low CR = lag ?

[Greg_S]

go for it paul. im sure it will evolve some more yet

[S2central.net]

go for it paul. im sure it will evolve some more yet

Spreadsheet attached

[AnLe]

rod 144mm
deck 220mm
bore 81mm
piston compression height 32,8mm
piston dish (valvepockets) 2,74cc
metalgasget 1,61mm
(papergasget 1,8mm in free air, will come down when head is tightened)
head gasget bore 82mm
combustion chamber in head 45cc

These numbers give 2226,1 sized engine, and give 8,9 of compratio,
please whipe your asses whith "stock 9.3" number.

I agree to that dynamic compratio is the thing to consider, I do not understand it, or know how to calculate it but that i know that changing camshafts and all your calculations needs to be done again.

To the topic, it is allways an compromise, wrc cars has over 11-12 cr but they have little more electronics and better fuel and they enginas has to last only 2 races. The first thing is to actually know what lag is, is it the time that when throttle is put down and full boost is generated, or is it the "feeling" that turbo kicks in or is it the time that boost is rising or is it the time that boost is at maximum. 99% of people think lag wrong, but I dont go in to that know.

You can not compare egine 1 rs2 spec versus engine 2 rs2spec but 8.0cr, of course the lower cr engine will behave even vorse than the bad rs2 spec. But the lowered compratio gives your oppotunity to change bigger turbo and cams without making the car sufficate at 5000rpms. Ofcourse this engine will generate boost slower and it is not as big kick compared to the engine1 but the car has 150 more horsepower and it has good rpm range raised but it is also longer range, and will be hellava faster if measured against clock.

same 99% that don't have a clue what lag is, they like the car to FEEL more faster, and thats ok, but you have just realize that and then its easyer to tune that direction, take few metal sheets of the metal gasget and rise the cr ratio, take turbo from 1.8t engine, and i can promise you that the low end torq is murder, this means of course remapping and so on.

The point I'm trying to make is that you cant compare just one item or value, everything links to everything, for surtain sircumtanses lowering the comratio will rev the engine better becouse the horse power that is used to compress 1bar boost to nine times smaller space (cr ratio 9) compared to 1bar boost to eight times smaller space, does not require as muchs power and there is more to turn the wheels, it is not simple.

I'm no expert on the matter so don't take my word for it, but crunch the numbers on cr ratio of an std s2 engine, it is not 9.3 and every calculationt that is based on that value is wrong.

[S2central.net]

OK - thank you for you considered scientific input. Real smooth... NOT !

Suggest you take the time to read the articles that are being referred to. Suggest you also look at the spreadsheet before you tell me it is wrong.

Audi have long since published a (static) CR on the 20VT engines of 9.3. Now the only way to get that is with a chamber volume of 53.65cc - because the complex shape of the piston tops and cylinder head area does not allow an accurate estimate of chamber volume (regardless of knowing deck height etc etc).

You can guess the compressed depth of metal or paper head gaskets all day long, but even if you measured it - you would need to burette the piston tops and head chambers to really know for sure what the chamber volume is. I'm taking Audi for their word on a paper gasket with 53.65cc.

We know that the later metal gasket reduced the CR to about 9:1. This is due to about 2cc more chamber volume created by the metal gasket. I've personally never tried to verify that with a measurement. Have you actually measured these things yourself - or just taking someone elses word for it ?

Anyway - in actual fact you are shouting about 8.9 and I'm trying to be somewhat more scientific with an approach that says 9.0 - why are you trying to take the piss ?

Dynamic CR is actually a very interesting subject - IF you take the time to learn about it. It is all about calculating the stroke of the piston after the intake valve has closed. Guess what - different intake timing affects this dynamic stroke length.

Go and do your homework, come back to us with some considered thoughts and opinions on the matter... Rather than comments like 'whipe your asses' which only serve to annoy me and make it harder to take you seriously !

These WRC cars with a static CR of 11 or 12 - do you know what their dynamic CR is ? I'll bet it is much lower than 11 - but it could be really hard to measure on any of the cars using variable valve timing !

The whole point of my discussion was to show that on an engine with 7A cams it would be advisable to run with a static CR of 8.86 - to yield a dynamic CR of 8.25... or if you want a DCR=8 then you need SCR=8.59 with the 7A intake cam.


