ENGINES - Metric Mechanic

Understanding Torque & Horsepower

Torque and HP are the primary measurements used to describe the power output of an engine. Torque is expressed in ft/lbs and is a measure of the engine’s twisting force exerted on the crankshaft from combustion. Horsepower, which is derived from torque, is specifically a measure of how much torque the engine produces in one minute divided by 5252. Therefore, the HP formula is:

HP = Torque x RPM (Revolutions per Minute) ÷ 5252

Both HP and Torque are measured either at the Flywheel using an Engine Dynamometer or at the rear wheels using a Chassis Dyno.

How MM Engines Generate Power

Basic engine components are the block (pistons, rods, crankshaft) topped off with the  head (cam/s, valves, valve train parts, etc).

The fill capacity (air and fuel mixture) of the Block’s cylinders, is what provides Torque while it’s the Head’s air intake that governs HP.

We use five techniques for modifying engine power! The first three occur in the block and the last two in the head.

1. Lighter Reciprocating Mass (piston, piston pin & rod)
2. Larger Displacement
3. Increased Compression Ratio
4. Improved Head Flow
5. More Camming

1. Lighter Reciprocating Mass

Weight is the enemy of acceleration! For example, in a BMW E36 M3 engine, turning 7000 rpm, the piston starts from the top, accelerating to 100 mph by 75° after TDC, then starts to decelerate and stops at the bottom. The piston starts and stops 14,000 times a minute over a distance no wider than your fist, 3.5″ or 89.6mm. The reciprocating mass squares with RPM. For instance, from 2000 RPM’s to 4000 RPM’s, the RPMs are doubled but the engine recognizes a FOURFOLD increase in the Reciprocating Mass! If the RPM’s are doubled again to 8,000, the Reciprocating Mass is 16 times greater than at 2000 RPM’s. This is why we take Reciprocating Mass so seriously and strive to reduce it’s weight through lighter components. The Reciprocating Mass in our engines is 15% – 40% lighter than stock. This reduction spools up the crank quicker and increases the reliability of the engine – less reciprocating weight (piston and rod going up and down) to tear up the engine over time.

2. Larger Displacement

Larger displacement is accomplished by boring the block and increasing the crankshaft stroke. Displacement is a direct ratio of torque output meaning, a 10% increase in displacement results in a 10% increase in torque. Boring an engine is usually the most cost-effective way to increase displacement without increasing piston speed – which increases wear. Boring a cylinder out by 1mm, equals adding 2mm to the crankshaft stroke. At MM, we typically stroke an engine by installing a crankshaft with a longer stroke. In a few cases, we offset grind the crank.

3. Increased Compression Ratio

A simplified definition for Compression Ratio is: a measure of the entire Cylinder Volume plus the Combustion Chamber Volume divided by the Combustion Chamber Volume. Increasing the intensity of the combustion chamber explosion is accomplished by increasing the compression ratio. At Metric Mechanic, we normally increase the Compression Ratio by 1.5 to 2 points. For example, in a stock M3, CR is 10.5:1* and we take it up to 12.1 CR. *Note: BMW states that the stock M3 CR is 10.5:1 but when we CC it out, we get a real 9.5:1 and we take it up to what measures out at a real 11.1. Since we typically discover an approximate 8% variance increase in all BMW Compression Ratios, we used their calculating method in stating the M3 goes from 10.5:1 to 12.1. We don’t know but have suspected that they are calculating a carboning-up factor in the combustion chamber which would eventually increase the compression. For street driving, all our engines are designed to operate with 91 Octane. During Driver’s Schools, we recommend adding 1 gallon of 105 Octane Race Fuel per 3 to 4 gallons of 91 Octane Pump Gas. The objective here is to avoid detonation under extreme driving conditions.

4. Improved Head Flow

Porting the head allows for more fuel and air to flow past the intake valves at a given lift, for greater cylinder filling. Our cylinder head flow increases are as follows:

• M10   18% – 24% flow increase over the stock head
• M30   16% flow increase over the stock head
• M20   16% flow increase over stock the head
• S14/S38   16% – 26% flow increase over the stock head
• M42/M44   6% flow increase over the stock head*
• M50/S52   6% flow increase over the stock head*
• M54   6% flow increase over the stock head*

*Newer engines have an optimal port size of about 85% of the valve head and come from the factory machine ported, leaving less room for improvement.

