Finn,
This is an often discussed topic to which there are two primary solutions; the first of course, is changing to an overdrive 5 speed transmission. There is the famous, rare (and expensive) ZF 5 speed; there has been discussion of a Getrag? 5 speed being available and installed, and one of our members, I believe, has put in a modern Mustang (USA-Ford) 5 speed. Of course the 5th speed is less then 1:1 making them overdrive which would provide you marginally less RPM at highway speed. Unless this kind of mechanical work is routine for you, this is an expensive and time consuming option to pursue.
The other alternative is changing your rear axle ratio which is far easier to do; I believe the term is to "shorten" the axle. You probably have the 3.92 standard gear set in your differential; you can go to the optional 3.69. If I am not mistaken, (I'm sure others will weigh in here) there are other ratios from other MB vehicles of the era that will fit, and I think you can even install an entirely different differential ex-sedan as well. Just wait, I'm sure you'll hear about other ratios and other experiences. Suffice to say you can make these changes, and I believe they are a lot easier to do in the differential then at the transmission. Before you pursue any of these options, you need to ask yourself the question, "How many RPM's drop at X km/hr will it take to make me happy?" The last thing in the world you want to do is spend a lot of time and money on a project only to find that the X RPM drop you "bought" isn't sufficient to quiet things down to your taste. For YOUR 1964 230SL, the factory charts show maximum output efficiency (150HP) of the engine at 5500 RPM, and peak torque at about 4250 RPM. At 3500 RPM, your efficiency drops considerably to 100HP. How do these RPM points compare to your typical highway speeds? How low do you want to go?
Now, nothing like a little controversy to get those flames a goin' on a "hot" topic before a holiday! (What? You think I don't KNOW a controversial subject??) To that end, let me offer my humble opinion: most Pagoda owners, myself included, have the same "complaint" about high highway RPM's, but I learned to live with it. Why? Well, I have my car in the condition that the designers and engineers intended. The engine revs higher, but remember it is a "system" of interconnecting concepts with torque, horsepower, drivability, etc. It's all in balance, whether you believe it or not. It isn't a design mistake. Giant diesel engines in ships turn just a couple of hundred RPM. Some racing engines routinely do 10,000 rpm. Our Pagodas do what they do by design. If you increase your rear axle, you'll have better "off the line" acceleration. If you reduce your rear axle, you will lower the highway RPM's at the expense of acceleration. Change anything, and your speedometer is off. You can't have it all ways with your car unless you do something drastic like an engine transplant! What I think bothers people is the concept of higher RPM's framed with their perceptions of what it should be, based upon modern vehicle experiences. I will suggest that there is nothing inherently wrong with the RPM's our engines turn at highway speeds. What's wrong, if anything, is the perception that its a problem. It doesn't hurt the car. If it hurts the driver, small foam earplugs (don't laugh, its serious: I wore them last year driving top down on the highway to Starfest!)are a very inexpensive and real solution! At highway speeds around here and in my car, the wind noise is far noisier then the engine.
My normal driver is a Ford Explorer, with a 4.0l V6. At highway speeds, 70MPH, it is just over 2,000 RPM and thus is extremely quiet. Engine noise is non-existent. This is an SUV, not particularly designed for quietness. My guess is sedans are even quieter. I think a lot of modern vehicles do the same, either with a combination of overdrives and axle ratios, and engines simply designed to turn fewer RPM's. But there's no way we'll ever see the Pagoda turn 2,000 RPM at 70MPH. It will always be noisier and higher revving then modern cars. Enjoy the Pagoda for what it is.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored
