ignition timing, high octane and high compression

[boosted3g]

I run an 11:1 compression piston with 105 octane fuel. What i would like to know is have anyone run high octane fuel with an 11:1 or higher piston with a cdi box with advanced timing or the sparks timing key. I am considering purchasing a cdi and rectifier from RMStator but want to know how aggressive the stock timing curve is. RMStator is one of the few that will provide you with the advance which is 5 degrees across the entire rpm band. Is the 5 degrees getting too aggressive?

[GPracer2500]

The info I have for the stock ignition timing is:

Under 2000rpm: 8 BTDC
2000 - 4000: linear increase from 8 BTDC to 27 BTDC
4000 - 6800: 27 BTDC
6800 - 9100: linear drop to 26 BTDC

That info came from MSD. I did not measure it myself but have no reason to think it's anything but accurate. I know the OEM service manual confirms the low rpm timing of 8 BTDC.

Should I assume that 105 is a MON rating? I'd guess 11:1, 105 MON, and +5 advance could be compatible without detonation occurring. Of course, the cam specs and squish clearance will matter (along with a whole slew of other and usually minor things).

If it were me, I'd try it. Go find conditions that favor deto and test out the setup. Knock was easy for me to hear in my 400ex. With no water jackets or anything to muffle the sound it's hard to miss when your looking for it. If it knocks then maybe measure the squish and tighten it up as much as your comfortable with. Re-test and see if everything is happy then. If not, go to another fuel or go back to the stock ignition advance. That would be my approach.

$0.02

[boosted3g]

Thanks for your response GPracer2500.

I would assume that the octane rating is the standard pump calculation (R+M/2) since it is pump e85. I dont know what the MON rating of e85 is since i feel it in more important since these engines rev high. I forget my squish clearance when i dropped the piston in but i do remember it was over the optimal .040. You seem to have a lot of info at hand. Do you know the rod ratio of a 400ex or even the rod c to c length. That could also play a part in this.

[drew416ex]

Dont use the advance key from sparks on compression any higher than stock, it will over heat. As far as a cdi and rectifier, i havnt had a problem.

[boosted3g]

Im not worried about heat, im more worried about knock. You say the cdi and rectifier are ok. The sparks key will do the same thing as a remapped cdi. Only it will deliver 6 degrees of timing rather than 5. Do you have higher than stock compression when using your cdi and what fuel do you run?

[drew416ex]

I have run the cdi with 11:1 on 93 octane and 12.5:1 on 110 octane with no worries. According to sparks the key does something different than the cdi but I dunno. I have the same opinion as you on that. They say you can use the key with any mode except higher compression and/or big bore piston. I have too assume they know what they are talking about on that.

[TRXRacer1]

Originally posted by boosted3g
Im not worried about heat, im more worried about knock. Heat is the sole reason for knocking assuming that you are equating knocking with predetonation. If you are worried about predetonation then you are indeed worried about heat. Higher heat is the reason for higher octane, all these things being discussed here are just contributing factors to more heat.:chinese:

[boosted3g]

I will rephrase, with my fuel of choice heat is not an issue. Since the switch to e85 the engine temps have been significantly lower than when running 93 pump. When doing an oil change i can idle for 10 minutes and the oil is just starting to get warm.

[GPracer2500]

Originally posted by TRXRacer1
Heat is the sole reason for knocking assuming that you are equating knocking with predetonation. If you are worried about predetonation then you are indeed worried about heat. Higher heat is the reason for higher octane, all these things being discussed here are just contributing factors to more heat.:chinese:

Sort of. End-gas temperature lies at the root of detonation. But so does time---->as in how long the end-gas must spend at X temperature before the flame front gets there to consume it. Plus, it's hard to draw a consistent correlation between engine temperature and end-gas temperature. A cool running engine can knock if localized end-gas temperatures are high enough. Or, an overheated engine may never knock, again because end-gas conditions won't allow it.

Basically, yes--I agree that engine temperature and engine knock share some connection. But I'd argue it's a mistake to so closely connect them that they always walk hand in hand.

Also, you should stop using the word "predetonation". The term is detonation. "Predetonation" is nonsense term that arises from a confused combination of the terms detonation and pre-ignition. Deto and pre-ign are separate phenomenon. They are both abnormal combustion events, but are distinct phenomenon.

[GPracer2500]

Originally posted by boosted3g
....Do you know the rod ratio of a 400ex or even the rod c to c length...

Nah, I sure don't.

[flauge]

Originally posted by GPracer2500
Sort of. End-gas temperature lies at the root of detonation. But so does time---->as in how long the end-gas must spend at X temperature before the flame front gets there to consume it. Plus, it's hard to draw a consistent correlation between engine temperature and end-gas temperature. A cool running engine can knock if localized end-gas temperatures are high enough. Or, an overheated engine may never knock, again because end-gas conditions won't allow it.

Basically, yes--I agree that engine temperature and engine heat share some connection. But I'd argue it's a mistake to so closely connect them that they always walk hand in hand.

Also, you should stop using the word "predetonation". The term is detonation. "Predetonation" is nonsense term that arises from a confused combination of the terms detonation and pre-ignition. Deto and pre-ign are separate phenomenon. They are both abnormal combustion events, but are distinct phenomenon. Man every time I read your posts I learn something..
You da man GP!

[TRXRacer1]

Originally posted by GPracer2500


Also, you should stop using the word "predetonation". The term is detonation. "Predetonation" is nonsense term that arises from a confused combination of the terms detonation and pre-ignition. Deto and pre-ign are separate phenomenon. They are both abnormal combustion events, but are distinct phenomenon. People add the "pre" to detonation in a slang sense to signify that the detonation is premature. Most commonly but not in all cases extra high combustion chamber temperatures prematurely ignite the fuel mix before it is time causing this knocking. This added heat can come from many things including added compression, advanced timing, poor plug choice and even carbon build up. Although I'm feeling you with your explanation, heat is what it's mostly about.

[GPracer2500]

Originally posted by TRXRacer1
People add the "pre" to detonation in a slang sense to signify that the detonation is premature....

Therein lies some of the confusion. "Premature detonation" suggests that there is some point when detonation is suppose to occur. When a gasoline engine is operating properly there is no detonation at all, ever. A normally burning mixture deflagrates--it does not detonate. The occurrence of any detonation whatsoever is abnormal. So, adding "pre" in front of detonation doesn't make sense. A mixture is either burns normally (deflagrates) or at least some part of it does not burn normally (detonates).

I realize people say "predetonation" all the time. I even see it in print occasionally. But it's not slang. It's a misunderstanding of what the terms mean.

Not trying to harp on ya--I just get frustrated with this subject being mangled so much. I don't have any problems with slang or jargon. But I no longer consider something to be slang or jargon when it misrepresents.

$0.02

[TRXRacer1]

Originally posted by GPracer2500


Not trying to harp on ya-- No problem! You make a good point and simple misrepresented things can be frustrating to anyone.

[boosted3g]

I guess im going to place the order tonight and try it out. Whats the worst that can happen. Honda makes more replacement parts everyday. Ill post the dyno when im finished tuning, this stock bore might surprise a few of the bigger bores out there. I ran out of fuel last week with the stock needle so ill be heading back with the HRC needle and this cdi box.