MBC... Tee'd to BOV!

[Slippi84]

Because you're a representative for a fairly major DSM shop, Joe. Instead of just another screwhead. You kinda should be expected to know your stuff better than the average newbie, if you want to portray the shop as experienced and knowledgeable.

Slippi, basic physics says it's not opening as fast. You sound like one of the 2Gers who insist that venting their BOV doesn't impact car performance, when they're only going on butt-dyno. It's simple physics that with a larger volume vacuum system, operation of the entire vacuum system will lag behind source conditions. This isn't hydraulics, it's just air. Air plays by certain rules. You may not be able to *notice* the problem as-is. That doesn't mean a problem doesn't exist. Try lengthening your vacuum lines by ten or twenty feet. Coil 'em up in the engine bay, and you'll see why having more volume than neccessary in a vacuum system, especially one with a single vacuum source is a bad thing. Just so you can notice/accept that there is a problem.

Please don't compare my experience with T-ing off the bov line to that of one of your ricer friends and won't refer to you going with what the mods say like a sheep. You keep saying this is just my opinion but its' not if yo makin a statement that by doing it our way we're putting our engines at risk and decreasing performance when plenty of people have come on here and said otherwise yet you still stand true to pure on paper experience. Your car was running like crap and you changd your mbc T location and it got beter and you "ASSUME" that cured your problem. Did T-ing off yoru bov line do any of the stuff you said it would? Cause unless someone can come on here and say it has sounds like you are shooting from the hip with theory and what if


edit: By the way if your so into theory and what the mods say why the findanza flywheel which they don't think is good for a turbo car one because aluminum can warp and break so chromoly is the way to go and on top of that the 8lb findanza unit s to light and doesn't have enough rotating mass to maintain boost after shifting?????

 

[staticbrainwash]

Please don't compare my experience with T-ing off the bov line to that of one of your ricer friends and won't refer to you going with what the mods say like a sheep. You keep saying this is just my opinion but its' not if yo makin a statement that by doing it our way we're putting our engines at risk and decreasing performance when plenty of people have come on here and said otherwise yet you still stand true to pure on paper experience. Your car was running like crap and you changd your mbc T location and it got beter and you "ASSUME" that cured your problem. Did T-ing off yoru bov line do any of the stuff you said it would? Cause unless someone can come on here and say it has sounds like you are shooting from the hip with theory and what if


edit: By the way if your so into theory and what the mods say why the findanza flywheel which they don't think is good for a turbo car one because aluminum can warp and break so chromoly is the way to go and on top of that the 8lb findanza unit s to light and doesn't have enough rotating mass to maintain boost after shifting?????

Christ dude, did you read anything? I came in here and said that moving it worked for me too. I was getting surge and spike. Both fixed by moving the line. As well as quicker bov response.

So don't get all huffy because you can't "follow" the crowd into the RIGHT thing.

And thank you for calling us sheep for using the compressor housing. I wonder why it was there from the factory. . . Hmmmm.

And i highly doubt ANYONE on here with more than ONE rep print or even ONE rep point would just move things around and assume it's better without trial and testing on their own. That how i KNOW FOR A FACT that it worked for me. Because it's the ONLY thing i changed that fixed my problems.

So thank you, i'm done here now, you "non-believers" are pissing me off with your refusal to even try something as simple as this.

 

[Slippi84]

Christ dude, did you read anything? I came in here and said that moving it worked for me too. I was getting surge and spike. Both fixed by moving the line. As well as quicker bov response.

So don't get all huffy because you can't "follow" the crowd into the RIGHT thing.

And thank you for calling us sheep for using the compressor housing. I wonder why it was there from the factory. . . Hmmmm.

And i highly doubt ANYONE on here with more than ONE rep print or even ONE rep point would just move things around and assume it's better without trial and testing on their own. That how i KNOW FOR A FACT that it worked for me. Because it's the ONLY thing i changed that fixed my problems.

So thank you, i'm done here now, you "non-believers" are pissing me off with your refusal to even try something as simple as this.

Again you ASS U ME I have never run a mbc off the compressor housing.

 

[staticbrainwash]

What i'm sorry, i don't respond to idiocy.

This should be closed, it's going no where. This is like arguing with a ricer about body kits.

 

[Slippi84]

What i'm sorry, i don't respond to idiocy.

