Author Topic: Cobra Jet head hardened valve seat porting  (Read 1706 times)

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390rpm

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Cobra Jet head hardened valve seat porting
« on: April 13, 2021, 10:39:13 PM »
My dad just got a valve job on his Cobra Jet heads. The hardened exhaust seats have some sharp steps. There seems to be a lot of "meat" to blend back toward the bowls. What do you guys think?

SMorton67FB

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Re: Cobra Jet head hardened valve seat porting
« Reply #1 on: April 14, 2021, 07:21:13 AM »
The ID of that seat seems too small for that valve. What does the chamber side look like?

390rpm

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Re: Cobra Jet head hardened valve seat porting
« Reply #2 on: April 14, 2021, 08:21:32 AM »
I don't have a picture of it at the moment, but It looked fine. I'm not sure if he used too small of a seat (i.d. wise), or if he was trying to compensate for lack of supporting material in the heads. The stock seat blend was better than this. Do you guys think the seats will retain enough elasticity/rigidity if blended smooth with the walls?

chilly460

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Re: Cobra Jet head hardened valve seat porting
« Reply #3 on: April 14, 2021, 08:47:21 AM »
That looks like a lot of material to try to blend, and you'd have to retain the thickness of the insert at the seat so it would be pretty tricky to grind it back without getting into the seat interface (if I'm seeing this correctly).   The factory heads have a lip there, with the flow going "out" past that lip I don't think the flow interruption is too bad that I'd risk screwing up the insert.  Did you happen to flow the heads?

1968galaxie

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Re: Cobra Jet head hardened valve seat porting
« Reply #4 on: April 14, 2021, 09:59:29 AM »
Looks to me like the wrong seat was used. The ID is way too small.
There is no "extra" support for the seat when there is nothing behind it.

Factory FE doesn't have great exhaust flow in the first place. This looks like a significant reduction in exhaust port throat size.
Perhaps and 80% throat size? Yikes!

390rpm

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Re: Cobra Jet head hardened valve seat porting
« Reply #5 on: April 14, 2021, 10:07:15 AM »
He hasn't flowed them. I'm not sure if he will. Being that it's an exhaust seat-- the step of the insert may not be that much of an issue being that the exhaust is flowing up and out. It's just a very abrupt change in area with some ugly ridges. I assume if the inside diameter is appropriate for the valve size then it may flow okay. I'm just not used to seeing that on a worked (albeit lightly worked) head.

390rpm

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Re: Cobra Jet head hardened valve seat porting
« Reply #6 on: April 14, 2021, 10:10:41 AM »
Yes! that's exactly what I was thinking. I suppose I could get him to take a valve out and measure the diameter, but at this juncture he may be finished fooling with them. I've heard that some Cobra Jet heads had an issue with hardened seats. I wondered if the machinist was trying to compensate for it?

jayb

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Re: Cobra Jet head hardened valve seat porting
« Reply #7 on: April 14, 2021, 10:56:51 AM »
I think it may be worth trying to measure the ID of that seat.  If it is a 1.65" exhaust valve, the seat ID should be right about 1.48".  If it's way smaller than that it would hurt flow.  I don't think opening it up to that diameter would cause any problems...
Jay Brown
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My427stang

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Re: Cobra Jet head hardened valve seat porting
« Reply #8 on: April 14, 2021, 10:59:05 AM »
Agree, and the machinist should be able to do it very quickly with a cutter and pilot without disturbing the valve job at all
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390rpm

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Re: Cobra Jet head hardened valve seat porting
« Reply #9 on: April 14, 2021, 11:23:13 AM »
Cool. So shoot for 88.0-90.0% of the valve diameter. If they were mine I'd take them back, but I think my dad is finished making trips back to the machine shop. He's is in the "...they're Cobra Jet heads, so it's going to make tons of power by default" camp. It's probably a 400ish hp build if the exhaust isn't hindered by the seats.

It's a 401 with approximately 10.07:1 compression (averaged between chamber sizes). Ferrea valves, 3 angle valve job-- probably 11/32.  Harland Sharp rockers, light porting (CO80-N heads), Rpm intake and cam, 800 Edelbrock carburetor, truck headers.

The machinist has supposedly built FE strokers that dynoed  in 500 hp range with stock castings, so he either knows what he's doing, or isn't truthful.

pbf777

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Re: Cobra Jet head hardened valve seat porting
« Reply #10 on: April 14, 2021, 12:53:07 PM »
     The C8OE-N heads are not a casting very receptive to very satisfying results (at least if only in appearance) in exhaust seat installation, this as shown in the photos.  Material is not available below (as with the head inverted for the process) on the one side of the port bowl to support the seat insert, which also provides for excessive exposure to the environment, but also limits heat transfer from the seat to the casting, this meaning that one wants to utilize greater sum of material in the seat to avoid distortion or loss of retention with heat cycling.  Note that increasing excessively the outside diameter for this seat material  sum is not a good choice as the casting is also not very thick before the water jacket is breached, and this is definitely not appreciated, so as often is the case some compromise may be in order for long term success!

     I'm not stating that all is correct, or that something is wrong in the execution of the workmanship as present here, but rather just presenting some thoughts to be considered before one who wasn't present and involved in the process with such observations draws a conclusion.      :)

     Scott.

390rpm

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Re: Cobra Jet head hardened valve seat porting
« Reply #11 on: April 14, 2021, 01:53:28 PM »
That's essentially what I was asking. I wasn't sure if this was a normal result in regard to hardened valve seats in those castings. I assumed excessive grinding would cause them to loose tension. I won't be home to measure anything for a couple of weeks, but I'll check them out when I get a chance. Hopefully-- with a little contouring they will flow fine. I assume the machinist has performed similar valve jobs on his dyno builds. If they performed as well as I've heard they did then there shouldn't be an issue in that department.

TomP

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Re: Cobra Jet head hardened valve seat porting
« Reply #12 on: April 14, 2021, 03:25:56 PM »
I've had several sets of Cobra Jets with seats over the years and all looked like that. The port cuts back under the original seats and because the seats are 1/4" or more deep it gets into the are where the original seats had the port drop away less than 1/4" below. I've just beveled that sharp edge with a die grinder and learned not to use seats on CJ or Low Riser heads, instead just go for bigger exhaust valves to restore a seat.

390rpm

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Re: Cobra Jet head hardened valve seat porting
« Reply #13 on: April 14, 2021, 03:32:37 PM »
That's pretty much the plan. I wonder if anyone has any flow data for a CJ head with hardened seats like these?