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Naff Cam timing
zilspeed - 8/6/05 at 08:06 PM

In the process of changing the head gasket on the GTM.
Part of that involves lining up the crank and cams before removing the timing belt.
Having set the crank into the correct position, the inlet cam lines up into position perfectly - however the exhaust cam is sitting 1 tooth retarded.
i.e. the exhaust cam would need rotated clockwise 1 tooth to be in the correct position. Cam pulleys fit into one position only, so there is no leeway for correcting bad timing by adjusting between cam and pulley.

Obviously, I will correct this when I put it all back together again, what I am curious about is, what affect will this have had on performance ?
Will it have flattened the characteristics - like fitting a cam with less duration ? In other words, a bit friendlier, but also less top end power ?

britishtrident - 8/6/05 at 09:57 PM

Haven't got a factory manual handy to check but I know Haynes isn't clear on this anyway

From memory the marks on the sprockets run slightly below the cam centre lines so if you put a ruller across both sprockets the 4 marks line up but the straight line dosen't pass through the cam centre point. Also worth check the bottom sprocket isn't floating on the shaft -- it dosen't have a proper keyway so that have been work loose and frett knocking the timining to hell.
I always check the crank position by measuring down the plug holes and don't rely on the crank mark

A retarded exhaust cam would increase the over lap pushing the power band up the rev range.

zilspeed - 8/6/05 at 10:09 PM

quote:
Originally posted by britishtrident
Haven't got a factory manual handy to check but I know Haynes isn't clear on this anyway

From memory the marks on the sprockets run slightly below the cam centre lines so if you put a ruller across both sprockets the 4 marks line up but the straight line dosen't pass through the cam centre point. Also worth check the bottom sprocket isn't floating on the shaft -- it dosen't have a proper keyway so that have been work loose and frett knocking the timining to hell.
I always check the crank position by measuring down the plug holes and don't rely on the crank mark

A retarded exhaust cam would increase the over lap pushing the power band up the rev range.


I was kinda hoping you would come in here :-)
I think what I'll do is put my own references onto the cam pulleys and head to coincide with the bottom marks lining up. That way if I put it back the way it was I'll be safe.
The E12 female torx tool and cam locking tool should be here tomorrow. That'll let me get on with finally getting the head off.
Compared to getting the exhaust manifold off, it should be a piece of cake. I'm black and blue from getting that bit done, although strangely all of this.