Combustion chamber mods

**jeffro ra28** · 03-09-2007, 09:15 PM

Perhaps i should have said "genearal production EFI".

No shit you can & probably would change injector timing if it was desired and had the ability to do so.

**rms** · 03-09-2007, 09:20 PM

True, but the idle is suprisingly good.
That is why we are useing piezo crystal injectors, to keep some control at low opening times (claimed down to .8 msec as against 1.2 msec for conventional). They will also operate at 100 bar pressure, so we could use a smaller injector.... but we can't afford a fuel pump that delivers this pressure.

I use the same stategy for road vehicles. Idle injection period is around 10 crank degrees centred around 110 deg ATDC, I try to keep injection within induction period as long as possible with the smaller injectors, moving injection end point towards IVC until injection start is before IVO.

But back to topic
You really need to improve the average flow from the lift at TDC overlap to max lift.
More times than not, mods that improve flow at low lifts are detrimental to flow at higher lifts.The combustion chamber is a part of the air flow system, particularly at higher lifts.

**jeffro ra28** · 03-09-2007, 09:33 PM

It will all depend on how you improve the low lift flow. IF you make the gains at low lift by cutting seat angles and seat approach angles. This can and most of the time will, improve both high and low lift. However it gets complicated at this stage because, WET flow will be most optmized with sharp angles whereas dry flow will see a descrease with the sharp edges, but be improved with smooth radiused angles.

**Ben Wilson** · 03-09-2007, 11:31 PM

Originally Posted by rms

You really need to improve the average flow from the lift at TDC overlap to max lift.

That's exactly what I thought (even similar words) when I asked about it here (post #36)...

I've learned to trust his answers on things like this......

**jeffro ra28** · 04-09-2007, 07:36 AM

There is no doubt that this is obviously a good way to improve the port.
My originall focus was to explain how to increase flow by modifying the chamber.

**Ben Wilson** · 04-09-2007, 08:27 AM

Yeah, and we did get a little off topic there for a while....

It looks like one of those things where there is no definitive answer. Pick a Jap bike and do a google image search for the chamber and you'll find as many different ways of doing it as there are bikes. Some have a very 4AG style chamber with large squish areas, others have most of the squish area machined away in the hunt for a bit more flow. I'm sure the correct answer lies somewhere in between the two.

Personally, I've found that it's possible to deshroud standard valves on a 4AG without carving much out out the squish area, with bigger valves that might get harder...