Are the 2V closed chamber heads worth the effort?

Can I get the pistons I need to make the 9.5 to 1 compression?

I know Tim at one point had them.

I have a 400 4sp in my truck now, the one pictured in my signature.

 

Yes, you can still get the pistons threw Tim, i got mine from him.

 

I agree with Bud. But I can't answer the question about the reward for the effort as that is a rather subjective thing. However, done properly a closed-chamber head gives quench and that reduces the tendency to detonate and run-on, meaning you can get by with a higher compression ratio than with open-chamber heads, like the stock ones.

But, "done properly" means the block has to be decked to zero-deck the pistons. That, plus the .039" head gasket gives the desired dimension for quench.

 

There were no 2-barrel closed chamber American Cleveland style heads.

That makes the only option the Australian 2-V 302 Cleveland heads.

These heads have about 60cc chambers.

To get 9.5:1 compression on a 351 you need 85cc above the piston.
Assume the 0.040" squish area had 10cc. You are still 15cc short.

But let's back up. Combustion chamber science. The combustion event, the actual flame travel, is quite slow in a chamber with no motion. To get complete combustion, you have to start the fire early enough (spark advance) to get everything burned. But big spark advance does two things.
1. Negative work. pressure pushing down on the piston coming up wastes some of the energy of the burn.
2. Destructive chemical process turns some of the molecules into different kinds of molecules that have a tendency to preignite, thus causing knock.

So by increasing speed of combustion we cut negative work and don't give the knock-prone molecules time to form.

We do this by physically mixing the flame in the chamber. We do this by arranging a squish area where two surfaces come very close together, squishing the mixture out and stirring up the contents of the chamber. Some people mistakenly think that the reduction in knock associated with these two surfaces coming together is that they cool off the knock reaction. that is why they called it "quench" Sir Harry Ricardo had the right idea and patented the squish principle in around 1919. Other low-knock heads have been offshoots of his basic ideas.

The clearance between the two surfaces needs to be quite close and it is generally accepted that this distance should be around 0.040", or 1mm. Closing up the gap even more seems to work but at some point the piston runs into the head and if it does this enough, the piston breaks. Bummer. on the other hand, the principle seems to work with clearances as large as 0.055", although not as well. If you get much larger than that, the chamber becomes more prone to knock.

It is not important that the pistons be at zero deck, although that certainly makes life easier as there are many gaskets with bout the right compressed thickness. But use of a 0.016" gasket and a piston 0.024" below the deck would also work.

Meyer has two ways to skin the cat, probably more. let's look at them.

The first way is to use stock 351M heads and a special piston that TMI is having produced by Keith Black. This piston has a step that creates the squish by protruding into the cylinder head. It also has a dish to keep the compression at a reasonable ratio. This is called a "step-dish" piston and it is used in other open chamber engines as well, such as the 360 Mopar engine. KB pistons have been built with steps and step-dish designs for years. Unfortunately, the piston is out of stock but should be available in another month.

The second way is to use a closed chamber head like the 302C head, and a piston with enough compression height to be at zero deck or so, also have enough dish to get the compression ratio to a reasonable level. This piston is currently available from TMI.

There is a significant benefit to getting the combustion chamber right in 351M/400 engines. That has been their weakness since Day 1. Get the chamber right and you can run 9.5:1 compression with pump gas, which will get you quite a bit more power as well as improve fuel economy dramatically. If you plan to drive your truck it is well worth doing. Total timing may need to be adjusted. Fortunately TMI is quite experienced in this modification.

I have no relationship with Tim Meyer or TMI. I am a student of combustion chambers and am happy to see another engine family benefit from the advances in understanding combustion processes.

R.