AmT_T

iTrader: (11)

Stock injectors on a turbo II are good to how much hp? how much boost on stock turbo?

RotaryRocket88

iTrader: (7)

In general terms: not much more than stock power and stock boost. The stock TII fuel pump will take a dive before the injectors do, but I wouldn't suggest keeping either stock if you're running anything but stock S5 boost (8 psi).

Here's a generic calculator: http://www.rx7.com/cgi-local/3rdgencalc.cgi. Keep in mind it's just an estimate and a stock ECU will limit duty cycle to 80%.

markusparkus

iTrader: (1)

Hmmm.... According to the wiki, the stock TII has about 40HP more than the stock NA. That's roughly 5HP/PSI. The calculator said 550cc injectors at 80% are good for 265HP, or 100HP over the stock TII. Doesn't this imply then that you should be able to run 20psi boost before the stock injectors tap out? I know from reading that this isn't true, but I'm curious where the math goes wrong.

RotaryRocket88

iTrader: (7)

Your math is way too simplified for these kind of calculations. This thread has info on all the fuel calculations you could ever want: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/feeding-turbo-rotary-horsepower-airflow-fuels-881992/

A stock S5 TII has 200 BHP, which is 65 BHP less than the estimate given by that calculator. You can't really say 1 PSI = 5 HP, as this ignores many things such as airflow characteristics, intercooler pressure drops, air temperature changes, compressor efficiency, etc... A stock turbo would also only make 20 psi in a tiny window around 4000 RPM, and it would begin to nuke itself due to how fast it has to spin to achieve that pressure. The efficiency of the compressor at this point becomes ridiculously bad, and you would end up with some seriously hot air being forced into the engine.

Another thing ignored is the fact that the primary injectors can max out before the secondaries even come online. A stock turbo running more than stock boost can create a lean spot in the mid-range, which means a big risk of detonation. The stock ECU's 3800 RPM transition point is too late.

Also, if you look at stock turbo dyno graphs, peak horsepower is usually made around ~6-6.5k RPM and at something like 6 - 8 psi. That's just stock "peak" boost. But on a 100% stock TII, the restrictive intake and exhaust can drop the boost pressure below those numbers even.

markusparkus

iTrader: (1)

all these facts are so inconvenient to my little fantasy world.

I wasn't clear enough in my question, I think ( but I doubt this will help ). I'm assuming that the HP calculator is simply doing a calculation based upon how much fuel the injectors can release. Gross example: N number of X size injectors can release Y total fuel. Y total fuel when burned with the right amount of air releases a certain amount of energy - converted to HP.

So then if someone opens up their intake and exhaust and gets an aftermarket EMS and a turbo that can deliver enough air to the intake to burn the fuel dumped by the injectors at 80% duty cycle, then the stock TII injectors ought to be able to deliver around 260HP.

Seems like what I've read though says otherwise, since people at these HP levels seem to be going to larger injectors. Am I still oversimplifying it?

Where'd you learn all of this, anyway?

LunchboxCritter

iTrader: (9)

I would guess that bigger injectors running at 50% duty cycle are more efficient at doing so then the 550 ones at 80%.

Sort of like how 220v ac is more efficient at higher amp loads then 110v is.

ex: A welder running on 110v will cost more money to run then same said compressor running on 220v.

Honestly I'm just guessing.

markusparkus

iTrader: (1)

It's not a bad guess though.

RotaryRocket88

iTrader: (7)

Are you mixing up brake HP/flywheel HP with wheel HP? 90% of the time when people talk about making X HP, they mean power at the wheels. But all factory specs are at the flywheel. It's hard to say what the exact drivetrain loss is, but the best estimate I've found is 17% with a manual transmission. 260 BHP * 0.83 = 216 WHP. That calculator will give you flywheel HP.

And yeah, running injectors at a lower duty cycle is better for them. Giving yourself some headroom is best if you have some way to control larger injectors.

The best way to pick this stuff up is to just read a lot on the forum. The single turbo and aux. injectors sections have a lot of info, as well as the Rtek section. Owning an Rtek 2.1 helps too, since it allows data logging and a look into how the stock ECU works. Before moving my secondary injector transition point down, my data logs showed the primaries capping out at ~3500 RPM and ~10 psi. I moved the transition point down to 3100 RPM to solve the problem.

Evil Aviator

Yes, it takes a simplified injector mass flow rate and estimates HP by using a given Brake Specific Fuel Consumption (BSFC). Most calculators do not compensate for injector dead time and fuel density, and some do not even let you input the fuel pressure and/or BSFC. Regardless, the simplified calculators are still good for general planning, and when in doubt just round up to the next larger size injector.

