C1 Octane question

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Oct 12, 2005 | 11:18 AM

69ttop502

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Le Mans Master

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From: Watkinsville, GA and Glen Cove, NY

C1 Octane question

I am really enjoying my new 62. I have ordered an AIM and various other C1 publications from the NCRS site. My question is, are you having any trouble running your C1's on premium pump gas. Mine is the 327-340 horse motor. From what I have seen this motor has a 11.25 compression ratio. Thanks for the help.

Oct 12, 2005 | 11:33 AM

Ironcross

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From: Taylor Michigan

It`s actually only 11-1. GM used two steel shim headgaskets to lower the compression a 1/4 point on 62`s. Pump premium is all that you need. If ever there is need to replace the head gaskets, you could recover that 1/4 point by only using a single steel shim gasket. Thats what I did years ago.

Last edited by Ironcross; Oct 12, 2005 at 11:39 AM.

Oct 13, 2005 | 05:41 PM

69ttop502

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Le Mans Master

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From: Watkinsville, GA and Glen Cove, NY

Thanks Ironcross, it does seem to be running great on pump premium. I haven't even so much as checked the timing yet. I am going to do a full tune up this weekend. I was going to ask about the lead substitutes but found all I need in the archives. Thanks again.

Bill

Oct 13, 2005 | 06:46 PM

SWCDuke

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Quote:

Originally Posted by 69ttop502

It's ONLY 11.25:1 IF the deck height is machined to the nominal 9.025" above the crankshaft centerline, but they are typically 10-15 thou high, so knock off almost half a point. Now install the second .018" steel shim head gasket as was done as a running change in '62 and you're down another half-point to no more than about 10.5:1.

If you have a double gasketed factory built engine and we took it apart to measure all the dimensions necessary to compute actual CR, I would be willing to make a substantial wager that the average of all eight is no greater than 10.6.

The bottom line is that if it doesn't detonate on pump premium there's no problem.

So, does it detonate on pump premium, and what is the octane rating of your pump premium?

Duke

Oct 13, 2005 | 07:47 PM

rgs

Drifting

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From: Iowa

I forgot who makes it, but someone produced an aftermarket knock sensor to retard timing on point and electronic ignition systems. A friend of mine used one for testing and would then reset the initial timing to cancel it out instead of using the retard function (it had a knock indicator on it). With a loud engine/exhaust combo, you may not hear minor knock under load.
Of course, maximum power, compression, and timing are inter-related. Change one, you change the others. If you're not concerned with maximum power, you can retard the timing and run higher compression engines on lower octane. Efficiency drops of course.

Oct 13, 2005 | 09:09 PM

69ttop502

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Le Mans Master

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From: Watkinsville, GA and Glen Cove, NY

Duke, I am glad you chimed in on this one. As far as I know this motor has never been apart. It is not detonating on the fuel in the car when I bought it. It was pump premium (93 Octane) with lead substitute. I have topped it off with 93 and no detonation, although I haven't put a timing light on it yet. It does need a tune up as there is crud in the fuel filter bowl and a little rough running at times. I will do a full tune this weekend. I was going to ask about the valve lash. I see the spec calls for .008 intake and .018 exhaust. Is that a good place to be? Thanks everyone.

Oct 13, 2005 | 11:19 PM

SWCDuke

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Quote:

Originally Posted by 69ttop502

I think the 985 distributor only has 24 deg @4600, so you want the initial timing in the range of 10-14. Bump it to 14 and if it doesn't detonate you can leave it there. Then, if you want you can quicken the centrifugal curve with lighter springs.

I recommed .008/.016 on the Duntov cam in a 327. E-mail me and I will send you the procedure. With the Duntov cam you can either use the 30-30/LT-1 procedure or do both valves at TDC since the Duntov cam it not on the clearance ramps at TDC.

Duke