Tuning for Torque vs Horsepower: Know the Difference

Normally aspirated racing engines (or naturally aspirated) are common in the motorsport’s world. They produce ample amounts of power if they are tuned properly. However, tuning can be problematic due to how a normally aspirated engine works. High performance engines in scenarios ranging from a dragstrip to watercraft racing utilize this. We highlight some of the added ways to measure performance and tweaks to max it out.

Engines for sprint cars, for example, have a specific fuel demand at the torque peak and a different one at the horsepower peak. Oval track course lengths – such as quarter mile or third mile – affect the fuel curve tuning.  Shorter courses have slower speeds in the turns. Engines accelerate from a lower engine speed with tuning often concentrating on the lower RPM for the most torque peak. Longer tracks may have gentle, faster turns where the engine accelerates out of the turn from a higher RPM. The operating range of the engine is narrower with tuning concentrating on the higher RPM for the most horsepower peak.

 

This stack-type Kinsler mechanical fuel injection on a sprint car racing engine.
This stack-type Kinsler mechanical fuel injection on a sprint car racing engine.

Key Performance Measures: Torque vs Horsepower

When analyzing performance measurements, such as with a dynamometer, a vehicle G-meter, or race performance data, it becomes apparent that the torque peak and horsepower peak have two different air mass flows through the engine. As a result, they have two different fuel delivery amounts. This causes two different tuning routines, especially for racing engines with mechanical fuel injection.

Maximum torque at an engine RPM: Torque is measured in foot-pounds of force.  That is a measure of pulses of force per revolution and at a specific engine RPM.  Note that the key is ‘per revolution.  The units are different than the maximum amount of air ‘per minute’ that may occur at a higher engine RPM coincident to the horsepower peak.

In normally aspirated racing engines, there is an engine RPM where torque is maximum. At that engine RPM, torque is at maximum because volumetric efficiency is at its maximum. At this point, the maximum amount of air per revolution is flowing through the engine.

MAX FUEL PER REVOLUTION, MAX TORQUE: This max torque is at an engine RPM that requires the maximum fuel per revolution specific to that engine combination. The torque pulse strength is maxed out as well if the correct amount of fuel is delivered. The combination of maximum air per revolution and maximum fuel per revolution, at a correct air/fuel ratio, produces the maximum torque.

  • For a good racing setup, this should be coincident with the lowest engine operating range in the race. For example, in drag racing with popular torque converter transmissions, the stall speed or staging and launch speed should be near the torque peak of the engine, such as a common value of 5,000 RPM.  Engines with torque peak around that RPM are at the maximum force per revolution and pull really hard from the start of the race.

 

  • In boat racing, propeller slip in the water allows the engine to flash (boil the water) up to an RPM where the propeller is hooked up to the water, and the race boat accelerates. A good propeller selection is one that flashes to the torque peak RPM when the throttle is hit for good breakaway acceleration from the low end.

 

  • In some cases, due to gear ratio limitations from class rules, the operating range may have to dip below the torque peak RPM. For example, a common high output 360 or 410 ci sprint car racing engine may have a torque peak around 5,000 RPM as well. Engine RPM coming out of a corner on a short ¼ mile oval track may be down to 4,000 RPM because of slower speeds coming out of the tight turns. That is below the torque peak RPM. A good engine setup will immediately surge up to and through the torque peak RPM when the throttle is mashed coming out of the turn.

AIR DENSITY EFFECT: Air density changes with the track elevation and time of day.  Tuning involves changing the fuel amount for that track air density. That is to maintain the optimum air/fuel ratio. Too much or too little fuel for the amount of air going into the engine at the torque peak will reduce performance. (Ref. Airdensityonline.com)

Tracking air density changes becomes necessary.

  • A sea level track may have an air density around 95% for the final heat in the evening. The fuel system needs more fuel for the sea level track to maintain an optimum air/fuel ratio.

 

  • A high-altitude track may have an air density around 80% for the final heat in the evening. The fuel system needs less fuel for the high-altitude track to maintain an optimum air/fuel ratio.

