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FBody 09-01-2004 09:22 AM

Car missing while on Dyno?

A local tuning shop had a car cruise last weekend and they were offering dyno sessions/runs for $50.

I decided to put my 69 350 Camaro on their Dyno (Mustang Dyno) to see what was my output at the rear wheels.

To my big surprise and disappointment, my car started to in the mid - 4K RPMs. The Dyno curves (HP & Torque) shows starting to go down around 4,000 RPM while the curves would be expected to be still climbing until it start reaching the 5,000 RPMs.

While speaking with other friends of mine that went the shop earlier that same day, they told me that most of the cars that they saw on the Dyno were also missing at high RPM.

The questions I have are:

1) What would cause my car to be missing on the Dyno.

2) Will my car be missing on all Dynos.


snkbyt 09-02-2004 03:31 AM

FBody, on a Dyno its under load. So that is normally a spark problem or lean mixture. It would miss on any Dyno. Spark plugs problem, wires, possible timing.(too advanced at rpm). You should get thje same problem pulling a steep hill/ So you wouldn't have to pay Dyno fees.

FBody 09-02-2004 08:43 AM

Thanks for your response snkbyt,

I got the Camaro 2 years ago, last year I changed the following:

o AC Delco Distributor Cap
o AC Deco Spark Plug wires
o Crane XR-i Electronic Ignition Module (replaces the points)

But I never changed the Sparkplugs, since they all looked Ok.

Last year, I purchased a Proform Adjustable Shift Light and it never worked on my car, before and after the above changes. The light is always flickering. I know that the shift light works as I tried it on a friend's car.

I will try changing the plugs and try my Proform shift light again, maybe it will work.

Thanks Again,

USA1 09-02-2004 12:33 PM

When you changed out the points to electronic, did you run a new power wire to the distributer? If not, you are running at around 9 volts to the coil when you should be running 12. Just a thought. That would make it miss under a load.

FBody 09-02-2004 02:08 PM

Yes, I did supply the Crane XR-i module with a switched 12 Volts, while the Coil power is still supplied through the resistor wire.


docvette 09-02-2004 02:23 PM

Doc here:pimp:

Most likely, what is causing the High RPM miss is when you converted the points distributer over to a high Energy conversion,

The Cap is not spaced Wide enough to Resist Cross fires across the plug pick ups, This is one reason why HEI Caps are so large.

Go to a Good Spiral Core 8 MM or higher wire too...And Wider Plug gaps.

You may Want to Consider, if you have the room, Getting a GM HEI Distributor ET&AL...They are good to about 7000 RPM.

Hope IT Helps!


NAIRB 09-02-2004 08:18 PM

If there is any weakness in your existing ignition system, it's gonna show up under extreme load conditions. Happens on the street, the track and the dyno. I always find a big hill to accelerate a customer's car up if I suspect faulty plug wires, plugs, etc.

Just remember that your ignition system is working exceptionally hard to fire a spark under these extreme cylinder pressures as the engine works hard against a dyno. Modern cars do have superior designs in terms of ignition energy, so the old school Chevys need all the help they can get. As mentioned by Doc, try to eliminate spark scatter with a wide cap, use high quality plug wires and fresh plugs, and make sure you have enough juice to throw the gap for the application you are running. Electricity will follow the path of least resistance, so as it encounters more in the combustion chamber, everything else has to be sealed up tight (so to speak).

FBody 09-03-2004 08:10 AM

Thanks for your responses guys,

I will try new plugs on the weekend. It's the only thing that I haven't changed and I really don't how old they are, as they were on the car when I bought it two years ago.

If this doesn't fix it, I will move on to the wires and distributor, although I would like to keep the stock ones.


4 Jaw Chuck 09-03-2004 09:51 AM

Running a reduced gap at the plug will help, try .035". Dyno runs load the engine beyond that encountered under driving conditions and for longer periods of time.;)

Litdave 09-03-2004 12:37 PM

"Car missing while on Dyno"

I hope you find it!!!


couldn't resist :)

TurboS10 09-03-2004 01:41 PM

I am going to vote ignition. I had the same problem at the track with my 57 until I put a hot coil and MSD6 on it. Under hard load the stock coil is just not going to cut it in a performance application. The reduced plug gap will also help out if it is ignition.


FBody 09-07-2004 10:17 AM

Thanks Guys for all your responses.

I'm not sure how I will be able to confirm if I fix anaything, as it's not making it while on the raod (even going up a hill).


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