Engine knock at 2200rpm in 1977 Porsche 911S

1977 Porsche 911S Noise

As these cars get to be antiques, there are fewer and fewer people that know, or care, about their little quirks, they just throw an SC engine in and go on. However there are a few things that can give new lite to an old frustration, as found through experience, here is one of them.

A 1977 911S with 77k miles came in for a buyers inspection, it passed all tests, on the way home the customer filled it with fuel, then got on the freeway and headed home, after a short while he noticed an engine noise, a slight knock, not a ping, not a rattle, at about 2200rpm (which is a nice freeway rpm). He returned and had me listen, and sure enough it was noisy, more from one side than the other, right side I believe, so I checked it over, pulled the tensioner covers, everything was great. I asked around and found that others had encountered the same noise in 77 911's and nobody knew what it was, but they all said they ran fine, lasted fine, so left them alone. I wrestled with it for a while and let it go. At 105k it came in again, Valve guides were rattling, and high leak-down, two common facts of 75-77 911 engines; soft guides, and warped alumasil cylinders. So we did it up, the best guides, Carrera tensioners, got rid of the thermo reactors, euro nicasil's, chains guides, you know all the stuff. Got it together and it ran great, of course,,. but it had this rattle at 2200, louder than before, lets put this in perspective, $4400.00 and it rattles worse? Ahhh. OK, so round the patch I go, checking tensioners, cam timing, ignition to specs, fuels, fuel mix, ah somewhere in the fuel mix it made less noise. Sounds like piston rock on one side, other side is quiet????

As I was running it one day in frustration I grabbed the distributor and brought the timing up, it got louder, and started earlier, backed it down and it came less and less, and went away completely, I rechecked the spec sheet, vacuum off, vacuum on, plug, hmm doesn’t make sense. So I checked into it, it seems that the vacuum unit on the distributor was never used! But there it was and a hose to it, with instructions on the engine tag, trouble is, the red vent/plug was missing!

If you, like I, set it according to the tag, you get timing that is about fifteen degrees advanced from where it should be! There must have been a last minute change that wasn’t fully thought out, or something else that caused them to hook the whole vacuum advance system up, but not use it, and to include that little red rubber plug that plugs it off and vents the vacuum can. (Never plug a vacuum advance unit, with changing atmospheric pressure it can advance/******, and be a bit of a mystery in itself)

So what people do is go by the sticker then leave the plug out and connect the advance unit, making a whole bunch of advance and making those right side pistons rock in their bores around 2200rpm, who would have thought it, we always check the maximum advance at 6000, but do we think to put the vacuum hose on before checking?

The car I found this on must have had some old gas, that mixed with a few dollars of fresh premium, made a combination that masked the noise.

We were able to inspect the records on this car at a later date and found that at 25k it went in for it’s 30k service early, next receipt was for a noise being checked out a few thou later, then a long history of complaints of noise, and various ways that shops waved it off and said yes it is noisy, but don’t worry about it.

So with this tip we all know; to time them like a straight centrifugal distributor, to plug the vacuum port, and vent the vacuum canister on the distributor. That’s my (expensive) story/tip.