911T cam timing

[snowman]

I entered all the 2.2L 911T engine specs in a program called Dyno 2000. I used actual measured values of airflow for the heads (I measured the air flow on a Superflow 600) as well as the measured compression ratio (8.6:1). The program usually predicts the acutal engine horseopower withing a couple of percent. I got 130 HP with the first simulation, which is just about what the factory states it should be.

Now for the interesting part. I retarded the cam timing 5 degrees (on the computer) and picked up 20 HP for 150 HP total. Has anyone done this on a real engine? If so does it pick up a lot of power? I suspect that it might be real as the T engine used the same heads as the E and S engine.

[TimT]

I have played with the advance/retard of the cam, Its interesting. If you advance the cam timing you get torque and hp earlier in the rpm band, retard the timing you get the converse.

The factory cam timing figures will produce the power at a certain rpm per the factory power curves.

A few years ago I built an engine and intentionally advance the timing outside the factory specs, this would make the torque peaks and HP peaks at a lower rpm ( I did check clearances before firing the engine). I then had the car dynoed and found out that my setting had the peaks at 6000 rpm, i thought this was good for a small displacement street car.

This year I needed to freshen up the top end of this engine. I am now going to retard the cam timing to bring. the power higher up in the rpm range ( the street car is now a race car)

What this all means is since I had two reference points (I know I make x hp at 6000 rpm, and the factory makes the same hp at say 6700rpm) I can interpolate the cam timing. Then set the cam timing to produce max hp at my shift points. There is no reason to shift your engine after you have passed maximum output ( exept to prevent it from going kaboom)

I dont think you ever increase HP by changing cam timing..

You just move it around.

[cstreit]

TIm,

Can you explain specifically what advancing or retarding the cam is? I realize that it's changing position relative to the crank... So is advancing the cam rotating it further than than the spec? (In other words, if the crank rotates clockwise, to advance the cam, you also rotate it past spec in the clockwise direction?)

[TimT]

Chris, if you advance your cam your measurement will be less, or less than the small side of the spec given, if you retard it the measurement will be larger.

Ill have to dig out my notes, but I timed the engine I mentioned quite a bit outside of the Porsche spec.

I was able to extrapolate the cam timing for my engine based on the dyno info I had and the info supplied by Porsche. A crude but simple proportion gave me the lift dimension to allow me to match the hp peak to rpm I chose.

I wanted to have max hp at 7000 rpm so I set the timing to suit.. If I wanted my max hp at another rpm I could calculate that as well.

This relationship is only applicable to the same engine and cam combo.

[snowman]

It is true that advancing the cam timing helps the low end torque, and retarding the cam helps the higher end hp. But with the 911T Porsche had a pecular problem, a marketing one, not an engineering one. Porsche had an excellent engine, optimally tuned for about 180 to 200 HP. The air flow in the head is the major determanent of the power. So they had to detune the engine for the lower perf cars. They put restricters in the exhaust for the 911E as well as changing the cam. For the 911T I am not certain what they did besides the cam change. But it appears that a minor change in cam timing can produce 150 of the 180 HP if the program is correct. There is no reason that I know of that the program is wrong at this point. These heads are a work of art and flow lots of air, ie make lots of power. Normally retarding the cam only helps a tiny bit in high end performance, but a gain of 20 or 30 HP isn't normal.

[911pcars]

Camshaft timing is always described in terms of crankshaft degrees even though the cams rotate at 1/2 the speed of the crank (engine). Gotta remember it takes two revs of the crank (1 for the cam) for one complete 4-stroke cycle (intake, compression, power, exhaust).

Cam timing is the timing of opening and closing of intake and exhaust valves relative to crankshaft rotation (we use #1 piston for a reference) near BDC or TDC. For example, if "normal" intake opening begins 20ºBTDC (before top dead center), advancing the timing 3º places the beginning of intake that much sooner in the pistons (crank) position - at 23ºBTDC. Typically, the intake sequence begins some time before the piston begins traveling downward due to the inertia of the gases.

