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Dear Dave


Dave Coleman
Sep 1, 2007
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Direct 'Dear Dave' tech letters to will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.

Q. Who Wants it Easy?
I have a few questions on the topic of compression and forced induction and how compression ratios relate to power in naturally aspirated and forced induction engines.

I'll start with the compression questions. When you up the compression ratio of an engine, from say 9.5:1 to 11.5:1, you see a pretty immediate boost in power and the opposite would apply to lowering the compression ratio, right? Well, when turbocharging a car, wouldn't it be counterproductive to lower your compression ratio? Since nine times out of ten, you add the turbo in search of more power anyways. I've been told that lower compression ratios are more forgiving and easier to tune when it comes to forced induction, but is it really worth forsaking 25-plus hp just to have an engine that's easy to tune?

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Also, I'm wondering what the actual challenges would be when tuning a turbocharged engine (specifically the 1.8-liter Mazda BP from the Ford Escort GT) with a higher 11.5:1 compression ratio at a relatively high boost level.

As far as useable horsepower in an autocross environment, my fellow tuners at have mentioned that a high-compression turbo build would tend to spool quicker and offer more off-the-line performance than a standard or low-compression build.Jason LightfootCrackleton, WY

A. Let me explain what "easy to tune" means. On an easy-to-tune engine, it's actually possible to find an air/fuel ratio and an ignition timing setting that produces power without causing detonation or raising exhaust temperatures to levels that will liquefy metal. On a difficult-to-tune engine, like a BP with 11.5:1 compression and 15psi of boost, there simply isn't a workable combination, leaving you the choice of pinging to death, melting something, or drowning the engine with so much fuel that it doesn't make any power.

Raising the compression ratio increases both the pressure and temperature of combustion. While this is good for power output, there is a limit to how much heat and pressure the air/fuel mix can handle before it takes matters into its own hands and ignites on its own. This is called pinging, and it's the sound you hear right before you rebuild your engine.

An engine on the verge of pinging can be tuned down from the edge in two ways. A richer air/fuel ratio will burn cooler, reducing the tendency to self-ignite, but it will also make less power.

The other option is to retard the timing. Ignition usually has to begin before the piston reaches top dead center to give the combustion enough time to reach peak pressure while the piston, rod and crank are in a mechanically optimal part of the power stroke. Retarding timing can let the piston move further down the cylinder before peak cylinder pressures are reached. Moving down, of course, increases the volume of the chamber, which decreases the pressure. Less pressure means less likelihood of detonation, but it also means far less power.

Now, take that 11.5:1 engine, which is already running rich and retarded just to keep its insides in, and pump a bunch of hot, high-pressure air into it. What do you think will happen in that combustion chamber? That's right, it will get less easy to tune.

Now, on the topic of boost response, your web.friends have a point about higher compression reducing lag, but not for the reason they think. A higher compression ratio also means a higher expansion ratio. That is, on the power stroke, there is a greater difference in volume between top dead center and the point, somewhere near the bottom, when the exhaust valve opens and power production stops. This higher expansion ratio extracts more energy from your burning fuel (which is why high compression is actually more efficient), and actually leaves less energy in the exhaust to spool your turbo.

The only reason, then, that higher compression usually reduces lag, is that the engine makes more power off-boost, and can therefore accelerate to a boost-producing speed more quickly.

Ultimately, if you want good boost response, good power, and an engine that won't blow up, you're better off with a modern, efficient turbo sized for reasonable power goals, and low compression that's easy to tune.

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Q. OBD Dyno?
What do you think of using OBDII software to monitor modifications without going to a dyno? I use the MAF signal to determine if intake and exhaust modifications make a difference. On my old 1998 Volvo V70 T5, the MAF went from 27 lbs/min to 30 lbs/min after intake mods. I want to use the same tool for mods on my new 2007 Mazda5 and my friend's 2007 Mazdaspeed3, but he is doubtful.Randy WatkinsAlameda, CA

A. Logging OBDII outputs to test your modifications can be a good idea, but only if you think carefully about what you're measuring. On your Volvo, are you sure the increased MAF readings were because the intake modifications actually helped the engine move more air, or could it be because you changed how accurately the MAF reads? Different airflow patterns through the MAF and different pressure pulses bouncing around in the intake pipe can affect MAF readings. Maybe the engine is breathing the same, but the MAF is now reading high.

This is especially true of the Mazdas you're planning to modify next, as both of them have MAFs integral to the air filter box. These, like most Mass Airflow Sensors, measure only a small sample of air entering the engine and multiply that reading by the proportion of sample tube size to the total cross-section of the MAF housing.

Say the sample tube is half an inch in diameter, for example, and the intake pipe is 2.5 inches. The cross-sectional area of the sample tube's 0.2 inch2 is only about four percent of the MAF's 4.9 inch2. This didn't matter in your Volvo, since the MAF housing is a stand-alone piece that can be integrated into your intake, but eliminating the air filter box on either Mazda means making a new MAF housing yourself. If you just dropped that half-inch sample tube into a three-inch pipe, the sample would now be just under three percent of the new pipe's 7.1 inches2. The MAF would now read low by 30 percent, and the ECU would respond by running lean by a similar amount.

Now, if exhaust mods increase your MAF reading, you can be pretty confident that difference was meaningful, because the exhaust can't directly change the accuracy of the MAF. But, again, you still have to adhere to some reasonable test conditions. Make sure you test in the same place, in the same gear and with the same weather conditions. You should also do multiple runs with each set-up so you see how much the reading varies with no changes at all.

Finally, be aware of the OBDII tool's sampling rate. Data transfer is notoriously slow through that port, and reading multiple signals at once will usually make it even slower. If you're only sampling at 1Hz, for example, you have a one-second gap between readings. If your peak airflow lands between two samples, you'll never see it and you'll under-report your performance. In addition to configuring the tool to sample as fast as possible, you should test in the highest gear possible so changes in airflow happen more slowly and are more likely to be captured. Use common sense, though, and keep it under 100 in the school zones.

By Dave Coleman
94 Articles



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