# When talking about boost pressure...



## dburone (Apr 2, 2004)

When talking about boost, and you say you will give the engine 7psi of boost, is that pressure before or after the carb? I have my boost meter after the carb, and the wastegate gets its pressure from above, before the carb, so i se it moves when the meter says about 5 psi when its regulated at 7psi. So where should i read the true pressure ?


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## chimmike (Oct 17, 2002)

boost pressure is whatever is made right out of the compressor outlet on the turbo.


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## dburone (Apr 2, 2004)

chimmike said:


> boost pressure is whatever is made right out of the compressor outlet on the turbo.


So it seems logical that if the turbo gives 7 psi, after the carb i get 5 psi, because of the small place that the air goes thru in the carb


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## chimmike (Oct 17, 2002)

no.

you need to measure boost right after the compressor to see if you're actually getting 7psi there.

if you've got a boost leak, you'd know it.


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## lshadoff (Nov 26, 2002)

When I'm talking about boost, it is the pressure in the intake plenum that I want to measure. This gives me an indication of what the engine is experiencing. I really don't care what the compressor is putting out. 

I actually measured the pressure drop between the compressor outlet and the intake plenum on my system by measuring the pressure at the plenum and then at the compressor outlet port. My system shows a 2psi drop at 6.5# boost at the plenum. It's probably even more at higher boost levels (although I haven't measured it). 

With my setup (SR20DE, stock MAF, and 370 injectors) most people find the air-fuel ratio leaning out at about 10psi. If I were to measure the boost at the compressor, and set it at a "safe" 9psi, I could damage the engine due to detonation because the engine would be experiencing 11+psi of boost. 

Lew


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## dave_f (Dec 11, 2002)

lew is right, traditionally boost values are read from the intake plenum (after the throttle plate) this is what the engine is seeing. waste gates get their boost signal from the turbo outlet. There is usually a few psi pressure drop between the two signals depending your intercooler, piping and air flow.
-dave


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## Joel (Jun 11, 2003)

On a blow through turbo carby system would the drop in boost pressure be contributed to the fuel cooling the compressed air down thus reducing the pressure?


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## lshadoff (Nov 26, 2002)

Joel said:


> On a blow through turbo carby system would the drop in boost pressure be contributed to the fuel cooling the compressed air down thus reducing the pressure?


The reduction in pressure is due to the resistance of the piping and intercooler (mostly the intercooler).

Lew


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## dburone (Apr 2, 2004)

Now i have the meter before the carb and it show 2 or 3 psi more.
Th only thing is that it should put 7 psi, but it does way less, around 3 or 4.
The wastegate is opening a little early, but its not adjustable and should open at 7, maybe ill trick it in some way.
Today i managed to make the car run very well, but i still dont have much pressure and i noticed that the carb i bought has to big jets. 140 and 220, that is a bit to much, i had to close the secondary and it works better if i use both it gets too much gas.


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## dburone (Apr 2, 2004)

no i have 130 and 150 jets and its running nice, a little stronger than stock but still needs more tuning.


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## Joel (Jun 11, 2003)

lshadoff said:


> The reduction in pressure is due to the resistance of the piping and intercooler (mostly the intercooler).
> 
> Lew


So boyles law has nothing to do with it? I find it hard to believe its not a contributing factor.


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## lshadoff (Nov 26, 2002)

Joel said:


> So boyles law has nothing to do with it? I find it hard to believe its not a contributing factor.


It would be a factor in a static system. Boyles law says that if the air is cooled, the volume decreases. If you cool air in a box, the pressure in the box would drop. But air is continuously flowing in the intake system so the decrease in volume is continuously replaced. It is essentially like having a big hole in the box. 

It is the restriction to flow (mostly in the intercooler) that creates a pressure drop between the compressor and the intake plenum. As the flow increases, so does the pressure drop. 

Lew


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## BII (May 13, 2004)

Isn't there a drop in pressure inside the carb (like 2 psi maybe?).


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## lshadoff (Nov 26, 2002)

bII said:


> Isn't there a drop in pressure inside the carb (like 2 psi maybe?).


