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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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Even G techs are unreliable though much more accurate than stop watches and speedos. The right test would be a track, you get all those nice intervals and the only really poor number is the trap speed, which is averaged over the last 60'. Racing TCs aren't particularly slippy once they're getting rpm. I use a 10" 3500 stall in front of a TH350 and the slip at 4000rpm is very small.
My wife has pretty much committed to taking the SSR racing at the track where I race the 62. Since I find a 12.75 car a little tame, I have no interest whatever in going 15s in a 5K# truck, but it will be interesting to see how she does and the truck does.

As far as rollout is concerned, something is amiss. The SSR is not going to gain .43 in ET due to roll out. I have a 60' of 1.75 @ 2250' CA actual on 9.25 Hoosier Superpros DOTs. I've experimented with deep and shallow stage and the ET difference was smaller than you've postulated by quite a bit. I do shallow stage for the simple reason that it allows me to go on the flash of the last yellow and cut .009 - .020 lights vs having to wait which is extremely difficult to time and leads to huge variations in RT. As a testing proceedure it's simple enough to stage shallow consistently. BTW, space between the beams NHRA is 7", typically.

I can't state what the SSR will gain in the last 60' reasonably since I haven't tried it and the final gear ratio in the 1/4 and powerband position is outside my knowledge, but if it's making 99mph in the 1/4 with what has to be an abysmal 60' then it's making up a lot on the upper end. I run 12.75 @ 104.5 @ 0' (per NHRA alt correction tables) on 26" tires, 3.42 gears, TH350 w/10" 3500 stall at 3250# and my speed is climbing fast since I'm just into the gut of my powerband at the traps. (compromises must be made in a drag car that gets daily use @3500' in NE OR and wore studs for some of the year)

Well, as I said, it'll be interesting to see, if she goes along with the idea.
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There are 2 reasons I stage shallow, one is RT as I mentioned, the other is control of rollout. I can bump into the lights softly enough to make the stage light flicker on, that's my control point. If I mess it up, I'm probably going to redlight or try to wait and get some horrid RT. I race NHRA Div 6 Pro bracket, I'm at the slower end of the bracket. Since brackets are about dial and cutting a light, I don't worry too much about paring ET, I'm fast enough to stay in my Class and minute ET gains are meaningless. Yep, I have a faster car project, but it'll be a dedicated car with 9.0 as the goal. The 62 can have traction issues out the kazoo if track conditions get poor, it's a street car, gearing, full exhaust, steel wheels, 215R60-15 V rates/Cragar SS on front, IROC strut suspension, you ge the idea. Marginal gains just aren't in the picture. It's a very cool compromise car, going any further either direction would just wreck it.

I wasn't thinking about the poor HP/# of the SSR, that would surely make rollout a bigger factor than I experience. 1.75 60' is a heck of a hit for a 3250# car on 26" tires/3.42. Torque is your friend.
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