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How Much Boost?

Old 05-30-2005, 01:09 PM
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how much boost can be safely ran on a 13B-T? engine is a stock j-spec engine with 35,000 miles. how much do they run stock? i'm gonna use a camden blower for autocross and dont really know how much boost i should run.
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Old 05-31-2005, 04:02 PM
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The stock ECU and injectors cannot handle much more than stock boost (~6 psi on an S4, 7.5 psi on an S5). They might allow as much as 8-9 psi with a fuel cut defender, but without some form of additional fuel control (and additional fuel injection, whether it be larger injectors or more of them) you'll rapidly run lean and detonate, destroying the engine.



Note that these values are for intercooled air, as well. Most blowers don't intercool and that is a BAD thing for rotaries. If you're gonna boost, cool it.
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Old 05-31-2005, 05:24 PM
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meh its all relative all i had was a walbro, and a small-ish t04e, straight exhaust an cone filter. i was boosting ~13-16psi on average and it wa slike that for 60k miles, never blew up, i just tore it down to start a new project with the motor.



what your car can handle depends on how you throw it at it.



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Old 05-31-2005, 09:01 PM
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i'm using a holley for fuel management now so injectors and ecu will not be an issue. what im trying to figure out is how much boost is reliable. i see people running anywhere from 6 to 20 psi. thanks for the stock boost numbers, that will help. also, how many degrees of advance are you all running on boosted engines?
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Old 05-31-2005, 11:21 PM
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A boosted rotary running a holley carb... heh... ok. Depending on your setup, your afr and timing, the way you drive, how you define reliable, and how you treat the engine, anywhere from zero boost to 21+ psi can be "reliable".



Advance varies based on air temps, type of fuel control, level of boost, and ignition setup. Most people don't run more than 15-20 degrees under boost, some run far less, some run a bit more. This is one of those areas you really need to study 'cos it'll blow your engine in a heartbeat if you aren't careful. There's no hard and fast rule, its your engine on your setup that determines how much is "safe".
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Old 06-01-2005, 12:15 AM
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Maybe I'm dumb, but boost is very relative...shouldn't we be talking about flow?
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Old 06-01-2005, 02:14 PM
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[quote name='Dysfnctnl85' date='Jun 1 2005, 12:15 AM']Maybe I'm dumb, but boost is very relative...shouldn't we be talking about flow?

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[/quote]



thats what i was trying to get at, but some dont really understand turbos, everyone thinks psi= power.



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Old 06-01-2005, 03:44 PM
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Well according to thermo- and fluid dynamics, its not even flow. Its air mass. Mass of a given "stream" of air is governed roughly by PV=NRT although air is not really an ideal gas, its close enough to illustrate the concept.



The volume of your intake manifold is fixed. The volume of air your rotor ingests per cycle is fixed. That's V. What makes power, for us, is the N, or number of moles of air (technically the amount of oxygen but we're assuming that the air has a fixed percentage of oxygen in it). So that leaves only P and T that we can change with our forced induction device.



Its well known that boost raises the temperature of the air. A higher T naturally cancels out a higher P in that equation, thus if we raise the P and the T by the same factor (T being in absolute temperature), we get pretty much no gain in N. To sum up in laymans terms, the more efficient your compressor is, the cooler the output air, the more dense it is, the more air it will pack in at THE SAME PSI.



So what you should really be concerned about is compressor efficiency at a given boost and flow rate. Again, remember, the VOLUME of air is roughly constant w/r/t everything else at a given RPM. Its the density of that air that determines the mass. And mass is what gives you power.



Thus, a stock turbo puffing out 12 psi of air will be heating it FAR more than a T60-1 compressor puffing out the same 12 psi. Thus the T60-1 will make more power, and probably be safer for the engine since the air is cooler and thus less prone to detonating.



The moral: bigger turbos provide safer power, and more of it, at the same time.
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Old 06-01-2005, 08:40 PM
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okay then, let me rephrase. how do i start setting things up so i dont blow the engine. i would assume that running a modest vacuum advance and an 8lbs pully would be a good place to start. since the engine was already set up for 6 to 7 psi. then i will have to watch the afm and jet the card accordingly so i dont get to lean. am i on the right track, what else do i need to watch? since there is no other way to aftercool a roots blower ive been looking into water cooling the intake below the blower. how do i know if im getting detination?
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Old 06-01-2005, 11:35 PM
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You need to figure out CFM or lbs / hour from the supercharger.

THEN, you need to figure out intake temps versus boost psi.



Without those numbers, we are only guessing what is "safe".



I personally wouldn't want to run anything over 10psi with an SC for your application.





-Ted
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