Turbo aficionado needed (now 'turbo lexus build thread' updated 11/8)

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phucheneh

Diamond Member
Jun 30, 2012
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The BOV doesn't open at a desired boost level. It opens when you let off the gas, IE, when you CLOSE the throttle plate. The condensed air needs somewhere to go, so the BOV opens to let it out. If it doesn't get let out it will expand and go back towards the turbo, causing problems.

The only thing that opens at a certain PSI is the wastegate. This is so you can keep a steady, specific, PSI.

That's what I was saying...the above is my understanding, too, but then I don't understand why BOV's need tuning. I've seen articles actually give PSI numbers for common aftermarket BOV's...like 'this spring setup with this many turns on the adjuster should open in this PSI range.' Which seems silly.
 

Danimal1209

Senior member
Nov 9, 2011
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I've never heard of BOV tuning before. It's pretty much just buy a BOV that is capable of handling the amount of boost you want to run and put it on. I'm no expert though.
 

JCH13

Diamond Member
Sep 14, 2010
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The BOV/BPV affects how much pressure is in the intake upstream of the throttle plate like you all know. What this means in terms of performance is faster throttle response when the throttle is cycled.

The quintessential example is after a shift during hard acceleration. Just before a shift the turbo is completely spooled and is making good boost with the TB wide open. During the shift the TB is closed and so the pressure upstream of the TB spikes because the turbo is still spinning reallyfast. The BOV or BPV relieves this pressure to prevent the turbo from stalling (also called "surging") and to reduce strain on the intake itself (these benefits have been argued over 9 ways to Sunday and are completely tangential to my point). When the TB is closed the turbo slows down due to increased back-pressure on the compressor side and reduced flow on the exhaust side. When the TB opens again the up-stream air contained by the BOV/BPV is immediately available to the engine before the turbo is spooled back up to full boost.

The higher the pressure the BOV/BPV contains, more air will be available to the engine while the turbo spools back up. This can greatly improve throttle response between shifts by supplying air to the engine when the turbo cannot. Engine size, turbo size, and intake volume all play a role in this equation.

On my MS3 I installed an adjustable BPV and have it tuned for something like 26psi, which works nicely with the engine running at ~16psi and my large intercooler volume. Other MS3 owners run 30-36+psi on their BOV/BPV because they have bigger turbos that take longer to spool up and/or operate at higher boost levels. JLee's MR2 with a smaller engine, bigger turbo, and higher boost levels, doesn't even have a BOV or BPV. This maximizes throttle response right after a shift.
 
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Danimal1209

Senior member
Nov 9, 2011
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I'm not sure I understand how a turbo'd vehicle cannot have a BOV. It could damage the turbo over time.

Also, I think the bypass valve is something else, different then a BOV.

The BOV is solely used to relieve pressure, nothing more, nothing less.
 

phucheneh

Diamond Member
Jun 30, 2012
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I'm not sure I understand how a turbo'd vehicle cannot have a BOV. It could damage the turbo over time.

Also, I think the bypass valve is something else, different then a BOV.

The BOV is solely used to relieve pressure, nothing more, nothing less.

It's essentially like comparing a wastegate that dumps into the exhaust versus one that dumps to atmo. They perform the same function.

No BOV/BPV can and probably will damage the turbo. I believe I've heard of it on racecars, where they don't want to lose their spool. In such cases, I believe the thought is similar to that behind a rally car's anti-lag system: yeah, this is not overall a great thing to do, but who gives a shit, it just has to last for the race (or a set series of races). I believe some street cars with factory turbos also have no valve...I would assume the forces involved are simply just too minuscule for it to matter.

JCH, thank you. That seems so simple now that you've explained it. I guess my biggest misunderstanding concerned just how high the boost in the charge pipe could get.

I mean, in the case of the car I'm dealing with, I know it was damned high. You could hear quite the 'bang' when the valve didn't open and the charge pipe popped. It was always at that last coupling before the throttle- I actually had to make that one. It was a solid pipe there before, but the new turbo caused me to need a little more length and a tweak to the angle. That means no flares at the ends, just smooth pipe. But it's still quite a solid connection...just the weakest link, and it happens to be right by the throttle...probably a good thing, as I'm assuming a pressure 'wave' kind of flows back from the throttle when it closes.

