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Discussion Starter #1 (Edited)
Here's the setup:

TURBO INLET > Transition tubing > Air Filter

With a properly sized (large as hell) air filter would the transition tubing see any negative pressure (vacuum)?

The only real way to know is to plug a MAP into the area. Do you people do this to insure your turbo is getting all of the air it needs? If undersized, are we talking like 2-3" Hg?

My answer is No, it will not see vacuum. My reasoning is the concept from a naturally asperated carb' motor. If you got WOT and measure vaccum, you have a restriction above the carb, or it's the cfm of the carb itself. For the above question, the compressor wheel is like the carb throttle blades of a N/A motor. But I am looking for some real world data, like: There is no air filter large enough or good enough to do this.

I have two 76mm S400 turbos, 5" inlet.

Thanks!
 

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For testing purposes you could hook up a $15.00 cheapie vacuum/fuel pressure gauge and accomplish the same thing as the MAP for your intentions. You will see a restriction and vacuum if the filter is undersized/dirty - most off road pieces of equipment have restriction gauges on the inlet tract.
 

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I feel your answer is wrong. On my international semi I have an HT60 holset. Turbo moves enough air to make 570 hp and right at 2000 lb feet. The air filter on my truck is 12 inches in diameter and 16 inches tall and 2 inches thick or so. I see 2-3 inches of vacuum with a brand new filter. My turbo doesn't move a ton of air but it moves alot, but that filter is WAY bigger than anything you could put on a car. I don't think you can have filters on a performance oriented setup and they not create some vaccuum in the intake pipe.
 

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Discussion Starter #4 (Edited)
Thanks John for that real world info. I am looking to use flexible ducting as the transition between the filter and compressor inlet. I need it for the flexibility it provides, but yet I don't want it to thin so that the vaccum might collapse it. Most flexible ducting has that sprial wire in it like a dryer vent tube, but some have two wires, and even wire mech impregnated into the silicon. There's tons of options out there.

Here's one place that sells it by the foot:
http://www.aircraftspruce.com/menus/ep/ducting.html

Here's another that doesn't so much sell by the foot, but has a lot of product options and good customer service:
What they recomend is: http://www.ducting.com/T_7_NW.html Notice how it's still smooth on the inside.
What I can buy by the foot: http://www.ducting.com/R_2_Clear.html

Just trying to make the right decision now.
 

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I don't know how that rough wall stuff is gonna work out. I also forgot to put in the post that I have 6 inch piping from the filter outlet to the compressor inlet. Filter outlet is 6 inch too. So this thing can move a ton of air, but my turbo making 40 psi can still pull vaccuum on it.
 

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Discussion Starter #6
Forgot to add, the R_2 clear can handle 5" of Hg, the T_7_NW can handle 17" Hg.
But they only sell it in 12.5' lengths and that's about $70 plus shipping from Cali.

Instead, I am going to use this stuff:
http://www.aircraftspruce.com/catalog/appages/scat.php

Has tight radius bend. The "No negative pressure" only refers to the fact that it will want to curl up back to it's compressed state. Kind of like a vacuum cleaner hose extension, which would stress the coupling. For aircraft, that's a concern, for my 5" of Hg on the ground... not so much.
 

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Discussion Starter #7
40psi. damn. I"m down in the low teens. Still wanting to flow enough CFM for 1200 HP from a 540" V8. Not sure how those two equate to pre-turbo vacuum.

The wall of the SCAT tubing isn't too bumpy. Just the thickness of the wire. I'm not going for a world record in power, so I think the flow will be there for my application. I just don't want to crush the tube, or worse yet, pull pieces of it through my turbo.

The more expension stuff (because of minumum length purchse only) has a flat inside wall, even as it bends.
 

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You could go to a big truck wrecking yard and scavenge the aluminum ducting and big rubber elbows to piece something together with too. It's typically all 5 inch diameter stuff and you can get the bends in 45 and90 degree depending on which model trucks you pick from. Just an idea.
 

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Going back to what I think is your original question - the answer is yes, a properly placed pressure tap will give an indication of pressure drop in the inlet system of a turbocharger. Someone suggested a cheap manual gauge. That may be okay to get some information but if you have 1000+ hp being applied things happen fast and you will only get a glimpse of the data. I prefer using a map sensor hooked up to a datalogger. Then you can look at how the inlet pressure changes as the flow (boost) increases. This data can also be extrapolated to predict how much restriction you will have at higher flow levels.

I have a tap on one of my 2 - 3-1/2" inlet ducts. The "inlet ducts" are actually 90deg tube bends going directly to a large K&N filter. Each feeds a small Garrett Turbo that are operating near the 600hp level. If you wish, I can hook the tap up to my datalogger later this week and give you the reading(s). This might give you an idea of sizing required for your setup.

One thing that has not been mentioned here. A turbo is an air pump and can overcome restrictions on both the inlet and the outlet side. Depending on where your application operates on the P1/P2 section of the compressor map, these restrictions may not affect total CFM flow.
 

