Thread: Curious Question..Long

Well for those of you who are not in agreement with the notion that a faster burn is not only better, but essential to a well crafted engine, let me try and prove myself

See the whole point of "fast burn" is that a faster burn, is the RESULT of tuned intake speed, as well as the right dynamics to not only create the proper combustion cycle, but allow ALL of the fuel and air to be mixed. An example used in the book, is that a flame travels like a burning square of paper, and any obstructions (poor mixture) will effect not only the burn pattern, but the area burned, and the speed at which the flame spreads. Turbulence is also very beneficial, because it causes for the fuel droplets to be the smallest and most uniform throughout the chamber.

This is to say, when I say a faster burn is the most important portion of the whole dynamic of the combustion process, I mean that the burn speed is the direct result of how well the motor is using its fuel and air (volumetric efficiency). With that, I then began thinking (the whole reason I wrote this thread). That if our motors need allot of timing (not desirable..LESS torque), then they most likely have slow burn. So the extra timing is needed to complete the burn effectively. This is bad.

Why? (I should point out the following is influenced/cited from HPA publishing "stock car racing, engine technology") 2007:

"Faster burn rates translate into lower exhaust gas temperatures. The net effect is more heat (work) is released within the combustion space. Therefore temperature in the cylinder will be lower at the time of exhaust valve opening."

"The later the spark, the less pressure will be exerted on the piston prior to TDC on the compression stroke"

"This improves "transient torque""

"The maintenance of turbulence until the time of ignition is probably the most important consideration of all, for upon turbulence depends the rate at which combustion takes place"

-That is to say velocity of the intake is less important then ensuring proper turbulence. And no turbulence doesn't mean just slamming mixture into the cylinder, that would cause aggravated mechanical separation of the fuel and air all the way down to the cylinder.

"As ignition timing is increased, so is the amount of negative BTDC work done on the piston"

The article goes on at length about this, but that is enough for me to try and convey my point for now. This article included an interview with Larry Widmer, a renowned engine builder and drag racer, who has countless hours of engine development and racing under his belt. Surprisingly he races imports too...

Dudeman

YESSS that's my whole point, glad we are in agreement.

Key words bold

Dudeman

I see no value in a 'faster' burn. I don't even see how it's relevant. you have to get your timing right no matter what then you have to make sure your cams are degreed in.

a faster burn at what rpm?

my motor was built for a controlled burn and good swirl and atomization. Maybe that's why i can run 12:1CR on a chipped ecu and 92 octane

what size motor are you talking about? stock bore and stroke? Things change when the pistons or crank changes.


the only thing im saying is that you have to compromise somewhere. You can have the highest velocity possible but if your cams don't have the lift/duration it won't do you any good.

you could have the best most complete burn but if you don't have the timing dialed in then it won't matter.

Are we talking only full throttle and/or will your part throttle airflow suffer because all you are concerned about is WOT above 5k rpm?

If I am not mistaken you want a slower burn. Higher octane fuels burn slower and that is where efficiency starts taking place.

Wow. You guys do realize ideal timing REVOLVES around how burn is conducted?

I guess you guys just read the first few sentences of my post then derived your own conclusion. With no facts to support it.

I guess that was the most i could have hoped for here...

Edit:


Fast burn is the whole point of this thread. Do you understand a faster burn means more POWER?

If you don't, go back and read through the thread, it will teach you things...you missed the entire point of my posting.





Dudeman

sigh

you want to quote a book by a guy who builds motors that only have to work within a narrow rpm range and get rebuilt/checked after every race.

I have built a motor, read SAE papers and talked to guys that build street motors. Build your motor your way and I have mine we will then see who is making more power.

way to gloss over everything else i said...but you want to talk theories and ill talk street driving

Actually volumetric efficiency is just a ratio of air taken in to air displaced by the piston (not related to the combustion other than by residual gases). You're talking about the fuel conversion efficiency which includes how well fuel is used.





