Difference between AFPR and RRFPR?
12-05-2005, 03:06 PM
#1
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Difference between AFPR and RRFPR?
I apologize in advance if this has already been asked too many times, but I looked through the archives and couldn't find the answer:
What, if any, difference is there between an adjustable fuel pressure regulator and a rising rate fuel pressure regulator?
I heard at SF05 that it is the best way to increase horsepower (especially for GT's) along with exhaust mods (X-pipe and high flow cats) for a reasonable "investment". I'm looking to increase power for my GT without breaking the bank.
Rick Redpath
'90 GT, Guards Red (currently motor torn apart - long story)
What, if any, difference is there between an adjustable fuel pressure regulator and a rising rate fuel pressure regulator?
I heard at SF05 that it is the best way to increase horsepower (especially for GT's) along with exhaust mods (X-pipe and high flow cats) for a reasonable "investment". I'm looking to increase power for my GT without breaking the bank.
Rick Redpath
'90 GT, Guards Red (currently motor torn apart - long story)
12-05-2005, 04:11 PM
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They are one and the same. The rising rate fuel pressure regulator is adjustable. The rate of fuel pressure increase with vacuum descrease is greater than stock.
12-05-2005, 04:36 PM
#3
Supercharged
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Welcome Rick! 
Glad to see you ponied up for a membership! It's the best $20-$40 you ever spend!
I see you have a 90 GT - with the motor torn apart. Personally, I like long stories, so what's the story?
Glad to see you ponied up for a membership! It's the best $20-$40 you ever spend!
I see you have a 90 GT - with the motor torn apart. Personally, I like long stories, so what's the story?
12-05-2005, 04:49 PM
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A stock fpr with a vacuum port decreases fuel pressure when vacuum is present. On your car it's 55psi with no vacuum (open throttle) and 47psi with vacuum (idle). An adjustable fpr is just that, an fpr that you can adjust the pressure. You set the high pressure and vacuum does it's thing at idle.
A rising rate fuel pressure regulator adjusts the fuel pressure disproportionally from the change in vacuum or more often pressure. They have set ratios like 10:1. For every 1 psi of vacuum/pressure change, the fuel pressure goes up 10psi. They are useful in boosted cars when you can't reprogram the injection for larger injectors or higher pressures. At idle, the car runs around stock pressures, with high boost and load it runs very high pressures.
Some people call an afpr an rrfpr. Some people call an rrfpr an fmu. What I said above is what I've seen as the most common interpretation.
A rising rate fuel pressure regulator adjusts the fuel pressure disproportionally from the change in vacuum or more often pressure. They have set ratios like 10:1. For every 1 psi of vacuum/pressure change, the fuel pressure goes up 10psi. They are useful in boosted cars when you can't reprogram the injection for larger injectors or higher pressures. At idle, the car runs around stock pressures, with high boost and load it runs very high pressures.
Some people call an afpr an rrfpr. Some people call an rrfpr an fmu. What I said above is what I've seen as the most common interpretation.
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12-05-2005, 09:24 PM
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Originally Posted by ErnestSw
I stand corrected. Thanks Matt.
12-06-2005, 12:52 AM
#9
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Thanks ErnestSw and Matt! I really appreciate your response.
So, it sounds like an afpr is a "fixed" unit that you set at a predetermined pressure and the fuel "system" responds accordingly.
A rrfpr is also set at a predetermined pressure but the unit itself has a "rate of change" effect from vacuum and pressure in the fuel system.
Does that sound correct?
Rick Redpath
'90 GT, Guards Red
So, it sounds like an afpr is a "fixed" unit that you set at a predetermined pressure and the fuel "system" responds accordingly.
A rrfpr is also set at a predetermined pressure but the unit itself has a "rate of change" effect from vacuum and pressure in the fuel system.
Does that sound correct?
Rick Redpath
'90 GT, Guards Red
12-06-2005, 01:13 AM
#10
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Thread Starter
Thanks for the welcome Andrew! I've been around the email list off and on for about 5 years, but just recently "found" the forum and knew I needed to "pony up" for membership 
My wife has been wondering why I'm spending evenings on the internet again... It's like when I first got on the 928 email list!