The effect of "advancing" the cam timing is to move the gases into and out of the cylinder sooner relative to the piston. This results in engine torque and HP developing earlier in the engine's power curve. For an autocross car, it may be advantageous to have peak torque occur earlier in the power curve for better low speed response. Conversely, for a high speed track car, retarding the cam timing will move the torque and HP curve higher up the rpm scale.

The early T engines are limited not only by mild cam timing specs (relatively low lift, low duration), but by the cylinder head port size (compression ratio too). For example, T heads have 32 mm intake ports. S heads are 36mm; a big difference in terms of air flow and volumetric efficiency at high speed (the actual volume of gas entering the cylinder vs. the theoretical amount). It's interesting to note that engine torque of the T engine develops earlier than the S engine (due to cam timing and port size). This translates into better low speed response (as in low speed acceleration). As we all know, this advantage doesn't last long.

Engine displacement is a great equalizer as with it usually comes increased torque, the main determinant that gets you from point a to point b quickly. When accelerating, an early 2 liter S can barely keep up with a 2.4 T until its superior breathing can take advantage, but that only happens far beyond normal speed limits.

Sherwood Lee
http://members.rennlist.org/911pcars

[snowman]

The heads on my 1970 911T have 46mm intakes and 40mm exhaust. The info that I have says that all heads from 1970 thru 1977 have the same size valves in all engine verisions, i.e. T, E, S. Thats from Bruce Andersons book " Porsche 911 Performance Handbook" Page 70. The port changes referred to are irrelevant because I used the actual air flow in my heads. I suspect that the difference is small in terms of performance. The dyno 2000 program shows that about 10 HP comes from a 9.5:1 compression ratio vs an 8.6:1 one. The rest of the power comes from mostly the cam changes. HOWEVER according to the dyno 2000 program the T cam with its short duration and mild lift will give over 150 HP at 6000 RPM if it is retarded. Retarding it 15 degrees (crankshaft degrees), if it is possible, without the piston hitting the exhaust valve, shows 157 HP at 6000 RPM. What this means is the head breaths so well it only takes a mild cam to make power. The S engine operates at much higher RPM consequently the ports are probably larger to keep the intake gas velocity correct at higher RPM and the S does have an additional 23 more HP.

Again this is an engine simulator, it can be wrong, but it isn't likely. I was hoping someone had tried it to see if it works. P.S. The S cam specs in the simulator behave as you have stated they should when advanced or retarded. The big pick up in power for retarding cam timing is very unusual and I have not seen it on any other engine I have simulated or with the S cam.

[TimT]

Err.... I have tried it and posted it above... I have Porsche dyno info (as available in the factory service manual) as well as like graphs published by BA, and I have my own dyno info.

Realize this only works when you are comparing apples to apples........ the same cam in the same engine... making no other changes to the parameters.

Yes you can move the hp peak up and down the rpm range, its quite easy.Porsche knew this as well.. Id bet they chose the cam timing specs based on the market they were targeting.. The T and S were sold to different markets.

Changing cam timing is nothing new.

[ChrisBennet]

Originally posted by TimT
Chris, if you advance your cam your measurement will be less, or less than the small side of the spec given, if you retard it the measurement will be larger.

Help me out here Tim, I thought if I advance the timing, the cam will have rotate more/earlier) and the measurement will be greater as I'd be measureing further "up" on the cam. I'm sincerely asking, not trying to be a wiseguy.
confused in NH,
Chris

[TimT]

Chris,

It might be easier to think of in degrees rather than a linear dimension.

If you have a cam that has 102 deg lobe centers, and find your cam intake centerline is 100 deg, the cam is advanced, if you found the cam at 104 deg the cam is retarded.

On the Porsche engine we measure the lift of the cam at a point. If your cam requires 5mm lift, to advance the cam you might set it at 4mm. Since the cam is symmetrical, that cam will be opening or closingin "advance" of when it was specced to.

