Introduction.
This thread describes methods to improve engine output by reducing the engines tendency towards knock. Control and elimination of knock is important to producing high output small displacement engines. The pinnacle of those probably being the 1.6L turbocharged engines from the 80’s Formula One era.
The methods detailed below come from two papers from Adam Opel AG written by Dr Fritz Indra. These were on the development of the Group A C20LET (2L 16v turbo) engine for the Opel Calibra, “Hochaufladung eines 2.0 Ltr. 4 Ventil-Ottomotors” and the development of the C20XE for Formula Three “Der Formel 3 Rennmotor von Opel”. Further information was taken from a paper by Mitsubishi Motors “Study of Engine Cooling Technologies for Knock Suppression in Spark Ignition Engines” and another by Toyota Motor Corporation “Effect of Mirror-Finished Combustion Chamber on Heat Loss”.
Cylinder and Combustion Chamber Cooling.
Enhanced cooling of the top of the cylinder and the combustion chamber is achieved by drilling two Ø3.5mm holes between the cylinders and machining a slot 1.5mm wide by 20.0mm deep between the two. This can be seen on the Group A block in Figure 1 below. Note that the oil return holes have been blocked off and the water jacket area has been machined out. This is not done on the OHC engines.
Figure 1. Group A Block.
Cylinder and Combustion Chamber Cooling Cont.
The holes are spaced 45 - 50.0mm apart and go through the head gasket (no slot is cut into the gasket) and then into the cylinder head. This is shown in Figure 2 below.
Figure 2. Cooling holes in cylinder head.
The purpose of the slot in the block is to allow coolant flow around the full 360° of the cylinder for the first 20.0mm. This is the hottest part of the cylinder and the slots allow the cylinder wall temperature not only to be dropped (by some 80°C in the Mitsubishi tests) but also stabilised over the entire operating rpm range. Heat from the piston, carried away by the piston rings, is better dissipated by the cooler cylinder walls.
The coolant flows into the head in areas close to where the combustion chamber cores touch, stimulating flow around this area and helping to control the temperature of the combustion chamber is this area.
Mirror-Finished Combustion Chamber and Piston Crown.
Testing carried out by Toyota, on the effects of reducing the surface area of the combustion chamber and piston crown, by reducing the surface roughness have shown improvements in torque of 3%, reduction in piston crown temperature of 6°C and increases in the exhaust gas temperature of up to 10°C (providing more exhaust gas energy for the turbocharger).
The testing also revealed that even if the surfaces where covered in combustion deposits the above benefits where still realised, at slightly lower values, due to the lower surface area available for heat transfer by convection. It must be stated that both the chamber and piston crown must be polished to approximately 0.8z surface roughness or an optical mirror finish.
Figure 3. Surface Roughness values.
Figure 4. Comparisons of Torque, Exhaust Temperature and BSFC.
Piston Crown Cooling.
Cooling of the underside of the piston crown has been used in high performance gasoline engines and turbocharged diesel engines for many years, possibly from before World War Two. Cooling the piston crown improves its durability and reduces a gasoline engines tendency to knock by keeping the combustion surfaces (the crown) cool. This method was used on the Group A C20LET engines in conjunction with the cooling modifications to achieve over 300hp (with a 38mm restrictor) on unleaded gasoline during long arduous rally stages. Due to the regulations at the time for Group A, Opel were not allowed to modify the cylinder block for fitment of the oil jets, so the oil jets and feed were incorporated into the sump which was free to design as shown in Figure 5 below.
Figure 5. Special Cast Sump With Oil Jets.
However, on the OHC engines “off the shelf” oil jets can be used and fitted to a modified block. The oil jets that are used can be sourced from various VW, Porsche, Isuzu, etc. turbocharged gasoline or diesel engines and are the press fit type. The oil jets are fitted into the crankshaft mains of the block fed from a slot from the mains oil feed. This can be seen in Figures 6, 7 and 8 below.
Figure 6. Oil Jet Feed Slot From Mains.
Figure 7. Oil Jet Pressed Into Position.
Figure 8. Oil Jet Nozzle Aimed Through Crankcase Blowby Window.
Conclusions.
All three methods can be used on the OHC engine to reduce the tendency for knock. This will allow the use of higher turbocharger pressure levels and more aggressive ignition timing for higher power outputs.
Combined with effective intercooling, higher outputs with more reliability can be realised for street driven vehicles.
This whole text can be downloaded as a pdf file
here
A detailed fitting guide for the fitment of the oil jets is given in a pdf document
here courtesy of Total Vauxhall Magazine.
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LT3 Powered Vauxhall Astra.
Intercooler, Mopar Super 60 Injectors, SDS Stand Alone EFI, Ported Big Valve Head, Cast T3 Manifold With External Wastegate + More
