Blocking off the exhaust crossover to keep heat off the carburetor

[Chester C.]

My friend has a 327 engine that the gas is boiling off. Someone told her to block off the exhaust crossover ports on the cast iron intake manifold. This should keep the heat down of the carburetor but will it have any negative effect on the engine. I believe the choke will still work OK. Anyone done this before and did it worked OK.

[Joe L.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

My friend has a 327 engine that the gas is boiling off. Someone told her to block off the exhaust crossover ports on the cast iron intake manifold. This should keep the heat down of the carburetor but will it have any negative effect on the engine. I believe the choke will still work OK. Anyone done this before and did it worked OK.

Chester------


Blocking the exhaust cross-over may have some effect on cold weather warm-up performance. Otherwise, there shouldn't be any negative performance issues. However, if the heat cross-over is blocked, the heat riser valve must be removed, gutted, or permanently wired open.

As far as the choke goes, it depends upon what year vehicle you're talking about. 1965 and older 327's using an exhaust manifold heat tube for choke operation will not be affected. However, for 1966 and later 327's with a divorced style choke (i.e. intake manifold choke stove), the choke will not work, at all.

[Gene M.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

Chester,
Depends, is the choke mounted to the carburetor and receive it's heat directly via a steel tube thru the exhaust manifold. If yes you will be ok for warm weather driving. Cold weather will not get you any performance till everything comes up to temperature. If the choke spring is set into the intake manifold as in 67 model the choke will not operate. And vehicle operation will be erratic at best unless modifications are made to compensate.

I must have been typing at the same time as post #2. We say the same thing..........

[John H.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

My friend has a 327 engine that the gas is boiling off. Someone told her to block off the exhaust crossover ports on the cast iron intake manifold. This should keep the heat down of the carburetor but will it have any negative effect on the engine. I believe the choke will still work OK. Anyone done this before and did it worked OK.

Chester -

What year is it, and which intake manifold does it have - 250 or 300hp iron, or 350 or 365hp aluminum? Yes, it matters.

[Patrick H.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

I'd first wire open the heat riser and see if the problem improves. If it does, you can leave it that way or decide to also block off the crossover. If it doesn't, I'd bet you money that the crossover trick won't help at all.

[Domenic T.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

Chester,
I live in CA and I use the crossover block off SS plates with no problem. The choke actually works at the base of the carb but takes a bit longer. It also keeps the manifold paint from burning in the cross over section. Another reason I did it was that I have found that the hot exhaust heating the manifold un evenly caused cracks in the manifold under the carb that, when hot mixed exhaust with the fuel/air mixture that would cause a power loss at operating temps.
This was all before the EGR system!

DOM

[Chester C.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

Chester------


Blocking the exhaust cross-over may have some effect on cold weather warm-up performance. Otherwise, there shouldn't be any negative performance issues. However, if the heat cross-over is blocked, the heat riser valve must be removed, gutted, or permanently wired open.

As far as the choke goes, it depends upon what year vehicle you're talking about. 1965 and older 327's using an exhaust manifold heat tube for choke operation will not be affected. However, for 1966 and later 327's with a divorced style choke (i.e. intake manifold choke stove), the choke will not work, at all.

The engine is a 1962, 250 HP. It has the original carb with the choke at the bottom of the carb. There is a steel tube that goes from the exhaust manifold to the choke. I thought the steel tube was where the choke got it's heat. Why does it need the cross over heat. We have already wired open the heat riser valve. thanks

[Joe L.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

The engine is a 1962, 250 HP. It has the original carb with the choke at the bottom of the carb. There is a steel tube that goes from the exhaust manifold to the choke. I thought the steel tube was where the choke got it's heat. Why does it need the cross over heat. We have already wired open the heat riser valve. thanks

Chester------


The choke does not need the heat from the cross-over. Your application is a pre-1966. As I described in my last post, pre-1966 applications use a heat tube for the choke and, thus, are unaffected by blocking the cross-over passage.

[Jim T.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

I blocked off the exhaust cross over a couple years ago and have had no problems driving my 68 327/350 4 speed in cold weather. I wired the heat riser open many years ago.

