Confirm your dyno results and in so doing keep your engine tuner honest.
The arguments for and against chassis dyno 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 too help you confirm your dyno results and in doing so educate the reader of how they themselves can verify, by means of simple calculations, as to how much horse power their current fuel setup can support and as to how correct the horsepower figures are they received form their high performance engine tuner.
Confirm your results with your own fuel injector and horsepower calculator:
Confirm your dyno results
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. No matter how well the heads are ported or how much boost you are running!
Brake Specific Fuel Consumption
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 B.S.F.C. is but only one of the reasons. B.S.F.C 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 B.S.F.C 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.
B.S.F.C numbers remain very 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 B.S.F.C number. However, engines of different classes like diesels and gasoline engines will have very different B.S.F.C numbers. Therefore we can also conclude that the B.S.F.C 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 B.S.F.C figures:
Gasoline – .45 to .50
E85 – .63 to .70
Methanol – .9 to 1.0
Boosted or forced induction:
Gasoline – .60 to .65
E85 – .84 to .91
Methanol – 1.80 to 2.0
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:
How much horse power can a 1000cc injector support on a turbo charged 6 cylinder Toyota Supra at 80% duty cycle with the fuel pressure set at 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 to convert fuel injector size from cc to fuel injector size in lb:
Therefore: 1000/10.5 = 95.24 lb/hr
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.)
Therefore: (95.24 x 0.8) / 0.6 = 126.99 HP per injector.
Therefore: 6 injectors will support 6 x 142.86 = 762 flywheel HP
(Note: Always multiply by the amount of cylinders your engine has)
Drive train loss is dependent on transmission, differential, tire size and width but let’s use 17% for a Supra with stock drive-train, manual transmission and stock wheels. (Bigger and wider wheels = bigger drive train loss)
To calculate how much horse power the above Supra can support on the wheels we now have to subtract the estimated drive train losses from the flywheel horse power figure.
Therefore: ( HP – 17%) = (762 HP x .83) = 632.5 HP at the wheels.
Based on this calculation we can say that a Supra with 1000cc injectors at 80% duty cycle and a fuel pressure of 43.5 PSI can theoretically support about 630 HP at the wheels on 93 octane, give and take 2 – 3 %. Factors such as humidity and temperature will affect the outcome.
The exact same setup will make less power on E85 and even less on Methanol, however it might make 6 – 10 % more on “Race Gas”
To make the same power on E85 it will need a 1200 cc Injector (115 lb)
To make the same power on Methanol it will need a 2650 cc injector (250 lb)
If your engine tuner hands you a 1000 HP to the wheel dyno sheet you have to question the following:
A: The accuracy of his equipment.
B: His understanding of fueling.
C: The honesty of your engine tuner.
D: All 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. It also have to bee remembered that as fuel pressure raise injector lag times increase.
Raising the base fuel pressure to 55 PSI will require a re-calculation to determine the new injector size or flow figures.
Therefore: (The square root of 55/43.5) x 95.25 lb/hr = 1.1244 x 126.99 = 107.1 lb/hr
Now calculate how much horse power a 107.1 lb/hr fuel injector can support at 80% duty cycle.
Therefore: (107.1 lb/hr x 80%) / 0.6 = (160.63 x 0.8) / 0.6 = 142.8 per injector.
Therefore: 6 injectors will support 6 x 142.8 = 856.8 HP
Calculate Drive Train Loss:
Therefore: (1445.74 – 17%) = (856.8 x .83) = 711.14.92 HP at the wheels
Based on the calculations above it is easy to check if your performance engine tuner is truthful with you or if his equipment is accurate.
After a tuning session always ask your engine tuner for the following:
What is the fuel injector size?
What was the fuel pressure?
What was the fuel injector duty cycle at peak power?
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.
In winter time the air is cooler therefore more power in winter.
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 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.
1000 cc Injectors at 80% duty cycle and 43.5 psi base fuel pressure can support the following horse power figures to the flywheel:
1 Cylinder – 126 HP.
4 Cylinder – 500 HP.
6 Cylinder – 756 HP.
8 Cylinder – 1008 HP.
Quickly confirm your results with your own calculator: