Technical SBC cylinder head cooling

Good evening, so why are coolant lines needed from front to rear corners of a SBC intake? I see this from time to time but never understood the reason. My rear ones are plugged. Is it for trapped air? Thanks

 

Promotes even cooling. Used most often in circle track racing.

 

That is the farthest point from the radiator.

If you think about cooling on many engines, the path is a U that starts and ends on the same end of the block/heads.

This makes the rear of the block and heads the hottest area in the flow path. One line from each corner gets the hot water/coolant back to the radiator sooner.

 

this makes no sense to me.

 

Also helps prevent air pockets in the coolant

 

That is what the simulations at GM showed, and why the LS series engines had heavily revised cooling over the SBC.

It was all there on the screen. Red means hot.

Remember the LT series SBC's from the mid-1990's? Remember what GM did to the cooling system on those? I will give you three guesses why.

 

An interesting point is that if you look at G.M. engineering pictures of the 265 as it was being developed, some of them show prototype engines with the reverse cooing system that the LT1 ended up with forty years later.

 

When I think about the coolant path being a U that starts at the bottom front, and ends at the top front, with the middle of the U being at the back...it makes me think that the coolant gets warmed up some on it's path to the back (the block having less heat to p*** to the coolant than the heads), then makes the turn up, and collects more heat as it travels forward through the head.

How does it get hottest at the back? I'm not saying it doesn't happen, just that you haven't explained how it happens. Or shown us the simulation that shows it happening.

 

My buddy's 34 with a 400 small block had a strange heating problem. His engine would exhibit a water knock like you somtimes hear in house plumbing. It mostly happened during off throttle. In addition, the coolant temperature would suddenly spike to maybe 230 degrees and then slowly come back down. He pulled the heads and found no steam holes in them. Drilling the holes and putting the heads back on made no difference. Replacing numerous thermostats and restrictors made no difference. Bleeding the cooling system any way you could think of made no difference. Running coolant lines from the back ports in the head to a raised thermostat housing cured the problem.

 

Venting potential steam pockets.

 

Don't know the specifics on the Chevy design. The head of any engine absorbs more heat from the combustion process than the block does. That was the reason for the reverse flow on the LT1 Chevy in the 90s. They could raise the CR and by having the cooler water go through the head first, it offset the heat made by raising the CR. It had aluminum heads and a cast iron block.
When Chevy went to the aluminum head/aluminum block LS it was determined that reverse flow was cooling the head too much and lowing the combustion temps. So they reverted to conventional flow thru the block first. This allowed steam pockets to form and cause hot spots in the head. They added small steam tubes that allowed the steam/air to purge by rising into the tubes and flowing to the radiator.
In the 500 Cadillac engines I mess with, it was discovered that the flow to the rear cylinders was not as good as it could be when the engines CRs were increased. It went unnoticed when the CRs were lower. The solution was to drill a couple additional holes at the rear of the block so the coolant had some additional paths to flow into the rear portion of the cylinder heads.
I guess its kind of normal that in any engine, if air or steam exists in the coolant system its going to seek out the highest point in the system. The question is whether it becomes trapped during the flow process. If an engine suffers overheating or detonation issues, adding an escape path for any possible trapped air or steam may help, but it shouldn't hurt anything. There may be other contributing issues as well that contribute, such as cavitation in the water pump inducing air into the flow.

 

V8 engine cooling has always been somewhat of a mystery to me. Like Jim said, coolant enters the front of the block and picks up heat as it flows rearward and up, but with most of the transfer openings between the block and heads being the same and symmetrical side to side and end to end, there seems to me to be no way that the coolant in the back of the block and heads would have similar flow as the front. It’s not really a “clean” U, more like a U with leaks along the inside. The pressure being reduced at the rear of the block and no real inducement at the back of the heads for the water to flow forward. Byp*** lines coming from the rear of the intake manifold to the radiator seemingly could enhance the flow to the rear by providing an additional outlet.
In most cases, the factory “in through the block, out through the intake manifold” design has worked well for a long time although I’m sure that a careful study of the heat distribution would show some significant variation from place to place. Early Pontiac V8s and Ford flatheads deviated from the common configuration for a while but Pontiac changed to the common design as did Ford when the flathead was replaced. Like most m***-produced stuff, it may not be perfect, but it gets the job done for the most part.

 

We’ve run a -6 from them to an extension on the thermostat housing for years on a dirt track high compression engines 13-1 or more where they are mostly needed. I doubt it’s important on an 8 or 9 to 1.

 

Not all the coolant goes to the rear of the block before moving into the Cylinder Head. Some coolant goes up into the heads through core p***ages in the deck. So it's not just one simple U pattern. The coolant that does reach the rear most cylinders is progressively warmer because it has circulated around the additional bores.

I think theory behind the external byp*** plumbing is to divert some of this warmer coolant back to the radiator through the thermostat housing before it picks up additional heat by p***ing back through the cylinder head. It would also byp*** the restriction through the heads allowing more flow to the rear cylinders.

