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zac the muss

RB25 build Piston dome volume

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Does anyone know the piston dome volume for an Neo r34 RB25de? Reason being wanting to put a basic RB25 block together for a turbo build, already have an S1 rb25 head and can get my hands on a Rb25de neo block fairly easily.

Have worked out the dome volume of the piston should be around 12cc by plugging numbers into a C/R calc and comparing results with r33 and neo head chamber volumes.

Going by this an R33 head on a 25de Neo block would have a CR of 8.3 and with .5mm taken off the block would give 8.7 which could be ideal for a turbo build ?

Does anyone think this is a good idea? My reasoning is the 25det neo has a better piston with stronger ring lands so am guessing non turb will be like this also

I know the better idea is to get an rb25det block and go from there but is more $$ and will likely be doing a basic rebuild anyway so dont mind trying something different

 

Any criticism/advice appreciated!

 

Edited by zac the muss

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More compression is better than less compression.  It is not 1986 any more.  10:1 is not too much to aim for, with modern fuels.

It would be wrong to assume that the NA Neo piston is as good as the turbo one.  It might be, but it might not.  The 25DET Neo also has RB26 rods in it, which the NA certainly won't.

I wouldn't be doing any mix'n'match without getting suitable custom pistons.  ~9.5:1 would be ideal.

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Personaly i THINK high comp with a turbo isn't as nice, it gives better low end response and spool but 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.

2lt na 14.7 atmosphere

Add boost of 14.7 = same 4lt na

Add another 14.7 = same as 6lt na

But as the turbo is pre compressing on the 2lt the motor can still maintain speedy rpm as it now doesnt have to do the extra compression that the n/a car does.

Compression ratio of 8.5/1 is 14.7 psi less than 9.5/1 and so on.
That means that your motor could handle more boost at 8.5/1 but would also need it to be as powerful and would also rev easily in high rpm with boost and lag in low rpm without an antilag setup.

Anyways. if ya got it throw together. Would be better just finding a cheap 30 i recon for that head.
Depends on funding too.

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Yes it will.

I thought you tune cars gts boy.

Compression ratios have a large impact on the turbo.

Every time you go up one on static compression you add 14.7 of atmospheric pressure.

The extra air goin in causes extra going out making the rurbo spool faster.

And also adding extra pressure to the compression stroke making the motor work harder as it has to compress from 14.7 upto the ratio compression factor.

10/1 147psi
9/1 133psi
8/1 119psi

The higher it is the more intense the discharge after combustion creating more charge for the turbo to spool from witch helps down low rpm but up high in rpm causes more chance of detination from more heat caused from the engine working harder to compress air as a compression rario instead of as boost from the turbo.

I am not a dyno tuner but i know it affects your rpm power and speed.

I actualy cant believe you recon it wont affect the revs gts boy. Wtf?
Tell me why it wont!

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Regardless of wether you have noticed rpm speed fluctuations whilst tuning due to compression ratio differences, it is desirable to match compression ratio to boost to avoid knock and the as gts boy said todays fuels ..... pay 2 play.

Higher boost lower comp or vice versa.

Here is a lil bit from cgc about the G.O.

https://gcg.com.au/component/rsticketspro/view-article/27-compression-ratio-with-boost-pressure

High comp na ☆ yerp its good
High comp boosted $ lotsa mods ta be standout.

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Its a forum. What was my advice?

None!
I said personaly i THINK.....

P.s.

Where is your actual input to the thread discussion?

Go ahead contradict me or give your input to the poster and have a discussion so people from all levels of enthusiasm can learn and dont talk down to me / say im wrong without eveidence or missquote me because you didnt understand me.

Theres way to many egotiscal controlfreaks on this forum nowa days.

Now heres my advice.......

Id build it mate and then boost it hard on a stock ecu hahahahaha cause thats me.

But these guys will say shid like you cant, the motor wont handle it, the ecu cant, and all sorts of crap to make ya feel shid so that they discourage you to post and maintain there rules of the skyline bible (sau) and what knowledge there isnt so that as tuners and mechanical engineers you have to pay up to them to tune your car when they cant even explain a voltage reading on a knock sensor without spazing out or unable to believe a car can idle off the idle maps and act differently... because theyre not the real deal. there is however very knowlegable people on here that can communicate well and understand when and where others missunderstandings are and that are the real deal but dont have much time to be on the forrums .

But realy ya can its not how they would do it and it just wont last as long as a motor you throw thousands into tuning and moding.

If you have it, use it.

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2 hours ago, Slap said:

Personaly i THINK high comp with a turbo isn't as nice, it gives better low end response and spool but 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.

2lt na 14.7 atmosphere

Add boost of 14.7 = same 4lt na

Add another 14.7 = same as 6lt na

But as the turbo is pre compressing on the 2lt the motor can still maintain speedy rpm as it now doesnt have to do the extra compression that the n/a car does.

Compression ratio of 8.5/1 is 14.7 psi less than 9.5/1 and so on.
That means that your motor could handle more boost at 8.5/1 but would also need it to be as powerful and would also rev easily in high rpm with boost and lag in low rpm without an antilag setup.

