is cam timing "upgradable" ?

[Olivier 3.0]

I have a friend (the "SSI are hotter?" guy from a previous post ) who owns a SC (180 or 188 hp, he's not sure); he wants to know if he can change the timing of cams, in order to obtain more high-rev power... My '76 Carrera 3.0 has a more aggressive timing, such as the 204 bhp ('81-->'83), and he like the way it revs, even if there is less torque. My crankshaft is different and have less inertia than the SC, but we guess the timing is also responsible for ...

So, If he wants this comportment, can he change the cam timing, without any other modification (cams are the same), and how can he do this ??

PS: I swear i'm not this "friend" , he is just uncomfortable with your language ....

[GIBSON]

For some vehicles,it is popular to add adjustable cam timing sprockets. They allow an adjustment to cam timing which trades low-end performance against the high-end. VW's are famous for this with adjustable sprockets selling big. As to how much difference they make, I have my doubts. Mostly they just look good.
The 911 has no such adjustablility that I know of. The spec books give a range within which you can still be "within specs".
I've tried using the extremes of these numbers to see if performance changes but it does not in my experience.
Going beyond these spec tolerances would risk valve to piston interferance, more on some engines than others but it is not worth the risk.
Different cam grinds are the best choice, choosing which one will work best with a given carb/injection system can be tricky.

Bruce Herrmann

[speeder]

Olivier, Disregard previous post, (GIBSON). All 911's have "adjustable sprockets" from factory, and it is very possible and not too difficult to change timing. (Assuming a reasonable 911 mechanical familiarity). Bennefits from doing this alone are debatable on an SC, in fact they have already been debated here, (check archives first with tech questions- many of the most knowledgable people have already chimed in), you are correct in that it does not change power or torque output- just moves it a little on the RPM scale. No amount of cam-timing change can make a stock SC real strong at the top, but that's OK w/ some of us because of a great mid-section.

[Bill Verburg]

Speeder is correct!, In addition I can add the following info; All n/a cams used on the 3 and 3.2 liter engines have the same specs(I duration 229, lift .455" & E duration 220, lift .402"). The 76 Carrera 3 used a retarded timing(I/O @ 1 BTDC) which enhances power at the top of the rev range, the 78-79 US SC and 81-83 ROW SC used the same specs. 78-80 ROW SC had the cam timing advanced(I/O @ 7 BTDC) as did the 80-83 US SC. The 84-89(all markets) Carrera split the difference(I/O @ 4 BTDC)

[pjv911]

Simply put when you advance the camshaft in relation to crankshaft position you will move peak horsepower and torque lower in the rpm range. Retarding the cams will move the power up in the rpm range. I don't have any experience with your exact application but can tell you that on ricers it can make a significant difference but its best to leave at factory spec for driveability. Engines that have multiple cams (one for intake and 1 one exhaust) benefit much more as you can also play with overlap and static compression. I do it often but I use my chassis dyno to determine the sweetspot. My suggestion is to leave it alone. I have not fooled with mine if that means anything to you (2.7 CIS).
Also its not exactly easy to change the cam timing on your car while on a dyno since its a chain driven (oil lubricated) system.
Have you considered a lightweight flywheel for the crispness you seek ? BTW I did not read the post on SSI`s are hotter but playing with the cam timing will affect exhaust temp . I have noticed a change in exhaust temp on the EGT meter when tuning the ricer motors.

I didn't know the french had good taste in automobiles
You learn something new every day.

Kurt Williams

[GIBSON]

"Olivier, Disregard previous post, (GIBSON). All 911's have "adjustable sprockets" "

It would appear that Speeder has never seen a real adjustable sprocket. The 911 cam sprocket has a series of holes which allow a pin to be placed to orient the sprocket to allow the cam to be timed to the crankshaft to meet a given timing specification. This is hardly an adjustable sprocket. It has to be designed that way to allow for setting of the timing but the range of timing specifications are rather small.
A real adjustable sprocket offers infinate adjustment within a range of timing marks either to advance or retard the cams timing.
It is true that a twin cam motor offers more potential for timing modificatioin but it is always a trade-off. The price paid will come in a power loss in a given power band.
In a 911, one can adjust the cam timing to improve low-end response but the factory setting is the best compromise for all-around performance.