I will add, I believe the quintessential example is nailing it, then going from full spool to decel. That's when I was getting the described 'bang'...clutching in kept the pipe from blowing off, assumably because the engine RPM is being allowed to drop freely.

But your description of maintaining boost in the pipe, to include pressure in excess of normal boost, makes sense. Obviously (obviously now, at least), if you went to shift, dumped ALL of the boost, then came back on the throttle, you'd have some lag in spite of your RPM level.

Essentially...you want the BOV to do as little as possible, no? Better to dump exhaust out the wastegate than to dump more charge air than you need to, especially when you're dealing with this amount of piping and a large turbo.
 

jlee

Lifer
Sep 12, 2001
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If your charge pipes are blowing off, they're not nearly secure enough - with good clamps and a ribbed end on the pipe, it should never happen. I use Breeze clamps and have a one piece hot pipe and one piece cold pipe, so I have very few connections.

I'm not sure I understand how a turbo'd vehicle cannot have a BOV. It could damage the turbo over time.

Also, I think the bypass valve is something else, different then a BOV.

The BOV is solely used to relieve pressure, nothing more, nothing less.

The force of the compressed air in the intake tract is really quite minor compared to the force on the turbo at WOT in boost - compressor surge while on throttle / in boost is really bad, but off throttle surge isn't. There's endless debate on the topic but I have yet to see someone actually show a turbo failure caused by off throttle surge. Crawford's kits do not have BOVs.
 

JCH13

Diamond Member
Sep 14, 2010
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The BOV's are usually oriented forward because they would recirculate in to the intake.
I have a 99gsx (turbo AWD) that I have done a lot of work to.

Some turbo cars ECU is counting on the air released from the BOV(when you let off the gas in boost) to be reused. Without recirculating, the AFR would be more rich that it is supposed to be.

If you are not recirculating, it does not matter what direction the BOV is facing, since it will only open when it needs to.

Also, your boost gauge should not be getting it's reading from the same vac line that the BOV is using. It should be T'd from the vac line from the intake manifold, atleast, that is how it is supposed to be on an eclipse.

Blow Off Valves (BOV) vent to atmosphere, ByPass Valves vent back into the intake upstream of the turbo. Terminology matters here.

Orientation of the valve does not matter in it's function, only for routing plumbing.

You are right that a boost gauge needs to be connected downstream of the throttle plate.

Also, random opinion request from JCH or other turbo guys:

With a turbo that big, how much do you reckon intake volume matters? This setup spools faster than the T67, using old dyno sheets as a mark for comparison. But it still has some obvious delay...which, as mentioned, I like for the side-effect of not throwing the car sideways into a ditch on accident. But you could argue that the insanely-violent spool might be just as dangerous.

I mentioned possibly adding some nitrous to help spool. But I think a significant shortening of the IC piping could also do a lot...but I'm not certain. I think, somewhat paradoxically, that with a SMALL turbo, it might do a lot; the small turbo having a much harder time shoving enough air to fill the large amount of space. This one, in comparison, can move crazy quantities of air, and once it hits the sweet spot...my god.

I guess the question is whether or not that sweet spot would actually come in sooner or more gradually with a drastic reduction of charge piping. This is possible because of the existence of 2JZ intakes that keep the throttle on the driver's side of the engine bay...not only does it make the intercooler>throttle pipe shorter (and avoid the clearance issues you can see in above pic), it removes an entire section of piping that must travel to the opposite side of the car and enter the intercooler on the side opposite the throttle (then the intercooler moves it right back in the other direction).

I really doubt that you'd notice the turbo spool at a noticably lower RPM by changing intake volume regardless of the turbo. Cobb had a great technical paper on their FMIC R&D that addressed this. The FMIC added a significant amount of volume to the intake and plumbing as compared to the stock TMIC. Their driving tests showed that this delayed peak boost by 20-30rpm in a 4th gear pull... trivial.

Throttle response will be helped by reducing intake volume. However, if you've got plenty of that, I wouldn't worry about it too much until everything else is sorted out.

It's essentially like comparing a wastegate that dumps into the exhaust versus one that dumps to atmo. They perform the same function.

