I downloaded my grid files from the weekend and am puzzled by what I see on the engine rpm graph. There seems to be a sharp dip in the engine rpm in first gear on 2 out of the 3 passes I made. The last pass I don't have the dip and the car picked up MPH 1.5 and I broke out. I am looking for suggestions as to what this could be and how to track it down to fix it. All passes were same day in the same lane.

Time Run 1


Round 1


Round 2


I can also share the grid files if someone wanted to take a deep dive into it.

Wild guess: Trans fluid temp was cold, warmer, warmest on the runs - raising converter stall / flash speed as trans fluid temp climbed.

It’s certainly a possibility. I am using a jones pump to cool the oil to 160 after each pass before letting it heat soak in between runs. I run Amsoil trans fluid.

To be more specific, i am referring to the dips in the engine rpm graphs at 1.25 and 2.0 seconds on the first two passes. The car went 4.82 at 143 first pass. 2nd pass wasn’t a full pass but i figured it was a 4.82. Last pass was a 4.815 at 145. I thought that was a big swing in mph. I broke out running a 4.815 on a 4.82. Good race .004 at the stripe.

This motor is on alky with an apd belt drive pump and apd carb. 192 jet. Crank trigger with a grid ignition. I figured some more info certainly wouldn’t hurt to have.

Do you feel it when it happens?
If it's fuel related you should see a fuel pressure issue, do you log fuel pressure?
If it's ignition then plugs, wires, crank trigger pickup etc.
I don't see the small dip killing 1.5 MPH either, so something is not right, I'd look at fuel and ignition.

Cylinder misfire.
Might be a plug crapping out or a fuel quality (dispersion) issue.
Got any O2 data? Fuel Pressure data?
How windy was it? 1.5mph is a pretty big swing in the 1/8th.

When I have seen dips that big in my graphs I feel or hear them.

I don't have a logger other than the grid on the car. I do have a go pro that I use. Here are the passes for the first and third graphs. The fuel pressure gauge is in the dash at the far right. Not the best angle to see it, but not impossible.

https://youtu.be/65gAwmEfeUE

There wasn't really any wind, maybe 5 mph. I can not feel or hear them in the car. Does anyone hear the engine cut out on the video?

I agree, it has to be fuel or spark. I will pull the cap and rotor and check them out. I will also pull the plugs. Plug wires and plugs were new at the start of the season.

I can't imagine it is the fuel system. It was all brand new at the end of last year. APD belt drive setup. Fuel cell is behind the seat so we are force feeding the motor...

I had a problem with this motor on the dyno. I couldn't get it to pull clean to 7500. It would miss and pop. Anyways, we changed plugs and wires, cap and rotor, jets up and down, crank trigger pickup, ran the valves, changed timing up and down, checked rotor phasing.... You guys get the idea. We swapped the distributor and loosened the fuel pump belt and the problem went away. The black plastic piece in the distributor, that the rotor screws to was loose. Not the rotor, but the piece under it. As you revved the motor up you could see the gap on the crank trigger pickup change from the belt pulling on the fuel pump bracket. We loosened it at the same time we changed the distributor.

I thought I had it fixed but it's looking like it may still have an issue. What's weird is it's not consistently there. I will revisit these items and see if I can find it.

If anyone has any other ideas of things to check I am open to ideas.

Post a photo of the grid voltage graph from the two runs, any difference?

Mike,

Good thought, I will look at that here in the next few hours. FWIW, we are running a 16 volt battery with an alternator. The Volt gauge is the one just to the right of the tach on the video.

Cory,

If the car is wired to shut off with the On / Off switch in conjunction with an alternator, might not be a bad idea to look at the grid voltage graph.

If I saw something there, I'd then look at the switch itself.

The glitch seen in the grid rpm graph, gauges on the dash won't have the capacity to detect / display.

i had this and it was a bad crank trigger pickup cable.

DRIVESHAFT GRAPH ?

I swapped crank trigger pickups on the dyno, but am not opposed to buying a new one or borrowing one from a buddy. Easy swap. Was thinking about putting my spare crank trigger wheel on too.

Unfortunately, I don't have a driveshaft graph. Just basic grid logging.

I checked the voltage on the log files. The battery volts never drop below 17.3 during the run.

quote:

Originally posted by CHampshire:
I swapped crank trigger pickups on the dyno, but am not opposed to buying a new one or borrowing one from a buddy. Easy swap. Was thinking about putting my spare crank trigger wheel on too.

Unfortunately, I don't have a driveshaft graph. Just basic grid logging.

Have you look at the "ignition in" graph? That tells you if it'd a input problem, if that is clean changing a crank trigger would be a waste of time and money.

Joe

quote:

Originally posted by sr4440:

Have you look at the "ignition in" graph? That tells you if it'd a input problem, if that is clean changing a crank trigger would be a waste of time and money.

Joe

I just pulled one up and looked at it, there doesn't seem to be a gap there when the rpm drops. The square waves seem to be symmetrical, and the timing out matches up to the timing in waves.

The files are here:
https://drive.google.com/file/...CxZ/view?usp=sharing

https://drive.google.com/file/..._j1/view?usp=sharing

https://drive.google.com/file/...AUi/view?usp=sharing

Feel free to download them and see. 607, and 608 had the dips in engine rpm, 609 didn't.

If someone else could download them and take a peek, I would be grateful.

I would go 1 step hotter on plugs, and if that doesnt completely fix it, put 1-2 degrees of timing in the engine. The swamp weather we have had this year has made it difficult on alky engines.

Had something like that drove me crazy. I could not feel it.
Finally found the main switch power lead to the MSD was loose in the crimp. Installed new wire end and recrimped. Blip is gone.

Here's an overlay - green is the TT, Blue is RUN 1, and Magenta is RUN 2. How much warm up did you do before the TT? Did you put it on the brake during warmup? What were the weather conditions during the TT? Anyway, I'd look for a spark plug that doesn't appear as happy as the others. In my brother's car, the intermittent misfiring plug would be a skosh darker, the center electrode wouldn't look right and the porcelain would sometimes appear a little glazed.

Quick update on this. We double checked timing and rotor phasing friday night. Everything looked good and solid on that. We turned the fuel pressure up one pound, just to try something.

Went racing yesterday. Same track, and very similar weather conditions. Got 3 passes and all three had a clean engine rpm graph. Car went 4.81, 4.82 and then 4.83 all at 143 mph. I will upload those files and all the time slips and weather conditions later.