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BOV is opening under boost

Old 09-09-2018, 05:42 PM
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BOV is opening under boost

Once I hit about 3psi (according to the ECU) I could hear the BOV squeling and boost wouldn't go up. My mechanical gauge showed nearly 5 psi but who knows.

Notice the jaggedness of the boost, yet RPM climbs.


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Old 09-09-2018, 06:58 PM
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Re: BOV is opening under boost

The rubber diaphragm might have a tear in it. If you have a vacuum pump, apply some vacuum to the vacuum port and see if it holds.
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Old 09-10-2018, 07:11 AM
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Re: BOV is opening under boost

what type of valve? sure it flows enough for the application?
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Old 09-10-2018, 08:42 AM
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Re: BOV is opening under boost

Originally Posted by 86CamaroDan View Post
what type of valve? sure it flows enough for the application?
Greddy type S. Not sure, but the problem isn't not enough venting, the problem is it's opening under boost. Which is odd cuz you'd think even if the spring didn't keep it closed the pressure on the vac line would.

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Old 09-10-2018, 09:09 AM
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Re: BOV is opening under boost

Yeah the vacuum line side is suppose to equalize pressure to help keep it closed. Is it tee’d into another source or on its own? Put it on its own. Check line and internals of bov
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Old 09-10-2018, 09:24 AM
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Re: BOV is opening under boost

Originally Posted by Orr89RocZ View Post
Yeah the vacuum line side is suppose to equalize pressure to help keep it closed. Is it tee’d into another source or on its own? Put it on its own. Check line and internals of bov
IT's on it's own source. It has an adjustment on the top "hard ---- soft".. I wonder if maybe it's backed out too much?

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Old 09-10-2018, 10:48 AM
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Re: BOV is opening under boost

do you not run a bypass valve?
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Old 09-10-2018, 11:32 AM
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Re: BOV is opening under boost

Originally Posted by DIGGLER View Post
do you not run a bypass valve?
Just a BOV vented to atmosphere.

-- Joe
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Old 09-10-2018, 12:54 PM
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Re: BOV is opening under boost

need a bypass valve that can open at idle with a supercharger. i would swap it out.
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Old 09-10-2018, 12:56 PM
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Re: BOV is opening under boost

Originally Posted by DIGGLER View Post
need a bypass valve that can open at idle with a supercharger. i would swap it out.
It opens at idle, or whenever there is more than 10hg or so of vac on it.

A bypass valve recirculates into the air cleaner, or at least that's the definition I've always gone by. (see below)

https://www.turbobygarrett.com/turbo...bov_and_bypass

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Old 09-10-2018, 01:13 PM
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Re: BOV is opening under boost

Yeah spring rate should be enough to keep valve shut until vacuum opens it.
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Old 09-10-2018, 01:24 PM
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Re: BOV is opening under boost

if its somewhat open at idle you are good. im talking about something like a vortech mondo valve or something. you can actually adjust the vacuum where the valve opens up at idle/part throttle on those, not a pressure where it opens, since you would usually adjust boost psi with your pulleys/blower size. i know the mondo fore sure opens at idle.
you could plumb either bypass or bov to vent back into the intake piping technically making them "bypass".

now that i think about it, the way a mondo is made, boost pressure actually forces the valve closed more and more as pressure goes up.
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Old 09-10-2018, 01:29 PM
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Re: BOV is opening under boost

Originally Posted by DIGGLER View Post
if its somewhat open at idle you are good. im talking about something like a vortech mondo valve or something. you can actually adjust the vacuum where the valve opens up at idle/part throttle on those, not a pressure where it opens, since you would usually adjust boost psi with your pulleys/blower size. i know the mondo fore sure opens at idle.
you could plumb either bypass or bov to vent back into the intake piping technically making them "bypass".

now that i think about it, the way a mondo is made, boost pressure actually forces the valve closed more and more as pressure goes up.
This valve appears to be designed the same way. It should have pressure in the top when under boost to keep it closed. Yet it's opening. In fact, at about 3psi it's screaming.

Otherwise, the car seems really reliable. Just need to tinker with the valve a little bit, and clean out my trailer. I'm hoping to get some passes at the track during October.

-- Joe
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Old 09-26-2018, 10:51 AM
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Re: BOV is opening under boost

I turned the screw clockwise a few turns, and now it's holding about 6psi.

It's funny, it just opens up and starts screaming lol.

I'll tinker with it a little more when I get back from vacation. I gotta clean out my trailer and maybe haul it up to the dragway before they close (end of October).

I've been driving the car as much as I can, and it's impressively reliable. Doesn't matter if it's 90 and humid, or 45 degrees out the temp stays the same, no issues. I'm kinda impressed.

