RotorMotorDriver

iTrader: (1)

Ok, heres the deal. Im just trying to learn for a turbo setup I wanna build on my 12A far into the future. My plans are a mid to low 12 second car, but if thats not do-able, then just something fun .

Fuel System:
I want to use a TWM throttle body on a Weber wrap around intake. The problem Im having, is figuring out what size injectors and stuff Im gonna need. Does anyone know some good websites for figuring out injector size? I went to RC Engineerings website, but I dont really know what all that means, and I dont really know what Im doing.

Turbo:
I want to put a turbo on this thing, but I have no idea what to shoot for. I was told to just put a 2nd gen turbo on there for now and call it good. Well thats fine and dande until I want to upgrade (remember, far in the future), so what do I need to know about turbos to choose the right one for my application? Does anyone have any good links or info for me?

So, there are my main questions, I hope someone can help. Thanks guys .

~T.J.

EDIT: Basically, Im trying to build THIS car, only I would be happy with 12 sec 1/4 miles . I noticed hes using 4 injectors with the TWM (2 in the throttle body, 2 in the manifold), so how do I figure out the primary injector size, the seondary injector size, etc. "You must teach me...How!!" Thanks .

Jesuscookies

iTrader: (1)

Here some info that I found very useful, for injector selection, and fuel pump help.

http://www.maxcooper.com/rx7/how-to/...tem/calcs.html

Check the entire site out. Max has some very useful information on it.

82streetracer

iTrader: (1)

use a stock tII turbo and make that work. There cheap.

when you want more power look into BNR hybrid upgrades. if you make the stock turbo work, these will bolt in.

A stock s5 turbo could make about 225-250 hp on a 12a. Ill have one to sell in a week or two.

RotorMotorDriver

iTrader: (1)

Yeah, I looked at Max's site, its very nice. The problem is, I dont know anything about the rail pressure, injector pressure, etc. Thats what I need taught to me.

As far as I know, the rail pressure is just that. The pressure of the fuel in the fuel rail. Now then, my thinking is that if the injectors are attached to the fuel rail, wouldnt they run at the rail pressure not the 45 PSI or whatever it is they are flow tested at? What regulates the pressure that goes through the injectors? I was on RC Engineerings website messing around with their website calculator, and its got me all confused. I know injectors sizes are based on the pressure flowing through them...I think. So like an -SE injector thats 680 cc's might flow 680 cc's at 45 PSI or whatever, but it you bumped the pressure up to say 50 PSI, it might flow 700 cc's, right? Jesus, I dont understand much about this at all... Then theres the duty cycle. For my best guess from reading some vauge explanations, this is just the percentage of time that the injector is "open". So when the engine is running, the injector "pulses" open and closed (like an intake valve in a piston engine) to allow the right amount of fuel in in quick squirts instead of one long one because that would drop the pressure too much, right? So the duty cycle is the percent of time that the injector is open over a period of time. So if out of 1 minute and 40 seconds (100 seconds, to make it easy), the injector is open for 75 seconds, thats 75% of the time, making it a 75% duty cycle, correct? As you can see, I wanna learn, but I need to know where and what to read. Thanks for all the help so far guys .

~T.J.

maxcooper

Injectors are flow tested at 43.5 psi to determine their size. They flow more at higher pressures and less at lower pressures. So, a 680 cc/min injector will flow 680*SQRT(50/43.5) = 729 cc/min at 50 psi. It is still a 680 cc/min injector, but you are using it at a different pressure. For such calculations, be sure to use what I call the 'effective pressure', which is fuel rail pressure minus the intake manifold pressure, as this determines how much the injector will flow. So, 50 psi effective pressure would give a rail pressure of 65 psi if you were running 15 psi of boost at the time. The fuel pump still has to supply the required flow at the full 65 psi, however, so use the rail pressure for calculating fuel pump requirements.

Duty cycle is what you described -- the percentage of time the injector is open. The injectors are pulsed to control fuel flow. You would get way too much flow at idle if the injectors were open all the time.

