Cobra Jet Carb and Heads on 427 GTE?
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Thread: Cobra Jet Carb and Heads on 427 GTE?

  1. #1

    Cobra Jet Carb and Heads on 427 GTE?

    Has anyone put a 428 Cobra Jet carb (735 cfm) and 428 Cobra Jet heads on a 427 GTE? I saw that with just those bolt on swaps, the engine dynos at 414 HP with headers. The stock HP rating was 390. The Cobra Jet heads also allow for the improved Cobra Jet exhaust. But, I wonder if the Cobra Jet heads work with the PI intake manifold with no modifications?

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    Re: Cobra Jet Carb and Heads on 427 GTE?

    GT-E heads are identical to Cobra Jet heads, so unless you have installed some other kind of heads, you already have the same heads as a 428CJ. Obviously the aluminum medium riser / 428 Shelby / 428 PI intake works fine. A 735 Holley also works fine, but the difference in performance is trivial through exhaust manifolds and mufflers.

    The exhaust bolt pattern on a 427 GT-E is drilled with 14 bolt holes. In order to use 428CJ exhaust manifolds two of those bolt holes (upper end ones) has to be plugged on each head using a cut off piece of 3/8" - 16 bolt installed with some sealant. Otherwise you have an exhaust leak at each of those four locations. One hole, the upper one at #4 cylinder, has to be drilled and tapped 3/8" - 16. And of course you will need to use a 428CJ H - pipe, and the entire exhaust system will need to be 2 1/4" instead of the original 2" system.
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  3. #3

    Re: Cobra Jet Carb and Heads on 427 GTE?

    Thanks! It seems that the Cobra Jet and 427 hydraulic W Code (GTE) heads flow the same. So, is the HP gain really trivial with just the Cobra Jet carb and exhaust swap? If you believe this, the Cobra Jet carb and heads on the 427 GTE makes 414 HP (with headers):

    "https://m.youtube.com/watch?v=D3eRVjH8kO0&t=87s"

    The stock 428 Cobra Jet seems to make about 365 to 385 HP with headers (based on various dyno results). I have only seen this one ^ dyno result for a 427 GTE with a bigger carb. Does the stock 427 GTE really make that much more HP over a stock 428 Cobra Jet? I know the factory ratings were 390 HP for the 427 and 335/340 HP for the 428 Cobra Jet. I know that just about eveyone claims the stock 428 CJ was underrated, with some claiming up to 411 actual HP. But what about the stock 427 GTE? Was it really 390 HP, or more?

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    Re: Cobra Jet Carb and Heads on 427 GTE?

    Click image for larger version. 

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    The heads flow the same because they are identical save the way the exhaust bolt holes were drilled and tapped. The early "135 series" 428CJ in fact used GT-E 427 heads unmodified in any way, they are casting C8AE 6090-J, and use the same 14 bolt mounting of the same exhaust manifold used in the 427 GT-E.

    Beginning in mid February 1968 the C8AE 6090-J head was replaced by the C80E 6090-N casting in the 427 GT-E. The same castings were also used on the production version 428 Cobra Jet, but the 428CJ got an improved exhaust manifold and larger diameter exhaust system.

    NHRA factors the 428CJ at a higher number because with the bigger carburetor it will make another 10 - 15 horsepower through open headers with no exhaust system. Both engines are identical in terms of displacement and the cylinder heads are the exact same on both. The camshafts are the exact same. Bore and stroke are slightly different but in both cases the engine is an oversquare design, the bore and stroke makes no difference in power output. Anything that you can do to one will yield exactly the same results if the same modification is done to the other.

    The main difference that is quantifiable is that the 427 block is stronger with its cross bolted mains. The 427 has a superior side oiling system. These are things that lead to better reliability, not power output.

    In my GT-E I used the 428CJ exhaust system. At the drag strip I have raced this car for many years at the Pure Stock Muscle Car Drags (PSMCD). I find no discernible difference between the C9AF-U (735 CFM Cobra Jet Carburetor) and the original C8AF-AD (600 CFM) carburetor. The car runs high 13's / low 14's all day long with either carburetor, totally stock including smog.