Generally speaking, the cylinder filling and power gains will be about 1/2 the air flow increase of the ported head. For example, a 20% flow increase would equal a 10% increase in cylinder filling and power.

5. More Camming

We regard the camshaft as the crux of the entire engine. When designed right, it co-ordinates all other engine components to work optimally. Displacement, head flow, compression and driveability requirements, all factor into the camshaft profile design. Driveability requirements are determined by the owner’s driving style and how they use their BMW. A Metric Mechanic Sport Engine is designed for enthusiastic street driving whereas a Rally Engine covers that of course, but is also ready for Drivers Schools, Autocrossing and other driving events. They both have smooth idling qualities (with or without AC on) at 700 to 750 RPM, will work with the fuel injection idle circuit, and pass emissions testing anywhere in the world (in our experience so far). We meet these requirements by designing our camshafts to NOT exceed the overlap of the stock factory cams.

Our Sport and Rally Engines are build with a wide power band. Generally these engines make within 85% of their peak torque from 3000 to 3500 RPMs up to 6500 to 7000 RPM. This is done by running an intake lobe that is larger than the exhaust lobe. The Intake Lobe controls the upper RPM HP range whereas the Exhaust Lobe controls the low to mid-range torque. In 1988 we made a departure from the conventional practice of using identical intake and exhaust lobes and started designing the cam with a larger intake lobe than exhaust – a practice we continue to this day. Interestingly, BMW began this same strategy in 1995 with the E36 M3 Engine. Previously, the M50tu engine had Intake and Exhaust Lobes of 224° /9.0 Lift. In 1995 M3 S50 engine had a large Intake Lobe increase up to 252° / 10.3 Lift. The Exhaust Lobe was mildly increased to 234° at 9.7 Lift. Both the M50tu and the S50 Engine had 25° of Vanos (Variable Valve Timing) on the Intake Cam.

In Summary

These five areas (Reciprocating Mass, Displacement, Compression Ratio, Head Flow and Camming) are the keys to unlocking the power output of our Normally Aspirated or Boosted Engines.

The Effect of Driving Style on Reliability and Longevity

Driving Styles: Street vs. Tracking

Imagine cruising down a two lane road going to a Driver’s School event in a BMW E36 M3 with a 3.2 Liter Engine. After doing Driver’s Schools for a couple of years, you’ve moved up to the Advanced Level. Holding a speed of 60 mph at 2700 RPM, your foot is barely on the gas pedal and your fuel mileage indicator is showing 30+ miles per gallon. Your throttle plate, via the throttle cable, is barely open, severely limiting the amount of air intake. The engine is under light load now, and only needs to produce 10 HP and 19.5 ft. Lbs. of torque to cruise at 60 mph. Just ahead is a slower moving vehicle that you’d like to pass. So, your downshift from 5th to 3rd gear, put the pedal to the metal, and within 10 seconds you pass the car in front of you.

In that time, the intake throttle went wide open from 60 mph at 4500 RPMs to 80 mph at 6000 RPMs. The engine made 240 HP and 236 ft. lbs. of torque. Torque is indirectly related to the force, in pounds, pushing down on the piston top from the combustion chamber explosion. In the 10 second pass, the engine’s torque went from 19.5 ft. lbs. to 236 ft. lbs. and the combustion chamber explosion intensity increased by a factor of 12. Formula: 236 ÷ 19.5 = 12.1. The number of explosions increased by 2.2 times. Formula: 6000 ÷ 2700 = 2.2. If we take the intensity of the explosions x the increased number of explosions, we see that fuel consumption increased by 26.62 times. Formula: 12.1 x 2.2 = 26.2.

The increase in fuel consumption can also be figured out from HP. At 2000 RPMs, the engine was making 10HP. At 6000 RPMs, it was making 240 HP. If we divide 240 by 10, again we’ll see that the fuel consumption increased by 24 times – similar to the 26.62 result. So, for 10 seconds, your fuel economy went from 30 + mpg down to about 1.5 mpg.

