94 cobra installing x303 cam Need Help Gramps helping Grandson street cred at stake

Diakrisis

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Apr 18, 2021
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I have installed an x303 cam in the 94 cobra bought a set of gt40p heads for it. What I did was take the original factory roller rockers (F3ZE-6529-AB Crane Cams) and installed them on the gt40p heads along with Comp cams (987-16) valve springs. I noticed the valves after I installed rocker arms, springs and new hyd roller lifters had a little play in them at rest. I completed the build fired it off and had a little rocker clatter then a ping sound. I immediately shut it down took valve cover off and had a broke inner valve spring and broke rocker arm stud.

My questions are these.
1) How did I screw up
2) how do I fix it
3) Can I go back to the original stock gt heads with original valve springs and what rocker ratio do I use on it with x303 cam

Should I have used 1.6 roller rockers instead of factory 1.7 roller rockers? Are the valve springs the correct ones. I will attach pictures.

Grandpa helping Grandson with his car. Street cred is at stake please help.
 

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  • broke rocker arm stud gt40p heads.jpg
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  • original gt heads.jpg
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  • springs.jpg
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HemiRick

I'd be looking at jacking under the house
Jun 28, 2020
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When installing/replacing springs, lots of things should be checked.....installed height, spring bind, etc....Did you do any of this?
 

Diakrisis

New Member
Apr 18, 2021
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Texarkana Arkansas
I bought the heads gt40 p heads complete with valves and springs. I took the springs off and installed a set of comp cams max 590 lift springs
  • OD Outer Spring: 1.430"
  • ID Outer Spring: 1.070"
  • ID Inner Spring: 0.697"
  • Seat Load: 121 @ 1.800
  • Open Load: 343 @ 1.200
  • Coil Bind: 1.150"
  • Spring Rate: 370 lbs/in
Installed x303 cam

Ford Racing Data Sheet


Part Number:M-6250-X303Rocker Ratio:1.60
Lifter Type:Hydraulic Roller LifterEngine Type:FORD 5.0 (1985-1995)
Lobe-Center Int.107°Lobe-Center Exh.117°


IntakeExhaust
Valve Lash:Hyd.Hyd.
Duration:286°286°
Duration @ .050" Lift:224°224°
Valve Lift:0.5420.542
Lobe Lift:0.3390.339


Valve Timing @ 0.05" Lift:OpenClose
Intake:5° BTC39° ABC
Exhaust:49° BBC5° BTC


Installed new roller lifters.
replaced stock rockers 1.6 that came on the gt40p heads with the original 1.7 roller rockers that were on original heads and no I did not check, but as I had #1 cylinder @ TDC on compression stroke I installed rockers and rotated 90 degrees and continued the install by firing order. Checking clearance all the way did not seem to bind. but had a tiny bit of slop thought it might have been because lifters where not pumped up. Would changing from 1.6 to 1.7 be the culprit because cam data says for 1.6

Do I need to buy 1.6 ratio roller rockers will that fix.

Thanks
 
Aug 23, 2020
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Seguin
Running the X cam with 1.7 rockers is going to bump your lift to ~.580, which is a lot for pedestal mounted rockers. .542 (1.6 ratio) is a lot, but I'm sure lots of people have done it in the past. Also the stock GT-40P valves utilize different retainers for the intake and exhaust valves. My spring package I bought from Alex's Parts had specific retainers for my GT-40P intake/exhaust valves. Not saying yours is incorrect, but worth checking. I'd take the heads to a reputable machine shop and have the valve spring installed height, valvesprings and retainers checked, or you could be spending money forever fixing broken parts chasing an unkown problem.
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
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I'm curious why you were changing from GT40 heads to GT40P heads. After the cost of that swap, the gains are not really worth it. You could have stuck with your original head if you wanted.

Also, those springs seem a little light for an X cam with 1.72 RR's . They are only 121# on the seat. It's been a while since i've messed with cams and such but i would think for the lift you'll see with an X-cam you would want something with about 140-150 lbs on the seat. Given that your lift would be very high, you might even want to change to 1.6 RR's to bring total lift down to a more managable range. Then you could run a spring like this which is good for max of .550" lift.
 
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Aug 23, 2020
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Seguin
I looked a little closer at your picture of the broken stud. Are those the factory GT-40P retainers? If so, that's a big part of your problem.
 

WhiteCobra95

5 Year Member
May 2, 2006
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From what I remember, the X303 is a pretty big roller cam. This probably isn't well suited for any non-ported GT40 iron heads from a flow perspective.

The lift profile alone from that cam will probably push a stock Cobra pedestal rocker arm beyond it's limits as others have mentioned. You may need to have the heads machined for proper screw in studs and guide plates. We used to do that conversion back in the day for running factory BBF heads, but that was when head choices were limited. It's kind of expensive and a bit risky if you don't have a good shop do the work correctly.
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
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If the heads aren't ported, they are probably maxed for flow right around .500 lift. SO really the cam is opening up the valve much more than needed for this combo, and stressing the drivetrain components.

This is a bit of a mismatch in parts and really should be rethought out. If you are intent on using the X-cam, you should change to 1.6 RR's to at least bring the lift down slightly.

The X-cam and 1.7''s are creating more total lift on a set of GT40P's than I am running for total lift on my TFS170's/custom grind.
 

2000xp8

SN Certified Technician
Aug 8, 2003
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If this is a real 94 cobra, this entire plan is ill advised.