It may also help purge trapped air in the system but not all of it. Trapped air beneath the decks and in the head can create hot spots which can aid unwanted detonation.

 

Have seen a few set ups that converted a Gen 1 SBC to reverse cool.
I can see a benefit to having heads cooled before block like factory later engines.

 

The block and head will indeed have large openings, those are required to hold the casting cores when they are cast, and then to get all the casting sand out of the water jacket. But if you look at the head gasket you'll probably find that it will block of such holes completely, or just have small openings to let air and steam rise up and get out with the coolant exiting from the engine.

I'm sure there have been lots of people noticing that the head gasket blocks off most openings, and trying to improve cooling they have opened up all those to match the block/head. Imagine the surprise when the engine boils far easier as the coolant now short circuits straight from the block inlet up to the head exit so there is almost no circulation through the rear of the engine.

 

@bchctybob ya sorta beat to it. But ya the factory system was good enough for the grocery getter but not sustained high RPM racing.

 

I can understand how a circle track engine that's under full load almost all the time would need a different cooling system calibration than a normal street engine. I'm also aware of the holes in the head gasket "along the way" as the coolant travels to the rear of the engine, and that these could be modified with careful testing to help with different load situations.

 

Smokey spent some time looking at this

 

Yes he did . Smokey Yunick also talked about plugging the core p***ages in the block with pipe plugs to divert more coolant (water) to the rear. They drilled small holes in them to let trapped air escape. Chevrolet drilled steam holes in the 400 blocks because the Siamese cylinders were particularly good at trapping air.

 

It doesn’t really flow in a U in a SBC. It flows from the pump through the cylinder block to the back and also from the block up into the head through all the holes at the same time. It relies on the restriction of the holes in the top of the block for water to p*** through all of them as the pump volume is greater than each individually can cope with. This leads to less flow at the back especially at low pump speeds. Some engines get past this whilst still having the same head both sides by using a non symmetrical head gasket, SBC isn’t one.

 

The SBC has a non-symmetrical deck, instead of a non-symmetrical head gasket. The result is pretty much the same.

Anyways, I like Smokey's solution....

 

My brain must be addled! I’ve built them and couldve sworn the block and heads were the same front to back and side to side!

 

If you look at a circulation diagram for a SBC engine the cooling comes into the engine thru the lower radiator hose, travels along the cylinder walls, and than up through the back to the heads. Then thru the heads and out the thermostat housing to the upper radiator hose.
If you add a crossover to the rear of the heads you've changed the flow pattern for no good reason. It does nothing for a street driven engine to have a crossover at each end of the heads, except to get less cooling at the front of the heads.
If a crossover was a good idea then I'd think after all the decades the SBC was around Chevrolet would have eventually changed the intakes to allow rear crossover.

 

I had a 1987 Monte SS, 305 with center-bolt valve covers and one-piece rear seal. It took coolant from the right rear of the intake as well as from your typical SBC sources. If I remember correctly, that extra line tied into an existing heater hose. No idea if it did anything worth while but it was there.

When I got my hands on a 1995 350 with center-bolt valve covers and one-piece rear seal (TBI engine), I swapped the heads and intake for earlier ones and had no problems not running that extra hose deal. I can't remember if the TBI intake had a coolant fitting there. To be honest, I don't even remember the 350 having it's intake when I got it so it could have had the fitting.

 

The later engines do take the heater supply from the p*** side rear of the intake. But they don't have a coolant exit on the driver side.

 

I modified a Performer RPM manifold to have access to the rear water inlet on the heads for my SBC 400 motor. I did several other mods to help with cooling as well.

 

I don't believe the Monte SS 305 took coolant just from the rear of the intake, I believe it took coolant from the front of the intake and water pump, same as earlier SBC's but it also took coolant from the rear of the intake and tied into an existing hose with a T fitting. I still have the T hose fitting in the garage somewhere. I could be wrong but that's what I am remembering. Actually, I still have the intake too. As I recall, the hose coming off the rear of the intake was definitely a smaller diameter. The extra hose on the Monte was p***enger side only.

Interesting ... a Google search isn't showing me any pics of the intake I have, guess I'll have to dig it out for a peek.

 

I found my intake. It is casting 14101074. Wife isn't home to help me load the pic so I searched and found this pic on the 'net. It is as I remembered, regular heater hose fitting near thermostat, other regular hose comes off water pump, and this third hose to the right of the dist that gets "T-ed" into one of the heater hoses. This one pictured is advertised as being off a 1987 el Camino and mine was off a carbed 87 305 Monte SS, both first year (in Canada) for center bolt valve covers while still carbed and just before everything went to fuel injection. Guess it's somewhat of an oddball.

Sigh ... can't get the pic to load.

Edit: Google the part number then images and pics can be found on eBay ads.

 

this one