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Oops! Problem timing runs!

1970 Views 18 Replies 6 Participants Last post by  JimGnitecki
Before I had the 4.56 rear axle ratio installed this week, I had done some "baseline" timing runs with the 3,73 gears.

Since I don't have a G-Tech or other computerized accelerometer, and I don't want to abuse my vehicle anyway, I simply did a few straightforward 0 to 60 runs using a digital stopwatch.

Even that turned out to be pretty inconsistent, as I am unwilling to do stuff like "preload" the torque converter. I simply mashed the throttle and the stopwatch start buton at the same time, and when the speedometer touched 60, i hit the stop button the stopwatch.

I used the speedometer even though I knew it was damped (so that it won't appear "nervous as we drive). I figured the tachometer was not going to be trustworthy since even if I calculate he rpm versus speed relationship in 2nd gear, I KNOW that torque converters and auto transmission clutches slip under full throttle and shift conditions.

Turns out, that I may have made a big mistake using the damped speedometer. It may lag a lot more than I thought.

I learned this today when I finalyl got a chance to hit the throttle hard on a freeway entrance ramp from virtually a dead stop (just turned onto the ramp from a city street). With the new 4.56 gearing, the transmission hit the top of 1st REALLY fast - much, much faster than before the gear swap. But what was really revealing is that I caught the flash speedometer reading right at the shift point. With the new gearing and the raised 6200 rpm shift point, it ACTUALLY shifts at 39 mph. However, the speedometer had only gotten to an indicated 30!

This means I cannot "repeat" the timing runs with the 4,56 gearing with any degree of accuracy at all. In fact, it's pretty likely that the times I got with the 3.73 were also longer than reality, because while the speedo would lag LESS with the slower 3.73 acceleration, it would still lag appreciably.

I guess what I need to do is borrow a G-Tech and at least get accurate times for the 4.56 gearing. That won't give us any comparison base, as I'm sure my "protect-the-SSR" and "be safe at all times" approach to acceleration won't come anywhere near what the magazine test drivers and Chevrolet do when testing. Sigh . . .

Jim G
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1 - 3 of 19 Posts
sure wish you could have gotten better before and after data,
not very scientific at all , i guess ''seat of the pants'' is still
the best method......................
this is not accurate info what about converter slip ? I have a road race Sonoma that
I have drag raced also, it has 600rpm of slip going thru the traps there is no way to calulate
your speed by your tach. when there is slip involved. Any way the speedometer shouln't
have any problem keeping up , these trucks are fairly slow !
The Sonoma I was refering to has a stock converter, and only locks up in over drive,
an automatic is coupled to the engine by fluid, there is always a speed difference between
the engine and the trans. gears unless lockup is happening in the converter which does not happen in 1st and 2nd in a 4L60E, its not actually ''slipping'' like a clutch would.
1 - 3 of 19 Posts
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