Keep it above 4,000 RPM?

Jay Zollinger · 02-23-2005, 01:34 PM

The most recent Panos has an old article about a new Porsche owner in which he repeatedly receives the advice that he needs to keep his revs above 4,000 to avoid carbon build up in the valves. Is this right, in particular with older cars?

84porsche · 84porsche's Garage

I read the article too. I would assume it to be true. Non related incident, my car had been rarely driven for 3 years before I purchased her and after driving at least once a week for the past 2 years, she runs so much better than she ever did when I first bought her.

RoninLB · 02-23-2005, 02:02 PM

Quote:

Originally posted by Jay Zollinger

Is this right, in particular with older cars?

a signature with you model may get you clearer info.

Jay Zollinger · 02-23-2005, 02:14 PM

I should fix that. Mine's a '73 S, targa.

speeder · speeder's Garage

You would not want to do the opposite, ie. lug the car around @ 2000 rpms all the time, but the idea that you have to drive around above 4k all of the time is asinine. It just depends on the circumstance, (is the car under load/are you just taking it easy/spirited driving/etc...)

The best advice for any motor is to not load it up, and to drive @ the RPM that the motor *feels comfortable* at, all good motors have their "sweet spots" and if you are a decent driver you should be able to feel them. When I hear some tool driving down the street @ a steady speed and 5k RPM, I know that someone told him to "keep the revs up".

SLO-BOB · 02-23-2005, 02:43 PM

I read that article too. Very funny. I remember how I got started on 911s. My uncle in Denver always had them. As a youth I recall riding in his 911s and wondering why he kept the rpms up so high. Must have been near 4k. I asked once, and the carbon issue was the answer. Maybe the older cars require it, but I keep my 77 around 3k or so. As mentioned above, I like to find the sweet spot. I will say that unless I'm moving along pretty well, my car doesn't see fifth gear.

speeder · speeder's Garage

Also, it has been many decades since anyone produced an engine so tempramental in a production car that it had zero power or fouled plugs below 4k rpm. Your '73 911S, while it produces maximum power and performance at high revs, was considered a very tractable and torque-y Porsche in its day, w/ great drivabilty @ all RPMs. There are a lot of misconceptions, (or short memories), regarding these cars, but that is the truth.

Back in the old days, the quintesential Porsche owner drove like an absolute gentleman on public roads when others were around, and like his ass was on fire @ the track. Nowadays I see the opposite a lot, ie. like an idiot on the street and slower than cow***** @ Willow Springs.

Fred999 · 02-23-2005, 02:59 PM

I just starting driving my '67 911S around ( with 300 miles since rebuild). After 100 miles or not really spirited driving (stop start and under 3,500) it started to run poorly. So I was told to "drive it" with a little more spirit. An hour or two of driving, taking it up to 4,500 rpms or so helped it no end. Seemed like a different engine. That said, I don't cruise at min. of 4,000rpm and the engine doesn't seem to need it to keep the carbon at bay. (haven't dropped the valve covers yet qualifier thrown in)

At least, this is my limited experience with what is regarded as the archetypical plug fouling early car.

pwd72s · 03:07 PM

Jay, I think you'll find that the "sweet spot" for your 2.4 S engine will be right at 4,000...in other words, the revs your car will love when cruising the gorge towards Pendelton in 5th. Just loafing around town? Try to keep it between 3 and 5, it'll be happy... But hey, what do I know...I've only had my car since '74. Maybe Denis has more experience with the 2.4 S?

(edit) The old article's reference to fouled plugs was with a '68 911L model. The CD ignition was introduced in '69, solving much of that particular problem.

donstevens · donstevens's Garage

Remember they used leaded gas and Zenith carburators back then!!!

I had a carburated S that ran lousy down low. All it wanted was full throttle.

The other idea about higher revs in air cooled engines could have been for cooling. A 911 engine tooling at 3000 rpm produces less heat per RPM than at 1200. Plus at 3000 there is much more fresh air flow over the cylinders and oil cooler.

Don

Zeke · 02-23-2005, 03:10 PM

Andial has told me on more than one occasion the 3K is the minimum rpm you want to see when you upshift. AFA crusing, I see no reason to "hold" high rpm in the lower gears. Anything over 2K works for me. But, when I want to accelerate, I first downshift and begin at 3K. I like 3K in 4th and 5th because I am pushing more wind and don't want to work the motor at much lower rpm.

This all sorta follows what Denis said. And, he's right about just knowing what is better. If you don't know or feel it, maybe get a Boxster.

buster73 · buster73's Garage

Especially with the older S cars its a good idea to let them breathe once in a while. At low revs the plugs tend to foul over. Similar for race cars and other high performance engines with agressive cam timing, lots of carburation and less powerful ignition systems. With modern EFI and ingition system this is no issue anymore. And it is a good excuse to have fun - nothing moves below 5000 anyway.

Wil Ferch · 02-23-2005, 04:48 PM

I think this was (partly) due to a cary -over of the early years where you shouldn't be lugging a roller-bearing Hirth crankshaft.

Neat device..."built-up" crank using con-rods with no bolts that would (normally) hold together two pieces on the big ends...just a big hole that slips over a portion of the crank that is "built-up".

These roller-bearing cranks couldn't take lugging...so people kept them zinged up prety good...usu over 4K RPM.

