Hi

Just wondering.
What is the typical compression ratio of 2 stroke engines?

Saw that ZDZ80RV has CR of 10:1 (quite high).
How about Fuji engines (e.g 64cc) or Zenoah engines (e.g. 63cc)?
Are they as high?

How about 3W?

Thanks!

Years ago I was inspecting G62 engines after a race..Most showed about 110 lbs on the compression gauge when spun by an electric starter..One showed 150...when I got home I tested mine, got 110, put some 30 wt oil in the spark plug hole, got 150..depends on how well the rings are seated in the bore......
A Q50 with a thick base gasket will lose 500 or more rpm compared to a very thin base gasket, one that almost allows the piston to touch the top of the cylinder....

the compression ratio - - not the compression in psi.
The ratios you see are calculations on total swept area.
Really of little value .
Why?
the porting/ and internal flow of the engine can really skew the density of the compressed charge.
for example - IF the exhaust port is raised from say----150 to 175-- the compression calculated (from top of exhaust port to top of stroke - is REDUCED.
However the density of the charge -with a tuned pipe - can increase- on the raised timing setup.
Then-- the pressure in the cylinder INCREASES.
Confusing?
Don't let the adv compression ratio influence how you about engine performance
some low compression setups run very hard.
Simply because they breath very well.

PSI was for reference only, don't know the actual number..A G26 has an 8.8 to 1 compression ratio according to the owner's manual, I would expect the other Zenoah engines to be similar...

I have a compression guage for model engines - as you said - the compression will vary all over the place with addition of oil.
What is strange is that my engines' peformance seems totally unrelated to the "compression test."
Remember Dykes ring setups ?
same thing.

Yup, K&B .40s, for one...Whatever happened to that ? How about Total Seal gapess rings on 4 cycles ? You think they might work on a 2 stroke ? Maybe with a custom made piston ? No compression leak at all.....

A bit delicate for the ham fisted "mechanics".

the Dykes had to expand and contract to seal .
I fixed some pretty screwed up K&B engines.

I just saw an old NIP K&B .40 dykes ring around here somewhere...The attic has way too much stuff in it

Thanks! I've been meddling with my car engine actually. Found that my CR is 11.1:1 ...... some Alfa Romeo JTS engine.
Which I thought was pretty high ...

Then I realised that a humble ZDZ had 10:1! For a 2 stroke!
So I started looking out for other 2 strokers just to confirm the CR.

Thanks for all the replies.

Wow, alfa, 11.1 to 1 is a lot for a car engine. Do you have to run aviation gasoline in that thing to avoid detonation?
Charlie

Well, 11.1:1 is ok nowadays.
Certain Honda VTEC screamers runs at 11+:1 too.
Our premium gas is 98 octane. So it runs just fine (for my Alfa).

The new BMW M5 (V10, 5 litres, 507hp) runs at 12:1!!
It is right at the edge. But they have very advance knock sensors.

Even the lowly old Chev small block can be setup to run that compression-- and work on 90 octane fuel.
Alot has been learned about rod lengths / chambers/ etc., which affect preignition .
with injectors and variable valve/intake runners - all kinds of previously impossible stuff can be done.
The ZDZ 10-1 compression rating is misleading .
If one is really keen on finding actual measured compression ratios - -- the old " measure the volumes with oil" is still the best bet .
the calculated ratios (like most calculations) simply tell you your pencil is making a mark on the paper.
The actual compressed charge changes in density as the engine goes from idle to full tilt .

Just read that 3W-75i TS QS needs 98 Octane or better!
Maybe compression ratio 11:1?


Anyway, does anyone knows how does the 3W-75i TS QS compares with the ZDZ 80RV?
[8D]