jerryv

The shuddering is not normal. Get it checked out. I cruise at 60 in 6th all the time and don't experience anything like that ..

JerryU

Hmm! At 72 I resemble that remark! However I would never try to accelerate from 1200 rpm in 6th or 7th! The crank uses sleeve bearings! They require a hydrodynamic wedge to build enough pressure to keep the metal parts separate! That requires speed! Lugging this engine by using large throttle openings and therefor high piston/rod bearing loads would make me cringe! These are not ball bearing cracks! The 30 to 40 psi oil pump pressure is just to be sure there is oil at the interface, it is not anywhere near what it takes to avoid metal to metal contact!
Also agree what fun is that!
I don't use Econo Mode so am never using just 4 cylinders. Guess if your cursing in 6th, in 4 cylinder mode and try to accelerate, the car would shudder just thinking about what that is doing to it's bearing to crack interface!!

meyerweb

I'm over 60, and definitely downshift if I want to accelerate in 6th gear at those speeds.

bob guzzy

You are wasting your time if you take the car back to the dealer for the problem you described, been there done that, changed the plugs wires and chased it around for a month or more.

What I can tell you makes it better is the diablo tuner 93 octane tune and shut off DOD, it doesn't take all of it out but it does help smooth out the pull.

If I wind up keeping my car, I am going to put the tune back in, otherwise I will sell the car with the tuner, if the next person wants it I want it to be their choice to put it back.

Gamoto

It's been established that something may be wrong with the OP's car. What I don't get is why anyone would shift into 6th at all. At 60 mph the car seems sluggish even in 5th gear. The only time I would go above 5th is if I'm on the freeway for a lengthy drive. Then it's the time to utilize the higher gears to save gas. ECO mode, cruise control set, 7th gear in 4 cylinder mode. Other than that I can't see any logical reason to shift higher than 5th when your actively driving the car. My car has seen 6th one time just to see how shifts.

Larry/car

Sounds like you might have a weak cylinder. At high power, low RPM (high gear) this condition will be more pronounced.

JerryU

Decided to look up pressures created when sleeve crank and rod bearings are operating. They can reach in the thousands of pounds at higher speed (one sorce said 6000 psi!) That is why lugging and engine (i.e. putting high loads at low rpm) is not good! Also why most wear occurs when you start. Here is a pic FWIW:




Lugging an engine, i.e. high loads at low speed, is not good for the engine! The high pressure built at higher rpm is what keeps the crack and bearing apart.

srobert910

I think we all know that lugging an engine is not good for it. Maybe I should go back and ask: does anyone have a lugging problem with their m7 in 6th gear at 65 mph?

I don't believe that lugging should happen at this speed in any gear. If it was designed this way, then why would there be a 7th gear at all? Would you only use 7th at 90mph? We all know the car hits top speed in 5th and that 6th and 7th are overdrives. So instead of explaining to me the dangers or stressing the motor by lugging it, maybe you can answer if YOUR car does the same thing (my original question).

JerryU

Sorry you took the extra info as being negative! I did answer your question in my first post, #11 it does not!
I was curious about the pressures in an automotive sleeve bearing so looked up the info-was not as easy as typing in google, "What is the pressure..."
Thought there were plenty of folks saying their C7 does not and those saying you no doubt have a problem. Thought there might be a few others who might appreciate knowing what keeps the crank away from the bearings!
We'll all be interested in what the dealer finds-good luck.

meyerweb

The fact that there's a 7th gear doesn't mean GM intends for you to accelerate in 7th (or 6th). That's why there are lower gears. 6th and 7th are perfectly fine for highway cruising, but if you need to give it gas downshift first! Or buy an automatic and let the computer do it for you. Why do you think transmissions have multiple ratios?

You don't believe lugging will happen when trying to accelerate in a tall gear at 1200 - 1300 rpm? I don't believe anyone with much experience with a manual transmission would say that.

srobert910

Sorry if I got a little ruffled and attacked you in any way. I can see that you were just providing some additional information.

I just asked a question about my car and other owners experience with their cars... Then it turns into, You don't know how to drive your car, should have bought an auto, you're too old, etc.

I think I've had enough good, honest answers about other people that don't have the same issue that I do. I appreciate all the constructive input. Now back to our regularly scheduled programming.

johnnypro

Just to finalize this, let's refer to the almighty manual for recommended shift points.

