Thread: Ghe770
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Old 28-10-2013, 03:25 PM   #18
Ice88
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Join Date: Jun 2008
Location: Auchenflower, Brisbane
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Hope this is ok, and better yet hope its correct. (To my understanding it is )

So I’ll start my post with an overview of the combustion process.

We all know that in a naturally aspirated engine that on the exhaust stroke (i.e. pushing the exhaust gas out) that the intake valve opens towards the end of the cycle. This effectively allows air to travel from the intake side across the top of the head and out the exhaust side.

What is important to note is that this intake cycle is crucial for operating pressure. The more air we can induce into the chamber the higher the dynamic compression and as a result the more fuel we can induce to create a bigger bang. In doing so we also have the propensity to create more heat, this extra heat is absorbed into the head, piston etc. This heat in a head will have the tendency to create hot spots, which under compression will cause the fuel to detonate.

You may have heard the term that Quench is the art of introducing mechanical octane to an engine. And to a degree it does, but first we need to work out why Quench is important and in what scenarios.

Quench (Squish) is something that is incredibly important for inefficient and crappy engines such as LS1’s or old school v8 engines. The reason for this is that the valves are offset, in a DOHC or modern engine, air enters one side and out the other.

In pushrod engines and direct acting over head cam (?) the valves are offset such that if air was to scavenge it’d need to enter the piston and flow sideways outside the exhaust port. These heads tend to have flat in nature. I don’t need to point out really that is inefficient process. (This is why lots of V8 guys boost though, not because NA is crappy but merely cause there engines suck ) As a result the V8 guys start working on other means to create turbulence in engines. Some diesel engines will utilise things called swirl flaps that create swirl in the intake chamber to aid in the mixing of the fuel.

In V8s (pushrod) to create lots of power tight tolerances and well designed (in the sense of component selection and engine builder) engines are required. In stock engines the distance between the flat top of the piston and the head is usually rather conservative this results in a distance between the head (flat top of the head) and the piston being further apart then would otherwise be ideal. By reducing the distance between TDC (piston to piston deck height) and/or utilising thinner head gaskets you can in effect push the piston closer to the head.

What this means for them is that as the piston reaches TDC the air is pushed with greater force against the top of the head, what this is does introduce vertical swirls inside the combustion chamber resulting in a better mixture. The force of this air being squished and the result of a better mixture cools hot spots in the engine and reduces the lean spots created from the burn.

This is where the term of mechanical octane comes from. In fact because you’ve improved the quench in your head you can in fact use leaner mixtures then you would otherwise be able too.

What this really means for pent shaped and DOHC heads is little in comparison IMO. But I draw you to the following points:

What decking does is change the combustion head shape, this in effect means that you’re changing the shame of the chamber which can result in one of two things as I see it. It’ll either improve your quench and thus your burn. Or alternatively it’ll make it worse in which case you will effectively introduce detonation prematurely.

Flame propagation is essentially the speed at which the flame burns. This is affected in numerous ways but the basics are that a richer mixture will burn slower and vice versa for leaner mixtures. You may note that previously I mentioned that you can use leaner mixtures in a well designed engine to achieve a similar power. This will also mean that the mixture burns faster.

From my knowledge this would be particularly effective on engines which have a poor rod ratio. As they spend very little time around TDC.

I’ve prolly forgotten something or got something wrong in the above. I’ll have a read later and add anything I think I’ve missed. Let me know if you have any questions…

EDIT: Oh and becausing your increasing quench the amount of advanced timing required is reduced too. There's no real rule of thumb as the head design will determine how effective the increase in compression is
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Last edited by Ice88; 28-10-2013 at 03:28 PM.
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