Well, you asked for the motor story. Here's the "condensed?" version:
After I got back from Sharktoberfest, car was CLEAN and running great. Entered the Wash & Shine portion of the concour for AZ Region PCA Phoenix Flight, car was even cleaner. Then it all began the next day - went out to the PCA autocross held out at McDowell Mountain Park. They had blocked off one of the picnic loops for a great course - paved course with no cones, etc. Problem was, the pavement had been recently chip sealed, so it was like driving on marbles. Track got a little faster as it was being cleaned off by lapping cars. I was running the second lap of my second run when I guess I was running the short back straight a "little" hot... started braking in a straight line... turned in for the apex of the corner... started to drift out a little... then in the loose chip seal and ZOOM... off the track and off for a ride in the desert! Came to a stop, axles buried, car sitting on the rocker panels. Couldn't get out of the driver's door because there was a tree right there! Climbed out of the passenger door (REALLY MAD at myself). Looked over the car, seemed OK. Had to be pulled out by a truck back the way I went in, but once on the track again, car ran fine back to the parking area. Really fortunate that NO damage was done to the body or suspension. As I kept going over the car looking for any sign of damage I saw some dripping from under the belly pan. Dan Warner of Exotic Motorwerks in Phoenix was there running their 944 and had me roll the car back, he checked the pavement underneath and said it was oil.
Since it was just barely dripping, drove the car back to Exotic and got it up on the lift to assess the damage. Belly pan was trashed and once removed, could see that the oil pan took a rock hit as did the external oil cooler behind the radiator. The front spoiler just flexed and took it, but the belly pan, oil pan and oil cooler were toast. We left it on the lift overnight, and if that wasn't enough, when Dan came in the next morning, under the car he saw a bigger puddle of coolant than oil !?! What the... Well, I had noticed prior even to Sharktoberfest I was loosing just a little bit of coolant, but couldn't tell where. Car like I said was running great. But, it looks like from over time and the car getting overheated recently (hose failure), the head gasket finally was leaking enough to really see it. Totally unrelated to the off road experience, but I guess ultimately timely to show up at the same time.
Anyway, so now what? Just do the one head gasket? No... should do both. Replacing the oil pan... may as well do rod bearings... may as well do main bearings... take the engine out to make it all easier... heads off ...may as well do valve guides... may as well do rings... and on and on. Briefly contemplated a stroker kit, but it's going to be expensive enough as is (unless of course someone knows of a stroker kit somebody needs to sell cheap!)
Rick Redpath
'90 GT, Guards Red/Grey & Black - new motor in process
My wife has been wondering why I'm spending evenings on the internet again... It's like when I first got on the 928 email list!
Well, you asked for the motor story. Here's the "condensed?" version:
After I got back from Sharktoberfest, car was CLEAN and running great. Entered the Wash & Shine portion of the concour for AZ Region PCA Phoenix Flight, car was even cleaner. Then it all began the next day - went out to the PCA autocross held out at McDowell Mountain Park. They had blocked off one of the picnic loops for a great course - paved course with no cones, etc. Problem was, the pavement had been recently chip sealed, so it was like driving on marbles. Track got a little faster as it was being cleaned off by lapping cars. I was running the second lap of my second run when I guess I was running the short back straight a "little" hot... started braking in a straight line... turned in for the apex of the corner... started to drift out a little... then in the loose chip seal and ZOOM... off the track and off for a ride in the desert! Came to a stop, axles buried, car sitting on the rocker panels. Couldn't get out of the driver's door because there was a tree right there! Climbed out of the passenger door (REALLY MAD at myself). Looked over the car, seemed OK. Had to be pulled out by a truck back the way I went in, but once on the track again, car ran fine back to the parking area. Really fortunate that NO damage was done to the body or suspension. As I kept going over the car looking for any sign of damage I saw some dripping from under the belly pan. Dan Warner of Exotic Motorwerks in Phoenix was there running their 944 and had me roll the car back, he checked the pavement underneath and said it was oil.
Since it was just barely dripping, drove the car back to Exotic and got it up on the lift to assess the damage. Belly pan was trashed and once removed, could see that the oil pan took a rock hit as did the external oil cooler behind the radiator. The front spoiler just flexed and took it, but the belly pan, oil pan and oil cooler were toast. We left it on the lift overnight, and if that wasn't enough, when Dan came in the next morning, under the car he saw a bigger puddle of coolant than oil !?! What the... Well, I had noticed prior even to Sharktoberfest I was loosing just a little bit of coolant, but couldn't tell where. Car like I said was running great. But, it looks like from over time and the car getting overheated recently (hose failure), the head gasket finally was leaking enough to really see it. Totally unrelated to the off road experience, but I guess ultimately timely to show up at the same time.
