Brake Mean Effective Pressure (BMEP) — A Yard Stick for Power

Every gearhead knows his way around horsepower and torque numbers. But what happens when you’re faced with the question of accurately comparing a couple of different engines of varying displacement, intending to have to choose between them? Or perhaps you’re looking at the potential advantages of a four-valve DOHC engine over a two-valve pushrod version, or a Hemi versus a wedge combustion chamber?

These are legitimate questions and over the years, people smarter than us have come up with several evaluators. Much like batting averages for hitters in baseball, engine geeks have a way of evaluating engines beyond just the actual horsepower and torque. There are several ways to do this and we’ll take a look at a couple of the important ones.

But before we get into this too deeply, this story will deal only with normally aspirated engines. Supercharging, nitrous, and turbocharging are outstanding ways to radically improve horsepower and torque, but to many hardcore internal combustion enthusiasts, these are cheats or short-cuts to improving power. Certainly, these engines can be evaluated the same way. It’s just that it’s not a fair comparison to normally aspirated versions. So, we’ll leave the evaluation of the power-adders to a separate story and stick with engines that rely solely on atmospheric pressure to fill their cylinders.

This author was involved with an early buildup of a 520ci Kaase Boss Nine engine that made 834 horsepower, which computes to 1.60:1 hp/ci. Kaase tells us that he routinely makes 900 horsepower with his 520 development mule. That pencils out to a solid 1.73 hp/ci which makes the Boss Nine downright stellar!

Efficiency Is More Than MPG

An engine’s efficiency at making power is subject to literally hundreds of variables that contribute to making horsepower and torque. This list probably could easily extend into pages, however, ours will feature only the most notable and can hardly be considered complete. The classic description of an internal combustion engine is that of an air pump.

This means that the more efficiently it can move air in and out of the cylinders, the more efficiently it can capture air for combustion, which will result in a more powerful engine. So, cylinder head airflow, cam timing, and compression ratio are all huge contributors in this game.

In comparison to the LS chamber, older Gen I small-block Chevys had to rely on domed pistons to achieve a decent compression ratio. Tall domes require higher total ignition timing numbers to make maximum power and one reason why newer engines offer more torque for the same compression ratio.

Before venturing further, we should establish some basics on how horsepower is measured. First of all, engine dynamometers do not actually measure horsepower. Instead, they measure torque, or the amount of twisting motion created by the crankshaft which is measured in pound-feet (lb-ft). This is merely a twisting motion that can be equated to work.

But in order to determine the amount of power created, we have to include a measurement of time in which this work is accomplished. Horsepower is the amount of torque delivered at a given engine speed measured in revolutions per minute (RPM). This gives us the time factor and an equation that any good engine guy knows by heart:

horsepower equals torque x rpm divided by 5252

One aspect of any internal combustion engine is that it will achieve a peak torque number someplace along its operating RPM curve. This is acknowledged as the point where the engine achieves its most efficient cylinder filling and its highest volumetric efficiency (VE). This evaluator is expressed as a percentage of the actual volume of the cylinder. For street and performance engines, this VE is usually between 85- to perhaps 90-percent.

When an engine is built using well-designed induction and exhaust components, it is possible to achieve VE numbers higher than 100-percent, but again, this will generally occur at or around peak torque. At engine speeds below peak torque, the inlet air is generally moving too slowly to achieve high cylinder filling efficiency. And at speeds above peak torque, there is less time available to allow cylinder filling. There are exceptions to this rule, of course.

Want a normally aspirated goal to shoot for when building a killer 4-valve engine? Try beating Chevrolet’s new production LT6 engine making 630 hp from a mere 333ci. That computes to a leader-of-the-pack 2.01 hp/ci.

Optimizing Volumetric Efficiency

There are multiple ways to achieve better cylinder filling at higher engine speeds. The most efficient way to achieve high horsepower numbers is to design the engine to move more air into (and out of) the cylinders at a higher RPM. If the engine can efficiently fill the cylinders to make a similar torque at a higher engine speed, the engine will also make more horsepower.

As an example, let’s use a pump-gas 468ci big-block Chevy making 585 lb-ft of torque at 4,000 rpm. Using our horsepower equation, that computes to 445 horsepower. But if we push the peak torque to 5,000 rpm, that improves the horsepower to 556 just by raising the peak torque RPM point. Let’s say that peak horsepower occurs at 1,500 rpm above peak torque (6,500 rpm) with a torque reading of 525 lb-ft. That puts this engine’s peak horsepower at 650 horsepower at 6,500 rpm.

There are several ways to evaluate how well an engine is performing relative to another of different displacement. Two of the standards are either torque per cubic inch (lb-ft/ci), often referred to as a torque ratio, or horsepower per cubic inch (hp/ci). In an earlier EngineLabs story, we offered a simple calculation for a typical performance engine on pump gas.

We won’t go into all the details of the equation here, but the key point is an assumption of a toque per cubic inch of 1.25:1. Multiplying the engine displacement times 1.25 creates the engine’s torque which can then be used to produce a horsepower estimate at a specific peak-RPM.

