66 L72 restoration rebuild

[Brett Faust]

After finally finding an acceptable "942" block and a set of "858" heads. I
am starting a L72 restoration rebuild for my 66 and have a few questions for
the list.

The heads are rectangle port, closed chamber and I believe they should be
106.9cc ? I have a set of TRW L2268 .030 forged pistons. Running that piston with a 106.9cc chamber I am told will produce 11.25:1 compression. I am curious if I can get away with this on today's premium pump gas?

The L72 solid lifter cam, does anyone have the spec's on this grind? Is
there another more efficient (with over 35 years of technology) or even more
aggressive cam that would still pass flight judging?

Is it true you can still get a correct oil pan and balancer from GM? If so
does anyone have the part numbers?

While looking at several of these blocks I have noticed allot of them have
been helicoiled for the head bolts. I have never really noticed this before
on other big block applications. Is it wise to go ahead and have this done
while machining the block?

Thanks in advance for your responses,
Brett

[Duke W.]

Re: 66 L72 restoration rebuild

It's unlikely the engine will run without significant detonation on pump premium unless you severely retard the timing, which will turn it into a dog. Consider having the dome ground off level with the crown - flattops essentially.
My FM Performance parts catalog lists the dome volume of the L2268-30 as 35.5cc.

Recompute the CR with this much less chamber volume and be sure to use accurate dimensions for your piston crown-deck clearance and head gasket thickness.

I think it will run okay on pump premium at an honest 10:1, perhaps with some minor initial or centrifugal advance tuning, but you should get confirmation from someone with a similar setup. The actual CR from the factory is probably about half a point less than what is quoted in the specs, which is 11:1.

The cam specs are in the old Chevrolet Power Manuals, but the way it is speced makes it impossible to compare the actual timing with aftermarket cams. Frankly, even after 30 plus years I don't think you can beat the factory SHP cams and companion valve train parts. They're proven, but if you want to pursue an R & D project, that's your prerogative.

I'll leave the rest to Joe and Clem.

Duke

[Brett Faust]

Re: 66 L72 restoration rebuild

I am curious on the compression ratio because several of my fellow chapter members claim they are running this high a compression ratio without detonation? Would you suggest if I try this that I at least run a think head gasket? Also would the fact that I am in a higher elevation area (Utah) make any difference?

Thanks,
Brett

[Duke W.]

Re: 66 L72 restoration rebuild

I don't think most guys have much more than a WAG of what their actual compression ratio is. Ask to see their meassurments and calculations. I'll bet the response will just be quizzical looks.

Yes, higher elevation will reduce detonation with the same CR and octane relative to sea level. The tough part is quantifying it with any accuracy given all the other variables and lack of data on actual measured ratios.

Several years ago there was an excellent article in The Restorer by a member who took careful measurements and computed the actual CR for all eight cylinders of a BB. The difference between highest and lowest was about half a point, as I recall. Small changes in clearance volume can have a significant effect on actual CR. That's what you're dealing with. If you make all the measurements and calculate your actual CRs on each cylinder, then take steps to limit the highest to a reasonable number, like 10:1, you should be okay. Anything else is just shooting from the hip.

The time to start measuring is when you tear the engine down. Then compute what the actuals were. Based on the degree of detonation the engine was creating you can then take steps to eliminate it by picking a target ratio and then detemining what machining operations are required to reduce it to your target value. Then when you put it together you measure everything again to insure yourself that you hit the mark. The only subjectivity in this approach is picking the target ratio. The rest is real science.

Duke

Duke

[Eric H.]

Re: 66 L72 restoration rebuild

Brett -

At higher altitudes the air is thinner and therefore there are less molicules of air drawn into the cylinder, so the "charge pressure" at TDC is less than it would be at sea level. The compression ratio is the unchanged, as this is a volume function.

In traveling around our country, I have noticed that at higher altitude gasoline stations, the fuel is a lower octane number across the board. In Houston (sea level), our premimum unleaded is 93 octane. I have noticed much lower octane numbers at stations in New Mexico, Colorado and Arizona.

So if you are counting on lower air pressures to lower your "charge pressure" allowing you to run a higher compression ratio, the oil companies may have already beat you to this "savings".

Ric

[Mark L.]

Re: 66 L72 restoration rebuild

Brett,
My experience with the 66 425 motor included a complete engine rebuild. We used the same 2268 pistons 30 over and the 858 heads which by spec are 106.9 cc. The heads were measured for cc in order to ensure equal volume of each chamber. The TRW book shows a spec CR of 11:1 for the 2268 pistons. I also used a COMP cam with the factory grind. Please be sure to have the rear journel of the cam grooved to allow proper oiling. This is a 66 requirement only. The only deviation from spec was to install 1.88 exhaust valves in place of the 1.72's. The engine was at 450 hp on the dyno test.
I am able to use 94 octane Sunoco pump gas for street driving at sea level without any noticable predetonation or dieseling. I do notice that when I put a pint of two of lead additive in with the 94 the engine seems to run "crisper".
I should mantion there was an article by Bill Sangrey in the restorer several years ago. He built a "detuned" 425 and was very unhappy with the results. You might want to check it out. Good luck. You'll love driving this car!!

[Brett Faust]

Re: 66 L72 restoration rebuild

Mark,

I have never heard about grooving the rear cam journal??? Do you have any more information on this?? Also I am glad to hear from someone who is currently running my intended combination. The car currently has a LS6 clone 454 (not sure what CR) that is lot's of fun. I don't want to be disappointed with the correct motor.

Thanks,
Brett

[Clem Z.]

Re: 66 L72 restoration rebuild

only 65 396 and 66 427 blocks need a grooved rear cam bearing journal. the groove is 3/16" wide by 3/16" deep in the center of the rear cam bearing journal. this is to get oil to the lifter gallery. i could never understand why chey did this because the did the same thing with the small block,grooved cam and later change to a groove behind the can bearing. you would think they would learn from the small block.

[Clem Z.]

PS

if you have a grooved cam and you want to use it in a 67 and later BB you must take the rear cam bearing,solder the oil hole shut, and then drill a new .060 hole. this is because the later 67 and up blocks have the oil groove in the block under the rear cam bearing. this is a easy way to tell if the have a 67 or later block is by the groove in the block.