C2 Intake x-over block off

[Steve D.]

I found a thread in the archives where Clem Z recommended .005-.007' SS shim stock for blocking the heat riser x-over (aluminum manifold only). The only source I have found so far is Fastenal, which sells a large roll which has to be ordered. Only brake shims (not SS) at auto parts stores. Any suggestions?

Steve

[Christopher R.]

Re: C2 Intake x-over block off

Speed shops used to sell these in a kit. See if Jegs or Summit carry them. I also believe they come with some Chevy intake manifold gasket sets. When I did this 25 years ago, I cut up beer cans. Not recommended.

[Brian Monticello]

Re: C2 Intake x-over block off

The nice fel-pro gaskets come with shields but there are still holes about the size of a dime in each. I went with these and do not have any problems.

Fel-pro part #FPP-1256 $16
Brian

[Scott Marzahl]

Re: C2 Intake x-over block off

Paragon used to sell them as P/N 9308K for 1957-65, I don't know why they wouldn't work other years.

[Jim S.]

Re: C2 Intake x-over block off

Why did Clem recomend them in the first place?

Jim

[Duke W.]

Re: C2 Intake x-over block off

My experience with blocking the heat riser passage was not at all good and was short lived.

It would take ten to fiteen minutes of in town driving in mild temperatures after the coolant had reached operating temperature for the starting stumble to go away. The fact that my 327/340 is pulling a 3.08 axle probably didn't help as it is tough to get moving in the best of condtions, but the lack of manifold heat caused a lean condition as you tried to get moving with minimum clutch slippage and the engine would stumble.

Using the gaskets with small holes or block off shims with holes might work better. Another thing you might try is just block one side. This will create a dead end passage that will allow some heat to the base of the manifold, but since there would be no cross flow, the amount of heat would be reduced.

For sure if you completely block one or both sides you MUST ensure that the heat riser valve NEVER closes. Otherwise, there will be no outlet for the RH bank exhaust!

Duke

[Clem Z.]

Re: C2 Intake x-over block off

not to get into a pissing contest with duke i ran all my aluminum intake manifold chevy engines with the heat riser crossover blocked even in PA winters and never had what i would consider a problem. with a divorced type choke you must disconnect it because you will get no heat for it. when you install the stainless plates bend a 90 degree 1/8" tab on the top so you can remove the plate if you do not like the way the car responds.

[Rob Dame]

Re: C2 Intake x-over block off

I have the blockoff plates in my 56. I set the choke so it would just slightly close when cold. The choke works great and will open within a few minutes. Even with a PG when cold i can put it in gear and go with no stumble. Heat riser is wired open.

[Duke W.]

Re: C2 Intake x-over block off

Reports on the effectiveness of blocking the heat riser vary. For me it just didn't work well at all, and that was in CA in mild weather.

More common gearing than my CR/3.08 or an auto trans would likely not make the stumbling problem I experienced as noticeable.

If you've ever seen a race car try to get moving in the pits - blip the throttle - ease out the clutch - stumble - blip the throttle -ease out the clutch - stumble - etc. Maybe on the third try the car was moving enough to engage the clutch and it would idle away without stumbling and maybe stallling. This is what I experienced every time I had to come to a stop until the engine had been operating for at least 15-20 minutes, and believe me, it was no fun at all, and not good for my clutch!

Since my engine has an AFB with an integral choke and dedicated choke air heater, blocking the heat riser passages had no effect on choke operation.

Duke

[Jim S.]

Duke.....Clem ???????????

So, now I have heard the problems it may cause,But I still don't know what the benefits are ?
Jim

[Steve D.]

Re: Duke.....Clem ???????????

Jim

As I understand it, it provides a cooler air/fuel mixture and can help alleviate fuel boil over problems associated with long idle times.

Steve

[Duke W.]

Re: Duke.....Clem ???????????

The purpose of the "hot spot" under the carb. otherwise known as a "heat riser" is to promote fuel vaporization for good distribution, and they usually improved driveability especially in cold weather, but they do heat the inlet air, which reduces inlet air density. The reason for blocking them has traditionally been for more power, and one would never leave the heat riser open on a racing engine. FI cars do not need them - witness the Rockester FI engines and any modern EFI engine.

Today the reason would be more along the line to cure a fuel percolation problem. If you don't have a fuel percolation problem, I would suggest not blocking it, but just wiring the valve open is a good idea, if you normally don't drive in cold weather it should not affect driveability and will make the RH exhaust system last a lot longer.

Duke

[Jack H.]

Plus...

if intake is cast iron & painted, you can expect the Chevy orange to cook and discolor in the area of the cross-over passages. Some like their engine compartments to look 'pretty' for as long as possible...

[Jim S.]

Re: Plus...

Oh ! Thanks Steve ,Duke ,and Jack

[Scott Marzahl]

Doesn't this also help prevent warping the

old holley base plates too.