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Discussion Starter · #1 ·
It's been for some times I am looking for what's wrong with the boost spiking with my AC3. Everything was put in the right place and tightened up. Put a bleeder valve in and... still spiking. When I am at WOT, it spikes to 9-10psi then drops to 5-6psi. With this kind of drop I am losing lots of hp which is not I want. Any idea what's goin on and what should I do?

My suspect is still my bleeder valve. Can anyone post a pic or two of their bleeder valve or something that I can use to stabilize the boost?
 

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Do you have a fairly long hose from the actuator to the valve?

I found that all the (optional) AC jets that go into the AC boost switch assembly gave me way too much boost. Even the smallest one I tried, which I think was 2 sizes smaller than what came standard in the switch, produced 14+ PSI before I let off. Who knows how high it might have gone?

I thought about an adjustable valve too, but what I ended up doing was capping the AC hose port at the actuator, and now switch jet sizes directly on the actuator to change boost. There is a jet about an inch to the right the hose barb on the actuator. The size of this jet is what would normally produce 6 PSI if the AC switch was in the closed position. It is a pain to have to jack up the car and change jets while laying on your back, but it works.

BTW I think spikes of upto 3 PSI is "normal", 4 or 5 does seem excessive.
 

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Discussion Starter · #3 ·
Thanks rod. I think the hose from the actuator is too long, I'll try shorten it later and see what happen. So 3 psi boost spiking is considered normal with bleeder valve?
 

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I don't know that the valve itself makes any difference? I would think an orifice is an orifice.

I am not sure about this but I think irregular boost output with a bleed system is due primarly to changes in air density/temp and its effect on the rate at which how much air escapes through the fixed bleed orifice.

For example with a given bleed jet I may punch it in 3rd and get ~8 PSI across the RPM range, shift to forth and get a spike to 11 and then have boost hold at ~10 PSI. There has to be a simple explanation for that.

In any case, I think it might be possible that the extra volume of a long (and somewhat pliable) hose could delay the build up of the pressure signal in the acuator slightly, increasing the tendency to spike.

BTW I have never noticed the "declining boost output" with RPM that used to be commonly talked about here. For example a given setting delivering 10 PSI midrange and then dropping to 8 PSI towards redline. Generally speaking, it seems to me that a longer period in boost will result in an increase of boost, if there is any change at all.

Edit: Wait a second, your setup DOES go actuator/hose/valve/atmosphere, not compressor/hose/valve/hose/actuator like with a standard ball boost controller, right?

Also, when you say bleeder valve, this is an adjustable needle type that normally holds the opening to which to have adjusted it, right? Not a normally closed spring loaded ball type valve that opens as pressure is applied, correct?
 

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Discussion Starter · #5 ·
This is also what I am concerned about. My bleeder valve is not the adjustable one. I am using somekind of hose barb, so how much boost is controlled by the size of barb. I am not sure if this is the correct way to do. I saw some post that an fish aquarium bleeder valve will work too
.

I'd really appreciate if someone can show me a pic or two of their bleeder valve or generally that can be used to control AC3 boost.
 

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Discussion Starter · #6 ·
Rod, I also heard that I can actually use an electronic BC with AC3. What do you think about that?
 

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If your AC is like mine, there is a passage that allows compressor boost into the actuator. The actuator has a hose barb and a bleed jet. When air is not allowed to pass through the hose barb (because it is capped or there is a hose attached with a closed switch on the end) boost is controlled by the orifice size of the bleed jet on the actuator. ("Low boost setting")

When a hose is connected to the barb, additional air can be bled off from the actuator. How much additional bleed may be determined by a second bleed jet in a switch connected to the end of the hose (ala original AC3 setup "Hi boost setting"), or by inserting a "plug" with a small hole in it into the end of the hose (sounds like what you did), or by attaching an adjustable valve to the end of the hose. But if using a valve like this, it should hold a constant orifice. You wouldn't want to use a spring loaded valve that is normally closed until a certain pressure is reached, such as with a conventional check ball boost controller.

In any case more bleed = more boost. A small change in orifice size makes a big difference in boost, so be cautious. Any of the ways above should work fine to control your boost. Like I said earlier, I capped the hose barb and change the jet on the actuator body myself.

BTW, one of the main reasons I did this was the toggle lever on my AC boost switch was rubbing the bottom of my hood, this resulted in crazy boost variations as the engine torqued under load. Something to keep an eye on.

Some ACs, such as P51's, have an external line carrying the boost signal to the actuator. With the external line, it can be cut and a conventional boost controller placed inline. These valves function by holding the pressure out of the actuator by way of a spring loaded ball. The user can adjust the spring tension. Sufficient boost overcomes the spring pressure and the signal reaches the actuator. The actuator then prevents boost from rising further. Not sure how the original AC actuator mounted bleed jet would factor in here. Hmmm... Anyhow P51 has (or had) his setup like this so he would know.

As far as using an electronic BC, I think you would be the first. I imagine it would be possible, just might require a fair amount of ingenuity if you lack the external signal line.
 
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