I Need to Tame a Cat

[speedreed]

I inherited a 1968 Cougar XR7 with a 390 2-barrel.  The car was in storage for quite a while, and the engine was seized.  The VIN says it has an "X" code engine with 10.5:1 compression ratio and requires Premium fuel.  My research showed that Premium fuel in 1968 was 100 octane.  This is not practical for me.  I would like to rebuild it to run 93 octane pump gas.  So the question is: What is the best way to do this?  I assumed different pistons would be in order but I am also wondering about the cam, rods, crank, etc.  I read somewhere (forget where) that 9.6:1 compression is a good fit for this.  The engine is completely torn down, so all options are on the table.  Any advice would be appreciated, and part numbers would be awesome.
P.S. The heads are C8AE-H.

[Yellow Truck]

Not an expert, but "X" code, 10.5:1 with a two barrel? Seems odd. In any case, you can run a 10.5:1 engine on 93. I think the important question to ask is - what do you want for the end result? Stock appearance, low cost rebuild and a reliable driver, or something in the max performance range and damn the cost?

If I were rebuilding a 390 right now I'd be placing an order with Jay for a set of his new heads.

[speedreed]

@ YellowTruck - Fair questions. The 2 barrel had me scratching my head too.  Apparently the "X" code is relatively rare. One story I read had something to do with a loophole in insurance rates at the time.  Another said it was marketed as a fuel-efficient big block, but not many people bit on that offer.  The goal I had in mind was to get as close to original as possible, while making it driveable with todays available (93 octane) fuel.  I don't want to completely de-claw it.  In fact, my first thought was to ditch the intake and carb until I found out about the rarity of it.
Are Jay's heads available yet?  I guess I missed that.

[chilly460]

Jay's heads are well beyond the scope of a stockish 390 rebuild. 

I'd dump the 2bbl induction, keep it on the shelf if you ever want to return to stock but doubt it really adds to the value of the car. 

[blykins]

Agreed, a 390 wouldn't know what to do with a set of 400 cfm heads. 

To the OP, a cam change could theoretically get you to a spot where you could run pump gas with that compression ratio, but if you have it apart, a simple piston swap would get you where you need to be without much fuss. 

[mbrunson427]

Our '68 Cougar started life in the same trim. It's an X-code 390. You're correct, marketed as a fuel-saver big block. The Mustang had the same option, even less were produced. A guy here in Denver has one.

Before the car was re-done, it was driven with the original engine, no issue with the fuel. I don't think you'll have any problems with it.....other than lack of performance. Ours has been completely gone through, changed the color from dark green to black, has a stroker 427 now. Which makes yours just a little bit more rare I guess.

[Yellow Truck]

Brent knows what he is talking about, but I meant I'd replace the rotating assembly with a stroker kit, new cam, aluminium (Jay's) heads, and new intake. I think a 390 block would be able to handle 400 CFM, but an otherwise stock 390 would be overwhelmed.

[blykins]

I haven't read all of Jay's thread, but with the 2.300" intake valve that he was discussing, it would take a large bore to accommodate. 

[chilly460]

I guess I'm confused how we go from a guy contemplating a light rebuild with a 2bbl induction, to a stroker capable of utilizing 400cfm heads...meaning 800hp or so. 

[thatdarncat]

... The VIN says it has an "X" code engine with 10.5:1 compression ratio and requires Premium fuel.  My research showed that Premium fuel in 1968 was 100 octane.  This is not practical for me.

One thing to keep in mind is back in 1968 U.S. gas pumps used the RON octane rating number, in the early 70’s we changed to the AKI ( R + M )/2 octane rating number.

[My427stang]

I wouldn’t sweat it, they were advertised as 10.5 but were generally less and the 114 intake centerline of most cams of the era and slow advance curve lets them run just fine on mid grade, even if not too fast lol

[Falcon67]

If the engine is coming apart anyway, change the pistons.  Measure everything as is and calculate a real reading.  Lot of Ford engines are "advertised" at a certain CR.  On most 351Cs to get 10.5:1 you need a zero deck figure with a 65 cc combustion chamber, which is typically well outside of "nominal" as-delivered parts.  Mine have all been .012~.016 down in the bore with stock parts and chambers well larger than advertised. 

Also, I would not use 93 octane as a target for daily on a rebuild.  93 is typically just a fuel spiked to hell with toluene which makes it sooty and nasty.  Used to be common here, now almost can't be found.  Pump gas is trash anyway and I'd not expect 93 to be universally available.  With a decent driver cam speced in by Brent and a 9~9.5:1 static ratio you'd have a good driver and not care about 0.5 or 1.0 point of compression not being there.  And I could pretty much guarantee that 93 octane now isn't near the "real", "good" 91 octane I could buy back in 1974.    I used to run 93 mixed with VP110 - the intake ports and backs of the valves looked like I'd been running on Aunt Jemima maple syrup spiked with coal dust.

[plovett]

Yeah, forget about Jay's heads.  They aren't even close to right for the application.  Jay knows this as well as anybody.

Just rebuild the engine with a true 9.5:1 compression.  Or something close to that. 9.6:1 would be fine. Can you go higher on pump gas? Yeah, but the gains are tiny and the risk is large. 

Use a mild high performance cam (~218@0.050"), headers, dual exhaust, have a good valve job done on the heads, pocket porting if you have the money for it,  a decent intake manifold like an Eddy RPM, Holley Street Dominator, Eddy Streetmaster, etc., and a Holley 3310.  It'll run fantastic.

JMO,

paulie

[Nightmist66]

I would say forget the stroker kit. Save your money. I also agree that sometimes the compression was less than advertised if you tear it down and measure. The old 390 I took out of my car was nothing special and ran just fine on mostly pump 93 with a little AV mixed in for good measure, 38° timing. It was around 9.3:1 I figured, when I tore it down. It was just .030" over with a windage tray, a mild hydraulic flat tappet(Wolverine 224/234 @ .050" .536"/.562" on 110/105), factory C6AE-R's with light hand port, Edelbrock performer RPM, ancient 4777 650. Headers, of course and a 4.33 gear. Judging from trap speed, it was making an honest ~400hp or so. Felt strong for what it was.


Falcon- If you are not recommending the O.P. use pump 93, what are you suggesting? I see no reason it won't run OK on pump gas. I have run it in mine with no ill effects and have been using it in my daily for 3+yrs. My daily, way off topic, is a 4.6 Crown Vic. I use Shell fuel exclusively. When I have pulled the plugs a few times, they are always clean and the tops of the pistons that I can see are also very clean. This is efi, though and I run a 4.10 gear. Maybe that helps, maybe not.

[Stangman]

If I remember correctly Jays heads won’t work with a shock tower car. I could be wrong. Now a set of Eddy’s or Barry’s Or BBMs will help out with the compression also. Yes a cam with the right specs will also help. Brent seems to have a niche for that information. Between the heads and cam you could have a very streetable combo that’s got some bite to it.