One variable that significantly contributes to power is a well-designed combustion chamber. All production LS engines enjoy a very efficient chamber that does not require as much ignition timing as older engines. This reduces negative work created with engines requiring higher total ignition timing. Factory LS ignition timing numbers for peak power are generally around 20 degrees. This indicates high combustion efficiency requiring a low total advance number.

If we have a 350ci engine making 1.25 lb-ft of torque per cubic inch, it will make 437.5 lb-ft of torque. The reason stroker packages like a 3.75-inch crank conversion for a 350 small-block Chevy are so popular is that it is an easy way to increase displacement and make additional torque (i.e 478.5 lb-ft for a 383 at 1.25 lb-ft/ci). It’s worth noting, this assumes the 383 achieves the same level of VE, which may not necessarily be true if the same cam and cylinder heads are used that were originally sized for the smaller 350ci engine.

We’ve created a series of charts using GM, Ford, and Chrysler production engines along with some performance crate engines listing their displacement, compression ratio, torque, and horsepower numbers along with their lb-ft/ci and hp/ci values. Notice that there appears to be a correlation between compression ratio and peak torque which should come as no surprise.

The key is creating an efficient combustion space that creates the ability to increase compression without suffering the ill effects of detonation. Late-model engines like the LS, Ford Modular Motor, and the Gen-III Hemi (among many others) all fall into this category.

Production Engine BMEP

GM Engines

Engine Disp. Comp. Peak Peak  HP Lb-ft /ci BMEP HP/CI
(cid) Ratio Torque (psi)
ZZ6 350 9.7:1 408 420 1.16 175.8 1.2
LS1* 346 10.2:1 365 350 1.05 159.1 1.01
LS3* 376 10.7:1 425 430 1.13 170.4 1.14
LT1* 376 11.5:1 465 460 1.23 186.5 1.22
LT2* 376 11.5:1 470 495 1.25 188.5 1.31
CT525 376 10.7:1 477 533 1.27 191.3 1.42
LS7* 427 11.0:1 470 505 1.1 165.9 1.18
LS364/450 364 10.3:1 441 452 1.21 182.7 1.24
LT376/535 376 11.5:1 470 535 1.25 188.5 1.42
LT6 333 12.5:1 460 670 1.38 208.3 2.01
632 Big-Block 632 12.0:1 876 1,004 1.38 209 1.59

Ford Engines

Engine Disp.

(cid)

Comp.

Ratio

Peak

Torque

Peak  HP Lb-ft /ci BMEP

(psi)

HP/CI
4.6L Mod 2v 281 9.4:1 305 265 1.08 163.6 0.94
4.6L Mod 4v 281 9.4:1 335 310 1.19 179.7 1.10
5.0L Boss 302 302 11:1 380 444 1.26 189.7 1.47
5.2L Voodoo 315 12:1 429 526 1.36 205.3 1.67
5.2L Aluminator 315 12:1 445 580 1.41 213.0 1.84

Dodge Gen-III Hemi Engines

Engine Disp.

(cid)

Comp.

Ratio

Peak

Torque

Peak  HP Lb-ft /ci BMEP

(psi)

HP/CI
5.7L Eagle 345 9.6:1 395 410 1.14 172.6 1.19
6.1L 370 10.2:1 420 425 1.13 171.2 1.15
6.4L SRT8 392 10.9:1 475 485 1.21 182.7 1.21
  • OEM engines are rated using the most recent SAE J1349 correction factor for horsepower and torque, which produces roughly 5-percent lower power and torque than the typical traditional “hot rod” correction factor derived from the earlier SAE J607. Using the newer correction factor also lowers the BMEP and lb-ft/ci and hp/ci in comparison to engines using the older factor. To be a fair comparison between the production engines and the 632 hot rod engine, we could reduce the hp/ci rating from 1.58 by 5-percent, making it approximately 1.51:1 and BMEP to 198.5 psi. Or, we could raise all the production engines by 5-percent which seems like much more fun!

Getting Specific

A man with a ton of experience building both championship-winning IHRA Pro Stock engines as well as designing his own version of the Boss 429 head — called the Kaase Boss Nine — shared some dyno numbers with us that were flat amazing.

This engine is a competition-oriented 598ci Boss Nine engine with 15:1 compression and a big cam with 280-degrees at 0.050 specs, but the results were what really blew us away. The torque-per-cubic-inch was calculated at 1.39:1 making 835 lb-ft at 5,300 rpm. But the 1,037 hp at 7,100 rpm delivering a robust 1.73 hp/ci is what really got our attention. Clearly, the BossNine likes compression!

In an example from the Engine Masters competition in 2014, several competitors achieved numbers exceeding 1.5:1 lb-ft/ci. Tony Bischoff’s 401ci Gen-III Hemi entry belted out a peak torque of 611 lb-ft, which computes to a torque ratio of 1.52:1. That engine also made 698 horsepower at 6,400 rpm, which puts it at an impressive 1.74 hp/ci. Those are stout numbers for a two-valve pushrod engine on gasoline.

