Boost = Vacuum?

[stuckwithgsx]

Like everyone else I have friends that come up with crazy questions. Well last night I was with someone and they claimed to have heard from a reliable source that whatever boost you are pushing, you should be getting the same number in vacuum, or mm/hg?

Now this made me wonder because every stockish dsm i'm ever been in pushes the stock 8-10 or close lbs of boost, but then reads between 16-19 of vacuum?

Then building an s13 rb25swap with another friend it ran stock 9lbs of boost and 15" vacuum.

It's been too many years since i've seen any car that pushes more that 20lbs of boost, and I have a horrible memory .

So to clear the question... Should the amount of boost that is pushed on ANY car match the number of vacuum??

 

[gusu]

Okay, so I just read up a bit more. From what I have read on several different sites now, most people read between 17-20. Which is normal, however sometimes with cams it does change your vacuum. Vacuum is not the same pressure as boost and a normal DSM should be at 17-22.

It also seems as though timing has a lot to do with it. Changing timing can be used for anti-lag so it makes sense.

 

[ChiTownHussla]

No Vacuum and boost should not be but could be thesame.

 

[RadDsm]

So to clear the question... Should the amount of boost that is pushed on ANY car match the number of vacuum??

The vacuum and boost do not have to be the same. They could by chance, depending on mods, but they don't need or have to be.

 

[GRAVEDIGGER]

That is quite possibly the funniest thing I've heard in a while!

Please tell me you ARE serious and this isn't a joke and it will be that much better.

The answer is a definite NO, in case you didn't know.

Imagine Brent Rau's car with like 80psi of boost and 80" of vacuum. That would be some serious damn vacuum.

 

[jrohner]

This thread doesn't even make sense.

 

[LiquidX]

Vacuum is all contributed by how well the motor breathes. Valve over lap with the intake and exhaust valves opened up at the same time will decrease vacuum while when one is closed and the other open, vacuum will increase. Valve sealing is factored in as well as internal seals.

You should be seeing 18-25 inHg at idle on a healthy sealing motor with little to no valve over lap.

I think I may be forgetting a couple things but that's pretty much it.

Vacuum =/= boost

Boost is just the amount of positive pressure that the forced induction setup is making. It's also best to state CFM as psi is not static between setups/turbos.

 

[turboglenn]

Your friend may want to re-define his sourse as less-than-reliable

Having the boost the same as the vacum in some/most nearly stock cars is purely a coincidence

 

[Jon91TSi]

Tell that to someone like Brent Rau who runs upwards of 90 psi and see what kind of face he makes at you. I've never seen more than 20-22mm/hg on decel and I see around 10 at idle. I'm sorry to say, but your friend is a dolt.

 

[stuckwithgsx]

Thanks everyone for the replies. And it definately didn't make a whole lot of sense to me but i'm not exactly highly educated in how vacuum entirely works, so it just led me to wonder.

And i could not find anything on this site relating to this subject, so I figured someone else may wonder somewhere down the road. I'm glad it is RESOLVED!

Now I just need to figure out vacuum in it's entirety.

 

[turboglenn]

Thanks everyone for the replies. And it definately didn't make a whole lot of sense to me but i'm not exactly highly educated in how vacuum entirely works, so it just led me to wonder.

And i could not find anything on this site relating to this subject, so I figured someone else may wonder somewhere down the road. I'm glad it is RESOLVED!

Now I just need to figure out vacuum in it's entirety.

DOn't sweat it man, the easiest way to learn what you can't find to read is to sask questions. At least you honestly asked and didn't do the troll move and try and convince a board full of knowedgable members that the "reliable source" was an expert and there-fore correct

 

[Doubleot]

Tell that to someone like Brent Rau who runs upwards of 90 psi and see what kind of face he makes at you. I've never seen more than 20-22mm/hg on decel and I see around 10 at idle. I'm sorry to say, but your friend is a dolt.

This is about what I see. Don't sweat it man, asking questions is never a bad thing if you have already tried to search the same thing.

Sent From My Sidekick 4g, Because I can.

 

[stuckwithgsx]

Exactly right guys. I'm not trying to say I know all and i'm always right haha.

This question seemed pretty far fetched to me, but in all honesty I didn't know that answer. So I asked here because there's people that do know, and have made it to where I now know. Thank you all!

 

[jed344]

think of it this way. My cummins is pushing 45psi threw a 66mm charger and sees 0 vacuum due to a diesel having no throtal body it sees full flow the hole time. So no boost does not effect vacuum. Also i run bout 18 in of manifold vacuum at idle if i remeber.

 

[Alex7747]

think of it this way. My cummins is pushing 45psi threw a 66mm charger and sees 0 vacuum due to a diesel having no throtal body it sees full flow the hole time. So no boost does not effect vacuum. Also i run bout 18 in of manifold vacuum at idle if i remeber.

Wait what!??

