afpr or rising-rate fpr
05-20-2002, 04:48 PM
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afpr or rising-rate fpr
I need to get a new fuel pressure regulator. But I'm deciding between an AFPR and a rising-rate fpr.
Maybe this a dumb question, but what is the main difference between the two. I understand that one is adjustable and the other is not. But aren't all fuel pressure regulators rising rate? Isn't that why the vacuum line is hooked up to the fpr?
86' 951 shimmed wastegate
Maybe this a dumb question, but what is the main difference between the two. I understand that one is adjustable and the other is not. But aren't all fuel pressure regulators rising rate? Isn't that why the vacuum line is hooked up to the fpr?
86' 951 shimmed wastegate
05-20-2002, 05:16 PM
#3
The only difference between the two is that one is adjustable - both are "rising rate". Indeed, that's why the stock one has a vacuum line connected to it.
I've got a Bosch-based afpr from Huntley Racing. I've also got a boost gauge and an Autometer fuel pressure gauge sitting in my center console. Let's say I set my afpr to 40psi at idle with the vacuum line disconnected. Then I plug the vacuum line back in and go for a drive. For each psi of boost, the fuel pressure rises LINEARLY by 1psi. Therefore, if I was running 15psi of boost, my fuel pressure would read 55psi.
Think about the pressure inside your manifold, right at the tip of a fuel injector. Let's say you want your fuel pressure to be 40psi, and your fpr wasn't "rising rate". Well, if you're pushing 15psi of boost, then you've got a net pressure squirting fuel into your manifold of 40-15=25psi. That's why ALL turbo fpr's are "rising rate", they need to compensate for positive pressure inside the manifold. So, now the afpr senses 15psi, so it increases fuel pressure by 15psi, to maintain the 40psi that you wanted.
I don't know about the Paragon unit, but the Huntley afpr definitely raises fuel pressure linearly with boost pressure.
I've got a Bosch-based afpr from Huntley Racing. I've also got a boost gauge and an Autometer fuel pressure gauge sitting in my center console. Let's say I set my afpr to 40psi at idle with the vacuum line disconnected. Then I plug the vacuum line back in and go for a drive. For each psi of boost, the fuel pressure rises LINEARLY by 1psi. Therefore, if I was running 15psi of boost, my fuel pressure would read 55psi.
Think about the pressure inside your manifold, right at the tip of a fuel injector. Let's say you want your fuel pressure to be 40psi, and your fpr wasn't "rising rate". Well, if you're pushing 15psi of boost, then you've got a net pressure squirting fuel into your manifold of 40-15=25psi. That's why ALL turbo fpr's are "rising rate", they need to compensate for positive pressure inside the manifold. So, now the afpr senses 15psi, so it increases fuel pressure by 15psi, to maintain the 40psi that you wanted.
I don't know about the Paragon unit, but the Huntley afpr definitely raises fuel pressure linearly with boost pressure.
05-20-2002, 05:44 PM
#4
I agree with you, Ian. I only meant to suggest that if you were to graph the output of a stock unit and the Paragon unit, the stock unit's "line" would be more or less a straight incline and the Paragon unit would gradually favor it's vertical axis over it's horizontal.
05-20-2002, 06:37 PM
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No - that's not true unless it is a "rising rate" fpr. All fpr's that I have used on 951s are standard type - meaning that fuel pressure is linearly increased with respect to manifold pressure as Ian described.
05-20-2002, 07:45 PM
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All adjustable fprs for 951s that I have ever seen have a linear relationship between fuel pressure and manifold pressure. This most likely includes the Paragon unit.
It's easy to assume that an AFPR is rising rate but that is not true unless it specifically states "rising rate". As Ian said, the Huntley AFPR is linear like the stock unit on our cars. With a standard fpr or standard afpr, be it either a 2.5 or 3.0 or somewhere in between (via adjustment), fuel pressure is linearly related to manifold pressure.
With a "rising rate" fpr or afpr (have yet to see on a 951), fuel pressure will increase at a greater rate than manifold pressure. So if you have 3.0 bar at zero vaccum, you may have 4.5 bar at 15 psi of boost, compared with the 4.0 bar you would have with either a standard fpr or standard afpr. On a turbo you can imagine how dangerous the "rising rate" can be for a car running high fuel pressures and boost.
You can make your own standard afpr that will perform just as well as the aftermarket units by cutting the top off your existing regulator and adding an adjustment screw to vary spring pressure. Making a rising rate would be more difficult because it would entail selecting a rising rate sping to control fuel pressure.
It's easy to assume that an AFPR is rising rate but that is not true unless it specifically states "rising rate". As Ian said, the Huntley AFPR is linear like the stock unit on our cars. With a standard fpr or standard afpr, be it either a 2.5 or 3.0 or somewhere in between (via adjustment), fuel pressure is linearly related to manifold pressure.
