bob256k

Hi, a few times while driving have accidentally downshifted twice to a much lower gear in Sport + mode ( two gears from 4th) and the engine jumped to 5k went up to redline and then shifted. Is this bad for the car?

My car is stock ; 2015 gs350 F-sport Atomic Silver

Ratchey

It's actually good to do that every now and then. It helps remove the carbon buildup in the cylinders. The redline is below the dangerous level for the engine and you have a rev limiter to prevent it from reaching a critical level. No damage was done and you can continue to downshift as needed.

bob256k

YAY! just asking , I don't always drive it like i stole it, only occasionally.

np20412

The car also won't physically let you downshift to a gear using paddle/shifter/accelerator that would damage the engine or put engine speed past the Rev limiter. Your car is smarter than you give it credit for

bclexus

Correct! And, to let the driver know that the downshift would redline the engine there is a bleep sound heard at the attempt to make the downshift, and the downshift is [of course] not allowed. Nice feature!

Knucklebus

Plenty of people have had their redline raised into the 7000-7200 range with no real danger. These motors are well built and safely spin to a very conservative redline with no fear of damage.

er34

Put it in M when you do your paddle downshifts and it'll do rev-matching too - makes it much smoother and quicker.

np20412

Yup - it's still not super smooth but it is definitely rev matching if you are in M and S+. My QX70 rev matches smoother than my GS with the paddles when downshifting, but neither compares to the dual clutch rev matching in the Audi S3 I've been driving around lately. That thing is butter going into any gear at any speed, completely seamless.

jonathancl

Is this true? And if so why?

Ratchey

It's probably something that was more of an issue in the past, but if you drive around and never go above 3K RPM, carbon could build up in the cylinders as the piston isn't hitting it hard enough to knock it off.

I did find this info which could be related:
"Rings. Many of today’s engines use aluminum pistons. Since aluminum pistons experience higher thermal expansion characteristics than cylinder bore walls, they must be designed to have sufficient clearance at the most extreme temperature conditions. Naturally, the expansion rates between the pistons and cylinder bore walls will be most extreme under full-load engine conditions, so under part-load operating conditions, the aluminum piston-to-bore clearance must be greater than ideal. This in turn increases the space between pistons and bore wall, increasing the likelihood of carbon buildup in the ring area."
https://www.quora.com/When-carbon-ge...s-it-result-in

Knucklebus

First, the Toyota/Lexus 6400 redline is very conservative for a V6 of 3.5L with 94mm bore x 83mm stroke. It could easily rev another 1000 RPM if there were cams and head work to support the air flow required.

Second, carbon builds up an almost any engine except racing engines or propane, hydrogen or compressed natural gas engine. During low speed operation, the carbon can accumulate on the tops of the pistons, inside the head's combustion chamber and on the valves stems. Carbon sheds heat poorly so it can lead to a hot spot. This hot spot can cause the air/fuel mixture to prematurely detonate while the piston is still rising in the cylinder. This causes extreme loads on the rod and pistons. You will hear it. It is pretty loud at the extreme.

The way to combat low speed carbon build up is the exercise the engine by hitting redline from time to time. The much higher velocities will often break up the carbon and you will see a black cloud behind you to indicate you've done what we call back home an Italian tuneup. Unfortunately, this really only helps the pistons and combustion chambers. Once carbon builds up on the valve stems, it is very difficult to get off while the engine is still assembled.

np20412

Does the car need to be redlined in gear or will neutral revs perform the same function?

Knucklebus

Free winding it out of gear doesn't produce the high cylinder pressures and uses very little fuel. You might see some black smoke but usually minimal unless the motor is so sooted up it looks like a diesel rolling coal.

If you do it in 2nd or 3rd gear, the engine load is greater and the velocities are higher, resulting in a more energetic combustion that is more likely to loosen carbon. The carbon is brittle and breaks apart under the higher pressures than it was formed under.

np20412

Thanks for the clarity! 95mph it is

I remember as a teenager when my uncle brought home is Ferrari 550 Maranello he had mentioned the Ferrari dealership told him to really avoid extended periods of driving at under 3k RPM becuase of carbon buildup. At the time I remember thinking it was hooey but I've since learned that there may have been some truth behind the advice after all!

Knucklebus

That's between you, the state trooper, the judge and your lawyer. You can perform this maneuver in 2nd gear without breaking any laws.