The MR engine family is known as an "open deck" design. This means that the cylinder sleeves are not supported in any way by the engine block at the "deck" or the mating surface between the cylinder head and the block. There are semi-closed deck, and closed deck designs. If you want to read up on the differences between all three, here is a link to a great article that explains it.

Open deck engines are typically designed to be lighter weight, cheaper to manufacture, and easier to have variants designed off a base block. In the case of the MR engine, the MR18DE, MRA8DE, MR20DDT, MR18DDT all share the same block casting, with variations on cylinder size and some threaded holes. This is mostly possible because the engine is open deck. Making the cylinders larger is just a matter of slightly machining the sleeve insert area to take a larger sleeve. This is great for manufacturers, but terrible for real performance applications.

Why is this bad for real performance applications? As cylinder pressures increase with added boost pressure, the forces pressing outwards on the sleeves can get great enough that they can actually deform the sleeve. They can bend and warp it to the point of cracking or shifting. This may present itself in a few ways: You may get combustion gasses in your coolant, similar to how a blown head gasket would show, you can get coolant in your oil if the sleeve is shifted or cracked enough to allow that. You can get inconsistent air fuel ratios and combustion events related to a loss of compression at higher loads. In any of these cases it is not good. So what is the solution? You need to reenforce those cylinder walls.

The best way to do this is to make the engine closed deck. But how? Well, there are two realistic options and one wild and outlandish option (which will be covered later). The two more realistic options are: Machine custom block guards and install them into the engine or use a silica-based engine support material. Both of these systems have been used with great success for many many years.

So, is one better than the other? Yes. 100%. Block guards are formed, machined, pieces of aluminum that are designed to an exact spec and exact fitment. Thats always best. But. They also come with downsides. The biggest one for a niche market like the MR is cost and time. 3 years ago we reached out to a company not far from us in NH about having a few block guards made up. We were given an estimate of 6-8 weeks for the first one to be made, and then it would be 2-3 weeks for the following ones to be made and installed at a cost of several thousand dollars. We can hunt down the real numbers if need be. Now. If you, or we, were building 1000 horsepower pure race engines, this is how we would have done it. 100%. In fact, we do still plan on getting 1-2 of these done in the future as our drag car progresses. BUT. For 1-2 engines a year that make just over 400 or 500 whp, this is just not needed. It is in fact over-kill.

We have not yet reached the limit where a cylinder has shifted or cracked, but we do not want to find that limit. Based on information we have from other similar engines; we have found that cylinder walls in some cases can start to become more susceptible to failure at or around the 320-410 ft lb mark. So, at those numbers it is better safe than sorry to support the cylinders somehow. But at the same time, it is a bit over-kill to get a custom machined clock guard to add a little insurance to the build. This is where silica based block fill comes into play.

What is block fill? Well, it is what it sounds like. It is a material that is mixed with water into a liquid state then added to the inside of a block to add support to that block. Depending on the application you can completely fill the block with the stuff, or you can just add a thin layer of material to support the top of the cylinders where they are most likely to be damaged. It takes only 2-4 days to cure and is as affordable as a nice dinner and a jug of water. There are dozens of different block fills out there, some dry green, some dry grey, some are smooth, some are course. At the end of the day they all do the same thing, but like oil brands, it just comes down to builder preference.

At this point you are probably just looking for a pro's and con's list of the two, and we will get to that. But, it is important that there is an understanding of how block fill is applied in the case of the MR engine. Before doing the install it is important to make sure the engine deck is completely level. Check it a dozen times. To add block fill you need to get salt. A lot of it. You fill the block up with the salt, with leading points to the water inlet and outlet on the engine. Then depending on your goals, you create a void at the deck surface as deep as you want the block will to be. On 400-500 whp blocks, we usually do about 1" deep. You then mix the block fill in a container of some kind and then slowly pour the fill into the areas you want it to be. If you pour too quickly, you do risk displacing some of the salt, so go slow. After you have the fill where you want it, take a rubber mallet and gently tape the engine block all the way around to get the fill to settle and to raise any potential air out of the mixture. Now, let it sit and cure for a few days. We suggest leaving a similar depth of material in the mixing container so you can check on the curing every day. Once the material is hardened fully, take an oem head gasket and mark where all the coolant passages are that you wish to retain. Then, using an appropriately sized drill bit, VERY SLOWLY drill these holes out. Once you're done with this, put the block on its side, put a hose into one of the coolant passageways and run water through it for an hour or so until ALL of the salt is dissolved and removed. This can take quite a bit of time, but it is important to get all of the salt out. Once the block is clean, have the deck re-surfaced at a machine shop and you're ready to rock and roll.

Now for the pro's and cons:

Block Guard Pro's:
If made correctly, a perfect machined fitment means maximum cylinder support.
Aluminum expansion is easy to control with changes in shape and coolant passageways.
Repeatable.

Block Guard Con's:
Expensive, especially on niche engines.
Extremely time consuming.
If you mess up the install, you can damage the engine block permanently.
Requires specialty machining processes to install on certain engines.
Over-kill on medium to low power engines.

Block Fill Pro's:
Cheap
Fast
Every install can be tailored to the needs of the build.
Anyone can do it.
Perfect for medium to low power engines.

Block Fill Con's:
Requires occasional inspection of coolant for loose material.
Non-perfect distribution of load support.
Messy.


Both systems work and both systems are in use today everywhere. From the BoostesBoiz running block fill on J and B series engines to big power engine builds from Cobb and Mountune on Ford Eco-Boost Engines running custom block guards. So long as you use a system suitable for your application, there is not right or wrong way to do it.

Anyone who feels the need to talk down someone using something like block fill watches far too many youtube videos on over-priced builds and has not spent enough time doing research on the matter to understand where each system has its uses.

Earlier we mentioned an over-the-top solution for open deck. That would be a custom billet block. You are talking about making some serious, silly power to be needing that.

Please feel free to leave any questions you may have below, and we will try to answer all that we can, or feel free to contact Moroso if you would like more information on their product, including chemical make-up and suited applications.