173 Head On a 202
Original submission by T Mar 11th 2007:
173 Head on a 202:
I ran a 173 Head on a 202 Engine in my daily driver between late 1989 and July 2000. The Engine ran in an HZ wagon during that time doing both city and open road driving without incident.
In March 2001 the Engine went into my UC sedan where it is at the present time (Aug 7th 2007).
In both cars the Engine was coupled to a Trimatic and 2.78 diffs with standard tyres in each car (HZ 195 75 SR 14, UC 185 75 SR 13). The Trimatics have power/Economy Switches fitted to enable them to stay in the higher gear longer (delayed kickdown). Keeping the Engine in a higher gear and making it work harder keeps the combustion chamber deposits to a minimum.
Identifying a 173 Head:
A 173 Head uses a 14 mm 5/8" reach tapered seat Spark Plug and it has a special recess between
Cylinders 1 and 2 where the head meets the block to make identification possible when it's
The compression ratio is higher than 10.5:1.
Clearly 98 Octane must be used since the compression ratio is higher than 10.5:1.
Who Else Thinks Such a High Compression Ratio Can Be Supported?
In the late '60's Audi marketed a vehicle that had a compression ratio of 11.0:1. They claimed it was achieveable by swirling the mixture around a specially shaped piston crown.
Detonation and Pre-Ignition:Clearly such a high compression ratio must place the Engine in the danger area for
Combustion chamber deposits are the last symptom of an engine in trouble not the first.
Weak ignition, insufficient advance, the wrong type of engine load and vehicle use are prime causes of these deadly combustion chamber pests. Fixing the real problem gets rid of them and keeps them out.
It is essential to fit strong ignition. If the ignition cannot cope with the compression
ratio then the combustion chamber temperatures will never become high enough to burn down the
combustion chamber deposits properly and pre-ignition/detonation will result.
Since the driver has complete control of the shift points he/she will be the one to control
the vehicle's operational variables.
Jackie Stewart (international racing driver), in one of his videos, indicated that best economy
and engine life is obtained by changing up early and stepping on the accelerator. Here is one way to minimise combustion chamber deposits.
If you uprate the Carburettor, I strongly advise you to uprate the Ignition too, since better
breathing makes for higher compression pressures which makes for greater ignition requirements.
If the ignition cannot cope the mixture will automatically richen and combustion chamber deposits will form leading to pre-ignition and detonation.
Dec 25th 2007: I've just found that stated "98 octane" fuel is becoming lower in octane rating and causing kick-back problems under high speed at full load full throttle. As a workaround I've found pulling the choke partially on stops the problem. Presumably it quench's glowing plug electrodes.