Announcing the Merc 888

[Yellowjacket]

Rods and crank will be weak links. Kiekhaefer did a great job of optimizing these engines: Nothing is beefier than needed. Should the rods make it, double duty on the crank journals will likely take them the crank out quickly.

Even so, it's a very cool project.

Actually the rods and bearing have a good bit of margin. In modified motors these parts are spun to much higher speeds than a stock motor. When you realize that stress and loads are a speed squared function, where a 20% increase in speed is a 44% increase in stress and loads, then the stock rods and bearings are actually pretty robust. In addition the big end forces peak at bottom dead center, and these are actually lower with a piston pushing the other way, so big end forces are higher, but they aren't twice as high. If big end bearing forces were a problem you could always go to titanium rods.

At some point you're going to have to get rid of the stock rod bolts and get something just a bit longer so that the area on the cap is a bit bigger in the area between the head of the rod bolt and the bearing area. That's a nasty stress concentration area and that's a failure location for sure.

Also think about making mods or getting rid of the stock crank. You need more area to hold onto the slave rod and I fear that there just isn't enough room to fit in what you need to do with thin area between the counterweights.

[Tim Kurcz]

Actually the rods and bearing have a good bit of margin. In modified motors these parts are spun to much higher speeds than a stock motor. When you realize that stress and loads are a speed squared function, where a 20% increase in speed is a 44% increase in stress and loads, then the stock rods and bearings are actually pretty robust. In addition the big end forces peak at bottom dead center, and these are actually lower with a piston pushing the other way, so big end forces are higher, but they aren't twice as high. If big end bearing forces were a problem you could always go to titanium rods.

At some point you're going to have to get rid of the stock rod bolts and get something just a bit longer so that the area on the cap is a bit bigger in the area between the head of the rod bolt and the bearing area. That's a nasty stress concentration area and that's a failure location for sure.

Also think about making mods or getting rid of the stock crank. You need more area to hold onto the slave rod and I fear that there just isn't enough room to fit in what you need to do with thin area between the counterweights.

Indeed, the Merc crank and rods have performed well in naturally aspirated stock, mod, and even alky applications for decades. Compared to stock however, the forced induction 888 offers the potential for a quadrupling of power. Load and heat dissipation become big issues. Obviously, the stock Merc crank wouldn't last long under those conditions regardless of rod configuration.

Your last paragraph sums up the issue: There isn't enough room to build a robust master-slave rod system without removing counterweight material. This is what is I meant by design optimization. Mercury engineers did an astounding job minimizing space, weight, and materials, all on paper without calculators or computers. That they didn't leave enough space to build the 888 must have been and oversight - LOL!

Given architecture limitations yet to be fully explored for the master-link concept, the question becomes: Which rod/crankshaft configuration offers the greatest chance for proof of concept success in the 888?

The "skinny" aluminum-bronze rods fit within existing architecture and require no modification to the crankshaft. They are limited by (four) #8-32 cap retention screws, offset loading, and lack of needle bearing support. Simplicity is gained at the expense of centerline thrust, cap retention, and friction reduction.

The "master-link" Mercury based rods, bearings, and bolts maintain integrity of the time-proven design. They will require machining .090" (+/-) from each counterweight, and machining and/or adding material to allow swing clearance. Centerline thrust and roller bearing support is retained at the expense of crankshaft balance and strength.

Given these parameters; which would you choose?

[Fastjeff57]

Then there's the balance issue. Those counterweights were designed for one rod and piston, not two. Furthermore, unlike a true opposed piston engine, the 888's pistons move together instead of in opposite directions. That means the 888's rods and pistons will experience DOUBLE the 'throwing forces' (had to call it something). This could be trouble as well.

But it's sure worth trying! Love to be there when she fires up!

Jeff

[Yellowjacket]

Hmmm

Quadruplng the power by FI is probably not the best idea... When I was in high school 40 years ago I read the book "The Sports Car, it's Design and Performance" by a Brit named Colin Campbell. One line in that book stuck in my head and it's been there ever since...

"As a means of finding the weakest point in an engine, supercharging has no equal..."

True then, even truer today with the advent of "turn the screw" boost and turbochargers that any fool can crank in more boost than is prudent...

Crank and rod forces are primarily speed driven and I think that you can live with that, but in order to get more power cylinder pressures are going way up and these engines are close to detonation as they sit. If you go to higher cylinder pressures you're going to have to reduce compression ratio and really, a deflector arrangement isn't the best thing since it is not only heavy, it results in a big chunk of hot metal sticking up into the combustion chamber that is a detonation anchor...

