GM TBI Manifolds

[bruce plecan]

As you may have noticed some of the GM TBI manifolds have a coolant
passage the full lenght (from front to rear in addition to the cross
over by the thermostat housing).  Also, on some GM cars the heater
core is full ON, All The Time.  So coolant is flowing all the time,
under the manifold.  What they do is use a blend door to bypass air
around the heater core.  Plugging off this heat by not running a
heater core, or even a bypass will make for a slow car, that gets
bad mileage.  However, running a 5/16-3/8" inline restrictor has
had good results for me.  Some others use the EGR runner to heat the
manifold.  On either type, I've gotten better hot weather perforance
by installing a lifter valley tray, to keep the lifter valley oil
splash off the bottom of the manifold.  If your car has the type of
heater that is always on, and you swap engines for one that uses a
water pump bypass, you can plug off the bypass hose, since the heater
core now does that function.