Highly modded engine. Where can I find more revs ? Detailed post.
G'day there all who are reading this.
I'm normally a bit of a lurker but now comes the time I need to ask for advice.
I've been working hard on making a weapon engine and so far have achived very good results but I just can't get the revs I am looking for. I want to get away from the doughy horrible nature of a CAG engine and get my baby screaming. It's mostly for the ability to achive a wider powerband so I can run shorter gearing, Just the scope for over rev past peak power would be awesome but here we go anyway.
What i've got so far is an engine built from scratch starting at the crankcase
33mm stroke full circle crankshaft 12mm pin
3D Ported 15:1 compression head kit (I believe this is a copy of the ADA kit with the single massive boost port)
NGK CR8HSA Spark Plug
Original reed plate with center bar cut out and a Polini sized Zocchi Carbon Fiber Reed
Dellorto 19mm Pumper Carburettor (this one is rare as. I can't find ANY info about it)
Fatboy Pipe (I have access to the Jetpro replica S-bend pipe, a few stock pipes and a modified stocker which has had the header cut by 3" and the belly lengthend by about 4.)
Crankcase has had some minor grinding done to help smoothen the air flow into transfer ports and the bottom of the cylinder has been cut at an angle to allow smooth transistion from the crankcase up the transfer ports.
The engine is a 4 port design. 2 Transfer Ports with a single large boost port cut at 90 degree as is the transfers.
Standard timings are as follows.
Exhaust opens at 112ATDC
Transfer Opens at 128ATDC
Total exhaust duration 136deg
Total transfer duration 104deg
Blowdown 32deg
As you would expect the engine had some good low down grunt but felt like a typical yard machine. Peak power down low and peak revs were rather low.
Since the first trial runs and dissapointment I have begun to raise the cylinder by making spacers out of aluminium in the shape of a base gasket. Currently i've got around 3mm extra on the base to raise the cylinder. The floor of the exhaust port has been cut to match the piston crown at BDC and the exhaust port widened about 1.5mm on each side. The exhaust port retains an oval like shape to remain nice to the piston rings. I have shaved the cylinder deck to bring the squish back into check as well. Squish is somewhere around 0.8mm at the moment.
With these modifcations engine timing is now around the following on last inspection. I say around because I haven't retimed the engine after raising the exhaust port height very slightly in effor to increase blowdown time.
Exhaust opens at 84ATDC
Transfer Opens at 65ATDC
Total exhaust duration 176deg
Total transfer duration 130deg
Blowdown 36deg
Now it appears I have hit a wall. Although the bike still pulls fairly strongly from down low it obviously no longer has the low end torque of before. I understand this compromise. But for such a radical increase in duration I would have thought I would get more top revs. The powerband is lovely now though. 7/76 gearing accelerates ballisticly and reaches a top speed of 49km/h in no time.
Am I unrealistic to expect more or is there anything else I can do ? I'm open to any ideas
Just thought i'd add i've currently got the spark plug gapped down to .021" from .026" to check to see if the spark was being blown out. No improvement.
The fuel I am currently running is Australian BP Ultimate 98RON with 25:1 Maxima Castor 927 and blended with 30% Toluene to bring octane up to 105 to suit the high compression.
The engine is running a standard timing key.
And finally here is a video link of me giving the bike a rev on the stand so you can hear how it's sounding.
http://crashdummy.ugbox.net/files/cagreving.wmv
Thanks for taking the time to read this.
Linear Mode
This is a new Minibike videos website which was launched - 