Gordon Gecko is quoted as saying that "Greed is Good", but if you get too greedy you're going to find lots of things that are going to lead to a trail of broken bits and that tends to take the fun out it....

If you get to maybe 150 hp total you've done as much as the basic architecture is good for. More than that and you're going to start breaking lots of things and sticking pistons and that takes the fun out of it.

I think you will probably have to bite the bullet and make a new crank. These cranks aren't that robust in the first place for reasons I'll discuss below, so it may be something that you will have to do in the long run anyway, and doing it up front may be the price of doing business... These cranks can bend and it would be an advantage to better support them with additional bearings in what was the reed blocks. Jeff was concerned about offset imbalance, but remember that the two cylinder pairs would actually not need counterweights if the crank was robust enough to transfer the forces between the two adjacent cylinders. Another way of saying it is that each crank pair is a mirror image of each other and the forces balance, but the forces are huge. Racers have for years trimmed the counterweights in the mod classes without issues, but as noted, the unbalanced forces in the crank goes up and that's something that you could readily calculate and see how much bending you have in the crank.

What I was thinking about was the fact that the reeds are now unnecessary and the area that was used for the reed blocks and for reed travel is now open for crank counterweights. This would also help address the issue that Jeff brought up in that the offset weight is increased. I don't know how you were going to get the charge into the case, but the reeds are unnecessary now so you can use that area for a new crank. Similarly for the middle bearing, while you don't want to reduce the bearing itself, there is area off of the centerline that could be trimmed back. Reed travel (per the APBA tech manual) is .185 (max) which when added to the thickness of the reed stop is close to a quarter of an inch. With the reed blocks gone you could replace them with a bearing support that is bigger in diameter than what is there now. The stock crank diameter was minimized in that area to minimize leakage and get intake area. You want the opposite, a bigger diameter and a bearing support there so that's another reason to make a new crank.

You'll have to look at the existing cases and see what you can fit inside and how it can all work with an additional bearing between the cylinders, but by using the space were the reeds were you may be able to get the area you need to fit a decent design of master/slave rod arrangement and that may get you over the biggest hurdle and let you break it in other areas sooner.

[daveswaves]

Anything that easy can't possibly work..... or it would have been done before. Back to back rods cannot be made to assemble, let alone rotate.

Tim, I still have trouble visualizing why they wont rotate if the cyls are directly opposed. Pretend its just a two cyl opposed with a single throw and common rod joined at the journal, why wont it rotate.??

[Tim Kurcz]

Hmmm

Quadruplng the power by FI is probably not the best idea... When I was in high school 40 years ago I read the book "The Sports Car, it's Design and Performance" by a Brit named Colin Campbell. One line in that book stuck in my head and it's been there ever since...

"As a means of finding the weakest point in an engine, supercharging has no equal..."

True then, even truer today with the advent of "turn the screw" boost and turbochargers that any fool can crank in more boost than is prudent...

Crank and rod forces are primarily speed driven and I think that you can live with that, but in order to get more power cylinder pressures are going way up and these engines are close to detonation as they sit. If you go to higher cylinder pressures you're going to have to reduce compression ratio and really, a deflector arrangement isn't the best thing since it is not only heavy, it results in a big chunk of hot metal sticking up into the combustion chamber that is a detonation anchor...

Gordon Gecko is quoted as saying that "Greed is Good", but if you get too greedy you're going to find lots of things that are going to lead to a trail of broken bits and that tends to take the fun out it....

If you get to maybe 150 hp total you've done as much as the basic architecture is good for. More than that and you're going to start breaking lots of things and sticking pistons and that takes the fun out of it.

I think you will probably have to bite the bullet and make a new crank. These cranks aren't that robust in the first place for reasons I'll discuss below, so it may be something that you will have to do in the long run anyway, and doing it up front may be the price of doing business... These cranks can bend and it would be an advantage to better support them with additional bearings in what was the reed blocks. Jeff was concerned about offset imbalance, but remember that the two cylinder pairs would actually not need counterweights if the crank was robust enough to transfer the forces between the two adjacent cylinders. Another way of saying it is that each crank pair is a mirror image of each other and the forces balance, but the forces are huge. Racers have for years trimmed the counterweights in the mod classes without issues, but as noted, the unbalanced forces in the crank goes up and that's something that you could readily calculate and see how much bending you have in the crank.

