Dear colleagues of passions, I write from Italy Bologna land of Ducati engines, Lambo etc, enthusiast of engines like you. I'm playing with a Honda gx420 clone, put it in order to run without breaking. Connecting rod, piston, nitrided and lapped crankshaft, rocker arms, and titanium valves, self-built rocker arms, cdi electronic ignition with advance curve variation, start 12 ° before pms 3000 rpm, 38 °
before pms., derivation pitbike, strong intensity spark 40kv at 12000rpm.
I looked at many forms of internal combustion chamber, on the internet, none of them seemed suitable, many courageous experiments .....
I read that talking about 50 hp with 390cc? 128 hp per liter, is a power of all respect for an elementary engine like this, reachable if you do everything very well, if done as I have seen on the internet, you will never have these powers.
In my opinion, the limitations of not having a shift prevent you from having a very powerful engine in the entire range of use. However these are variations on the theme.
The theme is the head, I have not seen one that brings together the specific conditions for use, the shapes of the roof of the head are imaginative but with complexity that affect the fluid performance.
Here there is a beautiful and functional roof.
I would like your comment on this topic.
The engines are like us! if you breathe well, run more!
50 hp out of 390 cc is going to be pretty hard.
Sounds like you have most everything in hand.
Excessive revs will require billet rod and performance camshaft. .
The roof of the combustion chamber extensive work to change for the gain .
Better to design a new one.
do I read posts where people happily talk about stratospheric compression ratios, 20.1, 16..1 etc, but use gasoline as fuel? Even if he had 130 octane he would detonate! engine broken after 3 seconds! With a very good internal combustion chamber, you get to work well with 12.5 compressions as long as the engine cooling is super efficient, that there are no hot spots in the combustion chamber and having a sophisticated CDI with programmable advance curves.
If an engine runs with 16.1 of compression, it means that the volumetric efficiency is absurdly low or uses methyl alcohol and other.
I have engines for the Moto3 World Championship, 250cc 55HP at 12500rpm. 4 stroke 4 valves with a wonderful head! With this stuff you get 220 horses per liter, but the numbers are other 12.5. 1 compression and high octane fuel.
I'm really surprised by certain numbers that I read ......
You would make a very good neighbor Fun and Smart.
Be aware that some of these numbers are possibly inflated.
Yes methanol Fuel is common here . For racing .
High octane gasoline is available as well though only 87-92 octane at the gasoline station .
Unfortunately the picture you posted did not show as intended .
I am curious as to what you feel would be a good combustion chamber shape .
The hemispherical is considered to be best by most .
Dear friend, thank you for your opinions, the hemispherical chamber has a good volumetric efficiency, but it does not offer an ideal shape at the speed of the flame front, I am talking about the standard hemi motors used and loved by you.
It is clear that if the shape of the hemispheric chamber is optimized, it will be very good, it will be necessary to reduce it to the maximum, then create around the valves of the compartments with constant radius to obtain the greatest area of mixture passage throughout the valve lifting range, this conformation will gives the possibility to reduce the space of the flame and increases the possibility of creating optimized squish areas.
The fact that disproportionate ignition advances should be used speaks clearly about the thermodynamic deficiency of the roof of the head.
One of my old Ducati bikes, formerly Mike Haylwood! It had a small hemisphere roof, with 860cc, 40 years ago, it had 100HP at 8200rpm
with a juicy torque, with this bike I won 4 SBK South American championships, but you arrived! with the bikes of Edy Lawson Honda Kawa Suzu with a displacement of more than 1200cc and these gave us the dust! But our frame was better, we weighed 35 kg less and consumed less gass, but it wasn't enough.
I made multi-spherical heads as explained before, two well-arranged spark plugs, USA connecting rods Carrillo forged mahle pistons! the ducati turned gleefully at 9600RPM and had 960cc and had 142 runaway horses! Then modified the "modern" Brembo brakes trail 50 years ago ....... I won with two of my riders 2 years more. Other! the advance had gone from 42 ° to 32 ° !! Best squish best gas chamber turbulence, best air fuel mixing, ergo more power!
I hope you enjoy this experience that I tell you with pleasure,
I'd like to stay there with you! it would be great fun !!
Sweet , those were some great racing days. now with that info I have several things in my head. the Rickman 4 valve head for Triumph . Was I believe a Pent roof design . Always considered the 4 valves being the improvement when it actually may have been the chamber shape . Interesting that you incorporated squish band into the hemi shape for such an improvement .
You know an important fact to keep in mind, are the minimum thicknesses of the harmful space inside the combustion chamber, below 4 / 4.5mm the mixture does not burn! if you do not have a good turbulence it is useless space that serves to generate with counter flame that induces detonation, it is very useful to have set the whole mixture on fire with a microsecond timing (very difficult for ordinary mortals)
However, taking this into account if you play with the advance ramps you can do it with a lot of effort, but it's worth it in terms of power and reliability.
In the past, playing with a 50cc brand, Garelli, ex Angel Nieto big world champion of small displacements, I discovered fantastic phenomena related to turbulence related to the ignition advance. this record engine, methyl alcohol fuel, with NOS injection and extra alcohol, delivered 27 HP to the wheel! 16000 rpm, the interesting part and this, up to 12000prm, advance ignition 35 degrees in ramp, from 12000 to 16000/17000 rpm retarded in ramp up to 23 degrees of advance and at maximum power we had to delay further 10 / 12 degrees !! penalty, violent seizure and / or big hole in piston. compression ratio 16 to 1, considered by the PMI.
This was a billet engine with malhe nikasil cylinder,
winner of two world championships.
max admissible water temperature 48/50 degrees centigrade.
The cdi was built by a German genius! krober brand! it did miracles, even they were 600 hp liter, no turbo or volumetric compressor, in the 2 troke it serves no purpose.
We had to study the shape of the head and squish very well!
this happened 45 years ago. We weren't stupid ...
Now I try to have fun with these big indecent pumps, jejejeje
A good friend of mine worked in the combustion chamber.
They had the camber design and were fitting the piston too it.
after a race or run they would inspect it.
anyplace that was clean with no carbon he would machine that spot down until they had a complete burn over the entire piston and chamber.
I suspect there may have been an easier way to do it , still the results were a national record in pro stock gas drag racing .
With as you say nothing more then a two cylinder pump engine .
Get you one of those 460cc Vegas kart engines. Then top it off with at least 160cfm going inside of it. Meaning at least a 38mm carb, and big enough cam that will allow that much flow inside of the engine.