Low RPM Engine Noise Question....
 

My car runs very well (1982 SC with 77,000 original miles), all maintenance and updates,etc. However, when I am at lower RPM's, as the car is approaching the "good" part of the powerband it sometimes will make an odd noise. If I could describe what the noise sounds like I would say, sort of like a couple of old style pencil sharpeners whirring away in a big metal pipe. Nothing, nasty or crunching or sudden but a gradual noise that tapers off once I hit 3,000 or so. Any thoughts on this? Could it be a bearing in the transmission?

No self respecting 911 owner would lug their engine... It is verboten and stated in the manual not to drive the car that way. Lugging an engine designed to run at high rpms ruins the main bearings and shortens the lifetime of the engines. The 915 and G-50 gearboxes may have some sort of inherent rattle during initial uptake which Porsche tried to solve with the rubber centered clutch disc and double-mass flywheel.

If you leave it (or put it) in second gear when coming to a rolling stop, which many people seem to do, and you are down to say 5 or 10 mph, would this be lugging the engine?

Yes, if the RPMS are below 2000 with feathered throttle.

On the street, put it in neutral and use the brakes. Downshifting is for the track: brakes are cheaper than synchros.

If it's any consolation, I hear the same noise coming from my '82 between 2000 and 3000 RPM. It bothers me too. It made this sound before and after transmission rebuild. I know it's not from wheel bearings, CV joints or clutch, I've replaced those. I now suspect the pinion bearings in the transaxle are worn.
I try not to lug my engine, but to get up to 3000 RPM I need to travel through 'the lug zone'.

-Brad

So how do you take off from a dead stop without lugging your engine?

Does dropping the clutch at 3K sound reasonable?

Both my older bother and youger sister do just that but, for some reason, their clutches end up slipping in less than a year and as far as I can tell, have never figured out where that smell was coming from.

-Joe

Joe,

Lugging refers to holding the car/engine in a higher than appropriate gear for an extended period of time, like driving along @ 30 mph in 4th gear. It does not include accelerating through the gears from a start.

You will be much better off not popping the clutch in a 911. And so will your siblings in their cars.

"holding the car/engine in a higher than appropriate gear for an extended period of time, like driving along @ 30 mph in 4th gear"

That's not what I consider lugging unless you're under load, otherwise, it's harmless as coasting would be under those conditions.


-Joe

Hey guys, I think I got us on the wrong track using the term lugging. I have since redited the title of the post. Yes, this noise takes place as I am going through the lower RPMs. Brad I do feel somewhat better you hear this even after the tranny rebuild. Let me know if it ends up being the pinion bearings!

John Cramer hits on something that has amused me for years: Down-shifting when you come to a stop anyway on a red light or stop sign. It's a stupid habit that will prematurely wear out your clutch and related components. Two types of people seem to do it: The ones who don't know how a clutch works, and the ones who want to look macho. Then there is the bad habit of riding the clutch by using it as a foot rest or using the clutch to hold the car on a hill instead of using the emergency brake. Double-clutching is another abuse; a fully synchronized transmission in good working order for street-driving does not need it. Poor things; repair shops love them!

My question on shifting to second was not about downshifting per se. I put the car in nuetral and coast toward the light/stop sign, but as often times happens before you are at a complete stop the light changes or you see the coast is clear - do you then put it in second since you are still moving, or first? If you use second and start off with light throttle, is this a problem since you may be at only 5 to 10 mph?

Not necessarily, if you don't apply too much gas. It all comes down to how fast you want to get away; if you are in a hurry, go with first. If not, use 2nd with a reasonable amount of throttle. The car will let you know if it labors, you can hear it knocking and sometimes feel it when the car is bucking. That's bad.

Joe,
Agreed. That's exactly what I was referring to.

This is an honest question (not argumentative) but... I always thought double-clutch on downshift (when needed to engage relatively significant change in engine speed) was easier on the transmission syncro's because you are matching input and output shaft/gear speed before engaging those gears (=less work/wear on syncro)? Other than some additional wear on pedal linkage/ clutch cable, T/O bearing, and possibly clutch, how is it abusive?

The way I've looked at it, it's easier/cheaper for me to replace those parts (at my level of mechanic ability) than open the tranny to replace (or have replaced $$$) the syncros.

I don't d/c in normal daily driving, but if I am downshifting more than one gear (i.e. 5th to 3rd after hard braking), I'll usually d/c. I will also d/c sometimes when the transmission is very cold to help ease into different gears. Is that bad, and why?

