Plugs,detonation question

1st of all, take out the APP45's and put in regular 45's. MSD recommends not using any platinum plugs,
You don't give your timing specs. What are they, initial & full advanced ? Which advance springs & stop bushing are you using ?

I would play with the timing a bit (retard total timing & slow the rate advance, etc.) before I went to gas additiives.

Z. Ray

 

My 68 GT 500 did the same thing had to run octane boster.
The PI heads have small valves and the CC of the combustion chamber changes the comp ratio.

 

Dear Mr 7040,

Is 91 octane the highest usually available in California?

Having only fueled rental cars there I never paid attention.

John

 

I don't want to dispute the numbers given to you by the dyno folks, but they don't add up to the MSD distributor specs. If you are truly starting off at 12 degrees then you have 4 choices of MSD advance bushings:

18 degrees: black bushing/largest diameter (plus 12 initial) = 30 total
21 degrees: blue bushing (plus 12 initial) = 33 total
25 degrees: silver bushing (plus 12 initial) = 37 total
28 degrees: red bushing/smallest diameter (plus 12 initial) = 40 total


None of the msd advance bushings will give you the quoted 28 degrees total advance, if you starting out at 12 initial timing. The "28" number they are quoting to you might refer to the "28" bushing. That would give you 40 degrees total timing. Probably way too much.

As far the msd advance springs go, the dyno folks say they are using the stiffest distributor springs. The stiffest MSD springs (two silver heavy springs) will not give you the total advance until 3500 to 5500 rpm, depending which advance bushing is used. To get total advance by the quoted 2700 rpm figure, you would have to be using either a heavy silver/light silver spring combination, or a lighter spring combination: 2 blue springs, or 1 blue & 1 light silver, or 2 light silver springs.

Z. Ray

 

http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/distributors/8594_instructions.pdf

not sure if this the exact dist. that you have, but all the timing info will be the same regardless. the illustrations might help with the bushing location, etc.

I use Autolite 45 in my small block, don't know if that's the right type for the big blocks.

Now that you know what springs are in there, try using both heavy silver springs at first. That will slow the rate of advance dramatically, and might solve your detonation issue. You can always work your way slowly back to lighter springs as desired. I would also start with the black 18 degree (largest diameter) advance stop bushing. With 12 degree initial timing that will give you 30 degrees total timing. You can work your way up from there with the smaller bushings until you get the detonation again, then back off. Get a timing light if you don't have one already.

Z. Ray