At wide open throttle (that's when boost occurs) in a blow-through system (the carb is on the pressure side of the turbo) the restriction depends on the area of the wide open carb vs the area of the piping in and out. You would want to work with a carb which is rated close to the expected air flow under boost to minimize the restriction. 

As long as you are well within the ability of the compressor to supply boost, it shouldn't make much difference. i.e. if you want the engine to experience 10psi, and there is a 5psi drop through the piping between the compressor and the intake plenum, the turbo should be capable of 15psi. 

Lew


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## BII (May 13, 2004)

lshadoff said:


> At wide open throttle (that's when boost occurs) in a blow-through system (the carb is on the pressure side of the turbo) the restriction depends on the area of the wide open carb vs the area of the piping in and out. You would want to work with a carb which is rated close to the expected air flow under boost to minimize the restriction.
> 
> As long as you are well within the ability of the compressor to supply boost, it shouldn't make much difference. i.e. if you want the engine to experience 10psi, and there is a 5psi drop through the piping between the compressor and the intake plenum, the turbo should be capable of 15psi.
> 
> Lew


I was just referring to the pressure drop in the venturi if you're trying to firgure out how much boost you're running. A former boss of mine used to turbo Corvairs and when he would looks at boost pressure, he always had to compensate for the pressure drop in the venturi and size the turbo accordingly. On a small carb I would assume it wouldn't go beyond 2-3 psi at the most. I've never seen piping _after_ the carb on a blow through, usually the carb is bolted onto the intake manifold. I've seen it on draw through however. In that case the drop in pressure from the venturis would be irrelevent.


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## lshadoff (Nov 26, 2002)

bII said:


> I was just referring to the pressure drop in the venturi if you're trying to firgure out how much boost you're running. A former boss of mine used to turbo Corvairs and when he would looks at boost pressure, he always had to compensate for the pressure drop in the venturi and size the turbo accordingly. On a small carb I would assume it wouldn't go beyond 2-3 psi at the most. I've never seen piping _after_ the carb on a blow through, usually the carb is bolted onto the intake manifold. I've seen it on draw through however. In that case the drop in pressure from the venturis would be irrelevent.


Yes, the 'piping' after the carb is the intake manifold. For a carb to work, there must be a pressure drop across the venturi, or the carb won't draw gas into the intake. 

The CFM rating for carburetors is measured at WOT and a vacuum drop of 3mm Hg (0.1psi) in a N/A engine. If the carb requires this small a pressure drop across the venturi, it is a minor restriction. 

I got this information here: 
http://www.carcraft.com/techarticles/56919/ 

I'm not sure how it relates to forced induction, though. 

Lew


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## BII (May 13, 2004)

lshadoff said:


> Yes, the 'piping' after the carb is the intake manifold. For a carb to work, there must be a pressure drop across the venturi, or the carb won't draw gas into the intake.
> 
> The CFM rating for carburetors is measured at WOT and a vacuum drop of 3mm Hg (0.1psi) in a N/A engine. If the carb requires this small a pressure drop across the venturi, it is a minor restriction.
> 
> ...


Yeah, I've had a hard time finding material on forced induction and carburation, the book on tubocharging by McIness (sp?) goes into it a little bit, but certainly not enough to setup a system. There's bound to be something, I mean VWs, Corvairs and Buicks were turbocharged back in the day. I almost want to turbo my B12 with a carb just so I can write a book on it.


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## Joel (Jun 11, 2003)

lshadoff said:


> It would be a factor in a static system. Boyles law says that if the air is cooled, the volume decreases. If you cool air in a box, the pressure in the box would drop. But air is continuously flowing in the intake system so the decrease in volume is continuously replaced. It is essentially like having a big hole in the box.
> 
> It is the restriction to flow (mostly in the intercooler) that creates a pressure drop between the compressor and the intake plenum. As the flow increases, so does the pressure drop.
> 
> Lew


Oh i see, so its similar to current (flow) and electric potential (boost) with the cooler being a sort of resistor. Makes sense - cheers


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## lshadoff (Nov 26, 2002)

Joel said:


> Oh i see, so its similar to current (flow) and electric potential (boost) with the cooler being a sort of resistor. Makes sense - cheers


YES! You can model systems like the intake system as electrical components in a circuit. 

