Discussion in 'Tuning & upgrades' started by 3.0CSL, Feb 5, 2019.
Hi all, as above, any good ? Or are twin SUs a better set up ?
On stock engine, twin SU's for driveability and linear power delivery.
On a worked engine, DCOE is the go. But triples Not twin.
Did you mean the twin downdraft carbs that get fitted to some American cars?
The downdraft ones just look all wrong on our cars.
What he said!
This is a polarising topic!!!!
People on SUs racing in SCCA series (USA) produce big power and swear by them.
People with triples also make big power and swear by them. Some have strokers (3.0+L engines with long stroke diesel crank) and others with modified engines like mine.
This a good discussion on the topic
This is a good read from your BMW buddies
My 2cents as someone with a worked L28 and sorely tempted by triple OERs from RHDJapan. This is a summary of what I've picked up speaking to many people and spending hours reading up on the topic
DCOEs were designed for racing and definitely flow better, especially at the top end.
People state that SUs are better for low to mid range and better for throttle response coming out of those low gear corners.
I asked the very experienced mechanic at CCK Classics (a garage I would recommend in a heartbeat who RR tuned my engine for the run in period) the question about the drivability of DCOEs and he laughed at me saying if set up correctly they will perform well in all conditions.
It seems that it's a compromise - I.e. To get the lower end you need to use smaller chokes but that defeats the object of better flow at top end. Getting a decent airbox like the one MJP Eastern sells seems to also make a noticeable difference to air velocity at the stacks.
People have reported the DCOEs coughing back over a hot header and setting the car alight. I would contend a decent, well designed heat-shield directing fuel away would solve / minimise that issue.
SUs do go out of tune over time as mine have in 1000 miles but probably my fault.
With proper good quality linkages DCOEs are far less likely to go out.
DCOEs sound and look superb and what every petrol head will look at and instantly regress to being a 4 year old.
If you go that route, make sure you get someone who knows their stuff and has a collection of jets etc to rolling road tune them.
And for balance here's an interesting response I received from Ztherapy who remanufacture the finest SUs for Zs ...
My advice is, take your time, speak to lots of people with them, the old lags with experience of both get the right mods on your engine to make the most of the money you'd spend on them.
IMO triple 40's on a mild engine can be made to perform superbly in all conditions and be very good for fuel economy if they are setup correctly. On my old 2.8 engine they produced nearly 200hp and on a run down to la mans in 2010 gave me quite a bit over 30mpg. It did take the rolling road a long long time to get the fuelling right though as I was after an across the board and not just top end tune.
The big problem with setting them up correctly is that most dyno's just don't keep an extensive range of jets, chokes, and emulsion tubes to keep to keep swapping over. You will probably need to go away and purchase them yourself then return for another couple of sessions if you want it right. The beauty of EFI is you can get it spot on without spending any extra money on parts.
SU's are nice and simple but most are worn so it can be difficult to get things perfect. If the engine has more fuel demand then the needle may require modifying and this will be time consuming and also difficult to get perfect under all conditions.
Yes Rob, exactly ! The twin downdraft Webbers that are oft seen on US cars.
I am assuming that these were supplied by maybe Amco in the States, as an upgrade to the smog carbs on the 260 Z and maybe to replace the injection system on the 280Z ??
And re triple Webbers, for a fast road and also a race six cylinder E Type, triple SUs are recommended and not triple Webbers.
My old S.1 E Type didn't perform any better on Webers than it did on its triple SUs, it just used more fuel. Yes, they sound and look great, but triple webbers and manifolds are not ££ cheap.
These are the twin Webers that OP is talking about - down-draft DGVs.
I have them on an RLS30/260Z in build at the moment (on the car when purchased) and will be initially retaining them to see what kind of performance they give after a rolling road set-up and tune. They are probably better than the federalised flat-tops that were original to the RLS30 when new but the L series engines work better with side-drafts such as DCOE or Hitachi SUs.
Those downdraft webers may be ok but the manifolds look really inefficient. I can only assume they are fitted because new OEM carbs are not available especially in the USA. It will be interesting to see how you go-on with them Chris.
SU/Burlen don't seem interested in looking after Z owners - I did try.
The debate re twin SU/ triple Weber/Dellorto etc really boils down to getting the 'set-up' right as John has said.
I have always been an SU fan because I got a pair of 2" carbs working well on my track car and it was a sort of inverse snobbery over the guys who had poorer performance with Webers. However my Samuri has triple 40DCOE and they work fine too even when chugging around. I love the induction roar. The only thing I notice is that if I have a period of over-run on a closed throttle they can cough when I open the throttle - not a big problem but suggests there might still be work to do.
3.0CSL I have a mate who is building a Realm DType replica and I have discussed carbs with him. After his IVA test with little SUs he's going to fit triple webers and reckons 'in their day' the factory could get 30bhp more with them over big SUs. I also had a neighbour who raced a Jag 'special saloon' (Mk 1) and he had triple webers.
By the way there is a member on here who runs triple Hitachi carbs i.e. one per pair of cylinders.
Triple Hitachi SUs - very interesting, assume standard 1.75 inch same as HS6 SU's ?
Wonder if said member has ever done a comparison with his original twins in terms of extra performance and inferior fuel consumption ?
Burlen/SU new offerings ?
I also have a pair of these on the shelf awaiting a suitable car to test out on.
Kit specifically designed by a guy in the States working with Burlen, they look like they are based on the HIF44 used on latter day BL cars.
My old red 260z had twin DGV's and was very nice to drive. Started very easily was very responsive. The biggest issue was the adaptor linkage that came with them was terrible. I ended up making my own and re-engineering it. The manifold "P" trap design looks pretty bad but actually worked OK.
I had quite a lengthy conversation with a gentleman from Burlen at Beaulieu Autojumble last year and they said that they don't do anything for Zeds as they don't get asked very much and they didn't have all the specifications to build them despite asking several people for them. I then sent them all the details they needed to build 1.75 inch HS6 carbs for a direct bolt-on fit for Datsun Zeds including the measurements of the connecting linkage between each carb and I did post up something at the time to say that they said that they would build to order. I built mine using used HS6 carbs but I had quite a lot of help & advice from Burlen as well as from a couple of ZClub members. Burlen did say that they could supply new/reconditioned carb bodies and build from there. The one thing they cannot provide or build is the inlet manifold or ballance tube but they did say they could possibly refurbish one if it was sent to them.
I am sure you would be able to get a bespoke build set of HS6 from Burlen much cheaper than you would pay to get those from ZCarDepot. From my recollection of the conversation, they estimated at around £700 for a pair of HS6 SUs and as I have said before in various posts, you can get virtually every part for them quickly even different needles if you wanted to try different options.
HS SU carbs are much better than the HIF and my recollection of a BL car with HIF carb with floats and chokes at the bottom of the carb and directly above the exhaust was of constant problems. I doubt that this has been completely removed even if you used a heatshield so I would doubt the blurb on the ZCardepot website. They sound a bit like the issues you would get with the Hitachi flat tops. I prefer the simplicity of HS SUs and manual chokes - easier to tune.
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