ADVANCED cam timing ?

[T77911S]

I have a 77 911S 85000 miles. the motor pings ( or detonates ) at around 3500 rpm. I retarded ign timing about 4 deg from the correct setting and now it pings around 4000 to 4500.
I am running as rich as I can and it doesn’t seem to help much. the distributer has been rebuilt, new plug wires, new WUR , new fuel filter. I ve checked the air flow sensor, the plate and the plunger both move freely. after changing the IGN timing I started to suspect advanced cam timing. someone did some work on the car before I bought it, possibly a top end. I don’t think they new what they were doing because they reinstalled the cooling fins on top of the motor.
ive got the motor out now so I checked the cam timing. the spec says .016"-.021". when checked I came up with .062" for both sides. I turned the crank another 720 deg but this time I stopped when the dial read .020". based on the distance the 5deg. mark is from Z1 on the pulley ( about a ¼ inch), I figure it is about 10-12 deg advanced, closer to 12 (about 5/8 of an inch from Z1). the book says my intake opens at 6 ATDC, now I believe it is opening 6 deg BTDC. is what I am doing and thinking correct?
can advanced cam timing cause the detonation, and if the cam timing is advanced should the IGN timing be retarded to compensate ( not that I want to leave it advanced ). I see Porsche played around with the SC's timing, about 8 deg, with the most advance at 1 BTDC.
I also have concerns about the vacuum line connections. all the emissions have been removed and I don’t trust the previous work done on the car. any place out there with good diagrams?
I got into a discussion with someone about the specs. he said setting the cams at one end or the other of the specs (.016" or .021) can change how the car runs. I disagreed because the specs only cover 1deg of crank rotation and that you could never tell the difference. I said the specs were so critical because of the importance of having both cams the same.

[ianc]

Hi,

It seems to me that whether the engine detonates or not will not be related to the degree of advance the cams have. If you think about it, it seems like detonation would only be related to combustion chamber temp, ignition timing, and the octane of the fuel used. I'm not 100% certain of this, so hopefully someone more knowledgeable will chime in, but that would be my speculation.

I've heard it said that more retarded cams will help bottom end response, and more advance will benefit the upper end, but Bruce Anderson says the effect is fairly negligible. As you've said, Porsche played with this over the years, and finally settled on a value in the middle of the previous extremes they had tried.

I would check the amount of ignition advance you're getting at full advance with a timing light and make sure you're not getting more than spec at higher RPM.

Good luck with this,

ianc

[SC-targa]

Interesting question. Advancing the cam timing definitely shifts the powerband lower in the rev range. Retarding the base cam timing bumps the power up higher in the rev range.

I suppose it could cause pinging by allowing better low speed cylinder filling from the advanced cam timing. 12 degrees of advance sure sounds like a lot.

I'd want to set the cam timing back to stock, or maybe retarded just slightly. Good Luck!

Jerry Kroeger

[Steve@Rennsport]

Great question,....

Detonation is caused by many things but the key point to remember is: Cylinder Pressure and that can be changed by altering cam timing. Given the profile of these CIS cams, you can experience detonation in the mid-RPM range which is precisely where these mild cams develop high cylinder pressures.

Premature combustion from high cylinder pressures can trigger the ignition event as well as overly advanced ignition timing, insufficient octane or too-lean fuel mixtures.

Restore the cam timing and split the overlap and that will help your detonation issue.

[David E. Clark]

You didn't say what octane rating you are using but if you are running "regular" 87 octane you might try upping it to "high-test" 91-93 octane. Of course this won't cure any mechanical issue but at least it should help with the detonation until you diagnose the underlying issue.

[Walter_Middie]

T77911S,

I think you may be doing your cam timing measurements correctly. I have a stock 1975 2.7L with "SC" cams in it. The timing for the "SC" cams is 0.055 to 0.067 inches. If you have these cams, a value of 0.062 inches would be right in there.

I would check your piston to valve clearance and make sure it is correct. This will give you a good idea if the previous owner left the correct clearance between the piston and the head. If this clearance is too small, it would raise the compression and you could get detonation.

Best of Luck
Rex

[2.7RACER]

The US '78/'79 SC cams were timed to open at 1 degree BEFORE TDC.
The ROW '78-'80 SC's were timed to open at 7 degrees BEFORE TDC.
In 1980 the US SC cam timing was advanced to the ROW spec, 7 degrees BTDC.
In 1981 they all went back to 1 degree BTDC.
1 degree BTDC is 1.1mm at Z1 or .044"
7 degrees BTDC is 1.55mm at Z1 or .061"
Your cams are set to open fractionally before 7 degrees BTDC.
Stock '77 pistons were rated at 8.5:1.
I'm guessing the pistons have been changed or the assembled deck height is less than 1.0mm.
The SC's ran compressions from 8.5 for the US to 9.8 for the '80-'83 ROW SC's.
Obviously the higher SC's need premium fuel. The 9.8:1 Euro SC's would need more octane than is typically available today, I'm thinking closer to 95/96 octane.

[T77911S]

i ve tried 93 octane, helped a very little

ive checked the timing, 30deg once all the advance comes in.
i retarded the timing 5 deg. from spec and that helped a lot more than the hi octane fuel, but i still cant floor it past 4500 rpm, although it did raise the rpm by 500 rpm, but i cant keep retarding the timing because that is just covering something up.

i retimed the cams tonight, my first time. peace of cake!
the left cam nut was barely on and the right side was a bear,.
had to heat it up ( i was using 2 4 ft breaker bars)
i hope to have it back in this weekend..few other things i need to do

thanks

[T77911S]

i reset the cams to factory spec.
no more pinging !!!! the car runs great. so nice to be able to my foot in it!
it looked like the cams were set 12*+ by the previous owner
because he was not reading the dial right. they were set so the dial indicated 020"( which eqauled 080" lift)