FUEL! Im stumped

[kane2g]

If i have a hi comp pistons, in order not to detonate and still be able to run pump gas (93) can i just upgrade the injectors to add more fuel? I know i need to when im running lean, but what about hi comp engine, like 12.5:1.

Or did i just answer my own question?
thanks

[SleeperTeg]

nah, detonation has to do with the volatility of fuel not volume. what happens in a high compression engine is the heat generated by the compression causes the fuel to ignite resulting in detonation or knocking. higher octane fuel has a higher activation energy and doesnt combust until hit with a spark. i am not sure what the threshold is for compression/octane, but i think 91-92 octane will hold to you about 11.5-12:1. if you put bigger injectors in all its going to do is reduce the pulse width because your o2 sensors are still going to be seeing the same a/f ratio, basically a bigger and shorter shot of fuel on every stroke.

Therefore i think there are a few things you can do. Higher octane fuel or some sort of additive (probably the best option) will definitly take care of detonation. I also think you can advance your timing to a certain extent. I have no idea how much, but i have heard of people doing this to take care of detonation.

Hope that kinda answered your question and hope others have ideas on detonation prevention.

[drift]

you dont want to advance your ignition to resolve detonation. that's silly.

retard the ignition 1-2 degrees to prevent detonation.

also, running more fuel will quench detonation. fuel's volatility is the main factor to efficent controlled combustion within high compression motors, but adding more fuel decreases it's volatility to a certain extent, by creating more gasoline to ignite at once.

you can do this merely by getting an adjustable fuel pressure regulator, like the B&M, Vortech, or AEM regulators. increasing your baseline fuel pressure by 5-10 psi should force enough fuel thru the injectors to prevent detonation on pump gas at 11:1 - 11.5:1 compression

[kane2g]

i guess i am going to go 10.5:1 route.
instead of getting fuel regulator couldnt i just get bigger injectors?

[SleeperTeg]

oops, i meant retard

[Tofuboy]

retard the ignition 1-2 degrees to prevent detonation.

detonation has nothing to do with spark timing, you can't prevent detonation by retarding your timing.

Knock and detonation is two different things.

You can only prevent knocking by retarding your timing. The reason you have knocks is due to your engine has excessive advance timing and your piston is fighting against the expending gas before it reach to TDC.

Detonation is due to excessive compression (heat) or in some cases unstable A/F mixture, and this causes the A/F mixture to ignite before your spark takes place.

[drift]

Originally posted by Tofuboy


detonation has nothing to do with spark timing, you can't prevent detonation by retarding your timing.

Knock and detonation is two different things.

You can only prevent knocking by retarding your timing. The reason you have knocks is due to your engine has excessive advance timing and your piston is fighting against the expending gas before it reach to TDC.

Detonation is due to excessive compression (heat) or in some cases unstable A/F mixture, and this causes the A/F mixture to ignite before your spark takes place.

that is incorrect. knock is minor detonation caused by insufficient fuel charge to match the air charge resulting in an uncontrolled burn. retarding the timing reduces dynamic compression, resulting in less power and also reduced octane/fuel requirements.

detonation can be caused by many factors.. but the leading factor is due to a lean fuel charge at high compression levels. your definition of "detonation" is the definition for dieseling... when the combustion chamber and compression cause the fuel to ignite without spark, which causes your motor to continue running after the ignition is switched off.

ultra high compression and lean fuel mixture will cause knock/detonation, uncontrolled burn. in the case of detonation, the lean fuel mixture with a lot of air is more volatile than a mixture with proper amounts of fuel added. basically, an uncontrolled burn caused by the fuel's flashpoint, and not by true controlled combustion

[Tofuboy]

that is incorrect. knock is minor detonation caused by insufficient fuel charge to match the air charge resulting in an uncontrolled burn. retarding the timing reduces dynamic compression, resulting in less power and also reduced octane/fuel requirements.