Fuel systems work like this:
- pump has to supply AT LEAST the amount of flow the engine needs at the pressure the regulator wants
- the regulator holds the 'effective pressure' constant, adjusting for changes in manifold pressure. It does this by returning some of the pumped fuel back to the fuel tank to maintain the right pressure.
- the ECU controls the pulse width (which relates to duty cycle) to deliver the right amount of fuel for the engine under the present circumstances (RPM, load, etc.)

Something is wrong if the fuel pump can't supply the flow required to feed the engine what it wants, which causes the pressure to drop below what the regulator wants. The regulator essentially closes under this condition and all the fuel goes into the engine. But since the pressure is lower than it should be, the engine doesn't get enough fuel.

The pump merely needs to be adequate. Doubling its performance should not affect the amount of fuel delivered, since the regulator should hold the pressure to what it wants and the ECU tuning controls the pulse width as tuned to the regulator pressure. The ECU does not know anything about the fuel pressure, so it will not adjust itself if the pressure drops. It won't adjust itself if you increase the pressure either -- you need to simply set the regulator to the pressure you want and then tune the ECU to have the right pulse widths for that pressure.

My web page accounts for all these things and gives some recommended values to assist in making your choices.

-Max

RotorMotorDriver

iTrader: (1)

Ok, can you talk to me more about the rail pressure? Sorry if these are stupid questions. I was just told the only stupid question is the one not asked .

So, assuming Im running N/A, and I keep a constant 45 PSI rail pressure, theres no change in regulator pressure, and the fuel pump can keep up, I have no problems right?

Now, If I go turbo on the same setup, or now boosting... The regulator brings the pressure up (1:1?) to compensate for the boost, so now Im running at say 5 PSI of boost, my rail pressure is now 50 PSI correct?

So, assuming thats all correct, and lets just hypotetically say I wanted 350 FWHP, that means I will need:

A total fuel flow amount of 3561 cc/min (right?). I want to run 2 GSL-SE 680cc injectors as the primaries, giving me 1360 cc/min of fuel flow, which leaves 2201 cc's that I need to run through the secondaries. I can almost get that by running two 1,100 cc/min secondary injectors. I plan on running it at a conservative duty cycle of 65% with a rail pressure of 45 PSI. Does this work? What would I need to do to find out other crap for when boost happens? What numbers do I change? Am I even close?

~T.J.

maxcooper

No problem -- that's why we are all here.

That is basically true, but the regulator may still have a manifold reference signal connection (nipple for vacuum hose) so it can lower the pressure at idle and cruising when the manifold has vacuum.

That's right -- I think your understanding of this topic is right on.

There are many ways to do it -- that's why I made the web page (play what-if quickly and easily). An NA rotary probably has a BSFC around 0.6 or so (probably a little rich, but may be as low as 0.55 -- not as fuel efficient as a piston engine generally) and you plan to run 65% duty cycle maximum, so you will need about (350*0.60)/0.65*10.5 = 3392 cc/min of fuel flow total. Since you are running 45 psi effective pressure versus the 43.5 that the injectors are size-rated at, you can divide that by SQRT(45/43.5) = 1.017, giving 3392/1.017 = 3335 cc/min. Subtract your 680s, and you get 987 cc/min as the target size for your secondaries. But it would be much easier to use some standard size injectors and simply verify that they will provide enough flow for the power you need. That's why I made the web page - you usually start with a short list of components and see if they will be large enough to meet your HP goals without requiring super high fuel pressure or duty cycles. You just want the fuel system components to be more than adequate. Then you do your tuning (change the fuel pressure and duty cycles) to get the mixture right.

You can also support 350 flywheel HP with four 680 cc/min injectors, 45 psi fuel pressure, and 80% duty cycle.

For running boost, the BSFC (pounds of fuel required to make 1 HP for an hour) will get a little higher as you need a richer mixture to guard against detonation, say 0.64 or so. That will increase the amount of fuel you need for a given power level. The rail pressure will go up since you are running boost (which will require the pump to work harder), but the effective pressure will still be 45 psi if that is what you set it to. The BSFC and the rail pressure are the two things that change most notably for turbo operation. On my web page, you enter the effective fuel pressure rather than the rail pressure, so BSFC is the only thing you need to change on there.

-Max

RotorMotorDriver

iTrader: (1)

Ok, I think Im understanding this for the most part...

