Does MS and a RR-FPR play well together? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 03-29-2012, 08:20 PM   #1
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Default Does MS and a RR-FPR play well together?

I recently converted my 00 to a return style fuel supply. Fabbed a return out of the damper on the end of the fuel rail and goes through an Aeromotive RR-FPR (#13109) and back to the tank. The question is whether or not to use the vacuum/boost port for tuning. If you set the fuel pressure at 43 psi then plug in the port it will regulate the pressure to about 39, and then rises with boost. Will the MS do a more efficient job by itself? If so, should the fuel pressure be slightly higher? The Aeromitive tech guy said use the port. Not sure how familiar he is with MS. Anybody have experience with this setup? If it matters, the injectors are Siemens 550's and a Walbro 255.

Last edited by 2manyhobyz; 03-29-2012 at 09:34 PM. Reason: more info
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Old 03-29-2012, 08:22 PM   #2
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Yes, you will absolutely reference the FPR to your manifold.

Edit:

1) It makes idling the car easier. In an ideal world, we would all use 2000cc injectors with 0 latency and infinitely small minmum pulsewidth. As it stands though, injectors have a minimum that they can spray per cycle based on opening time, closing time, pintle opening size, and fuel pressure. By referencing the manifold, you are dropping the fuel pressure in vacuum (as you noted) which allows for a smaller minimum, and therefore more room to play with pulsewidth at low load (better idle/emissions/etc).

2)The inverse of ^ is true as well. By increasing the pressure proportionally with boost, you are effectively raising the maximum amount of fuel you can inject. Without having the FPR referenced to boost, you will actually max out your injectors earlier than you want to.


So in other words:
Provided the fuel pump is up to snuff (not dying or overworked), the fuel pressure increase with boost pressure will always remain correlated. By referencing the manifold, you are just offsetting the change in fuel pressure by the change in pressure on the other side of the injector to allow for a full range of adjustment and proper high load performance.

Last edited by Jeff_Ciesielski; 03-29-2012 at 08:37 PM.
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Old 03-30-2012, 12:15 AM   #3
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Thanks you. That makes a lot of sense and you did a good job of explaining it.
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Old 03-30-2012, 06:45 PM   #4
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Quote:
Originally Posted by 90 Turbo View Post
Thanks you. That makes a lot of sense and you did a good job of explaining it.
+1 ya beat me to it.
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Old 03-12-2014, 01:02 AM   #5
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Quote:
Originally Posted by Jeff_Ciesielski View Post
Yes, you will absolutely reference the FPR to your manifold.

Edit:

1) It makes idling the car easier. In an ideal world, we would all use 2000cc injectors with 0 latency and infinitely small minmum pulsewidth. As it stands though, injectors have a minimum that they can spray per cycle based on opening time, closing time, pintle opening size, and fuel pressure. By referencing the manifold, you are dropping the fuel pressure in vacuum (as you noted) which allows for a smaller minimum, and therefore more room to play with pulsewidth at low load (better idle/emissions/etc).

2)The inverse of ^ is true as well. By increasing the pressure proportionally with boost, you are effectively raising the maximum amount of fuel you can inject. Without having the FPR referenced to boost, you will actually max out your injectors earlier than you want to.


So in other words:
Provided the fuel pump is up to snuff (not dying or overworked), the fuel pressure increase with boost pressure will always remain correlated. By referencing the manifold, you are just offsetting the change in fuel pressure by the change in pressure on the other side of the injector to allow for a full range of adjustment and proper high load performance.
So, over the winter I've upgraded the injectors (800's) and turbo TD05HR and was wondering if Jeff's explanation is still valid. It makes sense, but just checking for other opinions. Thanks
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Old 03-12-2014, 10:57 AM   #6
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Yes, it is valid.

Bottom line on combining RR-FPR and MS is that VE tables will have much less variation low to high kPA as the MS is more fine tuning, and, as pointed out before, pulse widths do not change as much.

Of course you still have to change REQ_FUEL for the injector size if you don't want to totally change the VE tables, but rather just tweak them. (Pertains to injector change more than turbo change... you may still have major revisions).
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