Keep your engine tuner in check. Verify tuning results.

The arguments for and against chassis dynos has always been which one is correct and which ones not. This article has not been written for, or against any chassis dyno, instead, it is written to inform and educate the readers of how they themselves can verify, by means of simple calculation, as to how much horse power their setup can make and as to how correct the horsepower figures are that they received form their high performance engine tuner.

Let us start with fueling. One pound of 93 octane unleaded gas can support a certain amount of horsepower. Although less power can be made, there is not an engine tuner in the world that can make more power out of that one pound of 93 octane unleaded fuel than what it can support.

It is also common knowledge that two engines, even built exactly the same, does not always make the same power. Brake specific fuel consumption or BSFC is the main reason. BSFC is the ratio between the engine's fuel mass consumption and the crankshaft power it is producing or simply put; Brake Specific Fuel Consumption is the projected amount of fuel required to produce 1 HP for 1 hour. This means that an engine with a B.S.F.C of 0.5 will burn 1/2 or 0.5 lbs of fuel to produce 1 HP for one hour. In the USA, the fuel flow for BSFC calculations is normally expressed in pounds per hour (lb/hr) while the output units, of course, are in horsepower (HP). Determining exact B.S.F.C for a specific engine is complicated and requires an engine dyno.   

BSFC numbers remain similar over a wide range of engine sizes. These numbers only change for different engine designs and compression ratios. For example, a small one cylinder 50 cc four stroke and a large V8 engine can both have the same BSFC number. However, engines of different classes like diesels and gasoline engines will have very different BSFC numbers. Therefore we can also conclude that the BSFC for a turbo charged engine will be slightly higher than that of its counter part in normal aspirated form.

Standard practice call for the following BSFC figures: 

NormalAspirated Engine = 0.5 lb/hr

Super Charged engines = 0.55 lb/hr 

Turbo Charged Engine = 0.6 lb/hr 

Based on the amount of fuel available calculations can be made to within a small percentage of how much HP a specific fuel setup can support. Based on this calculations you now can confirm if your engine tuner and his equipment are reliable.  

Let’s look at an example of a turbo charged 6 cylinder VR4 engine with the fuel pressure set at 43.5 psi, supporting six 620cc injectors with a maximum fuel injector duty cycle of 80%. The fuel injectors are rated 620cc @ 3 bar or 43.5 PSI 

Regardless of engine size and turbo size, how much powers can this fuel setup support? 

First we want to convert fuel injector size in cc to fuel injector size in lb.

 The Formula:

Therefore: 620/10.5 = 59.05 lb/h 

Now we want to calculate how much HP one injector can support: 

(Do remember that this calculation indicates HP on the flywheel and not to the wheels.) 

The Formula:

Therefore: (59.05 x 0.8) / 0.6 = 77.4 HP per injector. 

Therefore: 6 injectors will support 6 x 77.4 = 464.4 flywheel HP 

(Note: Always multiply by the amount of cylinders your engine has) 

Average drive train loss for the VR4 from the factory is about 28%. Tire size and width can impact this figure even more. 

To calculate how much horse power the above VR4 can support we now have to subtract the drive train losses from the flywheel horse power figure. 

Therefore: (464.4 HP – 28%) = (464.4 x .72) = 334.4 HP at the wheels. 

Based on this calculation we can say that a VR4 with 620cc injectors at 80% duty cycle and a fuel pressure of 43.5 PSI can support about 335 HP at the wheels on 93 octane, give and take 2 – 3 %. Factors such as humidity and temperature will affect the outcome. 

If your engine tuner now hands you a 428 HP to the wheel dyno sheet you have to question the following: 

A: The accuracy of his equipment.

B: The honesty of your engine tuner.

C: Both of the above. 

More power can be made by increasing fuel injector duty cycle or installing bigger fuel injectors or by raising the fuel pressure. When raising the fuel pressure great care must be taken to ensure that the fuel pump can supply enough fuel at the new pressure. 

Raising the base fuel pressure to 55 PSI will require a re-calculation to determine the new injector size or flow figures. 

The Formula:

Therefore: (The square root of 55/43.5) x 59.05 lb/hr = 1.1244 x 59.05 = 66.4 lb/hr 

Now calculate how much horse power a 66.4 Lb/hr fuel injector can support at 80% duty cycle. 

The Formula:

Therefore: (66.4 lb/hr x 80%) / 0.6 = (66.4 x 0.8) / 0.6 = 88.5 HP per injector. 

Therefore: 6 injectors will support 6 x 88.5 = 531 HP 

Calculate Drive Train Loss:

Therefore: (531 HP – 28%) = (531 x .72) = 382.32 HP at the wheels 

Based on the calculations above it is easy to check if your performance engine tuner is truthfull with you or if his equipment is accurate. 

After a tuning session always ask your engine tuner for the following: 

1. What was the fuel injector duty cycle at peak power?  
2. What was the fuel pressure?   
3. What is the fuel injector size?  

These three figures will help you determine if your HP figure is within the ballpark or not. Do the calculations and if it does not match ask your engine tuner to explain. 

Also remember the amount of power your setup can make will change between winter and summer and between a dry or humid day, but know that fueling will always help to determine where you stand. 

1. In winter time the air is cooler therefore more power in winter.  
2. When it is humid, air is denser therefore heavier and thus moving slower through the engine therefore less power when it is hot and humid.   

There is a small group of performance setups that can not use these calculations and it is those who spray nitrous, alcohol, methanol etc. However a simple request to the engine tuner to supply you with the base horsepower number before he starts to tune for the spray etc. can help you determine if your performance figures make sense.  

The engine tuner at EngineLogics take great care to ensure accurate, reliable and correct engine tuning results.