    Quote Originally Posted by 1968 View Post
    Thanks! It seems that the Cobra Jet and 427 hydraulic W Code (GTE) heads flow the same. So, is the HP gain really trivial with just the Cobra Jet carb and exhaust swap? If you believe this, the Cobra Jet carb and heads on the 427 GTE makes 414 HP (with headers):

    "https://m.youtube.com/watch?v=D3eRVjH8kO0&t=87s"

    The stock 428 Cobra Jet seems to make about 365 to 385 HP with headers (based on various dyno results). I have only seen this one ^ dyno result for a 427 GTE with a bigger carb. Does the stock 427 GTE really make that much more HP over a stock 428 Cobra Jet? I know the factory ratings were 390 HP for the 427 and 335/340 HP for the 428 Cobra Jet. I know that just about eveyone claims the stock 428 CJ was underrated, with some claiming up to 411 actual HP. But what about the stock 427 GTE? Was it really 390 HP, or more?
    Royce Peterson
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  6. #5
    It looks like the NHRA rated the 428 Cobra Jet at 380 horsepower (as opposed to the 335 HP factory rating). As we know, the 427 was factory rated at 390 horsepower for 1968.

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    Re: Cobra Jet Carb and Heads on 427 GTE?

    Last I heard the 428CJ was NHRA rated at 420 horsepower by NHRA, with the W code 427 rated at 400 HP. Back in '68 NHRA rated the W code at 410 horsepower and the 428CJ as you say at 380. After the racers had time to figure out all the ways they could make horsepower the 428CJ's rating has crept upwards. Bear in mind the NHRA rules are not related to anything Ford built originally. So comparing the factory rating to an NHRA rating is meaningless. They are as alike as packing crates and parakeets.


    Quote Originally Posted by 1968 View Post
    It looks like the NHRA rated the 428 Cobra Jet at 380 horsepower (as opposed to the 335 HP factory rating). As we know, the 427 was factory rated at 390 horsepower for 1968.
    Royce Peterson
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  8. #7
    Well, this shows 380 HP for the 428 Cobra Jet (1969-1970 model Mustang) as of 2015:

    http://www.nhraracer.com/content/gen...rticleid=61904

    Is it not current? And is the 1968 W Code still NHRA rated at 400 HP?
    Last edited by Royce Peterson; 3 Weeks Ago at 05:19 PM. Reason: Edited by admin to make link functional

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    Re: Cobra Jet Carb and Heads on 427 GTE?

    It's complicated - see the weight and power factors here:

    http://www.nhra.net/tech_specs/class...s/1968FORD.xls

    There are different factors for the 428CJ using manual or automatic, plus various factors for aftermarket DOVE F5WE heads, and now Edelbrock heads are also allowed. The 427 looks like it is completely uncompetitive as it seems to be rated at 390 HP while the 428CJ is rated as low as 335 HP and as high as 375 horsepower (depending on class and cylinder heads and intake used), which can yield a significant weight / lb advantage over using the 427 W code in the same vehicle.


    Quote Originally Posted by 1968 View Post
    Well, this shows 380 HP for the 428 Cobra Jet (1969-1970 model Mustang) as of 2015:

    http://www.nhraracer.com/content/gen...rticleid=61904

    Is it not current? And is the 1968 W Code still NHRA rated at 400 HP?
    Royce Peterson
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  10. #9

    Re: Cobra Jet Carb and Heads on 427 GTE?

