Ray Hall
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COMPRESSION RATIOS


WHEN we were first considering changing over to Methanol it was stated a small power gain would be obtained right away with just the change of fuel, but to obtain the full benefit the engine would have to be modified to do so.

Going back to our simple heat engine again as a basis, we can say by the use of Methanol we are getting twice the weight of fuel to ignite, at the same time we can increase the compression ratio to a much higher figure thus producing much greater power or force on the piston, and so in fact obtaining a more efficient engine.

This extra power will, however, do two things, one being to produce more heat so the engine will run hotter, the other being that it will create much higher mechanical stresses.

The extra heat generated we can cope with, for example by using a richer mixture so gaining the cooling action of the Methanol itself, but the mechanical stresses are another matter.

When the fuel is ignited the resultant force is applied to the piston top, also to the cylinder head, but since the head is fixed and the piston movable, the latter starts on its downward stroke, the closed valves making sure the whole force is so applied without possible escape elsewhere.

Now if the studs or bolts, holding the head down, cannot cope with the now increased power we are going to be in trouble.

On the other hand if the head is well and truly held down, the force will be applied to the bolts holding the cylinder barrel to the crankcase.

Provided all these hold, the extra force is applied to the little and big ends, plus the crankshaft itself.

This is why, as a good example, the dirt track JAP engine has the bolts that hold the head on extended right down past the barrel, or in some cases through the finning, right into the crankcase itself, making a really solid mechanical assembly.

HIGHER COMPRESS!ON

There comes to mind as another example a well known twin cylinder machine with the engine made as an alternative in light alloy, the barrels and heads being interchangeable, the makers advising the use of the iron barrels for Methanol due to the alloy barrels tending to fracture at the base.

We must also, at this point, consider if it is proposed at this stager or at a later point in time, to use Nitro-methane, the question of the actual compression ratio to be used will be determined by the amount of Nitro-methane in the Methanol and as a guide Chart 1 gives approximate values on the conservative side.

Methods of obtaining higher compression ratios depend on many factors, which in a simple case, may be had by the fitting of high compression pistons, if available.

In some cases a thinner head gasket may be the answer, or total elimination of the gasket and face grinding the mating surfaces.

Again it may be possible to have the ratio increased by removal of metal from the head, or the block for that matter, but in such cases you must check that the valves have enough clearance to miss the piston.
Another method is that of building up the inside of the cylinder head with new metal and then machining to the required shape.


Remember in the case of a V-8 engine if you have the heads skimmed to get higher compression, you will be in trouble with the inlet ports now being out of line with the manifold, due to the heads sinking lower in relation to the manifold itself, so give the matter some careful thought before going ahead.

CHART 1

Approximate compression ratios recommended for use with Nitro-methane / Methanol fuel mixes.
Compression Ratio

% Nitro in Methanol per volume

16 to 1

15 to 1

14 to 1

13 to 1

12 to 1

11 to 1

10 to 1

9 to 1

8 to 1

7 to 1

6.5 to 1

10%

18%

28%

38%

46%

56%

66%

75%

85%

94%

100%


It must be remembered that for all practical purposes 14 to 1 should be considered the maximum usable compression ratio in modern short stroke automotive type engines.



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