NG horsepower upgrades

Huw Powell audi at
Thu Aug 26 22:10:27 EDT 2004

I've just been running around measuring exhaust junk I have lying 
around, out of curiosity... although the only 5-2-1 downpipe I have 
access to is on my 90Q.

> Well, imho the downpipe...
> On my ng the outlet of the downpipe is very close to 2" diameter, I do not
> believe that to be large enough. The 4kq is 2.5" I believe....

The 4kq is about 2.25" (probably 55 mm).  The 90Q version is 0.2" 
smaller on the outside, so it is probably 50 mm.

However - the 4kq outlet is smaller than the collecor is a few inches 
forward of the gasket flare area, so if it is cut a bit shorter and a 
custom joint is prepared, it will be more like 60 mm ID.

> Also, stock 90q manifold.... 5-2-1..... there is no "tuned length" in that
> setup as far as I can tell. The 4kq is 5-3-2-1 iirc... in the "3" section of
> the downpipe one of the tubes is a smaller diameter iirc, that is because it
> is tuned for 1 cyl only, the other 2 downpipe tubes share a cylinder each,
> hence the differing diameters. 

You are half correct here - the 4kq piece (which is very nice for a 
production part), has equal *diameters* for each cylinder all the way. 
(and by the way, the 3 to 1 convergence is at the same point, there is 
no 2 pipe part).  It is not equal *length* at all, so it's not tuned for 
scavenging, etc.  The gas paths are longer as you go forward by the 
distance between the exhaust ports.

The 90Q junction where there are two holes is definitely a compromise on 
the equal diameter issue, of course.  1,2 and 5 go into one pipe that is 
about 0.2" bigger than the larger 4kq holes, and 3 & 4 share another one 
of the same size.

> Likewise it is very easy to see how the 5-2-1 example is not
> as good as it could be, and was made as a compromise on cost.  My opinion
> anyway.... Either way the small outlet on the stock dp is too small, imho..

I guess what I am really trying to say has two parts:

1. (irrelevant to your all-out project) It's really not worth *just* 
swapping in the 4kq parts to an NG set up car.  The 4kq parts, along 
with a bigger cat *and* a larger overall system, only gains 5 hp over 
the really crappy old coupe/4k "log and downpipe".  So there's not a lot 
to gain there over the better 90Q style EM/DP.  Although, if the engine 
is out, or the exhaust manifold off for some reason, it makes sense to 
throw on the slightly nicer 4kq parts, especially now that they are dirt 
cheap used.

2. (applies to your project) I don't think the 4kq set up is up to the 
standard of everything else you are doing - if cost or ability/tools 
permit, what would *really* make sense on your n/a monster engine would 
be a custom made, equal length free flowing header.  Years ago, I think, 
these were actually available (remember the one that Carter had and 
sold, which may even have been Javad's a long, long time ago...)

People are doing this for their turbo projects, and you don't even have 
to cram it all together into a turbo right away, you've got a bit more 
room to play with pipe.

Also, does anyone have the capability for determining the ideal back 
pressures from the exhaust on these n/a engines?  As I understand it, if 
the exhaust is opened up too much, you actually lose power (or is it 

Oh, and also remembering Javad's flowing of the 10v vs. 20v heads, that 
might provide a sort-of benchmark for getting rid of CIS, right?  If the 
ten valve engine was at 130 hp, and the 20 v at 160, the only other 
difference would be actual efficiency improvements from the EFI 
management - which you'd also get, anyway.  Oh, yes, the 20v also had 
5mm larger exhaust pipes.

I also like your idea of *really* opening up the intake tract...

Huw Powell

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