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Aug 12, 2015 1:07 pm
Aug 12, 2015 1:07 pm
Subject: Re: Tappet clearances - why the variation?
Hi All,
You must not assume that the only reason for valve clearance is to compensate for expansion.
Cams are designed with opening and closing ramps of differing profiles and having differing attributes.
cooking engines i.e. those used for shopping for food need to be quiet so the wife can be heard giving instructions and therefore the engineer designing the cam incorporates a "quietening ramp" this can have several forms but the most common is a slight increase of the cam's base circle dia at about 100 deg to full lift and staying all the way to the opening ramp. For this reason it is also not recommended to adjust your tappets at any other position than 180 deg from full lift on the appropriate cam lobe.
A gentle opening is also incooperated so cam to follower "slap" is reduced. RR were very good at this and their quality control was good also which helps to allow for tight valve clearances.
Fast road engines etc. are driven by men. Men don't mind not hearing the wife’s instructions and in fact revel in the fact that they have raughty exhausts and a little noise from the engine so they can feel the power they normally do not have at home. Performance cams open the valves faster and let them close faster, they rarely have quietening ramps. (Bentley designed his race cams with 0.025" clearance and they work well) Heat is surely one reason for increased clearance but only a very small part of it.
Valve recession is also another factor we must consider, the faster we rev and the higher the lift, the more the hammering of the seat and the more the valves recess. 1 or 2 thou can happen very quickly, especially on a newly built engine and it's much much better to have to ask the wife to repeat her last instructions than have to stop and listen to her telling you repeatedly that you should have gone in the Renault because your valve has just burned out on the motorway due to too tight settings.
Won't go on now so have fun and keep it at 8 John, although I know Clair is wonderful to listen too ;o)
Please give her a kiss from me.
Regards,
Julian
Hi All,
You must not assume that the only reason for valve clearance is to compensate for expansion.
Cams are designed with opening and closing ramps of differing profiles and having differing attributes.
cooking engines i.e. those used for shopping for food need to be quiet so the wife can be heard giving instructions and therefore the engineer designing the cam incorporates a "quietening ramp" this can have several forms but the most common is a slight increase of the cam's base circle dia at about 100 deg to full lift and staying all the way to the opening ramp. For this reason it is also not recommended to adjust your tappets at any other position than 180 deg from full lift on the appropriate cam lobe.
A gentle opening is also incooperated so cam to follower "slap" is reduced. RR were very good at this and their quality control was good also which helps to allow for tight valve clearances.
Fast road engines etc. are driven by men. Men don't mind not hearing the wife’s instructions and in fact revel in the fact that they have raughty exhausts and a little noise from the engine so they can feel the power they normally do not have at home. Performance cams open the valves faster and let them close faster, they rarely have quietening ramps. (Bentley designed his race cams with 0.025" clearance and they work well) Heat is surely one reason for increased clearance but only a very small part of it.
Valve recession is also another factor we must consider, the faster we rev and the higher the lift, the more the hammering of the seat and the more the valves recess. 1 or 2 thou can happen very quickly, especially on a newly built engine and it's much much better to have to ask the wife to repeat her last instructions than have to stop and listen to her telling you repeatedly that you should have gone in the Renault because your valve has just burned out on the motorway due to too tight settings.
Won't go on now so have fun and keep it at 8 John, although I know Clair is wonderful to listen too ;o)
Please give her a kiss from me.
Regards,
Julian