Adjusting the Cam Timing - What are the Pros and Cons?

[FL911SC]

This past weekend, I let a guy drive my '79 Targa at the year end auto cross. He generally sets down the time to beat in his Cayman and during the week he works at a local shop and is very familiar with the older generations.
I asked him to drive the car to get his opinion on how the car feels since it is the only Porsche I have ever owned and the only one I have driven for any amount of time. The good news is that he liked the car and said it was on par with what he would expect and thought it drove well.

One of the things he mentioned as a possible modification was to adjust the timing. My first thought was that he was talking about the firing timing adjusted at the distributor. However, after a little more discussion, he clarified that he was talking about adjusting the timing by adjusting the cams.
Truthfully, I didn't know that was an option.

My questions to the group are:
1) Does adjusting the cam timing really improve the car?
2) What is the compromise? (e.g. lower torque at low RPM; harder cold starts: lower gas mileage; etc...)
3) Is there a negative in the long term?


I thought I was ask here (opposed to the engine rebuilding forum) since I am looking for more general answers.

As always, thanks to all!!

[BK911]

Its not that easy to adjust. Not that difficult either, but takes special tools and removing a bit off the back of the engine.

[javadog]

Two different cam timings were used for the 911SC. The difference was 6 degrees. If you have a US 79 SC, your car has the retarded cam timing, which is best for high end power. The later US cars had timing that was advanced 6 degrees compared to yours, which gives a little more midrange torque. The ROW cars used the same cams and the timing varied sometimes from the US cars. They started out with the advanced timing and changed to the retarded timing.

30-some years ago, when I drove an SC as a daily driver, I timed the accleration through the gears and got a measurable improvement with the retarded timing. Keep in mind that when you are running through the gears, you are only using the upper third of the rev range.

Driving around town the advanced timing might feel a little better. Hard to say which would be better in an autocross, since courses vary a lot and I have no idea what gears, or rpm ranges, you typically see in a run.

Personally, I like the feel of the '78/'79 US cars better than the later ones. Keep in mind, you have the larger intake runners, which work better at high rpms. If you want to modify your car, you might consider bolting on the CIS components off of a Carrera 3.0, if you can find a set.


JR

[tobluforu]

Not sure about your car, but I set my car in the middle of the specs, it then took two other tries for me to dial it in. So yes you can gain some performance depending on what cams you have and where you like to be with your foot into it. For my car I retarded the cams some. I then installed different cams, ones that give more power in the top end and found the best performance was to advance cam timing for those.

[john walker's workshop]

You probably won't notice much difference, but he will have a fatter wallet.

[tobluforu]

Originally Posted by john walker's workshop

you probably won't notice much difference, but he will have a fatter wallet.

lol!

[Peter Zimmermann]

Originally Posted by john walker's workshop

you probably won't notice much difference, but he will have a fatter wallet.

lol!

[FL911SC]

All,
Thank you for the feedback.
From "javadog's" post I gather that there is no overall real gain or loss. By adjusting the timing, you move the power band to a different point in the RPM range. Since my car doesn't seems to come to life around 4K RPM, moving that forward a few RPM may be of benefit since it would come to life a little sooner. This would help around town and in auto crossing. However, the compromise is that there would be less power at the top of the RPM range.

Judging by the last three post, the cost of doing this may not be worth the expense incurred.
If I have any of this wrong, please correct me.

Thank you again and Happy Holidays to all.

[javadog]

Originally Posted by FL911SC

From "javadog's" post I gather that there is no overall real gain or loss.

Ah, not quite what I said...

It makes a noticeable difference, something you can feel in the seat of your pants and also something you can measure with a stopwatch.

Porsche rated the US 911SC at 172 hp, all years. Peak torque was rated higher in the earlier years than the later ones, but none of this really tells you much. It's not the peak values that are really of interest; it's the area under the curve, whether you are talking about the torque curve or the power curve.

If you spend a lot of time on a twisty road in second and third gears and like to run the car hard, you'll want one timing. If you just mainly drive the car around town and you seldom run it in the higher rpm range, you'll want another. The earlier timing is faster; the later timing make the power curve feel a little less peaky.

For example, have a look at the three road test that Road & Track magazine did of the 911SC. They tested a '78 coupe, an '80 coupe and an '83 cabriolet. All of the cars will get through first gear about the same time and distance. Figure 3 seconds and 100 feet. Once you get into 2nd gear, the '78 will start to pull away from the later cars. Third gear increases the difference. The '78 will get to 100 about 2 seconds faster than the '80, which in turn is 2 seconds ahead of the '83. This, you can feel.

Checking the timing isn't all that hard and it doesn't take much in the way of special tools. If you are into the motor anyway, to install different tensioners or tensioner guards/spacers, it is a no-brainer. It's also something I'd do when buying a used 911, just because I like to go through all the basics on the front end.

If I had a '78/'79 US SC, I'd keep the stock timing, bump up the midrange torque with a set of SSI's and a Monty sport muffler, then consider some of the CIS tweeks I mentioned above.

JR

[colingreene]

if your going to all the trouble to re time the car id put better cams in it while i was at it.
that would make a real difference rather than a speculative one.