The Chrysler Gen-III Hemi is actually not a true Hemi since it uses small quench flats on either side of the chamber to improve combustion efficiency. Plus, two spark plugs per chamber measurably reduce total timing requirements. Both of these improvements are reasons why the Gen-III Hemi is a strong engine.

Engine Masters rules at the time specified 101 octane VP race fuel and Bischoff’s engine was running 11.4:1 compression. One advantage the Gen-III Hemi enjoys is a wide modified hemispherical combustion chamber with dual spark plugs. This configuration demands less ignition timing, allowing the engine to accommodate a higher static compression ratio while avoiding detonation.

In addition to torque ratio or lb-ft/ci, it’s often common to see references to another method of computing torque ratio called brake mean effective pressure, or BMEP. This formula uses a constant to compute a theoretical average cylinder pressure number. In BMEP, 150.8 psi is the constant, multiplied by the engine’s measured torque number, which is then divided by the engine’s displacement. That means a torque output of 1 lb-ft per cubic inch would equal 150.8 psi as an average pressure. This is another way to compare engines of different displacement and designs.

We’ve included this calculation in our accompanying charts for comparison purposes. Using the previous 383ci small-block example, with 478 lb-ft of torque, this computes to 188 psi, which is actually quite good. Generally, for a normally aspirated engine on pump gasoline, BMEP numbers above 185 psi are respectable. Keep in mind that this BMEP number is merely an average pressure reference number and in no way should be viewed as an actual cylinder pressure number. This BMEP number requires a little more math than the standard torque-per-cubic-inch, but is often referenced in comparisons of different engines so it’s important to know where this number comes from.

Among the engines we looked at is the Ford Performance Aluminator 5.2L (315ci) engine making 580 hp at 7,800 rpm with a peak torque of 445 lb-ft at 4,500 rpm. That creates an impressive 3,300 rpm powerband with a hp/ci rating of 1.83 and a BMEP at peak torque of 211.7 psi. These are very strong numbers for a pump-gas street engine. This engine generates the highest BMEP number of all the normally aspirated performance crate engines offered by the OE performance outlets.

Other Than Peak Numbers

So far, we’ve looked at just the peak torque and horsepower numbers for a quick evaluation of an engine’s potential. But these numbers can be misleading, especially for engines intended for street use where low-speed torque is a valuable commodity. To illustrate this idea, we chose a test we performed many years ago on a 6.0-liter iron truck engine with some added performance parts. The baseline test used a set of TFS aluminum 225cc cathedral-port heads, a mild 227-degrees at 0.050 Comp camshaft, and a factory LS2 composite intake.

For comparison, we swapped the factory intake for a budget sheetmetal version that looked sexy. While the fabricated manifold made an additional 14 peak-horsepower, it gave up as much as 40 lb-ft in the torque curve. For a street engine that spends most of its time in the lower engine speeds, it became clear it was not a good choice. This illustrates how focusing on a single evaluator like hp/ci can be deceiving — especially when it comes to street engines that operate over a wide RPM area.

This means that while simplistic evaluations like torque ratios, BMEP, and other factors do have value, it’s best to also know how the engine will be used in the real world as a major contributing factor when deciding on a specific engine combination.

If you’re looking for sheer normally aspirated power, look no further than Chevrolet Performance’s awesome 1,004 hp 632ci big-block street engine. That puts its peak horsepower per cubic inch at 1.59:1. Peak torque comes in at a torque BMEP of 209 psi. While peak numbers are always impressive, take a look at the 632’s power curve. This beast makes 600 lb-ft of torque at a modest 3,000 rpm. That’s 0.94 lb-ft/ci at a low engine speed. That’s what a combination of huge displacement and high compression will deliver.

One final example of this is the latest DOHC LT6 engine recently debuted by Chevrolet for the 2023 Corvette. This engine is a purpose-built, near-race-engine for the street. At 670 horsepower at 8,400 rpm from 333ci with an astonishing 2.01 hp/ci, this would appear to be a fantastic street engine. With an equally impressive peak torque rating of 460 lb-ft and a torque ratio of 1.38:1, all these are fantastic numbers.

However, peak torque with this engine occurs at an astronomical 6,300 rpm, and with only 333 cubic inches, this engine needs deep gear ratios to provide decent acceleration at lower engines speeds. The evidence of this is the 8-speed transmission behind the engine and its 5.56:1 final drive ratio. No, that’s not a misprint.

While this engine could be used in a generic street car, the reality is that for most street applications weighing 3,300 to 3,700 pounds, this engine would not be a wise choice even with a 5-speed manual transmission and deep rear gears.

We could easily fill another few pages with examples but by now it should be clear how it’s possible to compare performance from different engines of various displacements to come up with the most efficient combination. It’s just another way to become a more discerning gearhead.

Greg Brown’s Hammerhead Performance Engines has created a hemispherical head for LS engines. With a conservative 10:1 compression, his engine made 543 lb-ft of torque and 630 horsepower. That puts the Hammerhead at 1.51 hp/ci with a BMEP at peak torque of almost 197 psi. Plus, at 3,800 rpm it was making a respectable 493 lb-ft or 1.18 lb-ft/ci at this low RPM.

More Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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