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Re: Round Ports for the win!
No life (a.k.a. DattoMaster)
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As always this is to the best of my knowledge not written in stone by any means!

Ok, like I've said a lot, everything in an engine is a compromise. But if we were talking purely about combustion, and not worrying about flow/cylinder filling, then the port shape (2v engine esp) would initiate/produce swirl, so that the mixture keeps in motion and the fuel is evenly suspended/distributed and in the smallest most finely atomised pieces within the air. then you'd have the intake valves flat on the head just barely missing the piston at TDC and the chamber itself would be in/around the exhaust valve. The flat piston area in close proximity to the flat sections of the head (including around the intake valve in this hypothetical situation) will as the piston approaches TDC squeeze and force the gases toward the chamber. This gives maximum mixture motion, so when the spark initiates the burn, it spreads as quickly as possible.

You 'want' the chamber to be based around the exhaust valve area since this is the hot spot of the chamber, pushing all the mixture there will help to produce the quickest light off/burn so the burn will be quick and more complete, thus pushing down on the piston when it's still high enough for the pressure to do good work..A quicker burn also means you need less ignition advance to get that push at the ideal time, so less advance means less pressure buildup before the piston is past TDC so less 'negative' work being done, more of the burn/expansion being used productively, so more power being made from the same amount of air/fuel, or less power being lost, so to speak. A quicker burn also means there's less burn going on (if any at all) when the exhaust valve opens and the exhaust is being pushed out so less heat energy transferring to the exhaust valve during the exhaust phase, and thus the exhaust valve is overall heated up a little bit less. This then means a reduction in overall chamber temps (or peak temp around said valve) which means you can then safely run higher compression without risk of pre-ignition..

So very generally for combustion, you bias things toward the exhaust valve. on 4 valve engines this can mean that there is more dish one one side, or less 'pop up' of the piston crown around the exhaust valves.

Of course then you can run into issues of pistons hitting the valves, so you can't have them too close or run this theoretical ideal chamber shape in practice (or if you did, you'd have to run shorter valve open times, so they didn't get near the piston crown which then of course hurts flow at higher rpm (even at moderate rpm depending on how far you take it). Then there's the issue of valve shrouding also hurting flow, so you can't run this 'ideal' chamber shape either, it's just a case of trying to get as close to it as possible (which is often still far away but it's also the case that you have to do it fairly 'wrong' before you lose ideal combustion, you can get 'most' of it without having to have massively shrouded valves etc etc.

Another thing which is important is to try and make sure there's no exhaust left in the chamber after the exhaust stroke. And therein lies and issue with your head design. If you have a big pocket like that (and I realise it's a conceptual drawing, not to scale) and the intake valve is high then there's a really good chance of a lot of exhaust gases being trapped in there instead of finding their way out the exhaust.

Obviously also raising the valve that much on any production head will likely remove any chance of getting the short turn radius, valve seat angles etc to a size/shape that will produce decent flow.

So within reason, and by tiny amounts so you don't compromise such things, if anything you'd want to seat the exhaust valve slightly higher (when it's on the car, or 'deeper' if the head is upside down actually being machined) than the intake. This will (very marginally) tend to see less chance of residual exhaust gases after the exhaust stroke, and (with the use of tuned length headers etc and sufficient intake/exhaust overlap on the cam for that particular rpm range) if anything you can end up using that outgoing exhaust to 'pull in' (not technically the correct term, but in laymans terms whatever) a bit extra intake charge. If anything, and again within reasonable limits, it's better to have just a tiny fraction of the intake charge pulled out through the exhaust rather than exhaust left behind in the chamber.

I haven't looked at the pdf files in literally a decade and don't even know if they are still around online, but there used to be an online archive of NACA tests/research papers (NACA was the predecessor to NASA and some of their aeroplane engine testing stuff esp around ww2 produced knowledge/tech that is still very much applicable and useful today). Off the top of my head I recall papers about swirl (in this case they used deliberate shrouds around parts of the intake valves to bias flow significantly, and even managed to do high speed photos of combustion) and how it lead to a quicker/more complete burn, and I'm pretty sure there's also stuff (though it might have been other ww2 research stuff I read on this) where they ran supercharged engines with extra cam overlap so fuel was deliberately pushed through the engine and out the exhaust, this extra air/fuel not being burned but used purely for cooling the cylinder/chamber and it did lead to more power at higher boost levels. Most people are probably aware of the NACA acronym for one of its more famous innovations - the 'naca duct' which allows high speed airflow intake with minimal disruption/drag on an otherwise flat surface. It only works marginally on cars due to the relatively lower airspeeds involved.

Anyhoo, generally you'd want to bias stuff as mentioned. If you were running high compression but couldn't run super high piston crowns due to detonation, then as long as piston to valve clearance and flow considerations wasn't an issue, you'd tend to slightly bias the crown so that it was closer to the intake valve but left less close on the exhaust valve side, so it would 'push' the gases toward the exhaust valve side as the piston rises to TDC before the burn. You also tend to want to not go 'too' high with crowns as this can compromise how the burn spreads. You'd also want as much of the flat part of the cylinder head matched by a flat top piston that comes within around 0.035"-0.045" at tdc (the exact amount will vary with bore size, stroke length, rod:stroke ratio, rod material, max rpm etc. Basically all variables will affect just how close the piston actually comes to the head once you take into account rod stretch (more for alloy rods and for drag racing only if at all), piston rock at tdc, heat expansion, and so forth. Ideally you'd want the things to come as close as they can when running at full power/rpm without actually touching. This will get maximal push of the gases into the centre of the chamber, maximum mixture motion, with no unburned gases left behind which can later detonate, and so forth.

