Engine Questions & Answers

• It sure sounds as if you're making some fairly bold acceleration claims by saying that your 4.0 Liter Sport Engine will out accelerate the new 3.6 Liter 24 Valve M-Engine.
• Why larger engines?
• I've noticed you don't offer any "M" engines. Do you have plans to offer Performance "M" Engines?
• What are your thoughts on Turbo Charging?
• I've got an ETA engine and I see that you suggest I change my rear gear ratio. Could you elaborate?
• Surface Turbulence and Swirling, What's the Difference?
• Why are you the only ones using Surface Turbulence - why didn't BMW or someone else come up with it?
• I've got an old style 6 cylinder head I'd like to get ported. My mechanic says it might be cracked and he's suggesting that I get it welded. My budget is limited. Still, I want more acceleration. I'm confused! Help!
• I'd love one of your engines or even the head alone but they are out of my price range, so what can I expect from just the sport cam?
• Your engines sound neat but I already have a ported and polished head with a hot cam. All I need is the block - will you sell it to me?
• May I buy just your pistons?
• What is the Difference between a HiFlo ST and a HiFlo ST Sport engine?
• Previously I purchased a 1800 cc 4 cylinder engine (1980 and later). Is there a problem getting one of your larger engines?

• I have a 2800 engine (2800 or 528i), is there a problem getting a larger engine?
• I love the smooth drivability and "manners" of my fuel injected Bimmer. How much of that will I have to give up?
• You guys are really high on fuel injection and seem to be targeting the folks with fuel injection for your engines.
• Would I be better off to throw away my fuel injection and go to a carburetor system?
• I've heard that if I increase the horsepower of my engine my fuel injection may not be able to supply enough gas to keep up with the added air flow demands.
• What about headers and custom exhaust systems?
• What's your opinion of running a lightened flywheel or an aluminum flywheel in a Metric Mechanic HiFlo Series Engine?
• If you run larger pistons and rods, won't that throw off the balance of the engine?
• How smooth are your 2500 and 4000 engines?
• You say you run a 92 mm piston. How thin are the cylinder walls on your 2200 engines?
• Will I be able to buy gas for your engines?
• Will a competent mechanic be able to service your engine?
• What are some of the more common mistakes you see people make when it comes to installing your engines?
• Are there any drawbacks to your engine?
• What's your view on emissions and fuel economy?

It sure sounds as if you're making some fairly bold acceleration claims by saying that your 4.0 Liter Sport Engine will out accelerate the new 3.6 Liter 24 Valve M-Engine.
Yes, we're saying just that. We feel the figures shown are honest and obtainable. By no means do they represent the fastest times we have recorded.

For example, the 535i we show was originally tested on a 75° day with about 200 miles on the engine. It posted an average low time of 3.68 sec on a 60-80 mph test in 3rd gear. Two days and a couple hundred miles later we did 3 more timed runs. The temperature had dropped to 40°, and the tank was almost empty (premium gas this time). Two were 3.38 sec. and one was 3.25 sec. With the engine a little more limbered up, we 1/4 miled the car at just over the century mark in the low 14's. We threw this test out for the following reasons.
The crisp 40° day - we usually test at 65° or above.

Premium gas and re-tuning - we tune for 90 octane or lower and test accordingly.

Low gas tank - we normally test on a half tank of gas or more.

Before testing a car, we usually try to put 20 miles on the engine under a very controlled break-in procedure. One thing we really like about the 60-80 test is that it is much easier on a new engine than 0-60 mph or 1/4 mile run. Usually our engines aren't fully broken in when we test them (this takes about 2000 miles) so our figures tend toward the conservative.

