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What Octane?
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pacerman View Drop Down
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    Posted: Jan/29/2017 at 10:01am
Ok, I'm reassembling (actually just adding the heads) to a 1970 390 to get it running and install it in my 70 Donohue to get it out of the way and out of the garage.  When I start driving it, (original 390 bored .030 oversize) with 291C heads, what gas will it take?  93/6 octane?  Will it run OK without backing off on the timing?  I am in CA with crap gas.  I do have other sets of cast iron heads I can install.  I will not make the investment for aluminum heads for this car.  

Joe
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I assume you have the original camshaft?  That and static compression ratio are the major factors in determining the octane requirement.
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The engine had one rebuild by a previous owner.  I don't know the source or specs on the cam.  I can look for a part number.  Thanks,  Joe
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Originally posted by 6PakBee 6PakBee wrote:

I assume you have the original camshaft?  That and static compression ratio are the major factors in determining the octane requirement.
If it is the original camshaft, and the engine build date is after Sept. 26, 1969, it has the Type II cam, which closes the intake valve 7 degrees later than the Type I cam. It's not bad for a stock cam.
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I have a 390 with stock pistons and heads and around 190 static compression. I run it on 91 octane no ethanol. I am at 2500 feet which helps, at sea level I used 94 octane no ethanol. Timing is total 36 degrees and a fairly slow advance. 

A true MD would have been made in Jan-april or so 1970 if memory serves, so it will have the type II cam, if original. 

Chris 
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Originally posted by ccowx ccowx wrote:

I have a 390 with stock pistons and heads and around 190 static compression. I run it on 91 octane no ethanol. I am at 2500 feet which helps, at sea level I used 94 octane no ethanol. Timing is total 36 degrees and a fairly slow advance. 

A true MD would have been made in Jan-april or so 1970 if memory serves, so it will have the type II cam, if original. 

Chris 
Correct about the MD build dates from what I remember. I missed the part about what car it was in.
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If I had other heads to chose from, I'd pick the later lower compression heads in order to reduce my dependancy on high octane gas.

The additional benefit (if you're keeping the valves and seat angles stock) is better flow at low lift. Something the 291Cs weren't that great for. 

Sure they were the heads offered back when muscle was king and the advertised numbers were the stuff of legend. This has always made those heads the most sought after. The reality though, is AMC offered better all around heads later in the `70s. Why not use them.

I know you didn't say you wanted to change the cam, but there are also way better cams that have come along since 1970. This would be a good time to consider the upgrade to one with a more modern "max area under the curve" type of profile. 
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Thanks everyone.  I might change the cam, but I do have some later heads, 090 and 502 as I recall.  I'll see what shape they are in, but might keep the 291s.  Joe
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Flat top single relief pistons or dish???

This is really important because
the 291C heads with flat top single relief
pistons can send the compression ratio
to over 13:1
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