Question on increasing Compression
 

I need some advise,
People say that if you raise the compression on the a SVX engine above 12:1 you need to use special fuel. What I don't unerstand is that turbo & SC must have the same effect and you don't need special fuel for them. If I operate my SC with 6# boast then what is the final compression that the engine is operating at.
Any help in understanding this would be greatly appricated.
Tony

Answer stoled from online:

The limited a/f mixture of Hi comp requires higher grade fuel because knock is increased a great deal. /guess

Tony, I can't give you the numbers on this because I'm not an engineer, but I can give you an outline of the problem.

As a general rule for naturally aspirated engines higher compression engines are usually in a higher state of tune. This is generally true, but not universally true. CR in the EG33 is relatively high at 10 to 1, moderate to high specific output [hp per litre, say] CR in my Honda Blackbird is 11.0 to 1, pretty high, and high output. However the Gasoline Direct Injection 1.8 engine in my Mitsubishi is lean burn technology, CR is 11.7 to 1, very high, but moderate output. The exception that proves the rule:rolleyes:

Generally though higher compression engines are more efficient.

The downside of higher compression engines is that they can overheat the fuel mix by compression alone which causes localised detonation in the cylinder before the spark goes off. We call this pinking or pre-detonation. When it happens it creates fierce heat and pressure in the cylinder, as the exploding mix is trying to force the piston back before it gets to top dead centre and spark ignition. Pistons can burn a hole through if it is bad enough.

Engine management detects the "knock" of these explosions via the knock sensor and works to offset them by retarding the ignition, which overfuels and cools the mix.

Better quality fuel resists detonation better, so higher CR engines often are tuned for best petrol.

In the case of a pressurised, turbo'd or blown engine, the pump does force extra mixture into the cylinder and increases the pressure in the cylinder mix. This adds to the chance that the mixture will pre-ignite. To compensate for this fact the engine builders leave more headroom in the engine for the bigger volume of mixture that will be forced in. This means these turbo or SC engines are designed with a lower CR of maybe around 8 to 1, in order to avoid pinking when the turbo spools up and adds the extra 6 or 16 lbs, whatever pressure it is designed to work at.

This also implies that if you force feed your 10 to 1 EG33 with 6 lbs of boost and extra mixture, the in-cylinder pressures are going to be a lot higher than standard, pinking [pre-detonation] will be happening and your engine management system will be detecting it and compensating by retarding the ignition.

Consequently unless you can keep the incoming pressurised charge cool enough not to pre-detonate, your power gained will be reduced by the engine management system making your timing retard all the time. This will mean you won't be seeing the amount of power you should be seeing for the amount of boost pressure you are adding to the cylinders.

Phew!! That was long winded.:rolleyes::eek:

Anything there you did not know, or that will help you?

Joe:)

Excellent write up Joe, and it will be appreciated. However with due respect, I would not agree with ---“Consequently unless you can keep the incoming pressurised charge cool enough not to pre-detonate”.:confused:

A cooler charge will in fact have greater density. I think you may have become confused as a result of the water injection stuff, expounded by the late Automotive Engineer. Young Tom can provide you with the real oil.;)

It's not often I do, but I beg to differ with you, this time, Trevor.
A cooler intake charge is less susceptible to detonation than a warmer one. This is because the charge cools the combustion chamber slightly as it enters, as well as it will carry more fuel. Threby increasing the cooling capacity of the charge.
The reason it carries more fuel is, that the ECU will see the denser charge as more air, and compensate accordingly.

I am ever open to enlightenment Trevor:D

I see incoming charge temperature as the bad boy here, with charge density as an also ran at least in problem stakes.

Either a heat exchanging intercooler or the latent heat extraction factor that water or methanol injection and subsequent evapouration will supply, both of these mechanisms cool the incoming charge so that the mixture is less likely to pre-detonate in the last milliseconds before top dead centre. They also in cooling the charge make it denser of course, and more oxygen equals more potential power extracted.

As I see it all the oxygen you might think you could cram in there using 50 psi is no good for producing power if the mixture is so hot it explodes before the ignition spark pulls the trigger.

Where do you see confusion in my thinking??