At the proper air/fuel ratio, both would produce the best punch for that location.  However, the high-altitude track would have less power. An air/fuel ratio of 5 to 1 would be a typical value for a normally aspirated racing engine running on methanol fuel. Other racing fuels have other optimum air/fuel ratios with the same challenges for torque peak tuning.

Graph of methanol fuel GPM (gallons per minute) delivery vs engine RPM for mechanical fuel injection on a 325 ci racing engine at different air densities. Ref. High Horsepower Tuning for Mechanical Fuel Injection, p. 35 Illustration

This graph shows the differences in fuel flow needs in different air densities. An optimum air/fuel ratio of 5 to 1 is typical for a high output normally aspirated racing engine on methanol fuel to maintain best torque at different air densities (AD). That air/fuel ratio is by weight, not volume such as cubic feet per minute (CFM) or gallons per minute (GPM).

Maximum horsepower at an engine RPM: Horsepower units are a measure of work. The work is the amount of force that is done. That amount would be in a unit of time such as the ‘minute’ in RPM.  Note that the key is ‘per minute’. That is different than ‘per revolution’.

In normally aspirated racing engines, there is an engine RPM where horsepower is maximum. At that engine RPM, volumetric efficiency is most often NOT at its maximum. It is some value below the maximum that occurs at the torque peak. However, the higher amount of torque pulses per minute are great enough to overcome the lower strength of the torque pulses. As a result, the maximum amount of air per minute is flowing through the engine.

MAX POWER, OPTIMUM AIR/FUEL RATIO: This max horsepower is at an engine RPM that requires the maximum fuel per minute at the proper air/fuel ratio for that specific engine need. The combination of maximum air per minute and maximum fuel per minute, at a correct air/fuel ratio for that engine, produces the maximum horsepower. An air/fuel ratio of 5.2/1 is typical for a normally aspirated racing engine on methanol fuel. Again, other fuels have different air/fuel ratios that are optimum for horsepower peak with the same challenges as described.  Ref. Fuel Injection Racing Secrets, p. 175

  • For a good racing setup, this is usually at the maximum engine speed in the race. High speed racing on land in one example. A good setup is one where the engine is geared to the ground so that the horsepower peak RPM is matched to the top speed goal.  For high-speed racing on water, propeller selection is consistent with matching the horsepower peak RPM to the top speed goal. The maximum horsepower is coincident with the speed against the various resistances such as wind resistance, friction, and racer weight. A 200 MPH top speed goal, for example, would be reached at an engine speed where maximum horsepower is reached. That maximum horsepower would be the amount necessary to overcome wind resistance and other impedes that occurs to get to 200 MPH.

 

  • In some cases, also due to gear ratio limitations from class rules, the operating RPM limit may be above the horsepower peak. This is common in drag racing where gear ratios are optimum for the most overall acceleration. More gear ratio is provided in high gear for more acceleration.  The engine is overrevved beyond the horsepower peak as it approaches the finish line. However, the combination provides the best performance.

Horsepower units are a measure of the amount of work done at an engine RPM – how many torque pulses of force that combine to produce work in a unit of time of engine revolutions such as ‘per minute’. It is in traditional horsepower units proportional to the work that is done according to an old definition that was developed prior to 1900.

More revolutions per minute (RPM) can lead to a greater frequency of torque pulses which can lead to more horsepower. But this only works up to a point. That point is where the torque pulses get weaker.  In normally aspirated racing engines, that is usually from breathing restrictions from intake port size and camshaft timing. Increased number of pulses per unit of time do not overcome the engine breathing reduction that makes the torque pulses weaker. As a result, horsepower starts to drop.

For example, a normally aspirated racing engine has a maximum torque of 500-foot pounds at 5,000 RPM.  That is at a volumetric efficiency of 100%. Maximum horsepower is reached at 7,000 RPM. The volumetric efficiency is only 85% at the horsepower peak. It drops lower than that at higher RPM where horsepower drops off as a result.

Popular fuel injection racing combination: Enderle Bird Catcher hat on a tunnel ram manifold.

In the photo above, fuel delivery is controlled by mechanical fuel injection in this setup. This system is fairly independent of the CFM demand of the engine. The Enderle hat type of throttle body on top of the manifold plenum is rated at 4,600 CFM – about 4 time more air than the engine will use. The fuel injection system has to be tuned for torque peak and separately for horsepower peak. Variables are engine RPM and air density.

