Interesting note about downshifting an LS7 - Camaro5 Chevy Camaro Forum / Camaro ZL1, SS and V6 Forums

JUSTIFIED · 07-22-2016, 10:24 PM

Found this on a old (2007) Corvette forum. I've read many opinions about downshifting to slow a car. Ben Collins (the former Stig) actually advises doing while driving on the street. He doesn't advocate a harsh downshift, but advises that downshifting at an appropriate RPM while braking is perfectly okay. When I do so there is definitely some engine braking occurring, but it's not overly harsh. Would like to hear opinions from others....

VERY IMPORTANT LS7 info
If you don't want to risk damaging your LS7, read this very carefully. If you're one who insists on using the engine to slow the car rather than the brakes, especially on the track, pay special attention.
Some of you may have noticed, read or heard about oil pooling in the intake tract to the point it was pouring out through the air filter. This has also resulted in the hydrocarbon collection pads becoming unglued. Here's why...
The LS7 does not utilize a typical PCV system. Instead, the dry sump system performs this function. If you look at the top of the oil reservoir, you will see some small black hoses. These hoses carry oil vapor scavenged from the engine crankcase to the intake tract whereby it is burned by the engine. Under certain conditions, this vapor contains not only vapor but also engine oil. This is by design, is normal, and can be cleaned during routine service, per GM. The hydrocarbon absorption pads in the intake absorb unburned hydrocarbons and further help reduce emissions. If they become oil soaked, they can become unglued resulting in a loss of power and possibly engine damage.
During sustained high RPM use, such as that typically seen on the track, more oil laden vapor than normal is drawn into the engine through the intake. However, a potentially serious problem such as hydro lock can possibly occur if the driver uses the engine to brake by downshifting instead of using the brakes.
Downshifting is for rev matching in order to balance the car and hence aid corner exit speed by being in the proper gear for acceleration, NOT TO SLOW THE CAR!
You are not driving a semi with a "Jake brake". If you do this with the LS7 dry sump system out of habit you may very well damage the engine.
By downshifting to slow the car while at a high RPM, such as at the end of a straight for example, an abnormally high vacuum is created in the engine crankcase due to the throttle plate being closed. This in turn can result in an excessive amount of oil being drawn into the intake tract when the reservoir is filled to the prescribed 8-quart capacity.
In this regard, you may have read or heard not to fill the oil reservoir above the middle dimple on the dipstick and this is why.
In summary: again, do not downshift to slow the car. Use the brakes, as that is what they’re for. This is mentioned in the manual and/or on the CD. We have had this discussion here on the forum several times and I have always cautioned against doing so. With a normal PCV system it isn't as much of an issue but bad habits will usually come back to bite you and this is one such example.
Edit: remember, this is a production street car and as a such must comply with emissions regulations. Race cars do not. Race car dry sump oiling systems therefore do not have vapor recovery systems plumbed into the intake. In addition, they don't have hydrocarbon absorption pads in the intake tract to come unglued.

69L48Z27 · 07-23-2016, 06:16 AM

I guess I agree and disagree somewhat. Downshifting to 'rev match' slows the car, period. That's why you'll see someone come into a corner and the rear snaps out. I downshift on the street but not to any degree that it stresses anything. Isn't that the point of a stick shift?

The best advice I got learning to drive was - brakes are cheaper than engines/transmissions.

I know on LS1's extremely high RPM downshift would hydraulic lock the engine. I had a 4th gen WS6 car with a windowed block and a rod snapped in half because of it. Bought it that way.

JusticePete · 07-23-2016, 08:59 AM

The LS7 drysump is a HUGE improvement over a standard oiling system, but it is not a five stage dailey with an air oil separator and segregated pan. You can overwhelm the OE 'recovery' system and that what he is posting about.

GREAT POST!

JusticePete · 07-23-2016, 09:00 AM

FYI -- don't forget that oil in your air fuel mixture reduces octane!

