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Re: mgb rpms

To: NKED65A@prodigy.com
Subject: Re: mgb rpms
From: GOFASTMG@aol.com
Date: Tue, 26 Nov 1996 12:28:41 -0500
Bob,
   The main reason the B (GT or Roadster) seems reluctant to rev to the
redline really has to do with the state of tune, not the 5 main bearings.  
  In it's stock form, the B engine simply runs out of breath at the upper end
of the rev range.  With a bit of judicious port, intake and head work, along
with the proper camming, the B will rev quite sufficiently to scare the
living Bejeses out of you.
  The proper point to shift is roughly as you stated, Shift from the lower to
the higher gear so that the engine speed in the higher gear is at or near
peak torque spec. This is a good rule of thumb for street use, and will given
the gearing on the B, pretty much keep you out of trouble.
  For performance driving, though, it's a bit more complicated. The proper
point to shift from a lower to higher gear is the point at which the rear
wheel thrust in the higher gear is greater than the thrust in the lower gear.
 Since rear wheel thrust is a function of engine torque, trans. gear ratio,
rear axle ratio, and tyre diameter, the formula gets a bit complex,
especially when you have to know not just the PEAK torque, but the specific
torque at all engine speeds (Ie. have an accurate torque curve graph). But to
make it simpler, because the rear axle ratio and the tyre diameter are fixed,
we can make do with a measure we'll call prop shaft torque. This is simply
the engine torque at a given RPM times the trans. ratio. What we are after
then is the point in the RPM curve in say, 1st gear, where the propshaft
torque is less than the propshaft torque at the engine speed when in 2nd.
 For example,
  1 engine speed in 1st = 5000 rpm
  2  engine torque at 5000 = 100 ft/lbs
  3 Tran ratio in 1st = 2.63
  4 Propshaft torque (#2 times #3)= 263 ft lbs
  5 Engine speed after shift to 2nd = 4100 rpm
  6 engine torque at 4100 = 150 ft/lbs
  7 tran ratio in 2nd = 1.87
  8 Propshaft torque (#6 time #7) = 280.5 ft/lbs
  
 As you can see, to get the optimal shift points for a given engine/trans, it
would be neccesary to build a table which gave the above information for a
range of engine speeds/trans gear ratios to determine the exact shift point
for a given shift. Or you could do what I've done, put it on the computer in
the form of a spread sheet. Much simpler)

  I don't have the specs on a 69 B engine, but it probably isn't too far off
the specs for my 68  GF engine which are;
    Max HP= 98 @ 5000 RPM
    Max torque = 110 @ 3000 RPM
  
 I hope I haven't confused you too much!!

Rick Morrison




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