what does compression ratio mean?

I was under the impression that since the stated compression ratio for the
bantam was 10:1 then I should be able to get 10 bar compression measured on
a gauge. Is that right?
I get a good 7 bar that seems quite solid. It is a cheapo colortune gauge
but it shouldn't be hugely out I don't think. Could low compression be the
reason it is difficult to start? Good spark, carburettor seems ok but still
very temperamental.

Thanks

 

Compression ratios on 2-strokes are notoriously hard to measure, and I
think I remember reading something to the effect that you'll never get a
high reading anyway because of ports, something, I dunno.

Same article said that a measured 7-8.5psi was about right, so yours
sounds OK on that basis. FWIW 10:1 on a Bantam sounds unfeasibly high to
me.

Remember, anyway, that when checking CR you kick several times (or spin
the starter) and do so with the throttle held wide open.

 

I would expect the peak pressure developed to be lower for a 2-stroke vs.
a 4-stroke, with the same ratio of swept volume to combustion chamber
volume.

 

The Older Gentleman said...

Compression and compression ratios are very different things.

Compression is measured in Psi, or bar. Compression ratios are
determined by the swept volume of the cylinder being worked as a ratio
against the volume of the combustion chamber with piston at TDC.

By their nature, 2-strokes do have considerably lower CR than 4-strokes.
If you subjected any stroker to a CR even approaching 12:1 it would be
self detonating all over the place and would melt the pistons in next to
no time.

You're right about 2-stroke compression being hard to measure, porting
tends to screw things up by leaving bloody great holes in the sides of
the cylinder wall for the compressed gases to escape through.

 

Listen to this, kiddies, because this is a man who owned RDs.....

 

The Older Gentleman said...

*owned* being the operative word

 

<curious>
Isn't the ratio measured from the point at which the ports are closed?
</curious>

 

Geoff Berrow said...

Not the way it was explained to me, but I may well be wrong, it has been
known before.

 

out of interest, I recently did a compression test on the (now-removed) V8
in the Lard Rover. half-way through, I remembered the thing about open
throttle, so tried the next pot with throttle open and with throttle closed.
made about 0.1 bar difference in about 8 bar.

 

It's a Bantam. They all do that Sir. Then they catch fire. JDAMHIK.

Buzby

 

Oi!

 

The compression ratio is a mathematical formula, stroke plus combustion
chamber volume over combestion chamber volume . So for a 500 with 100 cc
combustion chamber you would get 500+100/ 100 = 6:1
In the real world of valve overlaps and fucking great holes in the barrel(
like ports) the results at hand cranking speeds are never going to be
anything like the theoretical levels.
If your bantam wont start and its coil ignition , I would say crankase oil
seals they love to leak air in and weaken the mixture.
Steve the grease

 

Yep a ratio of the gas volume in the cylinder at BDC to the volume at TDC,
but in banties, I think the BDC volume in that equation is taken from the
point on the stroke where the transit port closes.
10:1 sounds a bit high for a bantie though.
I liked your guess at crankcase seals.
A lot of us old neat-petrol sniffing 4 strokers forget the secondary
compression in the crankcase on these 2 strokes.
Another give away is it'll drink fuel and lack power when he gets it going
if it's seals gone.

Rick

 

coo. ello Wayne. didn't know you read this as well.

 

Later D14/B175 bantams are 10-1 compression ratio,according to the Official
BSA manual.
They should also all start within 3 kicks! If everything is set-up and
working correctly.
Ive got a D3 with Wipac flywheel magneto that always starts 3 kick.
Try changing the Condenser to a Lucas DCB700 if its still using the original
Wipac thing.

 

said:

Not the same Wayne as collected those hood sticks from all those years
ago? I still remember you owe me a pint!