No BOV/BPV can and probably will damage the turbo. I believe I've heard of it on racecars, where they don't want to lose their spool. In such cases, I believe the thought is similar to that behind a rally car's anti-lag system: yeah, this is not overall a great thing to do, but who gives a shit, it just has to last for the race (or a set series of races). I believe some street cars with factory turbos also have no valve...I would assume the forces involved are simply just too minuscule for it to matter.

JCH, thank you. That seems so simple now that you've explained it. I guess my biggest misunderstanding concerned just how high the boost in the charge pipe could get.

I mean, in the case of the car I'm dealing with, I know it was damned high. You could hear quite the 'bang' when the valve didn't open and the charge pipe popped. It was always at that last coupling before the throttle- I actually had to make that one. It was a solid pipe there before, but the new turbo caused me to need a little more length and a tweak to the angle. That means no flares at the ends, just smooth pipe. But it's still quite a solid connection...just the weakest link, and it happens to be right by the throttle...probably a good thing, as I'm assuming a pressure 'wave' kind of flows back from the throttle when it closes.

I will add, I believe the quintessential example is nailing it, then going from full spool to decel. That's when I was getting the described 'bang'...clutching in kept the pipe from blowing off, assumably because the engine RPM is being allowed to drop freely.

But your description of maintaining boost in the pipe, to include pressure in excess of normal boost, makes sense. Obviously (obviously now, at least), if you went to shift, dumped ALL of the boost, then came back on the throttle, you'd have some lag in spite of your RPM level.

Essentially...you want the BOV to do as little as possible, no? Better to dump exhaust out the wastegate than to dump more charge air than you need to, especially when you're dealing with this amount of piping and a large turbo.

Yes, the less air the BOV vents the better the throttle response will be. Like JLee said, if the plumbing is made and put together correctly you should never blow a connection.

Note that the wastegate will only open under throttle because it's actuator is connected downstream of the throttle plate (or it should be, anyway). The wastegate is not involved in off-throttle pressure management like the BOV/BPV is. The wastegate does not need to vent extra exhaust gas because the throttle is (nearly) shut and thus there is little exhaust flow.
 

JCH13

Diamond Member
Sep 14, 2010
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If your charge pipes are blowing off, they're not nearly secure enough - with good clamps and a ribbed end on the pipe, it should never happen. I use Breeze clamps and have a one piece hot pipe and one piece cold pipe, so I have very few connections.



The force of the compressed air in the intake tract is really quite minor compared to the force on the turbo at WOT in boost - compressor surge while on throttle / in boost is really bad, but off throttle surge isn't. There's endless debate on the topic but I have yet to see someone actually show a turbo failure caused by off throttle surge. Crawford's kits do not have BOVs.

All of this. We/I did some maths looking at the forces on a turbo during off-throttle surge when JLee was planning the BPV delete on his MR2 and it's not as scary as you might think. It convinced me that it's okay to do at least.

Also, it sounds awesome.
 

phucheneh

Diamond Member
Jun 30, 2012
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Oh, believe me, fellas, I know the connection that's coming loose is not optimal. I dunno if I'd go as far as to say 'not nearly secure enough,' though. During normal operation (BOV reacting properly), I think I could exceed the current 22psi and still never have a loose connection. But yeah, I only had worm clamps left, and the pipes are smooth...so it's not good. T-bolt clamps might've held, when combined with that super-thick silicone coupler (man does the quality of those things vary).

But the one in question will probably get welded...that whole pipe needs to be modded and/or replaced so that it's not basically getting mashed between the hood and the turbo (clearance is tight). That leads me to another dumb question, though...if I make a connection that can't be welded, how do you go about getting the 'ribbed' ends? Just buy them pre-formed, or does some kind of flaring tool exist?

What kind of setups are you guys talking about with no BOV/BPV? What size engine/turbo and how much boost?

Wouldn't it be better to simply have a REALLY stiff BOV to vent only at the height of the pressure spike? Surely that couldn't have much effect on performance (I would think).
 

jlee

Lifer
Sep 12, 2001
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Oh, believe me, fellas, I know the connection that's coming loose is not optimal. I dunno if I'd go as far as to say 'not nearly secure enough,' though. During normal operation (BOV reacting properly), I think I could exceed the current 22psi and still never have a loose connection. But yeah, I only had worm clamps left, and the pipes are smooth...so it's not good. T-bolt clamps might've held, when combined with that super-thick silicone coupler (man does the quality of those things vary).