With a "rising rate" fpr or afpr (have yet to see on a 951), fuel pressure will increase at a greater rate than manifold pressure. So if you have 3.0 bar at zero vaccum, you may have 4.5 bar at 15 psi of boost, compared with the 4.0 bar you would have with either a standard fpr or standard afpr. On a turbo you can imagine how dangerous the "rising rate" can be for a car running high fuel pressures and boost.
You can make your own standard afpr that will perform just as well as the aftermarket units by cutting the top off your existing regulator and adding an adjustment screw to vary spring pressure. Making a rising rate would be more difficult because it would entail selecting a rising rate sping to control fuel pressure.
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05-20-2002, 09:44 PM
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Thanks for clearing the clutter for me. Think I'm starting to understand this.
But I wonder, if one were running a little lean at high boost, wouldn't the "rising rate" style be the one to get? Not sure why "rising rate" would be dangerous.
Seems to me that with a "linear" afpr one could not adjust for high boost/lean out problems without possibly running too rich during idle.
Thanks for all the great help guys.
But I wonder, if one were running a little lean at high boost, wouldn't the "rising rate" style be the one to get? Not sure why "rising rate" would be dangerous.
Seems to me that with a "linear" afpr one could not adjust for high boost/lean out problems without possibly running too rich during idle.
Thanks for all the great help guys.
05-20-2002, 10:40 PM
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1) You want to be real careful about running high fuel pressure at high boost because the injectors can "lock up". Conventional wisdom says 70 psi max. I tend to think that is a little conservative. Anyway, how would you know what pressure you were running with a rising rate unless, like Ian you had a fuel pressure display in the cockpit?
2) The high rpm leaning is mainly a function of the chips. The chips (factory and most aftermarket lean the mixture at high rpm to compensate for reduced boost pressure)
Bottom line - you need the right chips for your setup
Hope that helps
2) The high rpm leaning is mainly a function of the chips. The chips (factory and most aftermarket lean the mixture at high rpm to compensate for reduced boost pressure)
Bottom line - you need the right chips for your setup
Hope that helps
05-21-2002, 12:36 AM
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I think the difference between rising rate and linear has been cleared up. That is a rising rate increases the amount over manifold pressure as pressure goes up.
For awhile I was convinced that rising rate was the trick for me until I really thought about it. I kind of had in my head that rising rate increased with RPM/Load - it doesn't. So once you're at full boost you've got your full pressure, rising rate fpr or not.
So if you're working through the gears shifting at redline and then dropping down to the mid 4K range rising rate is only going to make a difference for the short time it takes you to achieve full boost. Maybe it will make a difference if you floor it at 3K, but I don't really care about that.
It was this revelation that made me realize that rising rate had little to no effect on your WOT air/fuel ratios.
Just my opinion.
--Brian Morris
89 951
For awhile I was convinced that rising rate was the trick for me until I really thought about it. I kind of had in my head that rising rate increased with RPM/Load - it doesn't. So once you're at full boost you've got your full pressure, rising rate fpr or not.
So if you're working through the gears shifting at redline and then dropping down to the mid 4K range rising rate is only going to make a difference for the short time it takes you to achieve full boost. Maybe it will make a difference if you floor it at 3K, but I don't really care about that.
It was this revelation that made me realize that rising rate had little to no effect on your WOT air/fuel ratios.
Just my opinion.
--Brian Morris
89 951
05-21-2002, 12:38 AM
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I think the difference between rising rate and linear has been cleared up. That is a rising rate increases the amount over manifold pressure as pressure goes up.
For awhile I was convinced that rising rate was the trick for me until I really thought about it. I kind of had in my head that rising rate increased with RPM/Load - it doesn't. So once you're at full boost you've got your full pressure, rising rate fpr or not.
So if you're working through the gears shifting at redline and then dropping down to the mid 4K range rising rate is only going to make a difference for the short time it takes you to achieve full boost. Maybe it will make a difference if you floor it at 3K, but I don't really care about that.
It was this revelation that made me realize that rising rate had little to no effect on your WOT air/fuel ratios.
Just my opinion.
--Brian Morris
89 951
For awhile I was convinced that rising rate was the trick for me until I really thought about it. I kind of had in my head that rising rate increased with RPM/Load - it doesn't. So once you're at full boost you've got your full pressure, rising rate fpr or not.
So if you're working through the gears shifting at redline and then dropping down to the mid 4K range rising rate is only going to make a difference for the short time it takes you to achieve full boost. Maybe it will make a difference if you floor it at 3K, but I don't really care about that.
It was this revelation that made me realize that rising rate had little to no effect on your WOT air/fuel ratios.
Just my opinion.
--Brian Morris
89 951
05-21-2002, 01:54 AM
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No. From what I have read here a rising rate regulator will have higher pressure at full boost than a linear rate regulator if they are both set at the same base pressure.
So a rising rate regulator is a way to add more fuel while under boost.
So a rising rate regulator is a way to add more fuel while under boost.
