Hi Tim I am starting from scratch when my cases appear . So my dilemma is do I have to go 90 bore to have a squish effect or is it not possible with the std head to obtain this without welding / fettling . On my Venom I used a Viper head with a landed piston , two plugs , comp 8 to 1 ignition 32 not 38 and the engine works well . I am hoping to do something similar on my Norvin project , advice much appreciated
E: Engine Squish
- Thread starter vibrac
- Start date
All the squish I have is the difference between the standard bore and 90 bore which at best is a circular 3mm protrusion of the head 'flat' ((90-84)/2) however I also went for a lowish 8.5 compression and when I was measuring the volume with my trusty play dough (around 75cc I think I remember) I checked the small land clearance and was about 35 thou A bit tight but it seems oK for 18 months (including a columbres rally)
I was going try twin plugs as I have all the BTH gear for that but as I was short of time and the head needed work I had a reconditioned shadow head so I used that instead Also since I dont use the standard knitting needles for a decompressor I was also unable to use a RE decompressor however amazingly driven by a small lithium battery in a standard dummy box the Comet has no trouble starting. Recently after a three month sitting in the garage (too much work on a certain racer) and without draining sump, renewing petrol or charging battery, I rotated the engine twice on kickstart tickled carb and pressed button and she started.
I was going try twin plugs as I have all the BTH gear for that but as I was short of time and the head needed work I had a reconditioned shadow head so I used that instead Also since I dont use the standard knitting needles for a decompressor I was also unable to use a RE decompressor however amazingly driven by a small lithium battery in a standard dummy box the Comet has no trouble starting. Recently after a three month sitting in the garage (too much work on a certain racer) and without draining sump, renewing petrol or charging battery, I rotated the engine twice on kickstart tickled carb and pressed button and she started.
I think that if your engine is only 8:1 you won't need any squish. My Rapide is about 8:1 (I have never measured it, but I have 8:1 pistons) and it has the top of the barrel chamfered as many people do, mainly so that the intake valve is not shrouded by the edge of the barrel. The top of the chamfer is about the same diameter as the recess in the head. There really can't be any squish at all but my bike runs really well. (It is fast too) My bike never seems to have any pinging or pre ignition, even when you try to force it by lugging it etc. Somehow the turbulence in the huge open combustion chamber must be enough. I have two front heads, I wonder if that really does improve the swirl as some people think. I have used different ignition settings 38 and 34 degrees, it doesn't seem to make much difference, my engine won't ping. I think that if it isn't pinging then it doesn't need more squish. With the standard bore, because the bottom of the hemisphere is more than 84mm there really is no way to get any squish without a lot of welding. It seems wrong that the head is bigger than the bore, but somehow it works.
My 8 to 1 standard head Rapide also does not ping.
The 9 to 1 bike which I sold never had that problem either.
Both of those bikes are points/coil ignition but for some extra safety you could add a modern EI with the two advance curves controlled by vacuum.
That way the advance amount drops down if the rider gets lazy/forgetful and lugs the engine.
Glen
The 9 to 1 bike which I sold never had that problem either.
Both of those bikes are points/coil ignition but for some extra safety you could add a modern EI with the two advance curves controlled by vacuum.
That way the advance amount drops down if the rider gets lazy/forgetful and lugs the engine.
Glen
Its all a bit of a fantasy in a way.......the thing to remember with a squish band like on the TP top ends is........look at the possition of the band, verses the overall height of the chamber........It's not really ideal.......A lot of these early engines with big hemi heads tend to have a squish area atop the piston crown on either side.......Look at some Manx type pistons to see what I mean........On our race engines we are tapering the squish on an angle to direct the flow of gases more towards the chamber roof than just straight in and flat....... I tend to feel that a flat very narrow squish area could actually cause detonation due to the "Dieseling effect" that this tiny area might cause........All experimental of course.
Hi, My racer is fitted with the 84 mm bore Terry Prince heads machined to fit my 90 bore Omega forged pistons. These pistons had an unmachined flat top and on the advice of an expert I machined them to a 20 degree shallow cone leaving the top of the piston flat. The Bathtub Heads were then machined to a similar cone which gives a huge squish area up into the combustion chamber. There are of course many other ways but in practice this has worked well since 2001 when competing in many races and is still being used.
Sorry I did not take any photos but at the time I did not know if it would be a success. Initially it was slower than the old setup and I had to retard the ignition and reduce the main jet size substantially to get the best power.
Sorry I did not take any photos but at the time I did not know if it would be a success. Initially it was slower than the old setup and I had to retard the ignition and reduce the main jet size substantially to get the best power.
Thanks for your reply
Hi Nigel You may have just saved me a lot of work Roger
Hi What was the CR of the engine . Roger