Why larger engines?
The displacement of an engine is directly related to torque output. It is very hard to get a torque increase without increasing engine displacement. Take a second and look at the 60-80 mph tests in 3rd, 4th and 5th gear. You'll see that the Alpina B-10 and the Hartge H-35 are only marginally quicker than the Euro 535i. Now both these cars have about a 40 horsepower advantage over a 535i (220 hp versus 260 hp at 6000 rpm). But they only have a 4 ft. lb. torque advantage (232 ft. lbs. versus 236 ft. lbs. at 4000 rpm). So, what is less than 2% torque going to do for acceleration? Very little.

So how do these cars gain acceleration? Well, the Alpina and Hartge both change the rear gear to a 3.73, the Euro 535i has a 3.45. This gives them an 8% gearing advantage. The final torque gain to the rear axles ends up being 10% (2% by the engine and 8% by the rear gear). Ask yourself, does this make much sense? How about dollars and cents?

I know of one German tuner who has broken out of the 3.5 liter mold and that's Schnitzer with its 3.7 liter engine. These engines, I've been told, have a fairly long stroke and I've also heard that the bore and stroke are about 92.5 mm x 92 mm (3707 cc). The engines have been getting great automotive press reviews because of added TORQUE! Big stroker motors feel great to drive on the street.

I've noticed you don't offer any "M" engines. Do you have plans to offer Performance "M" Engines?
Yes, we are prepared to build "M" engines but first we have to see that there is a market. Currently, I would say that 80% of the people who buy engines from us do so because their engines are worn out. The other 20% still have a good engine but they want to go faster. Because "M" engines are so expensive, we feel their owners will stick with bolt-ons (hotter cams, computer chips and hot exhausts) until their engines die. We haven't seen many "M" cars with over 100,000 miles. Also, developing a core bank of "M" engines is a bit of a problem. Therefore, anyone desiring a performance engine for an "M" car would have to bring it to Kansas City and be willing to give up his/her car for 4-8 weeks because we'd have to build up the original engine (usually we swap engines).

We feel that the "M" engines are basically too peaky and need more bottom end power (torque)! Therefore, our "M" engine would be a 2700 HiFlo ST "M” (94 mm x 94 mm = 2608 cc). The M5 or M6 engine would be a 4000 HiFlo ST "M" (94 mm x 94 mm = 3912 cc). We'd build the bottom ends using the same proven design that we use for our 2500 and 4000 HiFlo ST Sport Engines. We have stress tested our 94 mm strong arm stroker crankshafts under racing conditions at about 7100 to 7200 rpms with burst speeds as high as 8000 rpms using steel rods (570 grams) with no fatigue. We see no real problem running a 94 mm stroke so long as the customer maintains a 6400 rpm red line on the engine. By using a combination of our lightweight Alusil Forged Piston and a lightweight, longer, premium steel rod; reciprocating mass would be just over 1 kilogram. The reason we would go with a premium steel rod is that it would be about 40% stronger than the rods we currently use. Even though these rods are very expensive, they are a good insurance policy against a rod going through the sides of an "M" engine block.

Currently, we've ported the "M" head for 14% flow increase over stock on the intake side. The intake ports are rather small, 26 mm, so we enlarge them to 29 mm by mounting the "M" head in a custom made holding fixture and boring the ports out with a milling machine. The exhaust ports on the "M" head flow very well and they need no help.

The prices of these engines reflect their high parts' costs.
HiFlo ST "M" Engine = $11,500
HiFlo ST "M" Engine = $15,000

What are your thoughts on Turbo Charging?
Once on boost, nothing puts out torque like a Turbo, given a specific engine size. When it comes to forced induction, we feel that Turbo charging is superior to supercharging. With supercharging, you'll get a mechanical loss and a high noise level.

In the pre-planning stages for the 2500, 3800, and 4000, we did consider building complete Turbo engines. The reasons we didn't go into Turbo charging were:

1. There's no way one can keep the customer from playing with the boost.
2. The cost of building a smaller complete Turbo engine versus a larger engine was about 50% to 75% greater.
3. Tuning and plumbing problems.
4. The fear of higher product failure.
5. After weighing out the cost and liability factor, we opted to go with the bigger engines.