[Joe L.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

I blocked off the exhaust cross over a couple years ago and have had no problems driving my 68 327/350 4 speed in cold weather. I wired the heat riser open many years ago.

Jim------


Do you have the divorced choke connected? If so, how does that perform?

[Joe C.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

Blocking the crossover in an aluminum manifold will improve things as long as you don't drive in temperatures below AROUND 40 degrees or so. I can tell you that this is an excellent modification to do with an AL intake.

I've been told that blocking the crossover in a cast iron manifold is not a good idea, no matter the ambient temperature. Probably because it takes MUCH longer than an AL to heat up.

I use a race carburetor with NO choke or choke horn, and it starts immediately in 40 degree weather and needs less than a minute of feathering the throttle before it idles on its own. Warm up less than 1 minute and drive-off.

[Duke W.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

The engine is a 1962, 250 HP. It has the original carb with the choke at the bottom of the carb. There is a steel tube that goes from the exhaust manifold to the choke. I thought the steel tube was where the choke got it's heat. Why does it need the cross over heat. We have already wired open the heat riser valve. thanks

The purpose of the heat riser is to promote fuel vaporization, particularly durilng warmup, which is why the valve is closed for the first few minutes of operation, and some valves don't open all the way until there is significant exhaust flow, so it might be closed or mostly closed at idle.

The WCFB and AFB use a hot air choke system. Air is drawn through a rubber hose routed from the carb air horn to a steel tube that passes between the exhaust manifold and cylinder head. It connects under the exhaust manifold to a tube that is pressed into the exhaust manifold and then to the steel line from the top of the exhaust manifold to the choke housing, The choke housing and carb have a small o-ring-sealed passage that continuously draws air through this system via manifold vacuum, and the hot air unwinds the choke coil and keeps the choke open during fully warmed up operation.

Today's fuels have lower mid-range boiling points than way back when, and this promotes fuel percolation, which leads to hard hot starting and sometimes poor low speed driveability due to excessive rich mixtures.

If you want to block the heat riser, my suggestion is to block only one side. This creates a dead end passage that allows some exhaust heat to reach the bottom of the manifold below the carb, but its substantially less than if the heat riser is completely open, even if the valve is wired open or removed.

With WCFB or AFB carbs that have hot air choke systems, it doesn't make any difference which side is blocked. If a Holley with a choke stove mounted (usually) above the right side manifold cross over passage, block the left (driver's) side.

My experience with blocking both sides on my 340 HP engine was severe stumble starting from a dead stop for nearly 20 minutes of around town operation after a cold start, so I pulled the shims.

Cut a shim from about .010" stainless steel stock with enough exposed to grab it with a pair of needle nose pliers. If it doesn't work out you can loosen the manifold bolts and remove the shim. Then torque the bolts in the shop manual sequence and the gasket seal should be okay.

Some configurations have a "heat slot" that routes exhaust gas to the base of the carb. I'm not sure about your WCFB setup, but if it has a heat slot, plug the holes with Welsh plugs or drill and tap for pipe plugs.

Duke

[Arland D.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

Very similar symptoms as Duke described - cold blooded with a stumble for a few minutes. A few minutes of warmup before driving eliminates the stumble and it works very well in all other situations.

[Jim T.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

Jim------


Do you have the divorced choke connected? If so, how does that perform?

Joe, I am using the factory installed intake and it has the divorced choke and it is connected and works. It performs great. I have been using the same Holley spread bore on my 1968 that I installed in 1974. Original engine with 97K.

[Joe L.]

Re: Blocking off the exhaust crossover to keep heat off the carburetor

Joe, I am using the factory installed intake and it has the divorced choke and it is connected and works. It performs great. I have been using the same Holley spread bore on my 1968 that I installed in 1974. Original engine with 97K.

Jim------


That's great information. I had always figured that a divorced-style choke would not work with a blocked cross-over. In fact, I was planning to use an electric choke conversion for my "ZL-1" since the aluminum heads I'm using have a non-functional cross-over passage. Now, I'll just go ahead and use a stock-style divorced choke.