Joe:)

Crazy, I'm not sure that last statement is logical.

In the case of the SVX at any rate, the ECU uses the MAF to measure charge volume ingested. The speed of the air passing the maf gives an approximation of the volume of air being used.

As the cooling happens after the intercooler in one case, and in the intake manifold or even the cylinder with injection, the ECU has no way of monitoring this charge density, only the volume of charge passing the MAF. So no matter how cool/dense you make it after the MAF, the ECU has no way to see that, or react to it.

If it's not cooled and creates detonation of course, the ECU gets signals from the knock sensors, and can react to and compensate for that all right. And by adding more fuel to cool things down.

It's possible your statement might be correct for the MAP sensor that Impreza turbos use, but I have no familiarity with them.

Joe

THat's what I was basing my statements on, My only boosted engine was a Ford, with no MAF, only a MAP.

Then it's probably correct as it would be measuring things after the intercooler.

I get the impression that the MAP sensor is a better way of keeping an eye on charge than the MAF is for turbo engines.

Joe

Thanks guys it all sounds good, if the SC guys are having to put better head bolts to keep the heads on, then engine they have to be getting major CR. I will do a little research because 6# boast means more air in a give space. Anyone know what amount of air it would be.
Tony

A late thought a boasted engine has the air entering under pressure a NA engine the air enters under vacum. Okay that what the hell difference does that make to what we are saying. Does fuel detenate under vacum sooner then under pressure?

Normal air pressure is 14.7# per sq in, if the guy with the SC go to 16# and don't change the pistons then they are running at 20:1. NOW I see why they can't keep the heads from leaking. Does this make sense or have I lost myself.

[QUOTE=svxistentialist;501773]I am ever open to enlightenment Trevor:D

---------- it explodes before the ignition spark pulls the trigger.

Where do you see confusion in my thinking??

Joe:)[/QUOTE


Joe,

Within your posts I now understand that you are confusing two distinct issues, detonation and pre-ignition. :lol:

You have stated:-

“causes localised detonation in the cylinder before the spark goes off. We call this pinking or pre-detonation.”

“This adds to the chance that the mixture will pre-ignite.”

“ pinking [pre-detonation] will be happening and your engine management system will be detecting it and compensating by retarding the ignition.”

“pinking [pre-detonation] will be happening and your engine management system will be detecting it and compensating by retarding the ignition.”

“charge cool enough not to pre-detonate,”

What you have described is properly termed pre-ignition and pinking is the commonly used term for the outcome.

N.B. Detonation is a separate issue and occurs after ignition.

I now understand that you were incorrectly referring to a situation where detonation is in fact the problem.

Rather than become involved in a lengthy debate, I have co-opted Google and found this brief explanation:-

http://www.streetrodstuff.com/Articl...ion/Page_2.php

Cheers, Trevor. ;)

Hi Trevor

That is very informative, and thank you:D

My descriptions used terminology that does not agree/match with what is commonly used by engine people to describe the phenomena. Well, I did say at the start I am not an engineer.:rolleyes:

Essentially I was describing and dealing with pre-ignition detonation only.

Rather strangely, the term detonation in the Google article is restricted to spontaneous detonation that happens after the spark has fired. Almost as if what the spark had already triggered off a fraction earlier was not a detonation at all!!:eek::rolleyes::rolleyes:

In my defense I can only say that these terms are incorrect and imprecise use of English, even if they are understood by engineers.:tongue:

The word "preignition" refers to a state or to a time. It does not describe an event, which is the process of spontaneous detonation I was describing.

Likewise the term "detonation" as used in that article would be more correctly described as post-ignition spontaneous detonation.

Rant/

Thank you for pulling up the article, I will study it carefully.