From an efficiency standpoint, turboing bothers us slightly. When you're not on boost, which is most of the time, the Turbo will act as a slight restriction in the intake tract. Add to this, lower compression and cooler (inter-cooled) air and pretty soon, you've lost a fair amount of efficiency - when you're off boost.

On the positive side, we do see turbo-charging as a modern day accepted method for going fast. Because of this, we offer two Turbo charged models, the 2100 and the 3500 HiFlo ST Turbo. These engines have ported HiFlo heads (for an 18% flow increase) to increase the effectiveness of the boost. Also, Surface Turbulence has been added to reduce detonation and to increase fuel efficiency. Strong high silicon content forged pistons are combined with heavier cylinder walls and a cutting ring head gasket for reliability. These engines have been specifically designed for Turbo charging. Also, when dealing with a power level increase this great, you need to upgrade the cooling system, clutch, brakes and suspension.

I've got an ETA engine and I see that you suggest I change my rear gear ratio. Could you elaborate?
With a Metric Mechanic 2800 HiFlo ST "Baby" Six engine change-over, we would recommend changing the rear end gear ratio. Early 325e's and 528e's came with a 2.79 rear gear and late models had a 2.93 rear end. Many people don't realize that what makes a 325i quick is that BMW changed the rear gear to a 3.73. This is a big jump from earlier 3 and 5 series BMW's (see Gear Chart). With this gear ratio, high gear would feel like something between a stock 3rd and 4th gear, giving the ETA engine the sensation of a buzz bomb. Also an ETA engine has about 10% longer stroke, so piston speed would be increased by the same amount.

In our opinion, the best and most overlooked gear choice is the 3.25 ratio used in late 5, 6 series Bimmers. Also, the 3.46 ratio found in 6 series coupes from 1985 on is a nice ratio that feels like you've eliminated 5th gear. These gear ratios offer an attractive compromise. Also, these differentials should be much easier to find in salvage yards. Salvage price ought to be about half the price of used 325i rear end. Changing the rear aluminum cover and side flanges (they pry out) are all that's needed make the change over.

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?

Surface Turbulence History

• Metric Mechanic 1988
  The first most obvious reason we use it and no one else does is because it's ours - we developed and patented Surface Turbulence.
• Mercedes Benz 1970
  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:

1. Stationary Surfaces
2. 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.

In our Patent, most of our protection is centered on the Intake Valve. This valve makes up about half of our gain. So significant is the effect of the valve alone that it is the only Surface Turbulence item on the 2100 and 3200 engines.

I'd love one of your engines or even the head alone but they are out of my price range, so what can I expect from just the sport cam?
When adding a hotter cam to your engine, the upper RPM power will generally get stronger and the mid to lower RPM performance will fall off. Torque is basically a measure of how well the cylinder is filling and horsepower is a measure of high RPM torque (cylinder filling). So, by adding a hotter cam (longer duration and more lift) cylinder filling will be stronger at the upper RPMs and weaker at low RPMs. With a stock BMW cam, the intake valve closes at about 50° after bottom dead center and the piston has traveled up 11 mm on its compression stroke. With our sport cam, the intake valve is closed at about 70° after bottom dead center and the piston is now 25 mm up from the bottom. Now, obviously, this later closing will cause quite a bit of intake charge to be pushed back out of the intake valve opening at low RPM. As the engine reaches the upper range, the intake charge (due to its elasticity) has a fair amount of lag time behind the speed of the piston and the cylinder filling becomes greater.
Results? The hotter cam will make the engine stronger from about 4500 RPM on up, but below 3000 RPM, the stock cam will put out slightly more power. Cams are not just a magic stick that add power to BOTH ends of your driving range. To put it simply, what you gain in HP, you loose in bottom end torque. To overcome weak cylinder filling at low speeds (due to a hotter cam), you'll need to up the compression ratio (increase the compression) and/or port the cylinder head (flow more air into the cylinder initially). If you don't do one or both, the engine will feel slightly anemic at low speeds. Just adding a hot cam to a stock engine is not the cure-all that people think it is. Still, I feel our asymmetrical sport cam offers a good power increase over a wide rpm range and is very flexible.