But the one in question will probably get welded...that whole pipe needs to be modded and/or replaced so that it's not basically getting mashed between the hood and the turbo (clearance is tight). That leads me to another dumb question, though...if I make a connection that can't be welded, how do you go about getting the 'ribbed' ends? Just buy them pre-formed, or does some kind of flaring tool exist?

What kind of setups are you guys talking about with no BOV/BPV? What size engine/turbo and how much boost?

Wouldn't it be better to simply have a REALLY stiff BOV to vent only at the height of the pressure spike? Surely that couldn't have much effect on performance (I would think).

I was running a TD05 20G at ~23psi. Next is a GT3071 with boost to be determined, but I'll have a forged bottom end so likely at least 20, if not more (the 3071 is a larger turbo, so makes more power per psi). 2.0 liter 3s-gte 4cyl.

You can buy them pre-formed...I want a bead roller but they're super expensive. You can make your own that'll do the job, though: http://www.dsmtuners.com/forums/art...7563-how-bead-your-own-intercooler-pipes.html

I only ran the GT3071 for a few miles since I had broken ringlands on cylinder 3, but this is how it ran with a rough tune: http://www.youtube.com/watch?v=WA_Eo60fC2U

What size is your intercooler piping? I am using CT300 Breeze clamps and they're fantastic. I probably have a bunch of extras but it's likely not worth the shipping cost to sell them to you since you can probably get them at a local truck place (they're used on diesels, apparently) for $5 or so.
 
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phucheneh

Diamond Member
Jun 30, 2012
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I dunno of any specialty truck shops (like the ones that sell big rig stuff?) around here. There are a couple 'performance' stores, which, ironically, largely cater to (smaller) trucks...but mostly with chrome toolboxes, step bars, bull bars, ect. It's damned hard to find anything useful. I can't even buy muscle car stuff like carb jets locally.

IC piping is 2.5" for the long run from the turbo, then 3" on the other side of the cooler. I'll probably switch that to 2.5" to get more room and just stick a 2.5>3 adapter on the throttle body.

You like those clamps better than T-bolts? Even though those Breeze ones have the worm gear shielded to keep the hose from getting damaged, T-bolts still seem like they get tighter (fine, fully engaged threads) and distribute pressure better.

edit: okay, that home-made beading tool is genius.
 
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jlee

Lifer
Sep 12, 2001
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Yeah I went to a semi truck parts/etc place. I'm pretty sure that T bolts squeeze unevenly.
 

JCH13

Diamond Member
Sep 14, 2010
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Yeah I went to a semi truck parts/etc place. I'm pretty sure that T bolts squeeze unevenly.

T-bolts are good at making round tubes ovals. :p

What I to put a lip on intake piping was to run a weld bead around the OD. Not sure if that is practical at all for you though.
 

phucheneh

Diamond Member
Jun 30, 2012
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Sooo...I'ma keep pestering you guys. That's what you get for being useful/knowing what you're talking about. :mad:

What's the deal with 'Type something' BOV's? Type R, S, RS...I believe this is a RS, but I dunno if the letters connotate anything. It would appear that Greddy diaphragms are shit, which I'm inclined to agree with. I see some recommendations for NGR, but they only make an 'S' valve and seem rather fly-by-night.

Is there something I can put in this Greddy RS that won't get wrinkled/torn/stuck so easily? Preferably for less than $80? (that's what Greddy wants)
 

JCH13

Diamond Member
Sep 14, 2010
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I have no idea if letters like that mean anything... if they do I bet it's brand-specific.

I would be tempted to replace the Greddy with a Forge. They've got a solid aluminum plunger with a rubber o-ring that needs occasional greasing and seem quite reliable (I've had mine installed for around 40k miles). I know they're very highly regarded in the Mazda and Subaru worlds.
 

