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I recently bought a 66 corvair the crown conversion (327 in the back seat). I don't know much about the engine. It has the double hump heads and a solid lifter cam. I installed a atomic efi carb and a pro billet distributor with a 6A box. I had planned on just running race fuel mixed with 91 octane with no alcohol. I recently read that the lead in the race gas will screw up the O2 sensor. I would like to keep the engine in the car. If I run aluminum heads what are the chances i can get away with just 91 octane ? Thanks.
 

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Impossible question to answer since you don't know the compression ratio now....it could just be the common 250, 275 or 300 HP 327" that has flat top pistons and is just a little shy of 9.5:1 compression with those 64cc Double Hump heads on it...and it'll run just fine as is on 91 octane pump gas as is.

If it does happen to be a 10.5 or 11:1 higher compression 325 or 350 Hp engine, or something someone built up, the aluminum heads should make pump gas use a reality...plus you could get the aluminum heads with a 68 , 70, or 72cc chamber to lower the compression, if you get the heads off and find a big piston dome in there.

If you've got the available funds, from $650 to $1600 and up depending on the brand you want, the aluminum heads idea would be a great addition for three reasons...the fuel issue and it takes an easy 60 lbs right off the rear of the car PLUS it'll add about 50 hp at minimum....those hump heads ain't nothing that great anymore..
 

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Or just run a L33 with a 00-02 truck intake and be ahead of the game across the board.

500lbs. Half a dozen wires to run. 300hp 300ftlbs. Pump gas and better mileage.
 

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Or just run a L33 with a 00-02 truck intake and be ahead of the game across the board.

500lbs. Half a dozen wires to run. 300hp 300ftlbs. Pump gas and better mileage.
Yeah....good luck finding headers, motor mounts, trans adapter or clutch package to make that LS engine a drop in fit to a Crown mid-engine conversion on a Corvair.
 

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You reallyneed to know more about what's inside regarding the piston in terms of what the crown is shaped like, how far down the bore the crown is from the deck, how thick the current head gasket is.

Knowing the cam and its events would be pretty useful as well.

A problem with aluminum heads is the gap of the piston to the deck, generally to get in close to the preffered .040 inch realm between deck and crown requires decking the block down to zero clearance, or using .015 to .019 steel shim gasket to keep the clearance tight as possible and preserve compression ratio at the potential cost of accelerating the scrub wear of the aluminum against the gasket's fire ring. Although FelPro's coated shims are claimed by them to be suitable for this type of build. Aluminum is very much more tolerant of compression ratio than iron and bring hard seat inserts that wear well on a diet of unleaded fuel.

So you need to do some exploring into what you've got in this engine.

Bogie
 

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I would think that any 327 w/ double-hump heads (64 cc) would be at-least the 275 hp version, w/ 10.25:1 compression on flat-top pistons, too much for gas these days. Swapping them for same-cc aluminum would make a motor less likely to knock, going to a bigger chamber would help more. Considering the non-hardened valve seats on older heads anyhow, just swapping heads to new aluminum ones w/ the chamber of choice would be simple, effective, get you seats for unleaded fuel and finally save the sixties-vibe of using a gen I small-block.

Would love to see pics of the Crown conversion. A cool bit of history there.
 

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How about doing a compression check on that motor as is. If it has high compression and a shorter duration cam, then the compression will be too high, like over 200 psi. If it has lower compression and a wild cam, then the compression test will come out low, like less than 160 psi. If it tests out at less than 175 psi, then it'll be fine on pump gas.

When I want to get an idea of how good a match of components are in an unknown engine combination, be it 2 stroke or 4, I'll run a compression check to see how good a combination of compression ratio, to valve events, or port duration the engine has. Then, I'll run a cylinder leakdown to see how well the rings, and valves are sealing.

If the numbers are out of wack, then I'll year it down to investigate why.

When I was a kid in Alaska, the neighbor had a bug with a 327 in the back seat, I never got to go for a ride in it, but the owner said it tended to wheely 2nd gear, 1st had no traction. Your Corvair should be fun to drive. The stock ones I have driven cornered pretty flat. I heard that the turbo Spyder models could embarrass a Corvette.

Sent from my moto x4 using Tapatalk
 

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I would think that any 327 w/ double-hump heads (64 cc) would be at-least the 275 hp version, w/ 10.25:1 compression on flat-top pistons, too much for gas these days. Swapping them for same-cc aluminum would make a motor less likely to knock, going to a bigger chamber would help more. Considering the non-hardened valve seats on older heads anyhow, just swapping heads to new aluminum ones w/ the chamber of choice would be simple, effective, get you seats for unleaded fuel and finally save the sixties-vibe of using a gen I small-block.

Would love to see pics of the Crown conversion. A cool bit of history there.
10.25:1 compression with a flat top piston and 64cc heads in a 327 is a mathematical impossibility....even with either factory deck clearance of .025" and a .015" steel shim gasket or a zero decked assembly and a .041" thick composition gasket the result is the same, barely a tick above 9.5:1.

To get 10.25:1 on a 327, you either have to use heads with a 58cc chamber(Power Pack heads) with flat top pistons, or a domed piston with 64cc heads.

Math don't lie.
Any compression ratio calculator will tell you the same.
 

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Yes I'm likely confusing my back-in-the-day (67) 350 Camaro w/ my same-year Nova, each w/ flat-tops in there and the 327 factory rated at 10.1:1, not the 350's 10.25. https://www.gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Nova/1967-Chevrolet-Nova.pdf

Guessing at a CC+ for valves reliefs, the first calculator I take a stab at ( Engine Compression Ratio (CR) Calculator ) (input numbers 1-4-3.25-4.03-.015-64--1-.025), shows same 10.1.

Shrug.
The 1cc input for valve relief is what is giving the overly optimistic result.
Typical 4 relief stock flat top is about 4 to 5cc volume, a little over 1cc per eyebrow.
Get into aftermarket performance SBC flat top pistons and you'll find them even bigger yet, up to 7.5cc as they are figuring on end users often having bigger than 2.02" intake valves, longer durations, and advanced intake lobe centerlines...even the 2 relief high performance stuff is at least 5cc volume.

1cc is roughly 3/8"x3/8"x3/8", or the size of a sugar cube(actual is .3937"x .3937"x .3937")

Run the same calculator with a more reasonable 4cc valve relief and it comes out to 9.77:1

Calculator I generally use is here:
Compression Ratio Calculator - Wallace Racing
It has input for head gasket bore diameter, which can be a good bit bigger with aftermarket composite head gaskets.
 
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