Obviously you need the chamber big enough to clear valves and offer decent flow without shrouding, so you end up with a chamber that is 'bigger' around the intake valve, since the valve itself is bigger, but that's just the way it goes. The way some open chamber heads are, with large areas that are flat-ish but not inline with the cylinder head deck so no quench to be had (speaking generally not specifically/solely datsun heads) if you were running a boosted setup, you'd actually have more power and less chance of detonation with a closed chamber head and a flat piston with a dish in it that mirrows the chamber shape, than with an open chamber head (of this 'poorer' style, not all open chamber heads are bad) and a flat top piston. Other than that, obviously no sharp edges on the piston crowns, around the chamber (the chamfer/radius doesn't have to be ridiculous, but as a general rule, if it is nearly sharp enough to cut your finger if you run it along it, it's a potential area that will get hot and lead to preignition, or at least suboptimal chamber surface temps and the risk thereof. In other words, sharp edges here and there might mean you'd have to theoretically run 0.5:1 less compression ratio to be safe, or less boost, and that obviously is a waste of potential benefits.

I think that's about it, sorry that it is a bit all over the place. If there's stuff that isn't clear, ask away, I'll try and clear it up if I can.

Check out some of the NACA papers if they still exist. ACtually I just looked and here it all is:

http://naca.central.cranfield.ac.uk/


Also

http://www.theoldone.com/articles/default.asp

the articles down the bottom ('talking heads, swirl power and 'the soft head' are worth a read. The first two especially because back when they were written there was a lot of controversy about the info being presented and some people went anywhere from disagreeing to outright calling larry widmer nuts (or along those lines). Interestingly, now the stuff he was talking about back then is considered pretty much 'self evident' and well known. He's not perfect, nobody is, and there's more than a few detractors of some of his work, but I think he's on the money on this particular topic.

http://www.avweb.com/news/pelican/182146-1.html
http://www.avweb.com/news/pelican/182544-1.html

also look around on these two pages. There's a bunch of info on piston aeroplane engines and how running rich runs cooler only if richer than stoich and going leaner than stoich also runs cooler and related stuff, including a lot of early piston engine testing data, even books available. There's definitely some good info to be learned there about combustion. Not all of it applies to max power output but a lot of it can help with how to setup engines for things like 60-80% of max power output and why running leaner at these ranges is beneficial to engine life, fuel consumption etc. All of which relates back to combustion and of course the better grasp on has on that, well it's all part of the big picture I guess.

Posted on: 2016/4/2 11:20
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John McKenzie
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Re: Round Ports for the win!
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So the Ford Kent crossflow had a genuine advantage with it's flat cylinder head? I wonder why more engines didn't use that design, seems like it would be adaptable to most engines

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Posted on: 2016/4/2 12:04
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Re: Round Ports for the win!
No life (a.k.a. DattoMaster)
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yes and no. THe intake valve position 'can' be an advantage, but the problem then becomes that the chamber is all in the piston which can lead to odd piston designs/extra weight of a moving part vs a stationary one. Very generally the bathtub/wedge (some combination of those two) head tends to be the one to go for for 2 valve heads when it's all said and done.

there's probably a few diesels that have heads like that (obviously not pre-chamber ones) if for no other reason than to get at the required static comp ratios necessary for a diesel to work in the first place.

Posted on: 2016/4/2 15:18
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Re: Round Ports for the win!
No life (a.k.a. DattoMaster)
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I was thinking of almost the opposite of these chambers (late jag high comp v12) but with reduced swirl by making the inlet just a funnel and the exhaust flat with the concave on the piston to facilitate both the pull and push of both the fresh and spent mix but also having the inlet valve be cooled by the inlet charge using an oversized intercooler or e85 injection hitting the back of the inlet valves.

Posted on: 2016/4/3 3:50
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Re: Round Ports for the win!
No life (a.k.a. DattoMaster)
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The problem is that intake valve temp isn't a huge factor, exhaust valve temp is, because it's the one that runs much hotter relatively, which is why you need different material for said exhaust valves and partly why special hardened seats were initially fitted to exhausts (in cast iron heads, obviously alloy heads had seats for both from the getgo)

What's the engine combination based on - high compression NA engine or high boost turbo/supercharged or something else? If it's boosted, it becomes somewhat moot since you'd need a bigger chamber anyway (still within the basic shape/style already mentioned) to get the comp ratio down where you need it. Exhaust valve seat width would ideally be selected based on it too - turbo requiring the widest (adn it's all relative mind you) contact patch, followed by supercharged then NA. If you're running e85 it has a decent cooling effect on the charge in general and this can be a factor too (though it's often as not offset by the fact that it enables more boost to be run and indeed more generally therefore is run)

Posted on: 2016/4/3 13:43
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Re: Round Ports for the win!
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thank you for the history lessons Jmac, love reading that stuff and just trying to see whats possible with the cheap round ports but also with the FIA L series head I picked up for an NA application if I can finally get a sunny coupe of somekind here.

Posted on: 2016/4/4 15:38
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Re: Round Ports for the win!
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deep combustion chamber

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Posted on: 2016/4/5 11:33
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Re: Round Ports for the win!
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but swirl inlets?

Posted on: 2016/4/5 15:50
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Re: Round Ports for the win!
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Yes, it's got those too, for better burning and superior low-throttle response

Posted on: 2016/4/6 0:03
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Re: Round Ports for the win!
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old chevy with angled plugs biased to exhaust
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Posted on: 2016/4/6 2:27
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