JCH13

Diamond Member
Sep 14, 2010
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I think I made a mistake in an earlier post when I said that the wastegate actuator was plumbed downstream of the throttle plate. It should be plumbed right after the turbo outlet (for a single port actuator) and will open the WG when the turbo starts to surge off-throttle or make lots of pressure on-throttle (obviously).

Derp, my bad.
 

phucheneh

Diamond Member
Jun 30, 2012
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Wastegate controller is plumbed in just like AEM's instructions show. I've had zero problems with it. My hose for the compressor is about a foot from the outlet, though. I doubt that will cause any issues; it seems to respond plenty quick enough.

boostocn.gif


I like those Forge 'dual valve' deals (in theory...would like to see one disassembled). I would definitely like to replace this diaphragm valve with something more reliable. I was going to order a new diaphragm from NGR; can't remember if I mentioned that. A ridiculous $40, but it seems like a steal when you realize Greddy wants 80...I'm beginning to think they're just the 'Bose' of import tuning. But hell, for the price of three of the CHEAP diaphragms...

http://www.forgemotorsport.com/content.asp?inc=product&cat=0006&product=FMDV004A

I was thinking maybe pick up one of those, or something similar, and put it much closer to the throttle. If I trusted the other one to not leak (I don't), I could even leave it in place and just friggin' use both. But I'll just remove it...if I wanted to do twins, I might as well just use two of those Forge valves.

But the only reason I would use twin valves is if I feel like one simply can't flow enough air to purge excess charge pipe pressure...there is a LOT of volume to this piping setup, so at 22psi of nominal boost I know it's dumping a shitpile of air. The Greddy is definitely adequate for it, but that's part of the problem- a long-stroke diaphragm-operated valve just seems inherently unreliable. If they made the valve bigger with less travel, there probably wouldn't be all these damned problems with the rubbers tearing or bunching/wrinkling.

I'm kinda coming over to your side on the whole 'secure your piping better and don't worry about it so much' thing, though. Even if it can't dump fast enough to completely prevent surge, I'd rather have the more reliable valve and not worry about it so much. I guess I kinda 'bought into the myth' of surge damage. I've done a lot of reading, and it does pretty much seem like bullshit. On the turbo side, at least. I do also worry about damage to the throttle, simply because it's electronic. But I think that's just paranoia from dealing too many factory electronic thottle issues.

For right now, I'm doing some mildly ghetto fixes. The piping that was coming apart, I 'ribbed' by just expanding the pipe end then rolling the edges back in with some, ahem, precision hammering.

I think I might just grab one of those eBay piping kits. I'm thinking it's hard to fuck that up...these current pipes aren't exactly high-end, and they've held up through years of abuse. But that'll give me all I need to downsize the 3" piping and replace some of the sketchier bits with stuff that's got pre-formed ends.

I wish someone made an intake manifold that didn't cross over the engine...for less than $1500, that is. That shit's obscene.
 

phucheneh

Diamond Member
Jun 30, 2012
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So I was looking for that link for the DIY bead roller and decided to update this. I said in that tire thread that I was gonna just use that one to chart progress on this car (I can't update the thread title on this one; guess that only lasts so long), but meh...I have more turbo stuff and it might as well all stay here. In case anyone forgot, this is not even my car, I've just had it my place kind of a hell of a lot and have now put enough work into it that it is like most people's project cars: an enterprise in losing (okay, maybe just 'not making') money and a general time sink. And I find it to be both interesting and good experience.

Cliffs of other thread: car got slightly wrecked. Car being made betterfasterstronger and stuff. Putting Toyo R888's on rear, getting handling more dialed in, still have some tweaks to do on engine/drivetrain. Also have to finish up body repairs. I'll have had my hands in just about everything on this car, heh. Body shop will do the paint.

Protip: Greddy BOV's are shit (for the money, at least), but their main problem is easy to alleviate.

All the damn diaphragm tearing issues seem to be related to how it seats. Here are what the rubber bit looks like (click one of the types to expand and show a picture). That flat inner portion is sandwiched between two metal discs. The lower one sits JUST far enough down onto the valve that it that it forces the diaphragm to rub against the valve's housing. I do not see any reason for this. So I spaced it up with a washer, and sealed the old diaphragm with a layer of RTV just to test it.

