compression ratios

[scooble]

I know its been discussed before, but I can't ssem to find it on the search function.
But, I would like to know what ballpark Compressions Ratios are recommended for;

• Standard Super Unleaded
  50/50 Avgas Super unleaded
  100% Avgas

Many thanks

[scooble]

I believe the standard CR is about 7.4:1

[scooble]

I think I'll opt for more than 7.4:1 as a starter, but no so much that it detonates.

[KwakOn]

are you getting the head skimmed to raise compression? or the barrels.

excuse my ignorance, just curios.

[scooble]

first of all iam taking the whole engine to my tuner who will check the current port timmig. we have an idea whatit should be, so changing the barrel height may be an option, alternatiely, i may skim the head

[KR-1R]

.
.
the KR-1S SP's (C3A) used a skimmed head raising the CR to 8.0 (7.4 is normal)
Corrected CR vs Uncorrected CR
?? SP were slightly higher in exhaust port and running shaved head ??
treat 8.0 as fairly arbitrary

aim for between that, for a maximum and back down to Standard after acheiving correct squish clearances.

>>> 7.4 ....Scooble....... 8.0 <<<


Theory is one thing - but actually measuring what you have (+/- all the scientific errors of each measurement) versus what will have to get to gets fiddly when youre dealing with domed pistons, chamber volume, spark plug recess/volume...
Which is why most people just settle for achieving correct squish (simple assemble, measure, then subtract appropriate thickness of material)

?? consider also the effect of CR on engine braking on closed throttle at high revs (cushioning effect) - how many HP benefit %, would it make on Avgas etc vs limiting chance of detonation from octane and added heat.
I guess the characteristic you want to achieve is, clean burning, efficient and reliable over outright power.

TSI might have alot more input. (??he had a story about formula racing and head measurements??)

MAXIUM SQUISH VELOCITY more important than CR?
http://home.earthlink.net/~scloughn/id21.html
http://home.earthlink.net/~scloughn/id16.html


http://www.bridgestonemotorcycle.com/do ... ssion6.pdf

The effect of higher compression ratio on fuel consumption and power output was investigated for an air-cooled two-stroke motorcycle engine. The results show that actual fuel consumption can improve by 1-3% for each unit increase of compression ratio over the compression ratio range of 6.6 to 13.6. The rate of improvement is smaller however as compared to theoretical values. The discrepancies are mainly due to increased mechanical and cooling losses, shortcircuiting at low loads, and increased time losses at heavy loads. Power output also improves, but the maximum compression ratio is limited due to knock and the increase in thermal load. In addition, the investigation covered the implementation of higher compression ratio in practical engines by retarding the full-load ignition timing.

FACTORS LIMITING COMPRESSION RATIO
Higher compression ratio makes it possible to improve power output., but causes serious problems such as knock and piston thermal load increase. To solve the problems, several reports have proposed low compression ratio at high-speeds and heavy loads with variable combustion chamber
volumes.(6)(7). In this paper, retarded ignition timing is discussed as a remedy for the problems.


... leads back to the ??shortlived?? early 90's GP designs that had gas chraged floating heads/chambers ... so higher compression at low revs ... lower compression at higher engine speeds and loads.

[scooble]

thanks veru much, some good resources there.

[mj43]

Trisha

Not that I can find a copy but that spreadsheet I did a few years back will allow you to calc head volume and squish velocity. Easy enough to then calc compression ratio (not sure if it didn't do it for you anyway).

MJ

[scooble]

I am able to calculate the CR, but not sure what to set it to as something to to aim for though.

[TwoStroke Institute]

I dunno what your octane rating of the unleaded is or if you will be using the 10,500 rpm peak. Running this head volumes of 8.5-9cc would be possible.Not sure if anyone has run this low before??
Our 98RON fuel is good for 13.5:1 UCCR(10cc) for an 11,500-13800 rpm peak.Over 12500 they brickwall and loose over rev with small head volumes. Which is why I say you might be able to get away with more not reving so hard.

[mj43]

Looks like the trapped CR is 8.6:1

Volume of head at TDC is 8.32cc trapped volume is 63.05cc (this doesn't include the head volume)

[KwakOn]

my curiosity has been satiated with ample sufficiency.

thanks.