New Builds - Learners

hello eddie, heres a link to a lambretta site , it has a very useful bit to enter all your data bore stroke rod etcetc and also a compression calculator too , hope it helps , regards dan

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

A bit low Eddie for either 175 or 125....

The Head volume should be measured with the piston at TDC .... Pour in the oil thro´ the plug hole allowing the oil level to reach where the end of the spark plug would be, using a graduated beaker so that you know how much has gone in for Clearance Volume...


The formula is (piston) Swept Volume Plus Clearance (Head plus Bump Clearance) volume divided by Clearance (Head volume)...

The bump clearance ought to about 40thou (0.1cm) or slightly less (this is ancient tuning, mark you!) ... and the Bump Clearance volume will then be about 1.2.

In other words if the head volume is 30cc (including the Bump Clearance with a 125 engine the numbers will be 125 + 30 = 155cc. Now divide by 30 and you get CR = 5.17 : 1... Low!

If its a 175 it will be 175 + 30 = 205. And 205/30 = 6.83 : 1.

If you are on the limit for 175 formula -- ie at 180ccs then the the CR is (180 + 30)/30 = 7 : 1.

To be competitive -- on the best petrol -- the CR ought be over 8 : 1 and with a fuel over 101octane then 9 : 1....

With CR at 16 : 1 -- like the Walsh Bantam -- then its Methanol or one of the dope derivatives ....

If the Head Volume -- 30cc -- mentioned does not include Bump Clearance then you must include it in the calculation of CR----

NB. This is the nominal compression ratio (OK for 4-stroke engines) based on the whole length of the piston stroke.. For ported two stroke engines it is necessary to calculate the piston swept clearance from the edge of the exhaust port to TDC. Either measure the amount of stroke sacrificed to rhe exhaust port and subtract that from the cylinder capacity or -- as the Clearance Volume was measured -- With the piston crown at the exhaust port edge pour in oil and measure the amount. It does mean sealing the edge of the piston against the exhaust port edge whilst pouring the oil....

Eddie! The latter means the REAL CR is lower than the numbers calculated above..


I´ll let some of the others tell you what their engines have for Clearance Volume & CR... and somoen must have an eaier way than I have described.

Cheers Eddie!

PS -- Bump Clearance is the gap between piston crown & cylinder head. I know of some who used half that mentioned above and of courde the piston bumps the head at high tempeartures and high engine speed.

is this the new amos-pickering engine ed ? looking forward to seeing you on the Rodger Louis bike. best regards adam.

It could be why Trevor has been quiet of Late. I know an engine tuner who reckons low compression ratio, low engine friction (with a 2-stroke) and the engine revs on more to higher maximum speeds. In other words, if the gas flow dynamics (or whatever you call them?) are right at the higher speed -- plus what the exhaust pipe is doing at that higher speed -- the dynamic compression ratio will be higher than that calculated.

Come to think of it -- if the resonant pipe works properly at any relatively high speed the cramming-effect must increase the compression pressure
Over to you -- Trevor....?

Cheers!

Hello John ,
Spot on with that assumption ! To illustrate the point i have just the example , with numbers to back it up .
The disc valve Aprilia racer with a capacity of 125cc , churns out 53 bhp at the gearbox sprocket , as an experiment the exhaust system was removed , and the carburation and ignition were optimised for this new state . On the dyno the engine managed just 17.5 bhp , so the delivery ratio was almost entirely dependent on the exhaust system ! Small wonder they made 200 plus pipes , before settling on number 73 in that sequence . Makes you think !

Cheers , Trevor

Hi Guys,

Thanks Dan and John, I will look into those bits.

Adam, no this isn't that engine, I put these pics up after chatting to Ian about info on the Roger Louis Bike one evening.

Kind Regards


Eddie

The more I learn about the tuning of twostrokes the more I realise I know so very little about the ART...

What I could never understand about the Walsh Bantam was that with 16:1 compression ratio it would still rev on -- that´s according to the late Bill Lomas.

