Questions For Tuners

[MJTru]

hi i have just a simple question here.

add more fuel and air into the combustion chambers, does it take longer to burn, or does the more fuel and air actually burn faster? keeping in mind that all factors like fuel burn rate stays the same.

thanks.

have a happy holiday

[Missileman]

takes longer if da rate is da same, all things being equal and all.

[GTSBoy]

But the burn rate does not stay the same. There are non-linear slopes on simply every variable you can imagine. Increase the pressure in the cylinder (ie make the molecules closer together) and teh reaction rate goes up. Same with temperature. But the exact slope of the lines against each variable is not well known. Take it for granted that if you boost up an engine, the burn still happens in the time allowed for it (the combustion stroke). The total time may get a little onger or a little shorter, but it still takes about the same time. That's the best you're going to get unless you ask someone who designs engines for a living. And I mean designs engines....like an OEM engineer, not someone who builds race engines or whatever. Such a person may know what they're talking about, or they may not. But you'll be unlikely to be able to tell the difference.

[MJTru]

wow that last answer just made more light bulbs go on in my head. The reaction rate will go up. does this mean that from molecule to molecule it would burn faster, but the over all amount will still take the same time?? well the same time the combustion stroke takes? So GTSBoy in short it takes the same time??? and that's the best i will get. lol.

I'm asking because in my head i see people all over the net running made boost which i think is not practical and a trick. like around 50 plus PSI, please don't get me wrong here, but wouldn't it have to do with how much you can squeeze the fuel (octane) and to reach maximum cylinder pressure the whole fuel mixture needs to be burnt in full? If this maximum cylinder pressure happens BTDC then the piston would want to go in the direction opposite the rotation of the motor, and if it happens at TDC well this would be bad to because it would place all the force on the rod and main bearings, but the piston should be on its way down (ATDC) to make good power. Or if you can does the piston have to be moving down whiles all the fuel is being burn away?

Please bear with me here for a minute. I guess if i know when maximum cylinder pressure is reached that would answer my question. Is it when the whole fuel is burn away or is it whiles the fuel mixture is being burned (somewhere from the beginning to the end)?????

I'm thinking that on those high PSI motors the fuel never really reaches maximum cylinder pressure and is waited in the exhaust stroke. With a little better timing it would be used more efficiently.

Am i on the right track here mates??

please any help would be a good holiday gift lol thanks

[GTSBoy]

It's a big subject.

Suffice to say, there are limits. Stick with one fuel, say pulp 98, and stick with one basic engine type, like 4 valve simple jobbies like RBs. You'll never get to 50 psi boost with those baiscs because the fuel cannot cope and will detonate. In order to go to that sort of boost, you need to make the engine as det resistant as possible and then change to a fuel that will be able to be squeezed that hard. There's two sorts of such fuel. One is simply to push the octane up so that it is fundamentally det resistant. Examples are going to 1o4 avgas or heavily leaded race fuels or the various others like C16. The second type of fuel you could use would be one that takes a lot of energy out of the charge by cooling. Methanol and high ethanol fuel mixes are the obvious examples. Methanol and ethanol also both have higher octane rating as well, so you get some from column A and some from column B.

Anyway. That's a very simple summary of the basics. I've left out nearly everything. For example, high octane fuels tend to be slower burning than simple petrol. So the fuel change will bring along a change in the reaction rate. So the rate goes down on that variable, but it's going up because of the increased cylinder pressure. The end result might be slightly slower or faster, or exactly the same, or the difference could be big enough that you start getting unburnts flaming out the exhaust. Just depends on the bazzillions of variables you're playing with all at the same time.

[Lithium]

That's a very simple summary of the basics. I've left out nearly everything.

Sadly this usually proves the better way to go on forums as well, when I cared more I learnt what TL;DR meant. Regardless, your posts make you one of very few forum folks who I reckon would be good to have in a room for conversing about shite like this - as opposed to slap my forehead. Regardless, its nice having an OP asking interesting questions and having interesting quality responses

well the same time the combustion stroke takes? So GTSBoy in short it takes the same time??? and that's the best i will get. lol.

I'm asking because in my head i see people all over the net running made boost which i think is not practical and a trick. like around 50 plus PSI' date=' please don't get me wrong here, but wouldn't it have to do with how much you can squeeze the fuel (octane) and to reach maximum cylinder pressure the whole fuel mixture needs to be burnt in full?[/quote']

If you can get away with running more boost PROPERLY (ie, nice controlled combustion and an engine and turbo which are happy with it)) then excellent, more torque and power to be had. I know I may not be asking your question, am not entirely sure what is motivating your questions - presumably the big picture of making more power reliably like the rest of us... but for what it's worth the majority of the burn will happen with the piston dwelling physically near TDC, and the piston moving effects the flame speed more than the actual air/fuel amount.

Also, the peak cylinder pressure is imho a small moment in a big picture - so long as it isn't a dangerous spike then it is purely academic in terms of working towards a good tune, the torque you get is from the expanding gases and not the fire itself. The exhaust gases keep expanding well after the burn which caused them to - and the resultant cylinder pressure happens well after that too (even if there is still burning happening elsewhere). The more air and fuel you have, the more expanding exhaust gas you will have, so the more cylinder pressure spread throughout the expansion stroke.

