FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: speedreed on April 16, 2019, 09:55:16 AM
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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.
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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.
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@ 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.
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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.
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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.
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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.
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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.
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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.
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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.
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... 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.
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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
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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.
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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
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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.
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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. ;)
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I will make a version of my heads that will fit a shock tower car, and should have a 3D printed model of them to show at the FERR, but they will have the same intake port as the first set. Pretty much designed for high horsepower applications only, the other guys are correct that at least for this application, my heads are probably not a good fit.
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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.
I'm just saying that I would not count on 93 for saving my street engine. Design for lower - 91 at most. EFI is typically cleaner than a carb and my carbs are jetting usually a bit fat for racing so I expect more "grunge". Just saying that the pump fuel here with 10% ethanol is pretty much junk. To clean up the 351C in the Falcon, I run $9.50/gallon VP. Worth it not to have to deal with any combustion issues. If it was a street car, that'd be different. And it wouldn't be running 10.5:1. A previous open chamber 4V ran fine on 91 at 9.3:1 and pushed the car to high 12s on the strip. It's all in the tuneup.
"I use Shell fuel exclusively. " I fill up at Conoco but everything in every station here - no name, Shell, Sunoco, Conoco - all come from the West-T-Go mix plant. Same trucks, just a different blend. All comes out of the same big white tanks in Tye Texas. The only thing I see different is Flying J which runs their own tankers. Pump fuel is pump fuel, buy on price.
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VP Racing sells a fuel additive called Madditive that can add up 80 full octane points which is like turning 92 octane into 100 , my brother has been racing a SBF 12.5 comp. with pump gas and VP Racing's Madditive for over 5 years and works like a champ , one 12 ounces bottle treats 20 gallons
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Did anyone actually read the OP's post, or just jump on at the end? He has an X code 390. 2bbl. No mention of building a higHP build. Simply dealing with 10.5 compression which is fine for street use with a stock 390. The local 93 here in NE texas isn't loaded with toluene as stated. If you're on the right coastal states or the NE states you probably get 91 at the pump. I would highly recommend getting rid of points and go to the pertronix ignition. Weak ignition is problematic for any engine. If it seized it's likely you're going to be boring your block anyway. So dropping compression will allow you different fuel choices. I'm assuming your intent is more of a stock restoration Vs. a performace build.
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My point is that the difference between 9.5:1 and 10.5:1 on a mild build is not huge. We're talking 3-4% for a whole point of compression. So maybe 15 hp. Not worth the risk of detonation, in my opinion. Also, I believe it is better to have some more room in terms of ignition timing, rather than higher compression. A high compression engine with retarded timing is not going to run as good as a lower compression engine with aggressive ignition timing, in my opinion.
paulie
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Similar point - with the fuel available in Texas 10.5:1 without a huge cam and/or retarded timing isn't going to work. I know, I have a 351 with a pretty good size hydro roller and if I don't mix fuel it peppers the plugs. Consistently. Cranking compression is 165 in that motor. And yea, 93 is sooty - look inside your tail pipes. After running on it a couple of years and doing a tear down, I won't have pump fuel in my motors anymore - except for the trucks and the daily that are engineered to use base fuel. LOL, pump fuel it good for putt-putt and the mower. If you want to add fuel injection with timing control and detonation sensors then you could control things as needed to run higher compression.
Also, for ignition I'd look for a multi-strike unit that multi-fires below 3000. Not Pertronics. Update or install a good box, keep the points.
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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.
When I measured the combustion chamber on Jay's new head, it measured a full 4.250" wide, so it needs to go on a 427 block for sure. Joe-JDC
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The chamber size is actually a machining option; the casting is designed so it will fit down to a 4.05" bore if the chamber is machined to a smaller size. A smaller intake valve would be required to fit that bore though.
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That is good to know, thanks for the update. Joe-JDC
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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.
Likely you have C8AE-H heads?
And that 10.5cr "2v premium fuel" should be with flattop pistons, valve reliefs only.
The 1961-63 390-4v pistons with their shallow dish would give you about 9.7 CR with those heads. TRW L-2291F etc forged versions come pretty cheap, and forged can run a little more CR so 9.7 would be fine on 93 octane, even ok on 87 octane if you keep it clean & don't overjet.
The 390-2v FoMoCo pistons have a dish you could use as a salad bowl and will give about 9.0-9.2 CR. Theyre cast, and I've seen a lot of them punched through, which seems odd with the CR that low. Maybe the dish is so deep it weakens the piston. Much less forgiving than the early 61-63 TRW forged ones above.
The cam in there is 192-199-111 LSA and is fine if your goal is not to add power. We used to get 17mpg with that cam in a big C-6 Galaxie.
Depends on your goals. If the cam/lifters are good it saves you breaking in a cam.
If not, the Comp 252H is a 206-206-110 cam, about like the early 390gt cam, or the Melling "RV" cam is 204-214-112 which is about like the 428CJ cam. The Edel Performer (NOT RPM) is 194-204-112? which is about like the 66-up 390-410-428 4bbl vanilla cam.
I don't know why any big block EVER came with a 2bbl!
Yes, get a 4bbl intake, even a stock 66-up iron or Edel Performer. The 428pi alum intake also works well with that stock cam or the ones I mentioned.
All that and keep the CR about 9.5 & you're fine.
That engine can get away with 10.5 CR partly because of the 2bbl- the cylinders just cant fill, not enough carb, so it's more forgiving. Slap a 4bbl on there & it will spark knock like crazy, BTDT. It's too high, unless you run a monster cam which then needs headers, way better heads, etc.
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One point I’ll bring up, cause I haven’t seen it ( or I missed it )
Putting a good set of aluminum heads on will make it more friendly to higher compression without changing anything else.
However, as was pointed out, if the engine is seized it will probably need an overbite and new pistons anyway. But, having to rebuild the stock heads with new hardened seats starts getting pretty pricey.
So if you spend a little extra on a good set of aluminum heads, BBM, FElony, Trick flow, Edelbrock etc.
It’s a win win win, you can run higher compression, get more power and shave some weight.
Of course if you are wanting to leave it mostly stock, then the factory heads and a good cam will run just fine, and still make great power.
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No argument on the points made. However, the post seems he wants to change compression, yet keep it stock externally. I think some jumped it without reading the OP's question. I run 10.75 on pump 93. I do tend to run a bit of 110 during the summer as a mix.
-Keith