: Two S/C questions about the V



Jayfrigginjones
10-17-12, 02:35 PM
#1 -- What is the boost guage actually telling me? Is it Xpsi relative to sea-level, or is it Xpsi relative to ambient pressure. This matters to me since I'm over a mile in altitude, and wonder if the S/C is compensating, or if i'm suffering the same 20% power loss as the N/A cars.

#2 -- What would happen if I drove the car with the belt off the S/C? Would something overheat? Seize? Would mpg increase? Would the engine fall out and explode beneath me?

Thanks
Jay

V Wagon
10-17-12, 04:08 PM
There's really no reason to drive with the belt off the blower. Under cruise conditions it already opens a bypass and doesn't make boost. It takes very little power to turn it when it isn't compressing air. Nothing would overheat, or seize, MPG wouldn't increase any noticeable amount. The only thing you would notice is a lack of power when you floor it.

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Oooops, forgot this isn't like the Lightning was where it had a separate belt for the blower. The blower belt is the accessory belt. So if you take it off the blower, it won't be tight to the other accessories and none of them will be driven. So yes, you will overheat the engine, seize, it will blow up and all that other doom and gloom you mentioned.

I suppose you could get a shorter belt and then MAYBE you would be able to come up with an appropriate routing to drive all the accessories. But you would get no benafit from it so it'd be a big waste of time.

MacBuster
10-17-12, 04:29 PM
I'm curious about the altitude question, also, since I'm much higher than sea level.

N/A cars here lose something like 20% of their power. One benefit of Turbo cars here, is that they don't. What about the S/C?

Jayfrigginjones
10-17-12, 05:18 PM
I'm curious about the altitude question, also, since I'm much higher than sea level.

N/A cars here lose something like 20% of their power. One benefit of Turbo cars here, is that they don't. What about the S/C?

Maybe this is accurate, but maybe it's not. If the boost guage of *any* forced induction car is relative to ambient pressure, then it will loose power just like the N/A cars. If it is relative to sea-level pressure, then turbo and S/C cars should be able to compensate.

larry arizona
10-17-12, 05:26 PM
boost is relative to ambient atmospheric pressure. Yes you will lose power similar to any other NA vehicle, however, you can increase boost to make more power too. Your power will always be relative to your given altitude and pressure.

MacBuster
10-17-12, 05:36 PM
Maybe this is accurate, but maybe it's not. If the boost guage of *any* forced induction car is relative to ambient pressure, then it will loose power just like the N/A cars. If it is relative to sea-level pressure, then turbo and S/C cars should be able to compensate.

I think we are largely asking the same question. Basically, I want to know if the S/C gets us back up to "sea level"? I think your average turbo can just work harder to ensure the pressure going into the system is as close to spec as possible. With the S/C, I think they just spin at a rate relative to engine speed (?).

larry arizona
10-17-12, 05:52 PM
I think we are largely asking the same question. Basically, I want to know if the S/C gets us back up to "sea level"? I think your average turbo can just work harder to ensure the pressure going into the system is as close to spec as possible. With the S/C, I think they just spin at a rate relative to engine speed (?).

to answer your question no. Your SC only moves based on engine RPM and pulley size/ratio. So your SC speed is fixed and only can compress less dense air at the given SC speed. So at sea level you will have more boost than at 5000 ft. You will always have more boost at sea level than at any greater elevation with a given SC speed/ratio.

Jayfrigginjones
10-17-12, 05:53 PM
boost is relative to ambient atmospheric pressure. Yes you will lose power similar to any other NA vehicle...

Without knowing exactly how the engine management system measures pressure I'm uncertain, but L.Arizona's statement is my intuition. I hear claims similar to what MacBuster hears:"Turbo cars don't suffer altitude losses", but I've always been suspicious.


With the S/C, I think they just spin at a rate relative to engine speed (?).

Spin is relative to engine speed yes, but the S/C can create excess boost which is regulated by the ECM, so it can merely "throw away" less boost at a given rpm (above the rpm at which excess is possible for the load). At least I think this is how it works. I may have some part of that wrong though...

MacBuster
10-17-12, 06:14 PM
Without knowing exactly how the engine management system measures pressure I'm uncertain, but L.Arizona's statement is my intuition. I hear claims similar to what MacBuster hears:"Turbo cars don't suffer altitude losses", but I've always been suspicious.



