Cam timing - To mid tolerance or to one end?

[Gary71]

Any thoughts on if you should aim your cam timing in the middle of the spec or to one end? Does it make any difference, does it allow for a bit of chain wear?



I'm doing the cam timing on my standard 2.4T and the spec is 2.4 to 2.8mm lift at TDC. So plan to set to 2.6.

Any preference either way?

Thanks
Gary

[grannysmith]

I don't know if one end of the range is better than the other Gary so opted for the middle as you have. Not quite as close though

[Gary71]

That's just the set condition, it ended up at 2.5 once torqued and with a couple of crank rotations, hence checking to see if I need to undo that big nut again!

[DustyM]

One end of the range will give more torque, the other more top end.

[johnM]

Hi Gary

If you have new chains they will stretch. One end of the timing tollerance will give you good HP but poor torque, the other will give good torque but poor power. Set the timing towards one end so that when the chains stretch you end up in the middle. If that makes sence.

[Gary71]

So... Which end is HP and which end is torque

I'm assuming that retarded (smaller number) is HP and advanced (larger number) is torque. But in reality it makes no difference?

Allowing to settle back in the middle sounds a good idea so set advanced?

[fetuhoe]

Changing the cam timing will have an impact on engine behaviour and some tuners recommend increasing the advance as much as possible (to the limit of valve/piston clearance as an aid to drivability. Others who pursue Horsepower retard the cams.

I think opinions will be divided for as long as we tune engines

Advancing the cam timing

Starts the intake opening event earlier and hence opens the intake valve earlier
Produces more low end torque
Decreases Intake Valve to Piston clearance
Increases Exhaust Valve to Piston clearance

Retarding the cam timing

Delays the intake closing event and hence keeps the intake valve open later
Produces more high RPM Horsepower
Increases Intake Valve to Piston clearance
Decreases Exhaust Valve to Piston clearance

With a stock engine in terms of pistons, valves, cams, deck height etc. valve/Piston clearance shouldn't be an issue within the range of timing quoted by the factory.

If something that affects deck height has been skimmed/modified you may need to check.

Chain wear is always an issue as chains will change length during their life and at their maximum 'specified length' they will affect the cam timing by around 5 degrees.

We always fit Race Engines with 'Solid' tensioners and we tend to find that initial lengthening of the chains occurs within around 250 miles as 'high spots' etc' are removed.

Chains then stabilise in length for about 25k miles before they need re-adjustment.

The initial wear that can be heard as a slight rattle results in a cam timing change of less than 1 degree.

[911hillclimber]

I think the Bruce Anderson book covers which end to go to for bhp or lbft.
Bob Watson alway set mine mid way, and when I've done my 3.2
I have done the same.

Nobody seems to quantify the effect, 5 bhp or 15?
Bet carb tuning etc has a greater effect on drivability which must be the ultimate goal, esp on a T engine, so lbft is the way to err.

Good luck for the rest of the day.

[fetuhoe]

Regardless of which 'end' you chose I would try to set both cams to the same end.

I have seen a number of engines where both cams are 'in spec' but timed at different ends.

Engines with this feature always seem to exhibit a slight 'beat' at idle and are never seem to run smoothly.

Some early 911 cams do tend to have a relatively narrow lobe separation angle and this will make them more sensitive to cam timing than later engines and they do respond to changes.

The T cams are around 105 degrees compared to 113 degrees for a 3.0SC and 3.2 Carrera - Early S cams are 98 degrees and very sensitive to timing changes.

We recently re-timed the cams on a 2.2T and retarded them to the limit of the spec - They were previously mismatched.

We didn't use a Rolling Road but the car has noticeably better acceleration and performance has generally improved when driven briskly.

[Gary71]

Thanks all, seeing as I can't undo the pulley on the LH bank 2.5mm sounds like the target for the RH!

I think Andy's crowfoot spanner has done the rounds so many times it's a little worn. OK to tighten but jumps when undoing.

I've just punched myself firmly in the face trying!

[DustyM]

I've just punched myself firmly in the face trying!

Face or nuts, its always going to be one or the other

[Gary71]

Got the RHS to 2.6 and I'm going to stop there.

Interestingly (or maybe not!) I had 2.8, 2.4 and 2.6 all with the dowel in the same hole, just wiggle room makes the difference.

You can see why the Porsche tolerance is 2.4 to 2.8.

[fetuhoe]

How well have you blocked the tensioner?

The variation you have seems quite high unless the tensioner can move a little.

[Gary71]

Absolutely nailed. Any more and I'd break something! It just seemed the dowel had some play relative to the holes. All nailed down though now.

[Kneeslider]

A bit late to this thread, but I'm interested to find out more about SOHC Porsche engines.

Having played with cam timing a bit on other engines, all this talk of the 'middle' or 'one end' of the spec has me a bit flummoxed.

My plan always is to use the dial guage to measure the number of degrees on the crank pulley that corresponds to 1mm or 0.040" of lift on the opening and closing sides of the cam, and then divide by two to get a figure for the centreline of the cam, which doesn't always correspond to max lift, but is a good way of establishing the 'middle' of the cam, and do any subsequent calculations and measurements from there.

The total quantity of inlet advance is governed by how tight a clearance you want to run between the inlet valve and the piston, and a good rule of thumb that has worked for me on bike engines is to never go tighter than the piston to head squish clearance.

So, can anyone tell me about this 'middle' or 'one end' thing? Or am I just being thick?