It works beautifully. There is now such a good seal on the vacuum side that at the previous spring tension, the valve was opening at idle. In order to get good travel but also hold adequate tension on the valve, I ended up not only running both springs, but giving each a mild stretch to put it under a little more tension. Adjuster is backed almost all the way off. Again...works surprisingly well for a total piece of shit.

I'm got some bits coming in the mail to change the IC piping. I need a 2.5" pipe off the throttle body to properly clear the turbo, plus it will allow me to reinstall a strut tower brace.

My old understanding was that intake design did not matter on a turbo car. Not manifold or intercooler piping...it just had to not be restrictive. Fluid physics don't matter because of the boost; equal pressure is exerted upon all internal surfaces. Want more flow? Give it more boost...

Now I realize that was not-so-correct. I believe it is true when you're talking about being at full boost. But the transition from to said boost from a vacuum baseline is freakin' critical, especially with a big turbo and large intake/intercooler volume.

I'm wonder what effect going all 2.5" pipe would have, as opposed to mixing 2.5 and 3". Or all 3", for that matter. I think flow is good enough with 2.5", but I might be hurling air into the engine at fucking mach 2 or something. :awe:

Also of random note; I tried this with better pipe connections and the BOV purposefully not functioning. The compressor surge is kinda damn scary. It is LOUD and shakes the charge piping pretty violently. That's just revving the engine without a load...I can't imagine how much worse it would be after running up to redline under load then pushing the clutch in.
 

Danimal1209

Senior member
Nov 9, 2011
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Even though there is no proof that not having a bov/bpv in place can harm the turbo, why not have one there? It'll make you feel better.
 

JCH13

Diamond Member
Sep 14, 2010
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About mixing 2.5in and 3in tubing... if do that, do it with great caution. At high flow rates inner diameter transitions steeper than some small angle (was around 10deg in our FSAE cars) will induce big intake losses. I would suggest picking one tubing size and sticking with it.

Changing from 3in to 2.5in tube will knock your flow area down 30%. That is a really big deal, friction losses in your intake will at least double. I would anticipate considerable power loss if you go with 2.5in intake piping.
 

phucheneh

Diamond Member
Jun 30, 2012
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Got any links to science-y type stuff? I've read about this, but always more of anecdotal forum-y info.

Flow changes between pipe sizes is a totally relative thing, I would think...it's kind of like electricity...a shorter, fatter wire can carry more current, but that doesn't make it 'better' unless your load is actually exceeding the limits of a longer, thinner wire.

Obviously there's a bit more to fluid flow...but I just don't think you can say "2.5 flows 30% less than 3". I mean, there are plenty of people running little turbos on their Honda motor with 2" pipe, and I'm sure a random jump to 3" would serve no purpose other than to increase lag.

I wonder what about pipe transitions causes the losses? I'm trying to think through it, but I just get more pretzel-head.

Say you have all 3" off the turbo...then a big intercooler...then neck down to 2.5" after it. The air was already expanding in the IC, then getting crammed back through a 3" outlet. How is that transition any worse (as far as disrupting flow) than having a 2.5" outlet? What about [what the car has now, and is a very common setup] 2.5" into IC and 3" out? People consider that more acceptible, but it seems backwards to me. Hot charge air occupies more space than cooled charge air.

The whole thing, again, just kinda makes my brain-pictures hurt. There's that whole 'venturi effect' thing that I've never been able to 100% grasp...air goes into smaller oriface, air speeds up. Ok, got it.

But if the air accelerates going into 2.5 from 3, then slows down on the other side...with an accompanying rise/drop in pressure correlated with the speed...how does the the smaller bit of pipe actually cause a net effect?

Also, thing to remember: this is not actually a race car. If smaller pipe gets the air from the turbo to the throttle more quickly, that's a plus. Quicker (felt/percieved) spool. Earlier power means less sudden power later. And a drop in peak HP is not the end of the world. But I'm trying to avoid any grievous oversights here.

I kinda wanna do the 3">IC>2.5" thing just because it seems to make so much more sense than the so-damn-common inverse... :hmm:
 

jlee

Lifer
Sep 12, 2001
48,511
219
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Got any links to science-y type stuff? I've read about this, but always more of anecdotal forum-y info.

