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Discussion Starter #1 (Edited)
i have been thinking a lot about the possible direction i will take in modding my car, and have come up with the following.

What we've all been reading, is that the computer limits boost via the bypass valve.

What we've all been wanting to do is open up the breathing on the car (I/H/E).

When you do that, you lower the boost (because the air moves more freely throughout the engine), yet gain power.

So by opening up the exhaust tract, and improving the intake, we will theoretically see LESS boost, but more power.

Here comes my latest theory:

If in fact the computer limits boost by function of boost alone, by freeing up airflow, the computer will allow boost to come back up to normal levels (if it's true that freeing up restrictions decreases boost) by it's inherent nature to desire to see such boost (if indeed it limits boost), giving us MORE power. Sounds good to me. anyone want to elaborate?
 

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The MAF (Mass air flow) sensor measures the CFM entering the engine and is limited in range both by way of the PCM and its own voltage limits. If you exceed these limits the PCM will back off fuel and timing to prevent engine failure.
A header won't be enough to exceed the limits, but additional boost can be.
 

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Discussion Starter #3
SCdyne said:
The MAF (Mass air flow) sensor measures the CFM entering the engine and is limited in range both by way of the PCM and its own voltage limits. If you exceed these limits the PCM will back off fuel and timing to prevent engine failure.
A header won't be enough to exceed the limits, but additional boost can be.
so by increasing flow in the intake/exhaust tracts, we will reduce boost, but will the CFM's go up?

i would think so.

also, by what you say, the computer will limit the boost according to CFM not actual boost?

i guess we'll see if boost levels stay the same when people start having full I/H/E setups, because by opening up the exhaust, more air can get out faster, making the intake side "boost" less because there is less pressure in the engine. i may not be saying this in the right way, but i believe i'm getting at the right point. :rolleyes:

time will tell
 

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Vita said:
i have been thinking a lot about the possible direction i will take in modding my car, and have come up with the following.

What we've all been reading, is that the computer limits boost via the bypass valve.

What we've all been wanting to do is open up the breathing on the car (I/H/E).

When you do that, you lower the boost (because the air moves more freely throughout the engine), yet gain power.

So by opening up the exhaust tract, and improving the intake, we will theoretically see LESS boost, but more power.

Here comes my latest theory:

If in fact the computer limits boost by function of boost alone, by freeing up airflow, the computer will allow boost to come back up to normal levels (if it's true that freeing up restrictions decreases boost) by it's inherent nature to desire to see such boost (if indeed it limits boost), giving us MORE power. Sounds good to me. anyone want to elaborate?
Why not I/H/E + pulley? That could fix any perceived problem you see about lowering boost.
 

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Discussion Starter #5
shaneathan said:
Why not I/H/E + pulley? That could fix any perceived problem you see about lowering boost.
you missed the point. lowered boost is not a problem, if it is creating more power. my point is that opeing up airflow will lower boost, yet increase airflow for more power, less heat.

my thoughts are that the computer will allow the boost to come back up if it is indeed limited by the computer in the first place, thereby negating the effects of our "limited" boost, which is all hearsay IMHO.
 

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Discussion Starter #6 (Edited)
Lower total exhaust back pressure will decrease boost pressure, decrease blower discharge temperature, and decrease blower drive power, It will also give you a bit more detonation headroom.

i found the above while searching the 'net, it does all this while increasing power. so hopefully an I/H/E (3 inch) won't be at the limits of the MAF, and any boost being pulled will not be pulled anymore, and we'll be golden! i expect to see QUITE a bit of power increase from around $1000 in mods. should be quite impressive


anyone with a boost gauge have an exhaust yet? what are the boost readings if so?
 

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i think you are trying to hard on this thought. The open tracks for air to flow will not change boost. Its a good side you apply to it though. Heres the thing the engine will make 12 psi to the engine no matter how well you flow. the wastegate/bypass valve will be what determins boost. If you have a lot of flow the bypass will stay closed to make more boost at the desired level.
 

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Discussion Starter #8
goofyguy said:
i think you are trying to hard on this thought. The open tracks for air to flow will not change boost. Its a good side you apply to it though. Heres the thing the engine will make 12 psi to the engine no matter how well you flow. the wastegate/bypass valve will be what determins boost. If you have a lot of flow the bypass will stay closed to make more boost at the desired level.
not true.

boost is pressure, if you reduce backpressure, you decrease boost while increasing airflow. by opening up the exhaust, you WILL decrease boost, maybe not until a set of free flowing headers are on, but you WILL decrease boost.

let me try this again.

The engine has to push the air out through the exhaust, the more restriction, the more pressure. when you force air into the engine, you create a boost (pressure above atmospheric). if the engine has less trouble getting rid of it out the other side, you decrease boost as less pressure is required to push in the air. this is a fairly simple principle, and i believe it has a lot to do with how they came up with 270 hp for the sema cobalt. we will see, but money says that with the headers on, and a 3" exhaust, we will be seeing lower boost pressures, or MUCH higher power levels due to the computer not stopping the boost from building.
 

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with a few turbo apps in my belt. The cylinder pressure will be the same with the exhaust. Sure it will get out faster but the computer will keep it at the desired boost pressure all the time not just at x amount per rpms. Its a mechanical unit not a computer that actually tells it boost.