No signs of belt slip, and it' hasn't walked or thrown a belt so I'm pretty happy about my rev3 blower brackets.

Over this winter I'm going to be designing some blower brackets for my buddy's LT1 thirdgen build. (he's ditching the turbo). The LT1 fbody accessories leave the entire driver side of the engine open, so I'm going to run a double plate bolted to the block and head. A spacer bolted to the LT1 crank hub with independent blower belt. MS2 in batch fire + CNP on a .040 over build LT1. Should be a cool build.

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Old 09-26-2018, 04:03 PM
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Re: BOV is opening under boost

Hmm, what's a blow off valve lol? Ported shrouds FTW, plus one for us turbo guys...

Originally Posted by anesthes
Over this winter I'm going to be designing some blower brackets for my buddy's LT1 thirdgen build. (he's ditching the turbo).
Heh, so your buddy is going from slow.... to slower, ehh? j/k

Looking forward to seeing some track pulls Joe...

- Rob
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Old 09-26-2018, 04:17 PM
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Re: BOV is opening under boost

Originally Posted by Street Lethal View Post
Hmm, what's a blow off valve lol? Ported shrouds FTW, plus one for us turbo guys...



Heh, so your buddy is going from slow.... to slower, ehh? j/k

Looking forward to seeing some track pulls Joe...

- Rob
Haha I think after 5 plus years of reading about the turbo build blowing up on here or them never getting completed I think he just wants something that's reliable and works lol.

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Old 09-26-2018, 05:02 PM
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Re: BOV is opening under boost

Originally Posted by anesthes
Haha I think after 5 plus years of reading about the turbo build blowing up on here or them never getting completed I think he just wants something that's reliable and works lol.
Hey hey, mine is done lol. Mind you its not going to run nine seconds the way it is at the moment, but it's definitely on the road. I'm running a ten pound spring in the gate, and am waiting on my electronic boost controller to dial it in through 9 to 16 pounds. Will see that much boost when the E85 is in, but keeping it capped at 10 pounds with pump gas and 10* spark advance. Also bought a Flex Fuel Sensor that is still pending install.... but the car is on the road though.

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Old 05-06-2019, 08:29 AM
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Re: BOV is opening under boost

This is still happening.. I can adjust the BOV more but I'm thinking maybe it's just faulty?

The belt is not slipping. And while I cannot hear it blowing out the valve, it won't make more than 3.2psi at around 5k RPM.

Unless I've got the most free-flowing heads and intake, and the t-trim (rated at 1200hp) is too small for my 412" sbc, It's gotta be leaking boost.

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Old 05-06-2019, 09:12 AM
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Re: BOV is opening under boost

Unless I've got the most free-flowing heads and intake, and the t-trim (rated at 1200hp) is too small for my 412" sbc


Joe, this is more than likely the case. Remember that when superchargers advertise boost numbers, it is dependent on pulley size(s) in conjunction with how much the base engine is already making. Unlike a turbo which will force a predetermined boost pressure by way of excessive exhaust gases and spring pressure, regardless of engine power, the spring forces it to that boost pressure. Two sets of pulleys though in a centrifugal system will make different boost pressure when used on engines with significant horsepower differences. Example; your engine will make less boost pressure because with your advertised pulleys because it is already taking in that much air naturally aspirated from the getgo, whereas an engine making a mere one hundred and fifty horsepower as its' base will see higher boost pressure from the same pulley sizes and RPM you are using because it is taking in much less to begin with naturally aspirated. It's just a resistance measure. To know for sure, why not seal up the BOV momentarily and blip the throttle in Park/Neutral to see if it rises. This will at least eliminate the possibility of it leaking/bleeding off...

- Rob
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Old 05-06-2019, 09:29 AM
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Re: BOV is opening under boost

Originally Posted by Street Lethal View Post

Joe, this is more than likely the case. Remember that when superchargers advertise boost numbers, it is dependent on pulley size(s) in conjunction with how much the base engine is already making. Unlike a turbo which will force a predetermined boost pressure by way of excessive exhaust gases and spring pressure, regardless of engine power, the spring forces it to that boost pressure. Two sets of pulleys though in a centrifugal system will make different boost pressure when used on engines with significant horsepower differences. Example; your engine will make less boost pressure because with your advertised pulleys because it is already taking in that much air naturally aspirated from the getgo, whereas an engine making a mere one hundred and fifty horsepower as its' base will see higher boost pressure from the same pulley sizes and RPM you are using because it is taking in much less to begin with naturally aspirated. It's just a resistance measure. To know for sure, why not seal up the BOV momentarily and blip the throttle in Park/Neutral to see if it rises. This will at least eliminate the possibility of it leaking/bleeding off...

