Differences between stock C code and A code 289 - Vintage Mustang Forums
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post #1 of 18 (permalink) Old 07-09-2019, 06:22 PM Thread Starter
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Differences between stock C code and A code 289

In modifying a stock 1966 200 HP 289 C code to be a stock 225 HP A code, there's the obvious change of the intake manifold and carburetor. I have found the correct intake and Autolite 4100 CZF-B carb.

What else were the differences between the C and A builds? Did Ford use a different cam or heads? Was anything different in the short block like crank, rods, bearings?

I found a good C code 289 and am going to rebuild it to stock A code specifications. The cylinders will be bored .020" over and the crank .030" under. Since I'm having to buy larger pistons, I'm not sure if the C Code pistons were the same in an A code. Were the exhaust manifolds different? Did they have the same compression ratio?
Thanks for your help.

1966 2+2, 289 A code, AOD AT, 3.55 Currie Traction Lock, PS, PDB, AC, Pony deluxe interior & Rally Pac
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post #2 of 18 (permalink) Old 07-09-2019, 06:51 PM
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C code pistons were dished and A code were flattop I believe. This would have changed the CR. The A code was advertised as 10:1, the C code as 9.3:1 IIRC.

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post #3 of 18 (permalink) Old 07-09-2019, 07:03 PM
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The only other mechanical change, in addition to the pistons etc which are noted above was not the engine, but the rear end. C codes came with a 2.79 rear gear, A codes came with a 3.00 rear gear. How much effect that fractional change in gearing makes in the seat of the pants acceleration is probably more a function of engine condition, timing and tuning, but that's the way they came when new.

[/SIGPIC]67 Fastback GT -- original colour (Frost Turquoise), original 289 A code engine. Pic is of me and the Mustang taken in May of '67, with original F70-14 Wide Ovals. Same car is now restored to "as new" but 3 speed tranny swapped out for 4 speed back in the mid '80's, with tach dash, original Equalock rear, Koni's, Opentracker UCA, LCA, roller perches and idler arm, roller bearing pedal cluster, Cibie headlights, 4100 carb (old 4300 put in storage probably forever)
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post #4 of 18 (permalink) Old 07-09-2019, 07:27 PM
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The difference is the pistons. The A code had just 4 valve reliefs that displaced 3cc total while the C code had a dish in addition which displaced 13cc. That extra 10cc is worth about a full point loss. Once you actually calculate compression adding it gasket volume and so on the actual compresson of the A code is maybe 9.5 at best and the C code in the low 8’s.

The problem with these engines or one of them is they’ve more then likely been rebuilt and who knows what pistons have been used? Everyone assumes the flat top pistons advertised are the same as what Ford used. Many times they’re not and while they look like flat tops they have such huge valve reliefs that they actually displace 13cc. You need to be aware of this.

Externally the A code would have a 4 barrel carb while the C code would have a 2 barrel carb.

If you’re engine is in good shape I wouldn’t worry too much about the difference in compression. The money shot is going to be a 4 barrel carb, dual exhaust and distributor recurve
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post #5 of 18 (permalink) Old 07-09-2019, 07:28 PM
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The distributor is different, I assume its in the curve-weights & springs, but i don't know the spec of how its different.

The rear differences seem to trend that way but not sure it was a hard & fast rule but still it was ~9+% advantage with the 3:00 rear.

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post #6 of 18 (permalink) Old 07-09-2019, 07:46 PM
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Curious that you only want that ~10% more power when 2-3X that might not actually cost $1 more. I know for sure that a stock A w/3spd can feel like 50 HP over a C with auto OR a 4spd. Yet there are plenty of great cams can be had for under $100 or you could make some head changes or porting for free.

Anyone- Is it the cam that runs out or the flow capacity that runs out first on a stock engine?