That is why I said this:

You can only prevent knocking by retarding your timing. The reason you have knocks is due to your engine has excessive advance timing and your piston is fighting against the expending gas before it reach to TDC.

It's true that knocking is a form of detonation, but that form of detonation is cause by over setting your advance timing or lean fuel mixture. In reality, under normal circumstances the ECM will automatically set your A/F ratio, therefore in most cases the knocking is cause by advance timing.

What I meant that they are two different things is, you can only prevent knocking/Ping by retarding your timing (if you have over advance your timing), and you cannot prevent detonation (cause by ultra high compression) by doing so.

Drift-I am not sure what do you mean by "retarding the timing will reduces dynamic compression." To my understanding, compression is a fixed process, you can't reduce compression by changing timing. If you have excessive compression that causes the detonation, it doesn't matter if you retard the timing and that is why kane2g started the thread, I believe.

[drift]

Originally posted by Tofuboy
Drift-I am not sure what do you mean by "retarding the timing will reduces dynamic compression." To my understanding, compression is a fixed process, you can't reduce compression by changing timing. If you have excessive compression that causes the detonation, it doesn't matter if you retard the timing and that is why kane2g started the thread, I believe.

dynamic compression is the true compression ratio of a motor while in operation.

static compression... 10:1, 11:1 is the compression ratio of the cylinder with the piston at BDC as a ratio to TDC. dynamic compression is based on how much compression you're actually running when the spark plug ignites the charge.

retarding the timing retards spark, thus allowing the piston to move away from the cylinder head before ignition further than an advanced ignition, thus reducing the dynamic compression.


best example... take a 10:1 compression motor and a 12:1 compression motor. reduce the 12:1 compression motor timing full retard then stuff it with pump gas. guaranteed, it will run without detonation, depending on how much retard you can set... but it will only make as much power as the 10:1 motor. this is due to the fuel charge being lit off at a further distance from the head, therefore reducing the dynamic compression.

retarding the ignition prevents minor detonation by reducing a motor's compression thru a change in dynamic value. this works to a certain extent before retard is so far off that the motor fails to run properly. but this is how retarding can stop/prevent knocking without actually adding more fuel.


this is basically the "why" to your original response to the topic.

[drift]

Originally posted by kane2g
i guess i am going to go 10.5:1 route.
instead of getting fuel regulator couldnt i just get bigger injectors?

yes, but that's wasted money, considering your stock injectors on stock compression are only working at 60% or less of peak efficiency.

all injectors can pump up to 80% before they get stuck open.

if you need 80% on 12.5:1 compression, you've got fuel system problems. my prelude is 10:1 static compression with 7psi of boost on stock injectors.

[VTECseoul]

what would water injection do on N/A motors? i know that some ppl use it on S/C motors, like from Jackson Racing.

[drift]

Originally posted by VTECseoul
what would water injection do on N/A motors? i know that some ppl use it on S/C motors, like from Jackson Racing.

water cools the intake charge and creates density. for an NA car, it wouldnt do much unless it's a hot summer day... in cool weather, it wont help as the air cant get stuffed into the combustion chamber any more effectively without pressurisation.

[bjdm151]

Let me see if i can get this all,
Volatility has nothing to do with knock, Octane is the number assinged to a fuel in reference to its resistance to knock. Yes pump gas usually has a higher volatility (lower boiling points) than higher octane fuel but there is a difference when you get to race gas. You can look at the distialation curves of many differtent high octane fuels and see that theres is no correlation to from octane to volatility.
Dynamic commpression ratio has less to do with where you light the fuel than how much a/f mix you can put into it. The dynamic compression of an N/A street engine with 85% volumetric effiency will be much less than that engine tuned closer to 100 %VE or even possibly higher in turbo applications.
As far as the injectors why not go to a dyno shop and let them check your A/F ratio to see what you really need. I'm sure that dyno charge will be nothing compared to the cost of rebuilding that 12.something:1 motor if you screw it up.