Valve timing

[Zetomagneto]

I have a standard cam with 3/4" radius followers.Generally speaking, advancing the valve timing will increase power in the lower rev range, the tradeoff being loss of power at the top end.
Does anyone have firsthand experience of varying the timing, if so was it worthwhile, how much advance was given( crankshaft degrees, or inlet valve lift at TDC) and how accurately were the percieved gains measured.
Is the gain because more mixture is being drawn in, or is it that the dynamic compression ratio is increased by virtue of the inlet valve closing early.
Any informed comments would be appreciated.

[Alan]

To perpetuate an analogy started by Jack French, engine breathing works in exactly the same way as a public house. Therefore it’s far more important that the inlet valve closes late than opens early.

[Speedex750]

Hi Z, yes the Jack French pub analogy is a good one, later inlet closing gives a little more at the top end - but depending on how much you've retarded it - you might loose a little bottom end. I believe the gain comes from getting more charge into the cylinder before the valve shuts - the dynamics in the inlet system are still trying to get past the valve when it shuts, just like the pub landlord slamming the door when you fancy that last minute pint.

I would start by checking what you've already got in terms of timing with an accurate positioning of TDC and the opening and closing times for an inlet and exhaust with a DTI. There seems to be a variability in the accuracy of timing gears, keyway to gear tooth, and you could try different crank gears and see what effect they have. The definitive way is to rotate the gear and broach a new keyway but a simpler alternative is to make an offset key from an old flywheel or halfshaft key. From memory a 10thou offset gives about 3 degrees change to the timing.

As to what to set it to - I've seen a number of different standard timings so I'd be interested in what you find. It seems to me that the 'right' timing is for maximum inlet lift at 110 degrees ATDC. I wouldn't go more than about 5 degrees retarded from that but I'm sure we'd all be interested in how you get on?

Hope helpful. Dave

[Bob Culver]

Several have observed that timing with many aftermarket gears varies greatly. But few seem to have closely observed any effect. 
The first Sevens were very light and were not expected to be driven much beyond 25-30  mph, so if any model suited advance it would have been these in their day. The later heavier cars expected to be driven faster relied on revs to go at all so any advance would seem less appropriate.

The inlet timing duration of the Seven is modest but other cars have been less ie Morris 8 ohc and early sv. The  LIP Vauxhalls circa 1950 4 34 47 4 is similar the Seven advanced. (The timing of Mercedes about 1980 2.5l 123 is given in manual as 15ATDC 21 21 11 and the 2.7l 110 engine as 7ATDC 21 30 12!?)
 
When checking timing cam wear somewhat confuses. The mid point of two places of reasonable lift the best indicator.

[Stuart Giles]

have never knowingly advanced the cam timing, the only car I came across where the cam timing was advanced: mis-matched timing gears "timed" by lining up the dots, had very poor performance.  I always "dial in" the cam and usually set the maximum inlet lift to occur at 110 degrees ATDC -the standard timing- Certainly shouldn't  hurt on a light weight  car with decent breathing to retard the cam by a couple of degrees, I understand from someone who knows a lot more about this stuff than me that it's the late exhaust closing that delivers the top end power increase.

[Tom Coates]

I have a Paul Bonewell modified cam which has a stepped key to retard the valve timing by a few degrees. He suggested making up a vernier arrangement on the cam gear which would be a nice thing to have but a bit of a fiddle to make.

The car goes very well, but sounds a bit flat compared with the previous pigsty cam.