COMPRESSION RATIOS

[Adam_Winstone]

This is the text that first got me thinking about corrected ratios, rather than sticking with the geometric ratio that I had been considering. Please note that I do still calculate geo rate first, as it is quick/simple and you do form an informed opinion over time (just as you have) about what geo ratio will probably suit a barrel's state of tune, so I am in no way suggesting geo is obsolete!

http://www.amrca.com/tech/tuners.pdf



Here is Eden's calculator, which can allow you to simply plug in your data:

http://lambretta-images.com/archive/porttiming.php



Regards and Season's Greetings,

Adam

[Adam_Winstone]

PS - I would normally look to run my road motors on a geo ratio of between 8 and 9:1, which reflects the state of tune that I run most of my road motors on and the experience of what works for that state of tune. I'm not surprised to see that your experience has led you to a figure in that range, suggesting that we've both found what works for us.

Unfortunately, this has proven to be completely unsuitable for use on a couple of over-ported cylinders that I've worked with/on, that have needed a much higher geo ratio to suit the long exhaust duration. This is why competition motors, with much longer exhaust durations, start to use 10:1 (+) heads, as the resultant correct ratio starts to get the balance back. All part of my learning curve.

Adam

[Adam_Winstone]

Chris,

Also, if you want to see how much this influences the end result, a quick way is to put some basic figures into Eden's calculator and then put in the sorts of figures that many people are running on today's kits, rather than just 1 or 2 mm, e.g. this thread started about the Rapido Race kit, which can have an exhaust port up to 28mm tall (same as AF quote for a TS1), whereas a standard 200 will be more like 18-20mm tall. When you start to plug a 10mm port height difference into the fugures then you will really see how the corrected comp ratio starts to change as a result of port difference.

When considering results, don't try to compare the figures to those that you expect for geo ratios. A few years back a well respected UK dealer/tuner gave me the ballpark figures of 7.5:1 for Race motors and 6.25:1 for Road/Tour motors. I've found that many of the geo ratios that I would expect to run do then translate into something close to these figures for corrected ratio.

Adam

[Chris in Margate]

That is all great reading.
Seen some pages of that before but nevertheless good techy read when my tolerant and loving wife is engrossed in a TV soap !
Eden's page looks simple to use but I don't currently know my measurement from top of exhaust to top of cylinder so can't calculate. (Some great priced burrs from China advertised there...Anybody bought any ?).
Got a 225 Rapido top end on the bench with a 24mm exhaust port so the calculator will be very useful when I come to build it....If I ever make my mind up whether or not to reed valve it.

An intersting and informative thread and I am grateful for the information.
Many Thanks

Chris

[Trojan]

WOW, Thanks again Adam,certainly a lot more to this.....the set up I ve got is Race Rapido with a ballpark setting from AF 30mm open Dell orto and using a OTT Fresco as thinking ,keep it all how AF used to have theirs and also a 8.5:1 head which seems a lot easier to use when kicking over compared to the old head of 11.5:1 and using a B7ES Plug, and 3% mix on fuel, Now admittedly this is all being built as not on road yet, so along these lines would you think its a fairly safe set up?? cheers again.......

[Adam_Winstone]

Trojan,

Go for a B8ES as this should still be fine for running in and using once run in. 7 might be fine for around town running in, 9 might be necessary for longer high speed running once run in (although unlikely if you use an 8), hence the suggestion of an 8 to cover most applications.. during and after running in.

You have not mentioned ignition timing, which is a major factor to get right... getting it wrong can be dangerous and expensive.

Jetting should not be far off if you follow AF's suggestions for their kit.

Unless you are using very expensive 2-stroke oil, which does not help running in, I'm pretty sure that you'll find AF recommend 4% mix for all running in and for the life of the vehicles, which is the same as the Lambretta recommendation for any 200cc motor or bigger.

Take care and read up on plug checks and 'choke tests' for your running in and after, REMEMBER that 500-1000 miles at 45mph actually equates to ZERO miles at 55, 60, 65 mph. I've know a number of people run a bike in without issue, then seize the first time they open the carb up beyond 3/4 throttle for their first longer/high speed run!!!!


Chris,

I'm pleased to hear that some of that may be of use to you. I was afraid that I was waffling......... again! (lol)

Regards,

Adam

[Trojan]

Thanks for the updates Adam much appreciated, sorry I forgot the timing set up. BGM STATOR 120W set at 19deg although I have read and need to question this 17 or 18 deg ?? the oil im using will be Motul 800 full synthetic what I used years ago hence thinking at 3% mix....so hopefully along the right lines....cheers again.......

[rosscla]

200cc+ I'd always go for 4% regardless of the oil. There's an argument that says you should run in on mineral oil rather than synthetic, can't remember why right now...

Ignition timing I'd err on the side of caution at around 17 static or retarding to 17, if usin variable, at usual running speed.

Don't get obsessed with sticking to / below a specific speed for running in - vary it but ever don't labour the motor. The key to running in is bedding in the rings and taking the high spots of the hatch of the hone on the barrel without glazing it, both the piston and the barrel need to hold the oil.

[eden]

Someone may have mentioned this but if not....
When working out corrected ratio remember that if your using an expansion pipe that it will "stuff" the port when it reaches the rpm that it starts to work and will in effect give you closer to uncorrected CR.
Ever wondered why you get a powerband with an expansion and not with a box pipe.... the powerband is as the pipe starts stuffing the port and in effect increases your compression.

[Trojan]

Thanks again Gents, all this info is looking good,and will help in the long term, just want to make sure I ve got it right, thanks again................