hehehe sorry.
force of habit...

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Piston height (and also needle height in the jet) varies according to the amount of vacuum produced by the air flow through the throat of the carb. Air flow is determined by the butterfly position.

The amount of vacuum produced by air flow is determined by two holes in the piston base. The holes are recessed from the piston lower face. As air passes over the face, a vacuum is formed in the holes in the piston base and is applied to the piston top.

The SU piston being raised, also raises the mixture control needle in the jet. This richens the amount of fuel added to the air flow through the piston base and carb body, richening the mixture.

Mixture changes occur immediately the needle is raised (The piston reacts very quickly to changes in flow). No accellerator pump is needed on these carbs.

More vacuum = more piston rise against the spring. Stronger spring = leaner mixtures during accelleration and cruise. Softer spring = richer mictures during accelleration and cruise driving.

I would think in a turbo application, you woud need to pressurise the fuel bowls to manifold pressure and also the area above the piston damper. ????????

Any further thoughts?? I think they could work well with some thought.

You know theres nothing wrong with suck through carby on a turbo, but in this case you either use a standard maifold (inlet) or the twin su manifold as I described, or convert to Fuel injection. The thing I dont really like about blow through is the box over the carbies, imagine changing or retuning the carbies or carby. having to remove the box and sealing it again ect ect....and modifying the carbies if you're not using the box having to pressurise the bowls and the shafts. I have used the suck through method and the crossover pipe went straight to a standard inlet manifold, although I had a 45mm Side Draught Weber. Goes like hell

Try and keep it simple.

yeah i agree.

if you're dealing with carbies - keep it simple.

otherwise go EFI....sorry - had to slip that in.

yeah, feral's right. The SU piston never 'sees' engine vacuum because it's in front of the throttle butterfly. The vacuum it sees is created by the velocity of the air passing over the venturi.

The problem as I see it is, that as soon as boost is created, the velocity of the inlet air will increase alot, causing the piston to rise too much and run too rich.

Will pressurising the piston chamber with boost cancel this out? My reasoning says it should, but I still have a niggling doubt in the back of my head saying "there's too many variables here". Boosting the piston chamber could well overpower the vacuum that's lifting the piston?

If that was the case, you'd have to find the compromise of how far the piston moves relative to the boosted air speed with a stronger spring. But then the needle would have to be very sesitive in the first few increments to give decent off boost mixtures, because it won't move much at low air speeds (relative to the speeds under boost). The spring would also be related to the max boost level you run, and that would be a bit of a headache too. What about a variable rate piston spring?

That method would definitely require a very efficient pressure relief valve to prevent the piston being held open by manifold pressure when the throttle is closed. Or would it - no air speed over the jet orifice would prevent fuel being drawn out... Sorry, I'm just thinking this as I type it, and my head's starting to hurt, and I'm only coming up with more questions than answers. And no doubt confusing everyone else as much as myself, so I'm just going to stop now.

You know, you could put the carbs in front of the turbo, where SUs are known to work very well with turbos, and do away with all the headaches! Sorry rallycar.

I have never had anything to do with Turbos on petrol engines, so this will probably sound dumb. The suck through method seems to be the easiest, but wouldn't the heat generated cause problems with the fuel? I know that most superchargers are suck through, and work well with carbs, but I think there would be more heat in a turbo. I'm guessing you can't intercool the suck through method?
Cheers

Now my interest has been stirred, my A15 has been built with nothing but compression in mind & a turbo would be a backwards step for it (too much work). I have a good little A12 sitting in a corner, maybe I could build it into an old school (read:Carbed) rev happy hi boost little number saving the A15 for reliabilitie's sake. (all in the name of education, of course)

Turbos do make heat, as do blowers, but a carburater has a refrigerater effect. So heat from the turbo would help prevent the carb from freezing. No way would I try to blow through an SU, Draw through is soooo much easyer. Intercooling draw through is difficult but not impossible/

Quote: If pressure is the same both sides of the piston (by using a pressure equalisation hose to the top of the piston), then the pressure differential should be the same as from normal air flow passing over the orifice.

Quote: If the boost pressure is the same as the pressure on top of the piston then the only thing that can provide a pressure differential to the upper side of the piston is air movement over the orifice.

Variable rate piston springs are done using different length and differnt rate springs. The longer spring acts for the first part of the piston movement and the second spring cuts in when the piston rises sufficiently to add its rate to the first.

I think you would have to lock the main jet in posittion, as after pressurising the fuel bowl, the main jet is subject to a piston like effect. The rubber seal on the main jet, would allow the pressure on the body of the jet to make it push down against the choke spring under boost. This would richen the mixture ?????? Hmmmmm this could be a bit of a bonus if the spring rate was varied to use this to advantage. Any other thoughts.

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not difficult, just dangerous

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never felt a cold carby afte a cars been running

im a big blow fan, my intercooler makes a huge diffrence, you can tell by touching the pipes either side and feel a huge diffrence, not only does that give more power but it helps prevent detonation, and theres no fuel going through the turbo, surely ppetrol going through the turb is bad for getting an even mixture

im sure suck gives a good power increase but if your gonna go to the effort you may aswell blow, i dont see how one is easier than the other really, rellocating the carb would be a pain in the arse, just like boxing a carb is,

but hey why box the carb????, you dont need to unless you use over 10psi, but without lowering comp you wouldnt use more than 10psi, so unless your building a motor specifically for turb in which case you wont mind the little extra work, then i say blow is easier!!!!, the only harder bit then is the fuel system