Talking Crankshafts with Tom Molnar – Part 2

Click Here to Begin Slideshow In our last issue, we started a Crankshaft Tech Q&A series with Tom Molnar of Molnar Technologies (www.molnartechnologies.com). If you recall, Tom possesses a wealth of information with regard to crankshaft (and connecting rod) technology and he has been extremely forthcoming when it comes to sharing it. Here’s the second part of our series. And once again, you may be surprised at some of the answers to our questions: (7) What is the job of the crankshaft counterweights? Counterweights have two jobs on a crank. One, of course, is to counter the out-of-balance forces produced by the mass on the rod pin side of the centerline. The other is to reduce the bending forces. You can have a perfectly balanced crankshaft that runs smooth, but due to improperly placed counterweights, the crank could be seeing large bending forces that will break the crank due to fatigue. An example of this is on a 4-cylinder crank. A 4-cylinder crank can be balanced without any counterweights, but at high RPM with the rod pins, rods etc. pulling in one direction and the center rod pins pulling in another, it induces huge bending forces that over time will break the crank. (8) There are different designs when it comes to the oiling layout in a crankshaft (for example, cross drilling). Which layout works best? Cross drilling was the hot deal a lot of years ago, but as RPM went up over the years, it started causing problems. At high RPM, the oil wants to prevent the oil entering into the oil hole in the main bearing surface. If you cannot get any oil in, you will not get any oil to the rod bearings and you will burn them. We will not make any cranks with cross-drilled oil holes. (9) Is crankshaft stiffness more important than light weight? A stiffer crank will be stronger and have higher fatigue life. You can think of it this way: If some of the energy produced is used to bend and twist the crank, that is energy that is not getting to the tires to move the car. (10) Is there any advantage to gun drilling a crankshaft? Gun drilling the mains will basically take about 2 pounds off the nose of the car. There are no performance gains. (11) Is there any advantage of using smaller than stock diameter rod journals in a crankshaft, other than reducing bearing speed? Smaller rod pins reduce bearing speed and crankshaft weight. Is there any HP gain? Yes, but we are talking about numbers that are a lot closer to 5 HP than 30 HP. (12) Is journal overlap critical? Overlap is what keeps the crank in one piece and helps reduce bending/flexing. The smaller the rod pins, smaller the mains and longer the stroke, the less overlap and strength you will have in the crank. Check back next week for the last installment in this series!

Talking Crankshafts with Tom Molnar – Part 2

Click Here to Begin Slideshow

In our last issue, we started a Crankshaft Tech Q&A series with Tom Molnar of Molnar Technologies (www.molnartechnologies.com). If you recall, Tom possesses a wealth of information with regard to crankshaft (and connecting rod) technology and he has been extremely forthcoming when it comes to sharing it. Here’s the second part of our series. And once again, you may be surprised at some of the answers to our questions:

(7) What is the job of the crankshaft counterweights?

Counterweights have two jobs on a crank. One, of course, is to counter the out-of-balance forces produced by the mass on the rod pin side of the centerline. The other is to reduce the bending forces. You can have a perfectly balanced crankshaft that runs smooth, but due to improperly placed counterweights, the crank could be seeing large bending forces that will break the crank due to fatigue. An example of this is on a 4-cylinder crank. A 4-cylinder crank can be balanced without any counterweights, but at high RPM with the rod pins, rods etc. pulling in one direction and the center rod pins pulling in another, it induces huge bending forces that over time will break the crank.

(8) There are different designs when it comes to the oiling layout in a crankshaft (for example, cross drilling). Which layout works best?

Cross drilling was the hot deal a lot of years ago, but as RPM went up over the years, it started causing problems. At high RPM, the oil wants to prevent the oil entering into the oil hole in the main bearing surface. If you cannot get any oil in, you will not get any oil to the rod bearings and you will burn them. We will not make any cranks with cross-drilled oil holes.

(9) Is crankshaft stiffness more important than light weight?

A stiffer crank will be stronger and have higher fatigue life. You can think of it this way: If some of the energy produced is used to bend and twist the crank, that is energy that is not getting to the tires to move the car.

(10) Is there any advantage to gun drilling a crankshaft?

Gun drilling the mains will basically take about 2 pounds off the nose of the car. There are no performance gains.

(11) Is there any advantage of using smaller than stock diameter rod journals in a crankshaft, other than reducing bearing speed?

Smaller rod pins reduce bearing speed and crankshaft weight. Is there any HP gain? Yes, but we are talking about numbers that are a lot closer to 5 HP than 30 HP.

(12) Is journal overlap critical?

Overlap is what keeps the crank in one piece and helps reduce bending/flexing. The smaller the rod pins, smaller the mains and longer the stroke, the less overlap and strength you will have in the crank.

Check back next week for the last installment in this series!

Talking Crankshafts with Tom Molnar – Part 2 1

Counterweights are important in a crank. As Tom Molnar points out in the Q&A, they obviously counter the out-of-balance forces produced by the mass on the rod pin side of the centerline. They also reduce the bending forces that are present during engine operation.

Talking Crankshafts with Tom Molnar – Part 2 2

The oiling system used in a crankshaft is critical. In the old days, the hot setup was a cross-drilled crank. As it turns out, cross-drilled cranks can actually kill bearings, particularly in high RPM applications.

Talking Crankshafts with Tom Molnar – Part 2 3

It’s very possible to lighten a crankshaft (considerably). But that doesn’t come without consequences if the lightening goes too far. For example, Molnar tells us that a stiffer crank will be stronger and will have higher fatigue life.

Talking Crankshafts with Tom Molnar – Part 2 4

Reducing the size of a rod pin on a crankshaft will reduce bearing speed and simultaneously reduce the weight of the crankshaft. There’s a bit of power to be found by reducing the rod pin size, but not as much as you may have thought.

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