LT4 vs. LT5 - Automatic Camaro ZL1 takes on a manual C7 Corvette ZR1 from a roll showing how important shift speed is

[Sticky]

Obviously the ZR1 Corvette is lighter and more powerful than the Camaro ZL1. The ZL1 has the LT4 V8 with a smaller TVS1740 blower. On top of that, it is heavier. This should be a very easy win for the 755+ horsepower Corvette ZR1.

It is not easy for the ZR1 though. Why? The transmission. The ZL1 with its 10-speed auto shifts super fast. This just goes to show how big of an advantage modern automatics are over the manual transmission.

Is the ZR1 faster? Of course. But you can visibly see where the manual shifts hurt it in the video.

Skip to the 6:12 mark to avoid the garbage and get to the runs.

[subaru335i]

Yeah that ZR1 needs driver mod. With No Lift Shifting there is no reason he should be falling so far behind in the shifts.
It looks like he is losing boost through the shift and takes a while to recover.

[langsbr]

Does a PD blower even lose boost? Ever PD blower car I've ever driven, before you get the pedal to the floor the boost gauge is pegged.

[subaru335i]

Does a PD blower even lose boost? Ever PD blower car I've ever driven, before you get the pedal to the floor the boost gauge is pegged.

If you shift slow it will lol. The bypass will open up and lose boost when the throttle closes.

[Sticky]

Does a PD blower even lose boost? Ever PD blower car I've ever driven, before you get the pedal to the floor the boost gauge is pegged.

It is technically always making boost. This is why you see them generally pull through a throttle body rather than blow into one.

The same reason you never see positive displacements on ITB BMW M motors because if it were to keep blowing into open throttle bodies = runaway throttle.

A bypass valve could solve this but imagine if there were a bypass failure.

Anyway, a positive displacement is always making boost even right off idle but that airflow isn't exactly uniform through the curve but in theory it is close. For every rotation of the impeller the same amount of air is compressed. Boost is linear because the same amount of units of air are sent to the motor per revolution of the crank but it's not perfectly the same as the pressure changes through the rev range and depends on factors like the valvetrain, cams, exhaust, etc.

[subaru335i]

It is technically always making boost. This is why you see them generally pull through a throttle body rather than blow into one.

The same reason you never see positive displacements on ITB BMW M motors because if it were to keep blowing into open throttle bodies = runaway throttle.

A bypass valve could solve this but imagine if there were a bypass failure.

Anyway, a positive displacement is always making boost even right off idle but that airflow isn't exactly uniform through the curve but in theory it is close. For every rotation of the impeller the same amount of air is compressed. Boost is linear because the same amount of units of air are sent to the motor per revolution of the crank but it's not perfectly the same as the pressure changes through the rev range and depends on factors like the valvetrain, cams, exhaust, etc.

Kind of correct but not exactly. For the LSA/9 and LT4/5 the PD blower rotors are always spinning but they are NOT always making boost.
At part throttle cruising there is a bypass valve completely bypassing the blower. You can even completely remove the supercharger belt where it wont be spinning and as long as you don't use much throttle it will drive around exactly the same as normal because all the air is going through the bypass to the intake. If you try to push the throttle an the the bypass closes then without the blower belt you have to backdrive the rotors from engine vacuum and the engine will feel like a dog.

The blower is clearly not always making boost that is how they can get efficient cruising, otherwise the thing would be drawing 60+ horsepower from the engine at all times and you would get like 4 mpg cruising lol.

And if you shift slowly enough you will open the bypass and stop making boost and it will take a second to recover.

[Sticky]

Kind of correct but not exactly. For the LSA/9 and LT4/5 the PD blower rotors are always spinning but they are NOT always making boost.

Ok always compressing air then however you want to phrase it.

[subaru335i]

Ok always compressing air then however you want to phrase it.

When the bypass is open its not compressing the air, that is the whole point. Compressing air takes power and generates heat.
Its just spinning there basically in a vacuum.

[Sticky]

When the bypass is open its not compressing the air, that is the whole point. Compressing air takes power and generates heat.
Its just spinning there basically in a vacuum.

When the bypass is open it's bypassing where the air goes.

I'm not sure what you mean. If the rotors are turning on a roots it is always compressing.

[langsbr]

When the bypass is open it's bypassing where the air goes.

I'm not sure what you mean. If the rotors are turning on a roots it is always compressing.

That's how I thought it worked - the rotors are compressing the air; the bypass valve may be bypassing its ingestion into the motor though. I guess I misunderstood the "lose boost" part. I thought that as soon as you open the throttle back, it's at full boost, unlike a turbo that has to recover, and build boost again.

[Sticky]

That's how I thought it worked - the rotors are compressing the air; the bypass valve may be bypassing its ingestion into the motor though. I guess I misunderstood the "lose boost" part. I thought that as soon as you open the throttle back, it's at full boost, unlike a turbo that has to recover, and build boost again.

That's how I understand it as well.

GM just disconnects the blower for fuel economy reasons but a roots is always compressing air as long as it is spinning.

I experienced this when I test drove Terry's C7 Z06. In the cruising modes there is a slight lag before the blower re-engages.

[subaru335i]

That's how I understand it as well.

GM just disconnects the blower for fuel economy reasons but a roots is always compressing air as long as it is spinning.

I experienced this when I test drove Terry's C7 Z06. In the cruising modes there is a slight lag before the blower re-engages.

With the bypass open air coming in from the intake completely bypasses the rotors...so they are just spinning there with no access to air to even compress.

There is no way to "compress" air when it is under vacuum...because in a vacuum there is no air.

When you push the throttle the bypass closes again letting air rush into the rotors and then they start compressing the air and putting it into the motor.

This is basic physics...compressing air takes lots of power and when the rotors are driven off a belt if you are compressing air it is generating heat and taking power which burns tons of fuel. It would be stupid for them to design a system where the rotors are constantly compressing air no matter what. You would literally get 5mpg no matter how you drive. lol.

You can even hear it, when the rotors are not compressing air (even at higher rpm by just giving it small amount of throttle but revving it out) there is almost no blower whine because there is no load.

[Sticky]

With the bypass open air coming in from the intake completely bypasses the rotors...so they are just spinning there with no access to air to even compress.

Isn't the bypass after the bower, not before? That's how this one was done: https://www.bimmerboost.com/content....8-supercharger

This is basic physics...compressing air takes lots of power and when the rotors are driven off a belt if you are compressing air it is generating heat and taking power which burns tons of fuel. It would be stupid for them to design a system where the rotors are constantly compressing air no matter what. You would literally get 5mpg no matter how you drive. lol.

It doesn't have to be a lot of air but the point is a positive displacement spinning is always compressing air. How is a twin screw going to be in a vacuum when it compresses air internally?

You can even hear it, when the rotors are not compressing air (even at higher rpm by just giving it small amount of throttle but revving it out) there is almost no blower whine because there is no load.

From what I understand the rotors are compressing air but it is vented with the bypass valve. So it is compressing air the bypass opens while idling or under low load to unload the high pressure side. The blower is never physically disconnected therefore it is always spinning and therefore always compressing air.

[Sticky]

Here is the Z06 bypass valve, it's integrated into the supercharger:



The rotors are always spinning and compressing air the load is alleviated with they bypass valve. The valve is what limits boost pressure.

[Sticky]