Anyway, so now what? Just do the one head gasket? No... should do both. Replacing the oil pan... may as well do rod bearings... may as well do main bearings... take the engine out to make it all easier... heads off ...may as well do valve guides... may as well do rings... and on and on. Briefly contemplated a stroker kit, but it's going to be expensive enough as is (unless of course someone knows of a stroker kit somebody needs to sell cheap!)
Rick Redpath
'90 GT, Guards Red/Grey & Black - new motor in process
12-06-2005, 11:20 AM
#11
Captain Obvious
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Super User
Originally Posted by FlyingDog
A stock fpr with a vacuum port decreases fuel pressure when vacuum is present. On your car it's 55psi with no vacuum (open throttle) and 47psi with vacuum (idle). An adjustable fpr is just that, an fpr that you can adjust the pressure. You set the high pressure and vacuum does it's thing at idle.
A rising rate fuel pressure regulator adjusts the fuel pressure disproportionally from the change in vacuum or more often pressure. They have set ratios like 10:1. For every 1 psi of vacuum/pressure change, the fuel pressure goes up 10psi. They are useful in boosted cars when you can't reprogram the injection for larger injectors or higher pressures. At idle, the car runs around stock pressures, with high boost and load it runs very high pressures.
Some people call an afpr an rrfpr. Some people call an rrfpr an fmu. What I said above is what I've seen as the most common interpretation.
A rising rate fuel pressure regulator adjusts the fuel pressure disproportionally from the change in vacuum or more often pressure. They have set ratios like 10:1. For every 1 psi of vacuum/pressure change, the fuel pressure goes up 10psi. They are useful in boosted cars when you can't reprogram the injection for larger injectors or higher pressures. At idle, the car runs around stock pressures, with high boost and load it runs very high pressures.
Some people call an afpr an rrfpr. Some people call an rrfpr an fmu. What I said above is what I've seen as the most common interpretation.
Hmmm, that’s not accurate at all. The RRFPR is totally different than an FMU. They don’t work with boost at all, only designed to run under vacuum. I asked the same question a few months ago and that’s when I was told that contrary to what a lot of people believe, the two are totally different in operation. I’ve tried to look for that post but I can’t find it.
12-06-2005, 12:51 PM
#12
Captain Obvious
Super User
Super User
Originally Posted by redpathtribe
Thanks for the welcome Andrew! I've been around the email list off and on for about 5 years, but just recently "found" the forum and knew I needed to "pony up" for membership
My wife has been wondering why I'm spending evenings on the internet again... It's like when I first got on the 928 email list!
Well, you asked for the motor story. Here's the "condensed?" version:
After I got back from Sharktoberfest, car was CLEAN and running great. Entered the Wash & Shine portion of the concour for AZ Region PCA Phoenix Flight, car was even cleaner. Then it all began the next day - went out to the PCA autocross held out at McDowell Mountain Park. They had blocked off one of the picnic loops for a great course - paved course with no cones, etc. Problem was, the pavement had been recently chip sealed, so it was like driving on marbles. Track got a little faster as it was being cleaned off by lapping cars. I was running the second lap of my second run when I guess I was running the short back straight a "little" hot... started braking in a straight line... turned in for the apex of the corner... started to drift out a little... then in the loose chip seal and ZOOM... off the track and off for a ride in the desert! Came to a stop, axles buried, car sitting on the rocker panels. Couldn't get out of the driver's door because there was a tree right there! Climbed out of the passenger door (REALLY MAD at myself). Looked over the car, seemed OK. Had to be pulled out by a truck back the way I went in, but once on the track again, car ran fine back to the parking area. Really fortunate that NO damage was done to the body or suspension. As I kept going over the car looking for any sign of damage I saw some dripping from under the belly pan. Dan Warner of Exotic Motorwerks in Phoenix was there running their 944 and had me roll the car back, he checked the pavement underneath and said it was oil.
Since it was just barely dripping, drove the car back to Exotic and got it up on the lift to assess the damage. Belly pan was trashed and once removed, could see that the oil pan took a rock hit as did the external oil cooler behind the radiator. The front spoiler just flexed and took it, but the belly pan, oil pan and oil cooler were toast. We left it on the lift overnight, and if that wasn't enough, when Dan came in the next morning, under the car he saw a bigger puddle of coolant than oil !?! What the... Well, I had noticed prior even to Sharktoberfest I was loosing just a little bit of coolant, but couldn't tell where. Car like I said was running great. But, it looks like from over time and the car getting overheated recently (hose failure), the head gasket finally was leaking enough to really see it. Totally unrelated to the off road experience, but I guess ultimately timely to show up at the same time.
