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Heated Intake Manifold Crossover

Posted By Ted Last Month
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Ted
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Because the original thread regarding the heated intake manifold crossovers is taking place in a Classifieds post, I’m starting a new thread to make it easier to find when doing a search.  Here’s the link to the original thread.
http://forums.y-blocksforever.com/Topic148499.aspx 
 
Feel free to post with additional information.


Lorena, Texas (South of Waco)


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PF Arcand (11/29/2019)
Going back on the postings here. Why on a truck would you block off the crossover on the Blue Thunder or "B"manifolds?  Assuming the exhaust system doesn't have a Heat Riser, it's very unlikely that there will be excessive heat, particularly in the area where U live.. Some degree of heat helps vaporize fuel which helps warm ups & avoids card icing.. it's not a race car.

 
I get a large number of requests for blocking the intake manifold heated crossovers in the Y-Block intake manifolds.  One of the reasons is simply due to the paint burning at those crossovers on the painted manifolds.  Another reason is to eliminate the potential for carburetor flooding in hot ambient temperatures (Texas and other southern states) that occurs when the engine is shut off due to the manifold being overly hot.  Both reasons are valid but must be weighed against where and when the vehicles are being driven.
 
Both carburetor icing and that transition between where the choke has opened and the engine is still not fully up to temperature are two of the reasons why the intake manifold crossover passage is left open for heat.  When the intake passage is left open, it works best and as designed with the heat riser valve at one of the exhaust manifolds.  Unfortunately those exhaust heat riser valves only have a ten year life expectancy which is why so many older vehicles opt for running without them once the valves start to give trouble.


Lorena, Texas (South of Waco)


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Tedster (11/29/2019)
Ted: Can you help and try to explain if/how or maybe the lack of a heat riser impacts the crossover system? For example, say a set of Rams Horn exhaust manifolds. I've never have had a real warm and fuzzy on exactly how it all worked in the OEM setup with the crossover exhaust.

In other words for simplicity's sake there were two (2) possible conditions, flapper open or closed in the original setup, now does the installation of dual exhaust manifolds without any heat riser result in some kind of weird condition that is neither good for hot weather nor good for cold? There's no actual diversion of exhaust through the intake, or does it alternate through exhaust pulses, i.e. is it an identical situation to "flapper open" in the old system?

It does seem a little cold blooded. Not bad, though need to run with the choke full out for maybe 5 or 10 minutes in cold weather. So why not block the manifold ports off completely? This would seem to be better for summer ops.

 
The heat riser valve located in the exhaust in tandem with the crossover passage in the intake manifold helps to heat the intake manifold plenum much quicker than just relying on the heat soak from the engine.  Carburetor icing and fuel vaporization are the primary reasons for having heat riser valves and intake manifold crossovers but they also aid in that transition point where the automatic choke has fully opened but the engine is not fully up to temperature yet.  It’s in that transition area that the intake manifold is actually heating up quicker than the rest of the engine by using the heat riser valve and the carburetor is not as finicky as a result.
 
The heat riser valve is controlled by a bimetal spring that simply expands as it gets hotter.  When cold, the valve is closed and it takes about eight-ten minutes for the bimetal spring to expand enough to fully open that valve (flap).  While the flap is closed, the exhaust flow if diverted through the crossover in the intake manifold and out the other side of the engine.  For those engines where the automatic choke is connected to the tube going through the intake manifold crossover, then the automatic choke works more quickly also.  On the earlier Y engines with a single exhaust, that exhaust heat riser valve is located on top of the engine at the crossover pipe.  In this case, the exhaust flow from the left bank is diverted through the intake manifold and into the right bank exhaust.  On a dual exhaust engine, it can be located on either side of the engine and be effective.  The bimetal springs on those valves do get weak over time, the valves (flaps) themselves become frozen and inoperative, or the stems have worn to the point that there’s an exhaust leak present.  Hence the reasoning why so many of those heat riser valves are not being used when it comes time to work on the exhaust or rebuild the engine.
 
Running without the heat riser valve simply delays the heating of the intake manifold.  Assuming the intake gasket is not blocked at the crossover, then there will still be some exhaust flow occurring but without the heat riser valve, it’s not being concentrated at that point for a quicker intake manifold warm up.  Blocking the intake gasket at the crossover eliminates the exhaust as a form of heating the intake and now the intake manifold just takes longer to come to temperature as it relies now only on the transfer of the heat from the cylinder heads and the front water passage to come to temperature.  Once the engine is fully at temperature, there’s no need for the exhaust crossover as fuel atomization will be sufficient without the aid of additional exhaust flow through the intake manifold.  Once the heat riser valve is fully open, exhaust flow through the intake is minimized by design.  Not eliminated but at least minimized.
 
Even the heat riser valve and intake manifold crossover wasn’t enough as the various auto manufacturers relied on additional means to preheat the carburetor when the ambient temperatures dropped.  Many of the vehicle manufacturers came up with tin work that surrounded the exhaust manifold and the heat generated there was ducted up to the air cleaner with temperature controlled flaps.  Remember the flex tubing that went from the air cleaner down to an exhaust manifold?  That was part of that system.  Those systems were used for over a decade by the various car companies before electronic fuel injection became a way of life.  Electronic fuel injection solved all those cold start and warm up issues to the point that even preheating the intake manifold was no longer required.
 
From a performance standpoint, a heated crossover in the intake manifold is not desirable as it does kill power.  From an exhaust flow stand point, the cylinder heads do not benefit from those extra passages required to get the exhaust to the intake manifold.  To improve the flow in those particular ports, it’s not unusual to weld or fill those exhaust passages going to the intake manifold and then port the exhaust bowls accordingly to restore the exhaust flow values.  All the better flowing cylinder heads do not incorporate the heat crossover ports and likewise, you’ll not find the accompanying crossover passages in the intake manifold for those cylinder heads.  For those of us still running carburetors, then cold weather still presents a challenge until the engine is fully warmed up.


Lorena, Texas (South of Waco)


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a option that works on dual exhaust is different mufflers,lower restriction  on 1side.in bc Canada the heat riser works the best,but 1 that works,re the spring is designed to loose strength with heat.i can see that southern us is very different and agree different remedies.  I choose to have a bit of burnt  paint but a engine that runs well threw warmup


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