Chamber filling

[Tooleeda]

Read a few posts back about filling in the combustion chambers with weld to increase compression. Ive seen a couple of heads done like this and was wondering how this affects gas flow around the valves? Has anyone done any research into this? Im just curious because changing the shape of the chamber will greatly affect things like cam and ignition timing. The maximum advance of the distributor would need to be reduced to avoid pinging (not as simple as retarding timing because you'd need increased advance at idle) which means changing the mech advance.
I would have thought that getting raised piston crowns would be the way to go - same gas flow plus compression increase. And also I would think the chamber would run alot hotter as the head thickness to the water galleries is greatly increased (By as much as 2cm on the head I saw)
Anyone care to comment?

[Slug]

Very simply, it definatly does change gas flow around the valves, etc but its not just a case of welding anywhere you like it must be correctly shaped, otherwise more harm then good will be on the menu. As for putting raised pistons in instead this also has it problems of gas flow and more importantly increased flame front propagation. remember that a gemini has a rather poor combustion chamber shape to begin with. As for dizzy changes, if you want to get picky then any change to the engine will have an affect on the advance curve cam, fuel, shaved head, carbs, etc.... Michael

[Benjamin]

From what perspective you say Gemini have a poor combustion chamber shape? I am interested to know.

From what I have figured (I am no engine expert for starters they are a hemispherical shape (to a degree) and they are crossflow, which I thought were good things.

My guess is they don't really have much in the way of swirl characteristics or squish? What kinds of simple things can you do to the combustion chamber to help matters?

[Slug]

The gemini head is whats called a semi hemi design, which is very good when done correctly (Jaguar used it). but you have to remember that these engines are designed for one thing, pollution. So the important things, as you said like squish ( which a gemini has none) is not there, swirl has been induced through a curved inlet port not through correct head shape or valve placement. So potental power with a gemini head is limited. So to improve a few of these things I belive that welding and changing the shape of the combusion chamber is a step in the right direction... please dont take what I have said in the wrong way, when you compare heads of the 70's era then a gemini head dosnt look that bad, but in the big picture its still crap.... Michael

[Ben Wight]

Filling the chamber with weld to increase comp ratio will not require a drastic timing change, if any change at all. It is very likely that if compression is raised to a point where detonation starts, then sure, reatrded total ignition timing will stop the detonation, but the timing that the deonation occurs at is optimum for the engine. It is not too much timing that is causing the detonation, is is the octane rating of the fuel, basically, it is too low. You need to use a better fuel for higher combustion pressures.

Changing the compression ratio does not alter the cam timing in any way, however, one way of reducing the effective dynamic compression ratio is to use a bigger cam with more valve overlap, which results in part of the cylinder pressure being bled into the exhaust port before the valves are shut and the compression stroke can be completed. This is why engines with big cams can run slightly more compression, and hence need slightly more compression, than an engine using the standard cam. Its often that people use the standard cams in high compression engines, and they wonder why it detonates so badly.

Ben Wight

[Benjamin]

Naaah, was just interested, and what you have said I can understand
The more wiser now.

[TXcoupe]

hey slug those head wif the welded chamber are the done to look likie a wedge shage??
and yeh the gemi chamber is a hemi design but he placement of the vevles and the spark plug do not help flow charicatreisctics at all but they peak at in flow hp at 233hp so they are good for 190 wif manifolds

[Tooleeda]

Thanks for the insights fellas. The reason I was asking was because I know that a true hemispherical chamber requires up to 44 deg. total advance, 'all in' at arount 3200 rpm. Changing to a wedge shape would reduce this total advance considerably, and I was just wondering if this has been tested? And does anyone know the figure for full advance of a standard Gemini dizzy? (under WOT no vacuum)

[Slug]

I didnt say the valves and plug placement helped swirl ??? I said they didnt ... and If you want to have a look at a average combustion chamber job then look here
http://www4.tpgi.com.au/users/bussani/slughead.html

Michael

[Slug]

I didnt say the valves and plug placement helped swirl ??? I said they didnt ... and If you want to have a look at a average combustion chamber job then look here
http://www4.tpgi.com.au/users/bussani/slughead.html

One more thing I dont know what flowbench you have had a head on but there is no way in a month of Sundays that a gemini head will flow 233 Hp on a 25 inch bench

Michael

[Ben Wight]

A gemini distributor has two slots, one for 24 degrees advance, and one for 28 degrees advance, every one I have ever seen has been set to 24 degrees advance. So if you run 6 degrees static, then you will have 30 degrees total at over around 3000rpm with no vacuum.

From the info on other engine, I believe this chamber design should run better at around 34-36 degrees advanced.

I have however never tested differnet total advances on a dyno.

Ben Wight

[Fastgem]

hahaha, what - Craig Watson wasnt much help/didnt do a good job?? That must have been the first time ever!!

[Fastgem]

Oh, i think i left out over charged.

[Slug]

Yeah thats why the prick has gone broke ... Michael

[Fastgem]

why did it take him a year and a half to work your heads? He only made me wait 2 weeks for a cam. Was he still finishing his apprenticeship or something