When I initially decided to play with the cam timing (I cant leave well enough alone) I got confused about which way to go with the lift measurements. Since I bought my cams from Web-Cam, I called Laurie at web-cam, she told me the to use a lesser reading to advance the cam, and that the limit to how much I could change the timing depended on the clearances.

did I help or just further confuse the issue

Tim

[ChrisBennet]

Sorry Tim, I must be retarded (or is that advanced?).
Here's what I'm "thinking" (for want of a better word). Point out where my thinking has gone astray (please!):

1. Given a point in time when the valve is supposed to open according to some specification, if I time the cams so the valve opens sooner (than the "spec"), haven't I "advanced" the timing?
Have I got advanced and retarded confused?

2. The valve lift "setting distance" using the Porsche method, goes by the valve lift. It seems like this point is very near where the cam lobe starts so if I turn the cam further, the lift gets greater i.e. it doesn't pass the tip of the cam lobe and get smaller.

3. If the "setting distance" is 1mm and but I use 2mm to time the cams, the cam has to open the valve earlier so I'm at 2mm of lift by the time the crank is at TDC. All along I've been thinking that this was advancing the timing because the valves were opening sooner.

still confused in Nashua,
Chris

"You know how they say There are no stupid questions? Well Chris, we're going to make an exception in your case."

[snowman]

Chris, you are correct.

[TimT]

1. Given a point in time when the valve is supposed to open

the dimension Porsche gives is absolute, without sign convention. If your cam spec is 5mm, you really read -5mm on the dial gauge. So if you set your cams at 4mm (-4mm), the cam has progressed further in its cycle (advanced).

2. The valve lift "setting distance" using the Porsche method, goes by the valve lift. It seems like this point is very near where the cam lobe starts

Yes, and that dimension has a direct relationship to the postion the cam will be in time.

Ill try and scan something tomorrow that may explain things better than I can.

[cstreit]

So basically to "advance" a cam's timing, it actually opens earlier... Meaning you will rotate it backwards relative to it's factory setting and against it's normal rotation?

Or in other words... At a given point in crankshaft rotation, a cam that's been advanced will show more lift (or earlier lift anyway) than one that's set exactly at spec?

[911pcars]

Chris,
Your summary of camshaft advance is right on. To add to that, advancing the camshaft timing begins and ends all events earlier than "normal".

Sherwood

[snowman]

Chris,
You are correct. The cam just starts to open and this is where Porsche says to set it. IF the cam is advanced it is open further than it normally is ie if it is specd at 4 mm at tdc and you see 5 mm it is advanced, if it is less than 4mm it is retarded. No majic in other words.

The marks on the cam are not accurate enouth to set the cam timing within 5 or 6 degrees, so Porsche uses the valve lift at tdc method, which will get you within 1 or 2 degrees. Less if the tdc mark is not correct. No big deal as with normal cam timing there will be no noticable difference when driving the car. The difference in driving for a cam advanced 3 degrees vs retarded 3 degress is in the mind of the driver, not the preformance of the car. For a normal camshaft advancing or retarding 8 degrees will be noticable. to the advanced driver, and may not be to the novice. For the change I see in the 911T, on the computer, if it is real, would be felt by any driver.

[ChrisBennet]

Thanks snowman. Good info.
-Chris

[TimT]

Lets put this to rest... I made a cad drawing at work today....
" a pictures worth a thousand words"

I know that you can move youre HP and torque peaks around....dont know about making MORE torque or HP

[snowman]

This is correct if the TDC marks were lined up and the rest unchanged.

[ChrisBennet]

Tim,
I see where you are coming from now. The "specified position" using the Porsche/valve lift method is very close to the start of the lobe (not the lobe center). Advancing the cam timing would always result in a larger value - unless you advanced the cam timing by like 50 degrees, enough to go past the lobe center.
Using your original example, the timing would be retarded, not advanced.
"If your cam spec is 5mm, you really read -5mm on the dial gauge. So if you set your cams at 4mm (-4mm), the cam has progressed further in its cycle (advanced)."

-Chris