This should be closed, it's going no where. This is like arguing with a ricer about body kits.

No one has blown a turbo or motor or damaged anything T-ing off the bov line period you show me otherwise and you may have a valid argument otherwise your throwing paper theory at everyone to justify logic that has been proven myth by just about everyone one here. Until you can do so I"m finished also as this is a battle of wits and you my friend are unarmed

 

[staticbrainwash]

I am physical proof it works? So how am i unarmed? And don't even begin to think you are better than i. Because i'm sure i could run circles around you in almost every category, except maybe douchebaggery.

Not to mention all the people oldman has helped with removing from the bov line as well. As i'm sure MANY MANY other people have too. I know a few other dsmer's in town here that have done the same with great success. So yeah, no physical proof at all.

And honestly, who's "following the crowd" eh? Doing it the old way. . . not the new better way. Yeah, you are the one without any kind of cannon fodder my friend not i. So just walk away before you embarrass yourself.

Close this down, it really is like arguing abortion or politics. Everyone's asshole just spouts what they think is right. But in this case, some people are wrong.

 

[black91awdturbo]

I think is what slippi is trying to say, I personaly have had trouble with spike when trying to use the compressor as a boost source. I then used the bov line and bam no more spike rock solid boost control. Does that mean my way is the best way to hook up a mbc BECAUSE IT WORKED FOR ME? No it does not and just because it worked for you the other way it doesnt mean your way is the best way, and you said it could be deadly to a engine just like slippi I want proof of that. And dont say well it worked for me so its the best way I want proof that it broke a engine or turbo. Or else the simple fact that you seem to think you have the best "new way" is nothing more than your way of doing things with zero proof that the other way is "wrong." And please do say well so and so says it is so it must be. And thats not a knock on any one here but intill some one can show a test that proves our way leads to broken or not working bovs with facts it means zip.

 

[Slippi84]

I think is what slippi is trying to say, I personaly have had trouble with spike when trying to use the compressor as a boost source. I then used the bov line and bam no more spike rock solid boost control. Does that mean my way is the best way to hook up a mbc BECAUSE IT WORKED FOR ME? No it does not and just because it worked for you the other way it doesnt mean your way is the best way, and you said it could be deadly to a engine just like slippi I want proof of that. And dont say well it worked for me so its the best way I want proof that it broke a engine or turbo. Or else the simple fact that you seem to think you have the best "new way" is nothing more than your way of doing things with zero proof that the other way is "wrong." And please do say well so and so says it is so it must be. And thats not a knock on any one here but intill some one can show a test that proves our way leads to broken or not working bovs with facts it means zip.

 

[Slippi84]

I am physical proof it works? So how am i unarmed? And don't even begin to think you are better than i. Because i'm sure i could run circles around you in almost every category, except maybe douchebaggery.

Not to mention all the people oldman has helped with removing from the bov line as well. As i'm sure MANY MANY other people have too. I know a few other dsmer's in town here that have done the same with great success. So yeah, no physical proof at all.

And honestly, who's "following the crowd" eh? Doing it the old way. . . not the new better way. Yeah, you are the one without any kind of cannon fodder my friend not i. So just walk away before you embarrass yourself.

Close this down, it really is like arguing abortion or politics. Everyone's asshole just spouts what they think is right. But in this case, some people are wrong.

W O W. Now it's no longer even about cars you have stooped to I'm better than you so I'm right. The OP has heard enough from me on the topic and others he can come to his own conclusion. I'm not having a pissing contest with someone who thinks because he has more green squares under his user name he is better good day to you.

 

[Car Cannibal]

I have had it routed both ways, with it off the BOV the line came loose and the boost spiked to an almost damageable range if I had not peaked over in time to catch it. I think that alone would be worth not running it off the BOV. When the MBC leaks off the compressor you get no boost, I like that better.

There is a noticeable difference when you make the hose really short off the Turbo housing and then to the wastegate in comparison to off the BOV, try it and post back.

And, think of this, which way will respond to opening the wastegate faster, it being off the turbo itself or at the very end of the boosts path ( intake )??

 

[SBR Joe]

Because you're a representative for a fairly major DSM shop, Joe. Instead of just another screwhead. You kinda should be expected to know your stuff better than the average newbie, if you want to portray the shop as experienced and knowledgeable.