Anyways. if ya got it throw together. Would be better just finding a cheap 30 i recon for that head.
Depends on funding too.

You think wrong man.

Need to slow down a touch. Ever heard of the Dunning Kruger effect? Should check that out

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You think wrong man.
Need to slow down a touch. Ever heard of the Dunning Kruger effect? Should check that out
That is relevant only to my rant and how you tried to belittle me in the gtr sale thread.

How about a post helping out the thread starter instead.

I gave you a crucifix remember.

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Well the common consensus is that of run as high of a compression ratio as possible, it is limited by the fuel essentially. It even says that in the link you posted.

Higher comp will always drive nicer overall.

 

3 hours ago, Slap said:

Personaly i THINK high comp with a turbo isn't as nice, it gives better low end response and spool but 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.

The bit you may be missing is the dynamic compression  the engine sees, Wether it is is made from low comp and more boost or higher comp and less boost the "piston speed" won't be impacted, as it's the same pressure on it in both situations. It's certainly not somehow way worse with higher comp.

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Slap, what Ben said is the truth.  BMEP is a function of dynamic compression ratio which is made up of boost + static compression ratio (and also a component of cam timing/duration/pulse tuning, etc etc that make cylinder fill less than 100%, 100% or greater than 100% at different engine speeds).

More than one way to skin the cat.  Either way the cat is equally pissed off.

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I would argue that the compression stroke can compress air with less effort when boosted with psi from turbo as opposed to it added to comp ratio.

Would a cammed high comp 10.5/1 na r33 beat a boosted r33 running 1 bar boost 9.5/1?

Why?
(Piston speed?)

Maybe ?- The boosted car could compress the air at twice the rate as it has 2x atmosphere pressure at the bottom of the compression stroke and is the same ltrs.

Means ? Less work for same power - surely quicker compression stroke.

Not talking combustion stroke... atm.

I never said fuel wasnt relative.
I backed yas on it.


I found my car better with my cams set to lower my compression and alow a more free flowing motor.


I would also argue that they are different pressures as dynamic.

9.5/1 + 29.4psi boost =?
10.5/1 +14.7 psi boost =?
11.5/1 = ?

Because you have doubled the sarting amount of air with every 14.7 of boost
And the piston still has 100% travell left.

Do they all have the same answer?
Or
Does the volume of air make a difference?
And
How fast can you spin your motor by hand with the plugs in compared to with them out?

So is it?

9.5/1 + 29.4psi boost =? 169.05
10.5/1 +14.7 psi boost =?169.05
11.5/1 = ?169.05

Wich would seem right if the volume wasnt doubled and tripled inside of the cyl area.

Maybe it should be:??

(14.7atmo+29.4)44.1psi@9.5/1 =? 418.95
(14.7atmo+14.7)29.4psi @ 10.5/1=? 308.7
(14.7atmo)14.7@11.5/1 = ? 169.05

Anyone monitored cylinder pressure under boost at pp. (Perfect pressure point)?

How do you recon it should be calculated?

It is desirable to match for how you want your car to react.
Boost provides more air than raising the compression so you can make more power from higher boost than higher comp.
In saying this im not against raising it if it suits the cars setup.

On an n/a car push it up for sure.

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5 hours ago, Slap said:

Would a cammed high comp 10.5/1 na r33 beat a boosted r33 running 1 bar boost 9.5/1?

Why?
(Piston speed?)

Maybe ?

No.  No it won't.  That is, a 10.5:1 NA will not beat a 9.5:1 turbo running 1 bar of boost.  Reason? The turbo is stuffing TWICE as much mixture into the cylinder, then compressing it and burning it.  It will make nearly double the power just because of that.

Think about that for a bit before delving into unnecessary arithmetic that is based on a wrong premise.

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No.  No it won't.  That is, a 10.5:1 NA will not beat a 9.5:1 turbo running 1 bar of boost.  Reason? The turbo is stuffing TWICE as much mixture into the cylinder, then compressing it and burning it.  It will make nearly double the power just because of that.
Think about that for a bit before delving into unnecessary arithmetic that is based on a wrong premise.
To be fair even though you answered the car race part. Well done

So if psi is added by boost then it adds more power?
Over adding it as a compression rate?

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You are thinking about it wrong.

NA engine cylinder inhales 1x swept volume per intake event.  Ignoring volumetric efficiency of course.  Put turbo onto engine, running 1 bar of boost.  Let's ignore the effect of increased exhaust manifold pressure for the moment.  That turbo'd cylinder still inhales 1x swept volume per inlet event, but now that gas is at double the density.  Twice as much mass of air gets into the cylinder.  And the ECU has to chuck in twice as much fuel to go with it.  You now have twice the energy (from fuel) released during the combustion event and hence twice the power.

It is THAT simple.  (Except that it is not, because of the various confounding factors, such as EMP, differences in cam duration & timing, static comp ratio, charge temperature, that all add up to drop us below a simple doubling of power.  But we still go up towards double the power, just because we're chucking in double the amount of fuel).

