griffon c/ratio`s

[dave higgins]

I would like to ask the panel ( druid ), what was the c/ratio of the 380 twinports at the standard setting, regards. dave.

[Brian Thompson]

380 Engine Data.

Hi mate,

Haven't got much on Griffons but I can offer you this (from the original sales brochure) and quotes the 380 CR @ 10.7:1.

You can find the rest of the brochure here; http://www.kivas.com/greeves/literat...n/griffon.html

Hope it helps, and that the mighty druid will take pity on my pathetic warblings and guide us gently towards true enlightenment once again.

Especially if it's wrong.

Brian.

[druid]

Griffon CR

Dave,
10.7:1 is probably right, but don't forget this was in the days of good old 5 star. Unless you're running race fuel (Sunoco or similar) I wouldn't go above about 9.5 it also makes for a more forgiving engine.

Druid

[dave higgins]

Thanks chaps

[Colin Sparrow]

My book researches came up with 10.7:1.

[John Wakefield]

Compression ratios

One thing to remember with two strokes is that the actual comp ratio is lower than as quoted in engine spec due to the fact that the compression does not start until the piston has risen up the bore to close off the exhaust port. So a 10.7:1 ratio may only effectively be 9:1
Here is a link to a compression ratio calculator http://www.torqsoft.net/compression-ratio.html

[Kim275]

John Good point on the difference between swept and trapped compression ratio's. And for a 2-stroke Trapped is what really counts....but assume Druid was referring to Swept as that is what manufacturers normally quote?

My QUB is set at 12.7 Swept and 7.6 trapped. As Druid is referring to Swept as the specification and I assume you want to check whether your CR is different to standard spec suggest you do any calcs based on Swept.

If you measure and you find your CR is lower than 10.7 - as Druid points out I would not worry. If higher you may want to adjust or increase fuel octane.


This is my method of checking.......if you want to stretch your brain rather than using the on-line calculators
First calculate your required net head volume for your desired CR of say 10.7
SO assuming standard bore of 82mm and swept vol of 380cc - your head volume required is HV = SV/(CR-1) = 380/( 10.7-1) = 380/9.7 = 39.2cc.

This volume is the Volume of your head (measured to the edge of squish band intersection of the deck edge) LESS the Piston dome volume PLUS the squish clearance volume (1mm of squish clearance is 5.2cc for an 82mm bore). and for a standard Griffon Piston dome height is 7mm approx 20cc


SO you are looking for a gross volume of 54cc to achieve a CR of 10.7:1 with standard piston and a squish clearance of 1mm.

If your Gross volume is different then reverse the formula's to work out what CR you have.

HV = measured volume less dome plus squsich clearance vol

CR = (HV+ 380)/HV

SIMPLES


Worth checking as folks may well have played around with your head etc.
If you need some more input give me a bell - or drop me an IM

[dave higgins]

Well read kim, but i have not actually got a problem..., i was merely asking druid what the standard factory setting was, because someone asked me & i have never calculated it.
But thanks anyway...

[Kim275]

Dave ,
Most welcome !! - Just couldn't resist a bit of maths on a Friday evening...now back to the Thai curry and beer
Take care Cheers Kim

[Brian Catt]

I've been waiting for a certain 4 stroke engine owner to point out that his engine's comp ratio is also dependant on closing the exhaust hole!