CT5BW Question about lugging engine

[Steve D14]

I have had my 2024 CT5 BW 6 speed since July 2024 and have put 4200 wonderful miles on it so far. I do have one question about 6th gear. The gap between 5th and 6th is much larger that between the other gears and when cruising at less the 70 MPH the revs are at or below 2000. If I downshift to 4th or 5th and hit it, it really takes off smoothly. If I stay in 6th and give it moderate throttle it picks up nicely, but if I apply more throttle it seems to bog a bit and I feel some roughness. Once I get above 2500-3000RPM that goes away. This is only noticeable in 6th gear when the RPM are below 2000-2500.

Has anyone else noticed this and is this considered lugging the engine? I ask also because I modified my previous 2012 CTS-V coupe and had it tuned on the dyno and when I left the tuner it did a similar thing in 6th gear but much worse. So I went back to the tuner he drove it around with a laptop connected and made some tweaks and that virtually eliminated it. Is this a characteristic of the tuning of the LT4s and could I do any harm to the engine by "lugging" it in 6th?

Thanks,

Steve D

 

[SS_Shooter]

6th gear is basically a fuel economy gear designed for MPG. It's fine lugging it in that gear, and once it comes on the power in 6th it does accelerate, but if you want good acceleration, best to go to 5th.

A lot of manuals have a big-ass 6th or 7th gear that is designed to maximize MPG so they can meet some kind of bureaucratic fuel economy standard nonsense. The CT5BW is really a 5-speed car with a slightly-overdriven 5th gear (that is still very drivable) and majorly-overdriven 6th gear (0.54 w/3.73 rear axle) that only really works if you are just cruising. (Side note: Early Caddy V's and Corvette manuals had a 1st-to-4th feature that pulled the manual into 4th gear AUTOMATICALLY if you shifted out of 1st gear under certain conditions. This created a dangerous situation where you couldn't really accelerate and was done for fuel economy reasons...)

Having a big overdrive gear was NOT the case with my 1999 E36 BMW M3. It didn't have a 6th gear, and its 5th gear was a performance gear, so you were pushing like 3,300 rpms at 80mph in 5th, Same was true with my 2007 Jeep Cherokee SRT8; that was 5-speed auto (all usable) that turned about 3,000 rpms at 80mph in 5th. I think the Blackwing RPM's at 80mph in 6th are something like 2,200 (low comparatively).

The Brits used to solve the 'good gearing vs. excessive rpms at high speeds' issue with the Laycock de Normanville (LdN) overdrive, which was an EXTERNAL overdrive gear mechanism with a switch - it bolted between the transmission and the driveshaft. Throw the lever, it engages the overdrive gear, and the rpms drop by like 28%.

This was done because cars like the MG-TD only had 54 horsepower, so they needed very aggressive gear ratios in their 4-speeds when on the track (or really anywhere given the limitations of 54 bhp). However, they needed to be able to go 60-70 mph on the highway. With this setup, they could engage the Laycock overdrive and drive home on the highway at much lower rpm (than without the Laycock), and hopefully avoid having their tiny 1250cc motor blow the hell up...

Also, here's something cool: The 1978 911 Turbo used a four-speed manual gearbox, designated Type 930/34 with these gear ratios:

• 1st Gear: 2.25
  
  
• 2nd Gear: 1.304
  
  
• 3rd Gear: .893
  
  
• 4th Gear: .625
• Final Drive: The final drive ratio was 4.220.

Funny story: I once had a loaner Jeep Compass with a 9-speed automatic.

• When I put it in 'manual' mode, it didn't have enough horsepower to accelerate AT ALL in 8th or 9th gear under wide open throttle.
• Even 7th gear was a stretch, but it would gradually accumulate speed in 7th.
• I don't really understand the nuances of automatics, but I know it would drop into those overdrive gears (8th or 9th) when I was cruising and totally off the throttle. Even around town. Weird, right? Maybe someone can explain how those 8th and 9th gears were making things better...