typical boost pressure at cruise?

[gadget73]

Just curious what sort of manifold pressure I should be seeing at steady cruise speed on flat ground. I'm getting 2-3 psi around 55. Only noticed this because I fixed the plumbing for the AC cutout, and anything over about 45 the AC stops working. The pressure switch kicks at 2 psi. I'm guessing thats why someone reversed the check valve in the first place.

[kingoftarmace2004]

whats your rpm at 55mph? I know i dont have a reading for you right now. but I know on my m21 stock turbo I would max 10psi at 2200-2300 rpm. it really depends on load and your right foot. i think cruising boost should be 2-3. but have no evidence for you.

[gadget73]

Not sure. No tach. An engine speed calc says it should be around 1840 at 55 though.

I never did find a really happy spot with that switch. Either it cuts out all the time, or never. Cheap switch from China, sometimes you get what you get. Need to find one that works so the AC cuts out when I try to merge. Not like the car has much power to begin with, but the AC really loads it down.

[kingoftarmace2004]

im doing an a/c delete. the systems been converted the r134 and did blow cold. Do you need parts? -Paul

[gadget73]

I don't think BMWs had the pressure switches that Ford used. I can't even figure out what the Ford part number is, because Ford.

[kingoftarmace2004]

what are your highside and lowside pressure with the a/c engaged at idle? there is a chance that you have a low charge keeping the compressor running more because of low pressure? I would need to see readings. -Paul

[gadget73]

The pressure switch I'm talking about is an intake manifold pressure switch. Ford used boost pressure to indicate when the engine was under heavy acceleration and it was supposed to cut the power to the clutch once it got over some point. No idea if or how BMW did this.

[kingoftarmace2004]

orrr, sorry, read it wrong. The only boost sensor I have is the overboost/oil light thingy.

[gadget73]

yeah I have that one too, it trips the "check turbo" light.

[kingoftarmace2004]

you could plumb in a newer style, mybe 1-2psi or 2psi which is 5 inhg switch to make sure it will turn off?

[gadget73]

The switch I have is adjustable but its got too wide a span. If I get it to kick out at heavy throttle, the only way to get the AC back on is to back off the throttle completely to get the manifold pressure down to nothing. It just becomes annoying to drive that way. Basically I need a different switch.

[kingoftarmace2004]

thats what I was thinking. Find a more modern switch with less of a range? and not adjustable? from a newer turbo or turbo diesel car or truck? might solve your headache.

[gadget73]

think I'm going to need adjustable, but most of those don't seem to tell you the span. This one is about 5 psi, I think I need closer to 1 psi. Without specs though I have no idea what newer application would have a useful switch. Not the biggest problem in the world, I'll find something that fits the bill eventually. If I could figure out what the original part number is, I might be able to get one but knowing is half the battle.