Bruce Herrmann

[speeder]

Well, with all due respect, there is a pretty limited range of adjustment possible w/ any given cam in which an engine will still run. 911 cam sprockets are adjustable within the range that anyone would want to adjust them, assuming that "anyone" has a clue what they are doing. I'm not sure that I understand the benefit of an "infinitely adjustable" cam sprocket?

Considering the price of P-parts I would at least put in the time to maximize the total engine combination, even adjusting the cam timing.
Test and document and keep testing untill you find a workable combination.
Chris

[GIBSON]

It seems to me the problem with "adjusting" a 911 camshaft is how to verify you have made any difference. Do we agree that no horsepower gains are going to come from camshaft "adjustment"? If you can gain overall horsepower from adjusting a cam I'd like to understand how that happens.
If not, do you use a dyno to find out where in the horsepower band the changes occur? If not, is it all "seat of the pants" measurement? If so, that's really subjective and I doubt very
accurate.
Using the term "infinate" in any camshaft adjustment is probably not accurate. I agree that all of these adjustments are made within a very small range. I'm just surprised that any of these
"adjustments" are that noticeable.
If we are"assuming that "anyone" has a clue what they are doing", I would hope anyone timing cams in any car knows what they are doing.
It's been my experience with 911 engines that most engine builders would do well to spend as much time positioning the cam sprocket in relation to the intermediate shaft sprocket as they do setting the cam timing. I've seen lots more problems with worn sprockets and chains than I have from improperly set cam timing.
Well, I think I've beat this subject into the ground long enough.
I agree, set the cams properly, "adjust" them if you want to, and be as careful with all of the engine rebuild process as you are with the cams and you'll have a nice engine.

[RoninLB]

Gibson...I did the best I could with adjusting the cam sprocket to intermediate shaft sprocket. I did not use Porsche tools for this and it was a bit difficult to do. Do you have any tips to ease the head ache??

[ChrisBennet]

Originally posted by GIBSON
It seems to me the problem with "adjusting" a 911 camshaft is how to verify you have made any difference. Do we agree that no horsepower gains are going to come from camshaft "adjustment"? If you can gain overall horsepower from adjusting a cam I'd like to understand how that happens.

I was under the impression that changing the cam timing would change the torque curve i.e. move peak torque to a different RPM. If that is true, moving the torque peak to a higher RPM would increase the horsepower if the torque value was the same (or even slightly less). Horsepower = torque * RPM * 1/5252
Have I been retarding my cam timing for nothing?
-Chris

[Olivier 3.0]

thanks everybody for your answers ...

"I didn't know the french had good taste in automobiles "

Er, Kurt , may I remind you that, even if it's arguable, we are supposed to have discovered the automotive, with Delamare Deboutteville, and a lot of famous brands and pilots were French:
Delahaye, Renault (four times F1 championship winner), Peugeot (Pikes'sPeak, Le Mans, Dakar, WRC and all African rallies winner) Citroen (with the first front wheel drive car, first 4-disc car, and so on), Bugatti (yes, even if he was from Italy, all what he did in cars was done in France), Alpine, Venturi for the manufacturers, and Trintignant, Behra, Cevert, Depailler, Arnoux, Alesi, Mouton, and of course Prost as drivers. I also remind you that Ferrari owes its success to a french guy, Jean Todt... Even Cadillac was from France !!

Noneless we've good taste, but we love and practice a lot cars; even too much sometimes (it's our latina side, I assume)...
Maybe it's why US cars are very rare in european countries ?

