POWER = FUEL ECONOMY ?

Please note I will be discussing Petrol driven engines and although a lot applies to Diesels it is in my opinion not cost effective for anyone but a Transport operator. Although Turboís are highly effective, our Temperatures and other conditions made me loose interest in them. Although this article is very technical in nature I have decided to split it up into various segments that might interest you.

If you increase your motors efficiency, you will invariably increase its Power and also get a better Fuel consumption and should also get a longer life out of it, if and this is important, the engine is matched to the right Vehicle and gears and it is not abused. Just as a matter of interest 1 gallon of gasoline can drive 45 Hp for 1 hour under ideal conditions. Most Engines have a thermal efficiency of 27 % of which cooling water carries of 30 % and out of the exhaust go another 33 % (this is what makes a Turbo "go") Your fan and water pump suck up to another 3 %. Before we get to the interesting part 1L Cylinder Volume can make an effective 90 NM Torque (Turboís 50% more). Low RPM Torque with the right gearing can be very Fuel efficient, since you have less Frictional and Pumping losses in your engine and less stress on your components- meaning longer Engine life!

A quick explanation of the 4 Cycle Otto Engine (Constant volume combustion) is in order here. Regulated air (restricted through a Butterfly normally) passes through an Intake and goes into the Combustion Chamber, by which time it should be well mixed with fuel. Now it gets compressed and Ignited whereafter the burning gasses expand and force the piston downwards and thereafter leave the engine through the Exhaust. As a tuner the intake gets Ported (gasflowing is a term to check the porting efficiency in L/min.) for minimum restriction. The Combustion chamber gets modified for better combustion and the "bowl" (area just before the Valves into the intake) also gets changed to ensure:

1 The least restriction

2 No Interference by the Valves

3 Better fuel mixture by Swirling or Tumbling

The Exhaust ports get freed up to let the most gasses out. Changing when the spark happens will also change performance dramatically, or "hole" the piston.

The Camshaft is the brain of an engine and controls when the Mixture may enter and when the gases may leave with a lot of overlap in-between. A Thorough understanding of gas Dynamics is needed for their design.

The Header or "Banana Branch" aids the gases to leave the Cylinder by creating a negative wave in the Primary pipe and a returning positive wave ensures that the fuel mixture now entering the Cylinder will not leave through the Exhaust port. After the header you need a "certain" length and Diameter of pipe to aid your wave theory before emptying into the Atmosphere or into baffles (to curb noise). "Backpressure" through Baffles does not fit into this picture and should remain in the "Kroeg".

All this is to ensure that we can harness most of the Latent energy in our Fuel Air mixture to release itself into pressure forcing the piston Downwards. This turns the crank and finally our wheels. The more effective this process happens the more Power we get at the wheels and the less fuel is wasted trying to generate our combustion Pressure. Sadly it is a bit more complicated than it sounds and therefore a lot of "Tuning" shops fail miserably to let it all "work together". If you hit a good one the following Scenario might happen. Before, your top speed was 140 Kph at 4.5 Km/L. After you laid down your Moolah you can now Cruise at 140 Kph and get 7 Km/L. So some time down the road you score if you donít "Hammer" the "scraal Pedaal".

If a vehicle should need 100 Nm to Cruise it is of no concern what RPM you are turning the Crank at, except that a Motor will be most efficient if this is close to its Peak torque point. Generally a Low revving motor is a happy Motor. Therefore it does not help if this motor develops 200 Nm at cruising as the other 100 Nm are "wasted". Sure you need more Torque to accelerate, for which you have Gears. The larger your spread between maximum Torque point and power point is the more "flexible" the Engine will be. Toyotas Hilux 2Wd Bakkies are a case in point with Gear ratios excellently suited to a "work" truck. If a similar Engine Job is done to that as I do to the 4WD 2.4L (30% more Power, 50% better Fuel economy), hardly any difference is seen. Simply it needs no more Power; it sure pulls harder but that is a moot point in a "work truck". Then you put in a Lower Diff ratio so it now does 160 Kph at 4000 Rpm, its Fuel Consumption goes to 10 Km/L and its top speed suddenly goes over 200 Kph! Why am I waffling on about all this technical stuff? So you will know what your needs are and power and Torque figures have no meaning if you do not know at what RPM they are happening, and your final Drive ratio.

Take the Land Cruiser with stock 3 F Motor, modified and 2 V8 Chevrolets in this Horsepower table:

RPM

3F

3F modified

307 Chevy

350 H.O.

231 Buick

2000

68

72

118

112

87*

2500

83*

90

152

161

108*

3000

90

114*

188*

200

130*

3500

93

129

216

253

144

4000

87

130

235

318

150

4500

70

124

245

387

148

5000

 

 

242

456

136

5500

 

 

226

510*

113

6000

 

 

203

553

84

6500

 

 

170

582

 

7000

 

 

 

595

 

7500

 

 

 

600

 

8000

 

 

 

587

 

Please note the discrepancies since the Chevy's were taken at another Dyno at Sea level and the 3Fís were taken at Rand level (I have already approximated all power to sea level above)). The * indicates the maximum Torque point. Experience shows the 307 to give me the best performance and fuel economy in a FJ 40 Plaasbakkie. Although the 350 HO looks impressive it will "Drink itself to Death" needs Clutch slippage to get underway and is generally a Bitch to drive below 3000 RPM. Depending on your gears and tires, you would cruise 120 Kph at 3000 RPM and could easily accelerate up to 160 Kph. While in an old FJ 40 you will be doing 3750 Rpm at 120 Kph. Consumption should be 7 Km/L, 3F modified will get about 6 Km/L, 350HO might manage 2 Km/L if somebody would attempt that conversion. Also the 307 max. Hp figure lies at 4500 Rpm pulling nicely to 6000 Rpm, which dispels the Myth that V8ís canít be revved.

To modify the 3F cost half that of the 307 Conversion and would be worth it if the vehicle does not need to work hard. A conversion though is cheaper than a rebuild. Know your needs and then choose what you are going to do. To often people fit the Biggest and baddest or listen to "Bar talk" in this respect. Take above Cruiser into account. A lot of people fit 350 Chevy Motors (including myself) but do not take into account that it is a thirsty motor (5 Km/L are the norm), and that it is more suitable for low RPM torque with a far more narrower "Powerband" than the 307. Yet if you are going to pull your 30 foot Yacht at 180 Kph, it will be just the ticket especially in a FJ 80 with a 5 Speed Box, since at that speed it has not even touched 4500 Rpm.

Thatís it for the technical stuff. Look at the Article on the Buick 3.8L V6 Motor, on what Vehicle for what engine and Pitfalls to avoid in your conversion. By the way compare it to the Hilux and Ford V6 Power output. 1Kw =1.341 Hp and 1 Lbs./ft = 1.3558 NM.