blow off sitting still?

[UncaringMrBC]

how do i blow off sitting still...i have an "open to atmosphere" bov but when i am sitting still with the clutch in or im in neutral i can rev my engine up to red line and it wont make a sound when i let off from the bov....i noticed when i was taking off on a hill once i had the clutch out alittle ways and was reving it up and it would blow off loud when i let off of the gas....is there a reason that it doesnt blow off when im just sitting still and reving?

[DragonofBC]

Boost pressure usually starts to build when you are wide open throttle. I don't think you can build enough boost pressure to hear blow off by the time you hit the red line. If you were to hold the pedal down to the floor and let pressure build while bouncing off the rev limiter, I'll bet your BOV will make a sound but I HIGHLY recommend you don't do this. It's not worth blowing an engine to get your BOV to sound.

[NateS]

Well I have a greddy type s bov that is recirculated and I can do that in neutral or with the clutch in. I only have to rev it too about 5k or so. Although when I had the stock bov it would not do it; at least loud enough to hear it.

[97GSTspyder]

it depends on the bov's spring pressure, also...when you are accelerating, you're putting a load on the motor, which builds boost. you can not build boost by sitting in neutral and revving.

[Blackcrow64]

Uhhh, I hate to burst your bubble but I can build boost when I rev my motor sitting still. lol I can hear my BOV also and I only rev it to about 4500rpm... Then again, I am running my t25 and stock BOV still. lol

[crunchymilk55]

The T25 can usually blow off in neutral, but only 1 or 2psi max

bigger turbos build even less pressure.

There has to be load on the motor to make significant boost

[scottsee]

Originally Posted by 97GSTspyder

it depends on the bov's spring pressure, also...when you are accelerating, you're putting a load on the motor, which builds boost. you can not build boost by sitting in neutral and revving.

Correct, unless you have a studder box.

[97GSTspyder]

Originally Posted by Blackcrow64

Uhhh, I hate to burst your bubble but I can build boost when I rev my motor sitting still. lol I can hear my BOV also and I only rev it to about 4500rpm... Then again, I am running my t25 and stock BOV still. lol

on the T-25 you'd be lucky to see 1-2 psi in neutral just like crunchymilk said. also on the T-25, when revving it in neutral the bov is louder compared to doing the same thing with say a 16g, etc. besides revving your car to 5k all the time in neutral is a waste just to impress people...then you get those other people that are like "what the fuck is wrong with that dudes car". although, i did it all the time when i first got my ssq i still love driving though the tunnels in pittsburgh, building boost and letting off, hehe...

[Blackcrow64]

Nothing tops a good blowoff in a tunnel! lol

[Nayr747]

Isn't this the 2nd thread on this in like a month? I don't understand why you cant build boost with the clutch in either though.

Originally Posted by crunchymilk55

There has to be load on the motor to make significant boost

But *why* do you have to have a load to build boost? Aren't the same amount of exhaust gases leaving at 3k rpm with the driveline separated and at 3k rpm with it connected?

[Blackcrow64]

You are able to build boost, but because of no resistance (clutch engaged to tranny, to wheels, to ground) the motor revs too quick and it doesn't have enough time to get the turbine spinning enough to build boost. That is why with a small turbo like my t25, the turbine is lighter and it can actually get spinning a little to build some boost before your motor can't rev any higher. Thus, resulting in the BOV having some pressure to release. Whereas larger turbos like 20g's have heavier turbines and doesn't have enough time to spin barely at all before you can't rev any higher... I think I explained that correctly... lol

[crunchymilk55]

Originally Posted by Nayr747

But *why* do you have to have a load to build boost? Aren't the same amount of exhaust gases leaving at 3k rpm with the driveline separated and at 3k rpm with it connected?

Sort of, but think of it this way, what takes more energy? Running in place? or running?

When the engine has to "pull" something, it has to work harder. This is why when you floor it in neutral you fly right to redline, but if you do it in gear, you won't just fly to redline, because you're pulling something, the car heh.

This working harder = building boost

[Nayr747]

Ok well I understand that it takes more work to do something when there is a greater load but I'm just confused how exactly that works in an engine. Are there more exaust gases leaving under load? Because if you measured the hp with no load wouldn't it be almost nothing?

Originally Posted by Blackcrow64

You are able to build boost, but because of no resistance (clutch engaged to tranny, to wheels, to ground) the motor revs too quick and it doesn't have enough time to get the turbine spinning enough to build boost.

Well what if you didn't floor it but just reved it to 5k and held it for a couple seconds then let off? And what about highly modded cars that go through the powerband as quickly as we do in neutral. And they have incredibly laggy turbos.

[Blackcrow64]

It will build just as much boost as it can if you bounce it off the rev limiter. Once again, we are back to the whole resistance theory...

[bryan_bigd]

if i were you i would check and see if you have any leaks going from the engine to the turbo. if not then you can rev your engine one time to 4500rpms and the again to 5000 and the again to 6000 and the turbo wheel should have started to spin and the vacume pressure should be strong enough to close the waste gate door and it should make a nice blow off sound.
on my 91 eclipse i rev 14 pounds of boost in nutral and it blows off nice into the air,
on my 95 tsi with the 16g it can rev 12 pounds of boost in nutral and it is still realised throw the intake and it sounds good