63 intake gaskets

[Boyan B.]

I'm finishing up the engine on my 340 hp 63. Do I leave the exhuast cross over holes in the intake to head gaskets open on both sides, or plug one, or plug both? Fel pro has a small hole in the metal disk, I'm using Paragon intake gaskets that just have a square block off plate. What should I do?

Thanks, Boyan.

[Duke W.]

Re: 63 intake gaskets

My experience is that blocking both of them is not a good idea. Once off the choke the engine would stumble coming off a stop until it had been operating for at least 15 to 20 minutes.

My suggestion would be to block one side. Cut a shim from thin stainless steel stock - say .015" and wipe it down with silicone spray. You should be able to loosen the manifold bolts and pull it without wrecking the seal if it doesn't work out.

Let us know what happens.

Duke

[Robert Jorjorian]

Re: 63 intake gaskets

Boyan
To plug the intake gasket heat passages makes no difference on your choke operation. The 63 340HP AFB uses a heat source taken from the RH exhaust manifold through a tube to the choke cover.The heat passages in a 340 intake do not extend up to heat the carburetor base like on a 300HP intake which reguires an insulator. I doubt leaving them open would have any effect on driveability so I'd just leave them open like GM did.

[Duke W.]

Re: 63 intake gaskets

Leaving them open has no effect on driveability, but closing them off DOES. With the passage closed off there is insufficient fuel vaporization at low speed, which is why my SWC stumbled until the engine was running for at least 20 minutes.

Though the exhaust heat does not reach the base of the carb, it reaches the base of the plenum and gets the surface hot enough to quickly evaporate liquid fuel. It also quickly heats the carb base because of aluminum's low thermal resistance. Without some heat the fuel is slow to evaporate, and this can cause stumble on transients.

Duke

[Robert Jorjorian]

Re: 63 intake gaskets

In Florida heating the plenum under the carb is not an issue .In my experience and in all probability oil splash hitting the bottom of the intake from the lifter valley does more to atomize fuel than anything.
If one decides to keep only one side of that heat passage open, you better make sure your choice is the right side provided you have a working heat riser valve.The left side is an exit route! The hot exhaust hits the closed valve and back pressure forces it thru the right head passage ,under the intake (plenum) and out the left head and into the left exhaust pipe.The LEFT side has very little back pressure with no heat riser valve, which is why it is the route that heated exhaust takes to EXIT when the valve is closed.Once the spring on the heat riser moves the valve open back pressure forcing gas thru that intake is gone so it makes little or no difference on anything.

[Duke W.]

Re: 63 intake gaskets

If just one side is blocked it doesn't make any difference which side, but the important point is that if you block one or both sides the heat riser valve MUST be disabled by either wiring it open, removing the valve, or substituing the FI spacer.

If the valve is intact and closed there is NOWHERE for the RH bank exhaust to flow with either side of the crossver package blocked.

Duke

[Boyan B.]

Re: 63 intake gaskets

I do have a heat riser valve and need to keep it intact for judging purposes. I'd leave the passages open, but I'm afraid of the high heat cooking the manifold and intake gaskets over time. What if I drilled a 3/8 in hole in the block off plates and put the plate in that way--the same for both sides. Would that still give me enough exhuast cross over to heat the plenum at the same time lowering the temp a little not to cook my manifold?

Thanks.

[Duke W.]

Re: 63 intake gaskets

Make it easy - wire the heat riser open. Remove the wire and spray it with GM heat riser spray before judging events, then wire it back open.

I drove my car over 80K miles with the heat riser wired open and never had a problem.

Duke

[Robert Jorjorian]

Re: 63 intake gaskets

If the left side is blocked the exhaust pressure has no route to exit other than overcome the spring tension on a closed heat riser on the RIGHT side which is NOT a very efficient way to remove exhaust gasses.
If the right side is blocked you basically get no heat under the plenum which is what I originally stated makes little difference anyway.
Wiring the valve open makes the whole system inoperative by not letting the gasses cross over(under)the plenum which again I stated makes little difference in driveability.

[Duke W.]

Re: 63 intake gaskets

Uhhh... Bob, it works like this.

If the valve is open exhaust gases are free to traverse the heat riser passage and do so both ways based on wave dynamics in the exhaust system.

If the valve is closed, all the right bank exhaust is forced through the crossover passage to the left hand side. This creates a "hot spot" at the base of the plenum under the carb to aid fuel vaporization, which improves mixture distibution for better warm up driveability. As exhaust heat warms the heat riser spring, it opens the valve to reduce inlet manifold heating, but exhaust gases are still free to flow between banks and considerable heat flows with the exhaust gas. The heat riser will only open under command of the spring because the valve pivot axis is not offset like a carburetor choke valve, so high exhaust flow will not open it - only the spring.

If both passages are blocked there is NO heat flow to the manifold hot spot and this can cause driveability problems after the choke opens, even after the engine has reached full operating temperature, and even in mild weather.

If just one side is blocked, either one, you have a dead end passage from one side of exhaust system that will transfer exhaust heat to the hot spot, but since there is no actual gas flow, heat transfer is reduced relative to the passage being open to both banks.

If you block the heat riser crossover passage - either one or both sides, the heat riser must be modified so the valve is either not there or NEVER closes.

Another consideration is that some engine models have a "choke stove" located on the inlet manifold crossover passage, so blocking the heat riser passage will slow down choke pull off, which could lead to poor engine operation during and/or after warms up.

Duke

[Robert Jorjorian]

Re: 63 intake gaskets

Duke........ I dont think so but what do I know other than by not wiring heat risers open my car runs right before 20 minutes.