automatics vs. manuals

[StraightSix]

Assuming you have a locking torque convertor, is there any disadvantage to running an automatic transmission, with regard to drivetrain power losses?

[John Beckett]

Yes
 
 JB

[StraightSix]

Okay.
 
 (Roughly how much %?)

[Sumner]

What automatic are you considering?
 
 Sum

[John Beckett]

Well all the automatics are somewhat different in reguards to power loss, but as an example a T-400 eats up about 50 HP.
 
 JB

[StraightSix]

Toyota A340E.
 
 So the power loss would be more of a set figure and not so much a percentage of the power transmitted?

[Sumner]

I got the following gear ratios from another site:
 
 Gear ratios for the A340E 92 SC400 US model are
 1st 2.804
 2nd 1.531
 3rd 1.0
 4th 0.705
 Rev 2.393
 
 That is actually a pretty wide ratio transmission with a strong overdrive gear.  Think how hard it is to pull overdrive on a big hill.  According to Costella, and I agree, "Bonneville can be like pulling a big hill".  So if you gear the car higher in the rear than you would on the highway what is going to happen when it shifts into 4th.  If it is like my truck it will fall on it's face.
 
 Some of the 5/6 speed manual overdrives only have about a 20% drop going into 5th and/or 6th.  Much better.  I see that GM/Chrysler have developed a 6 speed automatic jointly that will be coming out in future models.
 
 Still like mentioned above it is going to take more HP to turn the auto vs. the manual, so why give that HP up especially if you are pursuing a record.
 
 I guess is there a reason that for you the automatic is desirable?
 
 c ya, Sum

[StraightSix]

The only real reason is that that's what the car comes with.  I agree the ratios are rather wide and a peaky powerband could present a serious problem.

[doug odom]

Has anyone ever done any real testing of transmissions and or rearends on a chassis dyno? I have never seen one.
  I think it would be very interesting to run a T10 - lenco - Liberty - Richman - Jerico - TH400 - TH350 - C4 - etc back to back on a chassis dyno to see the real numbers.
  Maybe I'm just skeptical because for years I was told so many things about air flow that when I got a flow bench I found most of them were not true.

[John Beckett]

Forgot to mention OD. Torque division...unless you got a serious blower motor it's gonna be tough to pull a .7 OD gear regardless of whether is an automatic or manual.
 
 JB

[John Beckett]

Mike Powell from Tex Racing has done a bunch of testing, but mostly on the Super T-10, T101 and Jerico 4-speeds.
 
 JB

[doug odom]

And results of these test are..........?

[John Burk]

If an automatic transmission had 50 lb of steel , 20  lb of aluminum and 15 lb of oil and if it isn't water cooled every degree it heats up is 75 BTUs . 75 BTUs in a 1 1/2 minute run is an average of 1.2 HP (1.2 x degrees gained = HP) .

[John Beckett]

For actual numbers someone will have to ask Mike. I'll see what I can pry out of him the next time we talk. I can tell you that, acording to Mike, the typical NASCAR 4-speed is the most effiecient and least HP hungry when compared to any automatic and most 5-speeds.
 
 JB

[wmtsmith]

no torque converter== very little heating