Yes, 260bhp would be in the ballpark assuming that the engine is well-tuned, the engine system can operate at a BSFC to produce that output, and the fuel system could handle the increased load.

Lots of reasons for that. Here are a few...

Sometimes it is a good idea to use slightly larger injectors to allow for future upgrades.

Those who are not all that into math will just toss larger injectors into their car to make sure they will not run out of fuel. Oversized injectors may cause idle problems, may reduce engine efficiency (power and economy), may degrade throttle response, or may work just fine. Undersized injectors can destroy your engine. Given the two choices, the better one is quite obvious.

Most people on this forum are horrible tuners and buy cheapie parts that are not very efficient, so they will not get the same BSFC as a wealthy car owner who can buy top-quality parts and hire a professional tuner. Therefore, broke/ignorant/lazy people will need more fuel to make the same horsepower. This is why you should never try to keep up with the Joneses. Some of the RX-7s on this forum have well over $50,000 worth of parts and labor in them, so don't get upset if your car is not as fast as theirs even though it appears that you have the same modifications on your car.

A few aftermarket fuel computers drive the secondary injectors as a percentage of the primary injectors, so they are not be able to max-out the secondaries without over-driving the primaries. Also, most piggyback fuel computers are limited on how much they can increase the stock fuel injector pulsewidth.

Not all fuel injectors are created equal. Some are flow-rated at 43.5psi fuel pressure while others are rated at 36psi. Some are flow-rated with a test fluid which may flow more or less than gasoline. Some have a longer dead time than others. Some greatly vary their output at different pressures and pulsewidths, while others have a more linear output. Unfortunately, real life is not like a video game in which there is only one type of "550cc" injector that you can drag and drop into your engine with a compuer mouse and get an output of exactly 550cc/min.

You can learn this by taking classes at an engineering university, a local tech school that offers automotive tuning courses, or from specialized schools like EFI 101 or training seminars given by various EMS manufacturers.
http://efi101.com/
http://www.motec.com.au/trainingUS/datesUS/

I learned about engine performance at Embry-Riddle Aeronautical University.
http://www.erau.edu/
I also had some training at National Aviation Academy, which is a good school for those who don't want to spend the big bucks on a 4-year program at Embry-Riddle.
http://www.naa.edu/

markusparkus

iTrader: (1)

Followup question: For those of you that run aftermarket EMS, is there a place for you to input your injector size into the map calculations? That would seem like necessary information to do a decent tune job.

eage8

iTrader: (9)

yes, I would imagine. With megasquirt there is.

if there isn't in other EMSs you could just multiply the whole fuel map by the difference.

Evil Aviator

That would not work well because fuel injectors do not always flow to their exact rating specs. Most EMS computers have a main fuel matrix in which x = engine rpm, y = load (TPS, MAP, or AFM), and z = fuel injection time in milliseconds. Other inputs, such as fuel injector dead time, injector phasing, injector staging, fuel injector trim, etc., is usually addressed in secondary maps if available. Some EMS computers have a global trim function that helps reduce tuning time when changing fuel injectors, but the global change is only an estimate, so fine tuning is still required.

I think that maybe you don't understand how a fuel injector works. It has an electromagnet that opens and closes a valve, and that's it. The flow rating is based on how much fuel flows through the valve when the injector is open. If the fuel pressure is increased or decreased from the test value, then more or less fuel will flow through the valve than the rating would suggest. Therefore, the fuel pressure setting will affect the output, as will any spikes or lulls caused by the normal operation of the fuel system. Also, it takes a fraction of a second to open and close the valve, and this time is not taken into account in the rating even though it will affect the tuning of the engine. The open/close time is affected by the voltage it receives, so that is also a factor in engine tuning. Various brands and models of fuel injectors will have different operating characteristics, even if they may have the same static flow rating. The important thing to understand is that the fuel injector flow rating is a static value, and the dynamic use of the injector when your engine is running may not fully resemble that value.

Aaron Cake

iTrader: (1)

HPBooks just released "Performance Fuel Injection Systems" which is a great introductory guide to all things EFI. It is written by two of the DIYAutoTune guys; Matt Cramer and Jerry Hoffmann.

http://www.amazon.com/dp/1557885575?...H6SYK0185K0PZ&

While I have not read my copy, my skimming randomly through it has shown that it is indeed a great reference.

While you are there, also pick up a copy of "Street Rotary" which is a great introduction on rotary building and modding. Also by HP Books, written by Mark Warner, with contributions from little ol' me.

http://www.amazon.com/Street-Rotary-...3010522&sr=8-1