Joe:):lol:

I have oil?? I will agree that a denser charge is less likely to detonate. BUT only if the amount of air stays equal. For exampel a turbo car runs no itnercooler making 14psi of boost. An intercooler is added and he continues to make 14psi because of his wastegate adjustment. The denser charge is like running twice the boost so it has to be done in respect to the limitations. If you were detonating on 6 psi so lowered it 14.... You are going to more than likely detonate on 14 now and lower the boost pressures would be recomended. Thats how I see it and if I am wrong, please do correct me

Tom

Joe,

Please be sure that it not my intent to raise an argument, but it is desirable that the facts are made clear, so that others reading this stuff do not become misled. :)

It would appear that you are currently claiming that the dictionary is incorrect. :confused: -----

-----“In my defense I can only say that these terms are incorrect and imprecise use of English, even if they are understood by engineers.”

“The word "preignition" refers to a state or to a time. It does not describe an event, which is the process of spontaneous detonation I was describing.”-----

N.B. The word “preignition”, pasted from my computer dictionary --- noun the premature combustion of the fuel–air mixture in an internal combustion engine. The word IS a noun covering an EVENT.

Furthermore, the two words “pre” and “ignition”, together as a verb and noun, can be used to describe a relative point in time, but do not, and can not a be used in describing a certain state.

You also know say:-

“Essentially I was describing and dealing with pre-ignition detonation only.”

Therefore you have confirmed that you were previously dealing only with “pre-ignition/preignition” , i.e. a state resulting from of combustion being initiated prior to the intended point of ignition. (Ignition. Noun, the action of setting something on fire or starting to burn.)

Therefore you were not dealing with detonation. (Dictionary ---Combustion of a substance that is initiated suddenly and propagates extremely rapidly, giving rise to a shock wave.)

Finally you say:-

“Likewise the term "detonation" as used in that article would be more correctly described as post-ignition spontaneous detonation.”

An addendum could be applied to assist those not exactly acquainted with English. However the article is exactly correct. Again as per my computer dictionary. (Combustion of a substance that is initiated suddenly and propagates extremely rapidly, giving rise to a shock wave.)

I too found it a good article as it precisely sums up a subject which confuses many.

Cheers, Trevor.

Tom, thanks for oiling a crossed thread. :rolleyes:

There has been some confusion within the thread because, within the terms or reference applied in describing the subject being discussed, NA, SC and Turbo charging have become rather intwined.

In any event, there are so many aspects involved that it has been difficult to define which comes first, the chicken or the egg. :)

Confusion also exists regarding preignition as opposed to detonation. It is to be hoped that all interested, will scroll back on the complete thread, so that real oil is wasted.

Cheers, Trevor.

I beg to differ.....
 

Full well I understand that you have no wish to propagate an argument, and merely wish to clarify for those who want to understand the processes involved with internal combustion. Never the less, I have to take you to task on the use of the English language as it is employed by those who would be quite happy correcting the majority of us in what is correct, absolute and proper.

"The dictionary is incorrect."

Exactly what dictionary?

If your computer dictionary is describing the term preignition as a noun, it can only be doing so by accepting the meaning of the term as that which is commonly applied to the phenomenon of premature localised detonation pre ignition in an internal combustion engine.

To define this as the commonly accepted meaning of the phrase is quite acceptable.

However, there is a radical difference between accepting this phrase as commonly defining a specific event and the actuality in English of the term pre ignition, which precisely and absolutely defines a state or a time, not an event. Not even in Middle Earth English does the term pre ignition define an event.

As regards the use of the term "detonation", here in Ireland, once upon a time Bomb Central, we are only too well aware of what exactly the REAL word "detonation" means.

To suborn the use of this noun to merely describe a pretty unusual and little understood in-cylinder phenomenon is totally asking for confusion in the minds of non-engineers.

So if your intention is to enlighten, and I have no doubt this is your intention, then perhaps it would actually help if the misuse of these words in this context was actually recognised and explained, and the specific meaning in the context of the internal combustion process was explained clearly, and without the acceptance of such mental shorthand that engineers might use while usurping older and more correct definitions of certain nouns, words and phrases.

Joe

Please Joe,

Read accurately what I stated lest there be cause for expletives here. :(

The computer (Appleworks) dictionary is CORRECTLY defining the word “preignition”. You clearly state preignition is a PHRASE which is quite acceptable. Then you inexplicably insert the word “premature” and you derive the words “pre ignition”, in lieu of the subject matter, i.e. the single word “preignition”. Twisting my statements in an effort to score points, does not impress. :p

As I have pointed out there are two subject matters relative to the debate. Each is separate and spelled in common plain English, "preignition" and "pre ignition". Arguing otherwise is frankly stupid.