Story time
A few years back, a shop bought a 3500 sport engine from us and liked it a lot. Next, they bought a HiFlo Sport Head and liked it too. Then, they thought that what makes our head work so well must be the cam, so finally, he called to buy just the cam. I explained to him that a hotter cam will move the power band up higher but and sacrifice low end power unless the compression or cylinder filling is upped. A 535i is only turning over 2000 rpm at 60 mph in 5th gear so; torque will be down on a sport cam. He bought just the cam anyway and put it in a 535i which is a fairly low compression engine - 8.1:1. This time, they didn't like it. The next time I heard from him, he complained that at low speeds, where in reality we spend most of our driving time, the car was actually slower than stock.

Your engines sound neat but I already have a ported and polished head with a hot cam. All I need is the block - will you sell it to me?
We sell the bottom end by itself only to those who have already purchased a HiFlo ST/Sport Head from us. The charge for the short block is figured by subtracting the current price of the Head from the current price of an engine and adding $200 for 4 cylinders and $300 for 6 cylinders. This charge can be waved if the short block is purchased within one year of a Head purchase.

Adding a head as you described with our block is a mismatch of machinery and philosophy. For example, we use Surface Turbulence for fuel economy, emissions, and reduces detonation. Polishing the head and using a hot cam does just the opposite (wastes fuel, increases emissions, and can cause detonation problems). Also, an inefficient head shortens the life of the bottom end by allowing fuel to condense on the cylinder walls thereby wiping out both the rings and cylinder walls.

If your heart's desire is to have one of our engines, then we suggest that you sell your cylinder head and buy one of our engines. In our opinion, you'd be better off in the long run.

May I buy just your pistons?
No! The answer follows the same logic as above.

I love the smooth drivability and "manners" of my fuel injected Bimmer. How much of that will I have to give up?
Very little to NONE!

You guys are really high on fuel injection and seem to be targeting the folks with fuel injection for your engines.
You're right! We wanted to come to the aid of these owners and that's why we wrote the book BMW Fuel Injection - The Enlightened Approach, a 150 page book that covers all aspects of fuel injection maintenance. By the way, the book is written in plain English. We are currently under contract with Bentley Publications to rewrite the book and include motronics. Expect it out the end of 1991.

What about headers and custom exhaust systems?
Many states now require a visual inspection to pass a smog test. In these states, your car won't pass if you have headers. Headers tend to crack near the head flange area and high exhaust leaks like this can eventually burn an exhaust valve. Headers have a rather short life expectancy of about 2 - 5 years. If you are going to run headers, make sure you run at least 1 1/2" of primary tubing. Our personal choice is to go with a ported cast iron manifold. A ported tii manifold is worth 1.2 second over a stock air injected manifold on a 60 - 80 test run in 4th gear. That's as good as a header will do and it's a lot more reliable. Our favorite cast iron manifolds are:

• The tii manifold
• The 1800/320i manifold
• The 318 manifold
• The 6 cylinder 3 into 1 cast iron manifold.

Porting an exhaust manifold (Anti-reversionary cut) is available for an extra charge - see price list.

What are some of the more common mistakes you see people make when it comes to installing your engines?

