Really 10:1 CR??
 

Hi Folks,
Had my 8 for a couple of months now and still soaking up lots of stuff from the club here.
With 54,000km on my R3 and warranty I did not have the compression tested, but have been reading about the test and the acceptable results. I have 2 questions that I could not find the answers to..
1. Sensitivity to cranking speed, would I be correct to ass-u-me that this is due to the rate of leakage passing by the various seals, especially as the apex seals pass over the spark plugs?
2. I believe the "chart" only goes up to 140psi, this would be a pretty low number for a piston engine with a 10:1 CR, but then you would crank until reaching a max'm reading, which the rotary tester does not allow (I believe), so what would a conventional tester go up to if allowed to reach a peak reading from sequential chambers?
I use premium gas in my Suzuki, our old Audi A8 because of the high CR and my small 2 strokes, chainsaw, leaf blower to avoid the ethanol, so no trouble putting it in my 8, just wondering if it is really required...
The car has winter wheels on it but is not seeing much use yet, but it seems to enjoy having a work out; I have sensed a difference in how it pulls at that lovely 7200rpm point, always strong, but some times more of a surge??
Too many questions?
Regards
Trevor T

yes

Premium is required.
I pay .30¢/gal extra for E091.

I'm not too knowledgeable about the details of CR.

1) yes. My understanding is that these engines have a very long compression and decompression duration compared to piston engines so at slow speed enough pressure leaks out before you get to top dead center that the effective CR is lower. Especially when the engine is hot and clearances are wider and especially if a seal is compromised.

Keep in mind that 250rpm at the crank is 83 rpm of each rotor. A piston in a regular motor at 250 rpm is moving at 250 rpm ans covering less distance, so there is less leakage loss.

2)i'm not sure the question makes sense. The pressure the rotary or regular tester registers is the pressure at the spark plug at tdc. I'm sure it's higher at idle speed than at the starter's 250rpm, but you'd have to measure a running engine to know that, which would make your tester part of the combustion chamber, which would be bad.

Thank you for the replies... my understanding is that a conventional comp'n tester has a check valve in it, you test each cylinder until a peak reading is reached, the volume in the hose would not be that significant to the final number.. my memory thinks that a motor with a 10:1 CR would be up around 180psi... the rotary tester cannot do this as it has to read each face of the rotor sequentially.. but if tested with an old school gauge I would expect a "good" rotor/chamber to peak at a similar number.. my early engine experience was with ported 2 strokes, a calculated CR using bore / stroke dimensions might give a high CR number, but it was not "real" because at the bottom of the stroke the exhaust and transfer ports are still open, guess that's what I was wondering about our 10:1 ratio... is it calculated with the volume after the intake ports are passed?
I have always been fascinated by the RX rotaries, but this is my first, after close to 60yrs around piston engines, so please be patient with me.. lots to learn.
Cheers
Trevor T

I guess what I meant is that at the test rpm (250), the engine is not running effective 10:1 CR due to leakage across the various seal edges. At operating rpm, the leakage is reduced, building up to 10:1 pressure ratio. This concept is foreign to piston engines so there is no point in comparing.

The total seal edge length for a rotary chamber is much longer than piston ring length for a piston of same volume, although most of the leakage is above and below the apex seal.

There is a military study of rotary engines for UAVs floating around somewhere that explains all this very accessibly.

Thank you Loki for pointing me in that direction, found some interesting reading with mathematical stuff way over my head, but it confirmed the issue and the compromises of the design.
Get the point about time for compression, happens in 1/2 rev in a 4S piston engine, so 1/8 sec at 240rpm; approx 1/3 rev of a rotor at 80rpm, so 1/4 sec, twice as long.. and only single apex seals v typically 2 comp'n rings...
Which also explains why a low compression, hard to start motor can still perform well at high rpm when running.
Regards
Trevor T