Doing the math ( turbo flow with Injector flow)

[dscoobydoo]

Ok, so I have been researching the crap out of turbo upgrades and matching injectors for when we take the plunge this summer.

Our current (stock) turbo flows between 350-390 cfm and the injectors flow 370.

NOW, if I went with the Super 16G (according to Deadbolt's website) it would flow close to 585, thus I would want injectors between 555 and 650. ( if I am right)

I was thinking it would be better to go with the upper (650) range if I want to push the Turbo to 20psi.

Although it would be easy to get the Nismo 555's I figure there are some 650's out there.

Thoughts??

[Legacy777]

The 550's probably would probably be adequate. I believe that's what I'm going to look into when I upgrade to the 16G. I don't know if I'll go the super route though.

[Project_Legacy]

ive been thinking about a turbo upgrade to the super 16g also. thanks for some info on the injectors!

[93forestpearl]

I picked up a set of JECS 650 cc/min injectors on ebay from DeatschWerks for $275. They came with a nice report as well. It says they are within 1.8% static. They'll go on with the rails from my '93 NA, naturally.

[Legacy777]

Dan,

What EM are you using?

[sammydafish]

turbos don't flow in CFM, they are not fans, they are pumps. They flow mass (lbs of air). Injectors also flow in mass (lbs of fuel). There is an industry standard for measuring injectors though (to convert to cc/min) but there is none for turbos. Your math will be flawed no matter how much you try to make it work; the reason is simply that changes in air temperature will make a drastic change to the CMF a turbo will flow. The total amount of air that can be used by the engine is also limited by volumetric efficiency of the engine itself, which will be proportionate to the amount of pressure you feed it.

If you want to size injectors, use your target horsepower as a guide. 550cc (52lb) injectors on a 4 cylinder engine should be able to support a little over 300 horsepower. 650cc injectors (63lbs) can support about 375hp. Both of those estimates are at 40psi net injector fuel pressure.

[vrg3]

That's not correct... Turbos are neither fans nor pumps. They're compressors. A turbocharger produces a certain pressure ratio across its cold side.

The actual flow rate at any given pressure ratio is determined by the efficiency of the compressor (which actually depends on both pressure ratio and flow rate) and by the rest of the system (piping, intercooler, engine).

Centrifugal compressors actually don't care about mass really. Some compressor efficiency maps are drawn with mass flow as one of the independent variables, but that's really volumetric flow multiplied by the density of air at RTP.

When people quote "CFM" numbers for a turbo, they're typically talking about a particular engine (including intercooler, piping, exhaust, etc). You can only use that number if you have the same setup.

But, yes, sizing injectors by desired power is the right way to do it. As a very very rough guide, multiply injector flow (in cc/min) by two thirds to figure out how much power (in horsepower) they'll support on a 4-cylinder 4-stroke port fuel injection turbo gasoline engine.

[Splinter]

Doesnt the fuel pressure factor into the equation quite a bit?

[vrg3]

No; pressure and nozzle size and shape of course determine injector flow, but fuel flow is fuel flow.

[sammydafish]

That's not correct... Turbos are neither fans nor pumps. They're compressors.

true, typo on my part

in most terms, turbos/compressors will always flow mass. If you rate flow in terms of pressure ratio (most turbo flow maps) your resulting flow must me in terms of mass. Unless you assume RTP at input, but the compressor maps I've seen that list flow don't do this and don't list what temperature and pressure they were tested at, so I don't know what values they were using.


Basically what I'm saying is that turbos are very dynamic devices. You can't say turbo a flows x CFM. It's simply not information that would do anyone any good.

[93forestpearl]

Josh, I'm still on the fence as far as what I'm gonna do about EM. I wanted to go with the Megasquirt, but nobody has written the code to read the Subaru crank and cam triggers. I want a standalone, so I've been looking at the Haltec and the Link plus. Each has plusses and minuses.

I'll agree that turbos flow a mass of air. My GT2871R is supposed to flow 44 lb/min. I think that equation for injector sizing is very conservative. A couple shops and others in the know have told me that the 650's I got will be plenty for that turbo, which supports 400-420 chp.

Just my thoughts.

[sammydafish]

I think that equation for injector sizing is very conservative. A couple shops and others in the know have told me that the 650's I got will be plenty for that turbo, which supports 400-420 chp.

The equation is pretty conservative, especially for modern engines that have been designed with efficiency in mind as well as power due primarily to emissions concerns. Because of this, chamber designed have moved to favoring leaner burns. This means that many modern engines make more power on less fuel with peak power A/F ratios getting leaner without risk of detonation or drastically high EGTs and burning valves. A general rule of thumb is to use a BSFC value of 0.55 for forced induction engines but efficient engine designs like some modern Hondas and even Subaru’s can run closer to 0.40 - 0.45 on heavy boost. This makes those injectors good for FAR more horsepower than the 'general rule of thumb' provides. I'm still fiddling a lot with my EJ22 NA on boost, but it seems to like A/F ratios in the high 12's to low 13s for good power. All that said, as long as you can control the injectors and don't have idle issues, having a little larger and injector than you need isn't really a bad thing.

[Subtle]

I've been using Simple Digital Systems.


www.sdsefi.com

[vrg3]

in most terms, turbos/compressors will always flow mass. If you rate flow in terms of pressure ratio (most turbo flow maps) your resulting flow must me in terms of mass. Unless you assume RTP at input, but the compressor maps I've seen that list flow don't do this and don't list what temperature and pressure they were tested at, so I don't know what values they were using.

I don't understand -- how can you say there's a certain mass flow through a centrifugal compressor if all you know is the pressure ratio across it? I mean, of course you have to compute a density ratio in order to figure out how much the air is actually compressed in the compressor, but the compressor itself is still just creating a pressure ratio with an efficiency dependent on that ratio and the volumetric flow.

Anyway, the way I've usually done this sort of thing is along the lines of what I did in this thread:

http://bbs.legacycentral.org/viewtopic.php?p=72589

Basically what I'm saying is that turbos are very dynamic devices. You can't say turbo a flows x CFM. It's simply not information that would do anyone any good.

Wholeheartedly agreed, except in the unusual situation where you're comparing power produced with one turbo on an engine to power produced with another turbo on the exact same engine.

[93forestpearl]

Doesn't a pressure ratio correlate somewhat to a mass ratio, dependent on temperature? They're all in the same equation.

As in the more pressure a tubo puts out for a given displacement, the more mass it is moving, and vice versa. So a larger turbo can move more mass of air therefore being able to hold a higher pressure ratio per given displacement. And a compressor's effeciency has lots to do with the temperature of the air at the higher pressure ratio, thus affecting the mass moved per unti time.

Am I even making any sense?

[vrg3]

Yes.

Doesn't a pressure ratio correlate somewhat to a mass ratio, dependent on temperature?

Yes; pressure ratio divided by temperature ratio yields density ratio.

As in the more pressure a tubo puts out for a given displacement, the more mass it is moving, and vice versa.

Yes. Of course. I was just trying to say that all the compressor is doing is creating a certain pressure ratio with a certain efficiency. Mass doesn't come into play until you're trying to figure out what to do with the air yourself. Double the absolute inlet temperature and you halve the mass flow without changing anything else. The volumetric flow is the same, the compressor's rotational speed is the same, the pressure ratio is the same.