so wouldn't you want to do both, ideally? You have to create the hole before the restricted fluid can fill it right?

 

Yes in some cases. I have seen it done where the engine is stood upright. For example, if you stood the 944 engine upright, the air can get trapped as the outlet often becomes lower than the passages. This is the case with the BMW M42 race engine. As for the 944 engine in its stock position, there is no need to vent. The restrictor works great. Solid water in the head. I suppose you could do both. It won't hurt, but who wants to drill their head when a simple restrictor in the outlet will work.

If you look at the water passages in the MLS gasket, you will see ther are fewer towards the front. This forces more water up the rear and purges the air towards the front. Maybe the stock gasket does not have this and adding a vent is helpful. I think the better way would be to use the MLS gasket and never have the fear the vent line becomes lose and coolant loss causes the very thing its there to overcome.

I've read alot about the fear of using the MLS Gasket. Some can never get them to seal etc, or they are to strong and will cause other failures. The sealing problem is not the Gasket, as many engines like the 944 use this type of Gasket. The problem is always something else. Its not a gasket fault. As for the fear the Gasket will be to strong, and something else will fail, thats crap too. I do not know any engine builder that builds into an engine a part that acts as a relief valve. They want no failures.

 

I disagree.

A restriction in the coolant egress from the head would increase the coolant pressure everywhere from the water pump to the egress point of the head where it is restricted, not just in the head. The flow of coolant through the head will be exactly the same as at a lower pressure because the same volume of coolant is going through the same passages. The only place the flow would change is at the restriction point where the coolant will increase velocity as it passes though the orifice or restriction.

The only effects increasing pressure through restricting the egress of coolant from the head would have is to raise the boiling point and compress any steam that is created despite the higher boiling point.

You would do better by adding Water Wetter than changing the coolant pressure in the head.

 

You are correct. The restrictor will change the Block and head prssures. As the block pressure is typically set by the Head gasket, the restrictor usually adds more backpressure into the head.

I am not telling anything new here. This practice is common place. Most engine builders will do similar on any engine to help the cooling.

As for your statements, I am sure there is many ways to solve the same problem. For those who wish to add the bleed should do so. Those who wish to do a more simple solution can look at adding a restrictor. This has been done on these engines for many years and it works.


"The only effects increasing pressure through restricting the egress of coolant from the head would have is to raise the boiling point and compress any steam that is created despite the higher boiling point.

You would do better by adding Water Wetter than changing the coolant pressure in the head."

Not so. This practice is common and helps pushb the water out into all areas of the Cylinder Head with good results. Sure the higher pressure in the head will help raise the boiling point, but you do not want the water to boil in the head period. The system pressure is still at the cap pressure and this is really the pressure that controls the boiling point. Boiling water internal of the head is the enemy. We want the water cavity to be solid water, not steam.

Your last comment about Water wetter. Do not understand what you mean here. It appears you do not understand what I am discussing with you.

The addition of anything that helps keep these engine cooler is a good thing. For anything you do, you just need to do it because it works, not because it is soemthing else to sell.

 

m42,
I'd like to give the restrictor a try. Where can I find what I need? Can you post some info on the specific part, and where to install it?
~Eyal

 

The 951 head has a cavity that does not vent, it is a dead end passage. It is like the jar I described earlier. It looks like Porsche failed to connect it with another cavity close to it so it would have flow as there is another cavity next to it that is also a dead end passage. No matter how much "back pressure" you create in the system those areas do not recieve coolant, they are dead end passages. Again, increasing pressure in an area ie "back pressure" or whatever you call it, does not increase the flow to areas that would not normally receive flow in the first place. It is a closed system. For more pressure to fill something that doesn't fill already there would have to be only a restirction that causes the lack of filling that the increased pressure would overcome. In this case it is not a restriction but a dead end passage so increasing the pressure would not fill the cavity. That is why I used the jar example.

