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Neil,

That is very odd indeed.

Octane doesn't affect flame speed.

http://motorcycleriderz.motionsforum.com/t196-octane-is-not-how-fast-a-fuel-burns

 

Peter

Interesting read....

In that case it must have more to do with the change in squish characteristics.

When I pulled the rover engine apart to free a stuck piston ring (it had been standing for 8 years before I bought it), I found that it had been fitted with thick 3.9 head gaskets.

Despite it saying it was a 9.35 CR engine (from the numbers it was a factory new short engine) the compression measured 9.7 :1, but it is obvious the heads had been worked on. I tried fitting a set of original tin thin 3.5 gaskets and this lifted the measured compression to 10.5 :1.

 

No chance it was going to run on regular unleaded without doing it a mischief hence the move to super unleaded.

 

Even more confusing are my two Stag engines. Both set up on the same rolling road, both have the same spec camshafts although slightly different exhaust manifolds with the more powerful engine having larger bore secondary pipes on the manifolds.

 

The Stag engine in the TR has a 10.1 CR, the Stag engine in the Stag 10.5CR. The difference was achieved by decking the block to bring the pistons closer to the head on the higher compression engine.

Both engines started out with 28 degrees max advance which is the standard figure for a mk2 Stag engine. After the rolling road runs the higher compression engine needed 31 degrees for its best figures, yet the lower compression one was retarded to 25 degrees max advance without losing any power. The higher compression engine made 30bhp more than the lower compression one, but that is more likely due to better inlet and exhaust manifolding than differences in compression.

 

I would have thought that the higher output engine would have needed less advance due to higher cylinder pressures from a combination of better breathing and higher compression, but this was not the case.

 

For my next engine I intend to close up the piston to head clearance a bit more. The 10.5 CR engine had a 15 thou piston to head clearance but poking a camera down the spark plug hole has not revealed a combustion chamber imprint in the carbon on the piston crown so I will probably aim for 10 thou next time and see what happens!

Cheers

Neil

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Interesting read....

In that case it must have more to do with the change in squish characteristics.

When I pulled the rover engine apart to free a stuck piston ring (it had been standing for 8 years before I bought it), I found that it had been fitted with thick 3.9 head gaskets.

Despite it saying it was a 9.35 CR engine (from the numbers it was a factory new short engine) the compression measured 9.7 :1, but it is obvious the heads had been worked on. I tried fitting a set of original tin thin 3.5 gaskets and this lifted the measured compression to 10.5 :1.

 

No chance it was going to run on regular unleaded without doing it a mischief hence the move to super unleaded.

 

Even more confusing are my two Stag engines. Both set up on the same rolling road, both have the same spec camshafts although slightly different exhaust manifolds with the more powerful engine having larger bore secondary pipes on the manifolds.

 

The Stag engine in the TR has a 10.1 CR, the Stag engine in the Stag 10.5CR. The difference was achieved by decking the block to bring the pistons closer to the head on the higher compression engine.

Both engines started out with 28 degrees max advance which is the standard figure for a mk2 Stag engine. After the rolling road runs the higher compression engine needed 31 degrees for its best figures, yet the lower compression one was retarded to 25 degrees max advance without losing any power. The higher compression engine made 30bhp more than the lower compression one, but that is more likely due to better inlet and exhaust manifolding than differences in compression.

 

I would have thought that the higher output engine would have needed less advance due to higher cylinder pressures from a combination of better breathing and higher compression, but this was not the case.

 

For my next engine I intend to close up the piston to head clearance a bit more. The 10.5 CR engine had a 15 thou piston to head clearance but poking a camera down the spark plug hole has not revealed a combustion chamber imprint in the carbon on the piston crown so I will probably aim for 10 thou next time and see what happens!

Cheers

Neil

Neil, Yes the turbulence from swirl and squish will dictate optimum timing, and the compression tolerated by a given fuel octane. The effect of a compression ratio change (around 9 to 12) by itself doesnt make a huge difference but it is often difficult to alter compression without altering squish etc at the same time.

http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/fig5OttoEfficiencyVSCompressionRatio_web.jpg

Peter

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