Standard Primary Compression Ratio?

[Adam_Winstone]

Hi all

I don't suppose that anyone has measured and calculated the standard 'factory' compression ratios have they?

The motors that might be of most interest would be 175 and 200, however, it would also be interesting to know of 125 or 150, if anyone has done so with these.

As with padding crank webs or fitting crankcase HPC plates, as per 60s tuning, there is a school of thought that this ratio is an important factor for efficient 'pumping' from casing into top end, particularly in the pre-expansion days.

The Honda 205 conversion was one that seemed to deliver higher performance than just cc and port timings normally delivered. With the shorter compression height piston, the primary compression ratio would have been raised slightly, increasing crankcase pressure and increasing speed of transfer process.

Anyway, this thread wasn't intended to start a debate over the pros and cons, rather, to raise the question of does anyone know what Innocenti worked to... as a datum for further consideration?

Cheers all

[Knowledge]

A good question Adam, but I think you should of stopped your post after the first sentence, as everyone is now poised to start the whole HPC/LPC debate again. You just know they are.....

(I am)

[MickYork]

Would i be right in thinking the only way to calculate it is by filling with a measured amount of oil *same as cylinder head volume) ?

[Knowledge]

I was thinking the same Mick. You would need to mount the engine vertically and fill it with some light oil, but where do you stop?

I guess the crank would have to be at BDC and the you would fill until the oil was visible in the top of the transfer port. And what about the inlet port? Would you treat piston-ported and reed valve engines differently? What about air trapped under the crown of the piston?

It actually is quite a hard thing to do, which is perhaps why Adam was asking in the hope that he wouldn’t have to do it himself.

[Adam_Winstone]

Yep, you take the engine out of the frame, remove manifold, put the piston to TDC, then fill it and measure the ml/cc. You can then calculate the ratio by comparing it to the cylinder volume at BDC.

I've now done this with mine but don't have any benchmark standard figures to compare it to. There are figures in 2-stroke tuning texts but they're likely to be a world away from the benchmark of a standard Lambretta configuration.

Yep, the HPC vs LPC debate is for another thread, this is trying to ascertain whether we have a Lambretta datum/benchmark to start from!

Ta