Spin is relative to engine speed yes, but the S/C can create excess boost which is regulated by the ECM, so it can merely "throw away" less boost at a given rpm (above the rpm at which excess is possible for the load). At least I think this is how it works. I may have some part of that wrong though...

Some google-fu revealed this same discussion on the audi boards. Audis have S/Cs that produce more boost than is utilized. This boost is then dumped by a waste-gate to achieve the "target pressure" as specified by the engineers. As a result, at higher altitudes, this valve simply closes more, releasing less (relative) pressure, ensuring that the manifold pressure stays quite close to sea level. The Audi guys are saying they only lose about 0.5% power per 1000 feet of elevation, compared to 3% for a N/A car.

I have no idea if the V has a similar system.

baabootoo
10-17-12, 10:31 PM
The SC belt ONLY drives that, nothing else. I had to drive mine a few weeks w/o it, and no issues at all. I never floored it though either, so I'm not sure if it run rich or lean at WOT. I also heard the 3%/1000' for NA cars a lot, and max 1% for SC cars.

Jayfrigginjones
10-18-12, 10:17 AM
Some google-fu revealed this same discussion on the audi boards. Audis have S/Cs that produce more boost than is utilized. This boost is then dumped by a waste-gate to achieve the "target pressure" as specified by the engineers. As a result, at higher altitudes, this valve simply closes more, releasing less (relative) pressure, ensuring that the manifold pressure stays quite close to sea level. The Audi guys are saying they only lose about 0.5% power per 1000 feet of elevation, compared to 3% for a N/A car.

I have no idea if the V has a similar system.

Nice googlestyle. I think this makes sense. The V should experience altitude power loss through the low rpm only.


The SC belt ONLY drives that, nothing else. I had to drive mine a few weeks w/o it, and no issues at all. I never floored it though either, so I'm not sure if it run rich or lean at WOT. I also heard the 3%/1000' for NA cars a lot, and max 1% for SC cars.

Good to know. Thanks

M5eater
10-18-12, 11:03 AM
Some google-fu revealed this same discussion on the audi boards. Audis have S/Cs that produce more boost than is utilized. This boost is then dumped by a waste-gate to achieve the "target pressure" as specified by the engineers. As a result, at higher altitudes, this valve simply closes more, releasing less (relative) pressure, ensuring that the manifold pressure stays quite close to sea level. The Audi guys are saying they only lose about 0.5% power per 1000 feet of elevation, compared to 3% for a N/A car.

I have no idea if the V has a similar system.

It should. TVS makes both the units for the LSA and the 3.0T.

larry arizona
10-18-12, 01:06 PM
The easiest way to come to understand this is no matter if your car is NA or FI, for the same set-up/mods you will always make more HP/TQ at a lower altitude. Density Altitude is a huge factor in performance.

6104696
10-19-12, 06:03 AM
in other words, the best 0-60 and performance times for any given CTS-V (or any other car with an internal combustion engine) will be attained in New Orleans or in Holland (or in a sub-sea level tunnel?), in cool weather (and proper tires for the temps).......:)

larry arizona
10-19-12, 06:35 AM
that is why DA (density altitude) is important when understanding 1/4 mile times.

MacBuster
10-19-12, 06:57 AM
in other words, the best 0-60 and performance times for any given CTS-V (or any other car with an internal combustion engine) will be attained in New Orleans or in Holland (or in a sub-sea level tunnel?), in cool weather (and proper tires for the temps).......:)

But your V will have an advantage at higher altitudes.

Frankly, it's one of the reasons I don't own a C63 right now.

M5eater
10-19-12, 08:01 AM
in other words, the best 0-60 and performance times for any given CTS-V (or any other car with an internal combustion engine) will be attained in New Orleans or in Holland (or in a sub-sea level tunnel?), in cool weather (and proper tires for the temps).......:)

Or @ MIR. I think DA was -1347 that day.

I managed 12.001 with cold PS/2's and me as a crappy driver.

Jayfrigginjones
10-19-12, 11:27 AM
The easiest way to come to understand this is no matter if your car is NA or FI, for the same set-up/mods you will always make more HP/TQ at a lower altitude. Density Altitude is a huge factor in performance.

OK now you mentioned density altitude. This is an aeronautical term for what your aircraft's engine "thinks" the altitude is (the way I understand it). This is a reference to Standard Pressure which is at sea-level.

