Balancing the crank.

[CrazyB]

I am about to start a rebuild on my m20 for the next autocross season.
I am racing in the DSP class.
I was going to send the crank out to get resurfaced and was wondering if there was any point to getting the rotating assembly balanced since I cant do any lightening and cant modify the top end to rev to higher RPMS

Should I save the money or bite the bullet and pay the extra to get the crank rods and pistons balanced?

[Dj Buttchug]

any reputable machine shop should balance and polished your crank shaft. it is normal to do this with any engine build.

how much are they charging you?

[CrazyB]

well 48 to check the crank for spek and polish it.
If it needs to be ground it will be 140
for balancing the rotating assembly it would be 250. Now I am not talking about just balancing the crank i am speaking of getting the crank, rods and pistons getting balanced together, i do not think this is a standard rebuild procedure.

[jlevie]

Balancing the rotating assembly and balancing the rods/pistons is a major factor in getting the most out of the engine while still remaining stock.When we built the engine that I have in my Spec E30 the rotating assembly (harmonic balancer, crank, and flywheel) were balanced as an assembly and the pressure plate and disc separately balanced . The rods and pistons were balanced to less than .5 grams error. And since crank throws, rod length, and piston height have slight variations, those parts were juggled to get be most uniform deck height. Oh, and the block was bored out 50 over.

The other major factor when building the engine is the head. We had the cam blueprinted with the lobes at the high end of the specification and the other side at the low end. That's not much of an increase in lift and duration, but every little bit helps. The springs were matched and a three angle grind done on the valves. To eliminate stress risers the rockers were polished. And the head was decked the maximum amount by BMW's specs, which bumps the CR up to almost 9:1. That means that the cam timing is off and that was remedied by installing an eccentric bushing in the cam sprocket.

Coated bearings were used and the oil pressure relief valve shimmed to but the limit up to 80psi for a bit more flow at the top of the rpm band.

After tuning on a DynoJet the engine made 160hp at the rear wheels on a +100F day. Because of the rules for Spec E30, tuning consisted of picking the best DME from a set of 6 or so, tweaking the AFM clock spring tension, using an ECT sensor that reads about 10F lower than usual, and using an aged O2 sensor for it's leaner than real indication.

[NC325iC]

sounds like you went through the ole parts bin on that build, a mix'n'match to get the best setup

[Dj Buttchug]

sounds like you went through the ole parts bin on that build, a mix'n'match to get the best setup

sometimes used bearings and such are better than new when building a race engine or blueprinting one. The engine builder can then mix and match to get the exact clearances he wants for the build.

To the op. If your going to do a full bottom end balance I think the price is a little high, prices may vary with location ect ect so in your case it may be a competitive quote. It is a good idea to balance engine parts. It gives better performance and can also increase engine life.

[CrazyB]

So far sounds like one of the cheaper best bang for the buck things to do is to mill the had to maximum spec for higher compression.
Now I noticed you mention doing that would require a new eccentric bushing for timing, but since my class allows adjustable cam gears would that work instead?

[nando]

do NOT grind a BMW crank. if the bearing surfaces require anything more than a light polish, throw it away and get another good used one. You don't want to have to deal with re-hardening the journals and finding the right oversized bearings.

I'd do balancing as a complete rotating assembly if you're going to.

if your class allows it an adjustable cam gear is definitely the way to go to correct the timing issue

[CrazyB]

There is one Sp rules that confuses me, the rules state any intake manifold, does this mean any oem or can I extrude mine or make a custom short runner intake?

[nando]

I think you can run anything - alex s. was running ITB's for a little while on his M20 in DSP.

[CrazyB]

So that means I can modify the stock manifold, port the head 1 inch in from the opening of the ports and bore the throttle body.
I hope this might make a difference since I am running a MAF and headers

[CrazyB]

So that means I can modify the stock manifold, port the head 1 inch in from the opening of the ports and bore the throttle body.
I hope this might make a difference since I am running a MAF and headers

[nando]

I wouldn't touch the ports of the head - there is nothing to be gained by making the inlets larger, all the gains are cleaning up the bore, short side radius, and blending into the valve seat.

[CrazyB]

Which I believe is not allowed by my class