Originally Posted by Pyndon
Hey John, great to see some pictures of the progress, engineering from home at it's best, I love to see this!
A few questions I would have coming from a background of tuning supermono, single cylinder road racing engines would be: (and I'm sure you have considered them all but it'd be nice to know the thought process you have been through on them)
1) What are your thoughts on flame propagation and cylinder efficiency with such a complicated piston top? Obviously a flat top piston and well shaped combustion chamber helps with turbulence and combustion rate, but here you certainly don't have that. What did cosworth have to say about this, I mean they have the experience in this field so I'd be interested to hear what their thoughts were?
2) Reducing the valve shrouding on a piston helps to get gasses in and out faster which is particularly important when you have an over square engine, obviously you are fully shrouded during cross over points so you must have your cam design pretty mature? Interested to see your cam profiling and overlaps chosen for the 450 project and will this be different for the different options of engine?
3) Finally, how did you avoid piston distortion during welding and do you intend to heat treat the piston again before using it? Appreciate it's only for demonstration and measurement purposes but still, thought I'd ask.
4) Valve and port sizing is pretty important too, do you have a plan to reduce the port volumes or valve sizes in the future to try and get the gas speed up a little?
5) Balancing? Extra piston weight really does screw with the balancing, will the Cosworth piston weight the same as stock or are you going to get everything balanced?
My intention was not to overload you with questions, just genuinely interested as I'm sure the other engineers amongst us will be.
On the down-drafted CRF450 motors that I used to build, even with a raised piston, we still had to achieve a relatively flat top and put a load of work into the combustion chamber to make this possible. Granted, we were always looking for around 70rwhp from a 450 and I know that's not the goal here.
I for one am really interested to see this project unfold and succeed, probably less for the end result for me personally (if I am honest) but more for the journey that that the guys are going through. The shear determination, lessons learnt along the way and the fun and excitement of a project such as this will make it all worthwhile. I also hope that I get the opportunity to ride the bike one day as I think it'll be a unique little fella. Good luck with it all.
Bloody hell Lyndon,it's a good job I'm on holiday this week so I have time to answer all those questions!!!
In response,here is the best I can explain .........
1.Cosworth understood that our brief was to try and keep the cylinder head standard,so that we can hopefully supply a customer 450 kit,which will consist of a short-stroke crank,and new piston,therefore making the kit as simple(and cheap) as possible.To this end,we realise we have compromised the gas flow,as they had to design such a high intruder into the combustion chamber,in order to get the compression back to standard,due to the fact we are only now sweeping 450cc instead of 654cc into the same head.This wont become apparent until we build and run the motor,but we will have to cross that bridge when we come to it.
2.Initially we will try the std cam & 690 rally cam,but have offers of cam design help from outside if we need it.
3.We annealed the piston after welding and will stress-relieve it after machining,it did distort slightly,as the gudgeon pin was very tight going in,so we reamed it back to 20mm diameter.It is something we will have to live with for the prototype engine.
4.Cosworth engineers did suggest the reduction in valve & port size to get more efficiency,but again, we want to keep the head standard, for simplicity.
5.The new production 450ccCosworth piston will be slightly heavier than the OEM version.The other 2 versions(stock comp & hi-comp 690cc) will be OEM weight.
Like you say,we are on a journey,not quite sure of the result,but we are definitely going to give it our best shot at making a "clubman" Dakar-spec 450 Rally bike, that is affordable and uber-reliable.We realised that from Day 1 it would never have 450RR power,but that is what we are prepared to swap for reliability and expense.
It will be great to get it on the road and trail ride it soon,come down and try it out.