Applications

There are different applications of engine performance characteristics:

Typical volumetric efficiencies (VE) showing torque peak & horsepower peak differences for different engine applications.

LOW RPM TRUCK ENGINE: For the truck, low RPM power is important. It is reached with a smaller intake delivery system that is optimum for good power at low RPM. Torque peak is high. However, horsepower peak can be limited by intake delivery restrictions. Volumetric efficiency at the horsepower peak can be good, but horsepower is low due to a lower RPM peak. It is further reduced by mufflers necessary for quiet operation.

MODERATE RPM PASSENGER CAR ENGINE: For the passenger car, moderate RPM power is important.  Moderate intake delivery design is made as a compromise for ‘the best of both worlds’ of low and high RPM. That combined with the proper transmission gearing and torque multiplication accomplishes good performance from a standing start all the way through the top speed. VE is also reduced by mufflers.

HIGH RPM RACING ENGINE: For the high-speed racing vehicle, the torque peak may actually be higher than the moderate speed applications because of the higher number of torque pulses from higher RPM. However, low RPM off idle power is usually very low requiring more gearing or torque multiplication for low end acceleration. ‘Or rev it up and spin the tires!’. In addition, VE and power is up from free breathing w/o mufflers.

Mechanical Fuel Injection (MFI) Air & Fuel

Graph of linear fuel delivery with increased RPM in mechanical fuel injection. Ref. High Horsepower Tuning for Mechanical Fuel Injection, p. 44 Illustration

Here is the problem. As the RPM goes above the torque peak, the volumetric efficiency becomes less. The optimum fuel amount per revolution to reach maximum power becomes less and curves down. The racing engine goes rich at high RPM from excess fuel that is delivered. That is indicated by the curved line. Excess enrichment reduces power.

Tuning for Maximum Horsepower

To achieve the best of both RPM worlds with racing mechanical fuel injection, the fuel delivery can be curved with a high-speed bypass. That is done with an added fuel system bypass. It is enabled usually just beyond the torque peak RPM.

Graph of fuel curve (dark line) in MFI chasing the optimum fuel need shown by the curve. Ref. High Horsepower Tuning for Mechanical Fuel Injection, p. 47 Illustration

Curving the fuel delivery plot to chase the fuel need differences between the torque peak and the horsepower peak is illustrated here. For mechanical fuel injection, it is done quite easily with a high-speed bypass.  It is enabled at an engine speed just beyond the torque peak. For a typical sprint car engine, drag race bracket engine, and several drag & circle boat engines with torque peak around 5,000 RPM, an opening point just below 6,000 RPM is common.

Engler Machine stack type mechanical fuel injection on sprint car racing engine.

In addition to high-speed bypass jetting, Engler has a unique stumble valve for added bypass tuning. It is a normally open valve that closes with pressure to further alter the fuel curve. It is commonly used for cases where sprint cars are slowed down during a race from a breakdown and idle around the track waiting for it to be cleared. The stumble valve helps to lean out the engine at low speeds to maintain engine heat for good response when the race is resumed. Under full power, it closes off restoring the engine to a normal fuel curve with a proper main bypass for torque and a proper high-speed bypass for horsepower.

CARBURETORS: Air/fuel is controlled with proper design for the air mass flow going through the carburetor venturis. Carburetors provide fuel control transition between torque peak and horsepower peak, sometimes with added barrels (secondaries) for higher mass air flow at the horsepower peak. Adjustment is needed for significant air density changes, such as from lower to higher altitudes. Venturi flow restrictions may limit top end power over fuel injection in some cases.

This vintage sprint car with six 2-barrel carburetors has ram tubes feeding the carburetors – all on a ram manifold for tuning flexibility. Progressive linkage can be added to provide center carburetors for low RPM, tipping-in the outside carburetors for high RPM.

Conclusion

After the work of figuring out the best setup for the torque peak and the horsepower peak, all that’s left is to run the vehicle and monitor the results. Monitoring the spark plugs for problems in the fuel system can help.  However, if the spark plugs are clean and the engine is performing at peak form, you can rest assured that setups for each situation are working right.

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