These definitions can get confusing so please tell me if and where you disagree with the following:

Your gen2 CTS-V is in Florida, therefore ambient pressure is ~1bar. The ECM will allow the S/C to create a max additional 0.6bar so the cylinders experience 1.6bar.

You drive to Colorado where the ambient pressure is ~0.8bar.

Now there can be 2 scenarios:

1 -- The ECM measures boost relative to ambient pressure so still only allows an additional 0.6bar, therefore the cylinders experience 0.8bar + 0.6bar = 1.4bar (a 15%loss)

2 -- The ECM measures absolute pressure and wants to see total pressure at 1.6bar, therefore it allows the S/C to compensate 0.8bar + 0.8bar = 1.6bar

I would like to think scenario 2 is true, but i'm not so sure. It completely depends on how the ECM measures pressure.

larry arizona
10-19-12, 12:12 PM
First off the SC is dumb to the ECM. The ECM and MAF only communicate pressure and temp for proper fueling. The bypass valve is only open under part throttle to help mitigate heat and boost that would effect driveablity. At WOT the bypass is closed and the SC can only spin as fast as the pulley ratio/ engine RPM. So with that understood your two scenarios are simplified:

At sea level you will have close to 9 lbs of boost stock at WOT and at elevation it will be less obviously.

Density Altitude is just a formula that takes elevation, temp and humidity on any given day/condition. The lower the DA the better power you will make.

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Now a turbo at elevation is a different story, The wastegate can be in the PID loop to allow more boost to compensate for elevation losses. SC's are dumb and have nothing to give feedback and control or have a surplus of boost (overboost) on tap.

That would be nice if that is how the SC bypass would work, but they don't. If the bypass was wired shut for example at part throttle and say 70MPH steady state on the highway, you would be creating boost which wastes fuel and creates heat. That heat would soak the IC system and when you wanted to go WOT and get max performance, the KR mode would kick in heavily hurting RWHP. So bypass is for part throttle MPG and driveability on a SC car.

Jayfrigginjones
10-19-12, 01:50 PM
First off the SC is dumb to the ECM. The ECM and MAF only communicate pressure and temp for proper fueling....

Ok I was missing this piece of the puzzle. How does one learn these things? Where could I get some reference materials on how this engine works?

Thanks!

larry arizona
10-19-12, 02:05 PM
One learns by exactly what you are doing, asking the question . This forum is VERY question friendly. Nobody on here will keep knowledge secret and are happy to share. I have had a couple turbo cars and SC cars with and without a bypass valve, plus I am a Mechanical Engineer by profession so most things mechanical (some electrical) make sense to me.

MacBuster
10-19-12, 02:43 PM
First off the SC is dumb to the ECM. The ECM and MAF only communicate pressure and temp for proper fueling. The bypass valve is only open under part throttle to help mitigate heat and boost that would effect driveablity. At WOT the bypass is closed and the SC can only spin as fast as the pulley ratio/ engine RPM. So with that understood your two scenarios are simplified:

At sea level you will have close to 9 lbs of boost stock at WOT and at elevation it will be less obviously.

Density Altitude is just a formula that takes elevation, temp and humidity on any given day/condition. The lower the DA the better power you will make.

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Now a turbo at elevation is a different story, The wastegate can be in the PID loop to allow more boost to compensate for elevation losses. SC's are dumb and have nothing to give feedback and control or have a surplus of boost (overboost) on tap.

That would be nice if that is how the SC bypass would work, but they don't. If the bypass was wired shut for example at part throttle and say 70MPH steady state on the highway, you would be creating boost which wastes fuel and creates heat. That heat would soak the IC system and when you wanted to go WOT and get max performance, the KR mode would kick in heavily hurting RWHP. So bypass is for part throttle MPG and driveability on a SC car.

This is a great response...even though I don't like the answer!

So one can mostly conclude that our S/C is not helpful at altitude like, say, a turbo would be. That's actually disappointing. I'll avoid those new M5's now. :)

larry arizona
10-19-12, 06:34 PM
It does NOT mean the SC is not useful at altitude. It DOES help alot! The question was did it fully compensate and make a car run the same at high altitude as say sea level. Turbos do a little better at altitude without tuning due to a wastegate, but a SC CAN do better at altitude but it just means you need to increase ratio (pulley) to get equal performance for a given altitude. You can run more overdrive on the SC at altitude. No car runs better at altitude than at sea level. Engines are just air/fuel pumps. Less air (higher altitude) means less power.