So I was going over my log of my last run at the track. I come to find out that the reason i did so crappy was due to a fuel pressure issue. That may not be the only issue though. I'm going over the log in datamaster and I'm looking at the graph (normally i export to excel) and i see the RPMs look like an etch a sketch line. it increases but real sketchy. for example it will climb the rpm range then drop like 25rpm hold then increase then maybe hold increase then drop and so on on. it looks like a bunch of little mountain peaks. what could that be?

 

The RPM reads in 25 rpm increments, so a drop of only a few RPM can cause it to change by one data bit, which is 25rpm. That makes the changes look more dramatic than they might really be. Even in an engine running perfectly, using an ECU with finer RPM resolution, the rpm at WOT does not always increase uniformly or smoothly.

If the increments do become excessive, it may be a sign of a problem with the pulse pattern from the Opti, which is what provides the RPM signal to the PCM. Or it may relate to misfires, but you would feel those. Is there any knock retard when the RPM seems irregular?

 

Some of the squiggle is normal, I used to wonder about this too, my car did this since stock and I've seen it ever since but at different intensities. I just checked my last Drag run and one of the worst RPM drops which I believe are false came in at 115mph, RPM dropped from 4975 at 113mph to 4850 at 115mph, thing is before the start of the drop the RPM increase rate is faster than after the rate after the drop, I dont know what that means, could be slipping clutch that got hot and grabbed but it's a McLoed Street Twin that grabs like no other, or even tires doing the same thing but I doubt it, they are 315's.

Other things that cause that are things like the motor trying to REV like hell and traction holding it back, and without you knowing it things like bumps, rocks, ect all contribute to momentary loss of traction which all show up on the RPM graph, just in weird ways.

I think 25rpm error is fine, I've had some like 200 or more, dont remember exactly. It would be interesting for the ultra high HP cars show the RPM charts compared to lower HP cars (High HP cars at higher MPH's of course), or maybe even people with different Opti's, and people with aftermarket PCM's.

 

thanks the biggest drop i've seen is 150 but mostly they are smaller. no spark retard.

 

also the rpm wackiness was worse in 3rd gear(a4).

 

heres a graph i created

 

I see no issues. Go have fun!