- Rob
Rob,

The impeller is spinning at 40,000 RPM @ 5,000 crank RPM, which equates to 1,018 CFM.

At 100% efficiency @ 5,000 RPM the engine would ingest 596 cfm. (best cast scenario, it's not 100% efficient)

One would think the boost pressure should be about 1 atmosphere (or 14ish psi).

-- Joe
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Old 05-06-2019, 09:58 AM
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Re: BOV is opening under boost

What pulley sizes (crank/blower), and where exactly is your boost pressure reading (3.2)?

The equation to calculate horsepower using airflow is;

Air flow (CFM) = (Horsepower × 0.625) × 1.1104

... so 596-cfm equates to 858 horsepower.

... and 1018-cfm is 1466 horsepower.

What size injectors are you running as well?

- Rob
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Old 05-06-2019, 11:02 AM
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Re: BOV is opening under boost

Originally Posted by Street Lethal View Post
What pulley sizes (crank/blower), and where exactly is your boost pressure reading (3.2)?

The equation to calculate horsepower using airflow is;

Air flow (CFM) = (Horsepower × 0.625) × 1.1104

... so 596-cfm equates to 858 horsepower.

... and 1018-cfm is 1466 horsepower.

What size injectors are you running as well?

- Rob
I'm using a 7.8" crank pulley and a 3.33" sc pulley. Impeller speed is 40,000 RPM at 5,000 Crank RPM.

Injectors are 60lb deka @ 72% duty cycle at 5000. (although a little lean, I need to add some boost fuel).

If I work my way backwards from injector duty cycle (60lb injectors at .55 BSFC = 860 hp, 70% of that = 602 hp).

That seems too high to me..

The CFM to HP calcs you did seem waaaaaaay out of the park.. Attached is a T-trim compressor map for reference.

Maybe I'm way off on my CFM calc, but according to the specs it should flow 1,400 CFM at 55,000 RPM so at 40,000 RPM I'm coming up with 1,018 CFM..

-- Joe

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Old 05-06-2019, 11:30 AM
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Re: BOV is opening under boost

Quickly comparing S-Trim vs T-Trim from Vortech with real world setups (not going on the compressor map), I see that the 3.33" pulley seemingly reports that much boost at 5000-plus RPM from engines making significantly less than yours does Joe, averaged at about 300 to 350 horsepower from I am seeing, with the usual drop to the 3.12" pulley raising it a tad. This now points to the following with your information;

60# injectors at 72% DC, is roughly 630 Horsepower.

3.2-psi is about a 21% increase, so at 630 horsepower you are making roughly 520 naturally aspirated.

... so knowing the above about your engine, your same setup would provide 15-psi of boost pressure with an engine making 350 horsepower as its' base, which is normally what Mustang guys see at those boost levels with an essentially stock (albeit bolt on) engine, and at 6000-plus RPM

- Rob
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Old 05-06-2019, 11:53 AM
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Re: BOV is opening under boost

Originally Posted by Street Lethal View Post
Quickly comparing S-Trim vs T-Trim from Vortech with real world setups (not going on the compressor map), I see that the 3.33" pulley seemingly reports that much boost at 5000-plus RPM from engines making significantly less than yours does Joe, averaged at about 300 to 350 horsepower from I am seeing, with the usual drop to the 3.12" pulley raising it a tad. This now points to the following with your information;
When I ran the S-Trim on the 355 using the same pulley combination I made 14psi of boost and 565 crank HP. Although the engine is larger, I'd expect the boost to be similar because it's a much larger blower.

Originally Posted by Street Lethal View Post
60# injectors at 72% DC, is roughly 630 Horsepower.

3.2-psi is about a 21% increase, so at 630 horsepower you are making roughly 520 naturally aspirated.
That seems to high. As much as I'd like to pat myself on the back regarding the build, 520 n/a seems way too high for this combo..

Originally Posted by Street Lethal View Post
... so knowing the above about your engine, your same setup would provide 15-psi of boost pressure with an engine making 350 horsepower as its' base, which is normally what Mustang guys see at those boost levels with an essentially stock (albeit bolt on) engine, and at 6000-plus RPM

- Rob
I'm confused at what you are saying. A base 350 hp mustang adding a T-Trim ?

