't Dyno tested it so can't comment on BHP or fuel mixture, I just measure it by ear and how the engine runs & accelerates. Recently at Castle Combe, I did stand a £1 coin on edge on the rocker cover of my engine at 2k revs and it stood there for 30 seconds so it can't be that bad.

    I have heard that 1.75 inch carbs on a bigger L28 engine could use DX fixed needles.
     
    AliK likes this.
  7. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    So uncle Paul, you are truly the Club’s SU king and I was wondering how long it would be before you drop in some genius!! :thumbs:

    I love the £1 coin idea - wouldn’t make it a second on my engine, but I also doubt my cam is conducive to a smooth fast idle either ;)

    The Burlen book is indeed a great resource however I have 700+ profiles on my S/S and wanted a dynamic tool for matching my existing profiles and simulating the effects of how many flats up or down.

    You, unfortunately, stole my thunder early. :thumbs:

    I have arrived at the same conclusion. I will continue to write up how I got there and the outcomes. I suspect a little polishing may well be required to achieve the ideal tune though for my engine’s specific behaviours.
     
  8. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Time for some road testing ....

    This is what the three needles look like going left to right in richness ...

    [​IMG]

    Noting the jet heights set by the tuner first so that I could always return to where I started, I went out to play. Leaning the mix a flat at a time, I found that a half turn lean was the best point for a happy engine and good low to mid range AFR.

    [​IMG]

    There are two torque dips / holes in the power band It’s always too rich between 2.1k - 2.5k rpm and 3.1-3.5k rpm, where the AFR goes to the 9s and 10s and the engine bogs down before coming to life again. This is probably due to cam overlap and header combo causing a reversion condition. I first noticed it when I went to the bigger diameter headers but the effect became much more pronounced post final RR tune. It is likely accentuated by over-rich needles.


    [​IMG]

    I logged the behaviour of the CCK needles (I call them that after the tuner who modified them) by positioning the AFR and iPad in the glove box, with my iPhone recording video, electric taped to the passenger headrest! Poor man’s data logger but effective! The iPad shows the 123 ignition app with MAP, revs and MPH.


    I then dropped in the SB needles and did the same with the jet height set to what my spreadsheet had predicted as a good place to match the CCK needles.

    The AFR was lovely and rich (11.5 - 12.5) at the top end but still a little too rich mid and low down as it practically tracks the CCK needles there.

    Next up, I dropped in a super rich KV needle to prove that there isn’t a flow / intake air speed problem. Sure enough it was mega rich on WOT with the jets wound nearly all the way up, able to hit low 10s / high 9s at 6K RPM. It proved that we don’t have a carb unable to deliver enough fuel / air problem. The issue is the needle profile.

    Here a a visual comparison of the SB and KV needles, you can see why the KV is SOOOO much richer at the top!!

    [​IMG]

    So I went back to the CCK needles and jet heights set 2 flats leaner than the tuner and did a bit more data logging. This time measuring cruise AFR (steady throttle) to see how rich the needles are at different points around the rev range . The results below show that the needles are over rich in low cruise but good at high cruise (over 4krpm).

    [​IMG]

    Back to the spreadsheet and found a formulae error that explained why the SB was richer than I expected. Once corrected I went through the database again and found a new shortlist.

    On the upside, I found two needles that are a pretty close match with what I am aiming for: DX and KD.

    From station 5 up, both needles have the same taper. The difference is the idle, moderate and light cruise areas.

    Using the Burlen resources/info, I marked the anticipated needle heights at different revs on my spreadsheet. (Vertical red dotted lines).

    [​IMG]

    I am leaning towards the DX as it is a leaner needle which enables me to profile it down and runs just richer at the top than the CCK needles’ average. It does make for a very lean idle. The dotted red line is the CCK (tuner reprofiled needle) average between front and rear carbs. The blue is the original SM needles for a baseline on leanness.

    Here is the KD for comparison ....

    [​IMG]

    If in the real world the KD turns out to be too rich at the bottom I can’t add material so would have to profile the rest of the needle at the top end. Having said that, I still may make the final 5 stations richer to avoid the max torque/power lean conditions we saw on the RR.

    Here is KD 1 flat leaner and it really works well for my 3-3.5 krpm toque hole.

    [​IMG]

    By contrast here is the DX set 1 flat richer and the idle / cruise is still very lean.

    [​IMG]

    Going even richer on DX will get the torque hole close to too rich again.

    My car will spend time idling in traffic and queuing for entering events like Silverstone or at Goodwood so I don’t want a lean and hot idle.

    For fun and some perspective for those using stock 240z needles, here is the comparison N27 needle (black) against the SM (blue) and my current CCK (dotted red) needle.

    [​IMG]

    For now I’m leaning strongly towards KD but the DX is a very strong contender, so I may buy both and return the one I don’t use.

    All this being theoretical, I need more real data before ordering more needles.

    Next up ....

    Enter stage right, the web cam. The plan is to pencil mark the rear carb piston in 1/8” increments from the resting point down; place a webcam pointing into the carb mouth (without blocking airflow) mounted where the engine inspection lamp is. Then setting up the timing light with rev counter within the field of view and recording piston positions at various revs - cruising and WOT. It could of course all end in a horrible mess of molten plastic ;)
     
    Last edited: Jun 13, 2019
  9. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Those HD8s look beefy Rob! What spec is that engine and what power are you producing?
     