    Quote Originally Posted by Royce Peterson View Post
    It's complicated - see the weight and power factors here:


    There are different factors for the 428CJ using manual or automatic, plus various factors for aftermarket DOVE F5WE heads, and now Edelbrock heads are also allowed. The 427 looks like it is completely uncompetitive as it seems to be rated at 390 HP while the 428CJ is rated as low as 335 HP and as high as 375 horsepower (depending on class and cylinder heads and intake used), which can yield a significant weight / lb advantage over using the 427 W code in the same vehicle.
    Thanks, Royce, that makes sense. By looking at the various dyno results on stock builds (on the FE forum and elsewhere), my best guess is that the "real" factory HP (with headers) was about 400 HP for the 1968 W Code 427 and about 380 HP for the 1968 428 Cobra Jet. Swapping in the Cobra Jet intake and carb on the 427 might get that number up closer to 420 HP. There apparently was a plan at some point to use the 427 as the Cobra Jet platform for 1968. Too bad they did not go that route, but those performance upgrades are fairly easy to replicate. In addition, it seems that the Cobra Jet exhaust manifolds and exhaust setup saves a substantial amount of HP loss over the stock 427 GTE exhaust setup.
    Last edited by 1968; 3 Weeks Ago at 11:52 PM.

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    Re: Cobra Jet Carb and Heads on 427 GTE?

    I don't agree. Stock they are very close. If both had headers on the dyno the 428CJ is going to make more power because it has a bigger carburetor and a better intake manifold. A stock 428CJ makes about 400 - ish horsepower on the dyno with headers. I am guessing if the 428CJ carburetor was plopped on top of a 427 then it would make about 5 horsepower less due to the aluminum intake being not quite as good as the cast iron 428CJ part. With the stock carburetor it's probably more like 375 horsepower on the dyno.


    Installed in the car, my personal results are they run very close to the same in stock condition. Please bear in mind NHRA race cars are not anything that can be compared to stock performance.
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  12. #11

    Re: Cobra Jet Carb and Heads on 427 GTE?

    Quote Originally Posted by Royce Peterson View Post
    I don't agree. Stock they are very close. If both had headers on the dyno the 428CJ is going to make more power because it has a bigger carburetor and a better intake manifold. A stock 428CJ makes about 400 - ish horsepower on the dyno with headers. I am guessing if the 428CJ carburetor was plopped on top of a 427 then it would make about 5 horsepower less due to the aluminum intake being not quite as good as the cast iron 428CJ part. With the stock carburetor it's probably more like 375 horsepower on the dyno.


    Installed in the car, my personal results are they run very close to the same in stock condition. Please bear in mind NHRA race cars are not anything that can be compared to stock performance.
    OK. But here is 414 HP from a 1968 GTE 427 with a Cobra Jet carb and headers:

    https://m.youtube.com/watch?v=D3eRVjH8kO0&t=87s

    And here is 368 HP from a stock 428CJ with headers, with some mods it was up to 394 HP:

    https://m.youtube.com/watch?v=gjQRllFs5qY

    These are dyno results from the same shop, same dyno. So, it sure looks like the GTE 427 has more horsepower with the same carb as the 428CJ.
    Last edited by Royce Peterson; 2 Weeks Ago at 05:11 PM. Reason: Removed quotation marks to make links active

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    Re: Cobra Jet Carb and Heads on 427 GTE?

    LOL he's got a roller cam and a 735 CFM carb on the 427. I would be embarrassed, that's terrible for a modified 427 and open headers. Ditto the 428CJ - was a plug wire off?
    OK it's Barry Robotnik, and he has a notoriously stingy dyno. Again, if both engines had exactly the same parts I would expect exactly the same results because at the end of the day cubic inches are cubic inches.

    Quote Originally Posted by 1968 View Post
    OK. But here is 414 HP from a 1968 GTE 427 with a Cobra Jet carb and headers:

    https://m.youtube.com/watch?v=D3eRVjH8kO0&t=87s

    And here is 368 HP from a stock 428CJ with headers, with some mods it was up to 394 HP:

    https://m.youtube.com/watch?v=gjQRllFs5qY

    These are dyno results from the same shop, same dyno. So, it sure looks like the GTE 427 has more horsepower with the same carb as the 428CJ.
    Last edited by Royce Peterson; 2 Weeks Ago at 05:18 PM.
    Royce Peterson
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