Im still waiting for proof of it being the "wrong" or "incorrect" way. There is TONS of people out there with it hooked up to the bov line...with none of the listed "problems" that come from having it this way, its works fine. PERIOD. If in your mind its improper than so be it, im not telling anyone they have to run it my way, if you feel like its a problem, then im sorry you feel that way but its not. And if you get more peace of mind running it from the compressor housing, by all means do it. Its works fine both ways.



Joe
SBR

 

[Talesin]

Look at it this way. Most of those in this thread alone who say to do it the 'new' way are being rather reasonable. Others are resorting to ad hominem attacks, insults, and essentially saying 'you can't PROVE it's wrong!' in the face of basic physical interactions broken down to a level anyone should be able to understand. As for proof, go back and read through my deductive list. If there's a glaring error, please point it out, as I'd asked. Slippi's counterpoint can only be substantiated by ignoring the laws of physics and volumetric displacement. By that, it is irrelevant.

Short version, the logical progression proves that it's wrong. Reduction in boost spike for the majority of users proves that it's wrong. If we want to take a mean time to failure study, to prove that excess backflowed air and overspinning will kill a turbo faster than proper operation, I really doubt that would be logistically possible (much less to also account for all the extra lifespan-affecting variables). But I really doubt that anyone would disagree that either of those conditions is BAD for a turbo.

What am I saying. Even if it came back and showed that teeing off the BOV line shortened turbo lifespan by a year with a deviation of a half-month, Slippi would still say that it didn't prove that HIS would fail early, because his hadn't failed yet. I keep forgetting how some people think. Or refuse to, and justify it by any means possible so as not to be seen as 'wrong'.

 

[SBR Joe]

Look at it this way. Most of those in this thread alone who say to do it the 'new' way are being rather reasonable. Others are resorting to ad hominem attacks, insults, and essentially saying 'you can't PROVE it's wrong!' in the face of basic physical interactions broken down to a level anyone should be able to understand. As for proof, go back and read through my deductive list. If there's a glaring error, please point it out, as I'd asked. Slippi's counterpoint can only be substantiated by ignoring the laws of physics and volumetric displacement. By that, it is irrelevant.

Short version, the logical progression proves that it's wrong. Reduction in boost spike for the majority of users proves that it's wrong. If we want to take a mean time to failure study, to prove that excess backflowed air and overspinning will kill a turbo faster than proper operation, I really doubt that would be logistically possible (much less to also account for all the extra lifespan-affecting variables). But I really doubt that anyone would disagree that either of those conditions is BAD for a turbo.

What am I saying. Even if it came back and showed that teeing off the BOV line shortened turbo lifespan by a year with a deviation of a half-month, Slippi would still say that it didn't prove that HIS would fail early, because his hadn't failed yet. I keep forgetting how some people think. Or refuse to, and justify it by any means possible so as not to be seen as 'wrong'.

Im just curious about you logic of it shortening the life span of the turbo, and in what way would the conditions that you state have any direct relation to where the MBC line is tapped? Even it we will say, the vaccum line came off the T on the BOV, es, there would be a vaccumn leak and yes the bov would not function properly, but you would also have no boost, which you would mmore than likely notice right away, or if it did boost you for sure would notice a change in how it comes on...you would definately noice a situation..



Joe
SBR

 

[staticbrainwash]

I'm just getting heated with all of this. I'm ignoring sippi's whole response to me because he's not worth it.

I didn't mean to get insulting, but i don't respond well to getting personally attacked or called out.

I'm now done with this thread, run how you want, do what you want.

 

[Talesin]

Read back on page 2, Joe. I walked through the whole chain. Short version, increasing the volume of a vacuum system slows actuation on the remote end(s). Running two actuators off a single vacuum tap (when two seperate, very low-volume vacuum systems were intended, instead of one large, unwieldy setup) only makes the problem worse.

In any case, the BOV response will be slowed, allowing more positive charge to backflow to the compressor. It might not be much, but every little bit will eventually add up. Combine this with a slower wastegate actuation, which is more likely to result in initial spike and overspin. In the event of a boost leak, the turbo will be forced to overspin to provide a steady psi level to the manifold, overcoming any amount of boost leak inherent, instead of providing a steady output pressure from the compressor, which at-worst would result in a lower peak boost, and draw the driver to investigate why (s)he's only bumping along at 8psi, when the MBC was originally set for 16psi.
Teed to the BOV line it would still *read* 16psi at the manifold, but there would be a dramatically higher actual flow, higher charge temps, richer running, slower running, fuel cut, and all that nastiness before the driver might think to check, unless it's part of a monthly maintenence routine.


