'66 327/300 Tuning

[Mark G.]

The resent discussions involving small block timing have been very helpful. However, I am still experiencing pre-ignition around 1500 rpm on warm days using 92-octane gas. My '66 Corvette has a 327 base engine with powerglide and air conditioning. It did have A.I.R. (K19) originally and now has a 1967 Holley 3810 carburetor. The initial timing is set at 6 deg. advanced at 500 rpm vacuum advanced plugged. The engine pulls 21" Hg. at idle. Full vacuum advance adds about 16 deg. for a total of 22 deg. at idle. The mechanical advance does increase about 15 deg. at 1500 rpm. I could not reference full mechanical advance at 4100 rpm. The carburetor was adjusted with the engine at temperature, in drive and the A/C on. I used the non A.I.R tuning specifications because the equipment was removed.
My question is what should I check/adjust next?

[Joe L.]

Re: '66 327/300 Tuning

Mark------

Corvettes originally equipped with K-19 were also equipped with a 195 degree thermostat. You might want to check to see if yours has one of these and, if so, change it to a 160 or 170. This will not make a huge difference since a thermostat only controls an engine's MINIMUM operating temperature, but it's a worthwhile change, anyway.

Another factor is this: in my experience, the 300 hp engines (both the 327 and 350 cid variety) are very susceptible to pre-ignition and, perhaps, more so than the higher compression engines that one would think would be more susceptible. That's because the 300 hp camshaft (originally GM #3896929) builds very high cylinder pressure.

[Duke W.]

Re: '66 327/300 Tuning

According to Corvette News, the centrifugal advance for you car should be 0*@900 15*@1500 and 30* @5100. Initial timing is recommened at 6 degrees with a range of 6 to 12, so at 1500 with the vacuum can disconnected you should read about 21 degrees. The centrifugal advance is the same for AIR, but the initial is recommended at 4 degrees AFTER TDC, with AIR.

I agree with Joe that the medium performance engines such as the 300 HP have a higher tendency to detonate that the SHP engines because the mediums were cammed for very high volumetric efficiency just off idle, which is exactly what you need with a two speed torque converter automatic.

You could try retarding the initial to about 4. That might just do it since your detonation is only signficant on warm to hot days. The other thing you could do is install at least one stiffer spring in the centrifugal advance mechanism. That might reduce the SOTP low end torque a bit, but it will also rid the engine of low rev detonation.

Because the SHP cams have lots of overlap, which creates a lot of exhaust gas dilution, and a late closing inlet valve which results in lower dynamic compression at low engine speeds, they can handle a higher static compression ratio, and we usually try to improve their modest low end torque by installing QUICKER advance springs - up to the limit of detonation.

Duke

[Mark G.]

Re: '66 327/300 Tuning

Joe & Duke-
Thank you for the assistance. The cooling system was flushed and a 180 deg. thermostat replaced the 195 deg. unit last year. The fan clutch, shroud, and radiator are original. It does not have a top seal (flap style). The mechanical advance specs. I was using came from the chassis service manual. The problem could be with the distributor. I can get it down to 4 deg. advanced as Duke suggested but the vacuum can hits the coil bracket. So, if I pull the distributor for more clearance, which way do I rotate the shaft? I want to stay reasonably close to the A.I.R. tuning specifications because I have the correct carburetor and a lead on a date coded pump with mixture control valve.

Regards,
Mark

[Duke W.]

Re: '66 327/300 Tuning

Set the engine at the initial timing mark for #1, then pull the dist. and check that the dimple in the drive gear points the same direction as the rotor tip. If its' 180 out, correct it. Then check that your wires are properly indexed in the cap IAW the CSM and correct if they are not indexed properly.

If the above two items are correct, you should not have any interference problems rotating the dist. to the proper timing point.

Duke

[Mark Ring]

Re: '66 327/300 Tuning

I would ask at what RPM the detonation is occuring? If it is below 3000 RPM or so, I would try a stiffer centrifugal advance spring as Duke suggested. If it is above 3000 or so, I would back the initial timing down a few degrees. You might need to do both.

-Mark.