Flow changes between pipe sizes is a totally relative thing, I would think...it's kind of like electricity...a shorter, fatter wire can carry more current, but that doesn't make it 'better' unless your load is actually exceeding the limits of a longer, thinner wire.

Obviously there's a bit more to fluid flow...but I just don't think you can say "2.5 flows 30% less than 3". I mean, there are plenty of people running little turbos on their Honda motor with 2" pipe, and I'm sure a random jump to 3" would serve no purpose other than to increase lag.

I must've missed the part where you were running a little turbo on a Honda motor.
 

Sunny129

Diamond Member
Nov 14, 2000
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Obviously there's a bit more to fluid flow...but I just don't think you can say "2.5 flows 30% less than 3".
A = πr²
= π(1.25)²
≈ 4.90625 sq. in. cross-section for the 2.5" pipe

= π(1.5)²
≈ 7.065 sq. in. cross-section for the 3" pipe

4.90625/7.065 = .69444, or ~70%. the 2.5" pipe can only push approx. 70% as much air/gas, i.e. it flows 30% less air/gas. granted, i've neglected to take into account the thickness of the piping (and therefore the actual inner diameter of the piping), but that difference is negligible, and the 2.5" pipe will in fact flow approx. 30% less than the 3" pipe, give or take a few %. there's your sceincey stuff...
 

JCH13

Diamond Member
Sep 14, 2010
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Throttle response concern-

First, let's look at the throttle response different between the pipe sizes. I'll take a WAG at some absolute numbers, just to make my point.

8ft of 2.5in piping is about 30L of volume
8ft of 3in piping is about 44L of volume

The turbo is probably flowing at least 120L/s of pressurized air (I did some fuzzy math for this number, 500hp requires around 750ft^3/min of atmospheric air, divided by 3 for the pressure boost, and converted to L/s)

basically that 14L difference would take less than 0.1s to fill. There's no reason to fuss about 100ms of throttle response, especially in a street car.

Piping loss concern-

Here is a good article that explains flow lesses in a somewhat simple manor:

http://engineering-references.sbainvent.com/fluid-mechanics/head-loss.php#.UYLkB7WR-So

... although the equations can look a little intimidating, we're just looking at their characteristics, not compute them exactly.

Major head loss - pressure drop due to friction with the pipe wall
Minor head loss - pressure drop due to fittings, turns, or transitions in the piping

There are a couple charts that show inlet and outlet minor head loss factors. Basically, if the pipe diameter transitions are abrupt or poorly designed they will induce much more minor head loss than the major head loss they reduce. Not that they can't be done properly, but it takes care and thought to do it right.

As for the major head loss from making the pipe smaller, let's do some basic math:

flow volume [m^3/s] = flow velocity [m/s] * flow area [m^2]

Let's assume a fixed amount of HP requires requires a fixed amount of air flow. So we keep flow volume fixed and look at flow area and flow velocity:

flow velocity3 * flow area3 = flow velocity2.5 * flow area2.5 = flow volume

algebra...

flow area3 / flow area2.5 = flow velocity2.5 / flow velocity3

pi*1.5^2/pi*1.25^2 = 1.44 = velocity2.5 / velocity3

That is, the velocity of the air in the 2.5in piping will be 1.44 times as fast a the velocity of air in the 3in piping.

Note equation (3) in the article where head loss is expressed as a function of fluid velocity squared.

with the velocity increasing by a factor of 1.44 the head loss will be (1.44)^2 = 2.1, thus the head loss through the piping will be over twice as much, both major and minor losses.

What does this head loss mean? It means a difference in pressure between the turbo outlet and the intake manifold. If the car was losing 2psi with 3in pipe, it will be losing 4psi with 2.5in pipe, but I don't know the absolute number.

What does this mean for performance? The turbo has to output the same flow rate at a higher pressure to create the same power. This pushes the operating point of the turbo straight up on a compressor chart, generally making it less efficient. It also heats the intake air more and restricts the exhaust more.

It would be unlikely that the engine will make the same power given all of these sad side-effects of restricting the intake with smaller piping.

Now, let's all be a little reasonable here, I'm not saying everything will melt and explode and kill a bunch of babies, just that everything will be a little hotter and a little more stressed and it will cost you power.

/scienced