Slippi, as for my mods, I have my own reasons. I've seen what happens to an overheated chromoly flywheel. I am not building this car to be a drag honey. I prefer road racing, so maintaining boost between shifts is less important than throttle response and the ability to rev-match quickly, most of the time. This is also why I went with a CFDF clutch instead of some ridiculously over-heavy ACT unit.. it's not going to be tortured in a straight line. If I am doing short-term drag type accelleration, the NLTS feature of DSMLink should suffice for 2-3, once I find a place to do the socketing local. Also, I got a pretty good deal on the flywheel, and felt the urge to keep $200 in my pocket.

 

[Slippi84]

I have never said running it your way was wrong or bad you are saying running it off the BOV line is wrong. I have run it off the compressor housing and it works fine just as it works fine of the BOV line. ONE kid came on and said pretty much he didn't have his vaccume line hooked up tight enough it blew off and would have caused damage to his car. If you don't hook anything up tight enough it could cause damage and I agree that in that situation you could damage your car. I have not tightend my alternator adjuster bolt all the way and seen the belt shoot off does that mean the dealer is wrong and we should tension the belt diffrently?? I never attacked anyone and I don't respond well to being called a ricer or any similar reference so I apoligize as well. My one and only point is the same as Slowboy and the other big companies who were quoted there has been no proof just theory, which is logical and makes sense, but is still just theory until someone can prove otherwise. I'm not being difficult and stubborn just I don't go along with everything I read or hear without some kind of proof to back up logic.

 

[SBR Joe]

Read back on page 2, Joe. I walked through the whole chain. Short version, increasing the volume of a vacuum system slows actuation on the remote end(s). Running two actuators off a single vacuum tap (when two seperate, very low-volume vacuum systems were intended, instead of one large, unwieldy setup) only makes the problem worse.

In any case, the BOV response will be slowed, allowing more positive charge to backflow to the compressor. It might not be much, but every little bit will eventually add up. Combine this with a slower wastegate actuation, which is more likely to result in initial spike and overspin. In the event of a boost leak, the turbo will be forced to overspin to provide a steady psi level to the manifold, overcoming any amount of boost leak inherent, instead of providing a steady output pressure from the compressor, which at-worst would result in a lower peak boost, and draw the driver to investigate why (s)he's only bumping along at 8psi, when the MBC was originally set for 16psi.
Teed to the BOV line it would still *read* 16psi at the manifold, but there would be a dramatically higher actual flow, higher charge temps, richer running, slower running, fuel cut, and all that nastiness before the driver might think to check, unless it's part of a monthly maintenence routine.

I do understand you theory and it makes sense, but realistically, it would affect it ever so slightly that it would never add up to enough to actually do anything to harm the turbo. But i do understand your logic behind it.


Joe
SBR

 

[Slippi84]

Read back on page 2, Joe. I walked through the whole chain. Short version, increasing the volume of a vacuum system slows actuation on the remote end(s). Running two actuators off a single vacuum tap (when two seperate, very low-volume vacuum systems were intended, instead of one large, unwieldy setup) only makes the problem worse.

In any case, the BOV response will be slowed, allowing more positive charge to backflow to the compressor. It might not be much, but every little bit will eventually add up. Combine this with a slower wastegate actuation, which is more likely to result in initial spike and overspin. In the event of a boost leak, the turbo will be forced to overspin to provide a steady psi level to the manifold, overcoming any amount of boost leak inherent, instead of providing a steady output pressure from the compressor, which at-worst would result in a lower peak boost, and draw the driver to investigate why (s)he's only bumping along at 8psi, when the MBC was originally set for 16psi.
Teed to the BOV line it would still *read* 16psi at the manifold, but there would be a dramatically higher actual flow, higher charge temps, richer running, slower running, fuel cut, and all that nastiness before the driver might think to check, unless it's part of a monthly maintenence routine.


