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Injector and pump sizing questions

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10+ Year Contributor
Feb 6, 2011
Vancouver, Washington
So to ask and amateur question and maybe this will help other new guys but according to this chart to be at 80%IDC(25.84lbs/min) of the 450cc injectors I could run (stock 2g pump) 18psi = 27.16lbs/min and be in a safe zone. I of course will be getting a MTX-L wideband before she ever see 18psi. Is this correct or am I just confused. I was always told that 15psi is where the factory fuel system leans out and cant keep up. Any insight would help and thanks in advance
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I'm deciding on what fuel pump best fit my needs. Probably invest in the wally 255 but that would be a little over kill. What ever I do I'm rewiring it. I'm looking to make 350 whp so I think a set of 750cc will get me there with room to grow. So my second question is how do you factor the upgraded turbo into the lbs/min? Its a MHI S16G
After doing some research abroad I found a rough conversion 14.47 CFM = ~1lb/min and on the same page it shows that a S16G flows 505 cfm at 15psi
Mitsubishi OEM turbo specifications (including flow rates) -
If my math is correct that is 34.89 lbs/min and would be putting me over my 80%IDC for the 450cc injectors I have. At that flow rate I would need to upgrade to a minimum of 650cc to maintain a 80%IDC. At the same time at 15psi the stock 2g fuel pump flows 15psi = 29.63lbs/min and would not support this amount also. So that is why it would need to be upgraded. Any of this making sense? Anyone know where to find the flow rate for the different psi levels so I could calculate for more than just 15psi?
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Good to hear you're doing the necessary research. All of that makes sense, and is correct.

As for the flow rates above 15psi, you'll want to look for a compressor map.

Any reason you're looking for injectors that are close to, but over your goals? What do you plan on using to tune the injectors? If something like ECMlink, just go big (e.g. 1150cc) and you won't have to worry about the calculations, and then will have room to grow in the future if you want it.
I plan on getting ecmlink eventually like most everything else. That has been the best tuning solution I have come across. I suppose your right about getting larger than I need for the sake of growth down the road. Anyone good at translating compressor maps? I am lost on this one and would like to know what the different cfm are at different boost levels. Whats the old saying " take a guy fishing and he can eat for a day, teach a guy to fish and he cant eat for a life time ". So if anyone could accurately explain how to read or at least make sense of this would be awesome.


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The link doesn't work, FYI.

Since you've been talking in CFM, I'll use this map.

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The X axis obviously has CFM values on it (airflow of the turbo).
The Y axis has Pressure Ratio values on it. To find your pressure ratio, use this equation:

PR = Atmospheric Pressure + Boost Pressure
......................Atmospheric Pressure

Atmospheric pressure = 14.7
Boost pressure = how much boost you are running (or want to)

So for example, I run 21psi on my small 16g.

PR = (14.7+21)/14.7
PR = 2.4

So if you follow the line across the map at 2.4, you'll see that the turbo at this boost level is on its way out of being efficient (which would be around the 1.8 to 2.2 PR values, since those spend most of their time in the map and pass through all the efficiency islands [ovals in the middle of the map]).

From a quick glance, it looks like the 2.4 PR will max out around 520 CFM, or 35.9 lb/min. All I did was follow the 2.4 line all the way to the right of the compressor map, and then estimate where that line is in relation to the CFM scale. I noticed for that point, it seems almost in line with the actual noted 520 CFM mark on the 2.0 line.

The calculated 35.9 lb/min comes close to what my log says, which is almost 33 lb/min. Now, it does spike higher (up to 36 lb/min), but I have no idea why as my airflow gets choppy above 6500 RPM. That's another topic though.

So you can plug in different boost levels to the equation and find out the max CFM then convert it to lb/min. Hopefully this makes sense, and that I didn't mess anything up :p

I'll also be moving our conversation to a new thread as it isn't really on topic here.
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I edited my last post and uploaded a jpg of the compressor map. Do you have a different one since this one looks like its in kg/sec. If you have a better one that would be much appreciated. So the cross lines on the map represent rmp and is that of the engine or the turbo? You almost need to go to school for all of this but IMO that's what makes it fun and keeps me interested. Always something new to learn and I haven't even started tuning yet. I don't know what I would do without dsmtuners haha I would probably still be venting to atmosphere and wondering why it idled like crap.
I did read the last post but I didn't see a map. Its says "Ill use this map" but there isn't a picture there and that's why I asked if you had another map. I assumed you had a map you were putting up other wise it wouldn't make any sense. Sorry I'm not trying to be difficult but I do like to learn and you are a great teacher
This really good for beginners, and the thread is clean without helpful insight

Nice, job fellas!
I have been doing some more reading on compressor maps and plotting points for different engine rpms and seem to me that no one explains how you know what represents engine rpm. I understand about the compressor rpm but I wouldn't be able to plot a point for a 3k engine rpm at a PR of 1.95 or the same PR at 7k rpm. If you have any information on this it would be very helpful. Thanks again.
* this is the page I was looking at the sparked the question if it helps any*
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A compressor map is made to show what the turbo can do at different pressure ratios (boost levels). This is obviously not going to be the same for every engine of every car that the turbo is used with, so there's no way you'd be able to plot engine RPM.

What types of things are you thinking that you'd want to plot engine RPM? This way I can try and help you understand more and/or guide you in the right direction.
That makes sense. If you read the link I posted he talks about plotting two points of different rpms and has a map he shows this on. I was just trying to do something similar and was unsure of how he figured out what the different rpm points are. I am assuming he didn't guess but there is no mention of how he came to that conclusion. More or less I was trying to simulate what he was doing on my own compressor map and see what the results are. I am curious to see where my red line is on the map and plot my curve so to speak.
We'll try this with the above posted small 16g map.

Where L = 2.0
RPM = 3500 and 7000
VE = 100%
Pr = 2.4

Red lines (guesstimated VE)
CFM = (2.0*7000*100*2.4)/5660 = 593.64 CFM (or 41.60 lbs/min)
CFM = (2.0*3500*100*2.4)/5660 = 296.82 CFM (or 20.80 lbs/min)

Green lines (VE average from 3rd gear pull attached - 91.8%
CFM = (2.0*7000*91.8*2.4)/5660 = 544.96 CFM (or 38.19 lbs/min)
CFM = (2.0*3500*91.8*2.4)/5660 = 272.48 CFM (or 19.09 lbs/min)

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According to the plots on the above compressor map, the turbo is not ideal at this PR. It will supposedly make 21 psi by 3500 RPM (which according to my log, I was unable to, but other mods might come into play here), but by 7000 RPM it will be way out of it's efficiency range at 21 psi.

I don't know how well this actually works for us, and if the equations are universal, or specific to the MR2 that the website was using. Either way, I hope this helps at least a little bit.


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Thanks for doing the conversion to our map. Ill have to do some log work when I get evoscan or link and see if the data correlates with what we found here. Thatnks again
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