Hi,
5900 RPM's in 4th gear with stock tire diameter (26.5 inches, more or
less) and a stock TR6 3.70 differential should equate to over 110 MPH.
100 MPH is something just over 5000 in 4th.
If your tach registers 5900 RPMs and your speedo registers 69 MPH, either
the tach is off or the speedo is off.
THere are some tests you can do.
1. as mentioned, compare your tach reading to a known good source. A
dwell/tach is a good place to start, but that only works up to 1500 - 2000
RPMs. After that, you need another form of tach. An "off the shelf" unit
from Sun or AutoMeter for under $50 will fit the bill to get you a larger
range of RPM readings.
2. connect an electric drill to your tach drive cable. Spin the drill up
to its highest RPM. The tach should move accordingly so 1500 drill RPMs
should be 1500 RPMs on the tach. I'm pretty sure this is right, because
the tach drive is geared 1:2 to give the proper cam/engine relationship.
If this reads wrong, get it calibrated.
3. do the same thing for your speedometer. 1500 drill RPMs should equate
to about 30 MPH on the gauge. If not, then the "muppah" gauge is probably
in need of calibration.
Now, 5900 RPMs with a TR6 motor is a visceral experience. When you rev a
TR6 motor that high, blelieve me - you'll know that you're turing some
RPMs! I suspect that your tach is reading high and the basis of your
incorrect reading. However, it might help us to help you if you told us
what size tires you use and what your rear gearing is (if you know it).
I suppose it is possible that you have some odd gears in the differential,
the ratios that I have seen or used are 3.45 (PI spec cars and TVR
2500's), 3.70 (stock Fed. Spec.) 4.10 (optional performance piece), 4.30
(optional performance piece), 4.55 (ditto) and 4.88 (from some sort of
milk truck :-)).
I've worked out the numbers on all of these, and the 4.88 is about 30%
more gear than the stock 3.70 thing. So with that setup, 5000 in 4th would
be 100 MPH indicated but only about 70 MPH in real speed.
So another possibility is that your diff. has been souped up with "gears".
If the latter winds up being the case, contact me off list because I'm
always looking for spare diff. gears for the race car!
rml
------------------------------------------------------------------------------
Bob Lang Room N42-140Q | This space for rent
Consultant MIT unix-vms-help |
Voice:617-253-7438 FAX: 617-258-9535 |
------------------------------------------------------------------------------
|