Thanks!SmileWavy

I've got the same noise on my 83SC (120K) with locally rebuilt trans at 105K. I like your description of the noise. From what I've heard or read so far, it points to pinion bearings. I just wonder how bad it has to get before damage is done to the ring and pinion.

"Double-clutching is another abuse; a fully synchronized transmission in good working order for street-driving does not need it."

Abuse?

Gunter,
I concur with Allen. Double-clutching (done properly) rotates the to-be-meshed parts at the same approximate speed. The closer they are in rotational speed before meshing, the less the synchromesh has to work in order to match the speeds. Conversely the more the synchromesh has to work, the more wear it'll have.

Have you ever tried downshifting into 1st at 15 mph w/o double-clutching. I's a difficult task using just the synchronizers due to their inefficient design. Not bragging, but if needed, I can do it by double-clutching. What does that tell you about the process?

YMMV,
Sherwood

I don't shift out of 1st until I'm going at least 20mph or when tach reads above 3000rpm. When coasting to a near stop and the car is still rolling, I go into 2nd before giving it throttle , engine is briefly in the lug zone 1-2 seconds. If I come to a full stop or less than 2 mph I go into 1st before releasing the clutch. In a 911 1st gear is good for 40mph and what a glorious sound it makes over 30 mph. No one should be shifting early in a 911 to save gas!!!

915 tranny makes lots of noises. I've never had one that was completely quiet.

Lugging is when you have too much throttle for the RPM you're at. Coasting down in gear until the engine is at idle speed is not lugging neither is idling along at 1500 RPM. Just be easy on the gas until the engine speed increases.

Double clutching is of course easier on the syncro's and lots of fun.

-Andy

Double-clutching with throttle: I doubt that one can match the synchro speed accurately; it's either to high or to low when using throttle to double-clutch. I can see some advantage in double-clutching without throttle. Without any accurate research either way, who is to say? I can point to several vehicles in my life time with well over 250k with good synchros not using double-clutch. There are myths out there like: "Don't re-set your odometer when still rolling", etc. These myths die hard. By all means, double clutch with gusto; half the people are doing it because it's cool.

"Double-clutching with throttle: I doubt that one can match the synchro speed accurately; it's either to high or to low when using throttle to double-clutch. I can see some advantage in double-clutching without throttle. Without any accurate research either way, who is to say? I can point to several vehicles in my life time with well over 250k with good synchros not using double-clutch. There are myths out there like: "Don't re-set your odometer when still rolling", etc. These myths die hard. By all means, double clutch with gusto; half the people are doing it because it's cool."

Gunter,
To each his own.

What is the advantage in double clutching without throttle? Unless I'm missing something, all you're doing is momentarily interrupting power flow during the middle of a down shift. This is the same effect as downshifting w/o double-clutching.

I agree there are vehicles with many miles on their gearbox with still functioning synchronizers. However, I have serious doubts those gearboxes are in a Porsche. I have not done any formal statistical analysis, but perhaps someone here can offer their experience with their 250K mile 901 or 915 gearbox.

Yes, there are myths and wives tales about auto tech., but double-clutching is not one of them. Maybe Wayne shouldn't be writing a book on gearbox repairs if synchro wear is not an issue.

I'd like everyone's gearbox to last a long time. Unfortunately, 1st and 2nd gear synchros (901/915) do not enjoy that longevity under "normal" circumstances.

Respectfully,
Sherwood

Gunter,

Under normal driving conditions there is probably not muched to be gained by double clutching. On the track however there are usually places where there is a heavy braking zone and a big change in wheel speed after pushing in the clutch at high RPM. This is where you really need to double clutch to prolong the life of the syncros. Most advanced to expert racers use this method and the really good ones can rev match very close to perfect.

Saving the syncros is not the only benefit on the race track as rev matching keeps the car balanced when down shifting. I realize that you can rev match without double clutching but why not go all the way if you can.

-Andy

If you intend to double-clutch at the track, isn't it good practice to double-clutch on the street to develop the skill? After rebuilding my 915, I double-clutch every downshift, even in my '88 Honda which is a POS. It's a good habit to get into as it doesn't do any damage, prolongs the life of the synchros, and develops your driving skills for more serious use. Is it necessary?? Heck no, not as long as your synchros are alright.

Dan