Lew


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## BII (May 13, 2004)

Joel said:


> Oh i see, so its similar to current (flow) and electric potential (boost) with the cooler being a sort of resistor. Makes sense - cheers


I've always thought of it in the reverse, modeling electric systems like mechanical/hydraulic systems.


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## dburone (Apr 2, 2004)

Well, my setup is blowthru carb and its working, not perfect yet but it runs, Its just been a week since i got the turbo runing, and am trying diferent jets to make it work better.


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## BII (May 13, 2004)

how did you seal the carb? is it boxed in or no?


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## Joel (Jun 11, 2003)

bII said:


> I've always thought of it in the reverse, modeling electric systems like mechanical/hydraulic systems.


Im an electrical engineer so i learnt the circuit theory first 
When first delving into turbos I was suprised at how many theories and practices are compatible - especially feedback systems


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## dburone (Apr 2, 2004)

bII said:


> how did you seal the carb? is it boxed in or no?


The carb is very simple, every vacuum accesorie has been removed, and all the conections on the carb except for the fuel inlet have been taped.
Thats all.
Jets are a little bigger but im still looking for the right ones.
The only problem i had these days was that the carb flooded sometimes at idle, i opened the carb and the float was stuck open, dont know for sure why thats happening, its not because of fuel pressure because at greater pressures it works and when that happens i can blow air into the carb with my mouth so its not pressure related, it also happens with the engine off and the pump on. I modified the hieght of the needle and now its runing good.
At WOT its a bit lean, so today im changing jets to bigger ones.


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## BII (May 13, 2004)

dburone said:


> The carb is very simple, every vacuum accesorie has been removed, and all the conections on the carb except for the fuel inlet have been taped.
> Thats all.
> Jets are a little bigger but im still looking for the right ones.
> The only problem i had these days was that the carb flooded sometimes at idle, i opened the carb and the float was stuck open, dont know for sure why thats happening, its not because of fuel pressure because at greater pressures it works and when that happens i can blow air into the carb with my mouth so its not pressure related, it also happens with the engine off and the pump on. I modified the hieght of the needle and now its runing good.
> At WOT its a bit lean, so today im changing jets to bigger ones.


You have to use different floats when boosting, copper or foam-filled floats.


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## dburone (Apr 2, 2004)

bII said:


> You have to use different floats when boosting, copper or foam-filled floats.


Why should i change the float if it isnt having trouble with pressure?
This carb already worked on a turboed engine and the float is still intact.
The problem i am having is that it gets stuck against something i still couldnt identify.


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## BII (May 13, 2004)

dburone said:


> Why should i change the float if it isnt having trouble with pressure?
> This carb already worked on a turboed engine and the float is still intact.
> The problem i am having is that it gets stuck against something i still couldnt identify.


The carb may have had the floats changed already if it was already used under boost. From what I've read the floats in a carb are designed for normal air density, under boost its too much pressure and they collapse or can cause the situtation your having, where they get stuck.


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## dburone (Apr 2, 2004)

bII said:


> The carb may have had the floats changed already if it was already used under boost. From what I've read the floats in a carb are designed for normal air density, under boost its too much pressure and they collapse or can cause the situtation your having, where they get stuck.


Its not boost related because it happens also with the engine off and only the fuel pump running.
It may take me some time, but ill get it right!!!

Friday i gota 170 jet for the second venturi and it started to run stronger, but still need a bigger one because the mixture is lean at WOT. But its very rich going slow so i guess the first venturi's jet should be a little smaller.


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## dburone (Apr 2, 2004)

Well, i´m trying to reach a conclusion with this thread, so finally where should pressure be measured??
Now im doing it before the carb, adn im putting in 5 psi, and in after the carb i read about 3 psi so i have a 2psi loss.
I wanted to reach 7 psi that is what a stock engine would hold, so i still dont know if i shoud have those 7 psi before or after the carb.


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## lshadoff (Nov 26, 2002)

dburone said:


> Well, i´m trying to reach a conclusion with this thread, so finally where should pressure be measured??
> Now im doing it before the carb, adn im putting in 5 psi, and in after the carb i read about 3 psi so i have a 2psi loss.
> I wanted to reach 7 psi that is what a stock engine would hold, so i still dont know if i shoud have those 7 psi before or after the carb.


