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How much vacuum


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Just out of curiosity, if I were to connect a vacuum gauge to the port on the throttle body where you connect to do a sync, how much vacuum would I see.

I know it will increase with RPM but what is the approximate range?

Thanks in advance.

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What you would see would be the needle swinging so wildly that it would be a burr. It would swing from 0 to well above 20"Hg, every time there was an intake stroke, then back to 0 again for until the next intake stroke! You would not be able to even see the needle it would move so wildly.


You need a restriction to damp put the vacuum pulses. I used a fine-pitch screw jammed into the vacuum line to force airflow along the restricted narrow helical path of the screw threads. Works great.


Once you have some damping, you will typically see a steady reading of somewhere around -8 to -12 "Hg (depending on the bike and the idle RPM). But the actuall reading is irrelivant. What matters is making it as close as possible between cylinders.

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A good gauge will probably have the damper built in, a cheap one won't.


A very expensive one won't either, I once, out of curiosity and cause I had the stuff on hand, hooked a $3000, 1000 torr absolute pressure transducer to the pressure line of my turbo K100. Now, that is a 4 cylinder bike and I had a pretty large intake plenum to work with. With a fast data logger connected to the output of the transducer I got nothing but a blur. Slowed the sample rate to 1/second and finally got some useful data.


I remember now, I was trying to see what was happening to the fuel injector pulses as manifold pressure (which I was using at the time to adjust fuel flow) transitioned from vacuum to zero pressure. I learned a bunch that day about fueling a turbo motor for the street with nothing but a MAP and TPS sensor, btw, it is tough to do. dopeslap.gif

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