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RB25 build Piston dome volume

zac the muss
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Slap Rising Star

Slap

Posted
Posted
It's not going to affect how the car revs.  It's just not.
This is where the argument begun.

It will and its power and temps and the fuel you use.

All of wich you know but had to say the above instead of acuratley showing me how the balance of pressure is the same at different volumes vs C/R to enable the pistons accelleration to not change.

As i said how fast can the motor be spun by hand?
... lower the compression to suit and its faster..or raise the power to suit and its faster.

Thanks for a detailed reply but im not arguing wich is better as its my personal prefference for me and yours for you.

Would be lovely to run e85 cept its not viable cost wise for on road high k use yet.
Slap Rising Star

Slap

Posted
Posted
Still wrong.  For every extra bit of compression working against the piston on the compression stroke, you have more power than that working against another piston on the power stroke.
What did i say wrong exactly?

"That more pressure will slow the compression stroke and cause more heat"

I did state i wasnt talking power stroke. because then power comes to play and volume is more effective for power.
The power vs resistance factor is now at play.
Wich then all comes down to air flow and fueling.

You cant take your turbo off and increase the C/R to make up for it unless you had superman fuel.
Do you see what im saying yet?


GTSBoy Grand Master

GTSBoy

Posted
Posted
38 minutes ago, Slap said:

What did i say wrong exactly?

What is wrong is the allusion to ....

1 hour ago, Slap said:

how fast can the motor be spun by hand?

Because it is not relevant.  Just because the higher CR makes it harder for you to spin it by hand, it does not mean that the piston motion will be made slowed when the motor is running and having to work to compress to a higher ratio.  That is because the compression is being powered by power that results from the extra compression in other cylinders, and there is more of that than is required to to drive the compression event.  You can't just take part of the engine cycle out of context of the other parts, say something about it is true (when it is true only in isolation, but not true when put back into context) and then go on to draw conclusions about how the motor will run.

Slap Rising Star

Slap

Posted
Posted
Because it is not relevant.  Just because the higher CR makes it harder for you to spin it by hand, it does not mean that the piston motion will be made slowed when the motor is running and having to work to compress to a higher ratio.  That is because the compression is being powered by power that results from the extra compression in other cylinders, and there is more of that than is required to to drive the compression event.  You can't just take part of the engine cycle out of context of the other parts, say something about it is true (when it is true only in isolation, but not true when put back into context) and then go on to draw conclusions about how the motor will run.
Your close but still not getting it.

If we add the cycle it works in favour of boost and low C/R according to fuel type.
You cant just say that basic physics doesnt exist...just because it can be hard to understand the exacts.

The explosive force of the extra air is mor than that of compressed with the fuels we use. And the motor has less resistance.
GTSBoy Grand Master

GTSBoy

Posted
Posted
19 hours ago, Slap said:

inevitably robs you of rpm speed in higer rpm as the engine has to work harder where if its lower comp it puts more of the work on the turbo allowing the piston to move quicker as extra compression is pre done as boost.

This is where the nonsense is.  The engine will rev according to how much power it is making vs the load applied to it.  If you are still maintaining that higher compression will make the engine rev more slowly, then we still have a problem.

discopotato03 Grand Master

discopotato03

Posted
Posted (edited)

Wow , all I can say is calculators don't build engines for manufacturers or race teams .

There are some very simplistic ideas being promoted here based on guestimations being pushed as fact .

All I can say is that very few engine are ever anything like 100% volumetrically efficient and trying to add a bar of pressure to a compression ratio is hmm - maybe optimistic . Way too many factors shoot that one down in a screaming heap .

My take is that detonation is a factor of heat and pressure , whatever starts an unplanned combustion event .

With static compression ratios , many things have run very high ratios but it is hard to argue that it slows down the way an engine revs . If you don't think so go search for some data on compression pressures vs combustion pressures .

Fact . What higher static compression ratios do for you is increase the dynamic or actual compression ratio at less than full throttle . ANY restriction in an inlet tract will limit how much air a cylinder can inhale and if it aint full when the valves are ALL closed good luck getting the measured or "static" compression ratio .

Then you can throw in hot cams that open inlets earlier and close exhausts later , and generally increase the overlap valve timing , and you start loosing trapping efficiency . The less there is to compress means you get a lower dynamic or effective CR . Hot cams are designed to allow an engine to rev higher , to make more power at higher revs .

Don't ever lose sight of the fact that all you have to charge an NA engines cylinders is atmospheric pressure - period . Real word an engine CANNOT suck air in , all it can do is create an area of lower than atmospheric pressure into which the greater 1 bar at sea level can push itself into .

 

Edited by discopotato03
  • Like 2
Slap Rising Star

Slap

Posted
Posted

Lol. No but it isnt as good at it as boost.

 

With boost and our fuels we are limited by materials to certain C/R. And it is easier to achieve a speedy motor by sacrificing C/R instead of boost on a boosted motor.

 

 

GTSBoy Grand Master

GTSBoy

Posted
Posted
2 minutes ago, Slap said:

Whats the first mod you do?

Up Boost!
Not C/R!

Yuh, because a bleeder is 11ty times easier than new pistons.  If I was building my Neo right now, I would be putting in higher comp pistons and you could not convince me to do otherwise.

discopotato03 Grand Master

discopotato03

Posted
Posted

Sacrificing CR is a way of saying no we haven't found a workable method of stopping our forced lump from detonating .

I'm not sure what range of compression ratios you talking about and I sense that you are not very aware of what factors affect measured CR  .  

Ben C34 Grand Master

Ben C34

Posted
Posted

Slap, this makes no sense. your posts are a mix of sensible helpful statements, confusing bits, and complete nonsense. 

This for example.

3 hours ago, Slap said:

Also back to piston speed...i should clarify i dont mean rpm or max velocity and enertia. I mean piston acceleration speed during compression stroke wich would be different from bottom to top depending on pressure.

if the rpm is the same how on earth can the piston accelerate or decelerate differently with a different compression ratio? the factors deciding piston speed are RPM, stroke, rod length. not compression. not possible. 

GTSBoy Grand Master

GTSBoy

Posted
Posted
7 minutes ago, Ben C34 said:

if the rpm is the same how on earth can the piston accelerate or decelerate differently with a different compression ratio? the factors deciding piston speed are RPM, stroke, rod length. not compression. not possible. 

I'll handle this one Ben.  The answer is compressible conrods.

  • Like 1
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Slap Rising Star

Slap

Posted
Posted

Rpm acceleration ben. Not all cars hit 8k the same speed.

The faster the piston can accelerate the quiker it can get to set redline.

 

I know im confusing and i dont mean to be but sometimes i feel like i have to defend myself before i can clarify and end up confused where your at only to go there and bring it back home in an evolving argument for you to see what im on about.

 

 

 

 

Ben C34 Grand Master

Ben C34

Posted
Posted
16 hours ago, Slap said:

The faster the piston can accelerate the quiker it can get to set redline.

You are confusing rotating and reciprocating mass.

nevertheless the speed at which an engine can increase in revs when it matters, ie accelerating a car, is really dependent on how powerful the engine is, (keeping gear ratio, car weight the same)

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