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Old 01-18-2009, 12:07 AM   #1
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Default Injector Size?!? (Greddy 10-14psi)

About to install a MS on my Greddy turboed 1.6L and want to run anywhere from 10-14psi when everything is said and done. I'd like to get some RX7 injectors but not sure what CC i'll need for that amount of boost.
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Old 01-18-2009, 12:09 AM   #2
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at least 440-460.
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Old 01-18-2009, 12:25 AM   #3
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Don't forget to have them cleaned/flowed... and make sure when you buy them you have the seller tell you the part number that's stamped on the plastic body and match it up with a compatible injector in the FAQ... ie, there are 440/460 RX7 injectors that aren't PnP.
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Old 01-18-2009, 12:29 AM   #4
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I run 14-15 psi on RX7 red tops, seems ideal.
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Old 01-18-2009, 12:33 AM   #5
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Quote:
Originally Posted by samnavy View Post
Don't forget to have them cleaned/flowed... and make sure when you buy them you have the seller tell you the part number that's stamped on the plastic body and match it up with a compatible injector in the FAQ... ie, there are 440/460 RX7 injectors that aren't PnP.
Sweet I'll have to check that out....I was about to ask that questions as well as if it mattered what year RX7 they were. Is there a decent source for these or just keep an eye out for them?
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Old 01-18-2009, 12:34 AM   #6
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rx7 forums are a good source. check out the faq here for year and part # info.
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Old 01-19-2009, 01:15 AM   #7
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RX7Club.com - Powered by vBulletin thats where i got mine
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Old 01-19-2009, 02:22 AM   #8
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If you know you want to go to 14psi, I would go with the 550's. That's the largest you can run on the MS I believe (MSPNP at least), and although the 460's can do the 14psi, if you are going to spend the $$ on a part get one that has some headroom and doesn't have to work very hard to do what you will need from it. I bought my 460's for my planned 10-11 psi so there is a bit of headroom.
-Ryan
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Old 01-19-2009, 08:01 AM   #9
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You can run whatever size injectors you want on any MS. Having a nice low, lean idle might become something you have to forfeit though. I could pop 1000cc injectors in my miata if I wanted and it would run fine. I would just have a rich idle.

As for the OPs question. The recommendations of the 550cc RX7 injectors are definitely the way to go.
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Old 01-19-2009, 10:41 AM   #10
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Got a PM for some 460's that have been cleaned and tested 3k ago....figuring I'll go with them unless some of the 550's of the same equivilent fall in my lap.

Last edited by danscreations; 01-19-2009 at 10:55 AM.
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Old 01-19-2009, 12:05 PM   #11
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Forgot to ask out of curiousity....what year RX7's are either comming out of? 2nd gens?
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Old 01-19-2009, 02:48 PM   #12
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Quote:
Originally Posted by danscreations View Post
Forgot to ask out of curiousity....what year RX7's are either comming out of? 2nd gens?
https://www.miataturbo.net/forum/t10821/#post126318
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Old 01-25-2009, 10:41 AM   #13
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440cc are just fine for 14psi
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Old 01-25-2009, 12:00 PM   #14
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Default Injector Sizing

The question is how much HP do you want to make? Using the the flow rate, number of injectors and maximum duty cycle and dividing it by the engine's Brake Specific Fuel Consumption will give you the theoretical HP you can make. It is then up to you to get enough air into the system to make that HP via Boost.
Basic equation used by most is below:

B.S.F.C.= .55HP/lb-min fuel (this is the most widely used number for 1.6 Miata)

.85= Maximum %duty cycle you want to run your injectors at, so you have some buffer from injector lock up.

Flow rate= 100% duty [email protected] PSI fuel pressure in cc's. In general divide by 10 to convert to lbs/minute

(4 injectors)x(Flow Rate/10)x(.85)/0.55 = XXXHP

The stock 1.6 injectors are rated at 230cc, so the theoretcal "safe" max HP is:

(4)x (230/10)x(0.85)= 142HP

By running up to 95% duty cycle, you could get another 10% HP ( 156HP).

HOWEVER, when you add boost to an injected system, the effective flow rate of the injectors will DECREASE under boost, if the fuel pressure is NOT increased in step with the manifold pressure (boost). This is basic physics, as the flow rate is dependent upon the relative difference in pressures from one end of the injector to the other.
Fortunately for Miata 1.6 owners, the stock Fuel Pressure Regulator will increase on a 1:1 basis the fuel pressure for up to 7PSI of boost, so this is not an issue at low boost levels.
This is also why you must go to larger than predicted injectors if you run more than 7PSI boost without some form of fuel pressure increase.
So in designing your system and choosing your injector size, you must first decide if you will alter fuel pressure delivery or not, and decide on a target Max HP based upon the capabilities of the entire vehicle ( and it's driver??)

In general, with a standalone fully programmable ECU for Injector control, once you are over 8-9PSI, you should run a fuel pressure system capable of raising the fuel pressure at least as much as the expected maximum boost you will run to avoid "running out of injector", or run seemingly way too big an injector.