Paul

[BRY]

Go and do your homework, come back to us with some considered thoughts and opinions on the matter...

I have and you chose to ignore them...........

Dynamic CR is actually a very interesting subject - IF you take the time to learn about it. It is all about calculating the stroke of the piston after the intake valve has closed.

As discussed previously there are different views as to whether this is the Trapped CR as it isn't dynamic.

[S2central.net]

BRY - I can't find links to sites about Trapped CR that describe it any differently than the Pat Kelley definition I have used. Here's an example -

http://www.hydrostream.org/ArticleArchives/Compression.htm

I know the Dynamic CR term is a bit of a misnomer because you end up with a different fixed value - depending mostly on intake valve timing.

I have plenty of other calulations that cater for airflow taking VE, rpm, boost and ambient conditions into effect - this is how you work your brain around compressor maps.

I don't see much point in trying to quantify another type of dynamic CR value across the engines entire operating range because it will differ widely depending on the prevailing conditions of boost and VE. Surely its enough to have a good estimate of airflow (and therefore mass when you do the temperature corrections) across the dynamic range (typically at full throttle against engine speed to build the data needed to plot the compressor map).

Maybe there are some situations when it might be useful to consider something like maximum air pressure in the combustion chambers - but that can be done quickly by knowing the DCR (derived from cam timing), maximum VE and maximum boost...

Roughly saying max cylinder pressure = Ambient pressure x boost x VE x DCR

So with 1bar ambient, 1.5bar boost, VE=85% and DCR=8 you get 10.2bar or just under 150psi

Is that meaningful ?


Paul

[BRY]

You originally asked about Dynamic CR and I gave you my thoughts on it. What you are considering to be Dynamic CR I think is Trapped CR. If anyone can give a definitive on this it would help.

With 1.5bar boost you would have a sh@t setup if you only got 85% VE BTW

I think you are a bit harsh with AnLe, I don't think he is trying to piss you off. He is only stating his views in a foreign language on a complex topic, I wouldn't begin trying to write about this topic in his language.
just my 2p,

[S2central.net]

Ahh FFS life is too short...

I only took the definition of Dynamic CR from the best presented websites that I could find on the subject in a very short space of time. Looks to me like the term Trapped CR is widely use to describe the same thing. I didn't come up with the terms on this stuff...

Have you measured VE on a 20VT cylinder head ?

I have absolutely no idea of 85% is true at 3000 or 6000 rpm on our heads - only specialised flowbench engineers could measure this. I would love to know for sure, but for now I'm happy enough to use that as a rule of thumb for the worst figure envisaged.

Certainly with reasonably modern 4-valve head design such as ours to be over the 90% mark in the upper rev range - but I have no evidence.

Maybe I was a bit harsh on AnLe - but I felt he was taking the piss with some of his language over 0.1 difference in static CR. If i was trying to post on a foreign language forum, the last thing I would be looking to translate is 'whipe your ***'.

I don't want to start a knob waving contest on this - I'm just dabbling with this engine theory stuff and trying to leverage whatever I can gleam from the published work of other experts into a way that I can understand the application of it into the real world of the Audi S2... and more importantly help others who aren't as engineering minded to get to grips with it.

Maybe I should have just stayed in the bar... actually that would be a better place to have some of these discussions... Cos at the end of the day - its all theoretical ****** that is irrelevant in the big scheme of things - if either of us (or both of us) are wrong - nobody dies !


Paul

[WabTheKing]

I don't want to start a knob waving contest on this -
Maybe I should have just stayed in the bar...

Paul

which bars do you go in?

by the way i am taking the piss

[AnLe]

hey s2central, did not mean you it was written with no particular user in mind.

I'm very pleased that some one is making same calculations as I did when I made my engine, making up my mind of piston to use, etc.

I have measured 20vt head and it was 45cc, and I have talked over and over of the paper is thicker theory, but that is not case here as there are no idea to use other than the metal gasget and as we know its factory installed part, it has to got the same ball park as orginal paper, can't believe that if an upgrade to part is made, it changes something as dramatic as cr ratios, that has to change emissions and that is big no-no nowadays.

as for the piston top, i have measured that also, dept and widht and valve pockets, of course it is an estimated result, I got 3cc, after that I realised that mahle catalog has cad drawings of each pistons they made, and voila there were the numbers 2.74cc I'm very pleased that my calculatios cave such accurate result.

