Hi,
Super 6 conversion and head milled .085'' for a calculated 9 to 1 comp ratio on my '72 Plymouth Valiant with 225 slant 6/automatic and a new Holley 2280 and stock exhaust manifold with 2 1/4" exhaust system.
I ruled out the vacuum advance by disconnecting and capping it. It has detonation when accelerating at part throttle at highway speeds. Can also be heard at full throttle. Does it climbing long steep grades too. I experimented with initial timing from 5 degrees btdc to 0 degrees tdc with same results. I just lashed the valves again tonite after I ran it on the highway for about 1/2 hour. I found them to be tight and realized the lower the idle the more accurate adjustment can be made. Is it possible that the tight valve lash was causing the detonation?
I plan to recurve a distributor in the near future but need to sort out this pinging/detonation issue.
I must say that the 2 barrel and bump in compression has made this into a different animal. Love the extra grunt off the line and low speed response and the ease at which it does highway speeds.
Thanks for listening and for your advice!....nick

What spark plugs are you running? With or without the metal ring washers?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Stock spec Champions with metal ring removed. I do have a set of the NGK's that I can put in.

Higher compression --> --> colder plug. Try an NGK BPR6ES (plain) or ZFR6FIX (fancy). Metal ring washer removed.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

copy that....Thanks!

If you modded the engine a stock carb may be slightly out of it's calibration as well, if the mix is somewhat lean it can also cause
pinging...(if it goes too lean, you'll start shooting ducks...and melting
stuff in the cylinder...)

Also, is your timing mark/damper accurate?

-D.Idiot

I'm in the lean mixture camp myself. Make sure the carb is A-1 (yes, you'll have to open it up) and there are no vacuum leaks.

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Joshua

I was also thinking about this and wondering if it is going lean. No known vacuum leaks. If I cover up the carburetor throat the engine quickly dies. All unused ports on the carb are capped. The car has a rock solid idle and smooth acceleration. I think the damper (car has 62K miles) is original and with the damper mark on 0 the rotor is pointed to number 1 and verified TDC when the head was off. I am studying the chrysler training video to familarise myself with this Holley 2280. I havn't really stood on it for a long period and most of the pinging that I have observed is at part throttle. Maybe I need to make use of the O2 sensor bung the exhaust manifold has in it and hook up a volt meter for the next test drive.

That and a long hose and vacc. gauge to a constant vacc. source at the carb base will tell you loads about what is happening while driving.

-D.Idiot

Valleyant
I am in Northern Ky.15 minutes away. Have wide band sensor, some spare carbs,distributors. Be glad to help.Sent pm

I installed the colder NGK plugs, thanks Dan for the suggestion, and it takes more throttle pressure to get detonation and it's not as loud as is was with the stock plugs. I noticed that I could ease into the throttle on the highway to gain speed and stay just shy of detonation easier. Making progress. Now I need to get A/F ratios and a vacuum gauge installed for further testing. Does this sound like a simple jet change or some other adjustment to remedy the problem?

Some other details I left out from before. The stock manifold was gasket matched at the downpipe to approx 2.25"/ full 2.25" exhaust system, pertronix ignition, vacuum advance out of the loop for testing purposes and it has Evans coolant. Thanks again!

What is the total distributor advance at 2800 - 3000 RPM? (without VA)
What is the cranking cylinder pressure in psi? ( Run a compression check)
DD

Good progress. Run those plugs for awhile, then take them out and read them. If they don't look fouled, you can probably make additional progress by moving down to a #7 plug (in the NGK range, higher number means colder heat range). Do follow Doc's advice on looking at the advance curves.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Is this the sign of a slipped balancer? With base timing set at 0 TDC using spark plug wire number 1, it shows 3 BTDC on wire number 6. Maybe I just need to continue retarding the timing in relation to prior settings in hopes of helping this pinging/detonation problem. The car runs strong with the timing at TDC (still have the vacuum canister out of the loop) and starts up right now!
Doc, I need to get a gauge to check compression and my very crude assessment of the mechanical advance with engine speed at 3000rpm looks to be about 20 degrees with my (see above) base timing of TDC. I need a better timing light. I am basing this on the balancer mark being about the same distance away from 10degrees BTDC as TDC is from 10 BTDC. hope that makes sense. Thanks again!

Well, shes cured. Timing is set at 5degrees ATDC and vacuum pod hooked up and 4 1/2 turns out. No ping/detonation with 91 octane fuel. Runs even stronger and has great passing power on the highway. Trial and error on the base timing. Whatever the actual value is, I have found what this combo of parts likes. Thanks again for all the help!