blow off sitting still?
UncaringMrBC
11-11-2005, 03:08 PM
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
11-11-2005, 03:16 PM
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
11-11-2005, 04:15 PM
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
11-11-2005, 05:32 PM
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
11-11-2005, 07:16 PM
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
11-11-2005, 08:03 PM
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
bigger turbos build even less pressure.
There has to be load on the motor to make significant boost
scottsee
11-11-2005, 08:55 PM
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.
Correct, unless you have a studder box.
97GSTspyder
11-11-2005, 11:41 PM
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 :icon16: i still love driving though the tunnels in pittsburgh, building boost and letting off, hehe...
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 :icon16: i still love driving though the tunnels in pittsburgh, building boost and letting off, hehe...
Blackcrow64
11-12-2005, 05:02 AM
Nothing tops a good blowoff in a tunnel! lol :evillol:
Nayr747
11-12-2005, 05:24 AM
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.
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?
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
11-12-2005, 03:26 PM
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
11-12-2005, 07:23 PM
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
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
11-12-2005, 08:30 PM
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?
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.
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
11-12-2005, 09:22 PM
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
11-14-2005, 12:31 AM
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
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
crunchymilk55
11-14-2005, 12:56 AM
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
not unless you have a stutterbox
are you just guessing the boost levels?
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
not unless you have a stutterbox
are you just guessing the boost levels?
Nayr747
11-14-2005, 01:04 AM
Where's Kevin when you need him? :)
kjewer1
12-02-2005, 08:37 AM
The only thing I see here worth answering is why the turbo doesn't build (much) boost with no load.
A turbine is not driven my air simply blowing over the wheel. It's driven by heat and pressure differentials. You have more heat before the turbo than after it (why EGT probes in the DP read a few hundred degrees lower) and you have more pressure before the turbo than after it (a better flowing exhaust will reduce the pressure after the turbo, increasing the pressure differentialmaking it spool quicker). Hot exhaust gases push against the wheel with pressure, and heat energy is extracted to do the work of turning the wheel.
In nuetral reving, airflow and therefore fuel flow will be quite low. Only enough air/fuel are consumed to overcome the resistance of the running motor. This is why our motors always idle at .27 g/rev no matter what the rpm or mods are. 2.3s will be closer to .31 g/rev because of the increased pumping loss and other factors.
In this free reving situation there just isn't enough exhaust gas energy produced to spool up the turbo and build meaningful boost. And if you have ever run an EGT guage you can see this in the EGT temps. They are quite low at idle and free rev. Once you get a load on the motor and the throttle angle is increased enough to let a lot of air (and therefore fuel) into the motor, then enough exhaust gas energy is available to put the turbo to work.
A stutter box will load the motor enough to build boost, but not nearly as much as accelerating the car (the cars stock rev limiter can act like a stutter box). A small turbo up to say 16g sized can build ~15 psi on a typical 5500 rpm stutter. My T67 on the other hand built zero boost at the same RPM even with a stroker. To build any boost on that turbo I had to use antilag, which pulls the timing back far enough that the combustion event is still occuring when the exhaust valve opens, so some of the combustion process occurs inside the turbine housing. The rapid expansion of the exhaust gasses provides plenty of energy to spool up the turbo. You can also add in a bunch of fuel to increase the effect. In fact some rally cars had a seperate injector to feed fuel into the ehxuast manifold just for antilag purposes.
The short version is that a turbo is a load dependant device. There needs to be some significant load on the motor for it to come into play.
A turbine is not driven my air simply blowing over the wheel. It's driven by heat and pressure differentials. You have more heat before the turbo than after it (why EGT probes in the DP read a few hundred degrees lower) and you have more pressure before the turbo than after it (a better flowing exhaust will reduce the pressure after the turbo, increasing the pressure differentialmaking it spool quicker). Hot exhaust gases push against the wheel with pressure, and heat energy is extracted to do the work of turning the wheel.
In nuetral reving, airflow and therefore fuel flow will be quite low. Only enough air/fuel are consumed to overcome the resistance of the running motor. This is why our motors always idle at .27 g/rev no matter what the rpm or mods are. 2.3s will be closer to .31 g/rev because of the increased pumping loss and other factors.
In this free reving situation there just isn't enough exhaust gas energy produced to spool up the turbo and build meaningful boost. And if you have ever run an EGT guage you can see this in the EGT temps. They are quite low at idle and free rev. Once you get a load on the motor and the throttle angle is increased enough to let a lot of air (and therefore fuel) into the motor, then enough exhaust gas energy is available to put the turbo to work.
A stutter box will load the motor enough to build boost, but not nearly as much as accelerating the car (the cars stock rev limiter can act like a stutter box). A small turbo up to say 16g sized can build ~15 psi on a typical 5500 rpm stutter. My T67 on the other hand built zero boost at the same RPM even with a stroker. To build any boost on that turbo I had to use antilag, which pulls the timing back far enough that the combustion event is still occuring when the exhaust valve opens, so some of the combustion process occurs inside the turbine housing. The rapid expansion of the exhaust gasses provides plenty of energy to spool up the turbo. You can also add in a bunch of fuel to increase the effect. In fact some rally cars had a seperate injector to feed fuel into the ehxuast manifold just for antilag purposes.
The short version is that a turbo is a load dependant device. There needs to be some significant load on the motor for it to come into play.
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