Okay, my patriotic minute is over and I'm gonna take my pills...

Well I cant speak for the left left left or left right left right cars.
But With my Pro/Super Pro Nova (Drag racing) You can test using 60' 1/8 mile and 1/4 mile times and mph. In my experience with the Nova cam timing was a valuable tool in making the car run faster. It's called maximizing for a particular RPM range, in my case 6000-8500 rpm. As was playing with intake manifold spacers, header primary pipe size and collector length. Also assorted head improvements porting etc. Then you get into ignition timing with multiple retards etc.
The point is that if you don't take the time to test the different variables you will never optimize your combination. Or you may make your car alot slower.
Also with the parts being GROSSLY overpriced for this car I would take the time to make sure I was getting the most from each improvement.
As far as the "10hp" add on stuff for big bucks goes I wouldn't waste my money. Change the trany gearing instead a thousand or so into the transaxle will reap more bennifits than overpriced add ons.
"If ya don't try you will never know"

[GIBSON]

Chris
I did a little checking to see if I was crazy or what. I called Jerry Wood's shop, he builds more 911 motors than most shops and specializes in performance/race motors.
First, on racing motors they never check the affects of cam timing adjustments because the range is so tight that they valves and pistons would touch if the settings were slightly off. They select the middle of the range of accepted settings a leave it.
They also said that the benefits to be gained by moving the cam timing around is negligable so they do not do it. It is set and left. Dyno adjustments have to do with fuel/air mixture, exhaust etc.
So, regarding your comment, they said it's possible to move the
horsepower curve up but at the expense of horsepower
in other ranges.
I do know that on CIS engines, cam timing is very important because if valve overlap is off at all it will adversely affect performance. This would be true for all CIS Turbo engines as well.
Probably the best engines to experiment with would be ones that are carburated. I'm all for experimenting with cam timing, I'm just not sure it's worth the effort.

RoninLB,
Pelican sells the tools needed to make these measurements. You need a long straight edge and a way of attaching the straight edge to the case. Once that is done, measurements are made to be sure the alignment of both intermediate shaft sprocket and cam sprocket is perfect. Bruce Anderson describes the procedure well in his book. If you just assume everything is the same as before when assembling a new engine (i.e. use the same shims behing the cam sprocket) you could easily be off. This results in excessive wear of the sprockets and increased noise.
RoninLB:

Bruce Herrmann

[pjv911]

Bruce , camshaft overlap cannot be change on a motor with only one cam per bank. The intake and exhaust lobes are on the same cam. You must have twin cams per bank. Basically what im saying is that Cis will not be affected any differently than any other intake system.

Kurt Williams

[RoninLB]

No experience with testing cam timing at different settings. I have read that my '77 CIS can be affected by Exhaust changes, again no hands on experience. I did change stock pistons at rebuild to RS 2.7 8.5 : 1 and every thing else remained stock. I did notice a very slight hesatation from stop which could not be tuned out. I surmise it's the piston dome which does not swirl the fuel like a stock CIS piston. I personally feel that even if there was no danger of valve to piston contact that the cam timing could affect the CIS if not set within spec. even though overlap remained the same. I usually have no confidence with adjusting any thing from spec. on a CIS.

[ChrisBennet]

Originally posted by GIBSON
Chris
I did a little checking to see if I was crazy or what. I called Jerry Wood's shop, he builds more 911 motors than most shops and specializes in performance/race motors.
First, on racing motors they never check the affects of cam timing adjustments because the range is so tight that they valves and pistons would touch if the settings were slightly off. They select the middle of the range of accepted settings a leave it.
They also said that the benefits to be gained by moving the cam timing around is negligable so they do not do it. It is set and left. Bruce Herrmann

Ah, this is just the kind of thing I love to learn about. Thanks Bruce!