Exactly. In a perfect engine, you would be able to ignite the mixture at TDC and have it burn completely and "instantaneously". This would provide the most power as you would not be wasting work compressing an ignited charge whose pressure is increasing very rapidly (as opposed to regular unburned mixture). The closer you can get to the ideal state by retarding the timing towards TDC (while still hitting peak pressure at TDC) the better off your engine will be.

This is true for any engine conditions... regardless of throttle, stroke, bore, RPM, etc. You always want those ideal combustion conditions. Needless to say of course it's impossible to reach, and also impossible to approach for all of those different variables (you have to tune based on the most important RPM range for example, because the flow rate of air into the cylinder changes with RPM). I know you stated you have to tune it right, but what I am trying to say here is that even when you tune it for a specific thing you still want the above.



They burn slower to prevent knock (pre-ignition from to much pressure), not to be more efficient. Obviously you have to make compromises sometimes, like sacrificing burn speed for more pressure and power.

I agree with you, what i take issue with is saying that the level of compromise needed is not addressed in this thread. And to say 'faster burn' is the ideal state for making power.

I didn't want to talk about V.E. since the majority of driving we do you will never fill your cylinders anyway.

yes it's all true for any engine conditions but to talk about what's the best way to maximize performance without taking into account every other aspect of the motor is not a very good way of going about making power.

Not to mention the fact that you could have cams that reach peak power at 9k rpm but you don't have an intake, head, valves, header or bottom end to take you there.

So back to the original question.

Do you believe the ve, needs so much timing because it has less then ideal (lets say compared to a K20) combustion chamber design/intake runner design?

Which would lead to a slower burn.

Meaning there could be the possibility for allot more power out of the ve if this is seen to..

Do you agree with my reasoning?

Rather then a huge discussion about terms used I was looking for a more on point talk about these FACTS and the possibility they apply to the ve motor.

Btw, the person I talked about who wrote the book wrote this article for street and track use. He has expereince with all sorts of racing, and if you understand what i am getting at in this thread at all you would see that I am not concerned about rpm range tuning at all. Overall gains is what I have in mind. Look him up, he is far smarter then us.


While optimal fuel mixture and burn HAS to be best at ONE rpm. That does not mean at all that you cannot improve an engines overall performance by optimizing the burn all over the rpm range period. I don't see why some take things so literally. Until the perfect combustion chamber is designed, we will always have room for improvement. And for those of you who don't understand why fast burn is not better (at least Danja does :bigthumbup, then i guess i cannot help you out.

Edit:

donttazmebro: I understand completely what you are saying, but while compromise is always something we must take into account so our motors are street drivable. I guess i just don't see why the idea that the ve's combustion chamber might have room for improvement is wrong? Let me know exactly what you mean if you can please? Because from what I have read, no matter the cams or other mods to a motor, having a faster burn is always better, providing better fuel economy, less heat in the motor and egt, as well as allot better powerband all the way up from 0 rpm to redline. And again, to connect that with my initial thoughts, needing allot of timing is an indication that the motor is not burning the fuel as fast as possible and that is robbing it of power gained by simply addressing where the combustion chamber is lacking?

Educate me on this compromise, I am interested to know what you think..


Dudeman

Fair enough, but that's something you do any time you do a mod, not just of this. It's always for a gain under specific conditions, and it rarely has no effect elsewhere. As an example, we sacrifice part throttle and lower RPM efficiency when upgrading cams for a big gain in the high end all the time, and increasing the burn speed is no different. Firing early might provide peak pressure at TDC at WOT, but will probably hurt your efficiency when there is less fuel and air in the cylinder under other conditions, there's no arguments from me about that. I think that dudeman may have a point when considering combustion chamber and piston design for a specific RPM point or throttle or whatever, not for all of them.

Obviously if you could adjust timing in real time based on throttle and RPM, it would be much more effective, but as it is it would only be optimized for one point.

Anyways, I'm not really trying to argue, just want to make my thoughts clear.