I was at an autocross and had GPS data acquisition in the car. Looking at the output one run reached 63.3 MPH. The run was done in 2nd gear. The car is a stock 1988, 2.8L, 5 speed. The circumference of the rear tires is 80 inches (unloaded).
Impossible to divine, as the GPSr accuracy of most consumer grade devices for such situations is hamstrung by a 1 CPS data recording interval. i.e. : Affordable GPSr(s) can't give accurate readings in situations w/extremely dynamic velocity changes. The topspeed recorded could be correct, but just as easily not. One second is a long time comparatively speaking. If your device records @ a higher rate, say . . . 10 readings per second then I'd give it more credence. For such a device . . . $ell your first born, or build it.
Norm
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12:08 AM
Primaris Member
Posts: 550 From: Oak Grove, KY USA Registered: Aug 2001
Impossible to divine, as the GPSr accuracy of most consumer grade devices for such situations is hamstrung by a 1 CPS data recording interval. i.e. : Affordable GPSr(s) can't give accurate readings in situations w/extremely dynamic velocity changes. The topspeed recorded could be correct, but just as easily not. One second is a long time comparatively speaking. If your device records @ a higher rate, say . . . 10 readings per second then I'd give it more credence. For such a device . . . $ell your first born, or build it.
Impossible to divine, as the GPSr accuracy of most consumer grade devices for such situations is hamstrung by a 1 CPS data recording interval. i.e. : Affordable GPSr(s) can't give accurate readings in situations w/extremely dynamic velocity changes. The topspeed recorded could be correct, but just as easily not. One second is a long time comparatively speaking. If your device records @ a higher rate, say . . . 10 readings per second then I'd give it more credence. For such a device . . . $ell your first born, or build it.
Norm
The peak speed recording would be the average between two consecutive sample points.
Therefore, indicated peak speed is less than or equal to the actual peak speed.
Consumer grade GPS is quite innacurate, generally speaking. But even the cheap ones can be precise.
Like you, I come up with roughly 6200 RPM (6181 RPM with no tire load, and 6236 RPM with a WAG for a tire load factor).
However, roughly 6200 RPM surprises me a bit because I thought those cars had a fuel cutoff that normally would have kicked in at a few hundred RPM lower than that.
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12:39 AM
Primaris Member
Posts: 550 From: Oak Grove, KY USA Registered: Aug 2001
Well this is good news and bad news I guess. My tach is still way, way off. But, at least I'm not spinning the car to where the needle was pointing! When I was riding along and I noticed the needle was pointing to the 3 O'clock position I made my co-driver shift to third!
quote
Originally posted by project34:
Like you, I come up with roughly 6200 RPM (6181 RPM with no tire load, and 6236 RPM with a WAG for a tire load factor).
However, roughly 6200 RPM surprises me a bit because I thought those cars had a fuel cutoff that normally would have kicked in at a few hundred RPM lower than that.
I've read here that the '88s don't have a rev limiter. This was also on my mind as the needle was doing its best to give me a heart attack.
Hmmm . . . T'would seem things have progressed quite nicely since I last looked into such devices(thanks to Moore's law). I stand corrected on the cost/availability of the higher sampling rate devices(that's a good thing). And, it interfaces w/commonly available display/analyze devices too. Good score; which one do you use?
Norm
[This message has been edited by gt88norm (edited 12-22-2009).]
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12:55 AM
Primaris Member
Posts: 550 From: Oak Grove, KY USA Registered: Aug 2001
Originally posted by Primaris: Well this is good news and bad news I guess. My tach is still way, way off. But, at least I'm not spinning the car to where the needle was pointing! When I was riding along and I noticed the needle was pointing to the 3 O'clock position I made my co-driver shift to third!
Just out of curiosity, I checked out the "3 o'clock position" on the tach. Although there are no RPM numbers shown there, a quick and dirty extrapolation suggests "the "3 o'clock position" would correspond to roughly 8000 RPM, which, were you actually revving the engine to 8000 RPM, might well be enough to propel one or more connecting rods to truly dizzying heights.
The weakest links in a stock configuration are actually the accessories like the alternator, water pump, & A/C compressor. Regardless, you wouldn't likely get to 8000 RPM under acceleration without running into valve float and subsequent loss of power with stock valve springs. You probably wouldn't be able to get there even on a poorly thought out downshift, because you'd probably lock your rear wheels with compression braking before the engine got to 8000.
you should make/get an aldl adapter and read the rpm's directly from the ecm feed. you don't have to drive the car to work out the error factor. The tach error could be as simple as a corroded connector to the flex circuit on the dash, that will make the tack run very high, and sometimes add some real exciting effects (hit high beams and tach goes from 1k to 5k) The most likely connector is the vertical one on the passenger side, the ground point on that is crucial for the tach. or even the connectos where the tach slides into the board can be at fault. I had an issue with that connector, and after cleaning everything up, the tach is pretty accurate now, within 50 rpm at any given rev.
And yes the 88 has no rev limiter. but even a 6200 rpm burst is not out of the realm of the poor little 2.8 in stock trim. (pre 88's tho will give you a taste of that nasty rev limiter at that point, my 86 scared the crap outta me the first time it did it)
Speed vs RPM? See cave... Gear ratio. Excel workbook
RPM way off? Tach is off Clutch is Slipping AT TC is not locking up yet or as a problem. Math is off... Probably the tire size.
A small error is probably ok... Big error, more than 500-700 and up, something is wrong.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
...The tach error could be as simple as a corroded connector to the flex circuit on the dash, that will make the tack run very high, and sometimes add some real exciting effects (hit high beams and tach goes from 1k to 5k) The most likely connector is the vertical one on the passenger side, the ground point on that is crucial for the tach. or even the connectos where the tach slides into the board can be at fault...
Can you add some further guidance? Is this connector in the passenger foot well or do you mean passenger side of the gauge pod? How much disassembley is required to get to this connector? Pic?
The connector he's talking about plugs into the back of the instrument cluster. To get to it, you have to remove the rear panel off the pod, just a couple torx screws. To unplug it though, you might have to remove the lower steering column trim panel which is held on by four 7mm hex head screws.
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08:33 PM
Dec 23rd, 2009
tjm4fun Member
Posts: 3781 From: Long Island, NY USA Registered: Feb 2006
yes, Blooz filled in the blanks. after you remove the lower trimplate under the pd, it is on the passenger side, and also impossible to get to without removing the whole pod. to do that you need to remove the top cover, 3 torx at the top, then there are I believe 2 10mm bolts on a bracket at the top, and 2 more on the bottom but don;t remember exactly where. be very careful with that plug, it is best to spend the time and remove the whole pod so you can get at it cleanly and remove it STAIGHT out. the circuit board has the copper traces down into the hole it locks into, and you can peel them off the plastic sheet if you are not careful. it really is a bad design. there are 2 connectors on the drivers side, one is for the speedo(C1), the other is more pod lights(C2), if you want to remove the whole assembly to give it a good going over in the warmth. here is a pic of the layout for the 88 and the connectors and what they do:
that image would be looking towards the back of the car thru the windshield. you are concerned with the C3 connector, pin 18 for the tach signal, and pin 11 for the ground.
[This message has been edited by tjm4fun (edited 12-23-2009).]