... I think there´s an awful lot in "reduction of losses due to friction..." in every moving & rotating part from the piston-ring to the rear tyre contact patch.

And speaking of tyres there are losses attributable to tyre type, form, tread and pressure.

We´ve heard a lot about wheels. What about tyres? Any published Lit... ?

CheerS!--

Hello john , my bike was in error running 16.5:1 at Lydden , it reved to 10680( the rev counter recalls the max reached) , true it did only last one lap though! But it still reved high, I've been wondering about it myself on and off

Hi Danso...!
... sound like me trying to tune my own engine when I had so many other things going on I was bound to get it wrong...
Icaus-2 revved on and an ... ate and destroyed nearly every piston it was provided. I used it mainly to experiment with Centre-Point-Steering -- or whatever it is called? -- and Handling, after attending a Ken Sprayson lecture. Lost some potential friends I am thinking!! That in effect made it seem as if I was the only one happy and willing to ride it...

My excuse -- I suppose that is what you might call it -- was that I was spending more time trying get the 250ABS right for Andy and took on far too much. Well, the ABS had 5 gears, lots of power & torqe plus a Greeves Griffon clutch and with my body-weight I COULD race with it...

It seems the lower compression-ratio is preferred when the resonant pipe is right for the job.

I made a lot of exhaust pipes and now (knowing what I have learned on here) positively feel I probably got that wrong!!

In another vein: It must be wonderful to have lots of reliable helpers -- on a race day -- where the rider´s main tasks are reduced to cleaning his teeth, finishing his toiletry and putting on his leathers -- and smile sweetly at the scrutineers.

Good luck on practice day!

Cheers!

Question for Ian,

I may be going mad but how would you drain the gearbox oil as there doesn't appear to be a drain plug?

Kind Regards


Eddie

Hi Edd
just take the egg off and lean it over on a side stand this mod would have been done to clear a low level exhaust that it would of had in the passed

Hi Robbie,

Cheers for that, makes sense now.


Eddie

very nice ed, i'm insanely jealous, see you at mallory

Nice fairing ! Nice tank ! Nice cylinder head !

i you put the fridge's in the garage you'd have more room for bikes eddie!

Ned, not gonna disagree with you there!

Dan, good thinking! I came up with a better one, dad didn't all that furniture in the other room really.........

What type of Ned tank is that on the Rodger Louis bike ? best regards luke

Hi Luke,

The honest answer is i dont know myself, hopefully Ned, Ian, or any of the guys with the knowledge may come on and explain which one it is.

Kind Regards


Eddie

Hi Eddie
Nice to see you are looking after her and keeping he warm lol.

To be honest I'm not sure who made the original tank but my guess would be Ned, when Rodger gave me the bike to ride the tank was a lot longer than it is now and was to long for me to move my weight forward. I had the mould for the tank and modified it to the shape it is now and then mick Nash made what u see and Andy Checkley painted it.

I gave the mould to jimmy so Alan has it now I think but as the original tank that I will give u practice day has a ford tractor cap on it I would say its a Ned tank

Ian

Correct assumption Ian. I made the original tank and seat to Rogers design. The original seat was similar to a TZ and can be seen in the photo Mike recently posted on face book.
Eddie I am guessing by all the photos your posting, you are just a tad chuffed with your new purchase.

Yes Ian she is being well looked after.

Thanks Ian and Ned for the information regarding the tank and bodywork.

Well the pictures are always useful for people to comment and yes I am chuffed with it.

Kind Regards


Eddie

Hi all

The tank and seat were both made by Ned a great craftsman at this.

My Bantam used the same tank,seat and frame as Rogers and I had until recently the original wooden buck made by Roger that Ned took the mould from.

I also have the panels for an alloy version of the tank made by Roger and not ever welded together.

Tried to post a pic of my Bantam but with no success.

Someone please tell me were I am going wrong. (with the pic only please )

All the best Ray Gray

Good Afternoon Ray,

How are you trying to put your picture up, are you using photobucket or something similar?

Kind Regards


Eddie