(Bit off topic) turbo lag...

TVS tkmvs at bigfoot.com
Thu Jan 11 00:11:47 GMT 2001


   Thanks for the replies. Perhaps I should go into a bit more detail as to
what I'm looking for and how I think I can achieve it. The car is a 1970 MG
Midget which will weigh about 1200lbs. I've heard that a T3 on one of these
engines wouldn't start making boost until around 4000rpm, the cam I've got
is a "Sport"  which give max torque at about 3,500 rpm and max power at
around 6,500rpm assuming std sized valves/ports. This together with a CR of
around 9.5:1 and using 98 'super unleaded' should give reasonable
performance off boost...during normal road use. I need adequate torque
between 2000 and 3000 rpm, which this should provide, to allow an overdrive
to be used giving 20mph per 1000rpm. I'm also planing on using a large,
separate, EGR valve to bypass the exhaust around the turbo to reduce
back-pressure during cruse. In all, I think this should give very good
economy.
   Once past 4000rpm it should start to make boost, which will be varied to
keep the torque at around 80/85lbsft all the way up to around
8,000/8,500rpm.... asuming it doesn't knock at 4000rpm/85lbstf then I don't
see why it should at 8,500rpm? (But I'm probably wrong!)  So it should make
120-140Hp. Boost regulation, retarted igniton, and perhaps water injection,
would be used to try and give the best power curve. This is the sort of rpm
range that would be used for hill climbing and sprints.... and overtaking on
the road!
  It seems that useing  a large throttle bodey to recirculate the air
thought the turbo during gearchanges would be the easiest way to reduce lag.
I wounder if air could also be circulated below, say, 3,800rpm then the the
buterfly progresivly shut....  turbo lag would also be reduced when when
accelerating past this point?
  Please excuse my ignorance, but I was unaware that exhuast header design
was particulary important on a turbo engine! Is it posible to take advantage
of tuned lengths? (I'll seach the arcives) Most turbo headers I've seen are
very crude, cast, and contain almost right angle bends!
  Many thanks, Toby

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