Yes in a perfect head their would be only coolant, but despite your solid water theory, there are "hot spots" that will create steam or water vapor no matter what the temperature or pressure of the rest of the coolant is. Hence the reference to Water Wetter. You may want to surf over to the Red Line site and read their technical paper on water wetter and why they developed it.

As for the 951 head, it does have a design flaw that is a dead end passage that creates a steam pocket near the #4 cylinder that will cause deformation and failure. I personally do not see how increasing the pressure in the head would have any effect on that pocket except to delay the inevidable by increasing the boiling point.

 

 

I understand what you are saying. You are quite correct about not filling a cavity if it is filled with air that cannot vent. Are we talking about the 2V head?

I have seen many of these cut into pieces, and cannot remember where you are talking about. Can you post a photo of the exact location. I'm in the process of building an engine, and believe me, I would like to know. If you have found something, well done. I have looked before and every cavity is connected. Is it possible you have found the one head that is not. This is my point. Just force the water out into all parts and your good to go. If there was a cavity that would not vent, Head gaskets would be blowing all over the place. The only Gaskets I have seen that have let go, seems to be the Fire Ring from detenation, not from the Head lifting from overheating.

Can you post the location of the pocket you speak of. The location where LR tell to fit the bleed must be the pocket area and the highest point right? So it must be accessable from the outside of the Head. I just cannot see where.

If there is, I stand corrected and have learned something here.

Thank you.

 

Every cavity is connected, but air gets trapped in the head unless you install bleeder holes. Like when you open the bleed screw for the coolant system, not all the air gets out.

If you have cut heads in half then you have seen the line where coolant stops, it is very clear when you look at any 2 valve 944 head.

Increasing backpressure will not raise this line more than the air can be compressed, which is not enough. Bleeders must be installed, backpressure helps water not boil and that is all. Boiling water isnt even the problem, it is air, not steam, that is trapped in the head.

 

Ok , but please post some pics. I have seen heads cut and never have I seen what you are saying. There is a small area at the back of # 4 but its connected. There is no way air can get trapped there as the water exits the Block atbthe rear mostly and any air will be forced towards the front of the head. If the system is filled correctly and the Header tank is at the highest point, I cannot see how the air can be trapped there, any more than any where else in the Head.

Still anything you can add to help must be better.

 

The simple solution is to hit the curbs hard enough to get the car up on two wheels (at a minimum of 45 degrees) – this will help release the trapped air in the head. When the car slams down it also helps to shake loose any bubbles started by nucleate boiling.
This way your head gasket will always last longer than your suspension….

BTW -the Cometic gasket has gone through several design changes - mainly where the water coolant passages are located.

Chris White

 

Can you do this modification and still stay class legal in a PCA club racer??

 

"The simple solution is to hit the curbs hard enough to get the car up on two wheels (at a minimum of 45 degrees) – this will help release the trapped air in the head. When the car slams down it also helps to shake loose any bubbles started by nucleate boiling.
This way your head gasket will always last longer than your suspension…."

Or'''' have a really strong person lift up the front of the car and spin it over a couple of times on its rear end. This way the bubbles will flow out the top and you've not only saved your Head Gasket but the suspension as well.

But seroiusly, I'm not against anything that will help. We all know we need every bit of cooling we can get. Just need to do things because they make sense.
Thanks for the input. It made me question and relook at my situation.

Thanks again.

 

I wish I could post pics, but I do not have access to the heads anymore.

You will have to take my word for it!

 

I'll have 1 of mine cut to see. tell me where to cut, so that there is no confusion. I do see at the very end the cavity appears to be higher, but it is connected to the rest of the head cooling passages. If this is what you mean, that is not a problem. I now wondering if you have been looking at this with a Head on the bench. Held upright, you may think this is the highest point, but installed on an engine and installed in the car, this is not the highest point. Could this be the case?

Thanks for your info. This has a least made me think and look at my situation. Its always good to get alternative opinions. They at least make you think and look.

Thanks again.