05-21-2002, 10:02 AM
#13
Sorry guys!
I guess I confused the issue right from the front. I always thought that "rising rate" simply meant that the fpr compensated for boost pressure, I didn't realize that it specifically referred to an fpr that increased fp exponentially. Personally, I'd rather have a linear afpr, and then work out lean spots using a fuel controller. It seems pretty hit and miss to me that you're going to cure a lean spot at the high end properly by going to a rising-rate afpr. You could very well end up with an even worse a/f condition.
I was part of a dyno day in Seattle a few months back. There were VERY FEW of us who were in the right ballpark with our a/f ratios. A lot of people don't pay enough attention to a/f ratios when they start adding bolt-ons to their car, but it should be at the top of the list of things to pay attention to!!!
I guess I confused the issue right from the front. I always thought that "rising rate" simply meant that the fpr compensated for boost pressure, I didn't realize that it specifically referred to an fpr that increased fp exponentially. Personally, I'd rather have a linear afpr, and then work out lean spots using a fuel controller. It seems pretty hit and miss to me that you're going to cure a lean spot at the high end properly by going to a rising-rate afpr. You could very well end up with an even worse a/f condition.
I was part of a dyno day in Seattle a few months back. There were VERY FEW of us who were in the right ballpark with our a/f ratios. A lot of people don't pay enough attention to a/f ratios when they start adding bolt-ons to their car, but it should be at the top of the list of things to pay attention to!!!
05-23-2002, 12:10 AM
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Hey guy's-
I just read over this thread, and I see where there is some confusion here. I may just make things worse, and MAY even be wrong entirely, but... I believe that "rising rate" can be either exponential, OR, linear. I mean think about it- both are raising at a rate- one linear nd one exponentially(?). Please correct me if I'm wrong, but, a conversation I had w/Jon Milledge a long time ago (~1yr) is where I originally heard the terms. I KNOW Jon knows more about this stuff w/these cars than most other engine builders/tuners put together- I could have misunderstood him though. I had told him I was running a Huntley adjustable, and he asked the question- "is it a rising rate"- I said I did not know, and he said it HAD to be a rising rate, like the Bosch. He said it probably didn't matter much since I had NO MODS, but that when I did chips, I should find out for sure if the pressure rose at the correct rate. He gave me the # of the rate of increase, and said he had personaly tested several adj ones that were NOT correct, but had never tested a Huntley one. At any rate, he was using the term "rising rate" and was saying that the 3.0bar fuel press reg that came w/his turbo kit was the of correct rising rate...
I just read over this thread, and I see where there is some confusion here. I may just make things worse, and MAY even be wrong entirely, but... I believe that "rising rate" can be either exponential, OR, linear. I mean think about it- both are raising at a rate- one linear nd one exponentially(?). Please correct me if I'm wrong, but, a conversation I had w/Jon Milledge a long time ago (~1yr) is where I originally heard the terms. I KNOW Jon knows more about this stuff w/these cars than most other engine builders/tuners put together- I could have misunderstood him though. I had told him I was running a Huntley adjustable, and he asked the question- "is it a rising rate"- I said I did not know, and he said it HAD to be a rising rate, like the Bosch. He said it probably didn't matter much since I had NO MODS, but that when I did chips, I should find out for sure if the pressure rose at the correct rate. He gave me the # of the rate of increase, and said he had personaly tested several adj ones that were NOT correct, but had never tested a Huntley one. At any rate, he was using the term "rising rate" and was saying that the 3.0bar fuel press reg that came w/his turbo kit was the of correct rising rate...
05-23-2002, 07:44 AM
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I think I know what you guys are confused about.
Jon Milledge probably was talking about the "rate" of increase in pressure.. this is a coefficient which is a linear increase.. 2x, 3x.. whatever it was.. this is still linear. (y=mx+b) where m would be the coeficient or rate of increase.
you may see an exponential increase because this is how boost is increased.. exponentially. It takes the same time to get 2psi to 4psi as it does from 4psi to 16psi. this is a power of 2. so the fuel will also increase at this rate...
so when the fuel pressure rises to match the boost pressure, it appears to do it exponentially as the boost does, but this is just because it has to match the boost pressure..
this is what i've gotten out of this.. but hey.. i too could be wrong.
Jon Milledge probably was talking about the "rate" of increase in pressure.. this is a coefficient which is a linear increase.. 2x, 3x.. whatever it was.. this is still linear. (y=mx+b) where m would be the coeficient or rate of increase.
you may see an exponential increase because this is how boost is increased.. exponentially. It takes the same time to get 2psi to 4psi as it does from 4psi to 16psi. this is a power of 2. so the fuel will also increase at this rate...
so when the fuel pressure rises to match the boost pressure, it appears to do it exponentially as the boost does, but this is just because it has to match the boost pressure..
this is what i've gotten out of this.. but hey.. i too could be wrong.