Any Benefits To Aftermarket Distributor

FastDriver said:

10.75 can run 87 octane, but what would be the point? He'll have to pull timing which will lose both efficiency and power. He might be able to get away without pulling timing, but would not be able to put the car under load. I'd be willing to bet that his cam has plenty of overlap, and that would definitely help with his SCR, which is more important than the DCR. BTW, the pressure before ignition doesn't matter. We're not worried about preignition, what we're concerned with is detonation.

Oh and 90LX, to me that's a mild engine combination so long as you're not running a radical cam. Running 12 or 13 to 1 would qualify you as not being mild to me and to most others. There are production cars running around 11:1 on pump gas, and I would certainly call them mild. Manufacturer's recommendation on my Corvette is at least 91 octane and it's running 10.1:1, does that mean it's not mild? Of course not.

Chris

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The difference in the new production cars that run "high" compression ratios is that they are fitted with knock sensors that ultimately control ignition advance, and variable valve timing that can also influence combustion characteristics. A traditional pushrod motor like 90lxcoupe's simply does not have the kind of technology to compensate for using low octane fuels. Like you said, he would probably have to de-tune the motor to run 87, and even then it might not work depending on his cam specs, operating temperature, combustion chamber geometry, etc.

And it IS the pressure (and temperature) in the combustion chamber before ignition we are concerned with, as that influences how the combustion process is going to occur with various levels of octane (or other compression-ignition resistant additives) present. I'm sure you understand this, but maybe you misunderstood my statement earlier.

I'm sorry that this thread is going waaaay off topic, haha. As much as I hate to admit it though, I love a good debate.

NikwoaC said:

The difference in the new production cars that run "high" compression ratios is that they are fitted with knock sensors that ultimately control ignition advance

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And what does the engine do when it senses knock? It pulls timing and adds fuel, which ends up exactly the same as what 90lx would likely have to do. There's nothing unsafe or less drivable, and it isn't as if tuning with an MSD Digital 6 or better box takes any time at all. Furthermore, knock sensors are not a prerequisite... they just make the tune idiot-proof.

and variable valve timing

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No offense, but it seems like you're trying to cloud the issue with technological mysticism. Variable timing doesn't change the need for octane in a high compression engine. Now you have 2 cam profiles that your engine will have to be able to run on without detonation, and interestingly the more aggressive profile is typically the least likely one to detonate on a given tune. It isn't as if the engine senses knock and then switches to a different cam profile to eliminate it.

A traditional pushrod motor like 90lxcoupe's simply does not have the kind of technology to compensate for using low octane fuels.

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But those other cars don't have the technology available to 90lx in his ability to manually control the tune. He can simply have seperate tunes for different types of fuel. In fact, he could simply switch to a new tune and run E85 if his fuel system were up to the task. Let's say he's going on a long trip, he could set up one tune to run 87 and another that ran premium. He might find that its cheaper to run 87, but when he wants to go back to 93 he just waits until the tank is low, refuels, and switches back to the more aggressive tune.

Like you said, he would probably have to de-tune the motor to run 87, and even then it might not work depending on his cam specs, operating temperature, combustion chamber geometry, etc.

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I respectfully disagree. The probability that he'd have any issue running 10.75:1 on 87 is nonexistant with enough timing pulled (and it wouldn't be as bad as you think). Plus, detuning the motor isn't some involved process. They actually make selectors he could mount in the cabin that allow you to switch from one tune to the next.

And it IS the pressure (and temperature) in the combustion chamber before ignition we are concerned with, as that influences how the combustion process is going to occur

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You said that you'd like to see him stand by his statement that he can run 87 octane in his engine because you were skeptical. Now you're striking me as the know-it-all type with no hands on experience tuning cars. If that's the case, then you're probably not interested in learning anything from guys like us, but I'll give it one more shot, and then you can choose either to learn graciously or to twist words and save face. I'm cool with either.

