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Support your local Porsche machine shop - Buy a 1976 911 2.7.
First, let me say, I like the 1974-1977 911. I like it's "shape"; The long hood, accordian bumpers, and narrow fenders all look good. I like some of the mechanical improvements as well. The new seats and the forged aluminum trailing arms were welcome changes. Porsche even addressed the problems with the ventilation system. Well, they tried to address them. The air still smelled like oil, but it no longer reeked like you were hauling 50 gallon containers of boiling oil.
The problem was the engine. How could the same 2.7 that ran so beautifully in 1973 go so wrong by 1975? I
think it was the combination of archaic smog equipment, the first gas crisis, and some questionable engineering
choices.
Saying the 1974-1977 2.7 liter engines had fundamental design problems was putting it lightly. It's almost as if the powertrain committees never worked together. The good news is, we're talking about thirty plus year old engines. At this point, they've been rebuilt at least once, and hopefully some of the problems have been corrected. The first problem was the valve guides. The original valve guides were made of copper, and lasted somewhere between 30 and 60 thousand miles. The guides lasted longer on cars whose owners removed the thermal reactors, but they still wore out prematurely. Porsche didn't change over to the silicon-bronze guides until 1977. Silicon-bronze guides can last several hundred thousand miles.
Another problem was heat. Aside from the aforementioned thermal reactors (dual thermonuclear blast furnaces
which were bolted directly to the heads of the 1975 and newer models), the 2.7's had an air pump, EGR valve, a
substandard five-blade cooling fan, no front oil cooler, and Bosch K-Jetronic (CIS) fuel injection rather than
MFI. Each of the items I listed makes the engine run a little warmer. When coupled together, they spell
trouble. Hot running Porsches may have worked well for the Alaskan clientele, but not so for those of us who
live in southern California - the largest Porsche market in the US.
Heat is especially troublesome on these engines. The case is made of magnesium and wasn't designed with 2.7 liters in mind. On top of that, Porsche installed steel head studs directly into this magnesium crankcase. Due to differences in thermal expansion, the head studs regularly pulled the threads out of the crankcase. This allows the heads to become loose, and the headgaskets fail. The fix for this is to drop the engine and install a set of timecerts and RaceWare headstuds. While the engine is out and the heads are off, a full valve job with new guides can be done.
Long term 911 owners are used to hearing "as long as the engine is out . . . . . ." However, taking care of a number of problems now will mean the engine doesn't have to be pulled for those same problems later. After decades of thermal cycling, magnesium cases will contract in several places. It's common for the main bearing saddles to warp. This will cause the bearings to wear unevenly. While the case is being machined so that the main bearings fit, the machine shop can look at another common problem - the center cylinder seats. The center cylinders have a nasty habit of sinking inward. If new cylinders don't slide right in to the cylinder seats, that may indicate the seats are not quite circular. Rather than fixing the well known issues, one way to catch all the problems is to have a good machine shop mic the case.
To fix the case, the machine shop will probably clean it, then reassemble it minus the crank and bearings, and
torque it to spec. Then the case can be machined. The mains can be brought back in spec, and the cylinder
seats can be machined round again. This is also a good time to drill out the head stud threads, and tap in new
threads. A little Loctite and some Timecerts, and the case is done.
There are still a few things that should be looked in to, but we're on the home stretch.
If the engine hasn't been upgraded with hydraulic tensioners, it should. Mechanical tensioners are known to fail. If it does fail, and the chain slips off one of the sprokets, a complete engine rebuild may be required to make things right. Performing this upgrade will result in a much more dependable engine. If you're unfamiliar with the tensioner issue, the old tensioner used a spring to keep the timing chain tight. When Porsche introduced the 1984 Carrera, it came with a better design. In addition to a spring, the new tensioners used hydraulic oil pressure. This design was so good, Porsche released an upgrade kit for earlier engines.