I have considerable in-depth experience draughting technical specifications which must stand up in law, and state with absolute confidence, that real engineers never use “mental shorthand”. What is more when speaking, they say what they mean, and mean what they say.

According to Funk and Wagnalls Standard Dictionary. Detonate, verb. --- To cause to explode suddenly and with violence. (i.e. Not combust slowly.) Detonation, noun. Detonator, noun.

Webster Pocket Dictionary. Detonate, To cause to explode. Detonation,
noun, explosion.

Examples illustrating similar nouns:-

The preignition resulted from carbon deposits.

The prefix enabled understanding.

The prescription was prepared.

The precaution was warranted.

You state that in Ireland you all are exactly aware of what the real term detonation means. I state that what you have set down is an Irish joke. :)

My intention IS to enlighten and I reject your sarcastic suggestion that I should alter any of that which I have accurately described. In fact I am annoyed that I have had to expend so much time, in order to make sure that readers are not advised in error, as a result of continuing absurdity.

With due respect, Trevor. *<)

"Arguing otherwise is frankly stupid."
 

I think not.

If there is "confusion" here to begin with it is because there are terms in use by the engineering community that use words or conjoined words in this context that may have other meanings in common English.

Establishing and explaining the fact that these words may have other meanings is important to those who wish to understand what is going on, but are not engineers and could not be expected to be familiar with or understand that the words mean something particular and different when used in an internal combustion context.

The word "detonation" is a particular point in this respect.

And the word "preignition" as a noun now has a different meaning from the phrase pre ignition, from which it is derived.

These things need to be made clear, Trevor, so that people not only understand what is taking place but the imprecise nature of the language that is used to describe the situation.

And while I am yielding the point [and did in my last post!] that the engineering terminology is commonly accepted, and thus "correct", I stand by my original contention that the use made of these terms is incorrect and imprecise English.

Arguing these points and explaining the use of these words in this context is not a waste of time. We are not explaining for a third level group of engineering students here, who have already accepted the mental shorthand. We are trying to explain to a group of SVX owners [including me!] who may not realise that the words and terms used in this context may mean something different from other possible dictionary definitions.

So there is absolutely no need for expletives. I genuinely thank you for explaining the use of these terms and enlightening me and for finding that great article. However, lighten up, the words used may be perceived to have different meanings to non engine people, and as you are always saying, it is important that the explanations we give make it clear exactly what these words are commonly taken to mean in engineering language [not English!!:rolleyes:] when describing these phenomena.

Joe:)

I have just read that SRS article through.

Twice.:eek:

That is a fabulous description of the process, the causes, the cures and adjustment strategy.

Thanks a million Trevor:) I'm in your debt.

[Hey, I still think the word detonation is not properly descriptive of what is happening, but now at least I know what's being described in this context;)]

I vote we coin a phrase here and call it "postignition".

:rolleyes::rolleyes::p:tongue:

Joe, you have clearly indicated that in your opinion readers here are unable to understand correct plain English, and do not have the wit to comprehend the meaning of the words --- detonation, pre ignition, pre-ignition and preignition. You claim that these terms are both incorrect and imprecise English.:confused:

I claim that any person able to read, possessing a normal understanding of the language and a reasonable level of intelligence, will be able to understand all of that which I have posted. However, provided that they have the patience to sort and bypass the confusing asides. No special or obscure engineering words have been included, all is plain exact English.

You claim that the use I have made of words/terms is incorrect and imprecise English. I claim that all words used have been carefully chosen as being exactly correct and precise. Damn it all the words are in common use, included in dictionaries, are not confined to engineering, exactly as I have already detailed. :lol:

Please, precisely, briefly in plain English, exactly indicate and in the context in which I have used them, (your words) “ that the use made of these terms, is incorrect and imprecise English.” The three terms in dispute being the words --- detonation, pre-ignition, or pre ignition. :p

I regret that my understanding must now be, that you are trying to justify your own misunderstandings and subsequent incorrect comment, regarding that which has been written carefully, logically and with due respect those reading.

