General question about FI
 

Before my Z I owned 2 Miatas ('99 and '06) and I toyed with the idea of a FI system for both though I never pulled the trigger. In general on those cars the FI systems, either supercharger or turbo, made around 6-8psi boost with upgrade options to get you into the 10-12psi range. That was the realistic max unless you wanted to start changing out large portions of your engine hardware to handle the strain. A quick look at a couple of systems for the Z seem to generally be in the same boost range.

I just finished reading an article on Inside Line about the new Sonata 2.0 Turbo. It makes 17psi boost. That seems ridiculously high to me. Then I remembered the turbo version Solstice had something like 18psi boost (I can't remember exactly). Why can these cars handle so much boost? I realize they are built from the factory as turbos but still that is a large difference in boost.

it comes from the factory boosted so the engine is built to withstand those type of numbers. The stock sti's run about 14 psi. There engine are built to withstand those numbers. Our cars werent built for boost straight outta the factory but I honestly wished they did.

stop thinking in terms of PSI... PSI is just a measurement, it doesnt equate to a static amount of horsepower or gains.

factory built turbo cars have engines that are built specifically for turbos. they have a lower compression ratio and actually require their boost to make any decent power. if you ever drove a evo or a sti, they drive like they have 50hp gokart engines until you hit full boost... a stock civic drives peppier in regular driving... this is because the compression ratio is so low in the turbo cars, drastically reducing how much power the car makes when the FI system is not pushing enough airflow to cause positive pressure.

a higher compression creates more efficient power from the same amount of air/fuel mixture. this is why the NA engines feel so much better when driving around town. Because of this, NA engines, when boosted, make far more power per psi (or more importantly, per airflow volume). They actually need less airflow to produce the same power as factory turbo engines.

This comes with a downside though. Higher compression engines, coupled with the increased heat of pressurized airflow, offer a far less stable air/fuel mixture. It is much more likely to pre-ignite in the cylinder, causing engine damage. This is why NA cars turned to BOOST have to be very precisely tuned/calibrated to not cause engine damage.

Its a compounding issue since NA engine internals are not as structurally beefy. because they use lighter parts to support making power at higher rpm (to somewhat make up for no boost), and because the pistons are typically built around tighter clearances for better emissions and longer engine life, rather then containing mass amount of HP.

NA engines and Turbo engines, while mostly the same, have slightly different specs throughout most major components.

To answer your question simply, why turbo cars run so much boost from the factory and dont really make all that much HP compared to boosted NA engines.. its because they have to. Between usually lower engine RPMs and heavier internal components, lower compression ratio, and camshaft specs... THEY HAVE TO or they would be slow as dirt. Take the turbo off a turbo car and you have redefined SLOW.

Also, those factory cars are generally running smaller turbo's. #'s of boost by itself is kind of an irrelevant number if you don't consider the size of the turbine, etc.

Please, only read phunks post, its perfect. That's all the info you need and its correct.

That makes sense phunk, while I understand compression ratios I didn't realize the factory turbos used different ratios than the same model car without turbo. I just equated psi boost to power because that seemed to be the biggest factor in the aftermarket units. With the '06 and later Miatas they did have trouble with pre-ignition because the compression ratio on the car was something like 11 to 1. They did finally overcome the problem some how but it took a while. Is pre-iginition a big problem with our Zs and FI systems?

rcz; thanks lol

axeman; typically, engine component strength will run out on the NA engine before you hit a dead end with pre-ignition. but, that only applies with proper tuning that has to be extremely conservative and leave plenty of margin for the factors mostly outside of your control (environmental conditions and fuel type/quality). Its important for owners of boosted NA setups to keep a very watchful eye on their tune as factors change and even as time goes on. Due to the poor fuel control in the factory gas tank in this car, i predict a lot of hurt engines due to lean spikes in hard turns.

Remember, a GT25 turbo at 8psi boost flows FAR less air than a GT60 at 8psi ;)

Second that....Phunk nailed it!

where is the boost pressure measured? how can gt60 flow more air at the same boost pressure? Pv=nRT, gas constant R, temperature T and volume of the engine V stay the same, so P = n, n is the number of mols of gas molecules, more air flow means higher mols of gas molecules so n goes up. If all the other variables stay the same, increasing the number of mols of gas molecules means increasing pressure. Right?

You are missing time in your equation. ;)

Pressure * Volume/time = #moles/time * Gas Constant * Temperature

GT25 = 22lbs/min@8psi at the choke line on the compressor map
GT60 = 125lbs/min@8psi at the choke line on the compressor map

Remember that horsepower is work over time. The more airflow you have (cfm, lbs/min, etc.), the more power you make provided you have enough fuel.

Boost pressure is the measure of restriction to airflow, which is why it is practically meaningless when trying to determine horsepower. Lbs/min and cfm are what matters...again, because you are relating a time based unit to a time based unit (airflow to horsepower).

Back on topic, Phunk nailed it.

Couldn't have said it better myself :tiphat:

so is it meaningless to say how much boost can the engine interals handle, rather is how much horsepower can the engine handle. Because different turbos at the same boost can produce different horsepower(different airflow.).

That is correct.