And you (s2central) are correct the piston top is complicated and i'm pretty sure that raised part of the piston is not included the calculations, the "squish" area of the pisto is few tenth of an millimiter above piston compression height, but that will not make any difference, BUT is vital for the knocking, pinging, that level is not to be lowered.


For the orginal question, for make it simple:

more cr means lower boost and lower advance to gain same hp
lower cr means more boost slower and more advance to gain same hp
To find the golden line to make most out of any engine, is more hp you want, more torg you want or more turbo kick you want - is trought trial and error.

btw. to get all three in same package, you need to add engine volume , and there are propally many examples to that little rule that it dont work, it just goes to show, nothing is simple - i'm going to go home and do some homework.

[S2central.net]

which bars do you go in?

by the way i am taking the piss

Nice one wab... well spotted... but no I don't frequent those kind of bars !

Paul

[S2central.net]

@AnLe - sorry I got a bit rattled earlier. I am so glad that you have been taking the matter of these measurements seriously. Can you describe exactly how you arrived at your total chamber volume (with piston at top of the cylinder) ? I can't quite follow the numbers you have provided so far - maybe I'm missing something (like enough brain cells on a Friday afternoon).

It would be useful practical info for anyone planning a serious engine build in the future.


Cheers,


Paul

[Jamo]

I really should stop reading threads by S2central, Pisobiker and Mark Halligan late at night.

My brain now hurts but great spreadsheet Paul i'm playing with the figures

[jogi730]

This is MY opinion on the subject - slightly influenced by what my tuner has told me but more so by my experiences.

Basically, my remit to the tuner was a fast car with usable power, especially as I lived in London.

He designed my engine to give me just that. I had 390bhp and as I have said many times on here before, until I changed from my RS2 EM to a tubular item, I just did not know what lag was. Power would kick in at 2500rpm and rise very sharply and hold. 360 lb torque by 2750 rpm! A Few on here have been or driven my car and hopefully they can confirm this. The power was also very smooth which actually made it feel not quite as fast as it really was. The CR of my engine is 7:1.
Now I stand to be corrected by the mechanics out there, but I am led to believe the difference between 7:1 and 8:1 from a turbo engine is in fact quite large.

My latest mods, especially the essential bigger IC have ruined the way the car drives.
I now have 550+(?)bhp. I'm sure I will get a better 0-60 and quarter mile time at GTI this year, but out on the road, I reckon from A to B my original set up would probably have been quicker

So my bottom line is, if an engine is designed well to do a specific task, be it high or low compression, it should achieve those tasks with no (or little) lag.

But low CR definitely does not mean a laggy engine.

Finally, do not try and ask an engine to do what it has not been designed for - you will be disappointed!

Cheers
CHris

[Jamo]

Good post Chris, I thik the difference is though if you dont alter anything else on the car but change the CR you will notice a change in off boost behaviour inparticular.

On boost as you can have agressive timing and a higher boost level it will seem quicker.

Regarding your EM, yes there is a trade of between flow and pressure ( due to volume ) this is from a theorectical chemical and physical view ( dont get me started on gas pressure within a large volume, you can have a IC to big. One simply cannot ignore the gas laws of Avogadro, Charles, Boyle, Daltons the specifically the Ideal gas law (damn how many are there there's more but I can't remember them! ) )

The smaller the diameter of the tube the higher the pressure of the gas ( lower lag, due to higher pressure exerted on turbine wheel. )

but this must be balanced by a small diameter tube can't flow as much gas ( higher the flow for the car theorectically the more bhp is acheivable ).

( The above is why you can also have a mismatched size of exhaust on a car dependant on BHP level)

Your expereicing lag as in a large diameter tube the gas isn't under the same pressure due to being at a higher volume it has a lower force to be exerted on the turbo exhaust wheel.

I'm going to have to dig out my books again to explain why you get pressure drop of a gas when it's in a large volume.due to the above laws but right now i'm off too GTI and no doubt all the people will tell me you can't have an IC too big or you can't have an exhaust too big etc etc!