You instruct me to “lighten up”. For the good of others, the issue here is the need for only accurate facts to be presented. This is the aspect which I take very seriously. N.B. My objective is objective. :p

Sincerely, Trevor.

P.S. Unfortunately the above was 'saved' before I had read your last sincerely appreciate post. Even so the content is valid and I trust that my motives are understood. *<)

So guys do we understand why the SC or turbo can run at a higher final compression then a NA?

;):D:D
I agree with this statement by and large.

And as I am logical and not unintelligent my quest for clarity on the definitions of the words used by engine builders came from the fact that although you seem to see the words as unambiguous, clearly they are not.

We will have to agree to disagree here Trevor.

What do you think of my better word "postignition" as a substitute for "detonation". eh??:rolleyes:

Joe:lol::lol:

Tony, as I said at the start I do not have access to the numbers on this, but if you read that article Trevor linked in his post, it very clearly explains what the problem is.

You are quite correct when you said that the NA engine gets its fuel charge pumped in by atmospheric pressure. Using a supercharger aka blower, or a turbo pumps in the mixture under pressure in order to get more oxygen in the cylinder.

If you look at pages 4 and 5 of the article it explains really well why higher efficiency engines [usually with higher CRs] are more prone to this detonation lark.

You have to remember that when the piston compresses the gas mix down to approx one tenth of its original volume this causes very high pressures and some heat in the cylinder, even in a naturally aspirated engine.

So when you add extra pressure [and more gas] using forced feeding by SC or turbo, you are only adding a percentage of extra pressure and heat to the equation.

However this extra pressure and heat is enough to push the combustion process beyond the safe limits the engine was designed at, and into the zone where this post ignition detonation is caused.

Bottom line, if you bolt on a supercharger or turbo, you need to lower the compression ratio to keep your engine safe.

Just as a rough guide, and this is not engineering gospel, mind, our SVX engines can probably manage 5 to 6 lbs boost without falling apart, at least in the short term. At 12 lbs boost you would need to be at about 9:1 CR for safety, and at 18 lbs boost you would need to be at 8:1 or so.

This latter [rough guide!!:ninja:] figure I'm quoting here is tried and tested, it is the level Subaru uses for the turbo EJ20 engine.

Naturally you would have to change other things, like using colder plugs and so on, I assume you are aware of these things.

Does that even partly answer your question?

Joe

Joe I agree with what you are saying and the artical was good. I am still not sure any one has explain why the NA motor gets problems at 12:1 up. May be I am missing some thing isn't it correct that when you talk 8 or 9:1 on boasted engines this is before you boast. If you then put a SC or Turbo on the engine the new actual compression a lot higher. 9:1 with 19# boast would be over 18:1 final compression.
Have I got the whole thing wrong.
Tony

Tony,

Joe has explained:-

“So when you add extra pressure [and more gas] using forced feeding by SC or turbo, you are only adding a percentage of extra pressure and heat to the equation.”

A measurement of forced pressure can not be considered as a ratio in conjunction with a measurement in respect of compression, as there is no common denominator. Each measurement is in a different ball park and do not relate on equal terms. Therefore one can not relate a NA engine on 12:1 with a SC engine in the way you describe.

Compression ratio measurement, illustrates a ratio in respect of maximum cylinder volume, as opposed to combustion chamber volume and indicates how much the charge becomes compressed prior to ignition. The same scale/description/denominator, applies in respect of both measurements.

Forcing the charge into the cylinder of an engine with a lower compression ratio does not increase pressure of the charge, prior ignition to the same degree as an increase in compression ratio. What is more the two measurements are unrelated, as they are not recorded by means of a similar scale.

On the basis of overall estimation/view as you describe, things are rather cock-eyed. ;)

Got it? :confused::)

I hope that I have correctly identified the point in respect of your confusion. If not, do not hesitate to again shout out loud. :eek::D


What about an article on your experiences, as I mentioned twice in the thread on tyres. :p:D

Cheers, Trevor.