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post #7 of 18 (permalink) Old 07-09-2019, 08:08 PM Thread Starter
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Originally Posted by Huskinhano View Post
The difference is the pistons. The A code had just 4 valve reliefs that displaced 3cc total while the C code had a dish in addition which displaced 13cc. That extra 10cc is worth about a full point loss. Once you actually calculate compression adding it gasket volume and so on the actual compression of the A code is maybe 9.5 at best and the C code in the low 8’s.

The problem with these engines or one of them is they’ve more then likely been rebuilt and who knows what pistons have been used? Everyone assumes the flat top pistons advertised are the same as what Ford used. Many times they’re not and while they look like flat tops they have such huge valve reliefs that they actually displace 13cc. You need to be aware of this.

Externally the A code would have a 4 barrel carb while the C code would have a 2 barrel carb.

If you’re engine is in good shape I wouldn’t worry too much about the difference in compression. The money shot is going to be a 4 barrel carb, dual exhaust and distributor recurve
Thanks, I now have most of the parts gathered before sending the block, crank and heads to the machine shop. It's a C6AE-6015-C block and I have a C6OE-9425-B 4V intake and the C6ZF-B carb. Dan at Mustang Barn has re-curved the distributor for the 4V. I'm planning to reuse the original C6AE heads with new intake and exhaust valves, springs, rockers and push rods and reuse the original C code crank. The cylinders only need to be bored .020 which limits my piston selection. There's more pistons to choose from if I had it bored to .030", but I want to save the block metal for future bores. The .020 cast aluminum pistons/rings I found have the 4 valve reliefs with no dish, but the reliefs look pretty big. Also, I'm planning to reuse the C code rods. Comp Cams says they have a flat tappet cam that's close to the Ford stock cam.

So does this sound like it should be equal to a stock A code?

1966 2+2, 289 A code, AOD AT, 3.55 Currie Traction Lock, PS, PDB, AC, Pony deluxe interior & Rally Pac
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post #8 of 18 (permalink) Old 07-09-2019, 08:17 PM Thread Starter
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Originally Posted by jfstang View Post
The only other mechanical change, in addition to the pistons etc which are noted above was not the engine, but the rear end. C codes came with a 2.79 rear gear, A codes came with a 3.00 rear gear. How much effect that fractional change in gearing makes in the seat of the pants acceleration is probably more a function of engine condition, timing and tuning, but that's the way they came when new.
Agreed. While trying to keep the car original, I decided to cheat 2 places to improve driveability and not be noticeable to the casual observer. I already swapped the C4 for a 1989 AOD and swapped the 3.00 open rear end for a 3.55 Currie Traction Lock. That's done and it performs better than I had expected. It's a really good match for a stock 289.

1966 2+2, 289 A code, AOD AT, 3.55 Currie Traction Lock, PS, PDB, AC, Pony deluxe interior & Rally Pac

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post #9 of 18 (permalink) Old 07-09-2019, 08:26 PM Thread Starter
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Originally Posted by 1ofAMillion+ View Post
Curious that you only want that ~10% more power when 2-3X that might not actually cost $1 more. I know for sure that a stock A w/3spd can feel like 50 HP over a C with auto OR a 4spd. Yet there are plenty of great cams can be had for under $100 or you could make some head changes or porting for free.

Anyone- Is it the cam that runs out or the flow capacity that runs out first on a stock engine?
I don't disagree. I had 1965 A code and a 1967 A code that I loved in my youth with C4's. I'm just trying to get a cruiser that can go to car shows or out to dinner while reminding me what it felt like long ago. The extra HP means nothing to me, but the originality does. I doubt the engine will ever see 4k RPM.

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post #10 of 18 (permalink) Old 07-09-2019, 10:07 PM
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I hope that was a typo when you said you were going .030" under on the crank. The most I would EVER do is .020"/.020". If it's that bad I'd look for another... you can still find "fresh" .010's.

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post #11 of 18 (permalink) Old 07-10-2019, 06:40 AM
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That AOD will suck the extra hp right out.

