I finally got around to pulling the original #'s matching 327 motor out of the car and dumped in a new fresh 355 small block. I have to say I was really happy with the power increase and the car runs great with one problem, DETONATION! It only happens at 3000rpm or above and it is pretty noticeable. I have retarded the timing, readjusted the carb idle mixture, gone to the next cooler plug (R44S) but it still is there. Now, I just have less power as well since I brought the initial timing down from 10 btdc to 2 btdc. I checked the total advance with my timing light and found it had 28 degrees centrifigal advance and another 18 degrees vacuum advance for a total of 46 degrees. I thought I had it set it in the past with the distributor springs for 37 degrees so it makes me wonder if this is the problem. Could the advance stop bushing in the distributor be missing? If so, where is it located? Any other suggestions?

 

I think you're on the right track. The bushing (if there is one) is on an upside down pin under the plate that the weights set on. You have to take the distributer apart to get to it.

BigBlockk

Later.....

 

The bushing might be missing or the plate that the bushing slides in could be worn. Mine was allowing me to get close to 40 degrees of centrifigal advance. Way to much. I had it put on a dist machine and the ybrazed up the slot so it now gets about 22 degrees of centrifigal. Don't worry about the vacuum can because under hard load it does not pull in any advance. What kind of compression ratio and what are the cam specs.

I have mine set at 12 degrees initial and 36 total. My vacuum can pulls in 16 more degrees at idle and cruise. So at idle I have close to 28 degrees advance. It runs much better since I have hooked up the vacuum can.

 

Is the detonation above 3 grand, is this hard throttle or most of the time?

 

The detonation is most of the time and under heavy acceleration. If I am cruising at 80, I'm pulling 3300rpm and there is no pinging.

My initial timing is set at 2 degrees btdc. I am going to try running it without the vacuum advance to see it that helps, hopefully without hurting performance. I have a comp cams 268 grind and I am running ported fuelie heads on top of 10.2 to 1 compression with high test gas. I'll let you guys know after I take it out for a spin later.

 

Darrell -
If you have your total timing set and verified to 28 degrees, the lack of a stop bushing is not the cause of your detonation. The stop bushing only affects the length of your advance curve, thus establishing the relationship between your initial timing and your total timing. If you have 28 degrees total timing and 2 degrees initial, you have a 26-degree long advance curve. This is indicative of a missing stop bushing, but installing the bushing will only serve to shorten the curve, thus giving you, perhaps, 8 or 10 degrees initial with your 28 degree total. Thus, it will not affect your detonation if you've set up and checked your total timing. But if the bushing is missing, I'd suggest replacing it so you can get the proper advance curve length. You can get a brass stop bushing from Mr. Gasket kit part number 927 or 928. You will need to remove and disassemble the distributor to instakll the bushing.

But to address the detonation, I would rather suggest you do 2 things:

First, richen up your carb a bit. If you've gone from a near-stock setup to a nice performance application using the same carb, you're most likely running too lean. A lean mixture will cause detonation just as bad as advanced timing and high compression. Bump your primary jet size up at least 2 sizes and see how the engine reacts. You may even be able to go as much as 4 sizes depending on the carb you're running and your new engine configuration. Play with it and see what happens.

Next, do a quick check and verification of TDC to make sure you're really setting the timing correctly. It's very common on a new engine that the line on the harmonic balancer is not compatible with the tab on the timing chain cover, so you may not really be at 28 degrees total... Always good to check and verify.

Lars

 

Lars

I have total timing with the vacuum advance at 46 degrees at 3000 rpm. I was going to try running it without the vacuum advance to see if that makes a difference. The plugs were burning clean with a little brown coloring on the insulator but that was after running the car and not trying to make it pin

I will be at LARSFEST NJ in one month so maybe I should save the carb jetting until then? BTW, where can I get the bigger jets for my Rochester 4bbl and that brass bushing so I can be ready for it?

Darrell

 

I will have all required parts with me at the Tuning Session. Stewart is gathering all info on all of the cars so I can assemble the parts list - if he has not contacted you for info about your car, please drop him a note to get the spreadsheet that outlines the info I need.

 

Lars,

I already gave all the info to Stewart. I'll try to work on this more in the meantime. Thanks and I'll see you in Sept.

Darrell