-- Joe
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Old 05-06-2019, 12:39 PM
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Re: BOV is opening under boost

Numbers for CFM were probably off in my first post. I was looking at the chart for the Ti trim not the T-Trim. The correct T-Trim chart lines up with the compressor map:

V-1
T-Trim
825hp
26
1200
Gear Drive
55,000
3.75"
2.38"




So in theory 60lb/hr flow is about 600HP right - and that's about 850 CFM. I'm putting my impeller speed calc at 872 CFM, so that seems to jive.But the pressure ratio on the chart shows 1.9, so it should be about 11psi. Not 3.2..-- Joe

Last edited by anesthes; 05-06-2019 at 12:46 PM.
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Old 05-06-2019, 12:43 PM
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Re: BOV is opening under boost

It's to your credit Joe, you're making great power. But the numbers I quoted were more on the crank horsepower side though, but regardless that is great power. What I meant by the T-Trim and S-Trim comparison is that most Mustang's with approximately 300-350 horsepower would be in the same range as you (horsepower wise) with your supercharger and pulley combo, but their boost gauge would be closer to 15-psi and not 3.2-psi to be making 630 horsepower (give or take a few), and more than likely closer to 6000-RPM and not 5000-RPM, which again is to your credit for selecting the right parts. If they went with the 3.12" pulley instead, it would be tad lower in RPM for them to make the same boost pressure/power. I mentioned the S-Trim because it is the closest in terms of your blower's specs for comparison, If you spun your engine to 6000-RPM you would of course see more boost pressure, but the 3.33" pulley is child's play for you. You should be closer to 8 to 10-psi with a 3.12" pulley at 5000-RPM, but we're talking a lot more power being made from your engine...

- Rob
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Old 05-06-2019, 01:05 PM
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Re: BOV is opening under boost

Joe, lose the BOV, or get one to handle more boost pressure, pulley up the supercharger, and slap a wastegate on it to control boost pressure down low.. This guy did that very thing with his Procharger, but he claimed he was losing power up top because of the wastegate. It had to be somewhere in how he set up the boost controller, either that or the BOV was bleeding off the charge in conjunction with the wastegate, Something he overlooked I bet. Either way, works like a charm...

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Old 05-06-2019, 01:07 PM
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Re: BOV is opening under boost

Originally Posted by Street Lethal View Post
Joe, lose the BOV, pulley up the supercharger, and slap a wastegate on it to control boost pressure down low.. This guy did that very thing with his Procharger, but he claimed he was losing power up top because of the wastegate. It had to be somewhere in how he set up the boost controller, either that or the BOV was bleeding off the charge in conjunction with the wastegate, Something he overlooked I bet. Either way, works like a charm...
That's a possibility, and I have considered that. I need to review the WG logic in the MS to make sure I can configure it to open whenever TPS is 0.

-- Joe
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Old 05-07-2019, 01:42 PM
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Re: BOV is opening under boost

I did some testing last night. I don't think the BOV is opening.

I am wondering however if my air filter is an issue. I was reading on another forum, a guy with a mustang was getting 3psi out of a T-Trim. He replaced the filter with an HKS mushroom filter and got 10psi...

I'm running this filter: https://www.summitracing.com/parts/SUM-F1344

Which is actually this one (believe it even says K&N but came in a summit box): https://www.knfilters.com/search/pro...x?prod=RU-2520

So then I looked at the test data and found that the filter is rated at 300 cfm...

https://www.knfilters.com/dynocharts/RU-2520.pdf

-- Joe

Last edited by anesthes; 05-07-2019 at 01:45 PM.
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Old 05-08-2019, 09:09 AM
  #30  
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Re: BOV is opening under boost

Intresting find. Im running a RU-3620 myself and i just barely fits between the blower and master, and i picked up 1.5PSI @ 6200.

Did you just post in the vortech/paxton/procharger FB page btw?
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Old 05-08-2019, 10:33 AM
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Re: BOV is opening under boost

Originally Posted by 86CamaroDan View Post
Intresting find. Im running a RU-3620 myself and i just barely fits between the blower and master, and i picked up 1.5PSI @ 6200.

Did you just post in the vortech/paxton/procharger FB page btw?
Yes I did. I've found facebook to be very helpful for EFI and FI tech.

There doesn't appear to be a flow test on the RU-3620.

Do you remember this guys setup:




I wonder if I should just fabricate something like that since I have a cowl hood, and run a flat filter like on my C4.

The only thing that makes me nervous is water. if I leave my car outside the trailer and it rains I sometimes have a puddle under my miniram!

-- Joe
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Old 05-08-2019, 10:53 AM
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Re: BOV is opening under boost

I messaged K&N but they dont have a flow chart on it so I'm out of luck. Its alot larger than the stock vortech filter though.

I thought to myself "This is the same problem joe is having on his 412! His names even joe...... wait a second"

Theres a megasquirt page thats very helpful also.