Wil

Jay Zollinger · 02-23-2005, 04:55 PM

I agree with all that has been said--I let my baby stretch her legs as often as is appropriate, but usually upshift between 3K and 4K when cruising around town. The car's "sweet spot" kicks in around 3.5 or so, but I've never felt compelled to drive on surface streets with the revs maintained at 5K, and thought it odd to see this advice in the Panos article.

Jeff Higgins · 02-23-2005, 06:10 PM

Interesting take on the Hirth crank, Wil. Makes sense that the roller bearings on those would be less tolerant of lugging than the plain bearing cranks. Most of my experience in air-cooled motors comes from Harleys, which use a similar pressed together "crank" (really just two flywheels with a crankpin between them). So many people think these are "low RPM motors" and tend to lug them a gear lower than they should be, trying to get that "Harley sound". Now these probably have more flywheel inertia than the Hirth cranks, and are probably more tolerant of this kind of abuse, but it still damages them. These old leaky way-out-of-balance twins like moderate RPM's, and "talk" to you more than most motors (having it right between your legs helps). They sure let you know when they are unhappy. I'm finding the same to be true with my little 2.4 flat six. I'm learning its "language" now, and boy does it talk to me. It's not hard to tell when it's happy. It likes to spin up a bit.

Grady Clay · 02-23-2005, 06:46 PM

I agree with most of the above. I am very careful below 3000.

The other issue is hot weather, A/C and engine cooling as Don noted. The cooling air is RPM dependent. In hot weather with the A/C on, keeping the RPM at the high end has all sorts of benefits. First it cools the engine better. It also lets the A/C work more efficiently by keeping the condenser cooler. The oil circulates to the front cooler faster. In hot conditions, I wouldn’t hesitate to keep it at 3500 at a stop light and buzz along at 4500-5000. Yes, it is slightly obnoxious but has good technical basis.

Jay, your issue is cams and fun. With a MFI 911S, keeping the RPM high (3500-4500) is keeping the trap ready to be sprung. If your passenger didn’t have a headrest, you could do injury when you suddenly stab the throttle. Paul is right on about the “sweet spot.”

I agree with Denis about the plug fowling issue. If there is any early 911 out there without a CDI, they should have their head examined (Fred, what CDI do you have?)

What Guenter said has bearing. These engines benefit from some full throttle leading to high revs each time they are fully warmed up. It is the combustion pressure that forces the rings to seal.

Of course Wil brings up Hirth. I had enough 547 engines (~15-20) to be lectured on this many times. I think there was also usually poor tuning, detonation, and way over carburetion in the pre-CDI days.

Best,
Grady

Fred999 · 02-23-2005, 07:00 PM

Quote:

Originally posted by Grady Clay

I agree with Denis about the plug fowling issue. If there is any early 911 out there without a CDI, they should have their head examined (Fred, what CDI do you have?)

Best,
Grady

Ahh...emm......as it was built in '67

Once my engine is run-in (first 1,500km) or 1,000 miles, I plan to upgrade to a CDI and I have mentioned it to my wrench already. Given that people are starting to regard these early cars as collectible (and value ascribed to originality), apparently the deal is to install a system that can be easily removed. (this is what I am told). Probably the 3 pin Bosch CDI (as per the suggestions of Early _S_Man).

Fred

speeder · speeder's Garage

Paul, I believe you that the *sweet spot* is ~4000rpm, (it was not much lower in my 911SC), but the question was whether you need to keep the motor up there all the time or can you drive around town or on the highway @ 3k rpm when you are just cruising, and jump down a gear if you want to accelerate quickly. I am just going by what the magazine road testers said when the cars were new, (I have a pretty serious old magazine collection).

I would exercise my 911 every time I drove it, (hell, I exercise my Volvo turbo every time I drive it), but I could also roll quietly when I felt like it. I think that this is one of those semantics things, and I always feel like people misunderstand the old "keep the revs up" advice. Some people on this board have posted that they never put their cars in 5th gear on the freeway, because of the "revs up" thing!! And these are people w/ SCs and Carreras! Low-revving, torquey cars w/ nothing at the top! Maroons!!

Anywhoo.....

Grady Clay · 02-23-2005, 07:37 PM

Fred,

I absolutely agree.

You can hide the CDI behind the air filter assembly and add some custom wiring (black heat shrink) that is easily removable and not apparent to any other than the most skilled. Paint it black.

The issue with the 3-pin Bosch is that it has such short spark duration. The 6-pin is better as are many of the current aftermarket CDIs. Fast rise-time is best for dealing with partially fowled plugs, long duration is best for smooth running. Hopefully Warren will chime in.

Good for you working to keep the ’67 original. I agree; there is significant value with originality. These 911s were collectable from the day they were built. Hopefully we will see yours at a Parade concourse in the States some day.

Best,
Grady

Fred999 · 08:27 PM

Truthfully Grady,
given what has happened with the early 911 car market, I wish I had kept the car more original than it is. I don't think my car will make a concourse as I intend to drive it ( a lot) although not as a daily driver.

Given that I am "running in" the beast, having my model ('67 911S) is very frustrating. You start to get the revs up to where its starting growl (4,500) and I have to shift up. Thats why I think this topic is very interesting.

Your point regarding the spark duration is a factor and I am not sure what my wrench will suggest. Actually its in with my mechanic getting its valves, etc done as I write. We plan a run this Sunday so I am hoping it will be ready.

I will say one thing with my ignition system (as is) - it doesn't help the drivabilty around town under 3,000rpm as it really lacks torque or smoothness. It seems to only really like it when I apply some load and revs (obviously a function of my S cams, too, I guess)