What I was thinking about was the fact that the reeds are now unnecessary and the area that was used for the reed blocks and for reed travel is now open for crank counterweights. This would also help address the issue that Jeff brought up in that the offset weight is increased. I don't know how you were going to get the charge into the case, but the reeds are unnecessary now so you can use that area for a new crank. Similarly for the middle bearing, while you don't want to reduce the bearing itself, there is area off of the centerline that could be trimmed back. Reed travel (per the APBA tech manual) is .185 (max) which when added to the thickness of the reed stop is close to a quarter of an inch. With the reed blocks gone you could replace them with a bearing support that is bigger in diameter than what is there now. The stock crank diameter was minimized in that area to minimize leakage and get intake area. You want the opposite, a bigger diameter and a bearing support there so that's another reason to make a new crank.

You'll have to look at the existing cases and see what you can fit inside and how it can all work with an additional bearing between the cylinders, but by using the space were the reeds were you may be able to get the area you need to fit a decent design of master/slave rod arrangement and that may get you over the biggest hurdle and let you break it in other areas sooner.

Fear not, Yellowjacket; I realize the crank train would never stand up to the 150-200 HP that the turbo is capable of delivering, and don't intend to run the engine anywhere near that hard. The factory crank & rods aren't strong enough (let alone the modified master-link pieces), the deflector pistons & combustion chamber would simply melt, and the lack of cylinder pumping may severely limit power anyway. Remember that the objective is to build a running proof of concept, zero crankcase pumping engine from Mercury parts. Given the obvious limitations, and those yet to be discovered, the project may not be overly practical, but it's still bad ***!

Tim

PS: You are correct that space now occupied by reed cages and center main could most certainly be used for a custom crankshaft & rods. I'm just not willing to chunk out $10K for something that might not work..........

[Tim Kurcz]

Tim, I still have trouble visualizing why they wont rotate if the cyls are directly opposed. Pretend its just a two cyl opposed with a single throw and common rod joined at the journal, why wont it rotate.??

Hi Dave,

Considerer that he Merc 4-cyl crank is two pairs of opposed crankpins spaced 90 degrees apart. So, two of the piston/rod combinations will actually assemble in-line (but not rotate). As you're lowering the upper block while attempting to stuff the second pair, you realize pistons are hanging way outside the blocks because the opposed rods don't swing because of the solid beam rod. Does this help?

Tim

[hydroplay]

Tim, I still have trouble visualizing why they wont rotate if the cyls are directly opposed. Pretend its just a two cyl opposed with a single throw and common rod joined at the journal, why wont it rotate.??

Its pretty easily to visualize why this wouldn't work. Imagine just two opposed cylinders with the rod you desire. It will fit together when the cylinder pair is at TDC-BDC and the rod is straight between both piston wrist pins. Now imagine it at some intermediate position where the rod would be at some angle to the cylinder bore. That works fine for a single cylinder but then add an opposing cylinder and a straight rod and you see the 2nd rod wrist pin would be pointing thru the cylinder wall instead of at the piston wrist pin. You could assemble an opposed twin like this but it wouldn't rotate. Two 4 cylinder opposed Mercs couldn't be assembled.

[Fastjeff57]

Ah, horse feathers! Slap the thing together, using your most favorable design (without breaking your budget)m and see what happens. Use a leaf blower to supply the air, the master rod design, and really modest power settings. If it runs, fine; if not, well... It sure was an interesting idea!

In engineering, it's called "Proof of concept".

Jeff

[Tim Kurcz]

Ah, horse feathers! Slap the thing together, using your most favorable design (without breaking your budget)m and see what happens. Use a leaf blower to supply the air, the master rod design, and really modest power settings. If it runs, fine; if not, well... It sure was an interesting idea!

In engineering, it's called "Proof of concept".

Jeff

Hi Jeff,

By Jove, you've got it. After about 12 hours design, mockup, and re-design work, I've settled on the master-link rod concept. Now it's a tussle between pivot joint options: 1) A .375" steel pin riding in a tiny full complement drawn cup needle bearing, 2) A .500" steel pin riding on a bronze plated bore, or 3) A .500" bronze pin riding in steel bore. I'm leaning toward a 1/2" pin.....

Assuming the proof of concept actually runs, only moderate power/RPM would be used: It would be fun to drive the little beast just to see the looks on peoples faces (and get a few pics, of course). If it behaves itself, a more robust version might follow.

Tim