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Old 05-08-2019, 11:39 AM
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Re: BOV is opening under boost

Originally Posted by 86CamaroDan View Post
I messaged K&N but they dont have a flow chart on it so I'm out of luck. Its alot larger than the stock vortech filter though.

I thought to myself "This is the same problem joe is having on his 412! His names even joe...... wait a second"

Theres a megasquirt page thats very helpful also.
Haha that's too funny.

I'm using 4" pipe so that filter wouldn't work but maybe a similar one.. I'm tempted to try it without a filter for a quick burst but with my luck it will suck in a chipmunk or something..

The megasquirt group is great as well as the 'Guild of EFI tuners'.


-- Joe
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Old 05-09-2019, 07:10 AM
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Re: BOV is opening under boost

Dan what are you running for a BOV?

I changed the filter last night to a huge chinese filter. I also adjusted the BOV a little because it won't open at idle, and that was apparently a mistake. When I got into boost it went up to 3 then to zero and it screamed. I'm assuming the BOV opened wide and dumped everything.

Does your BOV open at idle and stay open during cruise?

I've found that a lot of my cruise on this car is around 20kpa so I've been tuning that area of the VE map. Car really does drive nice on the street.. Once I get this peak stuff taken care of It should be great at the track.

-- Joe
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Old 05-09-2019, 07:31 AM
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Re: BOV is opening under boost

Originally Posted by anesthes View Post
Dan what are you running for a BOV?

I changed the filter last night to a huge chinese filter. I also adjusted the BOV a little because it won't open at idle, and that was apparently a mistake. When I got into boost it went up to 3 then to zero and it screamed. I'm assuming the BOV opened wide and dumped everything.

Does your BOV open at idle and stay open during cruise?

I've found that a lot of my cruise on this car is around 20kpa so I've been tuning that area of the VE map. Car really does drive nice on the street.. Once I get this peak stuff taken care of It should be great at the track.

-- Joe
Its pretty much open 75% of the time. Idle, light acceleration, cruise. Only when I'm into the pedal does it close and start to build boost. Im running a Tial Q with a -2 psi "Supercharger" spring in it. I notice all their springs for turbo cars are like -6 to -11 "psi" (no idea why they rate it in PSI vs in hg)
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Old 05-09-2019, 12:02 PM
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Re: BOV is opening under boost

Originally Posted by 86CamaroDan View Post
Its pretty much open 75% of the time. Idle, light acceleration, cruise. Only when I'm into the pedal does it close and start to build boost. Im running a Tial Q with a -2 psi "Supercharger" spring in it. I notice all their springs for turbo cars are like -6 to -11 "psi" (no idea why they rate it in PSI vs in hg)

Ok I ordered one and a -2 spring.


So I went for a drive with the big filter. Now it gets into boost than something screams and the boost goes to zero

I think it's the bov dumping . It could be the belt but it's a very tight 10 rib so I doubt it..

-- Joe
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Old 05-09-2019, 03:04 PM
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Re: BOV is opening under boost

Whats this thing look like? It got a small outlet port? I had a ZZP bypass valve two years ago that was VERY noisy. I unthreaded the outlet it it got quieter.
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Old 05-10-2019, 09:35 AM
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Re: BOV is opening under boost

Originally Posted by 86CamaroDan View Post
Whats this thing look like? It got a small outlet port? I had a ZZP bypass valve two years ago that was VERY noisy. I unthreaded the outlet it it got quieter.
Here is what we've got now:





I ordered the Tial q one you have. I'll see if that helps. Right now based on the screaming I'm hearing either the BOV is opening or the belt is slipping, but I'd find it hard to believe a very tight 10 rib belt is slipping. Guys run head units twice as big as me on 10 rib without issue.

Cog drives are stupid expensive, I'm not doing that lol.

-- Joe
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Old 05-10-2019, 10:36 AM
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Re: BOV is opening under boost

First of all. LOL they dont even make that bov anymore
2. I call BOV = bypass valve, use them interchangeably and prefer bypass. It still 'bypassing air' even if venting to atmopshere
3. That bypass (type-S) Has been replicated en mass by 'cheap copies' so be aware you might have that unit ($15 unit)
4. The original Greddy type-S I have used a hundred times. Their diaphragm will tear and performance will suffer. Not saying that is your problem but it is something that happens frequently.