 

[stuckwithgsx]

^^ X2.... ?? Can that please be elaborated, or deleted? Lol

 

[jrohner]

Wait what!??

are you saying you didn't realize diesels don't have throttles? or what is the 'what' about?

 

[talonDSMerr]

^^ X2.... ?? Can that please be elaborated, or deleted? Lol

Manifold vacuum is essentially created by the throttle plate found in gasoline engines. When the pistons move down on the intake stroke, they suck in air, but, with the throttle plate closed, there is just a little air inside the manifold that can go into the cylinders. PV = nRT. On the intake stroke with the throttle closed, you are essentially increasing volume, but keeping the mass of air the same. Thus, pressure drops, and you have vacuum.

Diesel engines, such as the Cummins mentioned above, do not use throttle plates. Whereas gasoline engines modulate airflow to vary engine power output, diesel engines modulate fuel flow instead. With the absence of the throttle plate, there is no restriction in the intake system and the piston can keep drawing as much air as it likes. There is still vacuum created, but very little compared to what is created in gasoline engines.

Long story short, his diesel engine creates massive amounts of boost pressure, and yet does not create much vacuum. Another way of saying "no, they are not related."

 

[ed1380]

The 18" he was referring to his dsm.

Everyone was confused because he said no vacuum and almost next sentence 18" of vacuum

 

[GSLENK]

Ill try and dumb this down on the deisel idea:

gas car combustion is a tad different than deisel, (you might know this because deisels use glow plugs, which have to warm up, as opposed to spark plugs)

deisel combustion depends on the cylinder to bring the AF mix to a combustible pressure, at which the mix combusts with help from the glow plug, (not a spark, just a const. temp *i think constant temp*)

so a throttle body is not really necessary. gas engines control air, and add fuel accordingly. Deisels control fuel, thus air is used as necessary. excess air, i presume, is wasted.

That also explains why people with cummins TD trucks can add a chip/tune, and get like 100+ HP (a significant amount) just from that.

Also think about deisels operate under a VERY narrow power band. thats why these big rigs have like 18+ gears (and hence why they seem to take forever to accelerate) with a very narrow powerband, it would make sense that there is no need to make air intake as variable as it is in a gas engine.

BAsically whatimsayinis: homeboy is on the right path.

Back to the op:
MAybe what he is asking, or what his friends are asking is:
If a turbo is generating 30 psi boost (in a perfect system) is it pulling 30 in/hg vac. from the atmosphere? fair question, that i dont have the knowledge or exp to answer, i would assume the answer is yes, because if boost>suction, boost will leak backwards to equilibrium. hence the BOV

If he is asking if the peak boost will be equal to peak vacuum, that is ubsurd.
Look at N/A cars, 0 boost and plenty of vac.
look at N/A to turbo conversions where boost is limited to 5-8psi, still 15+ vac.

 

[LiquidX]

Just adding in with the above post.

Diesels are at a constant WOT since there is not throttle body to control air flow. Airflow into the motor is always going to be unrestricted, how speed and rpm is made is just by injecting more or less diesel fuel.

Inject more fuel in the cylinder and the more pressure you will make which makes more power. More power is giving the ability for more speed. Diesel fuel combusts under pressure, not by sparking like a gasoline engine.

 

[ed1380]

And to add to that. Since nothing restricts the air, if your Turbo seals blow it'll run of its own oil until you block the air somehow or it runs out of oil=bad

 

[stuckwithgsx]

Wow, lots of information here guys! Thank you all.

The question was originally asking if the peak boost was or should be equal to peak vacuum. Obviously that is not true, and honestly I didn't think it was true for any amount of time. But it did make me wonder what the factual bases were for it. Well explained guys!

Onto the diesel subject, since we're there

I have always wondered exactly why they do not need a BOV or some sort of pressure release?

Now I know there is no throttle plate and the engine will only consume what it needs. Now some of these diesel's are pushing 70-90 lbs of boost though. So if you take 80psi inside of a pipe and the engine somehow blocks that pressure then it would need to go somewhere right? Where exactly does it go? I've been around many diesels and hear the whine after the throttle is let up, but again I don't exactly know what's going on there? If intake is open all the way to the intake valves wouldn't it make sense that the exess air is going to try and push its way through there? Wouldn't that be bad for them after time? And further leads me to wonder why a diesel will last an amazingly long time (given that it's taken care of). I've seen a few older 24v cummins last to 700k, of course injection pump, injectors, etc were all replaced. But the internals of the engine were factory.

WTF

 

[ed1380]

When a diesel driver lets go of the throttle real fast it stops the fuel flow. The motor still spins at the same rpm and no throttle body to create resistance so it consumes all the air= no compressor surge

 

[stuckwithgsx]

So where does the loud whine from the turbo spooling down come from?

It sounds to me like it comes from the engine bay. From my understanding and previous posts, I am led to believe the exess boost/air is going through the engine. Meaning that sound would all be coming out the exhaust?