07-22-2016, 10:24 PM	#1
JUSTIFIED Drives: 2015 black Z/28 Join Date: Nov 2015 Location: Texas Posts: 1,081	Interesting note about downshifting an LS7 Found this on a old (2007) Corvette forum. I've read many opinions about downshifting to slow a car. Ben Collins (the former Stig) actually advises doing while driving on the street. He doesn't advocate a harsh downshift, but advises that downshifting at an appropriate RPM while braking is perfectly okay. When I do so there is definitely some engine braking occurring, but it's not overly harsh. Would like to hear opinions from others.... VERY IMPORTANT LS7 info If you don't want to risk damaging your LS7, read this very carefully. If you're one who insists on using the engine to slow the car rather than the brakes, especially on the track, pay special attention. Some of you may have noticed, read or heard about oil pooling in the intake tract to the point it was pouring out through the air filter. This has also resulted in the hydrocarbon collection pads becoming unglued. Here's why... The LS7 does not utilize a typical PCV system. Instead, the dry sump system performs this function. If you look at the top of the oil reservoir, you will see some small black hoses. These hoses carry oil vapor scavenged from the engine crankcase to the intake tract whereby it is burned by the engine. Under certain conditions, this vapor contains not only vapor but also engine oil. This is by design, is normal, and can be cleaned during routine service, per GM. The hydrocarbon absorption pads in the intake absorb unburned hydrocarbons and further help reduce emissions. If they become oil soaked, they can become unglued resulting in a loss of power and possibly engine damage. During sustained high RPM use, such as that typically seen on the track, more oil laden vapor than normal is drawn into the engine through the intake. However, a potentially serious problem such as hydro lock can possibly occur if the driver uses the engine to brake by downshifting instead of using the brakes. Downshifting is for rev matching in order to balance the car and hence aid corner exit speed by being in the proper gear for acceleration, NOT TO SLOW THE CAR! You are not driving a semi with a "Jake brake". If you do this with the LS7 dry sump system out of habit you may very well damage the engine. By downshifting to slow the car while at a high RPM, such as at the end of a straight for example, an abnormally high vacuum is created in the engine crankcase due to the throttle plate being closed. This in turn can result in an excessive amount of oil being drawn into the intake tract when the reservoir is filled to the prescribed 8-quart capacity. In this regard, you may have read or heard not to fill the oil reservoir above the middle dimple on the dipstick and this is why. In summary: again, do not downshift to slow the car. Use the brakes, as that is what they’re for. This is mentioned in the manual and/or on the CD. We have had this discussion here on the forum several times and I have always cautioned against doing so. With a normal PCV system it isn't as much of an issue but bad habits will usually come back to bite you and this is one such example. Edit: remember, this is a production street car and as a such must comply with emissions regulations. Race cars do not. Race car dry sump oiling systems therefore do not have vapor recovery systems plumbed into the intake. In addition, they don't have hydrocarbon absorption pads in the intake tract to come unglued. __________________

07-23-2016, 06:16 AM	#2
69L48Z27 Drives: 2015 Z/28; 1969 Camaro SS Join Date: Nov 2015 Location: York, PA Posts: 814	I guess I agree and disagree somewhat. Downshifting to 'rev match' slows the car, period. That's why you'll see someone come into a corner and the rear snaps out. I downshift on the street but not to any degree that it stresses anything. Isn't that the point of a stick shift? The best advice I got learning to drive was - brakes are cheaper than engines/transmissions. I know on LS1's extremely high RPM downshift would hydraulic lock the engine. I had a 4th gen WS6 car with a windowed block and a rod snapped in half because of it. Bought it that way.

07-23-2016, 08:59 AM	#3
JusticePete Drives: Camaro Justice Join Date: Jun 2007 Location: Virginia Posts: 20,171	The LS7 drysump is a HUGE improvement over a standard oiling system, but it is not a five stage dailey with an air oil separator and segregated pan. You can overwhelm the OE 'recovery' system and that what he is posting about. GREAT POST!

07-23-2016, 09:00 AM	#4
JusticePete Drives: Camaro Justice Join Date: Jun 2007 Location: Virginia Posts: 20,171	FYI -- don't forget that oil in your air fuel mixture reduces octane!