Lift @ Tdc

[wardy18 33 1 Cars]

Hi all,

 

A quick question for those in the know, how accruate does Lift @ TDC need to be, i set this to the desired 3.65mm on my Catcam a few months but yesterday on removing the head ready for some work i re-checked and it had seemed to settle at 3.95mm now

 

What difference would this have made!?

 

I can only assume that the biggest variable which would affect this is the Cambelt so can the Lift ever be kept accurate to the mm?!

[welshpug 1,666]

how accurately have you marked TDC?

[James_R 3]

You want it to the nearest 0.05mm. It could have shifted due to the belt stretching, the tensioner moving, or the cam pulley moving.

 

when you measured it did you just do it once and go yep done?? I always find doing it at least 3-5x is needed to make sure it's correct as every time you wind the engine back and test it again it can be slightly different.

 

Oh and it would advanced the cam which in theory moves the power band down the rev range

Edited August 4, 2009 by James_R

[wardy18 33 1 Cars]

TDC is marked on my flywheel and lining this mark up with the TDC mark on the small plate sitting over the flywheel, i should do it with the gauge down the bore onto the piston but with the engine etc being ally i find it hard enough to set up one guage securely for the lift settings let alone another down the bore!!

 

i set it then re-checked it and adjusted it about 10x so it was definitely on the mark whn i set it last time!!

 

i think when i next set it with the new head on i will run it up for a while and maybe do an event and then re check it so i can then adjust after everything has settled

 

i assume 0.30mm is quite alot of extra lift

[petert 601]

That probably equates to approx. 2 degrees. Not a big deal in terms of power band, but what about valve-piston clearance? Having the clearance 0.3mm closer could be disaster if you haven't allowed enough.

[wardy18 33 1 Cars]

ooh well its run fine and hasn't destroyed itself but the head is coming off today anyway so i will check to see if they have been meeting at all

[James_R 3]

I thought on the 8 valve engine's there was about 5-6mm TDC valve clearance?? (before skimming)

[Cameron 16]

You shouldn't go winding the engine backwards to check again, you should just wind it forward until it comes back to the right place. That way the belt stays under the same tension.

[wardy18 33 1 Cars]

yeh Matt at QEP said there is plenty of valve clearance with these heads even with my 60thou head skim

[petert 601]

sorry to alarm you, I was thinking 16V.

[DrSarty 90 1 Cars]

sorry to alarm you, I was thinking 16V.

 

It's quite normal Pete, and arguably the correct train of thought.

[leon 1.9 0]

Nah, some people race their cars rather than ponceing around in them so have to stick to the rules!