[GIBSON]

Kurt,
I think valve overlap or cam overlap may mean different things to different people. For example, my S cam allows the intake valve to be open at top dead center. Most high-perfomance cams have this characteritic. Some even have both valves open at top dead,hence the strange, lumpy idle many muscle cars have. It's not so great for idle characteristics but is really nice at high rpm.
(It's also why it's a bit harder to get a MFI "S" car to idle really nicely that a "T", the camshaft profile favors idle conditions a bit more than the S.) This is produced by a single camshaft, it just depends on where the lobes are.

A CIS car cannot tolerate this condition because as the piston comes up to top dead, if the intake valve is open it causes the
sensor plate to go crazy, or flutter. This just does not work with
a CIS system. So, what I'm referring to when I say valve overlap is very possible with a single cam system, it's just a matter of where the lobes of the camshaft are. Take a look at some 911 cams and you'll see quite a variation in profile from,say an early 6 cam to a later CIS cam.

So, for a CIS car, trying to change cam timing can be somewhat counter-productive. RoninLB is right in my experience. CIS cars like to be set pretty much at stock cam timing.
S cams also have a fairly narrow range of settings as they are opening the valves so far that interferance can occur.

Anyway, all of this discussion is good because it gets people thinking. If I'm wrong, correct me. I'm glad to learn-that's why I spend time on a site as good as this one.

Bruce Herrmann

OOPS I assumed.
When I put a motor together I check several things.
1. Find true TDC and then check the timing pointer and adjust as required.
2. Check piston to valve clearance on INT and EXH. With the cam straight up, retarded 10* and advanced 10*
3. If the piston to valve clearance is acceptable then the motor is assembled. If not then the pistons will need to be flycut.

The other things to consider are valve spring pressure at installed height and at maximum lift. And at what lift does coil bind occur.

If a motor is assembled with a piston to valve clearance that is a wee bit on the tight side your just asking for trouble.
The cost of flycutting the pistons for valve clearance is nominal as is the loss of compression from the flycutting.
True you can't change basic cam specs once it is ground. You however do have controll over the cam in it's timing relation to the crankshaft. Also through rocker arm ratio changes you can change lift.
If cams and other valve train parts had resonable pricing then it might spure more experimintation.

Chevy race parts
roller cam new $270.00
roller lifters new $350.00
springs K-motion new (16) $250.00
titanium retainers new (16) $149.00
roller rockers new (16) $250.00
stainless steel valves new (16) $296.00 (Manley Pro Flo)
Total $1596.00 plus shipping.
Just a thought.
Chris
The above is not a complete list of items to check.

[speeder]

Bruce, You are absolutely correct about CIS motors not tolerating overlap, but the overlap you are refering to is a function of cam design, not the advance setting for that particular cam. Porsche used the same cams from 1978 to 1989 in the 911 with 3 different cam timing specs. Changing the timing does not affect the overlap- it has to do with the spacing of intake and exhaust lobes to one another.

In '78-'79, intake began to open @ 1deg. BTDC, for '80-'83 it was changed to 7deg. BTDC, (with all corresponding openings/closings moving 6deg., of course), for 3.2 motor they split the difference. This was done in 1980 to improve low-end grunt, but motors lost a little in upper range. Anyone who wants to can change theirs in either direction within limited range, but performance characteristic change is not dramatic, and power output does NOT change. Changing to 964 or 20/21 grind is another story, however, same overlap but different lift/duration. That will add horsies, especially w/ exhaust change,(SSI's), and other upgrades- higher CR, etc.

I respect Jerry Woods enourmously, but with the types of purpose-built motors they create, I would imagine that every component is carefully chosen to complement all others including cams, and he sets cams exactly where they belong for output characteristics of particular motor. I am under impression that they supply race motors which are engineered w/ balls-out performance emphasis over street character. Could be wrong on that, but for street car cam timing definitely affects power-delivery CHARACTER, hence the advent of variable cam-timing on virtually all high performance modern sports cars.