Regrind cams and eccentrics

decoupe

decoupe

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During the long and very detailed discussion about cams with Paul Burke (hope there isn't a test to follow) he mentioned that regrind cams often require oversized eccentrics on the rockers to compensate for the reduced amount of cross section (base circle) on the cam lobes. So, in effect, you end up with a somewhat "miniaturized" version of the grind spec - in this case a 282 profile.

My question is how would this effect or translate into the performance of the engine? Are the valves not fully opening? So getting the flavour without the spice?

Oversized eccentrics are available from IE. I may have to do this until Paul gets some cam blanks from Europe in order to catch up on back orders.

Thanks and kudos to Paul Burke and Don Lawrence for their patience and insights into the m30.


Doug
 
Small base circle cams are very common in stroked American pushrod V-8 engines where there is a chance of interference between the rod caps and the camshaft. You don't lose any lift because of the small base circle, in fact you gain lift if the lobe height remains the same as the tappet lift is basically the difference between the base circle radius and the radius of the circle scribed by the peak of the lobe.
 
When you reduce the base circle you basically move the follower closer to the center of the cam, which increases the lash. A stock eccentric will only accommodate a limited range of adjustment to take up the slack, so a larger eccentric may be needed. In a pushrod V8, you adjust the length of the pushrods to keep the valvetrain geometry correct.
 
HB Chris said:
Paul did not recommend larger eccentrics for the N21 cam I am buying. I should have it in two weeks.
Click to expand...

This is only for regrinds like IE offers - Paul has had run on his N21 cams and has no more blanks to grind from so I need to make the most of my 282 regrind in the short term.

Doug
 
Mike Goble said:
When you reduce the base circle you basically move the follower closer to the center of the cam, which increases the lash. A stock eccentric will only accommodate a limited range of adjustment to take up the slack, so a larger eccentric may be needed. In a pushrod V8, you adjust the length of the pushrods to keep the valvetrain geometry correct.
Click to expand...

Paul mentioned a base diameter of 1.040" as the diameter at which the oversize is necessary. Still not clear as to what effect the reduced base diameter has on the engine performance if any or is it must wear and noise?
 
I ran an IE reground cam in a race motor. It can be noisy.
The oversize eccentic is a comprimise. It changes the rocker geometry slightly to the valve head. The lift and duration should be the same if the cam is designed properly, but there might be more stress on the rocker arm. For mild cams (bellow 300) this is not a problem.

I think if I have gone tighter on my valve adjustment is would have quieted it down.
 
decoupe said:
Paul mentioned a base diameter of 1.040" as the diameter at which the oversize is necessary. Still not clear as to what effect the reduced base diameter has on the engine performance if any or is it must wear and noise?
Click to expand...

In an engine like the M30 with a fixed fulcrum on the rocker arm, reducing the diameter of the cam base circle requires that the rocker arm be rotated towards the cam and additional means used to take up the slack in the lash on the other end. Since both ends of the rocker arm scribe an arc in operation, you are changing the portion of the arc in which they operate, usually for the worse. Factory designs are such that the arc scribed by the rocker tip as it actuates the valve results in the minimum side loading of the valve. Changing this can cause accelerated valve guide wear.

Valve train geometry also has a great effect on the valve lift at any point on the curve. You want the geometry such that you are taking maximum advantage of the cam lobe. My main experience is with pushrod V8's, and they have a great deal of adjustability. You can raise and lower the fulcrum, change the length of the pushrods, add lash caps, change the rocker arm ratio, etc.

Here's a treatise by Racer Brown on OHC Datsun camshafts with a section entitled Rocker Arm Geometry, some very good info:

http://www.datsport.com/racer-brown.html
 
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