Turbo stuff

Mon Sep 10 03:33:52 EDT 2001

    I've been trying to not say anything because I am by far not an authority on turbos, but I had a freind that put one on an old Rabbit GTi with a stock motor from something newer.  He used a 1.8L 8V with low compression pistons.  It was completely custom that he did his self.  The turbo ran at 13 PSI, and everyone was amazed at why the car didn't blow up.  He said his secret was that the turbo was smaller but he specially picked one for it's CFM.  He said the CFM (Cubic Feet per Minute) was what made this setup faster then if he had a larger turbo, and it still spooled fast because it was smaller.  Like I said, I'm not a turbo expert, but it makes sense to me.

       Rave Racer
'89 Jetta 1.8L 16V GTX
 http://www.vwot.org/members/Pete.html
'87 Audi 4000 Quattro Sedan
 http://www.audifans.com/registry/view.php?action=viewCar&carid=110
'72 Triumph GT6
 http://motorcities.com/contents/01I3H011116682.html
'83 Toyota Tercel (yoda)     Possible future Sandrail donor...  Maybe
  ----- Original Message ----- 
  From: Scruggs Family 
  To: quattro at audifans.com 
  Sent: Monday, September 10, 2001 12:27 AM
  Subject: Turbo stuff

  I'm surprised in this round of turbo discussion that no one has mentioned "Maximum Boost - Designing, Testing and Installing Turbocharger Systems," by Corky Bell.  Published in '97 by Robert Bentley, it is very highly regarded reference on turbos.  There are chapters on intake design, exhaust manifold design, intercooling, fuel systems, sytem integration... everything garnered from his years in the automotive performance business.  He writes in an easy 'been there done that' style but the inclusion of many charts, graphs and formulae reveals his mechanical engineering background.  The 27 page chaper on intercooling even shows the best way to attach hoses and how to/not to duct cooling air.

  About three years ago I emailed Corky Bell to ask what he thought the best turbo combination would be for my '87 5kCStq.  I related the size and configuration of the engine and what I wanted from it... torque starting as low as possible building to about 325hp and holding there to 6krpm... we can dream, right?  Like the list seems to be I, too, was wrapped up in whether or not the K24 or K26 or RS2 or some hybrid is the answer to our needs.  His answer was surprising.  

  First he said that he was not fond of the KKK turbos as they were... I'm paraphrasing here as that email is long gone... a rather dated technology.  He said to go ahead and use the OE K26 as baseline reference and recommended a new turbo with a "one size larger turbine and a two size larger compressor."  Then he recommended the Garrett T3 series as being a modern, efficient, reliable turbo that can be had in a variety of trims to suit most any application.  

  I'm not intending to start any huge discussion on whether or not our beloved KKK turbos are up to the task.  Clearly there are tens of thousands of them out there whirring away providing service all over the globe... not the point.  I recognize the attraction of being able to mix and match the 24/26/RS2/hybrids then bolting them onto our existing manifolds and downtubes.  It is certainly the easiest route to more and tailored horsepower to feed our addictions.  However, if you are budget constrained like me then before we lay out $1500 for that RS2 alone why not see whether or not there may be a better answer?  

  I have not yet investigated whether or not the Garrett, or any other make, is built with the same in/out mechanical configuration as the KKK such that the adaption to our cars would be relatively painless.  I was kina' hoping that someone would begin making a better exhaust manifold for the 10V cars before designing the system around a new turbo.  The OE two-piece exhaust manifold is a significant upgrade from the single-piece one and may be the best short of having one made.  I also know that my goal of 325hp is outside the range of the CIS and... in the absence of a good mod to the CIS... the alternative is an EFI like Graydon Stuckey built for his former '86 5kCStq.  

  Back to Bell's recommendation... the suggestion that both components be larger than stock may seem counterinutitive.  General wisdom is that if you want a turbo to spool up quickly then it must be smaller... right?  Certainly there is less rotational inertia but that is not the only effect in play.  Holding everything else constant, the larger the turbine wheel the greater the radius at which the exhaust stream acts... longer lever arm means more torque to turn the compressor and spool-up is quicker at lower rpm.  In reality there are many other factors which impact the ability of the turbine section to do what we want it to do, among them are: efficiency of the exhaust manifold, size/mass of the compressor section that we are going to drive with the turbine, A/R of the turbine section (this is called 'trim' and is a little discussed but very important factor)... on and on.  

  Reason for the larger compressor is clear... to move more air at higher engine rpm... but move it at the point where the larger compressor is still efficient and unnecessary heat is not added to the flow.  Bell does a chapter on criteria for sizing and selecting the turbo proper.

  Recent conversation on the list have involved intercoolers.  I looked for a good used single-pass intercooler... with all the hardware... for a couple of years and came up dry.  The OE single-pass retrofit is a good idea for the ease of installation but not ideal because of the plastic endcaps and general unavailability.  I am now running an all aluminum single-pass intercooler made from the OE core... welded aluminum endcaps, with by-pass valve, aluminum crossover tube, silicon hoses ... made by fellow lister James Marriot.  James is nearing low-rate production capability and will notify the list when he is ready.  After I installed James' cooler I was greatly surprised to find that my clamped/strapped OE cooler had been leaking and, according to my A/F meter, changing the mixture at elevated boost levels.  Typically I run about 14 lb of boost governed by an HKS off board boost controller.  

  Regards, Gross Scruggs

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