11 replies to this topic

[mars red mike]

I was reading a Mini tuning book this morning specifically the section on cam timing. It says that the dot-to-dot method due to manufacturing tolerences can be anything up to 15 degrees out. Does this mean that on a stock 1275 MG Metro lump there is potentially more power to be gained if the cam is timed in more accurately?

[Cooperman]

A 15 degree error would be at least one tooth on the cam sprocket and I've never seen this much.
However, the tolerances are poor and sometimes the 'dot-to-dot' method comes up exactly right, whilst on other engines it can be a long way out. Up to about 5 degrees is not unusual, although 2 to 3 deg is more normal. I was surprised when I did a 1330 engine in July and it came out at 9 degrees retarded. That's the most I've ever seen and it was an MG Metro cam, so i guess the errors came from the sprocket key-ways and, maybe the crankshaft keyway slot
In the 60's almost everyone used the dot-to-dot method, which is, I guess, why some engines were so much better than others. With aftermarket cams and old BMC/BL/Rover tolerances it is always wise to do an accurate timing of the cam using either an offset woodruff key or, more expensively, a vernier cam pulley.
Yes, it does have a significant effect on performance if more than a couple of degrees out, especially with higher performance cams.

[GraemeC]

I have recently fitted a billet SW5 cam and that was over 10 degrees out.

The MG engine in question would need it's present timing using dot-to-dot checking and, if it was not ideal for the cam, correcting. This may well unleash a bit more performance.

[Cooperman]

On my own rally engines, with a new timing chain I usually time in my cam to 2 degrees advanced from recommended as the initial chain stretch will correct this. I once did a check after one rally and about 88 miles from new and the cam had retarded by about 1.5 degrees, so the 2 degrees retard will make the timing correct for longer. After about 6 events it has stretched to give about 1 deg of retard if set at 2 deg advance when new.
Graeme, the 10 deg error you found was worse than my 9 deg with the MG Metro cam recently.
I reckon that when doing it 'on the bench' it's easy to use the offset keys, but in-situ with just the rad and timing cover removed a vernier sprocket is easiest. Not, in itself, a difficult job so long as you have the right tools and a good DTI and engine protractor.

[GraemeC]

My problem was that I use Rollmaster gears which only give around 6 degrees of adjustment
Ended up with an offset key to put it back into the mid-point of the Rollmasters.

Admittedly the error could've been down to a stack up of errors in the cam and timing gears but I was suprised that a billet cam ended up so far out - a reground one I could've understood a little more!

[dklawson]

I deal with tolerance stack up every day (though not on engine parts). EVERY component contributes a lot. 9 and 10 degrees is a lot in total but easily possible with keyways cut in four components. I used an offset key when I installed my SW5 and it needed correcting by about 3 degrees. I did not set the additional advance that Cooperman suggested... but in hindsight I probably should have.

[Cooperman]

To be honest, I didn't actually come to the slightly advanced setting by a sudden inspiration. What happened was that after one rally I had an unrelated transmission problem (not unusual with my rally car!) and whilst the engine was out I decided to just check to see how much the chain stretch had effected the cam timing after such a short running period.
I was a bit surprised when it had retarded the 286 cam which I had set to exactly 106 deg ATDC with my vernier sprocket, by 1.5 degrees. When I fitted another new chain I set it to 104 deg and then, after about 6 events when the engine was due for a 'refresh', I checked again and the cam was at 107 deg, so I reckoned that 2 deg was just about the right amount of initial advance above the manufacturer's setting fugure.

[bmcecosse]

I have in the past laughed at people setting up cams to be 'spot-on' - knowing full well that after a very few miles the timing will have sagged back by several degrees! You are quite right to set in some advance - I would say up to 5 degrees! But some engines will run better with extra advance anyway - and some like the cam to be retarded - it's a matter of balancing top end power against lower end torque. Would need extensive trails on a RR with many different settings to determine what's best. I once set up a so called 'sports' cam a full tooth advanced to see what would happen - it ran MUCH better than before! Th easiest way to check the cam timing - is to measure the overlap lift at TDC on the inlet and exhaust valves - they should be identical!

[Paul Wiginton]

Personally Ive always done the 2 degrees advance with a new chain, but a lot of people Ive spoken to say they will never stretch. I (like Cooperman) have found they do. But since I dont use my car on the road so much now I tend to set it correct and reset it after each event as its a relatively quick and easy job when you have done it dozens of times.

Paul

[Cooperman]

I think perhaps 4 or 5 degrees would be OK for an engine which will be doing, say, 30000+ miles before it comes to pieces again and has the chain changed. Is it me, or do current chains stretch more than the original duplex ones fitted to the early Cooper 'S's.
Bmcecosse, you are right when you say that sometimes timing different from the manufacturer's recommendations can work well. Clearly the recommended settings are for average applications. You need a proper dyno to experiment if you are wanting to use the car for a specific branch of motorsport. I wish I had one here. Peter Baldwin is quite near here with his rolling road, but you can't really experiment unless you have a dyno.
I change my chain quite often, every 6 or 7 rallies, and it has always stretched by about 3 or more degrees in cam timing terms. You figure of 5 deg would seem fine for a long-life unit

[Paul Wiginton]

I have heard of a new chain that does not stretch, dont know anything else about it yet though

Paul

[lrostoke]

http://www.calverst....;products_id=71

This may be the one you are thinking of, remember reading about it in one of the mini mags, and claims of no stretch, and being better than most duplex chains.