5. How the bypass is supposed to work in turbo application (that bypass is designed for a turbo):
-Open at idle to allow compressor wheel speed to increase (turbo applications)
-Shut tightly at WOT to keep boost from leaking out

How do you test this system?
1. Perform a boost pressure test, Fill the plumbing with "boost pressure" using a compressor, in a quiet place so you can hear all leaks

This test is routinely performed as an essential step prior to tuning any engine, because a small air leak will rob power, increase IAT, increase EGP, and may result with broken engine parts. It is SERIOUS

Never run a high power forced induction application without a proper pressure test. My instinct tells me that the bypass is either damaged(internally) or connected incorrectly. Because they work great up to 18~psi in turbo application of the 92-02 era, prior to it being discontinued.

Tial makes a great unit, but you still need the pressure test.
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Old 05-10-2019, 10:50 AM
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Re: BOV is opening under boost

Originally Posted by Kingtal0n View Post
First of all. LOL they dont even make that bov anymore
2. I call BOV = bypass valve, use them interchangeably and prefer bypass. It still 'bypassing air' even if venting to atmopshere
3. That bypass (type-S) Has been replicated en mass by 'cheap copies' so be aware you might have that unit ($15 unit)
4. The original Greddy type-S I have used a hundred times. Their diaphragm will tear and performance will suffer. Not saying that is your problem but it is something that happens frequently.
Hey man.. I've had this thing for years and years and years. It was used on a buddies twin turbo Nissan probably 15 years ago. It's real, but really old and suspect. It opens when enough vac is applied but not at idle. I played with the spring preload screw and got mixed results, so I ordered the Tial Q.


Originally Posted by Kingtal0n View Post
Never run a high power forced induction application without a proper pressure test. My instinct tells me that the bypass is either damaged(internally) or connected incorrectly. Because they work great up to 18~psi in turbo application of the 92-02 era, prior to it being discontinued.

Tial makes a great unit, but you still need the pressure test.
Well.. It's either opening or the belt is slipping, or both. I can't tell if the screaming I'm hearing is the belt or the BOV, but I suspect BOV. When I swapped to the larger filter the other day it made it worse, so that leads me to believe BOV.

I don't know what the pressure is before the IC.. For all I know the IC is restrictive and I'm seeing 25psi before the IC and 3 psi after.. I need to add a second map sensor on the blower outlet and log on a spare channel. I'm gonna try and get that done next week, if I can remember which wire my spare ADC is in the harness.

-- Joe
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Old 05-11-2019, 08:12 AM
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Re: BOV is opening under boost

bypass needs to be open at idle on a supercharger to vent all the air pumping in from the supercharger. when you romp on it and vacuum drops to around 3-7" the bypass should close.
at idle you should be blowing leaves out from under the car because of all the air rushing out of the bypass valve.


https://www.procharger.com/procharger-bypass-valves
https://vortechsuperchargers.com/pro...ant=7813347649

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Old 05-13-2019, 07:23 AM
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Re: BOV is opening under boost

I just can't see a supercharger forcing in that much cfm at idle that would justify the need for a bypass valve needing to be used in this case. A bypass valve is primarily meant to keep back pressure off of the supercharger during idle to extend the life of the blower, but how much back pressure is even there when the bypass is designed to close between 3-7" Hg during its' operation. Do you guys think that removing the bypass valve that the engine will suddenly idle under boost pressure without it? That's impossible. Impossible with an engine of Joe's size, cfm and blower size. If anything, the incoming air through the throttle body during idle itself soothes/relives the blower of any back pressure as it is helping the compressor spin by pulling it from the other end, as the belt on the opposing end is pushing/forcing the compressor. I mean sure, slap that same blower on an engine flowing much less than Joe's engine, and I would see a possible need for a bypass valve at idle because it becomes more restrictive, but we're talking a lot less than Joe's engine. But in this case, I wouldn't run a bypass valve. An electronic wastegate would be better, not to mention with the added pinpoint accuracy in terms of electronic boost control at all times...

- Rob
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Old 05-13-2019, 11:18 AM
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Re: BOV is opening under boost

I think the issue is,

non-roots blowers take a full 'gulp' of atmosphere based on belt speed, which is based on engine speed, always based on engine speed.

For example a 5.7L Engine at 550rpm idle moves:
350x550/3456 = 55.7cfm, multiply VE: (55.7 * .80) = ~44.56cfm

Say a random supercharger unit flows 100cfm at the impeller rpm (around 5000rpm I get with 5" crank and 2" supercharger)