After the carb is what the engine is experiencing. That is what really matters.

Lew


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## Zen31ZR (Mar 24, 2004)

Maybe the fuel pump is overpowering the float. Just a guess, carbs and turbos together are not something I'm familiar with....... Going with simple carb theory, either the fuel inlet has a burr in it not allowing the float to push it closed, the plunger thingy itself is not sealing properly, inconsistencies in closing position meaning something is broken or "sprung", or inconsistencies in fuel pressure, large spikes in pressure will cause this.


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## BII (May 13, 2004)

dburone said:


> Its not boost related because it happens also with the engine off and only the fuel pump running.
> It may take me some time, but ill get it right!!!
> 
> Friday i gota 170 jet for the second venturi and it started to run stronger, but still need a bigger one because the mixture is lean at WOT. But its very rich going slow so i guess the first venturi's jet should be a little smaller.


You may have already damaged the floats, was the carb used in a blow through or draw through setup previously? Most factory systems are draw through, in which case, carbs need little modification to work, in fact many simply need to be rejetted to function.

And yes, measure boost post carb, because the engine experiences that, and that will be the number most important to you.


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## dburone (Apr 2, 2004)

The carb was previously used in a blowthru setup.
I guess that since i am measuring 5 psi before the carb im just putting about 3 psi in the engine and thats why i dont se that much power.
If i put the meter in the intake i will also see vacuum at idle, right?


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## Zen31ZR (Mar 24, 2004)

dburone said:


> The carb was previously used in a blowthru setup.
> I guess that since i am measuring 5 psi before the carb im just putting about 3 psi in the engine and thats why i dont se that much power.
> If i put the meter in the intake i will also see vacuum at idle, right?


You should see vacuum at idle, whether turbo or not turbo. I think superchargers are possibly the only mechanism which may cause positive pressure at idle.


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## BII (May 13, 2004)

[email protected] said:


> You should see vacuum at idle, whether turbo or not turbo. I think superchargers are possibly the only mechanism which may cause positive pressure at idle.


And not all superchargers do.

But I doubt you are "losing" 2 PSI through the carbs, but hey, if thats what the meters say, that's what they say.


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## velardejose (Sep 9, 2004)

Hi dburone
What car do you have? What engine do you have? What turbo do you have?
I am deciding if going turbo or supercharging my lil E16 engine
What boost figures were used in this engine, daily driver and all out drag race?
Thanks in advance


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## dburone (Apr 2, 2004)

velardejose said:


> Hi dburone
> What car do you have? What engine do you have? What turbo do you have?
> I am deciding if going turbo or supercharging my lil E16 engine
> What boost figures were used in this engine, daily driver and all out drag race?
> Thanks in advance


I have the same engine you have, E16s its on a B13 here it comes stock with that engine or the SR20.
I am trying to reach 7psi, but if your read this thread it seams that pressure should be measured in the intake between the carb and the block, and im taking it before the carb between the turbo and the carb.
And im also at 5 psi so i guess that the pressure thats reaching the engine is about 3 psi and thats why it doesnt run that hard yet. I still notice more power, but im having lots of trouble with the carb.
The idea is that the car should be a daily driver and im trying to have good fuel economy.
I recomend you think a lot about the carb, its a pain, im not using the stock carb, im using one of a VW Gol Brosol 2e, and its not in good condition.
Keep me informed of what you are doing and if you have a question just tell me.


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## velardejose (Sep 9, 2004)

I saw that some of the centrifugal superchargers kits include a 'box' around the whole carb...
Maybe you have a breather tube in your carb forgotten...
I am thinking about jetting the secondary more richer (GA16 stock carb)
Btw this is for my second E16, and there is no hurry
I am currently driving a mostly stock E16 with carbs and experimenting with the spark advance to reach high rpm at high power while the engine lasts a bit more...


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## Zen31ZR (Mar 24, 2004)

bII said:


> And not all superchargers do.
> 
> But I doubt you are "losing" 2 PSI through the carbs, but hey, if thats what the meters say, that's what they say.