As an example, at 14.5PSI (1 BAR), injectors are only putting out 2/3rds of their rated flow, and at 21PSI (1.5 BAR), they only are putting out 1/2 of their rated maximum. So a 440cc injector at 1 BAR is really a 290cc, and at 1.5 BAR it is a 220cc injector.

Plugging back into the original equation, you can see that a 440 injector, at about 15 PSI boost, with no fuel pressure changes, should make about 180HP safely. Going to 95% duty cycle, having a better flowing engine (improving BSFC) etc, and you can fairly safely reach 200HP.

Hope this helps.
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Old 01-25-2009, 12:40 PM   #15
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Thats what I thought. Im running an afpr 12:1 with supra 305s. It seems to do well.
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Old 01-25-2009, 01:16 PM   #16
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Any chance of a Dyno sheet with GT25 turbo? What type of manifold and downpipe size?
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Old 01-25-2009, 04:28 PM   #17
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Quote:
In general, with a standalone fully programmable ECU for Injector control, once you are over 8-9PSI, you should run a fuel pressure system capable of raising the fuel pressure at least as much as the expected maximum boost you will run to avoid "running out of injector", or run seemingly way too big an injector.
That's interesting. I don't run a standalone, but I thought one of the big advantages was the ability to run stock fuel pressures and eliminate rising fuel pressure as one of the tuning variables.

How does a standalone regulate fuel pressure without an FPR and high pressure fuel pump? Or, are you saying they don't usually have a means to do so, and should?
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Old 01-25-2009, 05:13 PM   #18
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With a stand alone, you don't have to have a means to increase Fuel pressure, but you'll need to run very large capacity injectors if you don't. As I pointed out at the end of the "Math Exercise", if you plug in your expected peak boost, and the injectors you have, you can see what your actually fuel delivery capacity and hence HP capability truly is with your system.
The disadvantage to running very large injectors is the ability to open them for extremely short periods at idle and get a clean idle. A smaller injector would in general stay open longer at idle and therefore have a greater degree of tunability.
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Old 01-25-2009, 06:29 PM   #19
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ditto on the rrfpr and 305 supras.

I've heard a lot of people talk about problems tuning with a rising rate fpr and stand alone, but I have no issues. I personaly feel that this talk comes from people that just try to plug in a map and call it a day. And of course the maps are way off. I also feel that the smaller injectors with higher pressure gives you a little more resolution (if you will) to make every day drivability better.

I have the vortech super fmu. it has the ability to lock out the pressure at level you wish. personaly I lock it out a 100psi. so I say if you have one on there with the greddy kit allready, and you aren't planning on selling/trading it leave that hooker on there and you could go with something a touch smaller. or just leave it on there for a little more head room, but swap out the disk for a little less rise in pressure so that you aren't running a thousand pounds of pressure by the time you hit 12lbsof boost. just for reference my static pressure is 45lbs(35 at idle), but by 10 lbs of boost it's at 100psi. so it can rise to dangerous levels it you don't have proper fuel lines and such. so I would recomend at those boost levels to back down the rate of rise a touch.
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Old 01-25-2009, 06:52 PM   #20
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Quote:
Originally Posted by drgoodwrench View Post
HOWEVER, when you add boost to an injected system, the effective flow rate of the injectors will DECREASE under boost, if the fuel pressure is NOT increased in step with the manifold pressure (boost). This is basic physics, as the flow rate is dependent upon the relative difference in pressures from one end of the injector to the other.
Fortunately for Miata 1.6 owners, the stock Fuel Pressure Regulator will increase on a 1:1 basis the fuel pressure for up to 7PSI of boost, so this is not an issue at low boost levels.
This is also why you must go to larger than predicted injectors if you run more than 7PSI boost without some form of fuel pressure increase.
So in designing your system and choosing your injector size, you must first decide if you will alter fuel pressure delivery or not, and decide on a target Max HP based upon the capabilities of the entire vehicle ( and it's driver??)

In general, with a standalone fully programmable ECU for Injector control, once you are over 8-9PSI, you should run a fuel pressure system capable of raising the fuel pressure at least as much as the expected maximum boost you will run to avoid "running out of injector", or run seemingly way too big an injector.

As an example, at 14.5PSI (1 BAR), injectors are only putting out 2/3rds of their rated flow, and at 21PSI (1.5 BAR), they only are putting out 1/2 of their rated maximum. So a 440cc injector at 1 BAR is really a 290cc, and at 1.5 BAR it is a 220cc injector.

Plugging back into the original equation, you can see that a 440 injector, at about 15 PSI boost, with no fuel pressure changes, should make about 180HP safely. Going to 95% duty cycle, having a better flowing engine (improving BSFC) etc, and you can fairly safely reach 200HP.

Hope this helps.
are you sure about this? as far as i know that 1:1 increase in fuel pressure continues for pretty much any boost level.

if what you're saying is true then what injector size would you predict i'd need for 320whp at 23psi of boost?

...because i'm running 550's with a stock FPR. they hit ~80% duty cycle at 7k rpm and full boost, tuned for ~11.5:1 AFR.
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