Cam Timing Confirmation

[Daviboy]

Hi guys,
I just want to confrim that what I have read I am understanding. So 3.0L SC engine with 964 cams, overlap at TDC should be 1.26mm. I have read that advancing the cam timing slightly will bring power on lower in the rev range. I want to have power come in around 2000 to 2500 rpm. Therefore what is the recommended overlap? I have read it can be up to 1.4mm however I was thing more like 1.3mm any ideas on where that would bring the power in? I dont care about losing power in the high range just want some oomph lower down?

[Flat6pac]

I always set cams to the high side. The left cam gets a number, say for example the 78 SC I did last night, high spec is 1.7 and I pegged at 1.7. The right cam after fooling with it slightly came in at 1.67, so its a match.
The spec for the SC cam is ROW .9 to 1.1 the SC USA timing is 1.4 to 1,7. Anywhere between .9 and 1.7 can be used successfully as long as the pair of cams match numbers. The 78 ROW and USA are the same pistons with .2 CR difference being the base gasket. I always go to the high side because of chain stretch and most time is spent in the 3000 RPM range not in the twilight zone..
Bruce

[lucittm]

During my rebuild I discovered that the range is plus or minus .3mm. So if you are at 1.26mm, then you could go to an advance valve overlap of 1.56mm for slightly more low end torque.

The more important value is that both sides should be exactly the same.

(Edit) I defer to Flat6pac because he has twice as many post as I do and my info is for the newer 964/965.

Thanks,
Mark

[cmcfaul]

When I asked my cam grinder about the correct timing of his cams (DC 30's) he replied with this. I have no idea what 3.8 mm is referring to.

"3.8mm will work fine with 9.5-1 compression. If you advance them, it will hurt top end power."

Chris

[Walt Fricke]

Chris

If you are building an engine, I think you are behind the curve. Do you have a copy of Wayne's engine rebuilding book? Or Anderson? Haynes, even? Take a look, and you'll understand. You will need to, because I am guessing you are going to have to time the cams on the motor you are building.

Generally, our 911 cams are timed by the amount of lift on an intake at the TDC where the exhaust cycle is ending and the intake cycle is starting. Both intake and exhaust valves are usually open at that point (hence "overlap."), though it doesn't matter practically as we just use intake lift, as the relationship to the exhaust is built into the cam. 3.8mm is the figure the guy who makes them says you should use for his cam. A lot of stock cams are in the ranges discussed above. Race cams can be a lot more - one of mine is 5.8mm.

If you used 4.2 mm, you would be advancing the cam. Normally this helps torque at the expense of high end horsepower. Retarding is the opposite (3.4, for instance). All of this is also constrained within the range where the valves won't be hitting the piston.

Or did I miss something in your question?

Walt

[Flieger]

3.8mm is how far open the intake valve is when measured at TDC using a dial indicator. Note that minimum piston to valve clearance usually happens earlier than TDC.

Edit: Walt beat me to it.

[truckinc]

you have to upgrade the engine to increase the RPM. so update the CC engine and booster.

[cmcfaul]

Ok, so 3.8mm is how much the valve should be open at TDC. This is not neceserly the total amount of lift possible. I think I get it. I was planning on having the guy who is rebuilding my heads do the valve timing, though I feel a little guilty about it (cheating on MY engine rebuild)

Many Thanks,

Chris

73 911 E

[Flat6pac]

that number doesnt work on a CIS engine, the piston crown it too close for that large a number. You need the valve recesses of an early carbed or MFI piston
Bruce

[cmcfaul]

2.4 MFI with JE pistons (deep valve pockets)

Chris73 911 E

[edgarcia737]

Hi,
just installed 964 cams in my 3.0L and set valve overlap timing at 1.4mm. nice lowend torque. I believe the factory spec is 1.26mm. Call John at Dougherty Racing Cams. Great guy and wonderful resource.

[Daviboy]

Hi Ed thats just the news I was needing to read! I am heading out to the garage now to do the timing. 1.4mm is what I am aiming for in fact rough timed last night and got 1.3mm on first reading. You know when you read instructions to the letter it is not that difficult.....

[304065]

Originally Posted by truckinc

you have to upgrade the engine to increase the RPM. so update the CC engine and booster.

Question for you truckinc. After which bird is our website named?

[cmcfaul]

Does more overlap = greater VE (Volumetric Efficiency)?

Chris

[Vin-barrett]

when you time the cams you will notice you really don't have that much flexibility to get them exactly where you want them.
If I remember the hole below 1.26 was 1.15 and the hole above ended up being 1.41.
1.26 was not an option. I have experimented with both ways and I will always err to the higher number for a better running car.

[Flieger]

Originally Posted by cmcfaul

Does more overlap = greater VE (Volumetric Efficiency)?

Chris

It means more VE at a particular, narrow, high rpm band. It will be because the air has enough momentum to keep coming in through the open exhaust valve even though the piston is coming up. The air only has that high velocity with ram air, velocity stacks, and a fast piston downstroke, or turbocharger pressure. The fast piston downstroke also means a fast piston upstroke, so there is a point where overlap is too much. Below that, there is more reversion so less VE. There are particular harmonics- bad and good ones, like a tuned exhaust system. Everything needs to work together. Cams, intake, timing, exhaust.

[Walt Fricke]

Vinny

There is enough slop in the "hole" vernier system that you can, with sometimes tedious fiddling, get to just about any number you want. It is not the pin which holds the cam to the gear - it is the friction the clamping force of the nut (or bolt) creates which keeps them together.

Which isn't to say you can't just use the pin and not worry - depends on how close to what you want.

[cmcfaul]

It's my understanding that timing the cams is like indexing the torsion bars. If you can't get the exact measurement you want because the holes don't line up (splines on a torsion bar) you rotate the sprocket 5 holes and measure again. No?

Chris
73 911 E

[Flat6pac]

If youre not getting the number youre look for because there is a .15mm diffrence between pin holes simply release the tensioner and rotate the chain gear in the chain one cog, reinstall the tensioner and you just increased the chances of getting a new number closer to the desired.
Bruce

[2.70Racer]

To clarify Flatpac6,
To advance the camshaft timing less than allowed by the pin holes, you jump the camshaft chain one or more teeth at the camshaft sprocket so the net result is the sprocket has turned counter-clockwise (CCW).
To retard the camshaft timing jump the chain such that the camshaft sprocket has moved clock-wise (CW).
You'll then reset the pin into a different hole to get the correct timing.
Remember the camshafts rotate CCW while the crankshaft rotates CW.
10 camshaft sprocket teeth equal the difference of moving the camshaft pin 1 hole.
Each camshaft tooth equals 1/10th the difference you get with 1 pin moved 1 hole.
This will allow you to time the camshaft exactly to whatever position you desire.
There is no need to accept one camshaft timing different from the other because the pin holes straddle the desired timing.
The Porsche engineers designed this ingenious process, they would accept nothing less than perfect camshaft timing. Nor should you.
Feel free to ask more questions.