• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

3500RPM on Tach @ 40MPH ???

Share this page

George Zeck

George Zeck

Jedi Warrior
Country flag
Offline
Just got my car running after a lenghty renewal. Among other things -- upgraded from Pos Earth to Neg Ground. In doing so, I was uncomfortable with the directions on switching the old tach to the new current -- so I bought one off ebay for another / later year.

Since I've never owned a LBC before -- I just find it unusal to drive @ 40MPH and pull between 3500 / 4000 RPM.

The engine doesn't sound as if it's all that "whinney" (it's does get "loud" -- but not 4000 RPM loud; then I've never owned a LBC before).

My friend and I did wire the tach ourselves -- directly off the ignition coil if memory serves me correctly.

Since adding the 1275 engine to the chassis of a 1098 -- I was wondering if somehow the older / slower differential had a bad effect on the engine. Planning on replacing it soon (from 3.9 to 4.1 -- maybe vice versa).

Any thoughts on this would be appreciated -- my gut says that then engine noice is not pulling 3500 / 4000 RPM -- but who am I to question what a gauge tells me.

Tx-

Geo
/ubbthreads/images/graemlins/hammer.gif
 
Spridget final drives are obviously different than what I run in the Mini, however, with my 1275 I turn 4k RPM at 65 MPH. I'd say the tach you installed is reading quite a bit high. I doubt your final drive swap would cause that kind of RPM shift/increase.

Tell us what kind of tach you bought and how you wired it in. The tachs with RVI on their face are current pulse sensing and are wired in series with the coil (+) wire using a loop of wire on the back of the tach (or inside it). On some tachs you can reverse the connections to this loop to see if your readings improve. The later RVC tachs are voltage sensing and connect with a single wire to the coil (-) terminal.

The RVC tachs are generally easier to install and use but like all these gauges, they're getting old. The electrolytic capacitors inside start to dry up and cause problems. I can send you links on how to replace the components if you're interested.

Another point worth mentioning is some RVC and some RVI tachs have calibration potentiometers on their circuit boards. Not all of the cases have an access hole to get to this trim pot. You can always take your gauge out of its case to see if you have this potentiometer and then drill an access hole for it. Having added the access hole, you'll be able to calibrate your tachometer once you borrow another tachometer that you trust to use as a calibration standard.
 
I looked on the back of the tach and saw no RVC of RVI markings. I think I just deal with the problem.

By the way -- replaced the differential on Monday eve and the car runs a lot smoother. Tach seems to operate more smoothly and doesn't jump as before and also seems to run with lower RPM's. 40MPH runs around 2900 RPM. Seems more in line than before.

Tx-

Geo
/ubbthreads/images/graemlins/hammer.gif
 
The RVI or RVC info will be on the face, not the back. Regardless, I'm glad it's calmed down a bit. However, an electric tach hooked to the coil shouldn't care what changes were made to the rear end. If you ever want some of those tachometer links drop me an email and I'll send them your way.
 
Back
Top