The next item to consider is what got us here to begin with - heat. The 2.7 was originally equipped with a bad oil cooling system. The factory put an oil cooler in the right front fender of other 911's. For the 1974 through 1977 model years, it was "optional". Those cars with the oil cooler option received what's called a trombone cooler. A trombone cooler is a pipe that loops around in the shape of a trombone. It's basically the same design that's used to cool power steering fluid. A real oil cooler with a fan, like one from an 1987 and later Carrera, will work much better. If you can't find one of those, any oil cooler will work better than the trombone cooler, For those cars without a trombone cooler, lines will have to be run from the engine to the fender, and back. Using brass lines will help cool the oil while it's in transit. Also, a thermostat should be used. Oil that's over cooled is as bad as using overheated oil.
The second part of the cooling problem is the fan. If the engine has the original five-blade fan, it should be replaced. The five-blade fan was only used on the 2.7, and it simply doesn't move as much air as the eleven blade unit.The fan is pressed on to the alternator shaft. To remove it, simply pull the alternator, remove the outer fan housing, and lightly tap around the fan until it comes off the alternator shaft. The new fan will tap right back into position.
The exhaust is also related to cooling. Any exhaust change that involves removing the thermal reactors will lower engine temperatures. Backdating 1975 or newer exhaust to 1974 spec will produce more power and allow the engine to run cooler. The downside is the car will no longer pass smog. Updating to a later SC or Carrera style exhaust with catalyst won't produce as much power as a 1974 exhaust, but the engine will run cooler, produce more power than it did with thermal reactors, and will pass smog.
The last two areas are related - intake and cams. Before going any further, a decision has to be made
regarding smog. It's illegal to remove the power robbing Bosch K-Jetronic injection system and replace it with
either MFI or carbs. Also, if you have a 1975 or later car, it came with thermal reactors. Thermal reactors
will not work with MFI or carbs. The same holds true if you update to a newer style exhaust with a catalytic
converter. Since many 1975 and newer cars have a requirement to pass smog, how you proceed is a personal
issue.
I'm going to proceed as if the car in question is a 1974 or something that doesn't have to be smogged. The 1974 through 1977 2.7 uses Bosch K-Jetronic fuel injection. This is also known as CIS. CIS is a more smog friendly induction system than carbs or MFI. One downside to CIS is it does not like overlap. If you're not sure what overlap is, my write up on Camshaft Basics will explain it. CIS doesn't like overlap, because it allows a pulse to flow up the intake tract. An intake pulse flowing the wrong direction confuses CIS, and causes a very bad idle. For this reason, 911's with CIS injection systems have very mild cams. The mild cams and CIS are basically why power dropped so dramatically between the 1973 Carrera RS 2.7 and the 1974 911.
How do we get the power back? Use cams with more overlap and an intake system that will support it. PMO or Weber carburetors will work with aggressive cams. By themselves, the carburetors will increase power by 10 to 15 horsepower. They will also drop fuel economy to the mid to low teens, require more maintenance, and run poorly when cold. I favor MFI
So what's the return on all this? A 2.7 that may outrun stock Carrera 3.2's.
| Specifications |
| MSRP for | 911 coupe | 911 targa | 911S coupe | 911S targa |
| 1974: | $9,950 | $10,800 | $11,875 | $12,725 |
| 1975: | $11,900 | $12,800 | $12,845 | $13,795 |
| 1976: | $13,875 | $14,795 | ||
| 1977: | $14,995 | $15,945 |
| Engine Design: | SOHC, two valve per cylinder, flat six |
| Bore and Stroke: | 90.0 x 70.4 mm |
| Displacement: | 2687cc (163.9 cu.in.) |
| Horsepower: | 150 @ 5,800 RPM (1974 911) |
| 175 @ 5,800 RPM (1974 911S) | |
| 165 @ 5,800 RPM (1975 / 1976) | |
| Torque: | 175 lb-ft @ 3,800 rpm (1974 911) |
| 167 lb-ft @ 4,000 rpm (1974 / 1975 911S) | |
| 176 lb-ft @ 4,000 rpm (1976 911S) | Compression ratio: | 8.0:1 (1974 911) |
| 8.5:1 (all except 1974 911) | |
| Fuel system: | Bosch K-Jetronic (CIS) |
| Transmission: | Five-speed manual |
| Length: | 168.9 |
| Wheelbase: | 89.4 |
| Weight: | 2600 pounds |
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