1978 200ci build.. Go...

Howdy Back Harris and All:

Distributor "Curve" is the function of the centrifugal advance built into the distributor. It allows the spark to increase, or advance, as engine rpms go up. The rate of spark advance is controlled by weight arms restricted by springs. As rpms go up the weights are thrown outward by centrifugal force. How much they are thrown out is limited or controlled by springs. The stock curve is very conservative/slow. Not ideal for performance. Recurving a distributors advance is a fine art, based on the engines attributes such as cam timing, compression, gearing, etc. The centrifugal is recurved by altering the weights and selecting the tension of the springs. The recurve is best done on a distributor machine by someone with experience is applying the various factors. In general, for better performance a centrifugal advance that starts above idle rpms and is all in by about 3,000 rpms.

Centrifugal advance is one spark timing factor. The others are the initial advance setting and vacuum advance. Ideally, all three should work together for the best in performance and economy.

Follow Chad's advice and check out the CI archives he listed.

Adios, David

Okay, I'm being told that if I mill the head .07 (seventy thousands) that I'll need adjustable rockers..

Any input..

I'm sure you could get around a 0.070" mill by shimming the rocker pedestals. I would recommend adjustables regardless. I just like the ability to fine tune things. It just depends on what your budget is. Washers are cheap. New rockers aren't. You might be able to find a set of questionable condition, used rockers from ebay. If you've got the dough for a newly rebuilt set, give http://www.vintageinlines.com a glance. These units have been rebuilt by a company here in the US that does fantastic work and has great customer service, as does VI.

Do you have your cam figured out? This will help you figure out where you need to go from a rocker ratio standpoint.

Edit: Just to be clear, I don't advocate shimming the pedestals. Just because you can do it, don't mean it's to be done...

don't go .070
leave some 4 nxt machining (if needed).
.050 is plenty - that will moer than do any clean up needed.
less is better - shoot for 9:1 or up to 9.5:1 to use 87 octane - U said "DD" not
'racer'.
Caculate ur CR so U can advocate w/yer machinist from a good knowledge base. Mine pushed me around (did it his way) cuz I had little knowledge to tell him what I (guys here) wanted & Y. It's far from a bad job but they don't know these motors like we do. The theory's all the same w/any engine but we have lousy intakes, other issues others have their own wrk around stratigies not accessable 2 us…

keep talkin

Howdy Again Harris and all:

No, You will NOT need adjustable rocker arms with a .070" mill cut. .025" will be taken up by the extra thickness of the thicker Composite head gasket. The hydraulic lifters can compensate for about .120", so the remaining .045" is covered.

I'd still recommend adjustable rocker arms.

Be sure to reestablish the oil transfer slot in the back of the head. And clean the bottom of the head bolt holes thoroughly. If the head bolts bottom out when torquing the head, use hardened washers as shims.

Keep it coming.

Adios, David

Why do you guy's always recommend adjustable's if they aren't needed? Seems like it's just another thing to fiddle with and worry about(read: screw up for me ). On my 250 I took .034" off the the deck and minimal off my head and went back to the non-adj, I didn't want to mess with them after having a 170 w/ solid lifters.

Thanks David. I was confusing shimming of the head bolts with the pedestals. Apologies for the confusion. Raising the pedestals would screw up the geometry....

I like the adjustables. Yeah, it's one more thing to set but I didn't think it was too bad.

Since the block is already at the machine shop; cut the deck instead of milling the head. It only takes about 2/3 of the same cut to achieve the same resulting rise in compression, and you're closer to gaining some quench. Milling the head doesn't bring the piston any closer to it.