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| Joined: | 03-12-2005 |
| Location: | Kingman, Arizona USA |
| Posts: | 532 |
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The traditional method for evaluating things such as damper oil and ignition timing, is to accelerate from an engine speed above idle but well below the torque peak, to the engine speed at which max power occurs, without shifting, and to use a stop watch to measure the time required. To ensure reliable results, the test must be done using the same stretch of road not less than three times in each direction (preferably five or more). You then vary the damper oil, timing, or whatever, in what you hope is the correct direction, and repeat the entire test sequence. Once you have identified what seems to work best for your particular car, you set it up that way, then repeat the test process on a variety of different roads (and, if you are unsure about intermediate characteristics, at several different partial throttle settings as well). There are usually unpleasant surprises along the way, so one generally settles on a compromise that seems not too likely to burn valves or puncture pistons.
Modern technology has provided the fuel-air meter. The only real advantage this offers is that it more or less accurately shows the actual fuel-air mixture during acceleration, thus eliminating a fair amount of guesswork; you still need to do a lot of tests. For carburetor work, you'd want a meter with high mid-range resolution, which means a fairly expensive unit. Price does not necessarily indicate performance, so it's prudent to understand the various specifications before spending any money.
As you can see, all this testing may take considerable time, and may involve activities that are neither safe nor legal on busy thoroughfares, but it does provide data that you can rely on, and reproduce if necessary. I have no faith in 'seat of the pants' estimations of acceleration: in past discussions, I've noticed that most owners upshift upon reaching the torque peak rather than at the power peak, which is where one should shift if maximum overall acceleration is to be obtained.
The 907 engine used in the JH doesn't have much torque below 3000 rpm, and the torque peak is at 4800 rpm, so if your carbs are working correctly, what you're thinking of as 'straining for air' is probably just the engine's normal characteristics.
At 7000 rpm, a 2-liter engine with a volumetric efficiency of 85% (which is probably better than a 907 achieves at that speed) can flow only 207 cubic feet of fuel/air mix per minute (CFM). A pair of street-grade Stromberg CD175s flows 388 CFM at a pressure drop of 1.5" Hg; nearly twice what the engine can consume. By comparison, under the same conditions, a pair of Dellorto DHLA40's with 36 mm venturis will flow 730 CFM. (Data is from John Palmer who quotes a Dave Bean catalog.)
Springs -- Unlike SUs, there are only three standard spring choices for the CD175 Stromberg, so your choice is limited. Since the spring affects both the transient and the steady-state fuel-air mixture, if you change springs you would most likely need to change mixture needles as well. I don't think you'd gain anything except trouble and inconvenience.