Slippi, as for my mods, I have my own reasons. I've seen what happens to an overheated chromoly flywheel. I am not building this car to be a drag honey. I prefer road racing, so maintaining boost between shifts is less important than throttle response and the ability to rev-match quickly, most of the time. This is also why I went with a CFDF clutch instead of some ridiculously over-heavy ACT unit.. it's not going to be tortured in a straight line. If I am doing short-term drag type accelleration, the NLTS feature of DSMLink should suffice for 2-3, once I find a place to do the socketing local. Also, I got a pretty good deal on the flywheel, and felt the urge to keep $200 in my pocket.

I can't hate a guy for trying to save $200

When you do get the link running you can calibrate the boost so that it will tell you actual boost not just estimated boost. I happen to have the link and have done this. I can tell you that there is no such boost loss as you speak of from T-ing off the bov line from comparing boost gauge to dsmlink.(If your wondering I calibrated with the connection on the compressor housing then switched turbos and went to a T on teh bov line with no change in actual and boostest)

 

[Talesin]

Joe, as I said, a small additional load will eventually add up. Just like keeping a penny (or quarter) jar, and being able to eventually change it in for a thousand bucks, it's not a large difference, but it does eventually make a dramatic one, given time. With any boost leaks and overspinning, it's like dropping in dollars instead, and you won't even notice until it's too late. Except in this case, the payoff is paying out for a rebuild or a new turbo.


Planning on swapping the stock gauge over for knock counts instead (damn 91 craptane), and just sticking with my mechanical boost gauge. Kinda annoying, none of the electronics shops in the area are willing to do the socket. May end up having to do it myself, and just be REALLY careful to get all of the solder off so none of the pads lift.

 

[dsm-onster]

I've run off the bov line, off the compressor nipple/j-pipe, off a throttle body nipple. directly off the pcv nipple (no other attached components) EVEN off of the fpr line. I've muffed w/ dsms for 7 years. . . Nothing was different. 14b, small 16g, t25, 18g, rs60t. I've work on / tuned friends cars where I've tapped in several different locations depending on the "architecture" of the engine bay. . . I'll change when I notice a problem. I suggest anyone do that. But None should change because of theory. When there's an occurance and you conspire a theory, you can make sense of things. But nothing makes sense where theory hasn't been verified w/ your particular setup. All if my particular setups don't seam to care which way the actuator gets it's boost reference.

Some theories of my own:
1. It takes almost an unmeasurably small amount of time to fill a 5+ ft pressure line of any reasonable diameter w/ boost or to evacuate it.
2. Surge is surge. If your turbo is close to surge but does not surge then it will last just fine. If a sluggish BOV makes your system ALMOST surge, then you're still not surging.
3. A MBC w/a bleeder hole only releases air from it's bleeder hole when the ball is open. A MBC ball vibrates and is not open the whole time you reach your desired boost level. A MBC is a typical check valve. The wastegate actuator sees no vacuum when the intake manifold displays vacuum.
4. A leak in a vacuum line (MBC bleader oriface) will lower the boost in the intake manifold. Thus the the MBC ball seats and closes. Once the boost exceeds the spring pressure, the ball opens back up. Thus in the line, the boost pressure before the ball and consequently before the MBC rises w/ the manifold boost and stays consistant as the ball spring pressure controls flow in the line to keep it consistant.
5. Since the typical MBC is a check valve, the ball closes before any measurable effect of the bleeder oriface can have an effect on the BOV response. If you feel that the additional line to the MBC affects response, see number 1.

All of my theory comes from occurances in my everyday experimenting w/ these cars for years. I SWEAR I wll change once I see negative effects. But since I havn't my theories hold true for my engine bay and environment. and all the other turbo autos in which I've modified the boost.

 

[wishihadatalon]

I think one thing most fail to realize is that the whole system pressurizes. So if you have a boost leak you will still have the same damn boost leak at the compressor housing. If you are unable to pressurize the whole system then how is moving the pressure signal any closer going to affect anything? Having a bleeder style bov is a leak so weather it is in the line for the bov or in the line from the compressor to the wg, you still have a leak.

Any good mbc or even better a ebc will/should take care of spike.

One quote that made me laugh was this

I don't get why people tee into the bov line AT ALL, but especially not for a gauge. Thats just silly. You want boost and vac typically, and that will not give vac. You need a manifold source for the most accurate boost and vac. But then again, what do i know. I only listen to all the wisemen and mods on this site that have been doing this shit for years and years.

Edit, running without pvc is also kind of silly but thats a whole new thread.