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Discussion Starter #10 (Edited)
Thanks for the offer Topfun99, but I don't want to put you through that effort. the only reason I am having to consider negative pressure in that area is because I am going to run a flexible tube for that transition piece and was worried about it collapsing under the negative pressure. I'll do some trial and error and learn as I go, mostly wanted to make sure I wasn't hosed from the get go. hosed. ha.

What also lead to this question was that the inital Cobra Transition I ordered caused a space limitation that resulted in an air filter size not much larger than a medium can of peanuts. That just didn't have the surface area to flow the CFM I needed. I do not have room for a 90" bend on a 5" pipe without cutting a cross section out of the outer corner. I am also building a street car, so I can't just cut big holes because I have head lights and blinkers such.

It'd be interesting to know if people actually found that a certain size/brand of air filter was causing a <5"Hg vaccum in that inlet transition and that changing to a larger surface area or better flowing filter lead to quicker spooling, more boost, and more horse power. Simliar to just pulling the filter off an making more power, which I think most people on here do is just run without a filter given the compressor wheels I see sticking through bumpers and such.
 

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I've had silicone couplers pre turbo collapse even with no filter on them at all. In my case I put a silicone 90 I had laying around on the turbo inlet since the turbo was mounted right in front of the radiator and I was hoping it would get some cooler air from the side. It was only a 500 hp turbo on a 4 banger, but as soon as it made any boost at all it sucked it shut. I would avoid anything flexible pre-turbo.
 

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Discussion Starter #12 (Edited)
Goodness. So I have two very different examples to go buy:

1) 570HP causing 2-3" of vacuum with a Holset HT60 that has a ~5" inlet and a very large paper filter.

2) 500HP casuing a silicon 90* transition to close shut on an unknown turbo with NO filter.

Kevin,
How did you find out the silicon coupler was closing, was it on a dyno?
Do you think the heat from the radiator softened an already week/cheap silicon transition?
What was he diameter? 500HP through 3" diameter over 2 linear feet is a lot different than 5" ID over 1 foot... but I can't get over that No filter thing and still resulting in transition closure!!

The ducting I selected has more collapsing strength than a typical silicon transition because of the metal spiral in it. But I will keep a close eye on it during heat cycles and on a dyno.
 

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Air filter restriction is in/hg on my truck. Fuel filter restriction guage is in/ h2o.
 

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I've had silicone couplers pre turbo collapse even with no filter on them at all. In my case I put a silicone 90 I had laying around on the turbo inlet since the turbo was mounted right in front of the radiator and I was hoping it would get some cooler air from the side. It was only a 500 hp turbo on a 4 banger, but as soon as it made any boost at all it sucked it shut. I would avoid anything flexible pre-turbo.

Had the same thing happen to a bmw i turboed, had a silicone 90 on the inlet and the car would fall over on its face on third, car dynoed a little under 500 hp thru a clogged/melted cat
 

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2) 500HP casuing a silicon 90* transition to close shut on an unknown turbo with NO filter.

Kevin,
How did you find out the silicon coupler was closing, was it on a dyno?
Do you think the heat from the radiator softened an already week/cheap silicon transition?
What was he diameter? 500HP through 3" diameter over 2 linear feet is a lot different than 5" ID over 1 foot... but I can't get over that No filter thing and still resulting in transition closure!!

The ducting I selected has more collapsing strength than a typical silicon transition because of the metal spiral in it. But I will keep a close eye on it during heat cycles and on a dyno.
The turbo was a T3 T04E 50 trim. The 90 degree coupler was 2.75", high quality 4 ply (really thick), and brand new. The 90 degree part wasn't helping. It was on the dyno that I found it collapsing, on the drive there I didn't get on it due to the rain. As soon as it made any boost it would just lose power. I've seen this before on the stock Talon/Eclipse intakes (made of stiff rubber, like the EPDM Rob mentions above) where the rubber spacer on the inlet tube gets out of place and blocks the compressor inlet, so I went right for it. If I had a pipe in the open end of it, it likely would have held it's shape. Being completely unsupported didn't help matters.

You are definitely going to get some vacuum on any tube in front of the compressor inlet even without a filter, there's no way around it. The question is really about the material you're using, can it stand up to it. If so, go for it! The metal spiral should hold its shape, but I would check to see if the covering material gets sucked in, between the spirals. It wouldn't block the flow in that case, but it might reduce the ID by an inch, or something. Keeping it as short as possible would reduce that effect. The dyno would be the best place to watch it, unless you have a GoPro or similar camera you can mount up under the hood and record it.

Another important point to remember is that any vacuum created at the compressor inlet is going to raise the PR, drive pressure, and shaft power required to run a given manifold boost pressure. Less is better. :)
 

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Had this exact same thing happpen to me with my sledpulling truck. Everything is metal in the intake tract except the boot / 90 that connects to the compressor housing. When I hit the top of 2nd gear that sucker fell flat on its face. That was fine, but what happened next was scary....as soon as the turbo spun down, it let the boot open and she came to life HARD and bad things started happening.

What our guys all say to do is go get a semi truck radiator hose and take the coil spring off of it, and wind that inside your soft parts, which helps prevent the collapsing if you are stuck with hoses.

And yeah it was the first time it's ever done it, and why? Because I put a phucking un-oiled K&N philter on it because the turbo was brand new. Bye bye filter.
 
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