Anyway, so now what? Just do the one head gasket? No... should do both. Replacing the oil pan... may as well do rod bearings... may as well do main bearings... take the engine out to make it all easier... heads off ...may as well do valve guides... may as well do rings... and on and on. Briefly contemplated a stroker kit, but it's going to be expensive enough as is (unless of course someone knows of a stroker kit somebody needs to sell cheap!)
Rick Redpath
'90 GT, Guards Red/Grey & Black - new motor in process
My wife has been wondering why I'm spending evenings on the internet again... It's like when I first got on the 928 email list!
Well, you asked for the motor story. Here's the "condensed?" version:
After I got back from Sharktoberfest, car was CLEAN and running great. Entered the Wash & Shine portion of the concour for AZ Region PCA Phoenix Flight, car was even cleaner. Then it all began the next day - went out to the PCA autocross held out at McDowell Mountain Park. They had blocked off one of the picnic loops for a great course - paved course with no cones, etc. Problem was, the pavement had been recently chip sealed, so it was like driving on marbles. Track got a little faster as it was being cleaned off by lapping cars. I was running the second lap of my second run when I guess I was running the short back straight a "little" hot... started braking in a straight line... turned in for the apex of the corner... started to drift out a little... then in the loose chip seal and ZOOM... off the track and off for a ride in the desert! Came to a stop, axles buried, car sitting on the rocker panels. Couldn't get out of the driver's door because there was a tree right there! Climbed out of the passenger door (REALLY MAD at myself). Looked over the car, seemed OK. Had to be pulled out by a truck back the way I went in, but once on the track again, car ran fine back to the parking area. Really fortunate that NO damage was done to the body or suspension. As I kept going over the car looking for any sign of damage I saw some dripping from under the belly pan. Dan Warner of Exotic Motorwerks in Phoenix was there running their 944 and had me roll the car back, he checked the pavement underneath and said it was oil.
Since it was just barely dripping, drove the car back to Exotic and got it up on the lift to assess the damage. Belly pan was trashed and once removed, could see that the oil pan took a rock hit as did the external oil cooler behind the radiator. The front spoiler just flexed and took it, but the belly pan, oil pan and oil cooler were toast. We left it on the lift overnight, and if that wasn't enough, when Dan came in the next morning, under the car he saw a bigger puddle of coolant than oil !?! What the... Well, I had noticed prior even to Sharktoberfest I was loosing just a little bit of coolant, but couldn't tell where. Car like I said was running great. But, it looks like from over time and the car getting overheated recently (hose failure), the head gasket finally was leaking enough to really see it. Totally unrelated to the off road experience, but I guess ultimately timely to show up at the same time.
Anyway, so now what? Just do the one head gasket? No... should do both. Replacing the oil pan... may as well do rod bearings... may as well do main bearings... take the engine out to make it all easier... heads off ...may as well do valve guides... may as well do rings... and on and on. Briefly contemplated a stroker kit, but it's going to be expensive enough as is (unless of course someone knows of a stroker kit somebody needs to sell cheap!)
Rick Redpath
'90 GT, Guards Red/Grey & Black - new motor in process
Unless you are ready to drop a ton of $$$ into the engine I would just have the head gaskets replaced, the valves re-lapped and if the oil pan gasket is leaking have it changed. If the timing belt/water pump is close to the change interval have them replaced too. These engine will run well passed the 300k mark without any need for a rebuild. Sounds like your engine is in an excellent condition and a rebuild is not necessary. Save your money and instead of fully rebuilding, just do the above and use the left over money for a turbo or a supercharger kit.
12-06-2005, 04:24 PM
#13
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Originally Posted by Imo000
Hmmm, that’s not accurate at all. The RRFPR is totally different than an FMU. They don’t work with boost at all, only designed to run under vacuum. I asked the same question a few months ago and that’s when I was told that contrary to what a lot of people believe, the two are totally different in operation. I’ve tried to look for that post but I can’t find it.
12-06-2005, 05:53 PM
#15
Rennlist Member
Thread Starter
What's that khalloudy... how's the saying go... something about the pot calling the kettle black!!! 
Rick Redpath
'90 GT, Guards Red/black & grey - my motor will last longer than Khaleds!