 There _has_ to be some sort of pressure drop across the carb, that's how they work. If there was no pressure drop, no fuel would come out of the jets.


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## BII (May 13, 2004)

[email protected] said:


> There _has_ to be some sort of pressure drop across the carb, that's how they work. If there was no pressure drop, no fuel would come out of the jets.


Right, I said that in a previous post. Lew found a site that stated like a 0.1 PSI drop over the venturi was typical. I just didn't think that pressure dropped 2 PSI, especially in a 4 cyl, the pistons are just not going to be sucking enough air to cause that drop, unless you had a REALLY long velocity stack or something.


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## dburone (Apr 2, 2004)

bII said:


> Right, I said that in a previous post. Lew found a site that stated like a 0.1 PSI drop over the venturi was typical. I just didn't think that pressure dropped 2 PSI, especially in a 4 cyl, the pistons are just not going to be sucking enough air to cause that drop, unless you had a REALLY long velocity stack or something.


I guess since the carbs barrels are small, 28 - 30mm that makes the pressure difference.


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## lshadoff (Nov 26, 2002)

dburone said:


> I guess since the carbs barrels are small, 28 - 30mm that makes the pressure difference.


Yes, that explains it! That's a lot of air to push through those small openings. You will probably need 10-11psi from the turbo to get 7psi at the intake since the pressure drop will increase with flow rate.

Lew


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## dburone (Apr 2, 2004)

lshadoff said:


> Yes, that explains it! That's a lot of air to push through those small openings. You will probably need 10-11psi from the turbo to get 7psi at the intake since the pressure drop will increase with flow rate.
> 
> Lew


Im pretty sure that what you say is right, in these days all give it a little more pressure. now i have 5psi at the turbo, and i solved some problems and things are starting to look good. :fluffy: :fluffy: :fluffy:


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## spdracerUT (Jun 11, 2002)

electric circuits and physical components can be modeled in a very similar manner. 

In my dynamic systems and controls class, we modeled mechanical systems sorta like circuits. Each component of the mechanical system was a junction with some type of function (resistance, capacitance, etc), and between each junction was a force and a flow.

Like if you look at the fuel system. The gas tank could be considered a capacitor, the fuel line a wire which has a flow resistance like an electrical wire has an electrical resistance. And the fuel pump would be like a battery which drives the flow. And the bigger the fuel line, the more gas flow like the bigger the wire, the more current. And then the flow velocity is like voltage. Etc, etc. Check valve like a diode, etc. 

ANYWAY!! The turbo is a pump forcing a fluid through a line with the intercooler and carbs along the path. The intercooler and carbs at like resistors and the boost pressure is your voltage. So after the air passes through the intercooler, you get a pressure drop, similar to a voltage drop and the same goes for the carbs.

So depending on where you measure determines what your boost gauge and wastegate will see. If you hook up your wastegate after the carbs, then it'll let the turbo spin until the wastegate sees that 7psi. stick it before the carbs... well, you get the idea


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## dburone (Apr 2, 2004)

spdracerUT said:


> electric circuits and physical components can be modeled in a very similar manner.


I was thinking about putting the waste gate sensor after the carb, so i wouldnt have to put the wastgate to high pressures.
where should it sense pressure?
If its before the carbs ill have the waste gate at 10 psi and the engine will have about 7 psi
if i put it after the carb i could put the waste gate directly at 7 psi.


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## BII (May 13, 2004)

dburone said:


> I was thinking about putting the waste gate sensor after the carb, so i wouldnt have to put the wastgate to high pressures.
> where should it sense pressure?
> If its before the carbs ill have the waste gate at 10 psi and the engine will have about 7 psi
> if i put it after the carb i could put the waste gate directly at 7 psi.



Every wastgate I've seen is actuated by manifold pressure, I'd imangine because you want it to see what the engine sees.


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## dburone (Apr 2, 2004)

bII said:


> Every wastgate I've seen is actuated by manifold pressure, I'd imangine because you want it to see what the engine sees.


I think im going to get the wastegate actuated by the manifold pressure (after the carb.)
Thanks


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