7A to 3B conversion using 7A head and MC2 block?

[Jim Green]

--- Ben Swann <benswann at comcast.net> wrote:
> Could I use the MC-2 Pistons and NG/NF crank and
> rods or???

Nope, MC-2 rods are 136mm long, 46 mm journals, NG
rods are 144mm long 48 mm journals.


> I'm trying to understand the differences more
> clearly.
> 
> I thought the 3B bottom end was nearly identical to
> the MC-2.  Where is the
> compression difference?

Ben, the compression is always changed with the
pistons if you stay in the same familly.  Now, the
volume in the heads is different between the 10v and
20v, so when you start swapping heads between the 10v
and 20v blocks, things change.  Putting a 20v head on
a 10v block will always lower compression.  That's why
if you put a 20v head on a high CR 10v block like the
NG, it drops to ~8.5:1

> Is there a major difference in the 7A head and 3B. 

The turbo heads have extra cooling jackets.
http://audi.newtsplace.com/forum/viewtopic.php?t=69

> I take it the cams are
> way different, or can compensation be made by
> sprocket 

Probably not, one tooth on a 20v sprocket is a pretty
large change in cam angle.
 
> I have the feeling I may be better off dealing with
> the MC-2 bottom and MC-1
> head and FI that I was going for originally. Then I
> can implement EFI
> solution.

See that thread Marc pointed you to.
http://audi.newtsplace.com/forum/viewtopic.php?t=84&postdays=0&postorder=asc&start=0

Dig through the three pages that are there, there's
some really good info.

With either of these you need to bolt on big power
parts.  450 hp on a 10v is entirely possible, on an MC
bottom end, maybe more.  The 20v is less limited
because of head flow, but the lower has to be beefed
up at some point.  So what's your goal?


=====
Jim Green
'89 90tq
'89 80q
http://www.mswanson.com/~jgreen/car_home.html

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