    Last edited: Jun 13, 2019
  10. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Got the web cam installed where the inspection lamp would be.

    [​IMG]


    and marked up the piston

    [​IMG]


    A nice view right there for a geek!!

    [​IMG]

    And then discovered that my torque holes are definitely reversion related. I have fixed the blowing on the collector joints but need to stop it blowing on he S pipe at the back where the exhaust silencer is connected to the rest to prove that the reversion isn’t due to the leaky joints. Here’s a video for your enjoyment ... you tube has reduced the quality so you can’t see the markings on the piston as I can on my phone / PC.



    I may revert back to the old header / exhaust to do a back to back comparison.

    So if time and children permit tomorrow, I will head out to the motorway and do some cruising recordings at different revs and with a head torch mounted next to the web cam to properly light the piston markings.

    Once I have some needle positions and can confirm my model on the spreadsheet is telling the truth then I will be ordering both the KD and DX needles. I may also order a stronger spring to see the effect of increasing intake air velocity on reducing this reversion effect before switching back to old header.

    On that note I measured my spring using kitchen scales and some books ...

    [​IMG]

    The Hitachi SU spring is doing 2 oz!

    Here’s a table of HS6 springs that are compatible - I think a 4oz would be a good test.


    [​IMG]

    More fun to follow tomorrow!
     
  11. franky

    franky
    Z Club Member

    Thats great work. I like the sultry voice over!

    You think Seans Zstory manifold is causing the torque dips? A good test would be to have it on RR now when you've tweaked it as far as possible, then with your older system?
     
    SeanDezart and AliK like this.
  12. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    Just don't go below 3500 rpm. Simple.
     
    AliK likes this.
  13. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Jon you never fail to crack me up with your flawless logic!

    The voiceover being sultry - I was shattered!! Blooming kids won’t sleep!!!!

    Well, Frankster - I don’t know enough to point the finger at one thing, at the moment it’s all lots of hypotheses. I started to notice the car being “flat” on the second dip after putting the new exhaust on but it could have been because the top end is livelier in comparison? But it became REALLY noticeable after the last RR tune. So I need to sort out the needles first.

    I don’t think I can say it’s the exhaust without back to back testing. As you know, an engine is an eco system and I imagine it’s a combination of many factors. Also I never had an AFR on the old exhaust so difficult to compare without subjective butt dyno feel.

    What I really do like though, is how nice it is to be able to drive and talk to my passenger without having to shout to be heard!
     
  14. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    Ali, that looks like an engine-bay fire waiting to happen. Interesting though.
     
    AliK likes this.
  15. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    I think the AFRs will be very different with the air filter cover on.
    Do you have an adjustable cam sprocket? It would be interesting to see the affect of retarding the cam a little.
     
  16. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Yes quite! Probably why the foam without air housing filters aren’t best for safety.

    My heath-shield does cover the header so I feel a “little” less nervous about a fire, even if I do carry an extinguisher behind the passenger seat. ;)

    Sadly I don’t Jon. It’s an interesting point you make, I haven’t been comparing AFRs with and without filter on.

    It would be interesting to see the same video for a stock cam-ed unmodified engine. I wonder how much of that happens regardless.

    Watching the video again, I wonder if the bit at the end when the lighting is poor is just high-speed shockwaves / air pulses moving the camera about!? Next time I’m doubling the frame rate and installing a light.
     
  17. Blue

    Blue Forum User

    Great stuff!

    The piston in the SU should be set up with mass and springs to travel its full excursion when "on song". You may wish to verify this before assigning stations.
     
    Last edited: Jun 16, 2019
    AliK likes this.
  18. Jimbo

    Jimbo 1978 260z in yellow

    there is a fantastic online size chart that allowed you to pull up a graph showing you the sizes of a needle that you can compare to others.
    this is so you wont have to file them or spend a fortune in needles as if you need to change the specific needle area to make it rich or leaner the graph will high light the correct needle.
    http://www.mintylamb.co.uk/suneedle/
    ive inputted my needles as i have sm needles but my tuner found the dx ones worked better
    the tuner i used is a very knowledgeable guy called allan who runs a company called carb care.
    he has mobile emissions test equipment and a lot of knowledge on these that happily will explain over the phone or a cup of tea most things about these carbs
    word of warning he is retiring very very soon so be quick, as long as you dont waste his time he is very happy to help.
    https://www.carb-care.co.uk/
     
    AliK likes this.
  19. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Thank you Mr. Blue. That is great advice that I will heed! As soon as the weather improves I will be back on the road to verify.

    BTW, love your site!! It’s been an invaluable source of info in my Z journey! :thumbs:


    Thanks chap, I too used that site and it is brilliant!!!!! I set up my own s/s mainly to model the effects of moving the jets up and down.

    That is great to hear - so it sounds like I may be heading in the right direction. Remind me of your engine spec again pls - I know it’s an L28 but what head / cam and compression are you running!?



    Again, thanks buddy, it’s rare these days to find someone who truly knows their stuff on carbs beyond the basics. I will be sure to call him and try to absorb more wisdom. With all these guys retiring, it just makes you realise how much intellectual capital is moving out of circulation and onto the golf course :(
     
    Last edited: Jun 16, 2019
    Jimbo likes this.
  20. Jimbo

    Jimbo 1978 260z in yellow

    keep up the good work alik
    its incredible what can be done with these carbs given the time and patience to understand them and adapt them for optimum performance
     
    AliK likes this.

Share This Page