Power, torque -> acceleration

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Yvan@office - 26 Feb 2007 08:08 GMT

I have 1.6 liter '87 BMW. I am having trouble with it's Pierburg 1B2
carburetor (cold start and idle). Replacement parts are ~200 euro. In
local Weber shop I was told that I can replace it with Weber 32 ADF
(can anyone confirm this?) for ~300 euro.

So now I have:
75@5800 [hp@rpm], ~80 [hp/ton], 110(80)@3200 [Nm(lbft)@rpm]

With new carb I should have:

90@6000 [hp@rpm], ~100 [hp/ton], 123(91)@4000 [Nm(lbft)@rpm]

Currently I feel lack of power during acceleration. I also have Audi 100
2.3 liter:

136@5500 [hp@rpm], ~100 [hp/ton], 188(139)@3000 [Nm(lbft)@rpm]

I am not a sporty driver, I drive most of the time in slow city traffic.
I was wondering how would compare new BMW setup with Audi with the same
hp/ton ratio, but different torque. I am not talking about 0-100km/h
(0-60mp/h). I am interested in acceleration in third gear during
overtaking. I knew that it is not just the meter of power and torque,
there is also a gear ratio... but I think that power/weight and torque
are the decisive ones.

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Don Stauffer in Minnesota - 26 Feb 2007 15:03 GMT

> I have 1.6 liter '87 BMW. I am having trouble with it's Pierburg 1B2
> carburetor (cold start and idle). Replacement parts are ~200 euro. In
> local Weber shop I was told that I can replace it with Weber 32 ADF
> (can anyone confirm this?) for ~300 euro.

snip

> I am not a sporty driver, I drive most of the time in slow city traffic.
> I was wondering how would compare new BMW setup with Audi with the same
[quoted text clipped - 3 lines]
> there is also a gear ratio... but I think that power/weight and torque
> are the decisive ones.

Actually, neither torque NOR horsepower is really that good a
predictor of performance. What is really important is the integral of
torque.d(rpm) carried out over a range of rpm (definite integral)
between recommended shift points. We can have two engines with the
same peak torque at some given rpm. But one is a very peaky engine,
and the curve drops rapidly on either side of that peak. Another
engine has a wide band, with torque almost as high for quite a band
below peak. The later will have superior performance.

Through the years there has been discussion about creating some
performance ratiing based on such an integral, but it has never become
popular. For now all you can do is look at the torque or horsepower
plots (versus rpm) and quantitively estimate.

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Dyno - 26 Feb 2007 23:56 GMT

>> I have 1.6 liter '87 BMW. I am having trouble with it's Pierburg 1B2
>> carburetor (cold start and idle). Replacement parts are ~200 euro. In
[quoted text clipped - 22 lines]
> popular. For now all you can do is look at the torque or horsepower
> plots (versus rpm) and quantitively estimate.

Or you could plot tractive effort (tire force vs vehicle speed) which
would take into account tire diameter, differential ratio, individual
transmission gear ratios and the engine's torque curve. The graph would
give you a series of intersecting lines, one for each transmission
ratio. The maximum force would then occur when you shift at the
intersection of each pair of gear ratio lines. It shouldn't be too much
of a stretch to add in vehicle mass in to get acceleration.

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Codifus - 18 Mar 2007 02:20 GMT

>>I have 1.6 liter '87 BMW. I am having trouble with it's Pierburg 1B2
>>carburetor (cold start and idle). Replacement parts are ~200 euro. In
[quoted text clipped - 24 lines]
> popular. For now all you can do is look at the torque or horsepower
> plots (versus rpm) and quantitively estimate.

Or, put simply, take a dyno plot, and measure the area under the curve.

CD

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