Can someone tell me
Turbo Mini Forums

How can you wind the boost up or down by 30 or 40% and not have to change the needle or spring in the carb?
This is a serious question!!
Regards
Dave

seriously i dont think it would be a good idea as fueling would be out prob on the lean side which is bad, unless you were running really rich before hand i wouldnt advise it anyway

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Plenty of people run variable boost ..... in fact almost every body with an in car bleed will have used it.


Think about how the pressure regulator and SU carbs work and I am sure you'll figure it out.

Every day is a school day ...........

How fast and how expensive ...... the same question...

Hi,
You get the carb and timing setup for the maximum boost posssible for your engine spec. Then you can adjust the boost whenever, wherever upto that maximum with a bleed valve....
Cheers,
Gavin :)

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Roger, I have but I draw a blank, Can you please explain.
Regards
Dave

i seem to remember turbo dave mentioning rising rate fuel preasure with boost.
no idea how though.

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Hi,
The more boost going into the engine the more the pressure regulator increases the fuel pressure. Also the air pressure in the plenum goes into the float chamber, both pushing more fuel up the jet.
Also I'd imagine the pistion in the carb rises higher with more boost and so lets more fuel out of the jet into the engine. That's how I think it works ...
Cheers,
Gavin :)

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The change in fuel pressure is indeed the key and so long as it's set to be sound at peak boost you should be ok everywhere.

Every day is a school day ...........

How fast and how expensive ...... the same question...

OK, I understand that the rising rate fuel pressure regulator is there to maintain a 3 -4psi pessure diferential over boost pressure to ensure a constant fuel level in the float chamber and I can't see how this is relevant to the original question.
The carb has to draw the fuel from the float chamber via the jet from the suction over the jet. If you vary the suction by changing the boost I would have thought that the needle can only be correct for one boost pressure or does the SU compensate in some way for variable boost on the same needle?
If this is the case should we all fit a needle to suit a 200 BHP motor and let the carb sort it or maybe do away with boost pressure adjustment?
Regards
Dave

Remember with the turbo application the float chamber is seeing the pressure in the plenum.

What you say danboy is correct except that it's not suction but pressure differential across the needle valve that controls the fueling. The needle valve after all is just a throttle.

Remember in nat asp mode the float is seeing atmospheric pressure (via the vent) and a Vacuum in the carb throat.
In a boosted application you are seeing pressure relative to the plenum pressure in the float and something lesss than that in the carb throat (the reason why the restrictor in the plenum is so important).

The net result is everything is balanced correctly is the same.

Every day is a school day ...........

How fast and how expensive ...... the same question...

Off to bed. Continue in the morning.
Regards
Dave

night danboy

If the carb and needle is set up for the highest boost you ever want to run, it will just run on the rich side at any boost level below that.