1 to 4 27 (17 mph)
4 to 5 40 (25 mph)
5 to 6 64 (40 mph)
6 to 7 72 (45 mph)

These are of course for maximum fuel economy, I don't really shift this early because I like to hear the engine purrrrr . With that said, if they're saying you can upshift to 7th at 45, you certainly shouldn't be experiencing any lugging at 60.

JoesC5

I'm 73. How do you downshift? I guess I'm to old to know how to drive a car(even though I started driving a stick shift 1952 Chevy in 1954(12 years old) and got my driver's license in 1956(14 years old).

But, to the OP, your car should not be doing anything other than smoothly accelerating in 7th gear at 60 MPH at light throttle application. It won't accelerate quickly, but it should accelerate smoothly.

I think you have something amiss that the dealer should look into.

AS for lugging the engine...
with the Z51 at 60 MPH in 7th gear the engine is turning 1250 RPM
with the base at 60 MPH in 7th gear the engine is turning 1100 RPM.

Unless you are trying to climb a 6% grade, or going WOT, you are not lugging the engine at those RPM's.

For reference, GM designed the car to do the 1X4 CAGS shift into 4th gear as low as 15 MPH. That's only 650 RPM after shifting into 4th gear. You just don't floor it at 650 RPM in 4th gear. That would be lugging the engine.

airmed2

Rocket Science.

speedlink

There are an awful lot of people that get these cars for the first time. They haven't really owned a sports car before.
They buy the manual because they feel they want the full sports car experience. (understandable) But they don't know how to drive a stick! They treat it like an automatic.

JerryU

First, clearing up the OP's issue of shaking or stumbling. Lets hope the dealer can define the problem beit ignition, fuel or perhaps a dual mass flywheel spring missing, which has been reported. Looking forward to seeing what the dealer finds.
Now as he said, "getting back to regular programming!"
The suggested shift speeds in the owner's manual were a bit of a surprise when I first got my C7 in October 2013. However I can understand GM's objective of getting a few extra tenths of a mpg to help meet their corporate CAFE requirements! The EPA mpg determining driving cycle is very specific and GM no doubt has done many tests on what can be done to optimize. The EPA cycle no doubt has modest acceleration requirements since it has to fit 1 Liter engines etc! Looking at the rpm's when shifting to the three overdrive ratios:
At 25 mph in 5th that's 780 rpm (all for Z51 as that is what I have calculated); 40 mph in 6th is 1000 rpm and 45 mph in 7th is 950 rpm. In 7th gear the equivalent axle ratio is 1.43:1, so for every 1.43 turns of the engine those large 20 inch tires turn once! I wonder if there is any other car like that?
The car idles at a lower rpm but there is minimal load, only overcoming friction. At 45 mph in 7th not much power needed either just a small amount of tire friction and modest wind resistance, so it's fine. The throttle is probably only open slightly more than at idle, so low cylinder pressure. However if one tries to accelerate or go up a hill then the throttle must open, the fuel air charge increases as do the loads on the piston, rods and bearings! If opened sufficiently that would be lugging!
I know I'm stating what most already know but wanted to get the definition. Just when would the loads increase to be a problem or shorten bearing life-don't know. Since the throttle is fly by wire does the computer just limit the amount it can open at those rpm's to avoid bearing issues? Perhaps.
But most would shift to a lower gear if they want to accelerate so there would be few if any warranty issues for shortened bearing life, IMO! Therefore GM gets a better CAFE rating and no one hurts their car!

NSC5

Jerry U properly defines the "lugging environment" in his last post. It is defined not just by RPM and load plays the more important role. On a level road with no headwind and no AC then 45 MPH in 7th won't be lugging but try the same RPM/vehicle speed scenario heading up to the Eisenhower tunnel and you will have very different operation. If the vehicle doesn't accelerate smoothly and easily at low RPM then the transmission needs to be down at least one range; although the tach is helpful if you just listen/feel the engine is trying to give you important feedback when it is being asked to do too much at that specific RPM.

A couple of weeks ago I read an accident report from a tourist in CO who cooked her brakes descending a steep grade. Although the typical signs reading, "steep grade use lower gear" were present she stated those didn't apply to her because she had an automatic transmission Maybe our driver's ed courses are spending a little too much time teaching proper seatbelt usage and missing overall operational awareness.

Woodson

^^

wgood

At 1200 in most gears you are lugging the car !!! 1500 is the sweet spot so 0-10 1st 20 2nd 30 3rd -----70 in 7th !!! Up steep hills 4th because 5-7th is overdrive !!!! Wayne

INTHERED

A blower should fix your 6th gear problem