NG horsepower upgrades

Alan Pritchard apritchard at
Fri Aug 27 04:01:07 EDT 2004

Thanks Huw, few comments...

>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.
Haven't physically seen one yet, only in photos, mine is still surface
shipping somewhere ;)

>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.
Oh, Ok, maybe not as good as I thought it was, only thought there was a 2
part from another listers comment...
Equal length headers are only good at producing max torque at a narrow rev
range, unequal lengths produce a wider spread of power.....iirc

>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...)
Yeah, if I could get one for a good price that would be the way forward. But
cost is a major issue here, most of the stuff I am doing is costing very
little, on a month by month basis, alternatively id have one fabricated, but
I simply cannot get my head around designing a 5 cyl header ;)

>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 
too large a diameter reduces the scavenging effects on a normally aspirated

>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.
Yeah, few other things there though... 10v being non crossflow...
The 20v having a better combustion chamber due to its pent roof (is that
right?) allowing a higher cr.....
>I also like your idea of *really* opening up the intake tract...
yeah, really considering some kind of individual throttle body setup, just
for fun ;)
or a bank of standard throttle bodies on the stock plenum... I dunno,
haven't thought that far yet.

Best Regards,
Alan Pritchard

Network Administrator
Mechanical Design Engineer
Seaeye Marine Ltd.
+44 (0)1329 289000

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