Now, what about turbo size selection? Are there any good websites that explain the A/R (area ratio?) of the turbos so I can try and figure out what would work best for me to put the power in the RPM range I want it?

~T.J.

RotorMotorDriver

iTrader: (1)

Ok wait a minute...

If I plug in a BSFC of .64
Rail pressure of 45 PSI
(2) 680cc/min primaries
(2) 1100cc/min secondaries
Max duty cycle of 65%
Drivetrail loss of 15%

That gives me 351 FWHP, and 298 RWHP. So that number is N/A since the rail pressure is only 45 PSI and thats the pressure that it would be at when the car is say, just idling? So if I was running 15 PSI of boost, that would make the rail pressure 60 PSI, and that would up the HP to 406 FWHP and 345 RWHP. Am I factoring in the boost in the right spot? Then after I factor in my 15 PSI or whatever, then I take the fuel pump size from the numbers after the increased rail pressure so I dont run outta gas when I boost, right? Holy cow, I think Ive gone stupid cause I just cant think this through for some reaon...

~T.J.

RotorMotorDriver

iTrader: (1)

Ok, after re-thinking this, I think that if I just leave the rail pressure at 45 PSI, then down lower if I add in however much boost I want, the web page automatically tells me how much more fuel its gonna take, right? so if I put 0 for the above settings, it says 38 gal/hr @ 60 PSI. If I say I want 25 PSI of boost (haha), then it says that Im gonne need a pump that flows 38 gal/hr @ 85 PSI, and odly enough, the 25 PSI difference between the two pumps is exactly the amount of boost I would be running, as in a 1:1 ratio. So Im guessing that the website automatically factors in boost on top of the information youve already given? So even though the rail pressure would be changing with the boost pressure, the website just doesnt tell you?

That could be an improvement if Im right. Have the boost pressure be zero when you first go there. Then, if they enter a boost pressure amount, have the webpage automatically update the rail pressure which would also update the amount of HP and what-not too because the rail pressure just changed.

~T.J.

EDIT: I had another thought. If when you hit boost, and the rail pressure bumps up from say 45 PSI to 55 PSI because you are using 10 PSI of boost, does the ECUs programming drop the pulse width so that the injectors still flow the "right" amount? Basically, do the injectors flow the same amount on and off boost, its just that the rail pressure increases so that the gas can actually get out of the injectors rather than be forced back into the rail? If this is correct, then disregard the comment above about changing the website because the HP potential wouldnt truly be affected by the rail pressure increasing if the injectors still flow the same amount (right?) .

setzep

To answer your question.... yes The whole reason the fuel pressure changes is to keep the ratio the same throughout the operating/load range. It's to keep the same pressure deferential across the fuel injector. So thats why when you're idling the fuel pressure is less than when your boosting. If you were boosting 50psi and your fuel pressure was only 45 then the boost would actually send air into the fuel injector. I don't know if this makes any sence but it does in my head

RotorMotorDriver

iTrader: (1)

Ok, so the injectors flow the same amount no matter what youre boosting? Just the rail pressure increases?

~T.J.

maxcooper

Yes, setzup's explanation is right on, and it sounds like you understand, RoMoDriver. The injector will flow the same amount for a given pulse width no matter what you are boosting because the regulator keeps the pressure differential across the injector constant.

I designed the web page that way because a person would normally disconnect the manifold reference vacuum hose from their regulator and set the base pressure (= effective pressure) on their adjustable fuel pressure regulator. The regulator will maintain that base pressure by reducing the rail pressure under manifold vacuum (idle, cruising, etc.) and increase the rail pressure under boost (at a 1:1 ratio).

The thing that isn't the same when you change the boost is the pressure at which the fuel pump must provide the necessary amount of flow. Increasing boost will generally increase the amount of power you will make, but imagine for a moment two setups that make the same HP, but one is NA and the other needs 15 psi of boost. If they both need 38 gal/hr and have an effective fuel pressure of 45 psi, the pump on the NA car only needs to supply 38 gal/hr at 45 psi, and the pump on the turbo car needs to supply that flow at 60 psi (45 + 15).

-Max

RotorMotorDriver

iTrader: (1)

I see the light!! Thanks guys, its making sense now .

So about those turbos...Anyone have any info on what everything means on those bad boys?

~T.J.