Replacing cams: break in?

dtw · 10-15-2009, 10:41 AM

I am replacing cams in an otherwise strong, healthy engine. The cams are freshly ground and the rockers have been re-surfaced. I'll pre-lube the cams with high pressure moly assy lube before starting the engine.

Of course, I will give the engine 20 mins at 2 krpm to bed in the cams/rockers. Beyond that, what other break-in procedures should I observe, if any? Any thoughts on oil change interval after this work?

BURN-BROS · 10-15-2009, 11:43 AM

There really is no break in procedure other than keeping oil pressure high to spray the cams with oil. 20 min at 2000rpm is from the domestic pushrod guys that have no oiling to the lobes other than splash....so they spin them to maintain oil in the area. We have spray bars so we do not have any issues. I have never invested any time into any break in procedure, and no failures as well.

dtw · 10-15-2009, 01:16 PM

Interesting, Aaron. The 20 @ 2k came straight out of Wayne's 911 rebuild guide.

Do you advise an oil change, or pretty much just business as usual, then?

BURN-BROS · 10-15-2009, 05:53 PM

Yea the moderate rpm for 20 min has been recommended for a long time. I asked around a while back why it was and no one had an answer for it. Most cam guys and builders recommend it just to be safe...but there is copious amounts of oil in that area.

As for oil changes, there shouldn't be very much matrerial floating around but a 500 mile oil change/ valve adjustment gives you the chance to check on the work that you did.

prschmn · 10-17-2009, 07:25 AM

A thought on the need for higher than normal brakin idle would be that it is required to develop the hydrodynamic wedge that keeps the rocker off the lobe.
Noone really thinks that the 60 PSI keeps rod bearing from scuffing do they?

BURN-BROS · 10-17-2009, 06:27 PM

Quote:

Originally Posted by prschmn

A thought on the need for higher than normal brakin idle would be that it is required to develop the hydrodynamic wedge that keeps the rocker off the lobe.
Noone really thinks that the 60 PSI keeps rod bearing from scuffing do they?

As long as you have oil on the lobe you have your hydrodynamic wedge. You actually increase loads on the cam as rpms go up.

As far as PSI goes, 60 psi is not the magic # to prevent rod bearing overload. That is determined by piston speed and the weight of the reciprocating mass(among other design factors). Porsche's unconventional oiling to the rods does require more pressure than other designs to maintain minimum pressure at the farthest critical areas.

prschmn · 10-18-2009, 05:14 AM

My point was that at low speeds that wedge is darn thin--higher than normal is just a safety plan with new parts.
If you want to better understand follow the link here.

http://www.utm.edu/departments/engin/lemaster/Machine%20Design/Lecture%2025.pdf

BURN-BROS · 01:05 PM

Quote:

Originally Posted by prschmn

My point was that at low speeds that wedge is darn thin--higher than normal is just a safety plan with new parts.
If you want to better understand follow the link here.

http://www.utm.edu/departments/engin/lemaster/Machine%20Design/Lecture%2025.pdf

No, I got your point. The actual reason stemmed from the complete lack of direct lubrication from the American pushrod engines. At Idle there is very little(read none)oil being supplied by splash....that is all. I have not followed the 2000 rpm rule, and have had zero cam failures. If this was not the case then I should have a 100% failure rate no?

Another thought;

Pushrod valvetrain design requires seat pressures starting at 90 psi on up. It is my experience that you rarely see a failure of the cam untill seat pressures are higher...say 140 lbs. At even higher pressures, engine builders cannot use the"break in" procedure or camshaft damage occurs. This situation stems from rules prohibiting the use of roller lifters. They must remain at idle and lubricate the cam another way. I'll try to find a link for that situation.

Our engines require much less seat pressures ranging from 50 lbs to 90lbs. Another reason the magic 2000 rpm does not apply.

The link was an interesting read, but was really directed at a fully enclosed bearing(circumference) not a open face with constant lubrication... and it is just a theory, one that seems to suggest the opposite of proven empirical data.