All the compression ratio does on top of that is make it possible to burn the stuff the way we want it to and get the mechanical force out of onto the crank.  If you have a turbo giving 1 bar of boost, and you have an intercooler that brings it down to near ambient (so we don't have to consider too much extra heat coming along for the ride) then you can compress the charge at nearly the same compression ratio as you would in an NA version of the engine, because the temperature rise associated with the compression event depends on the ratio of P2/P1, not simply the final pressure.  In practice, we still need to keep the turbo'd engine's CR lower than we might on an NA version, by maybe a whole ratio point, because it's not just the temperature rise that can lead to detonation.  There's a lot of blah blah involved in that, and it is EXCRUCIATINGLY well documented in millions of words on the subject already, so I'm not even going to try.

But what is important is that your simple arithmetic leading you to this psi value or that psi value is not the correct way to approach it.

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And then to complicate it even further is that your 1bar of boost is just the pressure that’s caught in the manifold and not an actual measure of the extra airflow/pressure that has made it into the cylinders 

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The extra volume gets read. Thats why it adds more fuel.
Do you ever think that i do these silly exersises to make you relise what your saying.

I do learn technichals from these discussions.

Now lets say you have too much pressure. Wich is better to reduce. Boost or compression ratio and why?

Example....11.5/1 @ 9psi.
What should be lowerd?

Can you see why it matters about suiting the setup.

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.

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16 minutes ago, Slap said:

The extra volume gets read. Thats why it adds more fuel.
Do you ever think that i do these silly exersises to make you relise what your saying.

The first line of your reply is obviously obvious.  Let's ignore the engine management aspects of that, and just agree that the turbo is making the cylinder swallow double the mass of air (and hence fuel), and hence power.

The second line of your reply is weird.  It seems as though you are trying to educate me to change my thinking.  Which is actually the opposite of what is actually happening here.

18 minutes ago, Slap said:

Now lets say you have too much pressure. Wich is better to reduce. Boost or compression ratio and why?

Example....11.5/1 @ 9psi.
What should be lowerd?

That is going to be different for different engines and different applications.  Too many variables involved to have a blanket answer.  Drag different to circuit different to hillclimb (think Pike's Peak altitude changes) different to Formula 1.  PULP different to E85 different to F1 rocket fuel.  2 valve head with big piston domes (ie, old ALFA, early Toyota twin cams) different to modern 4 valve pent roof, different to old school 2 valve V8 different to modern 2 valve V8 with lots of squish, etc.  Heads that cause a lot of swirl different to heads that cause a lot of tumble, different to heads with controls that allow them to switch those behaviours on and off.  Development stage different to finished engine.

Taking the last part of that first, if you have a given engine and it is detonating on your compression and boost combo, you would obviously reduce boost, because lowering compression is not really feasible.  Or you do something with your fuel, or you look for WMI opportunities, etc.  But if you're developing the engine, then the long list of application specific excuses I gave above comes into play.  The usage model for the engine is probably the first consideration.  A circuit engine needs good response, so you'd be less likely to want to reduce compression ratio on that than you would on a drag engine. All the other details I listed will make a difference as to whether you will get more total power (peak and average across the used rev range) using more boost or more compression.  It comes down to the detonation resistance of the head/piston design and the aerodynamics and the final trade offs between dynamic compression ratio and inlet charge temp and all that blah blah.  Actual prototyping and dyno testing would be required to find out what's happening.  It's not something that's going to be amenable to theoretical analysis without a lot of real world data to base the finger sucking on.

In your specific example though, if you are at 11.5:1 and only using 9 psi and reaching your knock threshold, it would suggest that you do have too much compression ratio and should probably drop that because you can probably make more power that way.  That's simply from an armchair view of the situation that says that 11.5:1 is a very high compression ratio for a turbo engine, probably exceeding the abilities of the sorts of fuels that we use, PULP certainly.  Maybe but maybe, E85 is suitable for that sort of turbo'd static compression ratio, because it is so good at suppressing detonation.  But that's a sidetrack discussion.

As a counterpoint though, with 9.5:1 compression there are plenty of engines like ours that are running 30+ psi on E85 type fuels.  So I'd like to think that you could run what looks like a silly high amount of compression, like 10.5 to 11.5 and still have a decent amount of boost (pick a number, say 20 psi) and make great power with an engine that is also much more responsive.  Remember that the fundamental thermal efficiency of an engine is limited by the static compression ratio.  The higher the CR, the more efficient the engine is - the more work it can pull out of the fuel. Make more gas flow when off boost and as boost is ramping and you will make better use of your turbo.

Again, the above is my ramblings.  There are millions of words published on these subjects already, that would hopefully not disagree with what I have written and also hopefully be a lot more accurate and precise.  If you are interested, it would be well worth your while chasing down the writings of properly knowledgeable performance industry people and things like SAE articles.  SAE articles can be hard going though.

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