There is an extra ~56cfm of airflow that needs to somewhere.
In a turbo application, the airflow backwards through the compressor applies forces to the blades of the compressor, and some of that energy is absorbed by changing the speed of the impeller. In other words, the airflow trying to flow backwards through the compressor (and into the intake pipe) is forcing against the blades of the compressor wheel in a non-operational, fundamentally inefficient way (its like putting your finger on the blades and causing friction between matter while the wheel is spinning) that we can say plainly is a simple set of vector forces along the blade while it is in motion, force magnitudes with directions along the shape of the compressor blade forcing it in directions it was technically not designed to tolerate. For example typically the tips of an aluminum blade will flex, and because of Al has no fatigue life cycle the way steel does they eventually break or start to break apart on the ends. The manufacturers know about this condition (called 'surge') and they do beef the wheel and blade and bearing and housing etc... all in advance to help prevent failure if the unit did happen to surge. That is why they seem so resilient without a bypass, surging all day long- it isn't because surge is harmless, its because the designer had taken surge into mind when creating the unit to the best of their abilities.
Anyways. These awkward set of vector forces along the blade apply that force against the rotational motion of the blade, causing it to slow down(turbo only) and possibly even change direction completely. At least, this is the primary difference between turbo and supercharger impeller speed- the turbine of the turbo isn't linked to the engine's rotational speed by a belt so not only can the turbo slow down to absorb and lessen the impact of some of those nasty improper vector forces on the blade due to surge, the turbine can also pre-accelerate the impeller to very high matching "engine speeds" that don't necessarily involve high engine RPM, thus making a high flow rate worth of boost available at an early RPM. The supercharger cannot slow down to lessen the impact of surge condition so it will take more damage, more abuse, more friction, during surge condition. There may be a warning from the manufacturer about this

If we look at Vortec site,

When should a race air bypass valve be used?

Any supercharger system that is operating in excess of 10 PSIG requires installation of a race air bypass valve to prevent compressor surge. All V-1 R, V-1 T, V-7, V-3, and V-4 superchargers require a mondo air bypass valve.

When should a standard air bypass valve be used?

A standard air bypass valve should be used on all supercharged applications where the supercharger is making more than 6PSIG. The advantages of using an air bypass valve includes eliminating compressor surge (the noise heard during deceleration) for quieter supercharger operation, and reduced heat soak in the discharge tube and the supercharger.
https://vortechsuperchargers.com/pages/faq

They are definitely worried about the friction and heat at the supercharger

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Old 05-13-2019, 12:48 PM
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Re: BOV is opening under boost

Originally Posted by Street Lethal View Post
I just can't see a supercharger forcing in that much cfm at idle that would justify the need for a bypass valve needing to be used in this case. A bypass valve is primarily meant to keep back pressure off of the supercharger during idle to extend the life of the blower, but how much back pressure is even there when the bypass is designed to close between 3-7" Hg during its' operation. Do you guys think that removing the bypass valve that the engine will suddenly idle under boost pressure without it? That's impossible. Impossible with an engine of Joe's size, cfm and blower size. If anything, the incoming air through the throttle body during idle itself soothes/relives the blower of any back pressure as it is helping the compressor spin by pulling it from the other end, as the belt on the opposing end is pushing/forcing the compressor. I mean sure, slap that same blower on an engine flowing much less than Joe's engine, and I would see a possible need for a bypass valve at idle because it becomes more restrictive, but we're talking a lot less than Joe's engine. But in this case, I wouldn't run a bypass valve. An electronic wastegate would be better, not to mention with the added pinpoint accuracy in terms of electronic boost control at all times...

- Rob
I would agree with you if it was a smaller blower, but this thing is constantly surging. In fact, if I give it a little throttle and back off, even with a TON of air rushing out the valve I get a huge fark out my air cleaner.

I can take a video if it helps.

-- Joe
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Old 05-13-2019, 02:01 PM
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Re: BOV is opening under boost

Joe, you're surging at idle with those pulley's? Last idle video I seen she sounded great. You're not running a bypass valve, correct? You're just running the blow off valve? Where is your kPa/Hg reading during idle while it is surging? You mentioned that the blow off valve opens with anything greater than 10Hg of vacuum (just read that), did you test to see where your kPa/Hg was hovering with the BOV tightened to max pressure keeping it closed? I'd be curious to know where the kPa is hovering myself with the tightened BOV. The low boost pressure is a given now knowing that...

- Rob
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Old 05-14-2019, 06:57 AM
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Re: BOV is opening under boost

Originally Posted by Street Lethal View Post
Joe, you're surging at idle with those pulley's? Last idle video I seen she sounded great. You're not running a bypass valve, correct? You're just running the blow off valve? Where is your kPa/Hg reading during idle while it is surging? You mentioned that the blow off valve opens with anything greater than 10Hg of vacuum (just read that), did you test to see where your kPa/Hg was hovering with the BOV tightened to max pressure keeping it closed? I'd be curious to know where the kPa is hovering myself with the tightened BOV. The low boost pressure is a given now knowing that...

