Why did they do this????

[Gonzostyle]

The attached picture is a head from an Alpina A4 motor (1970s 2002 BMW) that a buddy of mine is putting together for a customer. This is a very rare motor, that for the time put out a lot of horspower, approx 175hp. I am wondering why they put the divits in the combustion chamber and cut concentric circles on the intake valves (they did both sides of the valves). Maybe for heat dissapation? Anyone have any ideas?

-Jeremie

oh the motor is mechanical FI and NA

[JoshStratton]

I am not sure, but I would imagine it is supposed to act like a golfball and deliver improved airflow.

From http://www.hhp.ufl.edu/faculty/pbird/keepingfit/ARTICLE/DIMPLES.HTM

"The secret of dimples: The golf ball gets tremendous back spin from a drive shot, spinning as fast as eight thousand revolutions per minute. As the ball spins, the dimpled surface traps a layer of air that rotates with the ball, like a small whirlwind around the ball's surface.

Since the ball is traveling forward, the layer of air spinning on the top of the ball is moving "with the wind." And the layer of trapped air along the bottom of ball is moving "against the wind", so it is slowed by this wind resistance. As a result, air pressure builds up at the bottom of the ball, and the ball rises, much like an airplane wing.

The importance of the turbulence caused by the dimples is quite surprising. Without dimples, the ball would travel only about three-fourths as far. But there is a down side to dimpling--the ball has a greater tendency to hook or slice. "

[Madspeed]

Hmmm an odd idea Ill admit.
Here's some food for thought.
Has this ever been done again?

[Gonzostyle]

Josh: good call, I had not thought about that, the size of the intake ports are not just big they are HUGE.

Madspeed: I can only imagine what it would cost to have heads dimpled like that so my guess would be no. Although with a CNC machine........

I think if I could find another motor from Alpina around that time frame that the head might be similiar.
I have seen an Alpina B6 head from the early 79-80ish time period and I know that head was not dimpled.

Just an oddity from the early 70s I guess

-Jeremie

[MoeBawlz]

I remember reading a little while ago in a racecar magazine that Le Mans teams are going with a dimpled type of surface for the idea that airflow will improve much like a golf ball. There was a whole article on that. Another thought is that having those divots may act as a mini combustion chamber to maybe even the downward force out across the pistons face.

[JoshStratton]

"...But there is a down side to dimpling--the ball has a greater tendency to hook or slice. "

I just hope Audi's weren't known for hooking or slicing when they went down the road.

Geeze. I quoted myself, quoting someone else. :getout:

[myclone]

I am wondering why they put the divits in the combustion chamber and cut concentric circles on the intake valves (they did both sides of the valves). Maybe for heat dissapation? Anyone have any ideas?

-Jeremie


Dimpling the combustion chamber was all the rage several years ago and was touted as breaking up the boundry layer of gasses and improving turbulance in the chamber for a more complete burn (better burn=more power). I would assume that the circles cut in the intake valves were for the same purpose and dimples werent deemed a good idea for one reason or another (maybe it was just easier).

IMO dimpling has fallen out of favor since the introduction of better designed combustion chambers via aftermarket as well as production heads that use quench to promote a good complete burn rather than the crutch of using the dimples in a poor or moderate combustion chamber.

My .02 FWIW.

[OHCbird]

When I worked at Roush in the 80s, we had dimpled some of the Aussie / cleveland heads for the road racing program (at the time IMSA, but really a lead in to NASCAR). The dimples were 'tuned' to a specific flow rate (rpm band), and worked well, but were a bitch to get done. We went through about 30 heads trying stuff like that, even flow vanes and port 'canards' that were designed to steer the flow on the shorts side radius.

CNC porting with certain tooling can leave the surface with a desired ridge that can act like the feathers on a bird, which greatly controls the boundary layer air. Seen the stepped hulls on most new sport boats? same effect. Basically, the main flow isn't fighting the surface friction of the boundary layer air- allowing greater tuning of the ports.

[68Formula]

Bringing this back from the dead.

An engine build in the latest PHR had this done. They claimed about 40hp increase on a previous built engine(don't know the total hp it produced, so can't say %-wise how much it helped).

Anybody else have experience with this?

[cheapthrillz]

I remember hearing something like this about Richard Petty and a vinyl top on his racecar....