Earlier this year, in the heat of summer, I switched to an MSD Ready to Run distributor and MSD Blaster 3 in my 68 Cutlass S with the 9 to 1 compression 350. I had previously upgraded to a 4bbl Edelbrock 2711 and Holley carb. The distributor change really smoothed out the idle, and the car ran great.

I set the initial timing at 18 degrees BTDC, used the blue springs and blue stop which gave 22 degrees of advance by 3000 RPM for about 38 total since the springs allowed for about 2 degrees of advance at idle at 800-850 RPM.




Everything seemed to be working well. I could neither hear nor sense any pinging under WOT even when driving in 100F+ weather here in Texas with the engine running at 190-195F.

A few weeks ago, I had the rear diff changed from the 2.78 Open to a 3.42 Posi. After breaking in the new gears, I heard what sounds like pinging under WOT. The engine was running at 190F, and the outside air was a nice 72F. I did not sense any hesitation or loss of power, but I really felt like I heard some pinging. All of this was running 91 Octane, by the way.

My questions are:
1) Could the rear diff change affect the load on the engine leading to detonation?
2) Is it possible to install a data logging knock sensor on the car to find out if it is really pinging or something else?

I plan to back off the timing by a couple of degrees and test again, but I might want to keep the 18 degrees at idle and change the total advance.

 

#1 above should go the opposite way - the lower gears make the engine work less as the gears provide greater mechanical advantage. Think of riding a bicycle and the difference in pedal effort between a "high" gear and a "low" gear. It's the same with the engine.

One thing to consider is the gasoline. There are different blends between summer and winter, with different evaporation (volatility) rates.

So with all that said, try decreasing the advance by a couple of degrees to see if that removes the pinging.

 

Thanks, Kenneth! Regarding #1, that was exactly my reasoning, too. The lower gears should make it easier, which is why it was so puzzling. In regards to the gas, it is actually the same tank of gas. I mostly tinker with the car and drive it around to test the changes, so I have not yet used the full tank from when I changed the distributor this summer.

One possibility is that I am hearing some of the loose trim rings on my SSII wheels. It took two sets of originals for me to find enough to make one good set, but some of them are still loose. I can hear them very clearly when turning a corner, but I have never noticed them making noise on acceleration.

 

Are you referring to the 5 trim inserts? If so, those can be tightened up by removing the inserts and bending the friction tabs to make a tighter fit.

I stole info below from an old post about rattling wheel trim. Joe has a good description of how to tighten them so they don't rattle.


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Your fuel maybe getting old also. You can change to the purple bushing to knock a few degrees off your total and you can maintain your 18* initial.

 

Yes, these exactly. I tried bending them as you suggest, but some of them still rattle. It is probably worth removing any loose ones and trying the WOT tests again to eliminate this as a possibility. I should also invest more time in bending them to stop the rattle. ​​​​​​​

 

Agreed. The purple would be my first change if removing a couple of degrees stops the noise.

Good call on the fuel, too. The tank is getting low anyway, so I should refuel, too.

 

So I think we answered question 1 effectively. The gears really shouldn't have made any difference, certainly should not have made it worse.

Regarding question 2, has anyone ever installed a knock sensor on one of these older engines? Since pinging can be nearly silent, I wonder if it would be worth pursuing. I plan to add a data logging Wideband O2 sensor someday, too, and a knock sensor seems like a pretty reasonable thing to do, too, assuming that they make any for this purpose.

 

Weird as said, it should lug less and ping less. You may be closer to 40 degrees at full throttle, that should still be fine, apparently not. I ran that on a 9 to 1 350. What spark plugs are you running? Go to 18 degrees of mechanical advance, all should be good.

 

I’m going to double check the timing again before I do anything else. And maybe I’m just hearing things.

 

Well, I checked the timing and confirmed that it was 18 degrees BTDC at idle and 38 at 3000 RPM. I drove it again and did not hear anything.

I then backed off the timing to 16 idle / 36 all in and drove again. Sounded the same, and I noticed no difference in idle quality nor in the butt dyno observed performance.

It is better to err on the side of caution, so I think I’ll leave it there for now.

Thanks, all, for the feedback.