Twin Carbs blow through (purely theoretical)
Turbo Mini Forums

I was just having a think about the extent people go to get maximum power, thinking about electronic ignition/squish, etc, etc. Fancy cylinder heads, etc, etc........

I know you turbo guys just wind the bleed valve open for more welly, but I wondered, despite all the gas flow enhancing mods, why a lot of you used a crappy (?) log intake manifold. I figured that it has to be a resistance, even if it does homogenise the mix more.

NB:- I am fully aware that the set up you all use is great and cheap. I just wondered why people don't use twin carbs.

My thoughts.....

I reckon that a dense charge is going to have a tendency to increase droplet size/centrifuging/fuel separation as the boost goes up. This obviously means that excess fuel is blowing out of the exhaust and one pair of cylinders are running lean. This would limit advance and therefore power. Running twin carbs, would allow a central jet and no real direction change or separation.

What do you lot reckon?


Stu

(Working far too much to finish the supercharged blow through SINGLE CARB 66 mk1)

Bugger off, I'm getting there.

crappy log manifold apparently designed intentionally to prevent charge robbing, which i'm guessing would be worse on forced induction a-series

Crack on Stu, add it to the list of reasons not to finish the supercharger install....

Yeah, the mini is going out with a full race NA engine at the moment, the supercharger conversion is a lot more intricate than I thought, and a lot more EXPENSIVE! Should bring home the bacon eventually.

Patience rodders!

Edited by Hedgemonkey on 27th Feb, 2005.

Bugger off, I'm getting there.

space....

alex

AlexF

Sorry for giving yet another opposing opinion, Stu. If you look back some 2 or 3 decades there were never really good intake manifolds for twin SU?s, be it H, HS or HIF range. Those available usually need a lot of internal mods to make them worth your while. Currently Keith Calver is said to start supplying a specially developed manifold for twin SU?s.

Add turbo then. From what I have read and listened there seems to be enough confusion with single SU needle vs. spring selection. It is not that complicated once you understand what to change and where. But it is a question of getting one?s hands dirty and trying things.

Now, add another carb and you have no previous knowledge to begin with. What needle, what spring ? N/A setups for twin carbs are few and apart, best source I have located came from a German tuning manual and found them pretty accurate. For twin carbed turbo the BDD needle will most likely not work. It will be a question of guesswork, trials and errors and finally rolling road time. Needles are cheap but if you need to purchase a lot then it gets expensive. Add linkage: never an easy job to make the twin carb linkage really smooth. Been there done that. Third issue: space. Fourth issue: carb size, HIF38 (never available as sealed units) or HIF44 ?

FINALLY: what do you really gain from twin carbs ? Complications compared to factory setup, hardly any extra power.

But if you want to try it, do it. I have thought about it (and dismissed the idea), like I have many other things. I love engineering but you need to gain something. Otherwise it is either waste of time or what I call phallos factor (= something to show off at a cruising night but of little use).

BTW, one of the craziest ideas was to make a billet 16V head, for which I have an huge (1 ft x 1 ft x 3 ft) piece of alloy stashed away. I am crazy but not THAT crazy.

what about using one delorto (sp?), i.e off a lotus turbo?

dont you use one of these jukka? were there gains to be had?

Edited by Nic on 1st Mar, 2005.

Jukka, I was merely talking about the benefits of using a straight shot from the jet to the inlet port with forced induction. I suppose this would be present in a blown Weber to a lesser degree. Obviously the high pressure air has a different pulse velocity, hence resonance/inertia, this would obviously make a normal Twin Manifold behave in a different way to how it was supposed to (different balance pipe required, etc, etc). It was just an idle thought I had and I thought about it and without resorting to the pen and diagrams (per usual) I figured that the gains were significant. I thought that there would be a serious imbalance of fuelling in a single SU engine (can anyone back this up, especially at high boosts?more so than N/A).

The question wasn't directed about making a set up using existing manifolds, a new one would be required from scratch (With appropriately designed balance pipe-can't remember the method offhand). I reckon it's a valid point, I just wonder why no-one has tried it.

Regarding the 16V cylinder head, am I right in saying that the first one was machined out of a block of alloy by someone who worked at Weslake? Gordon Allen (of crankshaft fame) made a few special mini engines, he cast his own heads as well as blocks and also made a Billet 16V head as well as the Pinion 8 Port (Like a Weslake 8 port). I really wanted to speak to him about the designs but I believe he is dead. But I found another head designer in the process and got some interesting information about castings. (One of my splurges of research was about designing a head with a view to casting it). I was thinking along the lines of Semi-Downdraft like the Weslake as that was the best head by miles. (IMO) When my mate Big Mike gets his casting gear working I will make a mould. Have a few plugs I have made for core casting. I reckon casting is the way to go.

First thing's first though, I'm not really bothered enough about getting my car on the road, I'm more of an enthusiast about how and why it works and optimising the theory and exploring loads of different ideas. I am of the firm opinion that final design is completely achievable on paper and getting the ultimate result by actual testing.

I won't bore you with the results of my testing so far as it's not really worth talking about, BUT when I have something to talk about, it will be worth hearing......

Anyway, if you are interested in making your own head, I have lots of information. (per usual).

Meanwhile, back to the debate in hand.

Bugger off, I'm getting there.

More headache... A wealth of jetting experience... To be honest not really worth the hassle, Dellorto carbs are more expensive to buy, you need to have a good source for jets AND you need to know how a change affects the function of the carb. Trust me on this, SU HIF44 is a great piece of carburation regardless of how mundane it looks. It is simple to setup, just one needle to file. With webers and dellortos there are literally thousands of jet combinations and these carbs are pretty sensitive to e.g. temp and humidity. Natural if you consider that they are basically fine instruments. Get them right and do they sing ! Get them wrong and they are pain in the arse.

Hedgemonkey: -

read this site with interest a few weeks ago... could be of use :cool:

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

thanks for that jukka, i often wondered if there would be much of a gain, but ill take your word for it and the su will be here to stay

Bugger off, I'm getting there.

There are of course benefits from Weber or Dellorto, one of which is the accelerator pump which basically means that you don?t need choke when starting from cold. Just press the pedal twice and the engine is running. It is a touch smoother in action, with properly executed linkange is is a joy to drive. Sometimes SU linkages are best described as sticky. Then there should be better throttle response, can?t really see that with my engine since the internals are not that light. Infinite tuneability is one criteria especially for race engines but does not really apply here.