- Rob
I'm running a greddy type S valve that is vented to atmosphere.

So let me explain again.

At idle, when doing nothing, the valve is closed unless I back the tension screw ALL THE WAY OUT. However, with it all the way out I cannot make hardly any boost.

If I adjust it so that it holds about 3psi, it will ONLY open under heavy vac. I idle around 11-12hg, it probably opens around 15hg currently. 90% of the part throttle cruise driving on this car is around 20hg unless I nail the throttle. I can share a 30 minute driving log with you if you'd like, do you have megalogviewer ?

Even if I give it a little throttle blip at idle, it surges. My girlfriend calls it 'farting' cuz that's what it sounds like.

Anyhow, I ordered the Tial Q and the green -2 "supercharger" spring that Dan is running. I'm gonna give that a try.

Kingtal0n explained it quite well. On a turbo car you don't have a whole lot of resistance. On a supercharger when the throttle is closed you've got a lot of air trying to turn the impeller backwards, and the belt is not likely to give. I assure you every time I play with the throttle, engine brake, etc I get major surge. You can actually see it blowout out of the air filter!

Maybe next weekend I'll swap the BOV. Should be in this week. I'm trying to get the C4 back together. It runs but I need to bleed the clutch, readjust the valves, and I ran into a dumb dumb move with the steering column.

-- Joe
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Old 05-14-2019, 07:15 AM
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Re: BOV is opening under boost

Actually no Joe, a centrifugal is very similar to a turbo. When we're talking about a positive displacement supercharger (roots) I can see the need to vent the pressure to atmosphere during idle, but even at that low idle RPM, even the roots blower will bleed boost off due to the lower speeds even without the need for a recirculating bypass valve, unless we're talking about an enormously sized huffer. The centrifugal is not making any boost pressure at idle, and no resistance at idle means no compressor surge at idle. Reading this entire thread all over again, and now reading what you just wrote, I have to again agree with Dig. You need a bypass valve.

- Rob
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Old 05-14-2019, 07:19 AM
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Re: BOV is opening under boost

Originally Posted by Street Lethal View Post
Actually no Joe, a centrifugal is very similar to a turbo.
- Rob
It's similar in that it has an impeller that spins.

But like he said, The turbo impeller may be spinning faster or slower at any given RPM. The centrifugal is mechanically connected, it will always spin at the same ratio no matter what.

Also, the T-Trim is quite a bit larger than the 76mm turbo I was playing with a few years ago. It moves a lot of air.

-- Joe
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Old 05-14-2019, 07:49 AM
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Re: BOV is opening under boost

My thoughts on a bypass/blow off valve valve - My little V3 moves quite a bit of air at idle - More than what i would have thought before I purchased it. With charge pipe off, engine running there is no freekin way you plug it if you wanted, caps, clamps, ect. It generates so much flow/pressure its incredible. In the first two years i had to send it back to have the compressor seal fixed.... twice. I put a dual lip seal in it myself when the second one let go and the upgraded to the tial bypass/blow off valve with the new intercooler set up and I've been running it that way ever since. Point being from first hand experience, I find it really odd that a new, time tested unit had two failures of the original impeller seal and the "improved" seal that was redesigned specifically for seals that blew out while under warranty. I think they suggest a large bypass valve to prevent excessive pressure on the charge system when at idle, or decel. Pressure will always find a way out at the weakest point. The weakest point is a thin, flimsy seal - Not the compressor wheel, not the charge pipe, not the intercooler. It needs to relieve pressure in the system to prevent leaks - air or oil.
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Old 05-14-2019, 07:58 AM
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Re: BOV is opening under boost

Originally Posted by anesthes
But like he said, The turbo impeller may be spinning faster or slower at any given RPM. The centrifugal is mechanically connected, it will always spin at the same ratio no matter what...
That is not necessarily true, as well as meaningless because the pressure leaks past the impeller when the idle speed as well as the impeller speed is slow. This is why earlier roots blowers did not come with a bypass valve for any idle conditions, the slower idle RPM already allowed for the positive displacement's pressure buildup to leak past the blades. They were only added for longevity of the unit. A centrifugal is nothing like a roots blower in reference to total boost pressure, whereas the centrifugal, because of it's design, build pressure exactly like a turbocharger. Meaning it is not linear. The roots is more linear, meaning if pulley sized to 8-psi, you're going to 8-psi throughout. It doesn't work that way with a centrifugal, which is why they call it a belt driver turbo. Spinning a blower by hand at say 800-RPM doesn't mean a thing when you have your had in front of it feeling the air it is blowing out, because if you put your hand in front of the throttle body at the same 800-RPM the incoming air cancels it out.

- Rob
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