Combustion chamber mods, your views please

Competition engines and ancillaries - general discussion
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trickymex
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Combustion chamber mods, your views please

Post by trickymex »

Hi,

I was interested in people's views with regard to removing the squish area in the combustion chamber

This is a common mod on RB26 turbo charged skyline engines with a lot of power

Here are some pics for ease of understanding

Image

and a modified one

Image

This only shows one of the squish areas removed but a lot of people remove them entirely


What do you all reckon the pros and cons are??

They are there to control burn, so what happens when they are removed??

People claim to reduce the chances of det, how??

And at what point do they become necessary??

Thanks for any input

Ricky
Guy Croft
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Re: Combuction chamber mods, your views please

Post by Guy Croft »

Ricky, hi

I've written a fair bit on this, first of all put 'squish' into the site search (top right on home page), then have a read of some of the things I have written, most especially the reply to Chris at 'Fiat 127 engine build for sprints & hillclimbs'.

It is said that high rpm motors don't need much squish, but that is a pretty generalised statement. A lot depends on whether they have any at all.. From dyno exp I know that squish definitely does make for a faster and more efficient burn because of the squis-induced turbulence. I have never tested back-to-back with as-produced and then radical reduction in squish band on any normally aspirated engine.

It may be that in the case of your modified head they said
(1) ' if we get modify the squish band in the inlet side we will be improving the region where end gas can hide'. That's a fair assumption, because trapped end gas, of course, can cause detonation. That does, however predispose that making that region roomier will make it purge better under the influence of the vortices that form during compression, a big 'if' - unless proven in fact..
They may also/alternatively have formed the view that
(2) the loss of squish makes no difference to the burn rate, ie: the loss of squish-induced turbulence is not detrimental to power. Again a rather difficult thing to prove. Only 'step-by-step' tuning proves the efficacy of mods, and few really embark on it.
Or
(3) it may be that they figured the inlet band was shrounding the valve at lower lift.

I don't know why they did what they did really, try asking them!

Part of the reason the issue is clouded in mystery is that squish bands improve the homogeneity (mixing) of the fuel/air and the end-gas purging at the same time. And both reduce the tendency to detonation!


My own rule is really that unless their edge shape (where the bands join to the chamber) is rather abrupt - and thus disruptive to the flame front (in which case they should be smoothed out) or they are shrouding the valves to a measurable extent, leave them alone. I hope this makes a bit of sense..

Hope that helps,

G
axspeed
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Re: Combuction chamber mods, your views please

Post by axspeed »

Hi,
Not wanting to over symplify things but is it possible it's just done to reduce compression, thats certainly the case in a few of the other turbo projects I've come across. People do it to avoid having to use a spacer/thick head gasket, which seem to have a fairly poor reliability rate.
Just my thoughts,
Richard
Guy Croft
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Re: Combuction chamber mods, your views please

Post by Guy Croft »

You're right,

another for the list. I was just mentally 'revising' my post and thinking that very thing myself this morning.

G
trickymex
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Re: Combuction chamber mods, your views please

Post by trickymex »

Its funny you say that,

I have been thinking of doing exactly that for another engine i have been building that has a slightly high C/R, i need to add 2.8cc to the combustion chamber to get the desired C/R

I dont know if thats a lot as i try to avoid modifying the combustion chamber normally, but it does not sound like much to me so i was thinking of making the flat sharp edge of the squish area rounded and the 90 degree edge smothed over to help reduce hot spots

What do you think the pro's and cons could be, bearing in mind that the main reason for modification is to actually lower the static C/R

Ricky
trickymex
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Re: Combuction chamber mods, your views please

Post by trickymex »

I will add a couple of pics to make that a bit clearer

This not the head that im talking about but you get the idea

Standard

Image

And this

Image

The blue line on the last pic shows what i trying to explain, of course i would remove the material on the valve side of the line, this way i could keep some of the squish area and i would be removing the sharp edge on the squish area that could cause a hot spot??

Any views??

Regards

Ricky
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Re: Combuction chamber mods, your views please

Post by Guy Croft »

The region you've indicated may not create a hot-spot in the sense that you mean - but in standard form it is likely something I would myself smooth out because things like that cause disruption to the flame front. That is not only obvious but it also fact too.

What is not certain fact is whether reducing the area of the squish band will reduce the protection against detonation! I have written about this already on this thread and if you formed the view that the issue of squish is rather, er, vague, all I can say is you're right.

But back to hotpsots. Is, for example, a spark plug electrode a 'hot spot'? Compared with a bit of alloy sticking out and being ferrous - it ought to be, but the reality is spark plugs don't, under normal running conditions - cause detonation. Sure, if you overheat them by running the ignition out of spec, this is one cause of engines running-on after you turn them off. The chance of getting the alloy of the chamber that hot - unless you detonate repeatedly - is minimal. So the 'hotspot' thing, well, it really belongs in the same box as other things we take for granted as being 'good' or 'bad' practice. I've never actually seen a metal part or component in a chamber that I thought might actually 'glow' and set off the mixture. Things like 'oil in the fuel' can cause detonation or 'the ex header should be bigger than the port' and 'head gaskets should never be re-used' and so on! I can tell you that often, even with my exp, I struggle to any definite proof either way. So we' take it for granted' and don't do such-and-such and with a bit of luck the engine runs well. And every so often a 'bright spark' comes along who overturns all the accepted practice and gets away with it.

It's probably more important to consider that sharp edges and recess and other odd-shaped regions of the chamber could become 'carbon traps' (same happens in a ex port) and that carbon could glow, sure. Again, carburetted engines can run-on from that cause. Carbon builds up exactly like a snowdrift - in the direction of gasflow during burn and evacuation, and around edges it will layer up till there is a smooth ramp and then it just stays there. Not good to have that in the chamber. All in all, I shall suggest that sharp edges/intrusions in the chamber are 'bad practice' from a all combustion points of view and leave it at that!

GC
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