1. Too Rich
   The biggest problem with tuning is that those installing our engines tend to dump too much fuel into them. Some folks seem to think that if they run an engine richer it will run faster and cooler. Our surface Turbulence engines need little to no increase in fuel delivery. Running our engines initially on mixtures that are richer than a 13.2:1 fuel and air ratio (3.5% CO) can cause border line ring damage. Running the engine richer than a 12.0:1 fuel and air ratio will wipe out the rings and cylinder walls in short order (less than 100 miles). Here's how this happens. When an engine is built, the cylinders are honed in a cross hatch pattern that looks much like a "bastard" file. Now, these cross hatch hone marks are used to lightly file the rings into the shape of the cylinder. When excess fuel is put into the engine, oil on the cylinder walls becomes washed down by liquefied gas and the cutting action of these hone marks increases. This causes the rings and cylinder walls to become prematurely worn.
   To date, we haven't seen anyone who has run one of our engines excessively lean to the point of burning it up. Ideal spark plug color should be white to a very light tan on the spark plug insulator. To most people, that would look like the motor's running way too lean. If the plugs are dark tan, its way too rich. Although we use gas analyzers in our shop, we still check plug color at least once or twice before a customer picks up their car after an in-shop installation.
2. Too much advance
   Some folks also tend to think that running more advance will make their car go faster. The combination of Surface Turbulence and high compression (10.0:1) creates a rather fast burn cycle in the combustion chamber. The timing specifications listed in our tuning guide are for optimal performance and will sound rather retarded.
3. Break - In
   Our engines are built to fairly tight tolerance for long engine life. Pushing the engine hard before it's broken in will shorten its life or damage it. Typically, our engines take about 2000 miles before they are loosened up enough to take red line (for a burst). Also, some people think that the power of our engine is going to be at a much higher rpm band than stock and it really isn't. The difference is that there is much more power in the same range. Pushing one of our engines hard early can cause the pistons to collapse and piston noise to set in. Overheating or detonation can also collapse the piston skirts and cause noise.

Are there any drawbacks to your engine?
I'll explain one minor drawback. The Alusil (high silicon content) forged pistons that we use transmit a little more noise because they are very dense and lightweight whereas a cast piston is porous and heavy. An exaggerated analogy would be to "ring" a heavy cast iron skillet and a lightweight stainless (forged) steel skillet with a hammer. The cast iron will sound dull but the stainless steel will ring like a bell. When the engine is cold, it's not that uncommon to hear slight piston rattle until the engine reaches operating temperature and piston-to-cylinder wall clearance tightens up. So, noise generated before the engine reaches operating temperature, we consider insignificant.

The noises that come from an engine are made by clearances. Block noises are transmitted to the cylinder wall through the piston. Now, since our pistons generate a ringing effect, these clearances tend to be amplified. This is one of the reasons why we build our engines to low side (tight) tolerances - the main reason being longevity. We have worked hard at controlling noises and we very rarely get complaints.

What's your view on emissions and fuel economy?
Since the Fall of 1987, we have probably spent more research and development time on trying to understand how to make clean, high performance BMW engines. We started by reducing valve overlap and duration on our sport cams. In the Fall of 1988, we developed surface turbulence for improved fuel economy and emissions. In the late Fall of 1990, we redesigned our cams for yet lower emissions and more power. Our dual profile asymmetrical cam offers a radical departure in cam design. We developed new techniques for calibrating the airflow meter by reading the output of the oxygen sensor. The O2 sensor is one of the best pieces of tuning equipment built into late model BMW's. We've also spent considerable time flow testing catalytic converters and testing the emission output of our engines. In the '90's, Metric Mechanic's commitment is to build clean, lean, and mean engines.

Emissions

1. Toxic Fumes
About 1% of the exhaust is toxic in an internal combustion engine. These toxic substances are carbon monoxide (CO), Hydrocarbons (HC) and Oxides of Nitrogen (NOx).

• Carbon Monoxide
  can lower the oxygen content in our blood and because one can't see, taste, or smell it, CO can be extremely dangerous.
• Hydro Carbons
  in general cause irritation to the mucous membranes.
• Oxides of Nitrogen
  cause irritation of the respiratory system. Heavy concentrations can damage lung tissue. Again you have no sense of its presence because you can't see, taste, or smell it.

