Question on increasing Compression

[Dessertrunner]

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

[NikFu S.]

Answer stoled from online:

The difference is the total air/fuel mixture is much greater with super/turbo charging. High compression alone has far less mixture to work with. Lower compression supercharged engines still have very high compression (with the pressurized induction) and a lot bigger charge in the cylinder. Don't confused compression ratio and total charge available under high pressure.

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

[svxistentialist]

Originally Posted by Dessertrunner

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

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

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.

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

Joe

[Trevor]

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”.

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.

[crazyhorse]

Originally Posted by Trevor

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”.

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.

[svxistentialist]

Originally Posted by Trevor

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”.

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.

I am ever open to enlightenment Trevor

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

[svxistentialist]

Originally Posted by crazyhorse

The reason it carries more fuel is, that the ECU will see the denser charge as more air, and compensate accordingly.

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

[crazyhorse]

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

[svxistentialist]

Originally Posted by crazyhorse

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

[Dessertrunner]

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

[Dessertrunner]

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?

[Dessertrunner]

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.

[Trevor]

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

---------- 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.

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.

[svxistentialist]

Hi Trevor

That is very informative, and thank you

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.

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!!

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.

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

[TomsSVX]

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