Trevor I will stop talking compression ratio and talk head pressure at the time of ignition. Would it be correct to say that at that time the maxium NA pressure would be lower then the boasted engine?

In respect to lower or higher, you must state what is being compared.

If a NA engine and boosted engine have the same cylinder capacity and compression ratio, the boosted engine must have higher pressure at the point of ignition. Obviously there will be a greater volume of air forced into the combustion chamber to become compressed. What is more forced induction, will result in a small positive pressure at the moment compression commences, although this will have a very limited affect.

Yes, Tony, you are slightly mislead here in what you are saying, and Trevor has already answered.

Your idea on compression ratio is being distorted by your idea on the added pressure the turbo gives.

The CR, or compression ratio, is the ratio between the volume of the cylinder when the piston is at its lowest point over the volume when it's at TDC, the highest point just when ignition happens.

Once the engine is built, this ratio is fixed.

The in-cylinder pressures can go up if you add extra fuel/oxygen to the mixture, which is what the turbo is designed to do in order to give you more power. Our MAFs measure the amount of air the NA engine is taking in, and the computer can use this measurement to decide what the optimum timing may be for the required burn.

With engines that are turbo'd, they often use a MAP sensor. This measures Manifold Absolute Pressure, rather than the amount of air sucked in. It is a better measurement device for these force fed engines because the heat the turbo makes can vary[reduce] the density of the charge going in, and the intercooler if fitted cools and increases the density of the charge. So the MAP sensor gives an absolute measure of the pressure in the manifold which is a more accurate reading of how much oxygen is getting in there.

So when we say vary the compression ratio to suit the turbo, this requires engine work and will remain fixed when the engine is built.

Adding different higher boost pressures from the turbo will not change the CR on the engine. As you correctly suspect it will add more gas to the cylinder and because there is more in there, when the piston compresses this new denser mix, there will be higher pressures at maximum compression, just before spark.

To partly answer your question about high CR naturally aspirated engines, the following is the problem encountered; the very high CR, 12:1 as you mentioned, compresses the charge into a space 1/12th of original atmospheric pressure volume. This makes the mixture hotter and more pressurised than in a lower tune engine. These two factors create the environment where spontaneous detonation [pinking] will occur in the engine, so the easiest solution to counter the pinking is to use fuel [gasoline] that is resistant to detonation with a higher octane rating.

Joe

Is resonable to assume that in most cases the boast engine has a higher cylinder pressure at spark. If that's the case then the final gas temp before spark must be lower then on the NA engine. Do you guys agree those two facts are true.

Well I don't agree Tony.

The first is incorrect. The boosted engine is only compressing the charge in the ratio 8 to 1, so the in-cylinder heat of compression will be less. When you increase the rate of boost, you are forcing more mixture in there. This will cause the pressure to go up, but not much. The temperature will go up maybe 10 or 20 degrees F.

However, what happens if you go to very high boost pressures is you get pressure going up a bit like we said because of the extra volume of mix. However the temperature goes up a lot, disproportionately, because the pump [turbo or SC] is not 100% efficient. Lots of heat is added in the pumping process, and this unwanted heat causes the combustion temperatures to be out of safe range, which can cause detonation.

Pressures will be higher in a high CR naturally aspirated engine. However the in-cylinder temperature may not be as high as that in a highly boosted engine, primarily because it is not receiving the waste heat added in by the inefficient pump with the forced induction.

An additional factor that creates problems with the high CR engine is the exploding mixture at TDC is very small in volume. So it's difficult for the triggered detonation or burn efficiently or to burn up completely from the spark ignition. This makes it easier to cause secondary detonation after the spark.

The lower compression engine has a bigger area for the explosion to happen and depending on shape can ignite and burn more easily.

Joe

To put it logically and as simply possible:-

You have surmised that, “Is reasonable to assume that in most cases the boast (boosted) engine has a higher cylinder pressure at spark.”

Not so in general terms, because a well designed NA inlet arrangement, would draw air from a reasonably cold area. This would provide an advantage, in respect of offsetting the problems associated with detonation, thus allowing a higher cylinder pressure to be safely utilised.

