Kingtal0n

Ive noticed, I hit 5PSI before full throttle. Now if i step down even further, and the wastegates are set to 5 PSI, then does that mean that extra throttle movement doesnt add anymore airflow into them motor?

Ive also noted that at 0" of vacuum (right before boost) the car doesnt feel like it USED to (before turbos) at 0" of vacuum (full throttle, N/A)

Why does the car seem to make more power at the same 5 PSI of boost with more throttle than at part throttle with 5 PSI?

also, the temp is steady under boost, 130*F manifold temps at 5 PSI. Under normal driving it goes up to about 150*, but drops to 130* under boost. weird huh? Ill post that on the temp thread...

nightrider87

well, 5 lbs of boost isn't 5 lbs of boost i guess is the answer.

when at part throttle, you're seeing 5 lbs of boost but thats up against an engine thats set to flow at a certain rate, ie half its max rate. (you're pushign against the resistance of the throttle body which effectly limits the total flow of your engine)

At WOT, you're set to flow at full rate, and you get 5 lbs intake pressure too. Basically you are moving a lot more air at WOT.

Boost isn't the amount of air you are moving, its only the resistant pressure associated with a certain amount of air through a certain restriction.

At half throttle, you have less cfm of flow through the engine so 5 lbs of boost is easier to achieve (less air needs to be moved by the trubos). At WOT you have full flow cfms but your wastegates are still set to 5 psi so you've raised the total possible flow of the engine which means to get the same amount of backpressure you need to move much more air.

You could equate this to putting 5 lbs of boost on a stock smogger 350 (half throttle) and comparing it to running 5 lbs of boost on a big block 454 (WOT) Same boost level but much different air flow numbers.


This is terribly hard to follow lol but i hope one of these things i've tried to use to explain will help you out.

Steven89Iroc

From what I understand, 5 psi in the intake mani is the same amount of air no matter how much throttle you give it. The reason it's faster at WOT with that 5 psi is because the turbo's are not working as hard to get air around the restriction of the throttle plate like when it's at part throttle, so the air is heated less, and you know what cooler air does for you. Now 5 psi manifold pressure at part throttle should = more psi before the TB because it has to be to make 5 psi after that restriction, but once again there isn't that restriction at WOT.
It's the same idea as boost drop across an inefficient IC core, it has to make more pressure before it to get the correct amount of airflow after it.
I noticed this same phenomenon when I was adjusting my external WG on the Talon after I installed the (upgraded) turbo kit. When it stayed fully closed I would get ~20 psi at about 50% throttle (and would keep rising if I gave it more throttle), but when I got it dialed in, 20 psi at WOT felt alot faster.

I could be wrong about some of the theory, but it makes sense to me.

EDIT: nightrider, you got to it first, but where you went wrong is that you are thinking that the TB is restricting measured boost to the engine. Boost is measured in the intake manifold so the throttlebody won't create any restriction once it's already there.

Kingtal0n

I think your both on to something. My theory was alot like nightriders, but it didnt make any sense that 5psi in the intake manifold meant that the motor had a full 14.7+5 PSI of pressure to work with, if it overcame that pressure at part throttle then the boost would even be there at all.

I really dont know, im curious what other people will say.

nightrider87

a good point about the TB not being a restriction...i completely missed that. but i still disagree that 5 lbs of pressure in the intake is simply 5 psi of pressure.

maybe this explains it better, or perhaps opens something else up for discussion. Is this right??

Keep in mind an engine is just an air pump so at a lower rpm it is effectively more of a restriction to the air inlet than at higher rpms.

at 3000 rpm lets say your engine uses 300 cfm of air if its naturally aspirated. so at 3000 rpm, 5 lbs of boost means 300 cfm natural plus roughly another 1/3 (5/14.7 atmospheric) so roughly 400 cfm of air.

At 6000 rpm, assuming a perfect world, you'll be using 600cfm of air naturally but to be able to build the same amount of back pressure, you'll need to move more air with the turbo. Is this amount of air, at 5psi, 1/3 of the total moved air?

I know my very most basic of theory is correct but i don't know about the 5lbs of boost is 1/3 of the total cfm you're flowing or not.

It makes sense that if you have something that can only allow so much air in, ie an engine at lower rpm, then it'll be easier to build pressure. now bump up the speed of the engine allowing it to move more air and you need to push more air down it to make the same back pressure.
In other words, if you drive along with 5lbs of boost (and that's where your wastegates are set) at 3000 rpm and then goose it up to 6000, the boost pressure will stay the same, the engine's flow requirements will double and therefore the amount of flow from the turbo must also increase...caused by the increased amount of exhaust gasses which in turn leads to more intake flow, and so on.

nightrider87

exactly, if the turbo's flow did not increase, then you'd use up its 5lbs of pressure by going WOT and it would actually become a restriction to your NA engine.