At the Driver’s School Track

You finally arrive at the track for a 2 Day Driver’s School Event. For those of you who aren’t aware of what goes on at a Driver’s School, this is nothing like High School Driver’s Ed. At a Driver’s School, there are four levels of participation: Instructors, Advanced, Intermediate and Novice. Instructors and Advanced students drive solo due their experience level and their cars are well prepared; sticky tires, upgraded suspension and brakes. Advanced students and instructors are usually pushing their cars to within 90 to 95% of an SCCA (Sports Car Club of America) road racer. The main difference between a road race and a Driver’s School is that, before you can pass, the car in front of you must give a hand signal allowing the pass. Driving sessions are about 20 minutes long with 3 to 5 sessions a day over the 2 day event. So, for the weekend, you’ve now done 3 hours of hard Driver’s School time and burned up two tanks of gas and averaged about 7 miles per gallon. Going back to the 10 second pass, on the way to the track, the load on top of the piston is 12 times greater and the piston was exposed to 2.5 times the heat cycles as cruising down the road. Keeping this in mind, under these Driving School conditions, the life of a stock M3 engine would be about 50 – 75 hours barring breaking. Whereas a street driven M3 engine is good for 175,000 – 225,000 miles. In the end, high RPMs and high Loads, drastically shorten engine life.

Surface Turbulence and Swirling, what’s the Difference? Folks seem to confuse the two but they are very different. Swirling is a spinning of the air and fuel in the combustion chamber to gain better particle mixing.  Surface Turbulence is tumbling air flow over a surface to pull liquefied fuel off the surface so that it can be re-homogenized with the combustion mixture. Why are you the only ones using Surface Turbulence – why didn’t BMW or someone else come up with it? The first most obvious reason we use it and no one else does is because it’s ours – we developed and patented Surface Turbulence. But we weren’t the very first. During the “patent search” that was run during application for the patent, previous designs for surface turbulence on the valve seat did show up. On our valve seat, we had machined in a small ramp groove just under the 45° seat angle. As this area usually builds up carbon, we felt that tumbling air flow over it would keep it clean. Well, three patents had already been taken out on generating turbulence at the surface of the valve seat in a similar way. These international patents were all issued to Mercedes Benz. One, invented by Scherenberg described two concentric grooves (ramps) machined into the base of the valve seat. This was done to “facilitate detachment of fuel particles along the port walls.” Scherenberg and another inventor, Hardenberg, had both designed a valve seat that protruded into the combustion chamber. The seat was raised to create a tumbling action as the intake charge entered the combustion chamber. These patents were filed in 1970 and are the earliest record we know of where the concepts that we call Surface Turbulence were used. Petersen’s Yearbook 1988 – In a 1988 Petersen’s Yearbook Vol. 2, No. 2 an article appeared called the hurricane combustion chamber. A company called Air Flow Research had punched several little dimples into the combustion chamber. This article appeared about the time we were ready to make our work with Surface Turbulence public. In an attempt to further justify and validate our concepts on Surface Turbulence, we decided to incorporate the dipple idea from this article, which was widely read and current. After a few months, we switched over to grooves. Open Wheel Magazine 1989 – Then in December of 1989, in Open Wheel Magazine, an article appeared called Computer Porting. Pictures were shown of a stair stepped rough cut combustion chamber done with a CNC milling machine. This porting service was being offered by Kenny Weld (a famous sprint car racer) who owns Weld Wheels, a nationally recognized wheel company based in Kansas City, Missouri. Coincidentally, their retail store is just 1/2 mile West of our shop on Truman Road. One has to kind of wonder what would have happened if 20 years ago two Mercedes Benz engineers would have pursued the idea of surface turbulence in greater depth. The other two researchers and Weld are both involved in racing and who knows how many racers are using or experimenting with surface turbulence. Our Perspective – I think the main reason we’ve had such great success with Surface Turbulence is that we view it differently. By now, we can accept the idea that if atomized fuel particles come in contact with a surface, they will liquefy. But what kinds of surfaces do these particles encounter in an internal combustion engine. Two kinds: Stationary Surfaces & Moving Surfaces. Stationary Surfaces include the intake manifold, the intake port, valve seat, valve guide and the combustion chamber. Moving Surfaces include the intake valve, exhaust valve and piston. Of these two surface types, we’ve gotten the best results from adding Surface Turbulence to the moving surfaces. Previously, it doesn’t seem that these surfaces were regarded as very important – perhaps folks assumed that the fuel particles just shook off. We feel that all the surfaces are important – moving or stationary.