<VV> Turbo and Dual exhausts [LONG]

[FrankCB@aol.com]

Jim,
       Personally, I've always felt that the small YH carb is far more 
restrictive to increases in boost than is the stock exhaust system downstream of the 
turbo.  Someday, when I get the EFI installed on my 180 with a different 
throttle body but with the stock exhaust system, I'll find out.  Before then, I'll 
install a vacuum gauge downstream of the YH (upstream of the compressor) to 
find out just how restrictive the stock YH is.  I remember somebody (perhaps 
Bob Helt??) quoted the figure of 7 in. Hg. vacuum downstream of the smaller YH 
on the 150 engine.  I think the old "hot rod" rule of thumb with carbureted 
engines was to have somewhere in the range of 1/2 in.Hg. to 1-1/2 in.Hg. at full 
load.  Anymore than that and the carb was felt to be too restrictive and a 
major limiting factor in the quest for more power.
       Before you make any changes related to increasing boost pressure, be 
sure to install (even temporarily) a REAL boost pressure gauge.  The stock one 
that came with the Spyder/Corsa was grossly inadequate even when it was brand 
new.  Now after 40 years, you're lucky if you can get it to show ANYTHING in 
the boost range, much less an actual numerical value!  You want a gauge that 
has divisions for EVERY single psi of boost, some have divisions for every 1/2 
psi.  Incidentally, if it shows in.Hg (inches of mercury) that's OK since 2 
in.Hg is equal to 1 psi.
       Years ago, when I used a simple big Sears "tune-up" gauge, I was able 
to see the result of insulating the exhaust pipes upstream of the turbo.  
Driving the same long uphill run on the way to work, the boost increased from 8 
psi to 10 psi AND the boost "came up" much more quickly.  The disadvantage was 
that the 90 degree bend in the crossover DISINTEGRATED after about 6 months.  
The metallurgist at work, after his examination, said it was from 
overtemperature, not corrosion.  I also noticed that the crossover-to-manifold bolts had 
loosened on both sides, due to the greater expansion than normal of the 
crossover pipe from that same overtemperature.
       Anyway, the important thing is to measure BEFORE making changes then 
when you measure AFTER the changes, you'll really know the result.
       Frank "measure before changes" Burkhard     
       
In a message dated 3/1/04 7:27:59 PM Eastern Standard Time, 
simpsonj@bellatlantic.net writes:

> F.B.I. -- maybe, maybe not.  Yes, GM did design in back pressure to limit 
> boost (also the pressure retard, also the restrictive Carter YH carb, also 
> staying with small valves, etc...) but that's no reason not to open things 
> up a bit.  Of course, you do run the risk of grenading the engine, but if 
> you are willing to put the effort into really opening things up, installing 
> dual turbos, and so forth, then minor things like waste gates, knock 
> sensors, fuel injection, and so forth should be all part of the package.
> 
> Of course, as you point out, that still leaves the issue of cooling the 
> engine.  As they say, a short life, but a merry one!
> 
> Jim Simpson, '66 Corsa turbo (basically stock restricted...)