Doing a Dyno pull right now and we're having trouble getting an rpm reading using a plug wire and the positive side of the coil. Any advice?
Thanks.

Try terminal #1 of the coil

Do you run a MSD?

With the multi spark system, it throws off some magnetic pickups.
If you measure from the coil, then the RPM will read 4X faster than measuring it on one of the cylinders.

I have been watching your thread in the paddock and I am interested in the results because my engine is very similar to yours.

If your intake and exhaust aren't the same too, you can expect very different results.

Last time I was on a Dynojet the rpm reading was pulled off one of the plug wires with an inductive pickup.
The direction of the pickup coil can make a difference so if it doesn't work, try flipping the clamp around on the plug wire.
Any spark plug works for your purpose since you aren't concerned with timing.

We did try to flip the inductive pickup. Didn't fix it. My plug wire are putting off a lot interference. There solid. I guess that a problem.

"Do you run a MSD?"

If so, there is a wire that gives a clean tach signal that can be used by the dyno (white or gray cant remember)

If you are running MSD (6AL) here is a clip of the tach wire instructions:

Click to view attachment

Click to view attachment

Shane told me on the phone, no MSD box.

How about taking the signal off the (-) side of the coil, then? It'll read 4x higher than going off a single plug wire, but that shouldn't be a huge problem.

--DD

This will also occur if you have a spark plug thats trying to foul.. Due to resistance increases in the lead.

I keep an optical pick up handy for weird instances like this, or when I test a diesel. If they have an optical pick up, you can solve this issue in 5 minutes.

Swapped plugs. Worked out, thanks Jake.

Thanks everyone for the help/suggestions. We finally got it.
It was mostly on me. I didn't get the car ready for the pull. At first we were picking up interference from the plug wires and the spark plugs needed to be replace. Once we got this fixed, we got results. We ran out of time to tune the car. My only real goal is to get a class for Time Trails with the NASA group. Ill post up the dyno sheet in the paddock.
Thanks again Jake for the tip off on the plugs and thank you Chris for picking up the phone. Sorry I was so short. The shop needed me for another pull then.

No problem, I've been there.
So glad you worked it out and didn't have to abandon the effort.

Still curious about results! :-)

OK, some credit where credit is due; that was good, Jake. I get annoyed at you when everything is about selling or promoting and you're not actually helpful. But several times recently you've helped people out with information without doing any self-serving selling. That's a positive contribution to the community, something I both recognize and appreciate, so thanks. And something else of more practical importance, that's the kind of thing that enhances your reputation far more than the self-promotion ever could.

Good job.

Ditto. Can we see them?

If I didn't know the answer to this problem I'd just be another one of those people with a huge image that has no freaking clue as to whats going on. I've gotten where I am from proficiency, and thats all. :-)

If that dyno graph is squiggly and looks "nervous" even with max smoothing thats another indication that you need a set of plugs, or have way too high of resistance somewhere.

BTW- These issues often occur with higher cylinder pressures, so this engine sounds like it's got some balls. When I approach 14:1 CR this sort of thing happens almost every time and the plug wires and ignition system that are used are very critical. Thats why I have an optical pick up to fall back on.

Your always tough on Jake when he makes a reply cause you always think he's trying to sell his stuff. Well their are others on here that do the same thing and you don't go after them. I don't have a problem with people sharing what they have to sell because it does help keep our cars going. Otherwise it would be pretty hard to fine parts for our 40+year old cars.

I posted the numbers in the paddock. Feel the carbs are holding me back a bit. Take a look.
This is my second engine build and I've tracked it for two years now. Really proud of what we did here. This wasn't a unlimited budget build and It hasn't let me down on the track yet.
Thanks again for the help. It was a six hour trip and I didn't want to go home with nothing! The plugs (and a new set of wires) were the big help. Jake, thanks.

Not the case at all. I resent Jake's participation when he hypes his services or abilities but DOESN'T offer any real help. His participation back in the late 90's and early 00's was really as one of us, being helpful and making contributions. That was welcome and positive. Then at some point he decided he was revealing too many "secrets" (in his own determination) so he STOPPED sharing and the actual helpfulness in his posts dropped off to little or none. But this last post was different so I lauded him for it. As I said, credit when credit is due, but in fairness the flip side to that is criticism when that's what's due. For Jake or for any other vendor, exactly the same.