I don't know why you're sticking to your guns about the pressure/ temperature before ignition, but I'll say it again with more detail and clarification. It follows from the fact that we are trying to avoid detontation, as opposed to preignition per my last post, that our concern is the pressures and temperatures after ignition. The pressure and temperature prior to ignition has no direct impact on the likelihood of detonation or even on the pressure and temperature after ignition. What we are actually concerned with, if we are trying to avoid detonation as I believe is the point of this discussion, is the pressure and temperature of the end gases after ignition. This is where detonation occurs. By retarding the spark timing, we directly reduce the peak pressure/temperature of the end gases because the point at which the flame front approaches those end gases will happen later into the combustion stroke. By making this happen later, there is more volume in the cylinder as the flame front approaches the end-gases. That means that they will be lower pressure/temp. Hence if the timing is retarded enough, the end gases will not reach the detonation threshold and will burn uniformally until the flame front reaches the cylinder wall instead of the end gases all combusting at once at the detonation threshold.

Though these really aren't necessary for the topic of our conversation, here are some tuning and building factors that help to reduce detonation on high compression engines:
- tight quench, increases squish or swirl (or whatever vocabulary term you prefer to use) and reduces the likelihood of detonation.
- non projected tip plugs are cooler and so they resist det on race motors.
- debured and polished combustion chambers
- Aluminum heads conduct heat away from the cylinder much more quickly, which suppresses detonation
- long cam duration and overlap kills cylinder pressure, especially at low rpm
- a small cfm carb or intake reduces volumetric efficiency thus limiting peak cylinder presssure at higher rpm. ( high compression helps make up the diffference)
- richer mixtures cool the burn and help suppress detonation though at the cost of a little power. At high temps in the cylinder the rich mixture still has fuel trying to vaporize, which absorbes thermal energy that would otherwise increase the mixture temp
- Grooves cut on the head quench decks causes high swirl @TDC and increased mixture quality which allows a little more cr without knock
- At high RPM, detonation is less likely for several reasons: high piston velocity causes a better mixture, the flame front speed is the same, but the piston speed is much higher so the flame front reaches end gases further into the combustion stroke, at high RPM volumetric efficiency typically decreases, which results in lower cylinder pressures.

If you take advantage of all of these factors, you can run engines over 12:1 on 87 octane. The thing you'd have to ask yourself is why? Just about everyone willing to go this far, is trying to make power. You'll always be able to make more power with higher octane fuel and a good build. Why would you limit yourself to 87?

Chris

Here's an article that might interest you: 412 HP 350 ci AFR 190 Cylinder Heads, P4

FastDriver said:

You said that you'd like to see him stand by his statement that he can run 87 octane in his engine because you were skeptical. Now you're striking me as the know-it-all type with no hands on experience tuning cars. If that's the case, then you're probably not interested in learning anything from guys like us, but I'll give it one more shot, and then you can choose either to learn graciously or to twist words and save face. I'm cool with either.

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Is it too late for me to save face now?

I'm worried that you've already passed judgment on me, and anything I say from here out will make me sound like an *******. Maybe, but bear with me here.

Is it true that I'm young and young to the game, especially compared to some of you StangNet veterans, but then again I'm not just some kid sitting here with a Hot Rod magazine. I do not question your automotive knowledge. Notice in my above posts I was not directly disagreeing with what you said, but instead offering conversation-stimulating responses.

The pressure and temperature prior to ignition has no direct impact on the likelihood of detonation or even on the pressure and temperature after ignition

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This statement is misleading. Saying that the fuel/air mixture pressure and temperature before ignition has no direct effect on the pressure and temperature on the gasses after ignition would infer that adding a supercharger or turbocharger to an engine has no direct affect on power output, and ignores the thermodynamic fundamentals of the Otto Cycle altogether. I accept and agree with most of the information in your post, but I feel like you're fighting my P&T statement just so you have something to combat me with.

I am a bit of a know-it-all, but I'm dealing with it, and hopefully getting better. My girlfriend reminds me about it every day, believe me.

My efforts at "technological mysticism" were to avoid the work involved with posting a 500 word essay to get my point across. You did just that. Good info, by the way. I will read that article.

P.S. PM sent.

Ahh, I just realized that article is from Hot Rod magazine, haha. I'm a Jackass.