Now, the waste site for these toxic substances is the air we breathe. These toxic substances pose serious health hazards that can't be taken lightly. When doing performance modifications, one should consider these effects on the environment. Think about this - seriously - the performance freak who rips off the catalytic converter or guts it out, then pulls off the fuel injection and adds a set of side drafts, a hot cam, and headers (this can easily increase toxic waste by 10 fold) is being a royal jerk to rest of humankind who has to breathe this high level of toxicity they've created. Maybe there ought to be a law that says that annually you have to stand 10 feet behind your tail pipe and breathe your exhaust for 30 seconds - then tail pipes might get cleaner.

2. BMW Emission Systems
Before 1975, most BMW models had an EGR System. Generally speaking, 4 cylinder models used air pumps (exception being 1973 and 1976 - 49 state models) while 6 cylinder models didn't use air pumps.

In 1975 all BMW models were fitted with thermal reactors and air pumps plus an EGR System. Sensor relays were also thrown in for good measure. This basic way out continued on, to a large extent, for the next 4 years. This period, in my opinion, represented the dark ages of emissions for BMW's. "After burning" was used to reduce toxic emissions. What this meant was that any heavy fuel gases not burned off in the combustion chamber would be burned off in the exhaust manifold or thermal reactor. In practice, these systems used a lot of gas. Fuel consumption was 15-20% higher than later catalytic converter systems and the burn off left something to be desired. Also, this system was directly responsible for a lot of cracked heads on 6 cylinder engines.

Nineteen eighty represented the age of enlightenment for emissions on BMW's. By this time all BMW models had gone to 3-way catalytic converters and an oxygen sensor (Lambda Sensor). This system can convert 90% of the toxic emissions to harmless substances. So, instead of using a thermo reactor to torch these harmful substances to death - the catalytic converter changes their chemical make-up to harmless substances. This is kind of like the cave man approach "burning" versus the chemist who says "better living through chemistry". Once the converter is up to operating temperature (250°c), CO and HC become carbon dioxide (CO2) and water (H2O). NOx becomes nitrogen (N). A 3-way catalytic converter means that all 3 toxic substances CO, HC, and NOx receive after treatment. At the heart of the converter is a honey comb ceramic substrate that has been impregnated with platinum (to control CO and HC) and radium (to control NOx).

Now, many people have the notion that a catalytic converter is a real cork in their exhaust system (I once thought this myself). Most of this thinking was based on the old pellet converters of the mid seventies in which the exhaust had to travel though a bed of catalyst pellets. These catalytic converters were a big cork but the modern day 3 way catalytic converter usually flows as well as any muffler in an exhaust system.

Economy

1. Before Surface Turbulence
   In the past, with our earlier HiFlo engines, we would figure fuel consumption as follows;
   Under partial throttle, fuel mileage would increase by the same percentage that engine size increased.
   So, installing a 3500 HiFlo Engine in a 530i would result in a 15% drop in fuel mileage because engine size went up 15%. Now, running the engine at full throttle would drop fuel mileage by the percentage of horsepower increase. Under full throttle, this could be as much as 30% over stock.
   On the average, customers actually reported about a 10-20% loss in fuel mileage. In comparison to M cars, these increases really weren't all that bad.
2. After Surface Turbulence
   Again, surface turbulence has done wonders towards fuel economy and cleaner emissions. A surface turbulence engine can be increased 15% in size with very little change in mileage. It's very likely that the owners of our engines drive them "briskly". If one of our bigger engines was driven with a "light foot", I believe its fuel mileage would be even better than stock! One of customers drove from Kansas City to Chicago with a new 3800 HiFlo ST Sport. His cruise control was set on 55 mph - he got 33 mpg. Now, I would guess that a stock 533i would be at least 15% short of that figure under similar driving conditions. All and all, I'd say that Surface Turbulence results in exceptional gas mileage for a performance engine.