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351C Intake Manifold Leak
#1
I've had low vacuum since I got my car seven years ago (10-12"). I checked the intake manifold for leaks using all manor of sprays and didn't find any leaks. I had low vacuum (14-15") in a stroker FE engine which was due to a long duration cam so I planned a cam change. As the 351C engine was so extremely low I was sure the cam timing was off or the chain was shot. Today I pulled apart the front of the engine only to find quality double roller chain set straight up. On with the cam swap. When I loosened the first bolt I it made a hollow sound through the intake as did all of them. I've removed many intakes and none made that sound. Seems the only thing holding it down was the silicone on the end. The gaskets were black around the four inner ports. Looks to me like they were leaking from the lifter valley. I bet someone reused the gaskets and didn't tightened in sequence. I'm thinking to reseal and close up the engine and give it a try. Opinions ?
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#2
If there is evidence of an internal leak I think it makes much more sense to change the intake gaskets.

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.
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#3
Was there a valley pan installed ?
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#4
TommyK;126525 Wrote:If there is evidence of an internal leak I think it makes much more sense to change the intake gaskets.

Yes, that is a definite. I have a complete Felpro kit. I was curious if others found evidence of gasket leaks under the intake.

don29163;126527 Wrote:Was there a valley pan installed ?

No valley pan.
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#5
The 351C intake manifolds (especially the 4v) are difficult to install without being tilted a little, and will pull oil and air from the valley. It helps to temporarily put in some 4-inch long bolts, with heads cut off, in the four center intake manifold holes in the heads to help line up the manifold when lowering it into position. Because of the different expansion rates of iron and aluminum I don't believe the turkey pans work well with aluminum manifolds. The torque specs are different for iron and aluminum manifolds, as there has to be a little movement with the aluminum manifolds, due to the different expansion rates. For either manifold torque them down in 3 steps.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#6
Related question for you guys...

Can you tell me the function of the "valley pan". I just got my 73 351C 4V back from the rebuilder and need to install the intake. He provided the valley pan and was wondering if this should be used or if there is a better alternative.

Thanks
Jeff
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#7
Mr Mach1;126529 Wrote:
TommyK;126525 Wrote:If there is evidence of an internal leak I think it makes much more sense to change the intake gaskets.

Yes, that is a definite. I have a complete Felpro kit. I was curious if others found evidence of gasket leaks under the intake.

This seems to be a common issue with the 351c intake especially when using the Felpro print-o-seal gaskets. There are many posts on this subject on the network54 cleveland forum.

When I had my cleveland I used gasgacinch to glue the intake gaskets to the head to keep them from moving when installed the intake. Using all thread as a guide is a good idea as well.

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.
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#8
The turkey pan keeps hot oil from splashing on the manifold, helping to keep the air/fuel cooler. You can trim a turkey pan, and drill a couple of drain holes in it, to work with regular gaskets, or paint the bottom of the manifold to help reduce the heat in the manifold.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#9
i have been doing some research on the intake leak problem. On my Cleveland i have had the alum intake sealed 2x in the last 5yrs and i dont put 1000 miles a year on it. My engine builder specializes in Ford engines and does a LOT of Clevelands for street and racing. He doesnt like the pan gasket because he says it causes leaks. Of course he installs more high rise manifolds than OEM so the performance benefit of the pan gasket doesnt really enter into his build equations. For gaskets he just uses the regular Felpro's and a sealant. I have seen good comments on the Permatex product The Right Stuff for the front and especially the rear mating surfaces. Clearly ring blowby will cause increased pressures and you will have constant leaks. A clean and functioning PCV valve helps a bit also.
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#10
Well after tearing it 7/8 apart I thought for sure the intake wasn't sealed right. For years I was sure it was the cam as this had always been the issue in the big blocks I usually run regarding low vacuum. I should have listened to myself. I could have invested another 45 to remove the grille and a/c condenser but I didn't I thought I have evidence of another problem. I had nuts. Put it all back together and it's still only got 10" vacuum. As soon as the rain stops I'm going back to do the job right and swap the cam & lifters. I'm determined to drive this car this summer.

Leon
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