[wolverine]

IIRC peak in cylinder pressures should occur around 8 to 20 degrees ATDC. I had some notes written down on the subject from someone who logs them but I can't find them.

[t_revz]

IIRC peak in cylinder pressures should occur around 8 to 20 degrees ATDC. I had some notes written down on the subject from someone who logs them but I can't find them.

11-13 atdc iirc

Might have been in the "knock for budding tuners" thread or a link on the "water meth" main thread. Was an interesting read

[DVS JEZ]

More air and fuel in the cylinders (tightly packed in) burns faster

Also 12-15degree's atdc is where peak cylinder pressure is most effective

[MJTru]

sorry that it took me a while to get back to this topic being the new year and all i was a bit busy.

Like GTSboy mentioned witch i agree with it has a lot of variables like leaded fuel and so on. The one that I'm a hundred percent on is him hinting about talking to a OEM engine designer. I have done some research on this in the past, and i think that what your saying indirectly is the way a combustion chamber is shaped that has a lot to do with anti- detonation characteristics and so on. This is why you mentioned 4 valve heads???

Well I've found over the past years that OEM tends to keep the squish are as small as possible, and being a mechanic myself I've notice this as the (4 valve) motors over the years (late to date) did get smaller. The big advantage these days i believe has to do with special coatings that are proven to yield a great result in Anti- detonation characteristics. Most machine can do an OK jobs on reshaping the chambers to yield a better result, but i think that this will only be a practical results that is gained over years of experience, and not by any old bloke in a shop with machines. (pick them wisely)

On the other hand coatings are the new leading edge in reducing the detonation characteristics of a combustion chamber.

For you comment Mr. Lithium i'm dieing to find out more about TL and DR. in the past i can remember coming across these acronyms in my distant past. If i was to guess here it would be something like max torque required???? and DR i have no idea. lol. wow this can only be a clue that you are hinting that i can look up. I have tried on my smart phone at work, but with a minimum result. If you can widen the clue a little bit it would be most appreciative. Thanks in advance. I'm concerned that you mentioned that when you cared, I think deep down that you still do care.

As for the rest i do agree with the assumption that max cylinder pressure is around 10 and 20 ATDC dependent on multiple of verialbes

[Lithium]

TL DR - too long, didn't read... What most people seem to react like when you give everything they need to answer their question.

[GTSBoy]

Marcus,

I specifically mentioned 4 valve heads just so as to limit the discussion to something that was at least pertinent to our RB engines. Could have picked any other configuration.....but the other benefit of picking 4 valvers is that the combustion chambers are all very similar across all of them. There is a lot less variation in the overall design than there is across 2 valvers (look at the difference between old Holden/Chev/Ford motors and Hemis and various OHC Jap engines.....lots of different chamber shapes). If I'd picked a 2 valver I would have had to narrow down the description a lot more.

Anyway, with specific reference to 4 valvers, there is some variation in their designs. Narrow valve angles vs wide valve angles tends to give you deeper or shallower chambers for example. Bu the main thing with the whole 4 valve design was to try to get the max valve area into an engine, almost at the expense of a lot of other important things. Until recently, most 4 valve head designs have had precious little or no swirl on the incoming flow. That's because the inlet arrangement is symmetric with respect to the chamber as a whole. 2 valve heads give good swirl, and swirl leads to better combustion. So 4 valvers tended to rely on tumble to gain some in cylinder mixing. But tumble is the poor relation to swirl, and some engines got a reasonable amount and some did not. Then there were asymmetric inlet valve arrangements which were intended to cause one valve to start flowing before the other and so cause swirl, but they didn't end up doing as much as expected. There's lots of blah blah you could go on with on that sort of subject. The upshot of all that with respect to 4 valve designs is that there is not a lot of room left for a decent squish area, and worse, what area there is is available fairly equally on opposite sides of the chamber. It would be far better if it were all on one side, so you can squirt the mix across and have it tumble and swirl when it gets to the other side. The 4 valvers tend to shoot towards each other and the momentum of the two jets works against each other in the middle of the chamber. More and better squish would be a very desirable thing in our engines.

[ARTZ]

Bahahahaha.....

[ARTZ]

That was @TL;DR btw..

Do 4 valve motors have as much lift as 2valve engines..I always thought that as part of the reason they did it...to get more fuel/air in with less valve travel...

That is the whole point in making power, to safely get the biggest explosion in the smallest time possible

[GTSBoy]

Yes, that is part of the reason. You get a lot more valve curtain area (and can have more port area) with 4 valves, so to get the same total flow area you do not lift as much. If you lift more than that, you get more area. So you can have your cake, or eat it, or have some and eat some.

[_x_FiReStOrM_x_]

hi i have just a simple question here.

add more fuel and air into the combustion chambers, does it take longer to burn, or does the more fuel and air actually burn faster? keeping in mind that all factors like fuel burn rate stays the same.

thanks.

have a happy holiday

Adding more fuel + air in quantity is one thing, but then altering actual air/fuel ratios can be considered also - which have a direct affect on burn rates too.