Steven89Iroc

You are correct in the fact that more RPM's = more airflow for the same boost level.
I was, however, talking about the throttle position being the only variable, the rest of the engine being a constant comparison. At the same rpm on the same engine, 5 psi is the same amount of airflow no matter what position the throttle plate is in, it's just hotter air (for the sake of this discussion we won't get into how much more air is in there because it's denser when it's colder).
If I understood correctly, this is what his main question was about.

On the other note, 5 psi is definately not always 5 psi when there are other variables thrown into the equation such as how well air flows through the engine or what rpm it is at, etc.

As far as 14.7psi, that 5 is just added onto it basically, everything else still being the same.

askulte

To throw more fuel on the fire... When I originally did the turbo setup, I went to New England Dragway to get a baseline. It ran at 105 mph (up 1 mph from my NA runs). This was with 5 psi on the boost gauge. We found the throttlebody hose band clamp interfered with the throttle, and I was only getting about 2/3 throttle. This got fixed, it ran at 112, and all was well, still at 5 psi. IAT's didn't change much. This is on a SuperRam intake manifold, btw.

Kingtal0n

Thats very interesting. So it seems than regardless of boost pressure in the intake manifold, the throttle position still has the final say in what the motor is allowed to breath in, if you follow the logic we are seeing happen in all of our boosted motors.

But something that doesnt work out in my head, if the throttle body is halfway open, presenting a restriction to the turbo (or supercharger) then shouldnt the pressure before the TB be higher than the pressure after the TB since the motor is demanding more flow from the TB than it is actually providing? the fact that the pressures on both sides of the TB are equal, says to me that the motor is taking just as much airflow as the turbos are providing, at that boost level. if you follow THAT logic, the the motor is accepting whatever CFM @ whatever PSI appears on the compressor map of the compressor itself, which totally depends on the current motors possible VE and CID, meaning that opening the throttle any MORE would NOT provide more airflow, because your already allowing the motor to flow its MAX VE + Boost at that RPM. which makes no sense at all, since this is NOT what we are seeing in our personal experiences.

askulte

Pressure before the tb will be higher, since the TB is a restriction, and the measured 5 psi is after the TB. It's a similar effect to a restrictor plate on a NASCAR engine. I'd be curious to what intake manifold vacuum they're getting at WOT, considering a 800 hp is putting out only 450 hp...

83 Crossfire TA

iTrader: (2)

That’s part of the problem with this discussion, all of this depends on where you’re measuring boost and where you’re referencing your wastegate actuator to, since different locations in between the intake valve and turbo will have different pressure readings under different conditions. “after the TB” isn’t even sufficient since with a ram or harmonic tuned intake you will generate pressure (“boost”) at the valve at and around the torque peak even without any actual supercharger installed.

But barring getting too complicated (mostly because I don’t want to get into discussing static vs dynamic pressure, flow changes at different pressures…), boost changes the density of the air, which can be expressed as a density ratio (taking into account any intercooling, adiabatic efficiency, plumbing losses…). Besides the designed/built in qualities that effect the engine’s VE (that it will be a X% at Yrpm), throttle position changes the VE at that specific point; a partially closed throttle will result in a lower VE. Power produced is always proportionate to the density ratio * VE * RPM and torque is always proportionate to the density ratio * VE.

One other thing to point out, more restriction does not necessarily mean a lower efficiency. Especially when we’re talking 5psi in the intake plenum. The way most compressor wheels/scrolls are designed efficiency goes up in the 2:1 pressure ratio range.

Will's Fiero

Correct. The volume of flow will remain the same.

Your engine is making the same power with ambient pressure in the manifold as it did N/A, but the turbos are siphoning off some power to build 5 psi of boost in your intake ducting, which is then metered back to ambient pressure by your throttle body.

Your waste gate reference line presumably comes off your manifold. The turbos are limited to 5 psi in your manifold. However, they can make quite a bit more than that in the intake ducting, but the throttle body meters that down to 5 psi at part throttle. So at part throttle with 5 psi of boost, your turbos may be making 10 psi or more, with associated parasitic loss. That parasitic loss is why your car is faster at 5 psi WOT than at the same MAP part throttle.

Your turbos are always turning and your compressors are always working, but during normal driving they're way off the bottom of the efficiency map. Also, the air during normal driving is moving a lot slower through your turbos and intake ducting, and thus has more time to absorb heat.

Kingtal0n

<b>Your waste gate reference line presumably comes off your manifold. The turbos are limited to 5 psi in your manifold. However, they can make quite a bit more than that in the intake ducting, but the throttle body meters that down to 5 psi at part throttle. So at part throttle with 5 psi of boost, your turbos may be making 10 psi or more, with associated parasitic loss. That parasitic loss is why your car is faster at 5 psi WOT than at the same MAP part throttle.</b>

Well I meter the air pressure right IN the compressor housing, AND in the intake manifold. The pressures remain pretty much the same, within 1PSi of eachother. Until I get to 12PSI in the intake, at that point it seems to peak at the comrpessor around 14PSI. this was when the wastegate got stuck, now it stops at 8PSI compressor 150KPA in the intake on the 2Bar Map.

What your saying makes sense to me, but its inconsistant with what Im seeing as far as power production.