Makmov

Bumping (ie advancing timing) the cams on earlier cars was always the thing to do. My brother, who has worked in some the best Porsche tunning shops in the country said they never bumped cams on 964 and later for some reason.

I guess Porsche didn't want you advancing your cams on a 993.

However, they both have what looks to be a keyway or pin so I am not sure how it can drift away?

chris walrod

I've noticed 993's with recent top end work do seem to run smoother (idle mostly) and be a little more peppy. I've attributed this to simply the cam timing being close to where its supposed to be. Moreso equal side to side vs. set to some absolute spec.

AOW162435

Agree.


Andreas

race911

I've not seen the handful (as in 2 or 3) 993 engines I've had my hands in having deviated from "spec."

I'll also defer to those in the know who have played with 3.6L air cooled stock cam timing to set me straight. HOWEVER we used to do all manner of fiddling with T, E and S cams on everything from 2.0 through converted-to-carb 3.0 engines throughout their factory indicated adjustment range. Guess what? Didn't make a helluvalotta difference. No, I didn't have a dyno when I was a struggling 23 year old with a shop.............. (In Phoenix in the mid-80's, Bob Holcombe and the mega $$$$$$$$ from his benefactors was the only game in town for Porsche dynos.)

Bottom line is that the overlap spec for a 3.6L cam is, what, 1.5mm? Plus or minus basically nothing. For an old S cam the spec was .2 of an INCH. Plus or minus a few hundredths. And when all those in the know in the Bay Area (Woods, Anderson, etc.) back when I was learning pretty much said don't sweat it, I didn't.

NP993

Your cam timing would have to be VERY messed up to account for a 20 HP loss...

Stealth 993

I know most high HP cars the power is in the heads.

I'm trying to grasp this...
Early cars have the older style cam/sprocket that lock. Adjusting timing on these cars is useless?
Older cars have a key lock cam/sprocket & can move cam timing, to adjust you put in the older cam/sprocket. Or just retime every once in a while?

What is the advance/retard specs on stock cams? Are they different for the RS cams?
I also know there is no free lunch, if you gain 20hp, where is it? What does it do to the torque, & other places in the RPM range?

race911

The 993s lock, too. Just don't have the pin. So it's infinitely adjustable. Best you can do with the pin method is probably .002. Good enough for me, guess it wasn't for the factory. Or maybe in production it took 5 minutes less for Hans or Dieter to get the engine assembled?

The advance/retard on any post-MFI cam is pretty much zero. That means anything using CIS through the 3.6L air cooled. (NO idea about water cooled.) Tolerance on the high lift stuff from the dinosaur era is +/- .010, maybe .015? Been years since I've done one, and I don't have the spec books here at home with me.

johnsjmc

An advance or retard of only 3 or 4 degrees can make a difference you might be able to feel. If I remember correctly advancing the cam will move the power peak to a higher RPM and retard will move it down .

Stuttgart951

Backwards, methinks.

geolab

yes backwards

NineMeister

Actually Noah, I think that you are grossly underestimating the importance of accurate valve timing.

The first time I experienced this was when Porsche Club GB 993RS register secretary & fellow Speed Championship competitor Paul Howells told me of the problems he was having with the cam timing on his 993RS. To cut a long story short, the car had been previously remapped in Germany and was making around 310-315hp; following a top end rebuild (can't remember why but nothing serious, same heads/valves/cams/P&C's refitted) the engine lost power. Twice the local Official Porsche Centre set the timing with the factory tools, an independent specialist had another go and Paul also had a go himself; none of the attempts restored the previous power result & I think the best attempt was shy of 300hp. In the end he brought the car to me to be sorted.

Rather than go down the conventional route (again) I worked from Porsche's design specifications and set the camshafts to match the optimum valve opening and closing points as accurately as possible. Not only did the engine run smoother at idle & throughout the range, it also posted 317hp on the same dyno as previously used.

The bottom line is that valve timing events are critical and the real reason why swinging the timing on 911 engines produces indifferent performance gains is because 99.9% of 911 camshafts are either not made accurately enough or set accurately enough to be in perfect tune - if they were the difference between "right" and "not right" would be dramatic.

RDS928S

Wow 20 hp ~.
i hope it's true. However very embarassing to Porsche engine builders & camshaft vendors.

Rich

matt777

Cam timing should be checked on any high performance engine build. The question is whether our friction type sprockets have slipped or were installed out of tolerance from the factory. I guess a dyno run might indicate if power is down significantly although attributing it to cam timing would require further checks. Can it be checked with the engine in? Perhaps those that have done this can elaborate on the procedure (I look forward to learning a thing or two here). Correcting the cam timing may of course cost a little more.

NineMeister

In respect of camshaft manufacturers, is it shameful that factory cams have a 2 degree difference in intake to exhaust valve centrelines between the left & right sides and most aftermarket cams copy the error?

For the record, on a properly tuned engine valve timing events are critical to less than half a degree of crank angle, hence why I use this method to set up valve timing in the race engines we build:

matt777

Nice tools. I'm sure you'll loan those out to fellow Rennlisters?