CQ/7A turbo conversion questions

[Richard Hoffman]

Hello Chris!

As Denis mentioned, putting a 20v head on an NG or NF block will yield the
compression you are looking for.  If you want to keep the 7A around and have
to choose between new pistons or new rods, I'd go for new pistons.  Pistons
are already available from JE in the 8-8.5:1 range and a set of forged
pistons A)is cheaper than custom length rods B)addresses the weakest point
in the bottom end (the pistons) and C)guarantees a fresh bottom end.

Either way you look at it, you'll be in there, replacing the rod bearings
and such.  If you get new pistons say in 83.00 mm bore, you'll have the
block bored and honed and with a fresh set of rings you'll have a fresh
bottom end.  While you're there, might as well do a full rebuild - its not
much extra work at this point.  Or not, just do rods bearings and new
pistons/rings.  This is certainly a better idea than new rods and reusing
the weak NA pistons which may or may not be worn, oval'd, or whatever.  And
if you are throwing a turbo into the mix, you'll only shorten the life of a
high-miles motor (not that 130k is high, but it sure isn't low)

Another thing to consider is that shortening the rod affects how the motor
revs, how smooth is it, and also how much power it produces.  The critical
component here is the rod length/stroke ratio.  Ideal is 1.7:1.  Stock rods
are 144mm, stroke is 86.4 - R/S ratio of 1.667:1.  Sure 3mm isn't a lot
shorter, and it only lowers the ratio to 1.632:1, but the 20v is known for
strong high rpm operation.  Shouldn't it stay that way? :)

Anyway, planning a project like this is a lot of work.  Good luck
Richard

----- Original Message ----- 
From: "Chris Thorp" <thorp at spacia.org>
To: <quattro at audifans.com>
Sent: Thursday, April 15, 2004 9:32 AM
Subject: CQ/7A turbo conversion questions


> I'm looking to do a turbo conversion on my '90 CQ.  I'm planning on
> rebuilding a motor and going to an aftermarket EFI system (probably
> Javad's).
>
> What I've currently got:
> A 1990 CQ with ~130kmi
> A 5000cstq with ~210kmi
>
> I was planning on doing an MC1 swap into the CQ, but the more I looked
> at it, the better turboing the 7A looked.  I know there has been a fair
> bit of discussion on this, but I have yet to find solid information on
> what it would take.  It looks like I could run the 7A's head as-is
> because it already had sodium filled exhaust valves, reasonable cams,
> and is a 20v.  I would need to make a header with appropriate flanges,
> do the intake and IC plumbing, lower the compression, and maybe better
> crank bearings.
>
> To lower the compression, it looks like my options are: more/thicker
> headgaskets (kludge), or new pistons or connecting rods.  I did some
> calculations, and to get to 8:1 from 10.3:1 would take a rod that was
> 3mm shorter.  Is calculation correct and doable?  Or would new pistons
> be a better/cheaper route?
>
> Beyond these changes, what would I need to do?  As this is my first
> major project, am I missing something really obvious (it seems like this
> isn't really ever done)?
>
> TIA,
> Chris
> _______________________________________________
> quattro mailing list
> quattro at audifans.com
> http://www.audifans.com/mailman/listinfo/quattro
>