Whereas an engine driven supercharged engine would digest compressed air, which process involves heat as a by-product. What is more a turbo set up also involves close proximity with very hot exhaust gases, thus exacerbating the common issue involving detonation.

That said, apples must be compared with apples and poor design features, if compared to with excellence, could upset the above described apple cart in a specic instance.

If as you both suggest the head pressure in the NA engine is higher then the boast then why are all the SC guys like me looking for bigger head studs?

You have previously specifically stated, “Is reasonable to assume that in most cases.” We have been generalising accordingly, presuming that engines of similar power output are being compared.

In the event that a given engine is supercharged, without increasing its capacity, in order to increase the power output, most certainly the head pressure will be increased.

However the level is limited, hence the use of lower compression ratios at relatively high levels of boost. In this event the volume of the charge is increased as a result of the larger combustion chamber. At the same time there is the need to give special consideration in respect of detonation.

If the same engine remained normally aspirated and could be modified so as to develop the same power output, by increasing the compression ratio and using high octane racing fuel, the head pressure would be less than the supercharged engine achieving equal output.

You must appreciate that a substantial difference in head pressure is not involved, while both modified engines would be subject to a considerable increase in head pressure. Both engines could equally, very well require stronger head studs.

Trevor thanks for that I think I get it

Simple way to know what your cylinder pressures are like?

Look at the torque. Higher the torque output the higher the cylinder pressures. This assumes that you are comparing the same engine in size and basic geometry (stroke/bore).

So clearly if your forced induction is rasing the torque output of the engine, the cylinder pressures are higher. The talk about external compression and internal compression is largely invalid as compression is compression, with only the effiency of the compressor that matters.

Anyway pre-combustion pressures are more or less meaningless to this argument. What we care about is pressures during and after combustion. Which is what ah.. defines the torque. Detonation is bad as it creates huge spikes in the cylinder pressure that breaks things. Normal combustion pressures are more defined by the fuel type and the amount of fuel burned. Oh and how long it takes to burn. So Forced induction engines get huge cylinder pressures by pushing in lots of fuel and lots of air. NA engines can't match that. PERIOD.

Since cylinder pressure is related to torque, you might notice something. For a given cylinder pressure you make more HP at higher RPM. higher friction losses, but the same cylinder pressure does result in more HP the higher you go (to a point anyway).

Now for something new and fun to think of. Turbochargers are constant HP devices. They want to run at a fixed HP level. So the better you are at getting the turbo into making that HP level, th elower the rpm that it will do it at. So, if you can say make 400HP at 2,500rpm instead of 5K rpm, gues who just doubled the torque of the engine. Course that just doubled the cylinder pressure but hey, area under the curve wins races. BTW, the turbo is best thought of as a engine separate from the engine it is attached to. It is in fact a jet engine with the IC engine as its heat source/combustor.

All that said, match compression to use. 7.7:1 is great for running a turbocharger in the desert on pump gas. 14:1 is great for running leaded high octane fuel without forced induction. 22:1 is great for running street diesel engines. 50:1 is perfect for ultra hgih effiency in a compression ignition engine where easy starting isn't needed.

BTW just to warp your mind, diesel engines are the oppostite of gas engines. thier form of detonation is countered by higher compression or more boost. So no issues with running more and more boost till you lift the head off the block. Oh and running lean is running safe (and underpowered).

So what compression do you thing a SVX NA can run at on 98octane fuel, and if you have lower back pressure on the exhaust will it mean you can run higher Compression?

your 98 is about the same as our better 93 octane. (I'm assuming you use the higher number instead of avg like japan)

Frankly I'd leave it stock on the CR. Maybe push as far as 11:1 assuming I had full programing control over the ECU in some way. More so since Australia has proven to be a complete ***** with people finding nice blazing hot days with 0% humidity to blow thier motors on, when JDM tuning is all about more moderate temps, and humidity that is far more moderate.

I assume you are limited for some reason to NA instead of forced induction. NA is the road never traveled right now and thats a damn expensive and painful road.

Here's an interesting read-->
http://www.motorgeek.com/phpBB2/view...pression+boost

http://www.popularhotrodding.com/tec...on_ratio_tech/