Noise on RPM reading above 5000 rpm

[dorrisdormouse]

Hi

Fitted an MSD 7531 just before the start of the season and finally got to try it on the track last weekend. The car was a bit off its usual times and when i took a look at the new data i noticed its quite noisy and is triggering the rev limiter. The car is a Front engine dragster fitted with a 552ci big block ford with unported SCJ heads on Alcohol. It has no alternator, were not to sure if the battery is as good as it could be so will be looking to change it. The car normally launches at 5500 rpm, and goes throught the traps at around 6500, covering the quarter mile in around 8.4-8.5 seconds at around 157 mph - but over the weekend we struggled to 8.81 @ 151mph. Is this likely to be due to interference on the power line or crank trigger signal? Would fitting a noise filtering capacitor help? Copy of the trace attached

[dorrisdormouse]

Can anyone help with this?

[msdtech1955]

crank trigger or distributor?

how old are the plug wires ?

is the magnetic pick-up lead rum by- itself?

[dorrisdormouse]

Thanks, this is with an MSD Crank Trigger. The plug leads are several years old, closest one runs at worst 6 inches away from the crank trigger lead - the one with a single ground that came in the pack with the 7531. The 7531 and coil are brand new though. The main power lead for the coil does come closer though to the crank trigger signal lead (again the sheilded one) and these actually run to the box relatively closely.

[msdtech1955]

first ohm your spark plugs wires...( what kind of reading you get ?)

second...lets make a better ground path ...example:
to ensure , we have a true path to ground.
FLOATING GROUND1.JPG

[77bill77]

sorry to cut in here but is that showing that you want the ground on the coil to go to battery ground

[msdtech1955]

it's to make sure you have a good clean path to ground.

[dorrisdormouse]

Thanks All. Weve done a whole host of things heading into our next possible testing time (this weekend). We have fitted a new, far stronger, battery, new MSD Super Conductor Plug leads, weve run a better ground from the battery to the motor / coil / ignition box as per the diagram, a capacitor in parallel with the power to the ignition box, and also used the over braiding off some braided hose as a shield for the trigger leads... hopefully out of all these things we should see the problem solved :-)

Will report back how I get on.
Thanks again :-)
Dave

[dorrisdormouse]

Signal was _much_ better over the weekend. Didnt get many runs as we had starter problems. Still a few error spikes though.