The bov line is off of the manifold which deffinately sees both vac and boost. I am not saying you should tap off of your bov line for your boost gauge nor have I ever but I don't think it would matter. Hell if you use the same logic as the used for using the bov line then what about any delays or leaks for your fpr?

In the end the differences are what? Probably nothing. To get all pissy and start name calling because someone doesn't agree with you (because you think that your way is the only way, even though a car shown that was build buy a dsm master proves that you don't need to do it your way) is rather childish and along the lines of something my 9yr old sister would do.

P.S. Perhaps DSMER's should be more worried about the other hack job shit that they do than to worry about a stupid line location that has proved to be opinion based only.

 

[SBR Joe]

I think one thing most fail to realize is that the whole system pressurizes. So if you have a boost leak you will still have the same damn boost leak at the compressor housing. If you are unable to pressurize the whole system then how is moving the pressure signal any closer going to affect anything? Having a bleeder style bov is a leak so weather it is in the line for the bov or in the line from the compressor to the wg, you still have a leak.

Any good mbc or even better a ebc will/should take care of spike.

One quote that made me laugh was this

The bov line is off of the manifold which deffinately sees both vac and boost. I am not saying you should tap off of your bov line for your boost gauge nor have I ever but I don't think it would matter. Hell if you use the same logic as the used for using the bov line then what about any delays or leaks for your fpr?

In the end the differences are what? Probably nothing. To get all pissy and start name calling because someone doesn't agree with you (because you think that your way is the only way, even though a car shown that was build buy a dsm master proves that you don't need to do it your way) is rather childish and along the lines of something my 9yr old sister would do.

P.S. Perhaps DSMER's should be more worried about the other hack job shit that they do than to worry about a stupid line location that has proved to be opinion based only.

^^^^ Spoken like a true prophet!!

Joe
SBR

 

[oldman]

I can't believe this is still being argued. For those of you who needs proof, run a boost leak test with your MBC intact and see your BOV opens as the desired pressure is reached and the MBC bleeder hole is exposed. Running your MBC to your BOV line is really no different than creating a rip on the line itself. Not tapping the BOV line is not a matter of opinion, it's a fact.

 

[Slippi84]

I think one thing most fail to realize is that the whole system pressurizes. So if you have a boost leak you will still have the same damn boost leak at the compressor housing. If you are unable to pressurize the whole system then how is moving the pressure signal any closer going to affect anything? Having a bleeder style bov is a leak so weather it is in the line for the bov or in the line from the compressor to the wg, you still have a leak.

Any good mbc or even better a ebc will/should take care of spike.

One quote that made me laugh was this

The bov line is off of the manifold which deffinately sees both vac and boost. I am not saying you should tap off of your bov line for your boost gauge nor have I ever but I don't think it would matter. Hell if you use the same logic as the used for using the bov line then what about any delays or leaks for your fpr?

In the end the differences are what? Probably nothing. To get all pissy and start name calling because someone doesn't agree with you (because you think that your way is the only way, even though a car shown that was build buy a dsm master proves that you don't need to do it your way) is rather childish and along the lines of something my 9yr old sister would do.

P.S. Perhaps DSMER's should be more worried about the other hack job shit that they do than to worry about a stupid line location that has proved to be opinion based only.

 

[Talesin]

Wish, as a crude example take a garden hose and punch it full of holes. Run water down it. You're going to get a lower pressure at the far end than you would at the source end. The same holds true even in a positive-pressure system... quite possibly why everyone who uses an EGT taps the thermo probe into the #1 runner, which always seems to run hottest out of the four. Odd that it's also the furthest from the feed on the rail. Sure, if it was a closed volume that was being filled to a certain pressure, gradient drop wouldn't matter in the least.. but the intake tract is a flowing pressurized system. Same reason that intercooler pressure drop from the hot to cold side is real.

I'm running from the compressor outlet elbow on my setup. I see a minor boost drop at higher altitudes. I used to run BOV teed, which solved the 'problem' (gave me a solid 16psi no matter if I was at sea level or 8,000ft instead of dropping to about 12-14psi), which was primarily caused due to a few boost leaks that have since been fixed. I still get some drop at-height, which I'm blaming on the lower ambient air density until a better explanation comes along. Swapping over also solved an annoying spike-to-19 issue that would kick in fuel cut like crazy if I wasn't careful with how I romped on it.