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I've been trying to do a lot of research on boost lately because I'm planning on running a moderate turbo setup on my daily/light-duty-autox car in the future. I'm trying to get into the physics of things so that when I do my build, I'm not just slapping parts on and hoping for the best, but instead engineering a motor to work.

The question

My main question is this. I've been reading this article and while it's deepened my understanding of compression, it leaves me with this question: I know that engines running a higher static compression ratio require higher octane fuels to prevent detonation, so why do motors with higher effective compression ratios not seem to require higher octane fuels?

I usually hear about people running turbo setups and simply using regular pump gas and having no trouble, even though the effective compression ratio would be mucher higher than most naturally aspirated engines. For instance, the setup that I was considering would be a turbocharged Honda d16a6, which has a static compression ratio of 9.1:1, with 10 psi of boost, giving it roughly 15:1 effective compression.

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I might be answering my own question here, but a thought just occurred to me. Would this be because most turbocharged setups use some sort of intercooler, lowering the intake charge temperature? –  Annath Mar 22 '11 at 21:19
you will need higher octane fuel for that engine. See my hugely long answer for why. Tuned properly, you should be fine on mainstream premium (93 octane where I live). –  Bob Cross Mar 23 '11 at 15:42

3 Answers 3

up vote 16 down vote accepted

tl;dr: They do. It's just harder to tell how much.

The longer answer is that they do and that effective compression is failing you as an approximation for actual effects.

Think about detonation (AKA premature ignition of the fuel-air mixture). Normally we consider two causes: compression (the change in the space enclosed by the cylinder as the piston moves up and down) and temperature (e.g., measured temperature of the intake air).

In reality, there is only temperature.

Let's back all the way to the ideal gas law:

pV = nRT

where V is volume and T is temperature (in degrees Kelvin, remember!) and the rest are interesting constants that aren't germane to this discussion. Compression causes that V value to decrease and, therefore, the T value to increase, maintaining equality on both sides.

Think of it this way: you have a fixed temperature budget for a certain octane gas. If T gets higher than T_ignition, bang. So, as you point out, you can add an intercooler to the system, reducing the input air temperature.

Likewise, you can change the amount that V changes. This increases the amount of temperature increase that your engine can tolerate before detonating.

Now, adding a turbo on the intake air compresses the normal atmospheric pressure to something significantly higher, resulting in a change in those other constants that I previously brushed off (check turbo volumetric efficiency for more information) and increases the temperature.

That eats into my temperature budget. If I used lower octane gas, that would lower the threshold for detonation and, at boost, I could be looking at engine damage.

So, after all that, what do you do?

  1. Research research research: don't build in a vacuum. Copy other people's layouts or improve upon them.
  2. Measure your air intake temperature, before and after the turbo.
  3. Find the best gas that you can.
  4. Tune the engine computer to keep your engine from blowing up.

On tuning: one thing the ECU can do is add extra fuel to the mixture, thereby cooling the mixture down. Admittedly, using fuel as a coolant is not conducive to absolute efficiency but shouldn't be an issue when driving around out of the boost. As always, less right foot = less gas spent.

All of the above is discussed in Corky Bell's Turbocharging book Maximum Boost - a very entertaining read for geeky people like myself.

Following up some time later: I just noticed the specific question about 9.1 static compression ratio running 10 psi of boost. As an example, my WRX runs 8:1 at about 13.5 psi so, on the face of it, 9:1 with 10 psi seems achievable.

Let's look at one of the more arguably sensible equations for effective compression ratio (which, as we noted, is still an approximation of fairly complex thermodynamics):

ECR = sqrt((boost+14.7)/14.7) * CR 

Where ECR is the "effective compression ratio" and CR is the "static compression ratio" (what you started with before adding boost). boost is measured in psi (pounds per square inch). Remember, the goal of this equation is to tell us whether our proposed setup is feasible at all and will it be able to run on gas that I can purchase on the street vs. the racetrack.

So, using my car as an example:

ECR = sqrt((13.5 + 14.7) / 14.7) * 8 = sqrt(1.92) * 8 = 11.08

Using this equation, the implication is that my effective compression ratio is about 11:1 at peak boost. That's within the bounds of what you could expect to run a normally aspirated motor on with pump gas (93 octane). And, proof by existence, my car does run on 93 octane just fine.

So, let's look at the setup in question:

ECR = sqrt((10 + 14.7) / 14.7) * 9.1 = sqrt(1.68) * 9.1 = 11.79

As cited in the reference, 12:1 is really about as far as you can go with a street car so this setup would still be within those limits.

For completeness, we should note that there is also another ECR equation that wanders around the internet that omits the square root. There are two problems with that function:

  1. First, that would result in an ECR for my car of 15:1. That's a bit ridiculous: I wouldn't even want to start a motor like that with street gas.

  2. ECR is an approximation anyway: the real answer to the question of "how much boost can I run?" is derived from critical factors such as intake air temperature and compressor efficiency. If you're using an approximation, don't use one that one that immediately gives you useless answers (see point 1).

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+1 This is exactly what I was looking for, thanks! That explains a lot. –  Annath Mar 23 '11 at 17:40
@Annath, you're welcome. –  Bob Cross Mar 23 '11 at 19:27

One of the reasons that a turbo setup with the equivalent effective compression is more forgiving of low octane gas than than a static compression setup is that you're not at that compression ratio all the time. Take that honda, for example. At 9:1 static ratio, you can run 87 octane all day as long as you don't push any boost at it. When you do start pushing some boost down its throat, the knock sensors will go off and the engine SHOULD respond in various ways - perhaps cutting fuel, spark, or retarding timing, which should force the boost down (not that I recommend it).

In the static compression case, even when you're just trying to idle or drive nicely, you're still going to be predetonating on lower-than-required octane gas. This would apply to clutchless superchargers as well, there's no "off" switch or "I'm driving nicely" benefit. You're locked into that higher compression ratio.

Again, not to recommend the practice, I had a 270hp Ford Probe 2.2L turbo, and at full boost (~21psi) and 7.8:1 static compression ratio, I would never dare attempting to reach it on anything but 93 octane. However, sometimes on long trips I would fill up with 87 octane and set my boost controller at 7psi or lower, and not get any knock sensor activity logged. Even if I didn't lower the boost controller, you can just 'drive nice' if you want to risk it (but the temptation is rather strong). I was able to get 36MPG out of 87 octane when I was nice to it (quite economical). I compare that with my father's supercharged 427hp 4.6L V8 gets 12MPG when you're nice to it, 8MPG when you're not, and you don't have the option of anything but premium.

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'When you do start pushing some boost down its throat, the knock sensors will go off and the engine SHOULD respond in various ways" - right, you're hoping that a reactive system will detect the symptom and react in time to prevent catastrophic damage. The problem is that it just isn't going to work in time for some non-zero percentage of the situations. –  Bob Cross Apr 18 '12 at 18:02
Oh I 120% agree, and I put in twice I don't recommend doing this - it is, however, why you can 'get away' with lower octane gas in turbo/some supercharged setups that you could not in a static compression ratio setup. –  Ehryk Apr 18 '12 at 18:24

In addition to good answer by @Bob:

There are some tricks that can be used to ease the problem:

  • A knock sensor for detecting premature detonations (and adjusting boost pressure). E.g. Saab APC allows safe use of lower octane fuels.

  • Injecting water to cool the combustion chambers (instead of excessive fuel)

  • Per cylinder exthaust thermometers (and sequential injection / ignition)

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