Why not throw in a 302 crank, better cam, hipo exhaust manifolds and some mild head work?

Still look original and perform at least as well as an A code performed back when they were new.


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post #12 of 18 (permalink) Old 07-10-2019, 09:53 AM
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Originally Posted by 1ofAMillion+ View Post
The distributor is different, I assume its in the curve-weights & springs, but i don't know the spec of how its different.

The rear differences seem to trend that way but not sure it was a hard & fast rule but still it was ~9+% advantage with the 3:00 rear.
The distributor was a matter of adjustment. IIRC, the weights were the same, the springs slightly different, or perhaps they were just adjusted differently.

There were exceptions, but typically the C codes had 2.79, the A codes 3.00, and the K codes 3.50.

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post #13 of 18 (permalink) Old 07-10-2019, 09:57 AM
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Originally Posted by poqman View Post
Thanks, I now have most of the parts gathered before sending the block, crank and heads to the machine shop. It's a C6AE-6015-C block and I have a C6OE-9425-B 4V intake and the C6ZF-B carb. Dan at Mustang Barn has re-curved the distributor for the 4V. I'm planning to reuse the original C6AE heads with new intake and exhaust valves, springs, rockers and push rods and reuse the original C code crank. The cylinders only need to be bored .020 which limits my piston selection. There's more pistons to choose from if I had it bored to .030", but I want to save the block metal for future bores. The .020 cast aluminum pistons/rings I found have the 4 valve reliefs with no dish, but the reliefs look pretty big. Also, I'm planning to reuse the C code rods. Comp Cams says they have a flat tappet cam that's close to the Ford stock cam.

So does this sound like it should be equal to a stock A code?
That'll be identical to an A code, if the cam matches the split-profile C3AZ-V. If you want to cheat a little, and end up with more power, better mileage, and a smoother running engine, clean up the exhaust ports. It'll only take 2-3 hours, and well worth it.

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Last edited by 22GT; 07-10-2019 at 10:00 AM.
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post #14 of 18 (permalink) Old 07-10-2019, 10:14 AM
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Originally Posted by Huskinhano View Post
The difference is the pistons. The A code had just 4 valve reliefs that displaced 3cc total while the C code had a dish in addition which displaced 13cc. That extra 10cc is worth about a full point loss. Once you actually calculate compression adding it gasket volume and so on the actual compresson of the A code is maybe 9.5 at best and the C code in the low 8’s.

The problem with these engines or one of them is they’ve more then likely been rebuilt and who knows what pistons have been used? Everyone assumes the flat top pistons advertised are the same as what Ford used. Many times they’re not and while they look like flat tops they have such huge valve reliefs that they actually displace 13cc. You need to be aware of this.

Externally the A code would have a 4 barrel carb while the C code would have a 2 barrel carb.

If you’re engine is in good shape I wouldn’t worry too much about the difference in compression. The money shot is going to be a 4 barrel carb, dual exhaust and distributor recurve
In other words...a C code is a perfect engine to throw a turbo(or two) on! It does make me wonder though...if a C code had low 8.x compression with 58cc heads...what compression would you end up with if you used 62cc heads? Not that I have any intention of building a boosted v8(have enough of that in my built 4 cylinder engines) but a little bit of extra knowledge never hurts.
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post #15 of 18 (permalink) Old 07-10-2019, 10:52 AM Thread Starter
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Originally Posted by 22GT View Post
That'll be identical to an A code, if the cam matches the split-profile C3AZ-V. If you want to cheat a little, and end up with more power, better mileage, and a smoother running engine, clean up the exhaust ports. It'll only take 2-3 hours, and well worth it.

Port-Matching
Thanks 22GT. That article was very informative. I have no problem improving the OE design for this build.

1966 2+2, 289 A code, AOD AT, 3.55 Currie Traction Lock, PS, PDB, AC, Pony deluxe interior & Rally Pac
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