I see where your going though and your plan is right the maf will read x amount of air and assume this is where to let the bypass open but no there are many sensors to make sure we are getting the right boost. I will have the header and exhaust done after christmas the week before nex years and have a guage by then as well. I tell ya how it goes.
 

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Discussion Starter #10 (Edited)
goofyguy said:
with a few turbo apps in my belt. The cylinder pressure will be the same with the exhaust. Sure it will get out faster but the computer will keep it at the desired boost pressure all the time not just at x amount per rpms. Its a mechanical unit not a computer that actually tells it boost.

I see where your going though and your plan is right the maf will read x amount of air and assume this is where to let the bypass open but no there are many sensors to make sure we are getting the right boost. I will have the header and exhaust done after christmas the week before nex years and have a guage by then as well. I tell ya how it goes.

i think i see what you're getting at, but i'm not talking about cylinder pressure, i'm talking about BOOST alone (if it's the same then i am), and you get to two points that i am pondering

1) its a mechanical unit that tells it boost. is it, or is it a mechanical unit that LIMITS boost. either way, by the logic that the computer/mechanical unit limits BOOST, when we open up the exhaust and boost drops (due to lower PRESSURE out the exhaust), will it allow it to go back up to "normal" levels?

2) sensors. i understand that the sensors take the measurements of how much air (that is the key, how much air, not how much pressure) is coming into the engine (even though there is probobly a MAP). but what i wonder, and as Adam said, will the computer see more air at lower boost, and lower where it limits it by use of a formula, or will it see more air, and say, "cool, just throw some more fuel in there to compensate, adjust timing just a bit, and vioala! we're back to our ideal stoich (sp?) mixture." and stop limiting boost. if it sees too much air, i'm sure it will throw a CEL, but if it can handle even more air, will it allow boost to raise to normal levels (this is under the assumption that boost is limited)

you have to under stand that i'm using boost here as what it is, pressure, PSI, not CFM, or airflow. if indeed the computer limits BOOST, and only at a certain point, not relative to CFM, when it's lowered, we should gain that back up when it was being limited.

if not, and it was only going to boost so much anyway, and the boost limit is urban legend so to speak, my theory is pretty much moot.

but again, IF the computer limits boost pressure for safety purposes, and the engine is known to be able to take X amount of boost, so they put the limit there. when we reduce boost (likely only a few PSI, unless it's REALLY restrictive) by lowering total backpressure in the system, and the computer sees Y amount of boost at the point it would limit it, in effect it would no longer limit it until it saw that X amount again, giving us back that few PSI when it previously had been opening the bypass, freeing up more power. again, all theory, and i wish i knew more about what the computer is doing.


ps - hopefully we get them by then!!! and with a turbo system it's different, because you still have the backpressure from the turbo, and always will, with the supercharger, there can be minimal backpressure ensuring the piston works less to get the spent gasses out, thereby increasing efficiency and power, while reducing boost
 

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Any time you can remove restriction from an air pump it becomes more efficient.
A Turbo is both a restriction and compressor all in one, but it's worth a few psi of negative pressure to say the least.
A S/C is both a restriction and compressor too, but it's restriction is in mechanical drag on the crank and not the exhaust. The weight of the whole moving assembly helps negate this restriction, but the general loss between the two are about the same.
With that if you were to make the intake or exhaust system of the engine more efficient you would by definition move more air through it resulting in more power.
Totally oversimplified, but accurate.

It's easy to see the benefits of intake and exhaust system with a MAF device and exactly why GM uses one in higher end, performance related applications.

Using a more efficient exhaust manifold will result in different exhaust pressures and if it's designed correctly will flow measurably more CFM.
The key I found with the LSJ is that the boost pressure stayed the same, but CFM increased a LOT more than I expected. The end result was a much higher HP and TQ number than even I expected.

I have not done enough high resolution testing (PSI gauge with any real accuracy) to see exactly why this was the case, but I know from other Eaton supercharged vehicles I have tuned over the years when you improve the exhaust flow the boost pressure lowers.

Everything else being equal the manifold pressure will decrease if you lower exhaust backpressure.
With the RedLine its more bypass controlled and if the PCM just sets WOT manifold pressure to say 12.5psi then you wouldn’t see a pressure drop, but you would see a major increase in power.
Bottom line CFM equals power. A properly configured system will make more power as you increase CFM.
 

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Great comments Adam, keep them coming.
 

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Discussion Starter #13
see that's exactly what i'm saying. and the fact that you didn't see boost pressure drop makes my case, and is very good to know.

now that i'm thinking of it, i don't think they did much to the sema cobalt, aside from opeing up some flows, and a retune to adjust for it (on the agressive side).

now i think it is safe to say that the computer/bypass valve limits boost pressure at higher RPMs, and that we'll start to gain considerably once a full I/H/E is on the car.

if the m62 is truly good to 20psi (hearsay, but a number i've heard), think about doing headwork and a more agressive cam profile! lightened pulleys too.

i'm getting more than excited by all of this
 

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Light pulleys, flywheel, and a smaller super pulley and some computer tuning.
 

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And some handy wipes cuz i think 2 drops of pee came out!! :p
 
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